NBSIR 73-252 MFPG

detection

diagnosis

and

prognosis

Mechanical Failures

Prevention Group

MEETING No. 18

NOVEMBER 8-10, 1972

PROCEEDINGS

fdi'fed by

r.R. Sfifves and W.A. Willard

Insfifute for Hiaterials Research

National Bureau of Standards

Washington, D.C. 20234

NBSIR 73-252 MFPG

DETECTION, DIAGNOSIS, AND PROGNOSIS

Proceeiiiii|s if the ISth Meeting of the

Mechanical Failures Prevention Group,

helil at the

National Bureau of Standards,

November 8-10, 1972

Gaithersburg, Maryland,

Edited by

T. R. Shives and W. A. Willard

September, 1973

The 18th meeting of MFPG and these proceedings were sponsored by the Office of Naval Research, the National Aeronautics and Space Administration, and the Institute for Materials Research of the National Bureau of Standards.

U. S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director
.J
FOREWORD

The 18th meeting of the Nfechanical Failures Prevention Group was held

at the National Bureau of Standards in Gaithersburg , Maryland, on

November 8-10, 1972, The program for this meeting was organized by the

MFPG Technical Committee, Detection, Diagnosis, and Prognosis which is

chaired by Raymond Misialek. There were four technical sessions, each

of which dealt with an aspect of the topic. The program is a tribute

to the committee, to the session chairmen, and, of course, to the

speakers

This volume consists of the papers presented at the 18th meeting as well as the discussions of these papers and the panel discussions.

The authors' manuscripts are given as submitted to us except for minor

editing in a few cases. The discussions were recorded at the meeting

and were edited somewhat to in^Drove readability. We are presenting

these discussions as faithfully as practical and we sincerely trust

that no one has been misquoted.

Special appreciation should be extended to Mr. T. R. Shives and Mr.

W. A. Willard for their editing, organization and preparation of the

"Proceedings"; Mrs. Sara Torrence of the Special Activities Office of

NBS for the meeting and hotel arrangements; Mr. H. C. Burnett, NBS

I^tallurgy Division for general coordination, and the entire staff of

the Metallurgy Division and the Institute for Materials Research for

their assistance in many ways. Special thanks are due Miss Bronny Evans

of the NBS Metallurgy Division for her diligent efforts in transcribing

the recorded discussions.

ELIO PASSAGLIA

Executive Secretary, MFPG

Chief, Metallurgy Division

National Bureau of Standards

III
TABLE OF CONTENTS

FOREWORD

SESSION I: BEARING CONDITION MONITORING

1. A Status Report on Sensors and Their Application t

Bearing Condition Monitoring. R. M. Whittier

2. Crest Factor Analysis for Rolling Element Bearings

K. A. Smith

3. Resonant Structure Techniques for Bearing Fault

Analysis. Richard F. Burchill

4. Mechanical Signature Analysis of Ball Bearings by

Real Time Spectrum Analysis. A. S. Babkin*

and J. J. Anderson*

Panel Discussion

SESSION II: DIAGNOSTIC SYSTEM TECHNOLOGY

1. Preventive Maintenance Measurements Analysis and

Decision/Action Processes. Charles Jackson

2. The Army's Aviation Diagnostic Technology Efforts.

G. William Hogg

3. Identification of Failing Mechanisms Through

Vibration Analysis. James L. Wotipka and

Richard E. Zelenski

4. Structural Dynamic Considerations in Signature

Analysis. David Brown* and Paul Volbreck. (No

manuscript was received from the authors for

inclusion in these Proceedings.)

Panel Discussion

IV
Page

3ESSI0N III: DIAGNOSTIC SYSTEMS APPLICATIONS

1. The First Automatic Fault Isolation Testing System 99

for Reciprocating Engines Designed for

Production Use. George R. Staton

2. Condition Monitoring for Aircraft Gas Turbine 109

Engines, Past, Present, and Future.

J. F. Kuhlberg* and R. K. Sibley

3. An Automatic Engine Condition Diagnostic System for 119

Gas Turbine Engines. Murray Hoffman

4. A Systems Engineering Approach to Effective Engine 142

Condition Monitoring. David W. Leiby

Panel Discussion 185

SESSION IV: NEW APPROACHES IN SENSING AND PROCESSING

1. Application of Acoustical Holography to Industrial 201

Testing. Byron B. Brenden

2. Applications of Time-Lapse Interf erometry and Con- 210

touring Using Television Systems. A. Macovski,

S. D. Ramsey, Jr.*, and L. F. Schaefer

3. Application of Exoelectron Emission for Detection 215

of Fatigue Damage and Plastic Deformation.

William J. Baxter

4. Application of Automated Diagnostics in Motor 227

Vehicle Safety Inspection. John L. Jacobus

5. Application of Optical Processing to Flaw 244

Detection. Bill Baker*, Hugh Brown, Bob

Markevitch, Dave Rodal

* Indicates speaker when a paper had more than one author.

LIST OF REGISTRANTS AT THE 18th MFPG MEETING 252

V

SESSION I

BEARING

CONDITION

MONITORING

Chairman: Paul L. Howard

SKF Industries
i
"A STATUS REPORT ON SENSORS

AND

THEIR APPLICATION TO BEARING CONDITION MONITORING"

By

R- M. Whittler^ Manager, Transducer Development

Endevco_, Pasadena, California

INTRODUCTION

Vibration and noise emanating from a rolling contact bearing carries

Information about the condition of the bearing. The total noise

environment is, however, extremely complex. Through analysis and

experimentation certain features of the total vibration and acoustic

environment can be found to vary with the bearing's operating condition. Experimenters and instrument designers have found that detection

of such signals can be practical, and in application the result can be

useful and quite repeatable. This presentation discusses the criteria

for sensors and the technology employed to achieve this type of bearing

condition monitoring.

VIBRATION AND ACOUSTIC ENVIRONMENT

In addition to the total vibration and acoustic environment which

envelops the bearing, the physical characteristics of the bearing

itself cause additional periodic and nonperlodic dynamic stresses.

Periodic inputs include both harmonic forced vibration and repetitive

force impulses. An example of a forced vibration input would be the

oscillatory motion caused by dimensional Imperfections, such as race waviness. Repetitive Impulses occur from material impacts at surface

discontinuities, for example, at a spall. Both of these types of

inputs occur at rates which are higher than the rotating frequency of

the bearing. They are a function of ball quantity and bearing geometry

multiplied by the rotating frequency. A second type of physical change

Is the ball, race, and structure free vibration which is excited by the

impulses. A third is the high frequency stress waves which are emitted

from material failure points. This is called acoustic emission; it occurs even well before damage is apparent by visual inspection. In short, these various dynamic changes, which might be sensed, do occur

at frequencies higher than the fundamental rotating frequency of the

bea r ing.

3
"A Status Report on Sensors and Their

Application to Bearing Condition Monitoring'

The transmission of these inputs through the bearing structure and into

the surrounding mechanical system significantly changes the input.

Stress waves are partially transmitted, partially reflected and self resonances are set up — all this is a function of the total machine

design. This means that placement of a sensor is a critical factor

to the detection system design. The sensor should be mechanically

well coupled to the bearing. This usually means that placing it

physically close to the bearing is important.

SENSOR ALTERNATIVES

Many types of motion sensors have been used for machinery vibration

measurements. A common application would be the measurement of machine

unbalance at frequencies typically within the range of 1 Hz to 300 Hz.

Noncontacti ng and contacting sensors are both in use for this. A non- contacting proximity/displacement probe operates best at low frequencies

where the motions do Involve displacements In the region of .01" to .1".

Contact sensors, those physically attached to the machine, Include all

types of seismic devices for sensing displacement, velocity, acceleration, or stress. Basic to such a contacting sensor Is the fact that

its presence does affect the dynamics of the structure. In short, the

structure Is changed by the addition of the sensor. This can be

expressed as an Impedance loading problem at the contact point. On

most bearings such considerations become Important at frequencies above

a few thousand hertz. When sensing dynamic motion, one usually keeps

the sensor mass small In proportion to the local mass of the structure

at the point of contact. More often than not, this desire results in a sensor of 50 grams or less. Because of this limitation, and because

of the low displacements at high frequency, the velocity coil vibration sensor is not too useful for detection of bearing faults.

Vibration sensors designed to respond to acceleration inputs are of

smaller size than the velocity coil devices. In general their use to

10-20 kHz has become almost routine. Some people even use them to 100

kHz. From 20 kHz to above 100 kHz contact sensors designed to sense

the acoustic pressure in the structure have also been used some. Because of the high frequency response capabilities of these types of sensors they are desirable sensors for listening to bearings. A simplified vibration nomograph showing the applicable relationships,

vibration levels, and types of sensors Is shown by Figure 1.
"A-Status Report on Sensors and Their

Application to Bearing Condition Monitoring"

PIEZOELECTRIC SENSORS

Piezoelectric transducers are desirable condition monitoring sensors

since their sensitivities are high, sizes can be small, and their outputs are self-generating (no excitation voltage required). In the last

few years new piezoelectric materials have been developed increasing

the temperature capability from about 500° F to greater than 1200° F.

This is very important since bearings and machines can often be quite

hot. A plot showing temperature capabilities of some of these materials

is shown by Figure 2. Long life and excellent reliability, much higher

than that feasible with most sensors, can be achieved because these

piezoelectric devices have few parts and essentially no internal

deflections (not subject to fatigue problems of many other sensors).

Also Important to the overall reliability is the cabling and connection

system design. These components, along with the sensor, must function

In extreme environments. In service temperatures In excess of about

500° F result In special cabling and connector designs to eliminate

organic Insulation, to reduce surface oxidation on contacts, and to

Improve strength properties of the metal parts. Special designs have

been produced to provide connection systems for at least 1000 F;

however, for minimum cost and best reliability the electrical connector

temperatures should be maintained below 500° F. To provide cabling at

temperatures in excess of 500° F the metal sheathed, mineral oxide

insulated cables are used. These have been produced In various electrical configurations and sizes from .01" diameter to .18" diameter.

One interesting method to place the sensor close to the bearing and

to provide fast Installation has been to place the sensor at the end

of a probe and bring the electrical leads through the probe to a con- nector at the other end. This connector end Is exterior to the machine

and the unit is simply Inserted Into place.

To condition the electrical output from these piezoelectric devices,

circuits which sense the electrical charge, not voltage, are usually

employed. By doing this the output is not attenuated by the cable

shunt capacitance. In addition, the circuits for these types of

systems do not require the high resistance-capacitance time constant

that the voltage amplifiers require. Typical Input resistance requirements are now 100,000 ohms. Both single ended and differential sensing

circuits are available, the choice depending on noise rejection needs,

shielding difficulties, and cabling/connector requirements. Sensor

signals are now being transmitted through standard multi-pin MIL-type

connectors into differential amplifiers. If the temperature at the

sensor is below +250° F, conditioning electronics are sometimes placed

within the sensor case. This can reduce cabling and connector complexity.

5
"A Status Report on Sensors and Their

Application to Bearing Condition Monitoring"

Many different styles and configurations of sensors are available.

Some are designed for placement on bearing housings within machines

and others for mounting external to machines.

6

I
FREQUENCY |^ H z J TYPICAL DISPLACEMENT GAGE~|

I TYPICAL VIBRATION PICKUP |

I TYPICAL ACCELERATION SENSOR |

Figure 1. Simplified Vibration Nomograph

Displaying Typical Machine

Vibration Levels and Sensor

Frequency Ranges

.

Figure 2 . Temperature Ranges and Charge

Sensitivity Changes for Several

High Temperature Piezoelectric

Materials

.

7
DISCUSSION

C. Jackson, Monsanto : Are the P14/P15 materials for high temperature

use in accelerometers of the lead zirconate or lead titanate construction?

R. Whittier : No, they are not. The information is proprietary. The

materials are manufactured by our conpany and we don't reveal what the

formulations are. They are ferro-electric ceramic materials.

John Sudey, Goddard Space Flight Center: Can you suggest a way to solve

the problem when the phenomena is transient and could possibly excite the

resonance of the accelerometer? How does one prevent reading accelerometer resonance instead of the failure of the bearing whic?! is the thing

he is after?

R. Whittier: A transient input may cause the transducer to resonate when

the sensor is quite undamped, as most of them are. Some people actually

use this in their system concept, and other people, by looking at it in

a different way, simply don't look at that portion if they know what it is.

John Sudey: I would assume that the signal would be so high in magnitude

that it would saturate the amplifier that follows the transducer, and, as a result, you would get a very big erroneous signal that does not say

anything about the bearing.

R. Whittier: No, quite often that isn't the case. In that spectrum of

energy, the frequency content is not white. At the high frequency, it

is down, and therefore really the amount of noise, if you want to call

it that, sometimes appears to be very small. Another technique that is

used is input filtering. In the charge convertor, if you have any

resonance, either structural or from the sensor itself, you can eliminate it by electronic filtering.

8
CREST FACTOR ANALYSIS

FOR

ROLLING ELEMENT BEARINGS

K,A, SMITH

GENERAL ELECTRIC CO

»

SCHENECTADY, N,Y.

9
All mechanical systems generate secondary effects during their

operation. These secondary effects, in contrast to primary effects

such as pressure and flow in a pump, are not directly utilized in

the operation of the system. Examples of typical secondary effects

are vibration, acoustic noise, heat generation, etc. In fact, often

these secondary effects are major operational problems, and one would

rather see a system operate without them.

Luckily, there is a positive side to these effects. Generated

inside the machine, they carry information about internal operation

to the outside where they can be detected by a sensor and analyzed

to provide insight into the operation of the system. Signature

analysis of rotating machinery is one use of these secondary effects

which has demonstrated great success in recent years.

DIAGNOSTICS OF ROLLING ELEMENT BEARINGS

Ball and roller bearings usually follow a characteristic wear/

failure pattern. Since the sliding between metal surfaces is

minimized, the wear rate is usually very low. But the rolling

contact load between balls/rollers and the race is instead very

high, and leads to subsurface fatigue. Therefore, rolling element

bearings have a tendency to fail by fatigue rather than wear-out.

Obviously, the statements above are very general, and can be

challenged in special cases. But they describe the typical situation

10
for bearings which are properly designed and maintained.

All bearings can fail through many other mechanisms if

they are not properly used, for example, through corrosion

or lack of lubrication.

Figure 1 shows a simplified but typical wear rate

curve for a rolling element bearing. Early in life the

wear rate is often at a maximum. This is the running-in

period. The maximum may be absent for very high precision

bearings. After this initial period comes a long period

with very low-wear rate. This represents the major portion

of the bearing life. During this period the wear rate is

low, but both races and balls accumulate fatigue cycles.

The end of the low-wear period is marked by the

appearance of some surface defects , usually on one of the

races. This may be fatigue spalling but also corrosion

spots or other effects. The surface defects are small at

the beginning, but disturb the smooth rolling process,

increase the friction, and therefore grow and increase the

wear rate. When a sufficient amount of surface defects

have accumulated, the bearing fails to perform its function.

It may jam or fail to hold the position of the shaft, so

that secondary damage occurs. It may overheat and burn the

lubricant. This is the end of the bearing life. Many

techniques are used to detect bearing malfunctions in

operating machinery.

Temperature: The increased friction which preceeds bearing

failure generates heat that increases the temperature of

bearing and lubricant. Oil temperature is often measured.

11
overall Noise Level ; When the spalling has proceeded sufficiently

the bearing gets noisy. The problem is that bearings are usually buried

deep down in machinery, and there may be many other noise sources.

Oil Contamination : Spalling of bearing surfaces generates metal chips

which can be collected or detected with spectroscopic oil analysis

methods

All these techniques and others have been tried with varying

degree of success in many cases. The problem is generally that •

the malfunction indication comes too late, so that there is no

time to take preventive action before- total failure has occurred.

Another major problem is that these techniques do not identify the

failure mode. This is unsatisfactory since failure mode identification is often necessary to decide what corrective action should

be taken.

In the following a few examples will be given to demonstrate

how bearing malfunction signatures can be predicted and extracted

from complex vibration -signals

DETECTION OF FATIGUE SPALLS AND OTHER LOCAL DEFECTS

A local defect in the inner or outer race of a bearing

generates an impact every time a ball Cor roller) rolls over it.

Likewise, if the defect is in a ball surface, an impact is

generated every time it hits the inner or outer races. If the

12
bearing rotates with constant speed, each defect generates a regular set of impacts with an impact repetition rate that depends

upon bearing speed and location of the defect. Assuming that no sliding occurs in the bearing, those fundamental repetition rates

can be calculated. The result is:

Outer Race ^ n ^ BD , . xn 1 £ ±. f=T-f tl~?ji=r COS 6) (cps) Malfunction e 2 r PD * f '

Inner Race

f . =

n

^ (1+ M cos g) (cps) Malfunction i 2 r PD ^ f '

Ball ^ PD ^ ,T BD,2 o2 , , Malfunction ^b = BD ^r " PD^ ^ <^P=^

where f^ is the relative speed between inner and outer race (revolutions per second), and BD, PD, 6, and n as defined in Figure

The vibration pattern, or signature, which can be expected

from a single local defect in a bearing then consists of a series

of transients with a repetition rate as calculated above. Figure 3

shows such an ideal pattern.

The problems in practical cases are that the signature from

the bearing defect is often hidden behind background noise and also

that the repetition rate can only be calculated approximately.

It has been discussed in an earlier paper how a diagnostic

system may be designed to extract this signature from a local

defect, and thereby make the diagnostic system more selective. In

addition to the mere transient repetition rate, there are many

characteristic features associated with each individual transient.

These details of the transient waveform depend on the load pattern

for the bearings and on the structural response to the impacts

between rolling element and defect.

13
Once a relevent signature has been extracted it is possible

for an engineer to analyze the result and evaluate the condition

suggested by the data. Often, however, it is desireable to have

on-line continuous monitoring with an automatic warning if a failure

is approaching. This requires a pattern recognition scheme to

detect the failure parameter. For detection of fatigue spalls in

rolling element bearings, the crest factor meter has been found to

be a sensitive indicator. By measuring the peak level of the

processed data and comparing that to the RMS level, an "impact

index" can be defined which increases with increasing spall size.

This measurement allows monitoring of spall growth rates as well

as providing an alarm when the defect indicator exceeds a preset

level . .

This technique has been the basis of several pieces of hardware

developed by General Electric and now in use by both government

and industrial organizations. In fact, the sensitivity of this

technique is such, that in cases of low noise and strong bearing

signals (such as where the sensor is mounted immediately on the

bearing housing) , it is possible to detect small spalls without

prior processing or signal extraction.

Ik
Wear

Rate

Figure 1 • Typical Wear Rate Function For Rolling Element Be

15
•i

OD = Outer Diameter BD = Ball Diameter

ID = Inner Diameter (3 = Contact Angle

PD = Pitch Diameter n = Number of Balls

Figure 2 Basic Dimensions of Rolling Element Bearin;

16
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BIBLIOGRAPHY

Weichbrodt, B. , "Mechanical Signature Analysis, a

New Tool for Product Assurance and Early Fault

Detection", presented at Fifth Relaibility and

Maintainability Conference, New York City, July 18

1966, sponsored by AIAA, SAE and ASME . Published

in the Annals of Reliability and Maintainability

vol. 5, Achieving System Effectiveness, American

Institute of Aeronautics and Astronautics (1966).

18
DISCUSSION

Oscar Horger, Brenco: What degree of spall can be picked up in roller

bearings by this method?

K. Smith : In some of our tests on roller bearings, we have inserted

artificial defects of various sizes into the bearings. We have found

in cases where the spalled area covered something on the order of 1/4

of the roller, there was no significant energy apparent in the analyzed

data. Where the spall did cover a significant portion of the rolling

element, the information was present just as in the ball bearing. Now

the thought occurs to me that probably in a spall which develops as a result of operation, you are not going to have the artificial situation

that we created where there is a very tiny spall on one end of the

bearing, of the race, of the width of the race, unless there for some

reason is high force or high loading at that point, and indeed there

is contact and impact generated. So I think in applications where the

defect is generated during the normal operation of the bearing, this same technique applies eqixally well to roller bearings as to ball bearings.

William Glew, Naval Eng. Test Establishment : Is the averaging technique

on real time analyzer systems equivalent to signal summations?

K. Smith : The analysis that I'm talking about is strictly a time domain

averaging process. However, you can draw very close correlations between

averaging in the time domain and using a comb filter with very narrow

band width which has response peaks at each of the harmonics of the

periodicity that you are using for the summation process. In that sense,

the techniques are conplimentary, are similar. It's just that the

averaging tJiat I'm talking about is performed entirely in a time domain.

J. Sudey : If in fact you did get some answers by looking at the envelope,

would you pursue envelope protection further? . It seems to me it would be

so much sinpler than the crest factor.

K. Smith: The crest factor measurement by itself is a very simple

technique . I think it's much simpler than doing the envelope detection,

particularly where the envelope detection may require something like the

summation processing in advance. It is very often possible to simply

filter out the resonant response of the accelerometer. If you have a

high enough resonance, you can get sufficient impact energy in a band

width that extends no farther than 20 kilohertz, and you throw away the

rest of the information above 20 kilohertz. One gets rid of the reso- nance of the transducer by that means.

Jerry Forest, Ontario Hydro : Your block diagram on crest factor instrument arrangements showed an integrator at the tail end. What is the

purpose of this integrator?

19
K. Smith : The integrator is not absolutely necessary in the overall

scheme of the crest factor monitor itself. In that particular case,

the integrator was used to make sure the crest factor was remaining

at a high level for a sufficiently long period of time to give you

a reliable reading for that application, but in fact it is not

necessary in most cases, and on the system that we have used on jet

engines and on traction motor bearings, the averaging or the integrating done singly by the meter response itself is adequate to give

you a stable reading.

20
RESONANT STRUCTURE TECHNIQUES FOR BEARING FAULT ANALYSIS

RICHARD F. BURCHILL, PROJECT ENGINEER

FIELD VIBRATION ANALYSIS, MECHANICAL TECHNOLOGY, INC.

This paper presents a rolling element bearing detection technique developed to

define initial signs of failure in an angular contact ball bearing for a space

gyro application. This was done by MTI under contract to Marshall Space Flight

Center at Huntsville, Alabama. Goals were for a detector which could evaluate

installed bearings with a minimum of installation difficulties, had minimum

speed, load, and temperature dependance, and produced an output which varied

with the state of bearing condition so end-of-life predictions could be made.

The development program was a rather straight forward experimentally based one

which used a variety of sensing methods to define performance of new and damaged

bearings which were operated in a simple test rig. The subject bearing was

mounted on a stub shaft which was supported by a high speed precision spindle.

A housing was pressed on to the test bearing outer race and floated on it.

Thrust load was applied, from 50 to 1000 lbs., and the bearing was operated at

6000 and 8000 RPM.

New bearings provided base line performance. To simulate the onset of fatigue,

a bearing was damaged by acid - etching a line across the inner race ball track,

1 mil deep and about 12 mils wide. This fault is similar in character to the

first fatigue spall which appears in a worn bearing and guarantees that a defect

indication will exist for any load situation.

21
The bearing fault indication frequencies for this particular bearing were computed from rolling contact geometry. For this 107 size bearing, they were 8.7

X inner race rotation for an inner race fault; 6.3 x inner race rotation for an

outer race fault; and 6.0 times inner race frequency for a ball fault.

We looked at a variety of sensing systems to detect the bearing fault

1. Sound - to 20 K Hz with a microphone near the test bearing

2. Narrow band frequency analysis of housing accelerometer outputs at fault

frequencies (to 2000 Hz) using real time spectrum analysis techniques

3. Bearing torque -

4. Bearing outer race strain measurements using strain gages

5. Ultrasonic translator output in the 36 K Hz to 44 K Hz range which several

experimenters have used as a fault detector

6. Broad band accelerometer outputs to 50 K Hz using a miniature accelerometer

mounted on the bearing housing

From this variety of sensing techniques, only the miniature accelerometer output

provided the kind of output variation we felt we could work with. Narrow band

frequency analysis of the faulted bearing at bearing ball defect passing frequency produced only very minor indications which were nearly lost in noise, yet

one could easily see the 8.7 defect ball contacts for each revolution of the

inner race. The first figure is of oscilloscope trace photos taken for three

different bearings.

22
The upper photo is data from a new class 9 bearing.

The center photo is a similar bearing which has operated several thousand

hours and is beginning to show slight wear indications which show up as a

general roughness increase.

The lower photo is another new bearing which has an acid etched fault across

the inner race. Note the distinctive impact and decay indications as each ball

contacts the etched line. Pulses occur at 8.7 x inner race rotation frequency.

It was found that the character of the damaged bearing signal was retained when

the data was passed through a band pass filter set of 28 K Hz center frequency.

This filter has fairly narrow band characteristics with a "Q" of about 13 which

produces a window 2400 Hz wide at the 3dB points.

Second figure -

The next set of photos are for those same three bearing accelerometers , but

after being passed thru the 28 K Hz filter. Note that the lower photo more

clearly shows the characteristic impact and decay phenomenon.

We concluded that the fault information we were after was being carried on the

28 K Hz response much like an AM radio signal, so a suitable demodulation

technique was developed to get at that information.

The results of this demodulation are shown on the 3rd figureo Again, the new

bearing on top shows very little discrete content, while the worn bearing

and the discrete fault bearing show individual ball contacts. The scale

25
Qf the bottom photo has been reduced to permit the data to be

photographed, but at this point the fault indications are about

3 0 times greater in amplitude than those produced by the new

bearing.

A broad range of conditions were evaluated to determine the

effect of this bearing response, and we come to the following

conclusions:

1st - the 28 K Hz response was characteristic of this bearing

and did not change frequency with speed or load.

2nd - the amplitude of response at 28 K Hz was only weakly load

dependent. A 20 times increase in thrust load produced

only a 3 times increase in amplitude.

3rd - the amplitude of response at 28 K Hz varied almost directly

with speed - a 25% decrease in speed produced a 20% decrease

in amplitude.

4th - the bearing fault indications were only mildly attenuated

when the bearing outer race was moved from a tight, tap-fit

housing bore to one with 1 mil clearance. This turns out to

be a real asset.

We took advantage of these characteristics to build a Fault

Detection circut which included the 28 K Hz filter and envelope

detection device. The output of the envelope detector was

presented to a tuneable narrow band pass filter which could be

set to inner race fault frequency, outer race fault frequency,

or ball fault frequency to detect the type of fault present. An

alarm system indicated when a preset level was exceeded.

2h
We had built the required fault detector without fully understanding the mechanism

of the indicator. My initial computation of resonance of an individual ball had

come out at 28 K Hz , but I had neglected "G" in the equation and the actual value

was 550 K Hz, much beyond the frequency response range of this equipment. A calculation of the response of the ball on its oil films as a spring-mass system gave

values from 28 K Hz to 34 K Hz , but it was dependent upon ball load , and the experimental data did not follow that trend. Scratch another good possibility.

I had, very early in the program, computed resonant frequencies of the inner and

outer races as free rings, but the first modes came out to be 6000 Hz for the

inner race and 2800 Hz for the outer race, and I had assumed that the first modes

would be the predominant one for each. Neither frequency was present in the

Accelerometer Spectrum Analysis. I later found that indeed we were measuring

race resonance but higher modes than I had expected. With a special fixture to

resonate the inner and outer races separately, it was found that both responded

strongly at 27 to 28 K Hz , apparently the third ring mode for the inner race and

the 5th ring mode for the outer race. Exactly why these modes are excited so

clearly we don't understand, but it may be that both being in resonance reinforces

the output.

It was found during race resonance testing that low order modes of vibration were

easily excited in the free ring condition, but these seem to be very sensitive to

race mounting conditions and are not transmitted through normal bearing mount

interfaces. It seems that the very small deflections involved in the acceleration output in the 28 K Hz region are not damped out by the energy dissipators

one normally finds in a composite structure.

25
We have taken this test procedure and applied it to a number of different bearing configurations to define bearing condition, and have always been able to

find a discrete fault frequency in the range from 20 K Hz to 45 K Hz which

carries the ball passing information we need. Outer race faults in line and

out of line with the sensor were evaluated without difficulty, and also individual ball faults were easily defined. Ball faults may be intermittent however,

because they may not be in the ball rotation plane and may not show up under all

operating circumstances.

On several occasions we have broad band filtered the demodulated output so that

inner race, outer race, or ball fault will trigger the single alarm. Often it

is not important which component has failed, only that one of them is defective.

General wear in the bearing shows up as a random increase in response at the

resonant frequency and an increase in DC level from the demodulator. We have

proposed using logic to pass or fail units based on combinations of conditions

of discrete faults and general roughness or wear. We have seen frequency components in the demodulated output which appear to be related to bearing mounting

conditions, once and twice rotation, so it appears likely that the same techniques might spot problems due to housing machining errors.

We have looked at a number of automotive grade transmission and accessory bearings with good results. The class one bearings produce significant amplitudes

at resonance from their apparently higher surface roughness, but the first

fatigue fault still shows up an order of magnitude greater in amplitude than

acceptable bearings, so a very reasonable range exists to permit limits to be set.

26
We worried that the etched fault we had been using to simulate a fatigue spall

was too great in amplitude until an automotive manufacturer provided us with

a sample bearing which had been removed because it was "noisy". Its level

checked out about 8 times greater than our bearing, so we felt much better

about it I

In summary, then, bearing fault information is available from a system which

uses high order race resonance as a carrier for bearing defect information.

With high frequency sensors and electronics, it produces a linear response

output which is related to the magnitude of the defect within the bearing, so

the rate of deterioration of the bearing can be monitored. The sensor can be

attached to an outside surface near the bearing to diagnose the condition of

operating systems. The frequency range examined is far removed from the areas

in which general machinery vibrations are present, so sensitivity is maximized.

General wear indications are present to permit study of system response to oil

contamination by dirt, or by general bearing wear.

The system can be applied to new bearings as a quality control check. Expensive used bearings can be evaluated to determine reusability. It can be used

to evaluate ball or race damage incurred during assembly or during non-operational shock condition. Finally, and most important, it can define the

performance of a bearing in a complex machine and give warning of impending

problems to allow replacement much in advance of catastrophic failure.

27
CD

az cr. o <i

OD

crCO o

LU

OJ

(D

fciO •H
DISCUSSION

William Glew, Naval Eng. Test Establishment : Have you managed to establish

any amplitude criteria for good and bad bearings in these studies?

R. Burchill : What we use as a general guide is the fault indication we

obtain from the first spall. When does one replace a bearing? That's

a real serious question. We are in the process of working with an automotive manufacturer who is attempting to define the difference

between the new condition and the rate at which the bearing fails, so we can look at it from the quality control standpoint. We expect that

this program will extend also to atteiipting to run the bearing to the

time of maximum useability and yet replace it before it fails. At this

point we have not really established how far you can go.

William Glew : You are looking at a very high amplitude vibration

measured in terms of acceleration. Are you measuring it quantitatively

in terms of G levels or any other definite quantitative sort of dimension?

R. Burchill : The levels that we showed on that damaged bearing were in

the range of about 10 G's peak. I haven't established that as any magic

number, however.

William Glew : Yes, it looks to me as if potentially this is a very

valuable tool in the early detection of bearing monitoring and the next

stage would be to establish these high frequency vibrational mode G

levels

R. Burchill : With the bearing mounted in a real live structure, there

are some impedance problems, or at least some impedance requirements,

that one need keep track of , so that in establishing levels, one needs

to look at the distance between the problem bearing and the sensor.

29
MECHANICAL SIGNATURE ANALYSIS OF BALL BEARINGS BY REAL TIME SPECTRUM ANALYSIS

By: A. S. Babkin, AVCO Lycoming and J. J. Anderson, Federal Scientific Corp.

Alexander S. Babkin John J. Anderson

Mr. Babkin is supervisor of the acoustics group at AVCO Lycoming, Stratford, Connecticut, where he is responsible for all phases of acoustics, including

design of silencers, noise testing, data analysis

and predicting noise levels from engine design data. He received a BME Degree from McGill University in

1960 and an MSME Degree from Columbia University in

1967. He has 12 years of experience in sound and

vibration testing and noise control. He is a member

of the Acoustical Society of America and the Society

for Experimental Stress Analysis.

Mr. Anderson is manager of application engineering

at Federal Scientific Corporation, New York, N. Y. , where he specializes in developing new techniques in

the application of real-time spectrum analysis to machine diagnostics and production- line testing. He received a BEE Degree in 1954 and an MEE Degree in

1957, both from the City College of New York. He

has 18 years experience in the design and application of instrumentation used in control systems and

dynamic measurements. He is a member of lES, IEEE and ISA and a Registered Professional Engineer in

the State of New York.

To fully appreciate the value of such a method,

one has to experience the frustration of having

disassembled a faulty mechanism, and not finding

a defect, and having the "fault" disappear after

reassembly. How much better would it have been

to have had a scientific "diagnosis" of the defect prior to disassembling the device for

visual inspection or testing of internal parts.

Modern instrumentation techniques are now available

to measure "mechanical signatures" of rotating de- vices. These signatures allow a diagnosis of internal health without having to disassemble the

mechanism.

This report will discuss the ability of mechanical

signature analysis to detect faults in high-quality

ball bearings used in aircraft engines. Included

in the discussion will be the technical concepts

needed to understand mechanical signature analysis,

the potential pitfalls to be avoided in performing

the measurement and a description of the equipment

selected for the test program. Mechanical signatures of a number of ball bearings will be illustrated and specific frequencies correlated with

mechanical defects.

INTRODUCTION

Instrumentation for early-warning fault detection is critically needed in many industries, especially air

transportation, where high reliability and safety

standards result in large costs for quality control,

expensive test programs, periodic overhauls and con- tinuous inspection and maintenance. Although all of

these activities are very important to the program

to prevent catastrophic failures that could result

in tragedy, some of the test procedures presently

used are based on long-established methods that are tedious and often suspect. Disassembling a mechanical device to inspect internal parts can be a very

thorough and meaningful method of preventive maintenance. However, it is costly and the potential

for a fault caused by a poor reassembly sometimes

outweighs the benefits of this type of visual inspection. If methods were available to determine

the "internal health" of a device without disassembly, then maintenance and inspection costs could be reduced and reliability improved by lowering the

potential for failures due to poor maintenance work.

FIGURE 1. EXAMPLE OF HIGH QUALITY BALL BEARING

USED IN TEST EVALUATION (SHOWN DISASSEMBLED)

DEVELOPMENT OF MECHANICAL SIGNATURES

A ball bearing represents a complex vibration sys- tem whose components are the balls, ball cage, inner

race and outer race as shown in Figure 1. Like any

other manufactured part, these components have de- grees of imperfection and generate vibration by con- tinually contacting each other during rotation.

The amplitude of the vibration is dependent upon the energy of the impact, the point at which the vibration is measured, and the construction of the bearing. In ball bearings, the continuous contacting

of the rolling elements results in a vibration pat- tern that can be mathematically correlated with the

geometry of the mechanical parts of the bearing.

The vibration signal will contain many frequency

components due to the complex nature of the generation and transmission of vibration in mechanical

structures.

30
Although bad bearings tend to generate vibration

levels greater than the levels obtained from good

bearings, the simple method of meacuring peak vibration levels fails to detect a bearing with a defect

that will shorten the life of the bearing and possibly cause breakdown failure. And yet, the defect,

while not necessarily affecting peak vibration

levels, is "crying" to be heard. Like one voice

masked by the many voices of a choir, it may have a distinctive pitch that can be singled out if the ear is tuned to listen. Modern instrumentation

techniques give us the means to listen properly to the wealth of information contained in the vibration

of a bearing.

since most of the vibrational frequencies are pro- portional to speed. Rather than trying to hold speed

constant, it would be better to have mechanical sig- natures independent of speed. This is accomplished

by normalizing all frequencies relative to the funda

mental rotational speed. This procedure is called

"order normalization" where the fundamental frequen- cy of rotation is called the first order. Applying

certain assumptions, mathematical relationships can

be derived between the rotational speed of the various bearing components and the rotational speed

of the non-stationary race. When expressed as a relative speed ratio to the fundamental rotational

speed, the component defect frequencies are identified in terms of orders rather than frequency.

What is a Mechanical Signature ? The vibration signal, as measured by an accelerometer or other motion

transducer, can be electronically broken into its

frequency components and their related amplitude

levels. It is this plot of discrete amplitude vs frequency (narrow- band spectrum of the vibration

signal) that is called the "mechanical signature" of

the ball bearing, since it identifies the bearing

and is unique to the unit selected. Figure 2 is a photograph of an oscilloscope display of the mechanical signature of a ball bearing used in this investigation. Many of the discrete frequencies con- tained in the mechanical signature can be related to specific mechanical occurrences within the bearing.

The amplitudes of these frequencies are a measure of

the energy transmitted in the occurrence and, therefore", the "sL.oothness" of operation. By measuring

the amplitudes of specific frequencies, and performing relative comparisons, a decision can be made as to the "health" of a specific ball bearing.

Use of Spectral Orders . A comparison of the mechanical signatures of two ball bearings of the same type

would require data obtained at identical speed,

Linear and Non-Linear Effects . The vibration spec- trum of the ball bearing assuming rigid body motion

is characterized by a large number of discrete peaks

that can be divided into two categories-- linear and

non-linear effects. Assuming linear deformation at

the contact point and flexurally rigid bearing races

the frequencies due to linear effects can be theoretically calculated from the bearing geometry. The

computed frequency orders can be readily found in

the mechanical signatures of ball bearings. The non linear effects are associated with the non-linear

elastic behavior of the bearing structure. Frequencies associated with non-linear effects are primaril

found at low speeds and with low loads. The source

of the non-linear vibrational orders are large ball

diameter variations, eccentricity and low-order

waviness of the races. As the speed or the load is increased, linear effects predominate in the mechanical signatures.

Frequencies Related to Bearing Geometry . The vibrations generated by a ball bearing will cover a wide

range of frequency. However, in deriving the equations for analysis of the mechanical signature the

ball bearing was assumed to have rigid body motion. For this assumption to be valid, signature analysis

will be limited to a frequency range including up to eight (8) orders of fundamental rotation. At higher

frequencies bearing defects excite the elastic modes

of the bearing races. Also, random noise due to friction dominates the vibrational amplitude levels

at the higher frequencies. Preliminary studies

showed that the analysis of low-order mechanical

FIGURE 2. MECHANICAL SIGNATURE OF A BALL

BEARING. AS SEEN ON AN OSCILLOSCOPE DISPLAY

From the geometry of the ball bearing, formulas can

be derived to correlate mechanical defects with vibrational frequencies. The geometry of the bearing

will be defined in the following terms:

W = inner race rotational speed, rpm

d = diameter of ball element

D = pitch diameter of the bearing

a = contact angle of the balls to the raceway

Z = number of balls n = 1, 2, 3... index number
The fundamental frequency of rotation assuming a stationary outer race is given by

The order of the rotating ball set (cage frequency)

relative to the stationary outer race and normalized

by the fundamental frequency, fr can be calculated

as: (Ref. 1)

= -T - (^) ° ]

The order of the rotating ball set (cage) relative

to the rotating inner race and normalized:

The order of the rotating ball relative to the sta- tionary outer race and normalized:

Using these three orders of rotation, the orders

associated with various bearing faults can be calculated. Table 1 lists the formulas for vibrational orders that would be found for various types

of surface imperfections.

MEASUREMENT CONSIDERATIONS

In obtaining the mechanical signature of a ball

bearing, there are four major sources of error that

must be considered in the measurement. First, it

is important to establish what frequencies are pres- ent in the measurement that are not associated

with the ball bearing, but can be attributed to ex- ternal forces or connecting drive elements. Second,

speed changes will cause shifts in the frequency

spectrum, and will cause inaccuracies in both the amplitude and frequency measurement. It is therefore important that speed be controlled very accu- rately or a means provided in the instrumentation to correct for speed changes. Third, the random nature

of the vibration requires spectral averaging to en- hance the signal- to- noise ratio and provide statistical accuracy to the amplitude measurement. Lastly, the time required to obtain the mechanical sig- nature must be minimized, so as to reduce variations

due to temperature, lubrication, wear and calibration. In selecting the equipment to be used in ob- taining mechanical signatures, these major considerations become interdependent due to the instrument

techniques applied to the measurement.

Drive Isolation - The measurement of the vibration

of precision parts is obscured in many cases by vibration generated in the drive elements and trans- mitted through rotary couplings and mounts. Even

when care is taken to isolate the drive vibration,

the amplitude levels typically associated with the bearings are so low that many frequencies evident

in the spectrum can be correlated with external

forces. When these externally induced vibrations

have sufficient frec^uency separation from the in- ternal vibrations, and are of low amplitudes so as not to limit dynamic range, the drive frequency

interference can be tolerated. This is not nor- mally the case, however, since vibrations associated

with the drive are often the same frequencies or very close to bearing frequencies. This is particularly true of the first 10 orders of rotation when

direct coupling is used between the drive and bearing. Beat frequencies (those frequencies equal to the sum and differences of mixed frequencies) also

TABLE 1. VIBRATIONAL ORDERS GENERATED BY

SURFACE DEFECTS ON A BALL BEARING

TYPE OF SURFACE DEFECT VIBRATIONAL ORDERS

BALL PART IMPERFECTION LINEAR THEORY NON-LINEAR THEORY

Eccentricity 1 Inner race Waviness nZfi±1 nfi±fc Rough spot nZfj

Outer race Waviness; rough spot nZfc (n±1)fc

Diameter variation fc

Ball Waviness 2nfb±fc

Rough spot 2nfb

32
tend Co interfere with the. measurement of mechanical

signatures.

Therefore, great care must be taken to minimize vibration not directly associated with the rotation of

the bearing. The test equipment selected in this

study to measure ball bearing mechanical signatures

de-couples the drive prior to the start of the measurement. The ball bearing is allowed to coast down

in speed, while loaded with a flywheel that rests on

the inner race of the ball bearing while the rota- tional axis is oriented in the vertical plane. The

kinetic energy available in the flywheel at the mo- ment of de-coupling from the drive will be slowly

dissipated in the torque disturbances occurring

within the bearing. These disturbances generate the vibration sensed by the accelerome ter mounted to the fixed outer wall of the ball bearing. A "rough"

bearing will dissipate more energy and, therefore,

the time to coast to a stop will be shorter.

Speed Stability - The 'selection of the flywheel

drive imposes an additional requirement on the spec- trum analysis equipment; i.e., the spectrum must be obtained independent of speed. A technique must be applied to generate spectral orders or amplitude

components associated with mechanical occurrences

per revolution (i.e., 1, 2, 3, 4.6, 5.2, 7, etc.)

and independent of the rotational speed. A technique known as "order normalization" can be applied

when a tachometer is available that generates a pulse train, the frequency of which is proportional

to the speed of the ball bearing. Order normalization was incorporated in the instrumentation and

further description of this technique will be pre- sented in the equipment discussion under test techniques. In this way inaccuracies in frequency

measurement due to variations in speed were eliminated.

Signal- to-Noise Enhancement and Repeatability - In addition to detecting faults, it is also desired Co establish a criterion for setting quality levels for ball bearings. Since each bearing varies in quality

and no bearing component is without some imperfections, the measuring instrument should detect rota- tional frequencies associated with bearing defects.

However, these frequency amplitudes may be obscured

by the broadband noise levels associated with bearing friction, external vibracions. and instrumentation noise. A spectrum averaging technique can be applied to enhance the signal-to-noise ratio of the periodic discrete frequencies generated by the ball

bearing under test. In addition to increasing signal-to-noise ratio, the technique of spectral aver- aging increases the statistical accuracy of the measured amplitudes. Exploratory tests were con- ducted to determine the number of averages required

to obtain repeatable results within the accuracy of

the instrumentation and the limitations of the mechanical test equipment. A total of 64 averages was determined as the optimum number to obtain a repeatable spectral density. This number of averages

-X-Ould easily be obtained by a real-time spectrum

analysis system during the two minutes it took to coast from J600 rpm to 2400 rpm.

Evaluation Time - The selection of a flywheel drive

imposes limitations on the time available to measure

a mechanical signature. Statistical accuracy and

signal-to-noise enhancement require sufficient

length of data (time) to allow spectral averaging.

However, the slowing down of the flywheel speed also

varies the amplitudes of the spectral frequencies.

This cimplitude variation introduces another limitation to the measurement. A compromise is required

in selecting the maximum speed range, therefore

limiting the test time available to m.easure the mechanical signature. The selection of a Ubiquitous®

real-time spectrum analyzer provided the high-speed

analysis to allow sufficient spectral averaging within the limited test time.

INSTRUMENTATION AND TEST TECHNIQUES

The instrumentation and test fixturing had to be selected with the intention of optimizing accuracy

and minimizing the potential errors discussed in the preceding section. Figure 3 is a photograph of the test set-up selected to obtain the mechanical sig- natures of the ball bearings investigated in this program. The equipment consists of a Kostron Bear- ing Tester that provides the motive force necessary

to get the bearing/f lywheel combination up to speed

before allowing it to coast. The vibration of the test bearing is sensed by an acce lerome ter mounted

to the outer casing of the bearing.

The vibration signal is conditioned by a charge

amplifier and then processed by a real-time, 500-

line spectrum analyzer to develop the mechanical

signature. A 500-line spectral averager provides

signal-to-noise enhancement and statistical reliability (repeatability). A photoelectric pulse gen- erator provides a speed-proportional signal that is used by the spectrum tracking adapter to normalize

the frequency spectrum in terms of rotational orders

independent of speed. For this trial program, mechanical signatures were continuously observed on the oscilloscope display and permanent records V7ere obtained by plotting X-Y graphs of the mechanical sig- natures digitally stored in the spectral averager.

The block diagram of the instrumentation package,

showing signal interconnections is illustrated in

Figure 4.

Koscron Bearing Analyzer - The mechanical fixture

used in these tests to impart the rotation to the test ball bearing was developed by Bendix under the trade name Kostron. The device is used to measure

bearing vibration characteristics in such a manner

that the rotating energy source does not introduce

significant spectral componencs into the mechanical

signature of the test bearing. This is accomplished

when a high- inertia fly\vheel is brought up to speed

by an electric motor and then lowered to engage the inner casing of the test bearing. The drive motor

is disengaged from the flywheel and the kinetic

energy of the flyi^'heel is allowed to dissipate itself in the friction and torque disturbances of the test bearing supporting the fly\\;heel. Once freewheeling, the fly\..'heel takes approximately eight

minutes to dissipate its energy in a typical sample

from the type bearings used in this program. The Kostron Bearing Analyzer 'incorporates a built-in

53
HA, %

OSCILLOSCOPE DISPLAY

UBIQUITOUS® REAL-TIME SPECTRUM ANALYZER

SPECTRAL AVERAGER

X-Y RECORDERKOSTRON i'i^h,;L MOVER UNIT BEARING j

^ 1 w i,U TRACKING

ADAPTER (NORMALiZER)

FIGURE 3. BEARING DEFECT DETECTION SYSTEM

MOTOR

FLYWHEEL

SPECTRUM

TRACKING

ADAPTER

(MODEL 24AX)

PHOTOMULTIPLIER

ACCELEROMETER

BEARING

CHARGE

AMPLIFIER

UBIQUITOUS®

SPECTRUM

ANALYZER

(MODEL UA-6B)

DIGITAL

SPECTRAL

AVERAGER

(MODEL 1015)

OSCILLOSCOPE

X-Y RECORDER

FIGURE 4. BLOCK DIAGRAM OF INSTRUMENTATION
optical tachometer to produce a digital display of

bearing speed. This tachometer, generating 60

pnilses per revolution, is used to provide order

normalization of the spectral data.

Vibration Transducer - The vibration generated by

Che motion of the ball bearing is sensed by a piezoelectric accelerometer , rigidly attached to the outer casing of the test bearing. The sensor must

have a frequency response sufficient to cover the

data range of interest. Limiting analysis to a range where rigid body motion of the bearing is a valid assumption, the measurements were made over

the first eight orders. Data is processed while

the bearing coasts from 3600 rpra to 2400 rpm. This

is equivalent to a first- order frequency ranging

from 60Hz to 40Hz. The eighth order would then

range from 480 to 320H.Z. The lowest order of interest is 0.4, this being equivalent to a frequency

range from 24 to 16Hz. Therefore, the sensor and

instrumentation must have good low-frequency re- sponse and be essentially flat from 16Hz to 480Hz

to provide the required signatures.

The accelerometer selected for this test program

w<^s a B & K type 4332, having a flat frequency -re- sponse from 0 to 6000Hz +2%. Loxv- frequency cut-off

is generally determined by the conditioning amplifier and high-frequency response is limited by the accelerometer mounting characteristics. To maintain. good low-frequency cut-off, a charge amplifier

was selected for conditioning the accelerometer output signal before processing the voltage in the real-time spectrum analyzer. The charge amplifier

provides low- frequency cut-off below IHz. The resonance of the accelerometer mounting used was assumed to occur at approximately 2000Hz. The fre- quency response of the input circuitry to the realtime analyzer has lower and upper frequency break

points at 1.7Hz and 600Hz.

Therefore, the instrumentation selected for the test

is essentially fiat over a range of 5Hz to 500Hz and accurate to within +ldB over the 16-to-480Hz range used in the determination of mechanical signatures.

Spectrum analyzers using analog sweeping methods

would take 10 minutes to generate only DIE independent spectrum, or over 10 hours for a 64-spectrum average. Not only is the slov/ness not acceptable to this program, but earlier analyzers proc- essed very little of the available data and many

of the identifiable frequency components were lost.

Spectral Averager - The mechanical signature of the

ball bearing is obtained by averaging a number of

statistically independent spectra using a Federal

Scientific Model 1015 Spectral Averager. Figure

5(a) is an oscilloscope photograph of the frequency

spectrum obtained from a sample ball bearing. The

spectrum was obtained at 3400 rpm using signal data

lasting 0.85 seconds or one real-time period (i.e.,

sufficient time to generate one statistically independent spectrum). Figures 5(b) and 5(c) show

spectra obtained at 3000 rpm and 2600 rpm, respectively, using approximately 0.96 and 1.1 seconds of

time data. In each photograph, the spectrum is speed- normalized, so that each trace represents 8.3

orders of rotational speed.

The effect of speed variation on the vibration am- plitudes of the major orders can now be compared.

The upper trace clearly shows the increased, broadband noise present at higher speeds. This noise

(a)

(b)

Real-Time Spectrum Analyzer - The heart of the in- strumentation is the Federal Scientific Ubiquitous®

Spectrum Analyzer. The accelerometer signal is analyzed for frequency/amplitude components by the

Model UA-6B, 500-line analyzer. A real-time spec- trum analyzer was selected for this test to enable

the high-speed spectral processing required in the study. The UA-6B is a hybrid spectrum analyzer

(analog and digital) that uses the digital timecompression technique to expand the low-frequency

input signal to a high-frequency waveform equivalent. This high-frequency signal is spectrtun ana- lyzed using a fast-settling broadband analysis

filter. (A technical description of the performance of this type spectrum analyzer is contained in

References 2 and 3.) The selection of a real-time

analyzer in this test program is based on its ability to process lOOZ of the vibration data available

in the short period of slowdown between 3600 and

2400 rpm. In less than two minutes the real-time

analyzer generates 64 statistically independent

spectra in the frequency range of interest to this

study.

(c)

FIGURE 5. FREQUENCY SPECTRA

OBTAINED AT 3400, 3000, AND 2600 RPM

tends to obscure the stable discrete frequencies

present in the mechanical signature. Figure 2 is

the mechanical signature of this same bearing obtained by averaging 64 spectra within the speed

range of the spectra sho'.vm in Figure 5. The study

proved that to generate repeatable mechanical signa- tures of ball bearings, both spectral averaging and

55
order normalization was required. A total of 64 spectrum averages was selected as a compromise be- tween increasing statistical accuracy and minimizing

the range of decaying speed for the test data.

Since vibrational amplitudes become very low at low speeds, test data were restricted to speeds

above 2400 rpm.

Spectrum Tracking Adapter - Spectral averaging is required to enhance the vibrational signals that are buried in the random noise and to establish repeatability by statistical averaging. Figure 6 shows

the time history of spectra during coastdown from

J400 rpm. The sliding frequency pattern, due to speed slowdown, is evident in the time history.

U^en the spectra are normalized with respect to speed, the frequency sliding is eliminated, as shown in the time history of Figure 7. The spectra are said to be "order normalized."

The Federal Scientific Model 24AX Spectrum Tracking

Adapter accomplishes order normalization by using

the tachometer pulse signal available from the Kostron equipment. The tachometer pulse rate, which

is proportional to bearing speed, is used to control

the sampling rate of the Ubiquitous'"^' Spectrum

Analyzer, In this way, the sampling rate of the vibration signal varies with speed and, therefore,

automatically compensates the spectrum for speed

variations. (This technique is discussed in greater

detail in Reference 4.) The use of order normalization is even more important when averaging is re- quired. Figure 8 shows the spectrum time history

without normalization and Figure 9 with normalization. In each photograph every 16th spectra only

is displayed for comparison.

When averaging the sliding frequency spectra, the energy of the orders is distributed over more than

one spectral address of the digital averager. The

effect is to smear the peaks and this is especially

evident in the higher orders. For example, the energy of the eighth order will be distributed in

eight times as many addresses as the energy of the first order. Figure 10 shows the same spectra as Figure 8; Figure 11 shows the same spectra as Figure 9, but, instead of showing every 16th spectra, each trace represents the average of the pre- vious 16 spectra. VJhereas the normalized time history shows consistency in the spectral amplitudes,

the time history not using order normalization

broadens the spectrum, peaks and does not maintain

the shape of the mechanical signature spectrum.

MECHANICAL SIGNATURES OF BALL BEARINGS

The bearings selected for this study were split

inner race, angular contact ball bearings used in

jet aircraft engines. Forty bearings x^ere tested,

six were new, 28 used and six were reclaimed bearings. The new bearings were purchased and inspected for defects both visually and by measurement. The used bearings were obtained from the AVCO

Lycoming overhaul facility and the load/life history

was not available. The reclaimed bearings are used

bearings that had defective parts replaced by new

parts. Present inspection methods for these bearings,, including visual inspection and probing with

styli are slow, tedious and, in many cases, unreliable, A faster automatic inspection system, not depending on operator judgment is preferred. An automatic high-speed system proposed for this meas- urement is shown in Figure 12. The tests of this

program were carried out to determine if bearing defects could be sensed and identified by the selected

instrumentation.

Bearing Frequencies - The bearings used for analysis had the following geometry:

Ball diameter (d) = 0.50000 inch

Pitch diameter (D) = 2.75565 inch

Contact angle (a) = 28.5 degrees

Number of balls (Z) = 14

Using these measurements in the formulas for bearing frequencies we obtain:

fc = 0.42

fi = 0.58

fb = 2.69

With these basic rotational orders, the formulas

for surface defects contained in Table 1 are used

to calculate all possible frequencies- that may

exist in the mechanical signature of the bail bearing. Table 2 lists all vibrational orders up to 8.54 using both linear and non-linear theory.

Test Results - The mechanical signatures of four of

the bearings selected for discussion are shown in

Figures 13, 14, 15 and 16. The amplitude of the spectrum is plotted in log scale to provide the greatest dynamic range on one chart. This will

allow the detection of small defect frequencies in a measurement containing a large frequency component. The dynamic amplitude range of the instrumentation is better than 54dB.

A total of 8.3 orders was selected for-plotting as this range includes all .najor linear orders. Above

this range, order- related vibrations tend to excite

the elastic modes of che bearing races. Also, ran- dom noise due to friction dominates the spectrum,

making it difficult to measure frequencies that can

be correlated with bearing defects.

The mechanical signature of a good bearing is shown

in Figure 13, The amplitude is calibrated for 9QdB

equals 0.26 g and this scale is used on all graphs.

The noise floor for all mechanical signatures is approximately 50dB or 0.0026 g. The first order is the only frequency evident in this mechanical sig- nature. Although the first order can be correlated

with inner race eccentricity, the 1, 2, 3, and 4 orders obtained on graphs are assu:ned to be caused

by the imbalance of the large fly\meel used in the Kostron.

56
Figure 6. Spectral Time History - No Order Normalization Figure 7. Spectral Time History - With Order Normalization

Figure 8. Spectral Time History - No Order Normalization - Figure 9. Spectral Time History - With Order Normalization - Every 16th Spectra Every 16th Spectra

Figure 10. Spectral Time History - No Order Normalization - Figure 1 7. Spectral Time History - With Order Normalization

16 Spectra Averaged 16 Spectra Averaged

57
TABLE 2. VIBRATIONAL ORDERS ASSOCIATED WITH A BALL BEARING

HAVING 14 BALLS, 0.5" BALL DiAMETER, 2.76" PITCH DIAMETER

AND 28.5° CONTACT ANGLE

LINEAR THEORY NON-LINEAR THEORY

BALL INNER RACE OUTER RACE INDEX INNER RACE OUTER RACE

1 U.ib 1 .UU 0 0.84

U.HZ 9 11 . 30 1.26 1.00 3 1.32 2.16 1 .68 i 4 1.89 2.74 2.10

5 2.48 3.32 2.52 6 3.06 3.90 2.94

7 O.DH AH .HO AR o.oD

ccLU 8 4.22 5.06 3.78 Q

5.38 9 4.80 5.64 4.20

CEo

— 10 5.38 6.22 4.62 Q

5.80 5.88 11 5.96 6.80 5.04

LU

h- 12 6.54 7.38 5.46 <

o

7.12 13 7.12 7.96 5.88 _1 14 1.10 8.54 6.30 CL 8.12 15 8.28 6.72 Q

16 7.14 9.12 17 7.56 18 7.98 1

BEARING

FLYWHEEL

T

MOTOR

OPERATORS

CONTROL

PANEL

ACCELEROMETER

PHOTOTACH

CHARGE

AMPLIFIER

SPECTRUM

TRACKI NG

ADAPTER

GO /no go decisions

FAULT IDENTIFICATION

UBIQUITOUS

REAL-TIME

SPECTRUM

ANALYZER

®

MINI-COMPUTER

TELETYPE

DIGITAL

TAPE

RECORDER

FIGURE 12. BLOCK DIAGRAM: PROPOSED INSTRUMENTATION FOR

HIGHSPEED EVALUATION OF BALL BEARINGS

58
1 2 3 4 5 6 7 8

VIBRATIONAL ORDERS

FIGURE 14. MECHANICAL SIGNATURE OF BALL BEARING SHOWING BALL DEFECT

39

A mechanical signature showing a ball defect is illustrated in Figure 14. The presence of two large orders (5.80 and 1.00) will normally generate

beat frequencies that can be identified (5.80 +

1.00, 5.80 + 2.00). This bearing also shows orders

associated with inner race defects as determined

from non- linear theory.

Figure 15 illustrates the mechanical signature of a ball bearing showing an inner race defect. This

signature also shov;s boat frequencies spaced one order apart with lesser amplitudes. The presence of

a slight ball defect 'is assumed since the signature

also shows a 6dB increase of the fifth order over

the fourth and sixth orders.

An outer race defect is illustrated in the mechanical signature sho^m in Figure 16. Linear theory

correlates the peak found at the 5.88 order. Numerous other peaks can be correlated with the outer

race defects calculated using non- linear theory.

Correlation With Defects - Bearings having mechanical signatures containing discrete frequencies that

could be equated to imperfection frequencies were

disassembled for inspection. In all cases, defects

indicated by large amplitude orders could be found

in the inspection process. HcwevTr, many of the de- fects indicated by the lower amplitude orders could

not be found by present inspection procedures. It

is felt that the instr\imentation developed for this

study has greater ability to detect small bearing

imperfections than present inspection procedures.

It remains for further studies to determine Viow much these lower order imperfections will reduce

bearing life.

Test Time - For the results obtained in this investigation, mechanical signatures could be recorded

at a rate of one bearing every five minutes, with

more than one-half of the test time expended in

setting the bearing in the test fixture. Recordings

of mechanical signatures stored in the averager can

be plotted simultaneously with the changing of test bearings. The time required for analysis is not easily determined because the factors to be evaluated (i.e., the number of spectral orders, amplitudes, go/no-go reference levels, and multiple

weighting factors) have not been determined.

The instrumentation is capable of faster measurement

and evaluation by introducing a mini-computer to the set-up. The computer is programmed for calibration

and transducer equalization modes. Data will be obtained for peak amplitudes at selected spectral

points and recorded on high-speed digital magnetic

tape or slower-speed paper tape for processing sta- tistical data off-line. Immediate decisions can be

made for quality acceptance testing. The mechanical signature is compared to a reference signature

stored in the mini-computer and a go/no-go indication given to the operator. With reduced handling

of ball bearings by improved fixture design, the rate of high-quality testing and evaluation can be reduced to less than two minutes. Incorporating a mini- computer into the instrumentation system not only provides a new technique for high-production

quality control but also allows for a statistical

data bank of bearing defects that can be. used to re- vise designs or alter manufacturing nrocesses.

i+1

Figure 12 is a block diagram of the instrumentation

proposed for high-speed evaluation of ball bearings.

Figure Studie s. It is planned to extend the investigation beyond the objective of the present study.

With the ability to correlate vibrational orders

with bearing imperfections, future studies will

attempt to develop a quantitative relationship

between spectral amplitudes and severity of the defect. Also, a criterion will be sought for pre- dicting the remaining life in a bearing showing a lower amplitude imperfection.

These studies will require a large sample of new

bearings, all tested and accepted for high quality

and displaying "good" mechanical signatures. The

bearings will be subjected to controlled service in test rigs and periodically tested to obtain a time

history of mechanical signatures. Bearings will be disassembled to obtain corroborating inspection data

when variations are found in the mechanical signature .

CONCLUSION

This study was undertaken v.'ith the knowledge that

earlier attempts at mechanical signature analysis of

ball bearings were unsuccessful, or at best, not practical for high-speed test evaluations. The

majority of the earlier studies suffered from limitations inherent in the instrumentation used for the

measurement of the signature.

The present investigation was successful in developing test techniques using standard comm.ercial instrumentation to obtain repeatable mechanical signatures of ball bearings. The frequencies found in

the mechanical signatures were correlated with vibrations related to specific parts of the ball bearing.

Within the limits of the statistical sample of ball

bearings tested for this study, a trend was indicated in the ability to measure bearing quality.

The amplitude of each frequency is assumed to be a measure of the severity of the part roughness or bearing defect. The detailed correlation of spectral amplitudes with the severity of defects, and more important, the remaining useful life, v;ill be

the subject of a future study.

REFERENCES

1. Gustafson, O.G. , eC al, Dec., 1963, STUDY OF VIBRATION CHARACTERISTICS OF BE.\RINGS , SiCF Industries, AD432-979, 177 pp.

2. Monograph ii3, June 1971, REAL TRE SIGNAL PRO- CESSING IN TlIE FREQUENCY DOMAIN, Fed.iral Scientific Corporation, 21 pp.

3. Bickel, H. J. , March 1971, "Real-time Spectrum

Analysis", SOUND AND VIBRATION 5(3):i4-20

4. Anderson, J. J. , Dec. 1971, "Real-time Speccrum

Analysis in Vibration Testing", ISA TRANSACTIONS,

10(3) :269-276
DISCUSSION

Lee Doubleday, Naval Air Systems Command : Do you operate the bearing

under any load at all? Yoia would have to know what kind of loading

caused the failure if it is loaded against the race.

A. Babkin : Loading in this case is a fly wheel which is about 25 lbs.

in weight. This is a thrust load applied to the bearing. The load

is constant.

John Jamieson, Kennedy Space Center : Doesn't the fact that you are using a thrust loading on that bearing make the track to one side of

its normal operating point, if that bearing wasn't thrust loaded in

the machine it was used in? Doesn't that change what balls run?

A. Babkin : This angular contact bearing, as any split race bearing

is, was loaded in the same direction as it is loaded in the engine.

It has to be, otherwise it can't operate. The whole thing is the fly

wheel. It is suspended on air jets so that we don't get any frictional

effect and we have a constant thrust load on the bearing.

Jerry Forest, Ontario Hydro : Diaring the run down of the fly wheel, won't

all the forces acting on the bearing change? Centrifugal loads will change.

I assume, like you say, there is some unbalance. The magnitude of the

signals at the higher orders should be somehow related to these forces.

How does this affect the analysis?

A. Babkin : As the fly wheel is coasting down, the magnitude of the

disturbing forces also decreases. You could see this from the slide that

showed repetitive average spectra. We based our analysis on 164 or 128

seconds. So basically, most of the information was derived in the

beginning when the bearing was under relatively high speed. As it slowed

down, useful information was lost. A time limit of 2 minutes was used,

because beyond that point, there was no information left.

J. Anderson: Data was obtained between 3 thousand and 2 thousand RPM's.

It took anywhere, depending on the quality of the bearing, from 8 to 15

minutes for that fly wheel to come to a halt. We limit ourselves to an area of 2 minutes.

k2
PANEL DISCUSSION

Panel Members : R. M. Whittier, K. Smith, R. F. Burchill, J. J. Anderson,

A. S. Babkin, and R. Misialek

Panel Moderator : Paul Howard

Jerry Forest, Ontario Hydro : It seems to me, in comparing all 3 basic

systems o£ diagnosis or detection that have been presented, that the

crest factor approach presents a very simple and straight forward technique.

Perhaps this is a good point to discuss because some of the other problems

that were mentioned, like changing frequencies, at least the way I understand it, should be taken care of by the crest factor approach, Mr. Smith

mentioned that crest factor can peak at 5 to 7 times that which is considered

normal or satisfactory. Is this a universal number? If it is, I think

it seems to be a very good approach to failure detection.

K. Smith : In the majority of cases that we have looked at where the signal

is detected from the accelerometer placed somewhere in very close proximity

to the bearing, the results have been very repeatable. The normal bearing

runs at something around 2 to 3, and the crest factor runs something around

6 to 7 for a small spall. So in that sense, it's probably repeatable. If

you modify the signal in some way be adding large signal components on top

of it, you're going to dilute that crest factor measurement, so it is

important that you have a good signal to noise ratio. It is a simple

technique, but of course like anything else, you never get something for

nothing. First of all the crest factor monitoring technique does not

discriminate all by itself whether the defect is located on the inner race or the outer race, or on the ball, or someplace else. If you want to

determine where it is located, you have got to look at some of the other

parameters that have been measured such as the impact rate or the calculated

resonance response of the structure that's being excited. In terms of just

detecting that a particular bearing has a fatigue fault present in the very

early stages, it's a very simple technique, and apparently from all our

results, a very repeatable one.

Paul Howard: Would each of the gentlemen on the panel highlight the

similarities and differences of your systems?

R. Burchill : Mr. Smith has commented that the background noise is a very

important facet in the crest factor approach. If the level is too high,

other conditioning is required. The advantage of the resonant structure

technique is that, by using a very selective frequency range, we've been

able to reject a majority of the background noise. The information that

we are looking at then is related to the bearing itself. By using a very high frequency range, we are also isolated from many of the problems

that are experienced by attenpting to detect bearing faults along with

other machinery problems. This has been either attempted or done by the

Federal Scientific people.

^3
A. Babkin : Basically, the technique that we described was more selective.

It not only indicates that there is a defect in a bearing, but indicates

which part the defect is in. Now the crest factor technique works but,

as Mr. Smith pointed out, it indicates only that there is a defect

present. You can do the same thing by looking at the amplitude distribution functions. If you plot the distribution of anplitude as a bell

chart you can see if there is a fault in a bearing, the plot will be

very flat with a lot of peaks, and if the bearing is acceptable the

plot would be much narrower, so your signal image will be further apart.

In our technique we use a normalization method to get away from a speed

dependency which other techniques utilize. In Mr. Burchill's case he

is using a higher mode resonance of a race to detect a peak. This will

be specific only to a particular bearing. If he changes the bearing,

the resonance will appear different. In our technique, peaks will occur

in different places if the geometry is changed, but our technique is also

independent of the way the bearing is held to a great extent because we

don't depend on the structural resonance. As you pointed out, you can damp out the first few modes of ring resonance just the way you hold it.

K. Smith: It's a little difficult for all of us to point out the differences because we are all looking at the same basic technique, the

same basic event. The fact is that if a ball rolling over a defect

generates a transient vibration which is then propagated outside the

structure and which, at the same time, excites various structural reso- nances, there are a number of features that can be looked at and different

ways of looking at them. I think that the pertinent fact is that there

are various levels of sophistication to which you can go. If you simply

want the lowest level of detection, then you can monitor a spall by

looking at the peaks of energy that are indeed present in the signal.

If you want to know where the defect is located, then you use tJcie knowledge about the impact rates, and you can use the normalization procedure

that has been discussed with averaging. You can use something like a tracking filter to zero in on the particular rates that you want to look

for. If you increase the problem by having a much lower signal to noise

ratio, you'll have to go even deeper into the signal in order to pull it

out. I think the differences among the techniques attempt to resolve

various complicating factors involved in looking at a particular bearing

system. It all depends on how you wish to handle those complicating factors.

EWayne Fry, Oak Ridge Nat. Lab : Our group at Oak Ridge has been involved

for several years in analysis of noise signals from reactors to try to

detect incipient failure or component degradation in the system. Has

anybody on the panel or in the audience had experience in the application

of continuous monitoring of these techniques and if so, how has it helped

you in the diagnosis of incipient failures in your systems?

kk
J. Anderson: I am not going to answer your question, but there is

someone else within Federal Scientific who is experienced in this area,

and there is an application note available from Federal.

R. Burchill : Since we have gotten into ball bearing activities, we have

been using these very high frequency response sensors more often to look

at varying kinds of machinery. We have been able to look at resonant

responses of other structures, using similar demodulation techniques to

the ones we've applied to the bearing. In geared systems, we have

seen resonant modes of gears where the gears are being impacted and they

ring like a bell. The repetition rate on this ringing frequency can often be related to the forcing function, and very often to a fixed

structural member that is vibrating. By using this modulation component,

one can often lead back to the source.

Dwayne Fry: I am wondering if anybody has installed any of these sensing

devices on actual operating machinery and used them to diagnose failures

on line. I^st of what I've seen here this morning have been, I believe,

laboratory type applications and on knoim failures . I am wondering if

you can detect these in the presence of the normal background noise in

an operating system?

K. Smith: We've been monitoring bearings in jet engines for some time.

At Randolph Air Force Base, bearing monitoring of J- 85 engines is being

done in test cells, but it's an internally monitored sensor so iVs always

there. During each test period the defects are monitored. In one case there was a defect detected on a #2 main bearing ball in the presence

of the operating noise of the jet engine and all the rest of the noise

from the test cell. It was identifiable as a ball defect from the

periodicity of the data that was taken. There are other applications

being made within General Electric 's Jet Engine Dept. There are a number of places where it is being used continuously.

William Glew. Naval Eng. Test Est?ib1 i^hmpint ! I think the point is well

taken that these are apparently laboratory tools that we have been talking

about. The GE system is very expensive. Our experience with the realtime analysis system on the outs ides of typical machines is that it's not

too good at telling bearing faults inside the machines and any Pratt §

Whitney guy around here will say that they're no good on jet engines.

However, we have recently had quite a bit of luck on simple machines with

an ultrasonic analyzer and this is why I was very interested in Mr.

Burchill 's presentation. We have a quite old Hewlett Packard analyzer

and Dave Watson, who is with me here today, started trying to tell us 5

years ago that he could tell when a bearing was bad one minute after

it was installed. We recently carried out a comparitive check between

this instrument and the fairly new SKF shock monitor and found that
Dave with his analyzer can tell a defect very much earlier then the SKF

shock monitor. This is basically because until you actually have got a defect on your bearing, your shock monitor, or your transient pulse

indicator, will not tell you that anything is going to go wrong. The

ultrasonic analyzer appears to operate in the 34-36 kilohertz range. By

the loudness in the audio frequency produced from these ultrasonic waves,

you can tell whether a bearing is misaligned or whether there is any

undue pressure on the inner race or the outer race right from the word

go. This is the only practical application I've seen of bearing monitoring.

Most of the systems are very much too complex and too sophisticated and

require a very high standard of interpretation. But there is this ultrasonic monitor which I would like the panel to comment on.

R. Burchill : I have some experience with this ultrasonic monitor. In

the program we ran, we were evaluating that instrument because it has

been used in several cases and the manufacturer is very cautious about his

claims for it. I believe the range is from 36-44 kilohertz and that data

is folded back from 0-4 kilohertz. With sensors that can respond in this

range, we didn't find exactly what was happening. I think that the over- all levels in this high frequency region do tend to go 'up with the defect,

and the unit does respond to it. We were not particularly enthused about

linearity and the skill required to operate it.

K. Smith : I would like to make just one comment since my name was taken

in vain. The GE system, the basic system of crest factor monitoring as

I have tried to point out, is very single and consequently very inexpensive.

For the spectrum analyzers or ultrasonic translators, we're talking in

the range of 100 's of dollars, not 1000 's of dollars. That system will

tell you when you have an initial spalling if you have a sensor located

very close to the bearing itself.

R. Whittier : There are a lot of different applications. I just wonder

if one of the differences between these techniques relates to the fineness

of the resolution of the detection. Some people are not interested in

detecting minor faults such as a little etching or a little spalling, but

they are more interested in knowing whether the balls are still in there.

A very high frequency detection or sensing system tends to pick out the

smaller items, the small irregularities or imperfections. A question

for these other fellows relating to their technique is how do they resolve

bearing faults from other faults in a machine, such as a blade problem

or a foreign object damage.

k6
J. Anderson : I would like. to defend realtime analysis and echo what

Bob Whittier just said. All these techniques have advantages and

disadvantages. You have to look at the application involved to see what is best for you. To use realtime analysis in a bearing failure

application, I 'm going to repeat what I said at a meeting of this

society a year and a half ago. In the initial application of realtime analysis, everyone expected too much. They went into the system

expecting to solve all problems. The comment I made a year and a half

ago was, we had to walk before we ran. No one took it upon himself

to take a discrete conponent and to see whether the technique was

applicable to that component before we went into a complete machine

and all the inherent problems in many forcing functions. Our paper

is showing you an application of realtime analysis where it was good

for that application. It was a discrete component where we isolated

all the other vibrations normally associated with a coirplex machine.

K. Smith : I want to fully concur with that. There are a number of

demonstrat ions that we have seen in our work where realtime spectrum

analyses have given a lot of information about the condition of a bearing immediately off the assembly line, whether there is any

built-in out of roundness or any built-in eccentricities or ball size

defects. All these things show up as spectral components, and indeed

it is possible and was done on a program for the I^riner Space Craft

evaluating long life rate gyros. It is possible to rank a group of

gyros in terms of how many of these built-in defects are present, and

thus perhaps make some evaluation as to which of the group is the better

bearing. We are talking about different applications, different

monitoring goals and it's a question of how early in the game you want

to know about it, how good you want the bearing to be when you finally

pull it out and do something about it. There are a lot of questions

that I think only the user can give an answer to. It's not really our

place to answer these questions for you. We are presenting techniques

that work in a variety of situations, and you are here to shop amongst

them.

Paul Howard: There is one thing that I hope all this brings out to you.,

especially the first time users, before you get hooked on diagnostics.

If there was only one way to go there would be one rich guy and nobody

else would be in the business. The thing that makes this a horse race

is that there are a lot of different opinions and every one of these

guys can point to a success. So you know their technique works on something, and it's very important to consider how close your application

is to that something. I would like to state that bearings do not come

off the line with defects. It's the users who smash them up on installation.
A, P. Brackney, Monsanto : We are believers that bearings are great

when they come out o£ the factory. But in addition, we might have a pump and a reducer and a motor. What are the individual speakers'

plans to help us out in the trenches where the problems are? I think

realtime analysis is very close.

Paul Howard: I would like to give a short overview in answer to that

and then turn it over to the panel. I think one o£ the problems that

everyone of us who is in this business has faced is the inability,

usually due to lack of data, to get a good description of the machine

and of the problem that you are trying to solve. You go in and try

your particular approach. If it works, you are a hero and if it doesn't

work for lack of data or any reason, you're shot down.

R. Burchill : In general, the kind of work that I do is diagnostic work

in machinery. It seems very often that you come iq) to a machine looking

for a problem that might be apparent from vibration data one can measure. Or the operator might say, "Oh by the way, we ran a little water into

it one day and this is what happened to it". This is part of the

problem. It's often a very complex situation. I think the realtime

spectrum analysis is an extremely valuable tool in getting at some of

these particular facets. If you can identify a cause and effect, then

often you can use less expensive, simpler equipment to monitor that

particular situation. If you can use the analyzer to determine what

kind of a monitor that unit needs, you've won.

Alan Duguid, Huntington Industries: We think in terms of the bearing

in any system. It is the weak link in the whole system and any of the

idiosyncracies that exist within the system are going to be concentrated

in the bearing. My interpretation of this morning's discussion is that

basically you are talking about fatigue failures in a rolling element

bearing. Fatigue failures account for only a very, very small portion

of the failures. My method for checking bearing operation is very simple,

and that's to use a heat sensor very close to the bearing. When you

start up a new system you will find that the heat output of the bearing

will increase very rapidly, and then it will flatten out and it will stay

that way. If it doesn't, it will increase very rapidly and I would say

anywhere in the period of 15 minutes to an hour, you are going to know

whether it is going to stabilize. I have seen some systems where the

heat generated within the bearing has been so rapid from the time you

push the button to start it till the time you could react to turn it off,

the bearing was conpletely shot. I have seen needle bearings that have

generated sufficient heat that they actually fused together and it was

1+8
difficult to determine the difference from one roller to another.

Sensing the heat that is generated in a bearing is going to create

the means by which you can check it for a lot of different reasons. The user specifically is not interested per se whether a ball has a pit in it, or the race has a pit in it, or what the failure is. All

he's interested in is that there is the possibility of a failure.

The failure in a rolling element bearing, whether it be a ball or a

roller, can be dependant upon just the intermittant skidding of the

rolling element which is a heat generator. I also found that a very

excellent tool to use is the torque that's required to drive the

system. This is not going to react as fast as the heat is, but it

will react. One of the major causes of failure of rolling element

bearings is the failure of the retainer. Maybe it's possible to

instrument the retainer, but I personally have not found a way. If

anybody has, I would be interested to hear about it. This is ray first experience where I have been to a meeting of an organization

that really could be the voice of the user. One of the basic problems

is the fact that the bearing manufacturer produces a product that

consists of inner and outer races, balls, rollers and the retainer.

When you the user put that bearing in your equipment, that's your

responsibility. I don't care what the bearing manufacturer has done

to the bearing, you can ruin it. And the problem is that most of

the time the user has a bearing failure and he blames the bearing

manufacturer. I was affiliated with a bearing manufacturer out in

the wilds of Michigan and they are very peculiar people. I have seen

bearings that had chips and dirt in them and I swear the only way

they could have gotton in there was for somebody to put them in there

purposely. If I can say anything to leave you with one basic thought

about the use of rolling element bearings, it is that each one of you

is going to have a separate problem and there isn't going to be anyone

else who is going to answer it for you but you. Engineering is an art

and you only learn it by doing it.

Paul Howard : I do have to give you a little bit of flak on temperature

because we've run a lot of grease tests for people and there isn't any

really good way to tell when a bearing is gone. There are a lot of

good ways to tell when the grease is gone and we have determined this

by temperature measurements. We can tell a grease manufacturer whether

grease A or grease B is better under a given set of conditions, and we

do use the rate of increase of the temperature and change in the rate

of increase (the second derivative)

Alan Duguid : I have seen cases where the oil was running at temperatures

around 150" and the bearing burned up.

1+9
Paul Howard: The oil must not have been in the bearing.

Alan Duguid: That's right. Just because lubricant is alongside the

bearing doesn' t mean that it's in the bearing.

J. Anderson: The torque approach is just another way of sensing, and

when you are talking about the torque required to turn a bearing, you

are talking about driving that bearing off line, not on line. So

immediately you are faced with the same techniques. The use of torque

is just another sensing technique. Dave Seneca from GE, Valley Forge,

has a paper that was published in Nature Magazine which was specifically

on that technique. But after the torque is sensed by strain gages it

is spectrum analyzed. It turns out that the overall torque level is not

an indication of incipient failure. But once spectrum analyzed, lower

orders down as much as 10, 20, 30 dB below overall were an indication

of incipient failure.

Jim Reis, Northrop Corp : I would like to ask a couple of questions

pertaining to the ultrasonic device and the crest factor measurement.

Has any study been done by you people to find out how these measurements

relate to the actual remaining useful life of the component, that is,

using these techniques in a prognostic sense as opposed to a diagnostic

sense? Also do you have any experience in separating or do you know the

effects of gear meshes in obscuring the techniques from working properly?

K. Smith: There is some indication, not yet fully confirmed, but I think that there is every reason to believe that it will bear fruit, that

the impact level will change as a function of the life of the bearing

once a spall has developed, growing with the spall i:^ to a certain level

where the spall is covering a significant portion of the race or the

ball. At this point you begin to increase the overall vibration level

of the bearing significantly. Thus, the crest factor, after reaching

some peak level, will tend to level off and then start to be driven down

as a result of this overall "vibration level occuring. So the thought

is that by monitoring this bearing over a period of time, after the

initial indication of a fatigue spall, it may be possible to extrapolate

something about the projected failure time by looking at the shape of

this curve and the rate of change of the crest factor.

Jim Reis : Currently you've made no such measurements, is that correct?

K. Smith : Very little.

J. Reis : Is the rate of change when it's failing an exponential function, or IS It a linear function?

50
K. SmitJi: I really couldn't tell you at this point. I think it's

probably an exponential function but we don't have enough information

to tell you.

Paul Howard: There are a number of techniques that keep track of

damage growth. That's one that does. Shock pulse does, and in the

shock pulse case it's usually linear.

C. Beachem, NRL : What is the size of the smallest bearings which you

have listened to or analyzed by your acoustic emission listening devices?

Is there some small or minimum size below which it is not worthwhile

doing?

K. Smith : The smallest bearing we have looked at is a bearing that is

m an x-ray tube and is about 3/4 of an inch in diameter. The largest

was a 40 foot diameter antenna drive bearing that the technique worked

equally well on.

R. Burchill : At MTI, our program has been a little shorter term than

at GE. So far we have looked at bores from 15 mm to 55 mm. This is

general gear box and engine range.

Bruce Baird, Boeing Co. : In answer to a previous question we have had

a system installed at a chemical plant in Baton, Texas since last Inarch.

It operates 24 hours a day and we detected one failure about 8 days

before there were any other signs of failure. We use a system that is

similar to the systems being shown here. I have a question for Mr.

Burchill. I found that I can put a spall into a bearing or a flaw in a bearing and detect the peak signals out anywhere from very low

frequencies to very high frequencies, and not just at the resonances of

the outer race. I've run them from down around 20 kilohertz up to the

megahertz range. I feel that it's mainly the resonances of the transducer

that I'll pick up rather than the resonances of the races or the balls.

R. Burchill : The transducers that we have been using have advertised

mounted resonances of at least 80 kilohertz and above. The calibration

data that I have shows that it's flat through the region in which I am

operating, so I haven't been concerned. I've felt that I have been far

enough away from that resonance that it hasn't been a problem. By picking

a particular resonance that we have demonstrated to be quite independent

of load, our system becomes more selective, and it's true that there

are many resonances one can use, and they all have this same pulse

character. But they have other facets that control their amplitude,

and if you are going to make any sense of amplitude, not having to

normalize the data in some fashion, there are some real bonuses in

being selective about what you look at.

51
Bruce Baird : Transducers are used resonantly at 120 kilohertz, and

that is where I made my measurements, and I did di:5)licate the data

that you have shown there with the spherical roller bearing.

R. Burchill : You detected your signal and got that periodic wave shape?

You just did an amplitude detection on that signal?

Bruce Baird : Right. Essentially we filtered out the high frequency

band and envelope detected it to produce this modulation.

R. Burchill : I did this also, and then you can do a realtime signal

analysis to increase your signal detectability. Also, you can detect

your high frequency peaks relative to your background friction noise.

I think that might tie your system in with the crest factor approach.

Bruce Baird: I think we've been frightened somewhat of using a system

that depended upon the resonant characteristics of the sensor because

of the problems that have been indicated in the range of the resonance. One may have a very pure and very large amplification, or you may have

other distortions in that region. We have stayed away if we could

from the sensor resonance itself.

R. Burchill : We haven't had any trouble. We band limit rig^t around

the resonance of the transducer, but we are using transducers that are usually used with acoustic emission techniques. That is really the

earliest sign of detecting a bearing failure because the bearing failure

starts beneath the surface and propagates to the surface as Mr. Whittier

mentioned. It generates acoustic emissions as it propagates to the

surface, and these can be picked up using higher frequency techniques

instead of lower frequency techniques.

L. F. Sturgeon, GE : I want to respond to the question on bearing size.

We are using techniques which are related to all the ones which were

described this morning. Our primary enphasis has been on the spectral

analysis approach. The application has been primarily to small bearings,

for the reason that the smaller the bearing, the larger the effects of

mounting and internal geometric variations will be on the loading. We

are primarily interested in predicting failure down stream. We were

doing this on our 165 bearings and we are routinely using this on

bearings of the R2, R4 size.

David Board, Boeing: You discuss the advantages and problems associated

with applying the crest factor analysis techniques to the high frequency

acoustic emissions of the short duration and rapidly attenuated type

that are generally associated with initial bearing defects. Could you

also discuss the cost aspects of implementing this in realtime, in a real world.

52
K. Smith : The basic technique of crest factor measurements can be used

on any kind of a signal. If short duration high energy pulses are the

indication of failure, then in theory the crest factor technique can be

applied to those kinds of signals. It will indicate the presence of

those kinds of signals in otherwise random or background noise types of

situations. The limitations are primarily those of instrumentation of

a sensor. The problems in getting a sensor that is useable over the

range which you want with the required sensitivity are (1) getting the

sensor mounted in such a fashion that you have good response over that

range, (2) obtaining reliable signals at those higher frequencies and,

(3) having electronic circuitry that can handle the higher frequencies.

Robert Boole, General Radio ; I would be interested in any comments on

the band width that users have found to be desirable, either the band

width of a tracking filter or the effect of band width of a realtime

analyzer.

J. Anderson: Once again this comes in the area of applications. In the

case of bearings, when you go through the formula there are approximately

8 to 10 frequencies in the linear domain that all appear within the first

10 orders. Now, depending on the geometry of the bearing, some of these

may be very close to each other. This predicates what band width is

required. Normally you try not to have too close or too narrow a band

width. It becomes more critical as far as speed stability.

Robert Boole : I am assuming a constant band width is required. I was

just wondering about some comments on how narrow they have to be for

certain applications?

R. Burchill : I think there are some real problems with a very narrow

band analysis technique, in that if you have data that is of the inpulse

nature, the energy involved is very, very low, and so one can be fooled.

If you have some data that has a high crest factor, a spectrum analysis

of that information does not give you anything near the amplitude that

you would measure from an oscilloscope. One need be cautious because

you can get in trouble even with the rapid data production capabilities

of realtime analysis.

J. Anderson: The wider you make the band width, the less benefit you

get from signal annoyance reduction.

Peter Kamber, Boeing : The fact that three Boeing people have so far

spoken up here should indicate that we have a very profound interest in this

business. My interest specifically is in seeing equipment coming forth

55
that can be put on airplanes and used to do realtime analysis. I have

a feeling that we cannot do this without totally miniaturizing the

normalizing equipment that has been presented here. One of the panel

members has mentioned the magic words, the concept of tracking filters.

Does anybody have applicable experience in electronic tracking filters,

specifically the types that use miniaturized phase lock loops?

D. H. Biggs, Bently Nevada Corp : As far as the tracking type phase

lock loops in general are concerned, there are two types, the digital

and the analog. The digital has the advantage of being limited to a

very small range which you can track. The analog has the disadvantage

of a slow rate capability for the time it takes to get v^ere it wants

to go. The kind of system you deal with depends upon how much time you

have to take the data.

Paul Howard: Will each panel member give a short summation of what has

been discussed.

J. Anderson : What I have learned from this panel is that there are more

similarities than dissimilarities among our systems and basically we all

have the same problems. Our technique was one of looking for a system

to give early detection. It is a screening type system. In spite of

the fact that it is on line analysis, the con^ponent is tested off line.

Taking this to an on line test of a large, complex device, still has to

be proven. The problem is one of economics. It is not economically

feasible for a si^plier who is selling a $200 accelerometer or a $5000-

$10,000 instrument to embark on a long term program to prove the capability

in conplex machinery that would involve possibly a $100,000 to $200,000

study project. I have tried and the other speakers have tried to outline

techniques. It is up to you to look at the techniques and the specific

application. Listen to the various advantages and disadvantages and

judge for yourself what technique is most applicable for your needs. I have found no magic panacea enabling you to sell a piece of equipment

that goes on line without embarking on some sort of a preliminary program

to study the approach.

R. Burchill : We've taken some very advanced accelerometer designs that

have been working adequately in frequency ranges that have not been

ignored, but let's say considered only in the noise spectrum, and found

that there are indeed some rational conponents that can be used to define

the performance of a bearing. By being selective in this frequency

location, we've been able to retain the capability of evaluating minor

degradation due to wear or to lubricant problems or dirt ingestion. It

also gives a progressive increase as the bearing deteriorates. They can

be applied to simple machines as a quality control system, or to complex

installations with multiple bearings continuously or periodically monitored.

3h
K. Smith : I can only echo what's been said already. There are techniques

available at varying levels of sophistication, but there's no one technique that's going to solve everyone's problem. It's incumbent upon any- one who wants to use these techniques to evaluate his diagnostic goals

and then through direct contact or through meetings like this, to com- municate those goals to the people who are doing the work developing

these techniques. In that way usable systems may be provided that will

satisfy your requirements for diagnostics monitoring.

A. Babkin : Basically as pointed out, there are different levels of

s ophis t i cat ion. In certain bearings you cannot do any maintenance. In

certain types of bearings you can replace components. It also depends

on the economics of a system. Our system is applicable if the bearing

cost is above $80 a piece. Below $80 the system will not be economical.

R. Whittier : From a sensor standpoint, there are many ways of signal

enhancement" that have been discussed. I see applications from low

frequency to high frequency. In the lower frequency areas, up to 20

kilohertz, we are talking about measuring the motion in the structure.

At frequencies above that, in general I see people actually making acoustic

measurements on these structures. A sensor may be resonant in that area,

and if the sensor is being used in its resonant mode, there is an inherent

limitation, basically because it is now a resonant system more susceptible

to changes. I will second Burchill's effort here in that there is a good

deal of effort going on in the higher frequency areas in nonresonant systems

or in systems with some band width to them.

55

SESSION II

DIAGNOSTIC

SYSTEM

TECHNOLOGY

Chairman: Jack L. Frarey

Mechanical Technology, Inc

PREVENTIVE MAINTENANCE MEASUREMENTS

ANALYSIS AND DECISION/ACTION PROCESSES

Charles Jackson, Monsanto

MFPG, Gaithersburg, Maryland - November 8, 1972

This is a summary of a 72 slide program presented to MFPG relating the

Petrochemical Industry in terms of measures, decisions, and actions taken

to prevent failures on rotating turbomachinery.

While one cannot relate to the entire industry, the rotating equipment is representative. The following types of turbomachinery operate at speeds up to

52, 000 rpm, horsepower to 15, 000, temperatures from plus 1750 to minus

340 degrees F, with conventional trains in three cases of equipment at 10, 000

rpm.

Steam Turbines

Centrifugal Compressors - Horizontal and Vertical Split

Expanders

The maintenance cost for our one plant is one and one -half million dollars on

rotating equipment. Machine weights to rotor weights will vary in a ratio

sometimes from 20:1 to 175:1. Examples of various problems with rotating

machines are recorded including rotor fouling unbalance, rubs, oil whirls,

misalignment, blade salting, thrust overloads, loose components, base plate

resonance and other sub-harmonic resonances plus many forms of bearing instabilities.

DESIGN PRACTICES:

Many of the practices outlined in the 1973 revisions of API 6l7 which refer to

centrifugal compressors for refinery services plus the soon-to-be issued API

614, seal and lube oil standard, are applied throughout our plants. A review

of various practices are listed.

1. Two eddy current proximity probes are mounted 90° apart at or on each

bearing observing the relative vibration of the rotor to its bearing. These

outputs connect to permanent monitors for two levels of alarm. The first

level is important because it warns the operating people that the normal

operating level has been increased and that one of two things generally

could happen. The first would be a shutdown because the second level

may soon be reached, or secondly, the machine may maintain this higher

level which would indicate that one of the mechanical technology personnel

should be called to investigate the problem for a possible correction with59
out shutting down. A single level alarm really doesn't give you too much

choice. If you put the maximum threshold level^then you must shut down.

If you did not, you hardly know what to do except call someone and wait>

risking serious failure if levels increase quickly.

Seismic sensors are used on slower moving equipment and generally

equipment with exposed bearing caps. These sensors sense the absolute

vibration measure of bearing caps to space.

A key phasor probe is installed on driver equipment so that a phase

reference might be obtained on each train of equipment, i. e. , a compressor will be related to its driver phase as would the driver be

related to its own phase. Installation of this phase marker on the

driven equipment would not allow a reference when the driver is run on

solo test.

All vibration instrumentation can be viewed on oscilloscopes, x-y re- corders, octave band or fourier type spectrum analyzers, tracking

filters, phase meters, and other supporting instrumentation such as a

digital tachometer.

Thrust displacement of rotors is sensed in both directions by eddy

current probes preferably viewing the thrust bearing collar but occasionally viewing the rotor shaft near the collar when it is impossible to

install a probe looking through the thrust bearings at the thrust collar

as on some barrel type compressors.

The oil systems are fairly large consoles using stainless steel piping

from the oil filters to the bearings with dual five micron, nominal,

full-flow filters in the main supply. One-half micron, nominaljfilters

are used in the supply to the continuously lubricated coupling drives

which, in operation, are effective 8000 G centrifuges. If oil or solids

of a density other than turbine oil are present they will separate in the

coupling in the form of a sludge which may become a critical problem

promoting the sticking of these couplings which will severely affect the

smooth performance of rotating machinery.

Steam turbines are equipped with vacuum leak-off glands and large condensers along with 10, 000 G centrifuging at the oil reservoirs to maintain control on water and dirt accumulations in the oil system.

60
8. Thrust pad embedded thermocouples, or RTD's, are used on turbine

thrust bearings. Generally the centrifugal compressor will use a static

pressure gauge in the balance chamber and an oil exit temperature

sensing the immediate discharge from the oil control ring of the thrust

bearings. At severe situations, we have embedded thermocouples in

the thrust pads of the compressor as well.

9. Oil exit temperatures and/or embedded couples are provided on radial

bearings and other instrumentation or design, too numerous to list, are

provided in the oil circulating system of both turbine and compressor.

INSTRUMENTATION:

The portable instruments used by our department now total $80, 000 and are

used for surveillance of approximately 100, 000 horsepower in turbomachinery

.

The by-line becomes MACE (Measurement, _Analysis, C^orrection, and Engineering). This equipment was obtained piece by piece over the past 10 years. We

tried to obtain equipment that we need and understand so as not to get ahead of

ourselves. Each piece of equipment has its place. Often the less expensive

equipment may do a particular job better in certain situations. A good example of this is oil whirl which might occur at 43 to 45% of the operating speed,

that is, the frequency measurement may be 43 to 45% of the synchronous frequency. A hand-held vibragraph, which is a rather inexpensive piece of equipment, prints out on pressure sensitive tape showing the waveform with a timing

mark that occurs every half - second. A four foot trace from this instrument

will very clearly show the non-periodic vibration form of an oil whirl frequency.

ALIGNMENT;

Sixty percent of the problems in rotating equipment were attributed to misalignment during the first 5 years of turbomachinery analysis. As a result,

original development of techniques in proximity measure from water cooled

reference stands started in 1965, later accompanied by company-owned optical alignment tooling, in 1970. Prior to this we had contracted our optical

alignment work and there are several good firms performing this service.

BALANCING;

Use of conventional seismic - strobe equipment in field balancing has been used

for many years. The current x-y probes with key phase markers allow orbital

balancing reported earlier in an ASME paper. The most recent field balance

was on a 14, 000 horsepower, 5200 lb. steam turbine rotor, operating at 1 1 , 000

rpm which was balanced from 2. 6 mils peak-to-peak to 1 . 1 mils peak-to-peak.

61
SEVERITY:

Severity and action points are the most difficult part of any analysis system.

No one criteria is absolute. No one machine operation is identical to another.

However, within the framework of rough guidelines and confining the limits

to high speed turbomachines ,the listed values can be considered conventional

threshold values. I feel it safe to say that when these values are reached,

strong study and consideration will be given toward shutting down the equipment; trying, of course, to obtain the most information possible in order to

make a decision as to the problem. When the no-go limits are reached in

many cases the shutdown is committed by automatic systems. Other times it is based on operator judgement or on mechanical technology personnel judgement. One exception is that all machines shut down automatically when the

second limit of thrust deflection occurs on the operating rotors. The thrust

bearing is the only mechanism separating the rotating elements in the

machine from the non- rotating elements and when 20 mils deflection from the

original design deflection point has occurred, these machines shut down automatically. The consequences are too great, in our opinion, to await the normal time to make a decent decision. A thrust bearing conventionally fails in

about 30 seconds. I have examined over forty failures.

LIMITS

Description

Seismic Bearing Cap Measure

Proximity Relative Shaft to Cap Vibration

Thrust Deflection-Kingsbury Type Thrust

Bearings - Measured from Float Zone or

Normal Deflection Point

Misalignment-Center Line Offset per

Inch of Coupling Spacer Length in Inches

Thrust Bearing Babbitt Temperature

- Bronze Backed High Tin Babbitt

- Oil Control Chamber or Ring

- Pivoted Pads

- Turbine Oil Viscosity at 150 ssu @

lOO^F and 43 ssu @ 210°F

Go No-Go

0-. 2 ips o/p 0. 5 ips o/p

0-2. 2 mils p/p 4. 2 mils p/p

1 5 mils 20 mils

1/4 mil

170-210"F

1/2 mil

240°F

62

I
Within limits, we have found the guidelines presented by the following persons

or companies to be good. We do not necessarily adhere to them sacredly, but

the values indicated in these guidelines approximate those used by Monsanto.

IRD's modified Rathbone chart for vibration severity based on

bearing cap readings only .

Dresser-Clark's severity chart for vibration on their centrifugal

compressors as measured by proximity sensors giving relative

measure, shaft to bearing.

Koppers high speed coupling misalignment limits.

Safe thrust bearing temperature limits, as published by the

Centritech Corporation in Houston, Texas.

BIBLIOGRAPHY:

Jackson, Charles - "Successful Shaft Alignment" , Hydrocarbon Processing,

January, 1969, Vol. 48, #1 (Ref. ASME paper 68 -Pet- 25, "Shaft Alighment

Using Proximity Transducers", by Charles Jackson, September 1968).

Jackson, Charles - "How to Align Barrel-Type Centrifugal Compressors",

Hydrocarbon Processing, September 1971, Vol. 50, #9, Pgs. 189-194.

Jackson, Charles - "Monitor Points on Turbomachinery", Twenty-Sixth

Annual Symposium on Instrumentation for the Process Industries, Chemical

Engineering Department, Texas A & M University Proceedings dated January

20, 1971.

Jackson, Charles - "Using the Orbit (Lissajous) to Balance Rotating Equipment", ASME paper 70, Pet-30, September 1970.

Jackson, Charles - "An Experience With Thrust Bearing Failures", Hydrocarbon Processing, January 1970, Pgs. 107-110.

Jackson, Charles - "Vibration Measurement on Turbomachinery", Chemical

Engineering Progress, Vol. 68, #3, March 1972, Pgs. 60-65 (AIChE CEP

Technical Manual, 1972, Vol. 14.).

65
Dresser Industries - Clark Turbo Compressor Division, Clean, New York.

"General Guidelines for Vibration on Clark Centrifugal Compressors",

(chart on last page).

Koppers, Inc., Baltimore, Maryland, "Instruction Sheet 1900-59", Fast's

Self -Aligning Coupling Recommended Limits of Misalignment for Fast

Couplings

Herbage, Bernard S. - "High Speed Journal and Thrust Bearing Design",

Centritech Corporation, presented at the first Turbomachinery Symposium,

Texas A & M University, October 1972, Proceedings, Pgs. 56-61, Figure

Page 61.

Sohre, John S. - "Turbomachinery Anaysis and Protection", First Annual

Turbomachinery Symposium, Texas A & M University, October 1972

Proceedings
DISCUSSION

David Biggs, Bently Nevada Corp . : Can you give at least one example of

putting together information on deciding when to shut down a machine?

You gave us real quickly two levels. I would like a more specific

exanople with a few more details.

Charles Jackson : That's difficult to answer. It's kind of like a domino game. Tt's like the fellow with the still. When he loses his

mash, the whole unit goes down. On some of this turbo machinery, we

consider 2.2 as a warning level and at 4.2 we bring it down. We have

two different types of people to address this situation or problem to: one is the guy operating the unit who is out there on the No. 3 shift

and he doesn't really know what to do. The level all of a sudden hits

4. We try to pick that level on a basis of no matter what's happening

it's too high, and he must bring the unit down, if the system itself

checks out. There are some instrument checks. If the detecting device,

is malfunctioning or not functioning, then of course he would not make

a decision to bring the unit off. If he can check another point on the

same bearing and it agrees with the first point, the system is valid.

He is doing the redundant checking very fast, in a matter of a few

minutes. There are other inputs such as increasing temperatures in the

bearing, or changes in load that can be checked before actually shutting

a unit down. But if the system is actually functional, we want that

man to bring the unit off if it has reached the shut down level. The

alarm level is about twice the level for normal operating conditions.

We accept a unit on test standards on less than one mil peak to peak

at trip speed which is 15% over rated speed. If the detection system

is functioning properly and the alarm comes on, we start our analysis.

We may decide to bring a unit down at the warning level if the system

is erratic and only getting worse. We like to bring a unit down safely.

We may want to abort a pump in favor of the boiler. Let the pump fail,

for God's sake, save tJie boiler. We want to drive as long as we can

until we get a spare unit on. It now becomes the "sacrificial anode".

We like some redundancy. That's why our radial units are running more

like 401 automatic shut down, 60% not automatic. But on thrust, we

have found you can't play that game. If it moves, you have to come down

with it. We'll then make a decision on it.

Alan Duguid: How many man years went into the development of this system?

Charles Jackson : It's taken 10 years and we have tried to come up from

vibrometers to handhelds to proximities, accelerometers . We do not have

a realtime analyzer. We are being criticized every day for it and I think

they are right. The first accelerometer I used, as an exaiirole, we put on a steam turbine that had a 48 KHz resonant frequency. Tne turbine was

running maybe 1/2 G. But all of a sudden, it went to 10 G's. The

accelerometer was in resonance with the blade pass frequency.

65
THE AiMY'S AVIATION DIAGNOSTIC TECHNOLOGY EFFORTS

G. William Hogg

Eustis Directorate

US Army Air Mobility Research and Development Laboratory

Fort Eustis, Virginia

A review is made of the Army' s aviation diagnostic exploratory

development programs. Current efforts being pursued by the Bustis

Directorate, USAAMRDL, are discussed and the areas of diagnostic

research which sho-uld be pursued in the future are cited. The

discussion includes analysis techniques, sensor technology, oil

monitoring, and prognostics.

66
THE ARMY'S AVIATION DIAGNOSTIC TECHNOLOGY EFFORTS

INTRODUCTION:

Records show that Army operational helicopters experience a downtime of 25% to 30%, downtime being the ratio of out-ofservice time to total available time. About one-fourth of this

downtime is caused by supply problems, and about three-fourths

is in waiting for maintenance actions. These large periods of

aircraft unavailability obviously have a serious impact on

military operations, from both an effectiveness and a cost

standpoint. The maintenance performed on the Army's helicopter

fleet is of such a magnitude that the average annual helicopter

maintenance cost is 4 0% of the original purchase price of the

aircraft. Included in this cost are overhaul costs, which are

substantial. As an example, it costs $3,000 to overhaul a UH-1

transmission and $10,000 to overhaul each of the CH-47

transmissions

Although there are a large number of factors which come into

play in these high maintenance costs, there are a few that seem

to stand out as large and identifiable items. Inaccurate and

untimely fault identification processes are a large contributor.

Another identifiable item is what might be called "the

supply system problem''. Inasmuch as no advance warning of a malfunction is received, no advance notice is given to the

supply system that a replacem.ent part is needed, so one of two

conditions must exist -- either a large inventory of replacement parts must be maintained at maintenance sites, or

long delays will be experienced in the acquisition of replacement

parts. At the present time, the former situation is attempted,

but the latter is often experienced.

67
The third factor which appears to stand out is the overhauling of components on a time basis only. This is not to

condemn the time-based overhaul procedure, because in the

absence of other information upon which to make a determination

relative to overhaul, time-basing that decision appears

reasonable. That fact notwithstanding, a large number of

components are removed from helicopters for overhaul unnecessarily. A recent survey of transmission removal data showed

that of a total of 251 transmissions removed for overhaul, 198

were removed for no reason other than the fact that they had

reached their established time for removal. It is reasonable

to expect that some of these 198 transmissions did have

distressed components in them but there was no indication of

deterioration sufficient to cause them to be removed.

The ability to diagnose the mechanical conditions of aircraft components and establish a prognosis of that condition

will have a great effect on the cost and usage factors.

Accurate and timely fault identification determinations will

eliminate most of the uncertainties and the large number of

unwarranted component removals. Full use of diagnostic and

prognostic equipment will allow the removal of components on

a condition basis, as opposed to the currently used time-based

removal basis. The use of equipment having these capabilities

will lead to more valid prognosis and will greatly facilitate

the logistics problem of having spare parts on hand when

needed

The universal use of diagnostic and prognostic equipment

will have a most significant effect on virtually every aspect

of aviation. The extension, and possible elimination of,

time-based removals of aircraft components by the concept of

removal for cause has pervasive implications. The air crew

68
will continually be aware of the mechanical condition of their

aircraft. Maintenance personnel will have a substantial

portion of their unnecessary workloads reduced or eliminated.

These factors will have considerable impact by reducing the

required replacement parts inventories and other associated

logistics problems. The advanced information on forthcoming

component removals and required replacement will facilitate

having the right parts at the right place at the right time.

Knowledge of forthcoming maintenance requirements will also

allow higher aircraft effectiveness through scheduling. All

of these attributes will contribute to assure mission-readiness,

flight safety, and conservation of materials, personnel, and

finances.

In addition to the direct payoff of reduced maintenance

costs, an additional benefit of a diagnostic system will be the

identification of material failure mechanisms. With this

knowledge, the designers should be able to design future

systems with longer life and better reliability. The

development of diagnostic and prognostic capability in the

Army will have a major impact on the maintenance concepts

which will be employed in the future.

OBJECTIVES

The objectives of the Army's exploratory development program

are to provide an adequate technology base for the advancement

of Army aviation diagnostic systems and to investigate new

diagnostic concepts and their feasibility for more effectively

diagnosing the mechanical condition of Army aircraft components.

This presentation is a review of the efforts currently being

undertaken by the Eustis Directorate of the US Army Air Mobility

Research and Development Laboratory located at Fort Eustis,

Virginia to meet those objectives.

69
Diagnostic Technology Investigations ; Past, current, and

planned efforts to advance diagnostic technology can be broken

into four broad categories: (1) sensor technology? (2) signal

conditioning and analysis technology; (3) prognostic techniques;

and, (4) diagnostic techniques.

Sensor Technology ; Sensor technology is not sufficiently

advanced to provide all of the needed usable diagnostic signals

from flight-type hardware. In some cases, acceptably accurate

signals can be obtained from existing sensors, but these sensors

are generally the more bulky and heavier laboratory type of

equipment. Quite often, critical characteristics are sacrificed

in the achieving of flight-sized hardware. Efforts are being

pursued to provide sensors possessing the necessary characteristics, such as accuracy, repeatability, reliability, stability,

light weight, long life, etc., which can meet the needs of

diagnostic equipment. In addition to advancing the state-of-theart of the measurement of parameters which can now be measured,

work must be put forth to devise ways of measuring parameters

which now cannot be measured; for instance, the important parameter of turbine inlet gas temperature cannot be directly

measured at the present time. Present day determinations of the

turbine inlet gas temperature are made by inference only.

We currently are involved in sensor efforts on flow,

pressure, and torque sensing. Panametrics, of Waltham,

Massachusetts, is investigating a concept of using ultrasonics

to measure mass fuel flow. It is envisioned that this will

result in the accurate measurement of this parameter using a

non-intrusive, no moving parts transducer.

In the area of torque sensing, two contracts have been

awarded; one to the Garrett Corporation and the other to AvcoLycoming. Both of these efforts are directed toward improving

the accuracy of torque sensors.

TO
A contract has been completed with Hamilton Standard

where an effort was conducted to develop a pressure sensor

capable of measuring pressures up to 250 psi. The transducer

used a vibration cylinder concept for determining pressure.

The thrust of the sensor technology investigations is toward

the derivation of technology which will produce meaningful

signals which relate to mechanical component condition. These

investigations must include not only the advancement or

improvement of existing sensor concepts to improve accuracy,

reliability, weight, etc., but must also include investigations

of new concepts of sensing parameters of interest. An example

of such an investigation to be conducted is the application of

acoustic em.ission sensing techniques for the determination of

incipient metal failures in the aircraft environment.

Signal Analysis ; The heart of a diagnostic system is the

analysis technique and the decision-making criteria which are

employed. The ability to accurately record a signal related

to a component is meaningless if the analysis procedure used

and the decision-making criteria upon which this analysis

procedure is based are erroneous. Basically, there are tv/o

methods of signal analysis now being employed.

One of these, the logic matrix, is generally used where a norm exceedence type of concept is employed with several different parameters being involved. For instance, information

related to the exceedence of some preestablished norms for

hydraulic system, components, such as pump pressure, oil

temperature, relief valve position, control lever position,

and actuator position, could be fed into a matrix to allow

inferences relative to the hydraulic system condition.

Implicit in the successful use of a technique such as this is

the establishment of norm exceedence as an acceptable procedure

and the establishment of appropriate norms.

71
The second type of analysis procedure is that of spectrum

analysis. In employing this technique, some characteristic of

the signal, as it varies with time or some other domain, is

analyzed for its content. This type of analysis technique is

mostly applicable to vibratory or acoustic signals, but

conceivably it could be applied to such signals as hydraulic

pressure vibrations. Work done to date on spectrum analysis

investigations has generally been on a fragmented basis, and

thus, a continuing coordinated effort to devise logicallyand technically-based analysis techniques is planned.

Analytical and experimental investigations are to be

conducted in signal conditioning and signal analysis

technology, with the intent of deriving techniques of

spectrum analysis and pattern recognition suitable for

diagnostic use. Analysis techniques which have been used

successfully in other fields appear to warrant investigation.

Although it is probable that these concepts can be applied

to both the vibrations and noise emanating from mechanical

components, it has not yet been determined whether they can

be used to identify a specific component experiencing

failure

A contract with the Ohio State University Research

Foundation has been awarded to assess techniques which are

applicable for vibration signal analysis of Army helicopter

power train components. It is anticipated that this effort

72
will result in the identification of techniques which show

the greatest potential for this type of application.

PrognosisPrognostic investigations will concentrate

on forecasting the progression of a malfunction once detected

and the life remaining of the component in question. Since

mechanical components do not fail totally without any prior

warning, it appears that the prognostic capability is a

big pay-off area. The capability of determ.ining the

seriousness of a malfunction and its prognosis will have

impact on all phases of Army aviation. The entire field of

prognosis is so broad and deep, and has been penetrated so

slightly that large amounts of work are called for.

Investigative efforts must be accomplished to establish

the factors, both internal and external to the component,

which have effect on the failure progression rates.

These factors must be verified experimentally. The investigations conducted must cover the range of the aircraft

components' subsystems (e.g. hydraulic system, power train

systems, structures, etc.). Failure progression rates and

factors associated must be established for items such as

bearings, gears, pumps, and mechanical connections.

These two contracts represent the beginning of a long

range effort.

73
other Diagnostic Investigations ; Other diagnostic

techniques which do not fit in the above categories are

being or will be investigated by the Army. Included in this

category are such items as engine power available studies,

air density measurement techniques, fatigue crack detection,

oil monitoring and analysis, and acoustic emission techniques,

which are actually combinations of the previous technologies

and would likely serve as subsystems in a diagnostic system.

These concepts and others of this type must be pursued to

establish feasibility and usefulness in Army aviation.

7^
Identification of Failins Mechanisms

Through Vibration Analysis

JAMES L WOTIPKA RICHARD E. ZELENSKI

IBM General Systems Division, Rochester, Minn,

INTRODUCTION

Vibration and acoustic signals have been

foTond to be valuable indicators of the general

operating condition of mechanical equipment.

Th\is, their measurement and analysis is often

part of a comprehensive maintenance program.

Methods have progressed from subjective Judgments

based, on listening to noise, to the use of microphones, accelerometers , and ultrasonic detectors

for signal m-easi-Ter^ent , and waveform and frequency analyzers for signal analysis. The types

of equipment being monitored have also become increasingly complex, e.g., from large gears and

bearings to the valves in internal combustion

engines. This increased com.plexity has placed

severe demand on failure detection and identification techniques.

Two approaches to failure de'cection are commonly considered (l).l The first is statistical, where many samples of operation are taken to establish a characteristic signal. Components

with known defects are used so that specific failures can be categorized. The second approach is

analytical in that the possible failure m.odes are predicted along with their resultant effect on vibration signals.

The analytical approach has gained the

widest acceptance. Detection of failures in large

rotating equipment has been accomplished by observing increases in vibration levels. For more complicated mechanisms and when monitoring the

f\mctional performiance of a machine, a primary

method of failure detection has been comparison

of machine timing charts and signal waveforms.

This established the normal signal contribution

of individual components and mechanisms during a cycle of operation. Any change in the wavefotm

can then be attributed to a specific component

failure based on correlation with the functional

timing chart. Unfortunately, many mechanisms and

components may be operating simultaneously, making it difficult to identify a specific one. Therefore, a study was initiated to determine if some other method for identifying malfunctioning

components could be foiond. The results of the investigation showed that the natiiral frequencies

of components making up a mechanism were of significant value as failure indicators, particularly on high speed, low inertia equipment. Previous

investigators have primarily considered frequencies corresponding to rotational speeds and caused by occurrences such as the balls of a bearing

rolling over the pitted surface of a race, or tooth contact frequencies in gears.

The study took the approach that to -onderstand the operation of any machines, it is necessary to know the fvmction of basic mechanism^s making up that machine. Also, to gain an insight

into the nature of vibrations of a machine, it is

necessary to understand the signal contributions

of its basic mechanisms and com.ponents. To ac- complish this understanding, the general vibration

characteristics of several basic mechanisms and

the effect of their typical failure miodes on these

vibrations were studied.

A unique digital computer signal analysis

system that provides a flexibility needed for

thorough analysis of vibration data was used exAL. MOUMTIMO

PLATE

1 Numbers in parentheses designate References

at end of paper.

Pig. 1 Spring-loaded radial cam, oscillating

radial follower, and aocelerometer \ised as cest vehicle in the study of mechanism and com.ponent

vibration characteristics.

75
N - -fFrequency (Hz) Frequency (Hz)

(a) (b)

Q

o.

C/3

O

a.

Frequency (Hz)

(0

Pig. 2 Power spectral density plots of cam-and-follower operating at (a) 1000 rpm, (b) I900 rpm,

and (c) 2000 rpm indicating no frequency changes other than in amplitude

76
Table 1 Natural Frequencies of Components Making

up the Cam-and-Follov:er Test Unit

Calculated

Experimentally

Determined

Observed in PSD Plots

JDs^cLl. ULigO 1. y t\nZ - / KnZ

Cam 6660 Hz 6500 Hz

Follower 10 KHz - 13 KHz 7. 9 KHz - 14 KHz

tensively. The equip.T.ent and computer programs

are described.

PROCEDURE

The mechanism chosen for initial investigation was a spring-loaded radial cam and oscillating radial follower operating at 1200 rpm. A high frequency (125 Hz natural resonance) accelerometer was mounted on the frame supporting the

cam-and-follower to monitor vibrations during

operation (Pig. 1).

Because of the investigative natiire of the

study, we wanted the capability to analyze signals in as many different ways as possible. We

felt that digital computer programming would best

provide this needed flexibility.

The system used for data collection and

analysis included an IBM 1130 digital computer, an EAI ^-lodel 580^ general purpose analog computer, and a real time channel (RTC).

The analog computer v;as programm.ed to provide analog-to-digital conversion and signal preprocessing. The conversion provided a six-bit

digital representation of an analog input voltage.

The signal preprocessing f'onctlons included am- plification, ' shifting of d-c level, filtering,

amplitude limiting, and rectification.

The real time channel provided data and con- trol interface between the 1130 CPU and the ex- ternal test hardware. When digitally sampling an analog signal, a n^jjnber of conditions and variables had to be specified and controlled. The

data collection program was required to synchronize the sampling time with the test cycle (-ma- chine cycle), control the desired sampling rate,

and store data where it could be retrieved and

identified. Tl"iis was accomplished through program control of the real time channel clock, read

register, signal gates, cycle counter, and instruction and data address registers.

A' num.be r of analysis programs were used to

2 Electronic Associates Inc., West Long

Branch, N. J. 77

perform the necessary data processing and to provide the appropriate output. Exam.ples were en- semble averaging, fast Fourier transform routines,

auto-correlation, and cross-correlation. A signal

enhancing technique known as ensemble averaging

provides a means for improving signal-to-noise

ratio and for waveshape recovery. It operates on

a number of signals, each synchjronized to the

function being observed but individually, obscured

by noise. This "noise" can be any undesired effect, random or periodic, which is not in synchronization with the period of interest. The inphase periodic signals are reinforced as a number

of signals are added point-by-point. The noise,

hovjever, which can be positive or negative, averages toward zero. A fast Fourier transform (FFT) routine was formatted in a number of ways to allow frequencyanalysis of digital signals. The first was the

discrete Power Spectral Density (PSD) plot. Power

amplitude versus frequency was plotted to show

the contribution of each frequency to the total

power of the signal being analyzed. This could

be accomplished for portions of a machine cycle,

or an entire cycle. In addition to amplitude, the

program also provides phase information for each

frequency. Thus, if a number of similar consecutive cycles are analyzed, the result represents

the power spectrum of an ensemble averaged signal

(random noise removed) when phase information is

considered. However, if phase information is

neglected, the result is the average magnitude of

each frequency including noise.

To investigate the frequency-time relationship for a machine cycle, a technique similar to the sound spectrograph or sonogram (used in speech

processing and based on the fast Fourier transform) was employed (2). A method of overprinting

on a line printer provides a variable intensity

frequency-time-am.plitude plot. The result is a display with the frequency and tim.e axes on the

ordinate and abscissa, respectively, and the intensity of prin-cing proportional to power amplitude.
o

a.

cQ

O

a.

Frequency {Hi) Frequency (Hz)

X X

E

Q

- Q. to

o

a

S

Frequency (Hz)

(b)

Frequency (Hz)

(d)

Pig. 3 Power spectral density plots of one revolution of the cam shaft operating alone (a) and

the cam and follower operating together (c) oompared to the average plots of many revolutions (b)

of the cam shaft alone and (d) of the cam and follower together. The significant retention of signal on averaging seen in (d) was attributed to the action of the cam and follower causing vibrations to occur at the same point in time for each cycle.

78
X

V)£

Frequency (Hz)

cQ

Q. CO

o

0.

Frequency (Hz)

Fig. iJ' Power spectra indicating the higher vibration amplitude of a worn cam follower, (a), as compared to a new cam follower, (b).

Signals stored in the digital computer were

analyzed for similarity using correlation programs. The auto-correlation function (correlation of a signal with itself) was used to find

any periodicity within a signal that may have

been obscured by noise. A signal is shifted

along the time axis and correlated with the un- shifted version of the same signal. The correlation product reaches a peak when periodic com- ponents are in phase.

The cross -correlation function was used to detect similarity and timing differences of any

to waveshapes by shifting one signal, point-bypolnt, with respect to the other and observing

the correlation product. The magnitude of the

correlation product and the amount of shift needed to reach a maximum indicates the degree of

waveshape similarity between signals and the tim79

ing difference with respect to a sync point.

Although signal analyzers capable of these same tasks are commercially available, they generally lack the ability to perform them on single

cycles of machine operation or portions of a cycle. This capability was required for investigation of functional cycles of mechanisms and ma- chines .Initial testing of the cam-and-follower

mechanism was designed to provide information on

the general nature of the units vibration characteristics. The tests included runs at different rpm's, rxms of the cam shaft alone, and runs

of both the cam-and-follower . The data from

these tests were extensively analyzed using the

analysis programs mentioned.

In later tests, typical failures and failure conditions were introduced to determine if
(a)

Time tmsecl

Pig. 5 spectrographs \ frequency - amplitude versus txme j showing the insignificant dirference in

high temperature, (a) and normal temperature, (b) operation on cam- and- follower vibrations. A (1)

represents a minimum signal amplitude and (a) represents a maximum.

detectable changes in vibrations would result.

In the first of these tests, a field returned cam- and-follower set was compared with a new cam-andfollower under conditions of lubrication and no lubrication.

The second test was run to determine the effect of cam-and-follower run-in and temperature.

A new cam-and-follower was run unlubricated for

10 min., with vibration signals collected dijring

the last minute. After a cool down, another minute of signal was collected. This procedure was repeated a number of times on the same cam-andfollower. During the course of this test, fretting corrosion of the unlubricated cam. surface

developed. The resulting effect on the vibration

signals was observed and analyzed.

The final test was run to determine the effect of cam wear resulting In a cam contour change, as opposed to v;ear causing rough spots. Approximately 0.002 in. of material was removed in two

places on the cam surface; at a point of low ac- celeration and force, and at a point of maximum

acceleration and force. The 0.002 in. Just ex- ceeded manufacturing tolerances.

RESULTS

The cam-and-follower mechanism was initially

run at three different rpm's (1000, 1900, 2000)

to determine if any changes would result in the

Power Spectral Density plot. Fig. 2 shows the

results of these runs. Only the amplitude of the

frequency components changed and no new frequencies appeared. This indicated that the frequency

content' of the signals being detected were independent of operating speeds, i.e., rotational

frequencies (such as the balls in bearings) were

not contributing significantly to mechanism vibrations . The test runs at varying rpm's, led to a further investigation of vibration sources. A

first step was to determine the natural frequencies of the various components in the cam-andfollov;er m.echanism. Because of their relatively

simple configuration, the natural frequencies of

bearing races could be calculated (3, U-) . The

cam-and-follower , however, required experimental

methods. The mechanisms -were caused to resonste

by being struck. Resulting vibrations were de80
Pig. 6 Spectrographs (frequency - amplitude versus time) showing the higher signal strength of a new cam- and- follower, (a) prior to run-in, (b) after run-in. A (1) represents a minimum signal

amplitude and fa) represents a maximum.

tected with a microphone and displayed on an os- cilloscope. Photographs of the scope traces v;ere studied to determine the frequency content of the

signals. The results of the calculations and

tests are shown in Table 1. Included in this

table are frequency components observed in the

Power Spectral Density plots of the cam-and-follower vibrations. A similarity between predicted

and observed frequencies is apparent.

In the next series of tests, the cam shaft was run alone and the cam-and-follower operating

together was run. The purpose was to determine

the effect on vibrations of a periodic occurrence,

such as the follower traveling over the cyclic

contour of a cam. Fig. 3(a) shows a PSD plot of one revolution of the cam shaft. Pig. 5(b) shows

the PSD of a number of cam shaft cycles in which

phase information was retained (random signals

are lost as in ensemble averaging). The significant loss of power indicated a highly random signal for a cam shaft operating alone. Fig. 3(c)

and 3(d) show PSD plots similar to 3(a) and 3(b)

for the cam-and-follower operating together. The

high retention of power in 3(d) indicated that

the amount of randomness from cycle to cycle was

lessened. This was attributed to the cam contour

causing the cam, follower, and bearings to be ex- cited to their natural frequencies at the same point in time in each cycle. Hence, a greater

portion of the signal strength was retained on averaging.

Pigs. 4 to 9 and Table 2 show results of the

three experiments in which failures were introduced. Pig. 4- and Table 2 illustrate results of an experiment to determine the effect of lubrication on a new and run-in (field-returned) cam-andfollower mechanism.. The radial contacting surface

of the fie Id -re turned follower was worn slightly

flat. Therefore, as it traveled through the concave portions of the cam. profile, higher vibration levels were recorded, as shown in the power

spectra of Fig. 4 and certain significant frequencies of Table 2. Table 2 shows the confidence levels of cer- tain frequencies which indicated by their amplitude that new and unlubricated produced higher

vibration levels than run-in. and lubricated cams and followers. All frequencies from a number of

PSD plots were statistically analyzed. A significant result was that the frequencies of greatest

81
1/e/=3.'

f+

•f Si f 4 iff 4- 4 f •f 4

•f f 4

> 'f 4

•f 4' 4

+ 4

4 4

•f •f 4 4

•• 4 4

f f 4 4 4 4 4

0) <^ t» -o o <*\ o> Ch rg r-l o «i> <* O •H 00 o to to <o oo "3. • • • «Nj o O o o o • • • • • • • • • • • o o o o o o o o o 1/

time

nA » m •-4 <o o e CD «> n n-l <0 O rf* o « *M r~ m r- o «s) if\ r~ o <M * <^ 9> 0« 0* o o o o o r-l •H 9^ ^ «M «V «V <M «x T\l «X IV €V

time

(b) Frequency (Hz)

* * * « * « * *

tf\ o eo eg eo 1-4 o O (M <r o< fM r~ >-< r-< ft «*• * <r o <n o o o rsi "too o • • • m • • • • • o o o O o o o o o

44

4 4 4 4

«> o> r\ •I ^ o < «> rg

k; <^ r- ^ •o o o OS a) rg 1 IM r- o <M r\ r^- o <M ^ r- c o> o O o o o •-< >-< <-« »-«

est rg IN rsi Cvi eg Cg Frequency (Hz) rHi.HC>»-4fvir>j.-«or- o»f-<rg 0>9><Dt*-Of-«(ntn<nrgio<n = t-<i-<n^(r\m<fi3Dorg«r\.-«

b 0'-<or^rgrr>;nmmr~«oo

M ••••••••••a fi, OOOOi>4f-i(vjrgoooo

* «

4> * * *

fM m ©> »n f» o vf oo pg «} «o .-1 <o o m o •D rg r~

rg *> f-- o rsi «r\ r» o> rg ^ o>

c» o> o o o o o r-t <H "H pg rg rg «vi rg r«4 rg rg »>j Frequency (Hz)

(a)

im<0(Oa)«ai-«>-i«ao<orgf-« .OOOfHOorgr-^i-ioo oooooorgrgoooo

oooooooooooo

^rgir\r-orgmr--org-<i-f^o> 0Ot>OOOOOO»-»<-t>-«»-4

^•-«t-»^rgrgrgrgrgrgrgrgrg

Frequency (Hz)

(0

Pig. 7 Portions of average power spectra (phase information discarded) (a), ensemble average wave- forms (b), and power spectra (o), of the cam-and-follower before (upper figures) and after fretting

corrosion occurred, (lower figures) indicating signal strength lost on averaging due to randomness

of vibration for a fretted condition.

confidence were those corresponding to the natural

frequencies of the cam-and-follower.

Figs. 5, 6, and 7 show results of the experiment to determine the effect of temperature and

run-in. Fig* ^ shows tliat vibration signals of an

unlubrlcated cam-and-follower were not significantly affecttfd by high temperature conditions.

Fig. 6 compares vibration signals of a cam- and-follower during initial 'run and a later run, and shows a reduced level of vibration caused by

82
run in.

Fig. 7 shows the increased signal amplitude

and greater signal randomness that resulted when

the cam-and-follower were allowed to run unlubricated until a fretting corrosion condition oc- curred. In part (a) the average power spectrum

(phase neglected) of ten raw cycles shows greater

signal strength for the fretted condition. In parts (b) and (c) the ensemble averaged v;aveform

and the power spectra of these waveforms indicate

that a great deal of the signal strength for the

fretted condition has been lost. This is due to the fact that the signal strength was random be- cause of the fretting corrosion and, therefore,

was lost during averaging.

Figs. 8 and 9 show results of the experiment designed to study the effect of a cam contour changed by wear. In Fig, 8, power spectra

of wear and no wear conditions are quite similar.

Fig. 9 shows average waveforms and power

spectra for wear at a point of higher cam-to-follower load than for the data of Fig. 8. An in- crease in signal strength is evident.

CONCLUSIONS

The objective of this study was to investigate a means of machine failure detection which

did not rely solely on the time consuming correlation of machine functional operation with signal

waveform. The general vibration characteristics

of elemental mechanisms (e.g., cam-and-follower

)

were investigated and the effects of certain

operating conditions and failures on the vibrations were then determined.

Based on the results of an experimental

study of a cam-and-follgwer mechanism, the following conclusions were drawn.

1 Most frequencies seen in a spectrum analysis cf vibration data are natural frequencies

independent of mechanism operating speeds.

2 Components, such as cams, followers, and

bearings, when operating as a part of some mechanism, are excited to their natural frequency

throughout a machine cycle.

3 A periodic mechanical action, such as a follower traveling through a cam contour change, causes the cam-and-follovrer to be excited at the same points in time for each cycle of operation.

4 Detection and identification of failed

components can be accomplished by monitoring their

natural frequencies for changes in amplitude.

A number of typical operating conditions and

failures of a cam-and-follower mechanism were

studied using various techniques of vibration signal analysis. The conditions studied were: lack

85

CM

CI

• « « • C m ^ ^ m a? <o o> o o -o CJ p> ^ ^ ^ o o o o o o

(0 • • • • •

i; o o o o o a

CO

i_ CD N <o -< m

aj m o) «v r- -< $ «o a> ^ <n <o O -t * <t^ if^ IT' a.

I

« • « « • •

«

• • • • « • • * « * •

«

O'-«nOO00'Mf~'^4"-<0OOO00O00rgO0OO

ooooooooooooooooooooooooo

^no-*c^^.^oof^J^^lr»0^^o^*^^-*>4•OOrf^o^a3l.>^^.-^ 0D»-.fntf>a>Ofntf^«o<nior-orginr-O'M-*r^org^rin>o^'0'*f^f^r-^-<Doo<Dflo<j^(7»(7^oooooo»-»»-i«-.

Frequency (Hz)

(a)

*

« • • * « « «««««« •«* ««*

^r*-ra«ocr>^0tf»0'4'<>r^«)0o -t^tftO-oaJO^jcor-o^^flOiArNj

^-0"0'*^'^-•-^4'»^f^J<^^(^<^^- coeoorsj(n<>moo'>cDr-r-o--«

O00'^o*»-«o»-<a>«>r- to o t~ o iOfn^»n#-*wi-«min<n*--4fOt-«^fsi 0O00•-<•-*^^^'»<-•<-•'^«-•O<^lcn.-*0OOO000•-•r^ cnoooo oooooooooooooooooooooooooooooo

^D^Ml0^^^no^o^-^rf^o^«><M'00'^<^l^r-*-^w^om«)^sl^o^

^-t«f\tf\»f\»f\^^>0'Or-r-r-r-co<DQoeooa»c>(7'OOooo^.-»i

Frequency (Hz)

(b) Pig. 8 Portions of power spectra of a cam within

manufacturer's tolerances (a) and one worn 0.002

in. out of tolerance (b), at a point of low cam- to-follower force, indicating an insignificant

difference between the two conditions.

of lubrication; worn follower; worn cam.; irregular cam siu>face defects (e.g., fretting corrosion),

and rxHi-in.

The effects of these failixres were predictable and appeared as increases in vibration levels

(except run-in which reduced vibrations). However, the nature of the vibrations was not the same in all cases. A lack of lubrication caused

increased signal strength throughout a cycle,

while a worn cam and a worn follower caused in- creases only at the points of wear. Fretting cor- rosion also increased vibration levels throughout

a cycle, although the am.ount of increase varied

from one cycle to the next and also within a cycle. It was" evident, therefore, that although

the effect on vibration levels from various failures may be the same, their effect on the nature

of the vibrations may be quite different. This

then would allow identification of certain failures in some cases.

The significance of the work done in this

study does not lie so much in the results pertaining to the specific cam-and-follower investigated but more importantly in the general type of
Fig. 9 Portions of average waveforms (upper traces) and power spectra (lower traces) of a cam within manufacturer's tolerances (a), and 0.002 in. out of tolerance (b), at a point of high cam- to-follower load, showing the increased signal strength due to the cam contour change.

Qk
Table 2 Confidence Levels of Significant Frequencies Showing that a Run-in Cam-and -Follower

Ran Quieter than a New One, and that an Unlubricated Cam-and-Pollower Produced Higher Vibration

Signals than a Lubricated one.

Follower at high dwell Follower at concave portion

portion of cam profile of cam profile

Frequencies Confidence levels of significance Confidence levels of significance

Run-in Lubricated Combination Run-in Lubricated Combination cam vs vs not of run-in cam vs vs not of run-in new lubricated and not new lubricated and not cam lubricated cam lubricated

1220 99

2441 99 99 99

,2685 75 2929 75 95 75 3173 75 75 341

75 75 4150 70

4638 75-90

5126 75-90

5615 99 75 5859 99 75 6103 75-90 70

6347 70 70

6591 90 75-90 70 70

6835 99 75-90 75 75-90

7080 99 99 75-90 75 75-90

7324 75-90 70 70

7568 70

7812 97.5. 90 95 8789 70 70

9033 90 75-90 90

92"77 99 99 99 10498 99 75 10742 97.5 75 10986 90 75-90 75 70 90 75 11230 99 99 99 70 75-90 75-90

11718 99 99 99 11962 75 75 75 1 99*

122 07 99* 99 99 12451 70 75-90 75-90 75 12695 90 97.5 97.5 70 12939 70

13183 99* 13427 75*

13671 99+ 99 99 14648 99* 99 99 14892 97.5* 95 95

1635.7 75-90 75-90 75-90

18554 99 18798 99 19775 99 99 99 20019 75

2 02 63 75-90

21972 99 99 99 22216 99 99 99

Denotes instances where the run-in cam-andfollower had higher signal strength because of

follower wear.

85
knowledge gained (i.e.. Conclusions 1 and H-)

,

This type of information can be applied not only

to other cam-and-follower types, but also to other mechanisms. This has been shown in the design of a probe for the detection of bearing

failures (5). The natural frequencies of the

bearing races are calculated, and then an accelerometer mounted in a spring loaded probe and a filter are used to monitor the levels of these

frequencies. The system has proven capable of

detecting extremely small race and/or ball damage

that could not be found by monitoring the overall

vibration level of rotational frequencies.

REFERENCES

1 Tatge, R. B., "Acoustic Techniques for

Machinery Diagnostics," Paper presented at the

75th Meeting of the Acoustical Society of Ameriga,

Ottawa, Ontario, Canada, May 21-24, 1968.

2 Rothauser, E., and Maiwald, D., "A Digitalized Soiind Spectrograph Using FPT and Multiprint Techniques (Abstract)," Journal of Acoustic

Society of America, Vol. H-'p , p. 308, 1969. Copies

may be requested from IBM Thomas J. Watson- Research Center, Post Office Box 2l8, Yorktown

Heights, N-. Y. IO598.

3 Harris, C, and Crede, C, "Shock and

Vibration Handbook," Vol. 1, McGraw-Hill, New

York, 1961, pp. l-lH-, 1-15.

U- Love, A. E. K., "Treatise on the Mathematical Theory of Elasticity," Dover Publications

New York, 1944, pp. 1+51-4-53.

5 Martin, R. L. , "Detection of Ball Bearing Malfunctions," Instriment and Control Systems

December, 1970.

6 Balderston, H. L., "The Detection of In- cipient Failure in Bearings," Materials Evaluation, June, 1969, pp. 121-128.

7 Gibbons, J. B., "Mechanical Signature

Analysis — Engine Diagnostics," Memorandum —

MSA 65-2, Advanced Technology Laboratories, General Electric Company, July 12, 19^5

8 Weichbrodt, B., "Mechanical Signatioro

Analysis, A Nev; Tool for Product Assurance and

Early Fault Detection," Report No. 68-C-197. .Gen- eral Electric Research and Development Center,

Schenectady, N. Y. , June 1968.

NOTE:

This paper was presented at the Vibrations Conference and the International

Design Automation Conference, Toronto, Canada, September 8-10, 1971. It was

published by the Design Engineering Division of the American Society of

Mechanical Engineers (ASME) as Publication 71-Vibr-90.

86
DISCUSSION

Jack Frarey, Mechanical Technology, Inc . : As I understand it, most of

the changes you saw were changes in amplitude and not changes in the

resonant frequency content. On spectrograms that you were showing,

what was the frequency region on the bottom? What was the highest

frequency in which you found resonances?

James Wotipka: The spectrograms that I showed were not really too

representative of the data we saw. We were seeing frequencies as high

as 20-25 KHz for bearing outer races. These are the higher harmonics

of these frequencies. For the particular cam we were studying, the

resonant frequency that we had the most success with as a failure indicator was about 6 1/2 KHz and for the follower it was around 13-14 KHz.

For the gears we were studying, we were in the range of around 20K and

for the geneva incrementer a very good resonant frequency was 13-15K.

So it's getting to the upper limit of the audible range, almost to the

ultra-sonic range in certain instances. That's where these very small

and sensitive accelerometers work as very good transducers.

Paul Craft, Hughes Aircraft : On the single cycle spectrum that you

showed and it's apparent not averaging to zero over long periods of

time, or at least apparently increasing the signal to noise, what type

of frequency analysis were you doing? Single samples or real Fourier

analysis is phase sensitive, so that mechanical synchronization is

probably mandatory. Random phase fixed frequency is computationally,

at least, cancelled out with the imaginary fast Fourier transform.

What type of Fourier transform were you using?

James Wotipka: I wish Mr. Zelenski was here to answer that question

(the other author) . I do know that phase information was available

from the fast Fourier transform that we did. The single cycle that

you saw is the resulting spectrum from only one revolution of the cam. What we did, many times was to throw away the phase information and

work only with amplitudes. Then we could add these to come up with

somewhat of an average amplitude value. Now this would get away from

the problem of randomness in the signal and give an indication of

what you really had as an overall signal strength and still give you

a frequency analysis. We also went the other way where we included

the phase information. If there was randomness in the signal, if you

did any ensemble averaging, for instance, this randomness would tend

to filter out the noise and you would only see those parts that were

synchronized to the event. So, signal strength was lost.

87
Paul Craft : I'm looking into a situation where I can not get an external mechanical synchronization, which means, essentially, with

the measurement interval that I am taking, my phase information is

going to be random. Have you done anything which indicated that

that was even possible, that you could get tlie resonant frequency

information without external mechanical synchronization?

James Wotipka: Just take the overall vibration signal. You don't

really need a syncronizer to do that. First of all you can take a long record of a signal, it doesn't even have to be a repetitive

or cyclic type situation, and do a fast Fourier transform of it.

You do not have to have synchronization to get an indication of the

signal strength at various frequencies. The comparison I made, before

we did a final spectrum analysis when we ran the cam and follower

together or when we ran the cam shaft alone for many cycles, was done

after we had actually done signal averaging. Vfe introduced the need

for synchronization there because that's what we wanted to do. Those

first 2 charts of the power spectrum were done on a single cycle, but

they could have been done for many cycles and the results would have

been essentially the same.

88
PANEL DISCUSSION

Panel lumbers : Charles Jackson, W. Hogg, James Wotipka, and David Brown

Panel Moderator : Jack Frarey, I^chanical Technology, Inc.

Paul Howard, SKF Indijstries, Inc. : What is the effect of wear on the

mechanical emissions? ~

David Brown: The wear on a given device normally doesn't have a large

effect upon the mechanical impedance because basically you're looking

at the natural frequencies and the mode shapes of the structure. If

wear has drastically changed the geometry of the system, it would have

an effect on the natural frequency. In general, the stiffness of a given mode of vibration would have to be changed, which means that a cross-sectional area, or something similar to that, would have to be

drastically changed by the wear problem itself. So wear normally would

not have a big effect upon the mechanical impedance in a given system.

Automobile manufacturers would like to use this for quality control, to

inspect a frame to make sure all the welds have been made properly. In

that case, there could be significant differences in the stiffness of

various modes of vibration. But normally the wear would not have a

big effect.

Paul Howard : I was thinking mainly about the hysteresis effect wear

in bearings and gears would give.

David Brown : Normally in bearings, you do have some sort of damping

term which depends on the type of contact in the system. I think that

that would be a better indication of the system. The stiffness of a bearing may change due to wear and it is a very non- linear type of

stiffness to start with. In that case, you may see a difference in the

frequency response plot, but in a general mechanical system, I don't

think that would necessarily be true.

Paul Howard: Is the mechanical inpedance independent of the size of

the forcing function?

David Brown : Yes, only if it's a linear system. If it's linear then

the frequency response or mechanical impedance is a unique function of

the system. If it's non-linear, then the mechanical impedance of the

system depends iq)on the preload or the input conditions of the system

itself. There are techniques for estimating the effect of the input

on the estimated frequency response.

Nforris Weber, Hughes Aircraft Co . : How do you estimate the structural

damping in your analysis?

89
David Brown : There are a nimber of ways of getting structural damping.

I wrote a little paper that indicated 20 different ways of doing it.

You can do it in the time domain which means you can take a signal and

basically do a log decrement type analysis. You can go in and excite

a certain mode of vibration and look at the rate of decay of that mode

of vibration. You can do it with a Z transform technique, the Z transform

being a discrete Laplace transform. The Navy has developed a complex

exponential technique which fits a curve having the form e'ct to the data

where a is a complex number. The real part of that is the damping co- efficient. If you measure the frequency response you can go into the

frequency domain, and you can measure the half power pointy which is the

frequency spread between the half power points. You can go into the

display of the frequency response plot where you have the real plotted

vs. the imaginary and measure the rate of phase change in that plot. That's

a measure of damping.

Morris Weber : I know you can measure it. How do you predict it when you

do it analytically?

David Brown : Analytically of course it's very difficult to predict damping

m any given system. Most of the damping will come in through joints and

things of that sort which are very unpredictable. In a casting, for example,

we sometimes detect cracks measuring the internal damping in the system.

In that case the damping is a little more predictable and you could probably

get a good handle on what the damping coefficient would be initially. But

in a general system the only way that I know to determine damping coefficients

is to measure them. It's very difficult to estimate damping coefficients.

They are so much a function of the geometry and boundary conditions of

the system.

Charles Jackson : I thought you had a real good correlation of high-Q

resonance response to fast change in phase. But I noted that some of the

phases were 90°, 180°, 270°. Could you comment on the various phase shifts?

David Brown : Actually in that plot that we were looking at the phase angle

really went between 0 and -180°. I just moved the plot so it would keep

from junping back and forth between -180 and +180°. The phase angle was

going between 0 and -180? Everytime you go through a resonance, it drops

180°. When you go through an ant i- resonance it goes up 180°. Now if you

get on a cross transfer function or a cross frequency response which is

exciting here and picking up there, then everytime the phase angle goes

through a resonance, it decreases 180°. But you don't have a resonance

followed by an ant i- resonance, so the phase angle can go down 180°, then

90
go through another resonance and drop down to 360°, and just keep on going down depending upon your frequency response plot. That frequency

response plot was obtained with a transient input into the system. We

actually hit it with a hammer and measured the frequency response that

way. To do that you Fourier transform the response of the system and

divide it by the Fourier transform of the input. Out near the end of

the Fourier transform record, you run into some transform areas that

cause a little bit of a problem.

Jack Frareyr I take it from the last comment of your talk, that if one were performing spectral analysis of a machine, and if he ran an RPM

sweep in the region of interest, and for that system found this anplitude

to be relatively stable over that RPM range, that this would then in

fact be a safe region to perform what we are now calling the traditional

spectrum analysis of wave form analysis. The primary problem here comes

in trying to look at a conponent over a speed range.

David Brown : In a very dynamic system, there are a lot of resonances. The

response of the system is going to change drastically as a function of speed,

not based upon the forcing function, but based just upon the characteristics

of the system that you are looking at. Somehow, you have to conpensate for

that change. A relatively flat region would indicate a good speed range

to obtain signature analysis from that device.

William Glew, Naval Eng. Test Establishment : What type of orientation do

you use to get such an effect to transform function from merely hitting

the gear wheel with a hammer?

David Brown: Basically, we have a load cell mounted in a hammer which

measures the force. We measure the response of the system with an accelerometer or displacement pick-up or velocity pick-up. The frequency res- ponse of the system is defined as a Fourier transfom of the output of the

system divided by the Fourier transform of the input. You can very easily

Fourier transform the input which happens to be nearly a unit impulse.

William Glew: How are you doing this across your vdiole frequency spectrum

in about 1/2 second or the time of your impulse?

David Brown: A unit impulse input into a system has a very broad frequency

content. For example, a hammer blow will have a frequency content depending ipon what type of hammer you are using up to 10,000 cycles. We have

different hammers and different heads. We have hammers that go up to

10,000 cycles and they are all calibrated.

William Glew : So this is a numerical analysis, not a shock spectrum analysis?

91
David Brom: It's numerical. It uses the Fourier transform. If you

have noise in the system and you use a statistical technique of averaging

the cross spectral density and the par spectral density and take the

ratio of the cross spectral by the par spectral, you will obtain the

frequency response. No matter what the input is into the system, you can

get the frequency response providing you can measure the input and output

of the system, and the system is linear. For example, in a machine tool,

frequently they mount a transducer in the cutting tool. Then they monitor

the transducer that measures the relative motion between the tool and the

work piece. The cutting force is the input and the relative motion is the

output. The frequency response obtained from this happens to be a very

important plot for predicting machine tool chatter. On an automobile you

can mount a motion pickiq) on one of the wheels, and you can mount another

pickup on the steering wheel. You get the frequency response between those

two points simply running the car over a rough road.

William Glew : Do you get better results from this numerical analysis than

with a standard impedance plot using a false head and a velocity measurement point?

David Brown : The mechanical impedance technique is sort of standard. It's

a very controlled test. You go in and put in very known force conditions,

you control both the static force levels and the dynamic force levels, if

you choose. You can very carefully control those levels. So I would say

that the results from numerical analysis and the mechanical inpedance technique are comparable, but of course mechanical impedance is still the

standard.

William Glew: We have been trying at the Naval Engineering Test Establishment to improve machine performance by alternating the resonant conditions,

and of course we're defining the resonant conditions by impedance plots.

In practice we find that non-linearities are such that you certainly cannot

apply the theory in the way that you've defined in the lecture. I would

like your comments on this.

David Brown : You can handle non- linear systems if you do it properly. It

requires some special techniques. For example. Structural Dynamics Research

Corp. has taken complete automobiles and trucks for Ford and General Motors

and broken them down in terms of their components, such as the suspension

system, the frame, the cab, the engine, etc. They measured the dynamics of

the components and then put them back together and got reasonably good cor- relation. An automobile happens to be reasonably non- linear; con^onents

such as tires, suspension system, and shock absorbers are very non- linear.

You can handle the linear conponents very nicely, but then when you put it

together with the non- linear components you have to put it together with a simulation system that can handle non-linearities.

92
Alan Duguid, Huntington Industries : Did you use preload bearings in

your test system, and if you did, did you change the preload?

David Brown : In the little gear system we looked at, we didn't use

preloaded bearings, we just used very simple ball bearings. A lot

of the big machine tools we test have preloaded bearings in the

spindles. These may be tapered roller bearings, although it depends

i^on the machine. We've tested a lot of machines with hydrodynamic

or hydrostatic bearings which are very non- linear as far as speed

is concerned, and to a certain extent, as far as preload is concerned.

Alan Duguid: Did you change the preload to see whether that affected it?

David Brown : Anytime we work on a system of that type, we always change

the input conditions to check for non-linearities. As I mentioned before,

the frequency response of a system is a unique function if the system

is linear. If it's non-linear, then it's a function of the input.

The input is varied to determine if there are any non-linearities.

Measuring the frequency response under several different loading conditions

will determine the non-linearities. The type of non-linearity can be

determined from the way the system is loaded.

Alan Duguid : At the end of his talk Mr. Jackson showed a device that

was used for checking the balancing of a rotating system. It was a disk

and I think it had 2 or 4 carbon brushes.

Charles Jackson: I was getting ready to discuss alignment. We align

with the non- contact pick-ups as well as optically. There were two eddy

current probes that were being gap calibrated, one horizontal and one

vertical.

Alan Duguid : In roll grinding equipment, several people have developed

balancing equipment which is essentially dynamic, since if the grinding

wheel is breaking down all the time, the balance of the system is

changing. There are several different ways of doing it, but they all

basically end up as planetary types of systems that reorient themselves

in space to take care of the breakdown in the wheels. Have you ever attenpted to do anything with any of that large equipment that you're

trying to balance?

Charles Jackson : The one example that I gave we balanced a 5,200 lb.

rotor at 11, OOO RPM' s which was operating above its second critical.

Here you really get into a conplex situation if you balance below the

first critical and then you get a phase shift above this. These shifts

93
don't always come as predicted. We had a real dilemma there. We

were running about 2 1/2 mils peak to peak which was causing a little bit of shaking in our governor system. Our question is,

what are we going to depend on? If we used a stroboscopic approach,

could we depend on the phase lag being correct, having gone through

2 resonances? We chose to use the orbit display to allow us to

position the correction weight. In other words, if we had a circular

orbit with a marker straight down, say at 6 o'clock, that means that

at the instant in rotation when the notch, the one event mark, passes

the probe, the shaft is down below the first critical. This is

actually how we placed our weights and we made a correction on the

rotor with 10 grams at a 6 inch radius slightly overhung at the

exhaust end bearing. This is on a coiq)ling side which is an exponential type of response. To place it statically we simply find the

marker at 6 o'clock, stop the machine, and roll it around until the

marker is indicated.

Alan Du^id: What happens on rolls for rolling sheet? In rolling

copper it was found that they could grind the roll, but they couldn't

see anything in the roll. You could see a blush in the material which

was due to the vibration caused by the out of balance, and by incorporating a balancing mechanism into the grinding system at speed, they

were able to eliminate this. I would assume that you have transient

loads at speed when you are operating that change the balance. Have

you done anything about this?

Jerry Forest, Ontario Hydro : You're talking of an automatic balancing

device, and there are such devices, but they work only below the first

critical frequency. In fact, there are commercially available devices

for balancing motorcycle tires. A silicone liquid is poured in the

tires, and when the motorcycle is driven, the wheels balance automatically

as long as you are below the first critical speed. I would like to

ask Mr. Brown, if it's possible to make an inverse of this whole approach

at the design stage so that an equipment manufacturer can predict even

in a complex structure, what his system dynamics will be. If this can

be done, he may be able to change his design so that his operating con- ditions will be in such a location that the system dynamics will not

cause any problems. My experience has been that equipment suppliers

perform usual natural frequency calculations for rotors and other such

things, but they really don't relate to the practical "as installed"

machine. There are definite problems later on that equipment manu- facturers usually aren't able to cope with.

9h
David Brown : I think the technology is essentially available. For

example, that big electrical motor that I mentioned was a very

complicated system consisting of 5 components, a base, foundation,

rotor, bearing, and stator, basically tied together and they did a very good job of predicting that. The base and motor were finiteelement modelled. That way they could incorporate design changes

in those 2 components and then determine what the effect on the system

would be. The idea was to keep resonances away from 3600 RPM's,

which was the operating speed of the motor. The only problem is that

right now this technique is still very expensive to use as a design

tool. If you set the motor on any foundation, you can predict how

that foundation will influence the overall dynamics of the system.

This technique has been pretty much developed by Structural Dynamics.

It's called the building block approach. This technique can be used,

but right now it's expensive and only people like the manufacturers

of very big equipment or equipment that is mass produced, can afford

to apply this technique. Presently it would take maybe 2 or 3 months

to do a conplete design analysis of a big motor and may cost 30-40

thousand dollars.

Morris Weber: How many degrees of freedom are there?

David Brown : The degrees of freedom in that system are tremendous.

In the 50 by 50 matrix: describing the system, each term was a conplete

frequency response which may have 10 modes of vibration in it. One

of tile reasons for using this particular approach is that if you take

a rather complicated system and describe it in terms of 10 modes of

vibration, you talk in terms of modal properties. That tremendously

reduces the effective degrees of freedom that must be carried through

the computation in the system simulation. For example, in finite

elements, you would have to carry a degree of freedom for each finite

element in that particular model. In this case, all you do is consider

the connecting point between two pieces of equipment. It tremendously

reduces the effective degrees of freedom that you have to carry through

your model.

Cha.rles Jackson : We tested our last 14 cases at 1,000 RPM increments

and we do take amplitude phase readings from the test stand; that's

while it's still at the vendor's location. We had 3 units that missed

the calculated first and second criticals by 21%. I don't really much

care what the calculated value is. main concern is what the actual

value is. Some of the actual values do change under load. We're

running under helium and we can't fully power our load. I think the

move is to do more testing and get this problem corrected before it

leaves the vendor's shop because it's difficult to correct it once you get in the field.

95
John Sudey, NASA: Did you ever look into what the coherence function

is between the input and the output that possibly could give you indications of non-linearity of the system?

David Brown: We use coherence function quite a bit. I wrote a paper

for the Intemoise Conference in Washington that used coherence functions

to locate sources of noise. It turns out that coherence functions are very difficult to use for non-linearities, merely because the coherence

function is very much a function of any noise in the system, and the

effect of non-linearities on the coherence function is very poorly

understood. We are going to do some work for GE Evandale studying co- herence functions because they would like to use it to identify sources

of noise in the jet engine. The noise generated in the jet engine has

to traverse through a shear layer, which happens to be a non- linear flow

field, so acoustically it turns out to be a very non-linear element.

They want to know the effects of non-linearities upon the estimate of

coherence functions.

John Sudey : In the motors, did you examine the current which could

possibly be an indication of a torque vs. the response?

David Brown : No, we were looking at the dynamics of the motor, not

particularly the operating dynamics. We were interested in modes of

vibration, that would be excited by any sort of magneto constrictive

effect or the rotation of the motor itself. We were looking for

resonances that would be excited by those forcing functions.

John Sudey : How do you know where to hit with a hammer?

David Brown : Normally you want the frequency response between two points

in the system, or you want to get a mode shape which would be a series

of frequency response plots. So what you do, is define the points of

interest in your system and that's where you hit it. If you want a frequency between this comer and the table and this point over here

because this is where a bearing is located or a force comes into the

system, and a frequency response over here where your pick up is, you

hit it either here or here, it doesn't make any difference. You use

Maxwell's reciprocals.

John Sudey : What do you do with the system you can't get inside of?

David Brown : Then, of course you're stuck.

96
SESSION III

DIAGNOSTIC

SYSTEMS

APPLICATIONS

Chairman: Keith R. Hamilton

Wright Patterson Air Force Base

THE FIRST AUTOMATIC FAULT ISOLATION TESTING SYSTEM FOR

RECIPROCATING ENGINES DESIGNED FOR PRODUCTION USE

by

George R. Staton

Fire Direction £ Diagnostic Systems Division

Frankford Arsenal

Philadelphia, PA

The system described in this paper is known as DepotMAIDS

Depot Multipurpose Automatic Inspection £ Diagnostic System.

It was designed to supply the depot , for the first time , with a

method for evaluating incoming assets to determine whether or

not incoming engines are candidates for rebuild; and if found

repairable, to indicate what repairs are required to place

them in a fully operational condition.

Previous to the advent of this system, all incoming engine

assets were torn down and completely rebuilt. In addition to

this diagnostic function, the DepotMAIDS also serves as a quality

assurance tool to insure that the repairs or rebuild have

been properly accomplishedj provides for automatic run-in after

rebuild or major repair, and a complete product assurance test.

The system as presently configured consists of two test

cells supervised by a general purpose digital computer. Complete instrumentation to acquire those measurements necessary,

as well as control subsystems and switching, are provided.

The system block diagram is shown in Figure 1. This system is currently being extended to two additional engines

cells and a cell for the testing and diagnosis of transmissions. When fully extended, the system will be capable of

testing and diagnosing engines in 1790 cubic inch class , as

well as two cycle diesel engines of the 6 and 8 cylinder

variety. It will also handle all cross-drive transmissions in

the Army inventory. Thus, the capabilities will extend to all

combat vehicle engines and transmissions

.

Figure 2 diagramatically shows the capability of the system as it is presently configured, and as it is being extended.

Figure 3 summarizes the software functions designed into the

system. A real time monitor controls the operation of the

99
entire system, providing capability for self-checking , testing of units under test, reporting, and includes supervisory

test plan monitor for each of four cells.

Each test cell operator thinks that he has full and exclusive use of the computer at all times . This capability is

provided by the test plan monitor. Test cells can run fully

asynchronously under this software system.

Figure 4 shows the instrumentation subsystem as it is

configured for the 6V53 2-cycle diesel engine. The test cell

subsystems are those permanently installed transducers and interlocks which are housed in the test cell. In addition to

this instrumentation, temporary instrumentation for testing purposes is housed in kits which are applied to the engine prior

to entry into the test cell. Also in the test cell are the

transducer enclosures for the temperature transducers . Housed

in the test cell operator station are the electronics , consisting of signal conditioning and control systems for the dynamometer and power supplies , which feed the individual test cell

instrumentation subsystem. Back at the Master Controller,

additional electronics are provided to interface the test cell

with the Master Controller.

Figure 5 shows a listing of the 6V53 engine measurements

used for runin and diagnoses. Two basic types of measurements

are taken in the testing process: those which are characterized as steady-state; that is, taken at a fixed power point, and

averaged over a number of engine cycles to obtain the mean value

of the reading ; and time varying measurements , such things as

vibrations, injection processes, valve closing events, and so

forth, which are taken instantanously , and analysized for proper functioning of the system under test.

Readings taken are then compared with prestored limits and

grouped in what is known as a "truth word" for comparison with

a prestored "truth table" to determine what specific malfunction, if any, is present. Results of this analysis are then

printed out in plain English for the operator.

A typical truth table is shown in Figure 6 , and the Printout

as Figure 7

The system has been operational since March of 1971, and

has been highly successful during that period. Over 30 0 engines have been tested by the system during this period,

either for quality assurance or for diagnostics . Significant

100
savings have been achieved in both areas. For those engines

tested diagnostically , the costs have averaged $2,000 per

engine, as compared to an average of $7,50 0, if the engine is

completely rebuilt. In addition to these savings, considerable

savings have accrued from the fact that the engines rebuilt are

carefully inspected so that no potential or latent defect escapes undetected. Therefore, their performance in the field is

significantly enhanced.

This system has been operated on a two-shift, 5-day a vjeek

basis , and -has experienced only three days down time in over

two years of operation.

This digital computer controlled automatic test and diagnostic system, which is installed at Letterkenny Army Depot at

Chambersburg , Pennsylvania, is contributing significantly to

the modernization of our depot maintenance program. This

system is the first of its kind in the world, and has been

cited in the literature, both here and abroad, as a prime

example of the future concepts in automotive maintenance.

101
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108
CONDITION MONITORING FOR AIRCRAFT GAS TURBINE ENGINES,

PAST, PRESENT, AND FUTURE

by J. F. Kuhlberg and R. K. Sibley

Pratt St Whitney Aircraft

INTRODUCTION

The cost and complexity of advanced turbine powerplants coupled with

requirements for lower operational and maintenance costs are the basis

for growing interest in propulsion condition monitoring systems.

Additionally, electronics is playing an increasingly important role

in turbine engine control and power management. The propulsion system

variable geometry inlet and engine controls, the condition monitoring

system, and the propulsion management system can all be designed

around electronic computers for advanced engine designs. It would

seem logical therefore to think in terms of integrating these systems

for cost, weight, and space savings through the use of common

electronics. This paper explores possible methods for integrating

condition monitoring systems with modern propulsion system control

and management requirements and identifies advantages and disadvantages of various integration schemes.

Before addressing the integration problem, let's briefly review what

conditioning monitoring is, how it's been used in the past, how it is

used today and how it may be used in the future.

What Is It

The monitoring of critical aircraft gas turbine engine operating

parameters to assess the engine health is achieved through a process

called condition monitoring. Condition monitoring is a diagnostic

process which results from the application of fundamental engineering

laws and specific empirical relationships to determine the condition

of the engine at any point in time, and provide necessary corrective

action. See the diagram on Figure 1. The system provides the gas

turbine operator with an understanding of the engine's condition in

terms of mechanical and structural health, aero and thermodynamic

performance, oil and fuel system condition and control system

performance. The application of engine condition monitoring can

afford the operator cost effective support in the maintenance and

operation of aircraft gas turbine engines. It permits the timely

scheduling of overhaul or repair based on actual engine condition,

rather than on the traditional time basis alone, thus extending

engine "up time" by avoiding unnecessary removals. Additionally,

operational cost savings can be achieved by the diagnostic system

through the avoidance of flight delays, inflight shutdowns, mission

aborts, and other costly occurrences.

109
Past

The concept of diagnostics has, in one way or other, been applied to

aircraft engines since the beginning of aviation. Figure 2 is a

picture of one of the earliest specialized diagnostic devices,

known as the "Magic Wand". In past years, diagnostics were employed

to determine whether anything had happened , hence mandatory

maintenance was needed before the engine could continue in operation.

These monitoring practices included walk around inspections of the

powerplant, periodic inspections of lubrication and fuel system

screens and plugs, and removal of the engine after a critical

operating limit (vibration or turbine temperature) had been exceeded.

Many of these techniques proved very useful and are still employed.

Present

Presently, additional diagnostic techniques are being used on aircraft

gas turbines to determine if an engine event is imminent, is happening

or has occurred such that engine maintenance is required. Included

are such techniques as monitoring engine aero and thermodynamic

performance data either on the ground or in the air to determine

whether the engine components are performing nominally, monitoring

mechanical systems by measuring vibration, oil pressure, oil quality

and quantity, and a variety of non-destructive tests such as borescope

or X-ray inspections. In addition some diagnostic equipment which

correlates the life of certain components has been introduced,

particularly with regard to gas turbine hot section parts.

Figure 3 is a photograph of a so-called "hot section analyzer" which

monitors time-temperature exposure of critical hot section (burner

and turbine) components. When the Hot Section Factor, a time temperature integration, reaches a predetermined level the hot section

should be inspected. Additional information regarding exposure to

various turbine temperature levels are also displayed.

Spectrographic oil analysis is another technique which has been

adopted by many operators. A sample of engine oil is analyzed at

regular intervals to determine the presence of metallic particles

suspended in the oil. A change in engine condition is indicated by

a change in the count of metallic particles in the oil.

There are other diagnostic techniques which have been used in the

past and present, and are too numerous to review in detail in this

paper. Many of them have been successful in some degree.

Future

The future of condition monitoring lies in the combining of many

present day and new diagnostic techniques in an automated system.

With this variety of information available to a common system,

substantially greater information on propulsion system present and

predicted condition will be quickly and accurately determined.

110

1
In the past, the automated engine condition monitoring system has

been difficult to economically justify as a system by itself. On

future aircraft, electronics will be playing an increasingly greater

role in the airbreathing propulsion system. Engine control, inlet

control, propulsion management and condition monitoring are all

requirements of these advanced propulsion systems. Each of these

control and management requirements is envisioned as employing basic

designs centered on dedicated digital computers. In the interest of

reduced cost, weight and size, combining of these various propulsion

related computer functions must be considered. The integration of

various condition monitoring diagnostic routines with the propulsion

control and/or management requirements through the use of common

sensors, signal conditioners computational elements and display

devices may provide automated condition monitoring as a relatively

inexpensive by-product.

Two potential integration concepts that will be discussed in detail

are

° Propulsion management and condition monitoring

** Propulsion control and condition monitoring

Before getting into these systems, let's define propulsion management

and control. Propulsion Management is a system which strives to

optimize mission performance by scheduling engine power setting for

take-off, climb, cruise, etc. and matching power between engines on

a multiengine aircraft.

Propulsion control is a system which modulates the fuel to the engine

while coordinating the propulsion system variable geometry such as

spike, by pass door bleeds, stators and nozzles. The control is also

designed to protect the propulsion system against destructive limit

exceedance such as over speed and over temperature

.

Condition Monitoring and Power Management Integration

One approach to the integration of diagnostics is an engine condition

monitoring system coupled with a propulsion management system. This

consists of both airborne and ground support units. Schematically,

the airborne system consists of the components shown in Figure 4.

Many of the requirements for engine condition m.onitoring are compatible

with those of a propulsion management system. For instance, the

engine instrumentation requirements for a propulsion management system

consists principally of engine aerodynamic parameters and comprises,

broadly 70% of that required for engine monitoring. Airframe parametric instrumentation (i.e., alt., IAS, etc.) required for propulsion

management more than satisfies the monitoring requirements. Another

compatibility point in propulsion management is in the signal conditioning requirements, whereby engine and airframe signal sources are

processed in order to provide proper input into the data processor.

The added engine monitoring data requirements will mean that a signal

conditioning system will have to provide some additional electronic

capability but the external design considerations, weight, volume,

and cost should not be greatly affected.

Ill
The computer function in a propulsion management system is to process

engine and airframe data, and conduct programmed calculations to

provide information that will result in optimizing the mission being

conducted. This will include information regarding optimum airframe configuration and engine power scheduling for the mission.

This same computer could be used to process engine data to monitor

condition. The major requirement is to provide proper software in

the computer so that the engine health can be assessed. This means

computer memory capacity in the propulsion management computer would

be increased but system weight, size and cost characteristics should

not be greatly affected.

The function of the display in the propulsion management system is to

provide the pilot with specific action required to achieve mission

optimization either through airframe or engine operating procedures.

This display can also be used in engine condition monitoring by providing a diagnosis of any engine condition that affects the mission

and the necessary corrective action.

Completing the airborne monitoring requirements are a tape recording

system and maintenance advisory device. The maintenance advisory

device provides a gross diagnosis, while the tape recorder acquires

data for detailed diagnosis. Some recent work has been conducted,

under a Navy contract, in evaluating the feasibility of some of the

airborne system just described. A picture of the airborne signal

Condition/computer and the maintenance advisory panel are shown on

Figure 5. Approximately 50 parameters are acquired in the signal

conditioner. Computations are then conducted on a real time computer

which provide a gross analysis to the advisory panel which has 43

different maintenance flags concerning the engine, fuel and control

system.

A ground processor should be used to augment the in-flight diagnostic

capability achieved from the monitoring portion of an integrated

system. The general function of the ground processor is shown on

Figure 6. The ground processing system must do detailed computations,

requiring a large memory. While immediate maintenance action requirements and a go/no-go status are provided through the airborne system,

a ground system should be available with a minimum time lag to provide

an up to date permanent record of engine events and much more detail

regarding engine condition and any required maintenance action. In

addition, the trending data acquired at pre-selected flight conditions

should be evaluated to see when engine shifts from nominal are

projected. The ground processor system can be dedicated to just the

engine data reduction function or, as an alternative, time sharing

of a base computer could result in a cost effective method of processing the engine data. A picture of the ground processing system

currently in use in the Navy program is shown in Figure 7. This is

a 16K memory computer. It provides a permanent record of the engine

condition items indicated on the advisory panel and can augment the

indications of the panel with about 2 0 more diagnostics.

112
Condition Monitoring, Engine Control Integration

Figure 8 illustrates a possible functional integration of the

propulsion control and condition monitoring system. In this example,

the condition monitor can diagnose the inlet, engine, and control

using the same parameters. Control inputs and outputs are supplied

to the condition monitor. Other non-control related parameters such

as vibration, oil condition, etc. are also supplied to the monitor

to fill out its repetoire. Discrepancies between control inputs

and outputs as determined by the condition monitor indicate control

malfunction and discrepancies between the control outputs and

inputs indicate possible engine problems.

The apparent attractiveness of the integrated electronic systems

must be carefully traded against important considerations such as

reliability, maintainability, and basic requirements of the systems.

Idealistically , the reliability of the condition monitor must be

better than that of the propulsion system. The use of common

sensors and computer carries with it the possibility that a control

failure also becomes a condition monitoring failure. Thus at the

crucial point when a system failure occurs, the condition monitor

also fails. Use of redundant sensors and computers can alleviate

this problem while lending a fail-operational capability to the

control system.

Data display constitutes another area where the pros and cons of

integration must be studied and traded. A data display under the

control of the condition monitor will provide the flight crew with

the most complete information regarding the propulsion system.

This type of data is probably desirable in that it provides

comprehensive data for mission completion and safe return decisions.

On the other hand, a condition monitoring data display is unsatisfactory in the event of a condition monitoring computer system

failure. In this case the pilot has no reference from which to

operate his engines. This suggests that a completely separate additional instrumentation system may be required to supply the flight

crew with the essential information needed to operate the engines

safely. Also the human engineering aspects of any data display

need to be thoroughly evaluated before settling on a final system.

Other considerations must be addressed when studying possible

integration of system control and monitoring elements. Paramount

among these is computer cycle time. Presently, a limiting element

in propulsion control by digital computer is the time needed to run

through a single complete computation of the control laws. The

addition of condition monitoring equations and logic adds to this

calculation time. In some cases the time addition can be reduced by

performing only a part of the condition monitoring laws on each

control calculation pass. This is acceptable because, whereas the

control calculations need to be updated on the order of 50 times

per second, condition monitoring data need only be updated once per

second and slower in some cases.

113
Computer accessibility must also be considered in determining the

advisability of system intergration . The condition monitor system

must be readily accessible for routine maintenance after each

flight. The data tape recorder must be changed and flag panel

checked and reset for the next flight after servicing. The.- engine

control computer, on the other hand may best be located on or near

the engine to minimize communication problems between the engine

and control.

The number of engine condition monitoring computers required for a

multiengine aircraft also must be considered. While it may be

desirable to maintain individual engine controls, a single

condition monitoring system may suffice for the aircraft.

Conclusions

Integration of propulsion system controls, airframe controls,

avionics and condition monitoring is possible; however, careful

trade studies must be conducted to insure that the reliability,

maintainability, vulnerability, and other aspects of the various

systems is maintained. In addition the trade studies must insure

that integrated computation and data management is in fact the

lightest, smallest, and lowest cost approach.

In all probability there will be some integration of the various

electronic systems on board future aircraft. Initial integration

will be system related, such as propulsion controls, inlet and

engine, followed by the integration of peripheral systems such as

propulsion management and condition monitoring. Condition

monitoring may be split between the propulsion control and central

power management systems. Central checks and engine limit exceedance

may be monitored by the control while general engine condition is

monitored by the propulsion management system with data provided by

direct sensing and from the control.

In summary, we have observed the following:

1. Diagnostic procedures have proven valuable in the condition

monitoring of operational gas turbines.

2. Advancements in electronics have now made possible the

combination of power management, engine control and condition

monitoring into an integrated system.

3. Engine monitoring is a cost effective by-product of the control

and power management system requirements being determined for

future aircraft.

4. Electronic systems made up of sensors, avionics, and computers

must demonstrate satisfactory characteristics in the areas of

reliability, accuracy, and maintainability if the full potential

of integrated electronic engine control/monitoring/management is

to be achieved.

Ilk
DIAGNOSTIC

Figure 2

115
HOT SECTION ANALYZER

RECORDER FACTOR COMPUTER

Figure 3

AIRBORNE MONITORING SYSTEM

AVIONICS COMPARTMENT CREW COMPARTMENT

JS136- 10

Figure 4

116
lECMS DATA MANAGEMENT UNIT

(DMU)

Pratt & Figure 5

re T^a^B-^ PERMANENT

MAINTENANCE

ACTION

TAPE

GROUND

PROCESSOR

MAINTENANCE

ACTION AND

TRENDING

HISTORY

Pratt &

Whitney

Figure 6

117
lECMS GROUND PROCESSOR

EQUIPMENT

MAGNETIC TAPE

(HISTORY TAPE)

HIGH SPEED PRINTER

MOVABLE HEAD DISK

2.5 MILLION WORDS

PAPER TAPE PHOTO-READER

Pratt S Whitney

Figure 7

INTEGRATED PROPULSION CONTROL

AND

CONDITION MONITOR

INPUT

Li

MECHANICAL

PARAMETERS I

II

Pratt &

Whitnev

INLET

AND

ENGINE

PROPULSION

CONTROL

CONDITION

MONITOR

OUTPUT

DISPLAY

DATA

RECORDER

Figure 8

118
AN AUTOMATIC ENGINE CONDITION DIAGNOSTIC

SYSTEM FOR GAS TURBINE ENGINES

by

Murray Hoffman, Program Manager

Hamilton Standard

Division of United Aircraft

Windsor Locks, Connecticut

119
The operator of gas turbine and/or reciprocating engine - powered equipment naturally

requires that his equipment has the longest possible operating life and does not break

down unexpectedly. To assure that this type of operation is realized, he applies

maintenance procedures to minimize the rate of equipment deterioration and monitors

its operation closely to detect the need for adjustment, additional maintenance or overhaul. Without this monitoring, engines which are deteriorating are very likely to run until secondary damage occurs necessitating a major overhaul. However, these

procedures for maintenance and monitoring are difficult to apply effectively and

efficiently and consequently early engine warning signs are inadvertently undetected.

Hamilton Standard's extensive ejqjerience with these systems has led to the development of an automatic engine condition diagnostic system for gas turbine engines called

TRENDSTM

TRENDS, an acronym for:

TRend analysis and

E Ngine

Diagnostic

^stem

is an automatic real time engine condition diagnostic system that serves as a maintenance tool for gas turbine engines. TRENDS provides an engine condition monitoring

system that ensures the earliest possible detection of an impending loss of performance

or malfunction, performs a diagnosis, provides a prognosis and indicates the corrective action required. It remains in operation continuously, 24 hours per day, seven

days a week even though the engines may not be operating. It is a completely independent

system in no way interferring with the operation of the gas turbine generator system.

Its successful operation, or failure, cannot directly affect the engine. TRENDS is a maintenance tool which provides significant timely information which may or may not

be immediately acted upon. It does not replace the annunciator alarm system.

The primary tasks of TRENDS are to give adequate warning of the approaching need

for maintenance action, be it routine maintenance or maintenance necessitated by

deterioration in jplant condition. The warnings are issued as English language instructions printed out on a "tear-off" strip printer. No data reduction or evaluation is ever

required by the operator. The warnings are repeated at intervals until the problem has

been rectified or, in the case of routine maintenance, the operator acknowledges, by

means of push buttons, that he has implemented his instructions.

To enable TRENDS to generate these warnings it must monitor engine system condition.

This is done by means of a number of sensors, many already installed on the engine as

120
part of the basic control and alarm systems. Temperatures, pressures, vibration

levels, oil quantity, speeds and other parameters are continuously monitored in

real time. Then, by the utilization of a number of different techniques such as gas

path analysis and limit comparisons against truth tables, the condition of the engine

is constantly evaluated.by the TRENDS computer.

In addition to Its primary tasks, TRENDS provides the operator with a direct printout

of the data being monitored. This STATUS printout contains the last steady state set

of data showing each measurement in engineering units and corrected for ambient conditions. Also included is a printout of all unexecuted maintenance instructions. It is

available on demand by merely pressing a button marked "STATUS PRINT OUT" which

is conveniently mounted on the front of the TRENDS cabinet. At periodic intervals,

set at the option of the user, TRENDS will print out for record or logging purposes. The

printout will also include every maintenance instruction flagged and completed since

the last record report.

The monitoring and evaluation techniques used are those which have been developed

over the past years by the Flight Operations Section of Pratt and Whitney Aircraft and

the Engine Performance Group of Turbo Power & Marine Systems, Inc. These techniques

were semi-automated by Hamilton Standard in their AIDS (Aircraft Integrated Data Systems)

programs and TRENDS takes it the next logical step to a fully automatic real time TRENDS

detection and diagnostic system.

Hamilton Standard decided to demonstrate the utility of these techniques by a practical

demonstration which would also provide first hand experience of in-the-field operation.

A demonstration was therefore arranged jointly with Turbo Power and Marine Systems,

Inc. and Northeast Utilities at the Hartford Electric Light Co.'s (HELCO) South Meadow

facility. The field site is shown in Figure 1.0-1. A TRENDS unit is installed on both

gas turbine engines of a Twin Pac Generator System. Figure 1. 0-2 shows the TRENDS

unit installed in Control House 12.

Now, after more than 3000 hours engine time on a pilot plant installation at Northeast

Utilities' HELCO station in Hartford, this "early-warning" diagnostic system can reduce

down-time, through controllable improvements in operating efficiency, and in-service

detection and correction of incipient failures. As a result, action can be taken before

gas turbine performance or component life is compromised.

Over the last 18 months, during which HS has been fine tuning the equipment at the

HELCO installation, diagnostic monitoring has succeeded in detecting a number of

problems before they became evident to station operators - and has determined the

maintenance actions required to correct them.

FOULED COMPRESSOR. Variations in the turbine outlet temperatures indicated the

compressor was fouling at an unacceptable rate and should be cleaned as soon as

possible. The compressor was subsequently cleaned without first making an inspection.

121
122
FIGURE 1. 0-2 TRENDS DEMONSTRATION UNIT (INSTALLED IN CONTROL HOUSE

AT DEMONSTRATION SITE)

125
and new data showed the turbine was restored practically to its baseline performance.

About 2 weeks later, data indicated the compressor was again fouled. This time an

inspection was accomplished (to verify the condition) and confirmed the warning. The

compressor was again cleaned.

BOWED NOZZLE VANES. Monitored data showed the compressor needed cleaning

and that there were signs of "bowed first stage nozzle vanes. " A hot section inspection

confirmed the fouled compressor but visually, at least, the nozzle vanes looked acceptable.

Just to make sure, however, the crew measured the vane angle and found they were

bowed, but still within acceptable limits.

PRESSURE BUILD-UP . Data showed a "rapid increase in breather pressure in the gas

generator oil system" which called for an inspection. Station operators shut down the

turbine, found the anti -spark filter was clogged, cleaned and replaced it. Pressure

was restored to normal.

LOW OIL LEVEL. At another time, the data showed a dangerously low level of oil

in the reservoir. Station operators shut down the turbine, checked the oil level, and

found the system needed replenishing.

TURBINE BLADE PROBLEM . Data showed a strong trend towards fouling of all

turbine stages which was followed by a small step change in gas generator vibration

and an increase in scatter of data. A hot section inspection confirmed a blade fatigue

crack which required removal and repair of the turbine wheel. A significant failure

might have evolved if this situation had been left undetected.

Normally, with the possible exception of the damaged turbine blade, those problems

would have eventually been detected by a maintenance crew or resulted in an automatic

engine shutdown. The early warning, however, prevented any rapid degradation in

performance and made it possible to schedule minor maintenance actions at system

convenience before situations could develcp into major problems.

The TRENDS diagnostic system is modular in construction and flexible to allow for

growth in the number and type of measurements and the number and type of engines.

A single central computer provides the core of the system. Basically, there is a Central Unit (CU) for each total system, a Data Collection Unit (DCU) for each grouping

of one to two engines, and a set of sensors on each engine.

The sensors are used to convert pressure, temperature, oil quantity, and speed signals

to electrical signals. Other electrical measurement signals are taken directly from

existing instruments. Figure 1.0-3 is the block diagram.

12k
125
The DCU collects the electrical signals, conditions them and converts them into a form suitable for transmitting in sequence along common cables to the CU. A

DCU will handle up to 2 engine installations. Each engine can have up to 56 measurements. Additional engines necessitate an additional DCU or DCU's. Each DCU feeds

directly into the CU, and the CU can handle up to 24 DCU's. (48 engines).

The minimum size TRENDS system could consist of one DCU, one engine set

of sensors, and one CU. The capability of the computer is such that a single

CU can handle up to 48 engines in a single installation over direct wire; or they

can be connected to the CU without geographic or wire length limitations through

the integral modems permitting direct communication over telephone lines or

telemetry links. The output of the DCU is a digital code, programmably compatible

with all present digital transmission systems, which, theoretically, can be sent

around the world.

Sensors mounted on the gas turbine engine monitor a variety of operating functions

including compressor inlet pressure and temperature (PT2 and TT2); compressor

discharge pressure (PS4); gas generator output pressure (PT7) and turbine outlet

temperature (TT7); low and high pressure compressor rotor speeds (Nl and N2);

compressor and turbine vibration levels; lube oil system pressures, temperatures,

and levels; electric generator bearing temperatures; Figure 1.0-4 and Figure l.OrS.

Periodically, about once every two seconds, the DCU samples all the sensor inputs

from each engine (takes about 1/2 second per engine), converts the analog signals

into digital form, and transmits them to the CU where they are sampled 16 times

and then averaged to reduce the effect of any spurious inputs to arrive at a "true"

reading. These 16-value averages are temporarily stored for data processing and

emergency retrieval.

This constantly updated "retrieval storage" provides a history of engine performance

immediately prior to any engine failure or abnormal deviation in performance. The

computer memory retains a 5- to 6-minute record by continually recording every set

of averaged parameters, which it does by deleting the oldest or first set of readings

each time a new set is added.

The permanent memory storage contains the baseline values of the various gas

turbine parameters being monitored. Baseline information includes fixed data such

as operating limits as well as variable data which varies between engines and with

engine time. This data is obtained from initial performance readings taken when an

engine is first installed new or following overhaul.

126
MEASUREMENT LIST

Time

Megawatts

Total engine operating hours

c

Number of engine starts,

L. P. compressor speed

H, P, -compressor speed

Compressor discharge static pressure.

Gas generator exhaust temperature

Engine inlet total pressure.

Gas generator discharge total pressure

Steady state operation.

Engine inlet temperature.

Ambient pressure.

Oil temperature to gas generator.

Oil temperature to free turbine.

Oil breather pressure gas generator.

Oil breather pressure free turbine.

Oil filter pressure drop gas generator.

Oil filter pressure drop power turbine.

Oil level gas generator.

Oil level free turbine.

Oil consumption gas generator.

Oil consumption free turbine.

Vibration Station #1

Vibration Station #2

Vibration Station #3

Anti- Icing

Generator bearing temperature coupling and exciter ends.

Generator cooling air temperatures in and out.

Exciter air temperature out.

FIGURE 1,0-4

MEASUREMENT LIST

127
GAS GENERATOR SECTION

ROTOR STAGE

-STATOR STAGE r-N, ROTOR

N, ROTOR

^5O X o u

FREE TURBINE

INIET

EXHAUST GAS - DISCHARGE V

EXHAUST

STACK

-10

POWER OUTPUT SHAFT -

EXHAUST ElBOW

S 7

T2 ^T7

TT7

SUFFIXES 1 THRU 9 SHOW INSTRUMENTATION STAGE

N = ROTATIONAL SPEED

PT = TOTAL PRESSURE I.E. STATIC PRESSURE PLUS DYNAMIC HEAD

PS = STATIC PRESSURE

Tt = TOTAL TEMPERATURE

FIGURE 1.0-5 SIMPLIFIED MECHANICAL SCHEMATIC

128
Each engine is assigned a small electrically alterable memory module, provided as part of the TRENDS hardware, in which is stored all the information relevant to that

serial number engine such as baseline details, engine hours, regression analysis

information, serial number, etc. ITiis memory is plugged into TRENDS' DCU so

that it can be interrogated by the computer to transfer its information to the main

core memory.

Each engine is also fitted with a coded socket which is connected to the DCU so that

the computer can ascertain the serial number of the engine and hence interrogate the

correct electrically alterable memory module. Thus, when an engine is changed,

TRENDS automatically detects the new coded socket and searches for the new engine

memory module.

With a new engine, or one that's just been overhauled, it's necessary to "calibrate"

the engine to generate the baseline information entered into the memory module.

This involves running the engine at steady state for at least 1/2 hour at various

calibration power settings so that the computer can monitor the running at each condition and enter the calibration readings as baseline inputs into the memory module.

The baseline values are used as reference points for comparing "as is" versus "new"

component performance when evaluating on-line service. The computer subtracts

the baseline readings from the sensor readings to generate "delta" values which re- present the current amount of change from the installed or baseline value. Depending

upon the parameter, the delta values are given as plus or minus percentage figures or in engineering units.

Trends in performance are determined by a method of linear regression analysis

which calculates the slope (m) and intercept (b) of the straight line which defines

the measured deviations from baseline over a period of time. Two trend lines are used to show the rate at which changes are taking place. There's a long term trend

based on measured deviations recorded during the last 150 hours of engine operation,

which gives an overall view of what's happening, and a short term 20-hour determination which magnifies the effects of more immediate changes. Figure 1. 0-6.

Thus, with a single point defined by m, b, each trend line, it's possible to store

data that would otherwise occupy literally thousands of bits of memory storage for

each parameter monitored. This is the reason TRENDS is able to match the data

processing capabilities of much larger and more expensive computers — and also

makes it possible for TRENDS to keep tabs on as many as 48 different turbine

installations.

Knowing the slopes and intercepts of the trend lines, it's possible to predict when

deviations from the baseline will become so large as to warrant maintenance action.

Whenever the short term "m" deviates from the long term value by more than set

129
150
limits, it implies a change in the rate of deterioration which is taken into account

when predicting the time available for maintenance action.

Two other characteristics which the computer looks for are sudden changes in the

short term "b" relative to the long term value, and a sudden or progressive increase

in the randonmess of the data collected. Either of these changes, suggests a fault

other than wear or deterioration. In such cases TRENDS gives an alarm, prints

out instructions for remedial action, and records the time of the message. Diagnostic

"truth" tables are used to program the computer so that it can analyze the pattern of

deviations to arrive at the most likely cause or causes. Figure 1.0-7.

Basically, TRENDS generates four kinds of outputs:

FAULT MESSAGE. Whenever the continuously performed trend analysis shows the

need for a corrective maintenance action within 75 hours or less, the computer prints

out a brief "fault message" which identifies the problem and gives instructions for

its correction. This can happen at any time of day so the system is designed to flash a red light on the recording console to attract attention and ask for an acknowledgement by the station operator.

Until action is taken, TRENDS will repeat the message every 8 hours (without the

flashing red light) unless performance begins to deteriorate at a faster rate calling

for more immediate action. It steps up the message intervals to every 4 hours when

only 24 hours are left, and then every 2 hours from 10 hours before the predicted

event until 10 hours after it has passed. Figure 1. 0-8.

STATUS REPORT. A daily status report is printed out for each engine installation

automatically, at a set time of day, which shows the value of each measurement

being made, the number of operating hours to date, and the "deltas" or deviations

between baseline and current performance values. It also lists any outstanding

maintenance actions waiting to be performed as well as all maintenance actions

completed since the last daily printout was issued. Figure 1. 0-9.

This same status report will also be generated upon request at any time. All you

have to do is press the "status" button on the recording console for an immediate

printout of the latest data being recorded and processed.

Many gas turbine station superintendents, particularly those with large fleet

responsibilities or with remote station installations find the daily status report a very attractive operational feature.

WARNING ALARM. Whenever an operating limit is exceeded, for any of the parameters being monitored, the system prints out the last 5 minutes of recorded data

for all the parameters to provide a history of what led up to the problem immediately

before it was detected. In addition, the system also prints out a relevant fault

message and instructions for remedial action.

151
= » * 13

J- 0 < B < L J- K 0. z

18 }2

H9IH 3Xa •

H9IH 910 •

aN3 U3X1X3 H9IH agx •

aN3 9Nndnoo h9ih a9x •

Jit ^ "X •

>IOVN •

J iS < """X •

03033N 39<XNy • •

aasoiD xoN 3AnvA • •

aasono 3AnvA •

N3dO XON 3AnVA • •

N3dO 3AnVA •

0NVMN03 39IXNV ON • •

H91H "^X •

H9IH Xd "d •

H9IH 99 'd •

MOT n3A3n Xd •

-MOT -13A3n 99 •

H9IH SNOOXd •

H9IH "SNOOgO •

03801 SXNOOM •

nVWdON Xd ""°X •

nVWdON 99 ""OX • •

H9IH XdA • •

H9IH XJ ""°X •

H9IH 09 ""°X •

H9IH d ddia Xd •

H9IH d ddia 99 •

H9IH 99A 39NVH0 ON »Sd •

9NIOna3d »Sd •

9Nrona3d 'n • • •

9NI3na3d 'N • • • •

9NISV3d3NI 'N • •

9NISV3d9NI -N •

NVSnO XSVn kSIOdd a3SV3d3NI "X • • • • •

ICLEAN

COMPRESSOR

1

q:8

inu

i!

o

z2

Ju 1

CLEAN

COMPRESSOR

1

1

BC'IWED

FIRST

STAGE

|

1

FOULED

TURBINE

OR

WORN

SEALS |

1

INSPECT

HIGH

TURBINE

BLADES

1

INSPECT

LOW

TURBINE

BLADES

|

CLEAN

OIL

FILTER

GG

CLEAN

OIL

FILTER

FT

INSPECT

GG

BEARING

INSPECT

FT

BEARING

1

LUBRICATE

MOUNTS

LUBRICATE

MOUNTS

1

INSPECT

CCMTOESSOR

BLADES

1

_i5

hi5

CHECK

FT

OIL

LEAK

RLL

OIL

GG

FILL

OIL

FT

CHECK

OIL

SEALS

GG

CHECK

OIL

SEALS

FT

INSPECT

BURNER

ANTICE

NO

POWER

ANTICE

STUCK

OPEN

ANTICE

STUCK

CLOSED

ANTICE

STUCK

MIDWAY

ANTtCE

ERROR

|CHECK

ANTICE

AUTO

[inspect

GEN.

BEARING

1

INSPECT

GEN.

BEARING

1

INSPECT

GEN.

COOLING

1

INSPECT

EXITER

COOLING

152
A QS CLFAK -74

aCk CODf I 3

TIM£:ii?i6 16 1?

A GG INS^tCT -^'^ BOwtD F I ''ST m Aoe

ACK COPE 3

TIMF. 3(36 16 12 "^6

A GG IN OFCT -71

FOULiD TURBINF

Ol^ ORN StALS

ACK CODS \ 2 TIME 306 16 12 55

A TIT

ACK CODET I Mfc 3(56

A SKC

ACK CODE

TiMfc 306

-3k3

2

16 12 55

-4 1

1 16 12 55

FIGURE 1.0-8 TYPICAL FAULT MESSAGES

155
T I ME 306 ! 6 12 56

MM 49*9:) 50*67

TtH 20t £0»

NS 4 1* 5

ART 0*3 4»0

EPR 2*309 2*435

Nl 5792* 0*

NIC 5752* 0*

N 2- 8 2 5 7* 0 • N2C 8199* 0*

PB 173.06 122*45

PBC 12*029 8*515

TT7 966*4 480*8

T7C 1405*8 927«1

T7S 1*1 2*0

PT2 29*288 29*ij77

PT7 3b«2l 41*86

SS 10* 7*

TAM 66.67 66*67

PAM 29*433 29*433

OTO 0*0 83*3

OTT 0*0 0*0

OPO 9.52 3?*73

OPT 11.01 42*13 NOTE : TWO COLUMNS INDICATED

DPO 8.63 23*78 DATA FROM EACH OF TWO

^Vli ENGINES OF A TWIN PAC

QUO 16.09 7*66

Out 1 6.05 3* 73 INSTALLATION.

Vii ,3.e'00 1.447

vr 1 k)»000 1.412

VT2 0 .id00 1.406

Di^WC t<).00 0.710

ONIC -0.7 -3.

DN2C - 1.7 -4'i.l

DPBC - 3.1 4.4

DT7C 3.6

DT7S - I 1 1 •e -3s 'ki DJTQ 13*5 -5b«2

DCTT -Ma .J 2^.9

D0P3 -1 V.91 8*53

DOPT -l*i.4l 5*27

DDP6 - 1.38 1 6»86

DUPT - 1.16 17*1J

D^IQ 6*34 - 1»75

DOLT 3.05 - 9«6B

DVO 0.499 - a.*^4l

DVTl 0.673 - 0*781

DVT2 0*972 - 0.494

DiiFC ?. 46 -37 *09

DTIT ^.3l-61*flt

DCEL - 1.65 141.15

DFT - I .68 -87 .24

DbFS 2M8 -k6»34

DAFL - 'J.?3 35*53

FIGURE 1.0-9 TRENDS STATUS REPORT
While the engine continues to operate above limits, the acknowledge button glows

red. At any time the operator can press the "status" button on the recording console for a printout of the last set of 16-second average points.

MAINTENANCE MESSAGE. As required, the system issues routine preventive

maintenance instructions for the station operators and maintains a status report on what's been completed and what is outstanding at any given time. Schedule

frequency and message content are tailored to the requirements of the site, and may

be based on engine running hours or elapsed time. Figure 1. 0-10.

Each maintenance message is identified by an "acknowledgement code" which provides a link between the station operator and the computer. Whenever the system

issues a maintenance message, it records the time, the action to be taken, and

when it's to be completed. Whenever the maintenance is completed, the station

operator enters the acknowledgement code into the computer as a signal that it has

been performed and should be removed from the next day's status report.

In looking at what TRENDS can do, the questions loom large in the minds of users:

How effective is it in covering the major problems most commonly experienced

during turbine operation? And, how accurate is it in predicting and diagnosing

these problems?

TRENDS can successfully anticipate at least 75% of the gas turbine problems now

being experienced in industry while they are still in the formative or developing

stage—early enough to be able to take remedial mesaures and avoid an unscheduled

shutdown. Figure 1. 0-11 is a tabulation of the estimated accuracy of fault prediction.

155
160 DAY Preventive

HA inTenAnce

ACK CODE 1 2 TIME 227 13 26 28

CHECK FIRE EXT

ACK CODE 2 TlHt 227 13 26 27

CLEAN Breather

ACK CODE 1 TIME 227 13 28 27

CHECK Exhaust

ACK CODE 12 4 5 TIME 227 13 ?8 27

CHECK Inlet

ACK CODE 2 4 5 Time 227 13 28 26

AIR STAkTER

MAINTENANCE

ACK CODE i 4 5 TIME 227 13 ?e 26

LOBRICAJP rUEL

SYSTEM HOT OH

ACK CODE 4 5 TIME 2 '^7 13 28 ?6

V0 DAY »'REVFNTIVfc

M A I N ' K N A N C ^ ACK CODF 12 3 5 TlMt 22 7 13 2 8 2's

CHECK "LTSr'NATOi*

Al,K CODE 2 i b TIME 227 13 28 ?5

CHECK FUtL FILTEK

ACK CODE 1 3 5 TTME227 13 28 2H

6® DAY ^'REVENTIVE

MAINTENANCE

ImSPECT CoMPRESSOK

ACK CODE 3 5 TIME 227 13 28 23

FIGURE t^Q-tO ROUTINE MAINTENANCE MESSAGES

156
ESTIMATED ACCURA.CY OF FAULT PREDICTION

The reliability of TRENDS to spot trouble before it shows up as an unacceptable

deterioration in performance or component failure can range from as low as 25 to 30%-

where, by the time the symptoms appear, it s too late to take corrective action—up

to 100% for classically perdictable failure patterns.

Iteni_ Technique Accuracy

Blocked/Streaky Nozzles or Damaged Cumbustor gas path analysis 100%

Fouled Compressor Blading temperature and speed 100%

Bearing Seal Deterioration pressure and temp 80%

Bowed First Stage Nozzles pressure and temp 80%

Fmilpd Tiirhinp BlariinP" snpprl and nTPRRiiT'p 70%

Damaged Compressor Blade vibration and temp 30%

Damaged Turbine Blade vibration and t^mp 30%

Bearing Problems vibration and ^emp 25%

Anti- Icing Valve switch position 100%

Oil Filter Blockage pressure 100%

Excessive Oil Consumption reservoir level 100%

FIGURE 1 0-11

ESTBAA.TED ACCIW GY OF ^AUI T PREDICTION

157
The TRENDS diagnostic system provides the following action messages as rapidly as performance changes are detected and evaluated.

MESSAGE REASONS FOR TAKING RECOMMENDED ACTION

Clean Compressor

Inspect- Fouled Turbine or

Worn Seals

Inspect-Bowed First Stage

Shutdown No Anti-Ice Power

Inspect-Anti-Ice Valve

Stuck Open

Shutdown Anti-Ice Valve

Stuck Midway

Override Anti-Ice Signal Error

Check Auto Anti-Ice

Increased EGT lowers the life of the hot section.

Increased specific fuel consumption increases

operating costs.

Increased EGT lowers the life of the hot section

Increased specific fuel consumption increases

operating costs.

TRENDS detects this problem, which is the

start of hot end deterioration, at an early stage

and thereby enables the cost of repair to be

minimized.

The problem is self aggravating in that it causes

increased EGT's which in turn increases the rate

of hot end deterioration.

Continued operation of the engine could result in

compressor damage caused by ice collecting on

the intake, breaking away and entering the engine.

Increased specific fuel consumption increases

operating costs.

Unable to obtain maximum power on warm days

Anti-icing system only partially effective.

Compressor damage could occur in icing conditions.

In non-icing conditions the increased specific fuel

consumption would cause higher operating costs.

On hot days maximum power would not be available.

Increased specific fuel consumption increases

operating costs.

Unable to obtain maximum power on warm days.

The engine is operating in icing conditions with

the anti-icing system manually switched off. This

could result in damage to the compressor by

ingested ice.

158
MESSAGE

Clean Oil Filter

Inspect for Bearing Failure

Inspect Compressor Blades

Inspect Turbine Blades

Lube Mounts

Check Oil Leak

Fill Oil

Inspect Oil Seals

Inspect Burner Bad Tt7

Distribution

REASONS FOR TAKING RECOMMENDED ACTION

The forewarning given by TRENDS enables the

maintenance action to be scheduled.

Clogged filters result in lowered oil pump inlet

pressures and have an adverse effect on pump

life.

Bearing failures are progressive leading to major overhaul. Early repairs result in major

cost savings.

TRENDS detects possible compressor blade

problems at an early stage enabling the cost of

the repair to be minimized.

Early detection and repair of turbine problems

enables cost of repair to be minimized. Damage

may extend to free turbine if allowed to progress.

Excessive vibration reduces life of engine and

components.

Vibration caused by badly lubed mounts could

mask vibration caused by more serious potential

problems. Early warning enables maintenance

action to be scheduled.

Loss of oil increases operating costs. Leakage

may be to a hazardous area or create a hazard.

Avoid unscheduled shutdown due to low oil level.

Adding oil when the engine is off line avoids an

unscheduled shutdown due to low level alarm.

Bearing lubrication may become inadequate.

Oil may be lost to exhaust causing infringement

of exhaust emission regulations.

Uneven Tt7 distribution may be the result of a burner can problem. Damage to the nozzle guide

vanes can result.

A second probable cause is clogged or streaky

burner nozzles. This can damage and distort

burner cans and nozzle guide vanes. May also

result in failures to start.

139
MESSAGE REASONS FOR TAKING RECOMMENDED ACTION

Generator Coupler (or Exciter)

Bearing Overhot Inspect

Generator Over Hot-Inspect

Exciter Overhot- Inspection

The TRENDSTM Diagnostic System is designed to provide users of gas turbine engines

with an automatic data analyzing tool which will:

^ Avoid possible damage to generator by appropriate

maintenance action taken before problem develops.

* IMPROVE ENGINE MAINTENANCE PRACTICES, which will tend to:

* Reduce maintenance costs,

* Reduce operating costs,

Extend engine life, and

Reduce forced outages

The TRENDS diagnostic system is "on-line" continuously, providing maintenance action

messages as rapidly as conditions change and are evaluated. The high accuracy and

short cycle sampling times detect changes in performance at their very onset.

This early maintenance action, taken when required, should result in less costly repairs and as procedures improve, resulting in better maintained engines, maintenance

action requirements will be reduced. In addition, the probability of damage due to

secondary failures is greatly minimized. This increased visibility into the engine condition and the improved maintenance actions permit confident "on-condition" maintenance

procedures which can maximize time between inspections. It permits scheduling maintenance actions at the convenience of the user and reduces the probability of unscheduled

outages.

The automatic and remote capability of the TRENDS system also provides detailed

reports without the requirement of an on-site operator.

As recommended by the manufacturer, routine maintenance tasks are necessay to keep the engine operating at optimum performance and to prevent premature wearout. It is important that they be performed on schedule. The TRENDS diagnostic

system provides a print-out when the actions are due and provides a record of when

they were executed. Keeping the engines operating at or near peak efficiency for most

of their life avoids the over-stress conditions which sometimes lead to excessive and

expensive repairs, reduced life, and unscheduled outages.

114-0
The real worth of TRENDS is the knowledge and confidence the station operator has

in knowing that his equipment is operating properly and is available for immediate

power demands. In addition, the savings associated with scheduled repair and maintenance and reduction of expensive failures make TRENDS a cost effective investmento

ll+l
A SYSTEMS ENGINEERING APPROACH TO

EFFECTIVE ENGINE CONDITION MONITORING

David W. Leiby *

Aircraft Engine Group,

General Electric, Evendale, Ohio

ABSTRACT

The development and application of a variety of engine condition

monitoring techniques and equipment is currently being accelerated as

a result of increasing demands for improved aircraft flight readiness

availability, improved mission success probability, and reduced

maintenance costs. Although individual monitoring methods provide

valuable information, optimum effectiveness can be achieved only

through their application and utilization in an engineered system

of complementary techniques. To achieve effective condition monitoring,

therefore, an integrated system of airborne and ground monitoring,

diagnostic, and inspection techniques and equipment applied to aircraft

turbine engines for purposes of problem detection, isolation, and

trend monitoring is required.

Currently, a wide variety of monitoring techniques and equipment

is being applied to aircraft turbine engines. These range in maturity

from well-established methods (such as borescope inspection, Spectrometric Oil Analysis Program, and radiography), through items still in

early application evaluation phases (such as vibration analyzers, in-line

oil monitors, and parameter trend monitoring), to advanced development

lk2
techniques (such as turbine blade pyrometers and associated maintenance

recorders) . The purpose of this paper is to present the system

engineering aspects of condition monitoring from an aircraft engine

manufacturer's point of view.

Presented as Paper at the Symposium on Instrumentation

for Airbreathing Propulsion, Monterey, California, September

19-20, 1972

* Systems Engineer, Conditioning Monitoring Systems

1^5
INTRODUCTION

The larger sizes and higher costs of today's aircraft turbine

engines, the growing economic impact of flight and mission delays and

cancellations, and increased costs of maintenance material and labor,

have made sophisticated condition monitoring economically attractive.

More and more attention is now being focused on the application of

condition monitoring techniques and equipment.

A variety of different techniques and equipment has been

developed. Instrumentation manufacturers have developed and improved

their equipment to provide desirable monitoring capabilities in an

effort to expand their markets; certain aircraft users reduced their

operating and maintenance costs through the application of cost

effective monitoring techniques and equipment; and engine manufacturers

have developed and applied new and improved monitoring methods to

enhance their capability to provide engines with lower costs to the

user in current and future aircraft applications.

Condition monitoring must be more than a random collection

of measurement, inspection, and diagnostic techniques and

equipment applied to aircraft turbine engines. It must be

an integrated system of complementary technology, which is

intimately interfaced with the engine design, coordinated with

the engine application and operation, and incorporated into the

engine maintenance plan.

Ihk
The purpose of this paper is to present a systems

engineering approach to condition monitoring from an engine

manufacturers point of view. The philosophy, history, and

technical requirements are reviewed briefly for background

information. The main body of the paper is devoted to a discussion of selected condition monitoring techniques and equipment,

including the monitoring rationale, system description, and examples

of its uses and applications.

1^5
BACKGROUND

DEFINITION AND PHILOSOPHY - For the purpose of this paper,

condition monitoring is defined as that Integrated system of

techniques and methodology applied to aircraft turbine engines

to achieve problem detection, fault isolation and trend monitoring. Through integration of the system with the engine design,

its installation and application, and its maintenance plan, effective

condition monitoring can be achieved.

The tools of condition monitoring vary widely in technology

and in application, encompassing such disciplines as pneumatics,

hydraulics, electronics, optics, mechanics, infra red radiation,

thermodynamics, and gamma ray radiography. Some monitoring

techniques are applied in flight, either full-time or at selected

points in the flight profile. Others use in-flight recorded data

with ground processing. Still others such as radiography and

borescope inspection, are used only during ground inspections.

MONITORING EFFECTIVENESS - Effectiveness and accuracy of the

overall condition monitoring system is enhanced by using the

indication of one technique to complement and verify that of

another. In line, full-time monitors may be used as early warning

devices, with other techniques applied for confirmation, isolation,

and detailed analysis.
Proper application, Integration, and utilization of a system

can result in significant cost effectiveness in aircraft operations.

Economic benefits from increased operational reliability, and reduced

maintenance costs can result. The early detection of engine distress,

followed by adequate corrective action, reduces the incidence of

unscheduled maintenance, flight delays or aborts, and inflight shutdowns.

Maintenance costs can be reduced through the reduction of trouble

shooting time, and by permitting more effective scheduling of

maintenance personnel, equipment, and facilities.

HISTORICAL STATUS - Much of the basic technology currently being

applied in condition monitoring has been in use for some time.

The monitoring capabilities provided by cockpit instrumentation,

ground testers, and non-destructive inspection are the basis for

the development of more sophisticated monitoring techniques. The

historical background of individual techniques will be discussed later.

Advances in electronics, such as miniaturization, high temperature

capability, and higher reliability, have aided in the general advance

of condition monitoring capabi^-ity. Advances in condition monitoring

have also been stimulated by increased knowledge of the mechanisms of

failure, failure progression, and diagnostic logic. It is now

possible to formulate diagnostic logic to provide more reliable

indications of engine condition. Automated routines are now being

developed which provide go/no-go types of indication, minimizing

the role of the human operator in the diagnostic operation.

IU7
DESIGN INTEGRATION REQUIREMENTS

In the establishment of design interfaces between the aircraft

gas turbine engine and the condition monitoring system, it is

necessary to consider the technical requirements which influence

system design.

ACCESSIBILITY - Access to the engine must be provided to place

sensors at positions where desired parameters may be obtained.

For those techniques where actual physical contact is required,

such as magnetic plugs, borescope inspection, and radiography,

access to the engine for the implementation of these techniques

must be possible with a minimum of preliminary disassembly and

removal

ACCURACY - The accuracy of engine condition assessment depends

directly upon the accuracy of the input data to the condition

monitoring system. The sensor and its signal conditioner must

be capable of accurately converting the parameter signal into an

electrical input signal to the monitoring sysjiem.

ECONOMICS - The application of a specific condition monitoring

technique, and the depth and detail in which it is applied, must

be economically feasible. Suffice it to say, engine modifications

and implementation of a specific condition monitoring technique

cannot exceed the economic gains to be realized through the

application of that technique. There is a danger that costs per

engine would be prohibitive if condition monitoring techniques

1^8
and equipment of insufficient maturity are utilized,

EFFECTIVENESS - In the case of techniques requiring sensed parameter

data, the proper parameters must be available, and they must be

sensed at their optimum location. For such techniques as borescope

inspection and radiography, the access and extent of coverage must

be sufficient to provide effective application of the technique.

MAINTAINABILITY - The sensors, instrumentation, and special

engine features required to accommodate condition monitoring

must be applied to the engine with ease of maintainability

and calibration as design criteria. In addition, the maintenance

features of the basic engine must not be compromised by the

presence of condition monitoring.

RELIABILITY - Additions and modifications to the basic engine

to accommodate the condition monitoring system must be selected,

designed, and applied with high reliability as a goal. The

integration of several complementary techniques to comprise a

system will make it possible to segregate false indications of

distress caused by failure within the monitoring system.

Coincidentally , incorporation of the system into the basic

engine must not degrade the reliability of the basic engine.

Ik9
Fully effective condition monitoring capability on a particular

aircraft gas turbine engine can be attained only through the

coordinated integration of several monitoring techniques . This

cannot be emphasized too strongly. Without such an approach,

condition monitoring techniques utilized individually fall short of

their potential impact upon engine operation and maintenance. Several

of the more effective monitoring techniques are discussed in the

following sub-sections, including monitoring rationale, historical

background, system and equipment description, and current application

status.

PARAMETER MONITORING - Trending of the performance of aircraft

turbine engines through the periodic monitoring of selected

engine parameters is one of the primary techniques used in

condition monitoring systems.

Monitoring Rationale - The mechanical condition of an aircraft gas

turbine engine can be deduced from engine thermodynamic cycle data.

Selected pressure, temperature, flow, speed, and position measurements

and their manipulation through proper logic routines can provide

performance indices, efficiencies, and margins which can be trended

to monitor rates of change of engine performance characteristics.

Such changes are indicative of mechanical deterioration of the

engine

150
Historical Background - A form of trend condition monitoring has been

the observation and comparison of sequential thermodynamic performance

data from the pilot's or flight engineer's log. The concept has nowbeen expanded by the application of more sophisticated data gathering

and processing techniques. Using airborne data systems, parametric

data are recorded at selected times and/or at selected points in

the flight envelope, and are processed through trending logic on

the ground. Activities are currently underway in several areas in

which an airborne com.puter is applied to the performance monitoring

system to provide real time trending of the engine during flight.

Technique Description - Trend monitoring of the relative operational

health of an engine by way of measurements on major sub-assemblies

and accessory systems (modules) requires the availability and

processing of a number of engine parameters. Fortuitously, the

majority of the parameters required for condition monitoring are

also used in control functions or are displayed on cockpit

instrumentation

In performance monitoring of the thermodynamic air cycle of a

turbofan engine for trending of overall engine health, and the isolation

of degradation to the major engine modules, the following parameters

may be used. -

151
PLA

VSVP

P2

T2

EGT

NF

NG

P54

WFM

P25

PS3

T3

Power Lever Angle

Variable Stator Vane Position

Fan Inlet Pressure

Fan Inlet Temperature

Exhaust Gas Temperature

Fan Speed

Core Speed

LPT Inlet Pressure

Fuel Flow

Duct Total Pressure

Corabustor Static Pressure

Compressor Discharge Temperature

The thexTnodynamic performance of the engine and its

modules is typically indicated by corrected values of certain

measured parameters and calculated performance indices or factors.

Included among the output data are:

These output data are expressed either in engineering units

or in dimensionless , normalized units based on a selected control

point

Application Status - Currently, these systems are undergoing application

and routine operational evaluation at General Electric to provide

monitoring of development and flight test engines. Long term trend

EGTK

PCNFK

EPR

WFMK

ETAT

ETAC

PCNK

FANQ

T2CK

P'4Q54

W4CK

FAR4C

DPL

DSVP

Corrected Exhaust Gas Temperature

Percent Corrected Fan Speed

Engine Pressure Ratio

Corrected Fuel Flow

High Pressure Turbine Efficiency

Compressor Efficiency

Percent Corrected Core Speed

Low Pressure Turbine Energy Function

Corrected Compressor Inlet Temperature

High Pressure Turbine Pressure Ratio

Corrected HPT Inlet Flow

Fuel /Air Ratio at HPT Inlet

Power Lever Angle Schedule Deviation

Stator Vane Position Schedule Deviation

152
monitoring is obtained off-line from recorded data through the

Characteristic Analysis Trending Technique (CATT) program. The

real time limit monitoring of the performance engines is being

accomplished through the application of the Trend Analysis Digital

System (TADS)

Characteristic Analysis Trending Technique (CATT )

This technique was developed to provide printed and

plotted trending information to assist the engine user in monitoring relative engine thermodynamic health by major subassemblies . The CATT program provides trend plots of selected

engine parameters by processing data from the most recent engine

parameters, and incorporating this point into a plot of historical

data extracted from the data file of that particular engine. Trending

of a given parameter is accomplished by plotting the algebraic

difference of the present value of the parameter and the same

parameter on the original point associated with the current configuration

of the engine.

The CATT program is applied to trending overall engine relative

health based on EOT and component efficiencies at the take off power

level. Analysis of flight test data from the DCIO has shown that

the most effective monitoring data was obtained during the take-off

roll, just prior to lift-off at 135 knots indicated airspeed, with a

data point averaged from 4 samples per second per parameter for a 2

second duration. The selection of this point in the flight envelope

155
and the averaging of 8 data points produced trending results with a

minimum of data spread. A typical CATT trend plot of EGT is shown

in Figure 1.

Trend Analysis Digital System (TADS) - General Electric's Aircraft

Engine Group at Evendale, Ohio, has designed and built, and is

currently utilizing in engine development testing, an on-line data

acquisition system providing continuous monitoring and display of

critical engine operating parameters. The Trend Analysis Digital

System (TADS) consists of a cathode ray tube display (CRT), an ASR33

Teletype terminal, and a two-bay cabinet which houses an analog

multiplexer, an analog- to-digital converter, digital magnetic tape

unit, and a GE/PAC 30-02 Data Processor. Figure 2 shows a typical

TADS equipment arrangement. The TADS presents on the CRT "real time"

calculated engine parameters corrected to standard conditions to be

used for engine safety monitoring while the engine is under test.

The TADS continuously scans forty-eight (48) engine

parameters at four (4) samples per second per channel. The

current complement of parameters includes four (4) positions,

eighteen (18) pressures, eight (8) temperatures, ten (10)

vibration signals, three (3) frequencies, and five (5)

miscellaneous parameters. Eight samples are averaged over a

two-second period, and passed through an error reject routine.

The data processor performs, in real time, the conversion to

engineering units, the calculation of critical engine parameters

I5J+
or performance indices, checking for out-of-limit values, and

computes "delta" values for out-of-limit checks. The CRT display,

which is updated every three seconds, presents five absolute values

of critical engine parameters, six "delta" values of the parameters,

and thirteen manual data items. The TADS will generate, upon request,

a ten-point trend plot on the teletype, a printed hard copy of

selected parameters, a punched paper tape of the averaged data,

and a magnetic tape record of the raw engine data.

VIBRATION ANALYSIS - Since many turbine engine problems are

characterized by an increase in vibration levels, considerable

attention has been given to the detection and interpretation

of vibration signals.

Monitoring Rationale - A gas turbine engine generates considerable

acoustical and vibration energy over a broad frequency spectrum.

The acoustical and vibration output is the composite of the driving

forces of once-per- revolution of the main engine shafts, the blade

passing frequencies, bearing rolling element passing frequencies,

gear mesh frequencies, pump element frequencies, mechanical resonances,

aerodynamic resonances and gas flow turbulance. By analyses of this

complex waveform signal through the segregation of significant

signal components from the background "noise", it is possible to

monitor the changing mechanical condition of the engine in a particular

area. A number of different techniques are applied to achieve this

signal analysis

155
Historical Background - Engine vibration has long been recognized

as an Indication of engine distress, and vibration sensors and

display systems have been applied widely. Generally, these systems

sense velocity or acceleration and display the vibration in rails

of displacement in a frequency band which includes the onceper-revolution signal of the engine shafts over their operating

range. However, by the time an engine has exceeded the vibration limits sensed by such a system, the engine distress may have

progressed to such an extent that secondary engine damage has

occurred and there is increased risk of unscheduled engine

shutdown. Therefore, various attempts have been made to obtain

incipient failure information at a much earlier point in the

failure progression to provide an early warning of impending

problems, and to allow timely corrective action.

Early vibration analysis techniques included methods using

acoustical data obtained from microphones placed near the engine and

vibration data from accelerometers mounted on the engine. These

analysis techniques concentrated upon the problem of enhancing the

desired signal, separating it from undesirable background signals,

and interpretating the significance of the signal. Among the

techniques applied vjere a variety of sophisticated pattern-recognitionprocedures, including digital and analog methods. Narrow band

tracking filters have been applied to improve signal-to-noise

ratios of selected frequency signals.

156
The complexity and cost of analyses equipment, the high skill

level of data acquisition and reduction personnel required, the

marginal signal-to-noise-level operation, and requirements for

tuning the system to a particular engine posed serious limitations

to their universal application. As a result, many vibration

monitoring methods which could demonstrate a certain level of

effectiveness on prototype laboratory demonstrations, run into various

problems in attempting to fulfill the requirements in the market

place of practical applications.

Technique Description, Implementation, and Application - The

detection of aircraft engine distress through the application

of vibration analysis techniques requires both the availability

of data-rich signals and equipment capable of processing and

analyzing them. Since the engine operates in a strong acoustical

and vibration noise environment, it has been found that best

results are obtained through the use of data from accelerometers close-coupled to the areas of interest.

The application of vibration monitors using waveform

pattern recognition techniques requires broad-band vibration

input data. Since frequency components up to 15,000 hertz are

significant in the vibration signal, sensors and sensor locations

suitable for once-per-revoXution AVM systems may no longer be

sufficient

Internal Accelerometers - Accelerometer/vibration system

development programs are currently being accomplished, with the

157
cooperation of hardware vendors, to improve hardware reliability,

reduce cost, and improve quality of data obtained from vibration

equipment and monitoring sensors. Changes in accelerometer

design, connector design, cable bending capability, routing and

clamping procedures and elimination of connectors have been

introduced to improve accelerometer systems. The current internal

accelerometer is an integral, stainless steel-sheathed, hard-lead

unit, secured with thru-bolts, preferably directly to engine

hardware. A connector is provided as an integral part of the

hard lead immediately outside the engine casing.

FOP Detection - Automatic detection of Foreign Object Damage

(FOD) and Domestic Object Damage (DOD) is oriented toward the

detection of impacts which fall between those causing no damage

and those causing such severe damage that long lasting disturbance

of other parameters, stalls, or shutdowns occur.

The impact of an object on the rotating parts of an engine

releases measureable energy in the form of large, fast-rising

transients in the output of an accelerometer, hard mounted internally,

near the rotor thrust bearings. These transients are detected and

compared to a preset limit by the FOD detector. Figure 3 shows the

test cell model of the FOD detector.

Bearing Monitor - Several types of bearing defects such as

race spalling, rolling element spalling, end wear and foreign

material damage, provide suitable signals for automatic detection.

158
The latter is accomplished by the recognition of invariant signal

patterns produced by internal, bearing-housing-inounted accelerometers

The measurement used to identify the patterns is called "Impact Index",

and can be continuously measured with the bearing monitor. Figure 4

shows the test cell model and the airborne version of the monitor.

Normal bearings of all engines monitored to date have produced impact

indices of 2.0 to 3.0. When a defect develops, the index is driven

past 3.0, and increases as the severity increases.

Mass Unbalance Monitoring - Engine rotor mass unbalance can be

caused by many different malfunctions arising in an engine.

To detect this unbalance condition, amplitudes of vibration in

the rotor rotational frequency range are measured and compared to

either fixed limits or on a trend basis. Monitoring systems for

this "Once-per-rev" signal in the past have suffered from lack of

sensitivity, extreme noise and general lack of useability to detect

engine problems

In the Mass Unbalance Monitor (MUM) system currently under

evaluation, the combination of the MUM unit with internally mounted

accelerometers is expected to give greatly increased capability for

unbalance monitoring. The unit is essentially a high-response,

digitally-controlled, narrow-band, synchronous, digital filter.

The response is sufficient to track the engine through throttle

bursts and chops. With the greatly increased sensitivity of the

internal sensors, a very powerful means of detecting unbalance problems

is available.

159
Gearbox Mon itoring - Defects arising in gearboxes, such as

gearshaft end bearing wear, spa]. Is, high clearance, eccentric

gears and bent gearshafts, may be detected by measuring a

fixed pattern characteristic of Individual gear meshes. The

measured characteristic is called the "Modulation Index". This

index increases as the defect appears, and continues to Increase

proportionally to the defect severity.

OIL SYSTEM MONITORING - Lubricating oil, in addition to its functions

as a lubricant and coolant, acts as a carrier of information as

it collects and circulates wear particles, dirt, and oil breakdown

products. When properly monitored, valuable data can be obtained

on the condition of oil-wetted parts. Numerous methods have been

developed and applied to sense and interpret oil condition and its

associated engine components, including magnetic chip detectors,

acid number, conductivity, viscosity, spectrometric analysis,

indicating filter screens, and light transmission/scatter techniques.

Monitoring Rationale - Generally, an engine's oil consumption

rate and its oil system capacity are such that lubricating oil

dwell time in the engine, on the average, is only a few hundred

hours. This eliminates oil breakdown and loss of lubricity as

oil monitoring factors. The main concern, therefore, is the

accumulation of failure material from the oil-wetted parts.

There are two functions required of the oil monitoring

system. First, it must detect the presence of wear material

160
in the lubricating oil, and must detect this material at such

a threshold that the proper maintenance action can be scheduled

before there is serious risk of unscheduled shutdown or possible

secondary damage occurs. The second function required of the monitoring system is the capability for indicating the area of

the engine in which the distress is located. This feature is

necessary to allow timely maintenance action to take place

without undue trouble shooting to locate the problem area. Therefore,

effective oil system monitoring requires both problem detection

and fault isolation capability.

The value of the engine condition information which can

be obtained from the oil system depends upon both the effectiveness of the data and the ease and convenience by which the data

can be acquired. In the integrated system approach to oil

system monitoring, two or more methods are applied in a complementary manner. A continuous, in-line oil monitor can be used

to provide an alarm that metallic and/or non-metallic particle

concentration has increased beyond established limits. With this

early warning, magnetic chip detector and filter screen checks can

be scheduled, and an oil sample can be obtained for spectrometric

oil analysis.

Historical Background - A variety of techniques and equipment

have been applied to the monitoring of oil systems during

recent years. Many of these applications have been addressed to

problems peculiar to specific engine models. One of the techniques

161
which has been applied extensively is the magnetic chip detector.

Magnetic chip detectors are comprised of permanent magnets mounted

in the oil stream, with provisions for easy removal for inspection.

The progression of engine distress can be assessed by trend

monitoring the magnetic material buildup on the plugs. This technique

does have some limitations in that it cannot detect non-ferrous

material, and requires access to the engine for physical removal

and inspection of the plugs.

Another oil monitoring method which has been successfully

applied to a number of different engines is the Spectrometric

Oil Analysis Program (SOAP). In this technique, samples of

oil are obtained periodically from the engine and the constituant

elements in the oil are spectrographically determined. This method

is very sensitive, and can detect the presence of excessive wear

long before the distress has progressed to a condition of significant

risk. The system does have its limitations, however, in that fault

isolation is limited, and the logistics of the sampling, shipment,

and laboratory analysis may cause delays between the taking of the

sample and availability of results.

Applications - One currently-applied, in-line oil monitor operates

on the principle of the infra-red light scattering and light

transmission characteristics of the oil. The unit is normally

installed in the oil line between the oil tank and oil supply

pump, with total oil flow passing through the sensor. Figure 5

shows the installation of such an oil monitor in the oil supply

line of a turbofan engine.

162
The condition of the oil is trend monitored by periodic

indications from the in-line oil analyzer. The indicated

data must be interpreted with the proper recognition of the

dilution effects of new oil additions. When the measured

level of particulate matter in the oil exceeds the established

alarm limit, additional oil system data should be obtained

from the examination of the magnetic chip detector and oil

filter, and the withdrawal and processing of an oil sample.

The growth of distress in the wear surfaces of the oilwetted parts is accompanied by the build-up of particulate

matter in the oil stream. Large chips and debris accumulate

on the pump inlet screen; smaller materials are caught by the

oil filter, and the small particles (smaller than the filter mesh,

such as 40 microns) continue to circulate and build up in the

oil system, resulting in an increased signal level from the

in-line oil monitor.

A promising development in oil system monitoring is the

indicating oil screen. This screen, which is constructed

of a special composite of conducting and non-conducting mesh,

material, provides a method for remote electrical determination

of the amount of build-up of electrically conductive debris

collected on the oil screen. Figure 6 shows a prototype model

of such a screen for use in the scavenge line input of a large

turbofan engine. The application of such indicating screens

should allow problem detection, as well as fault isolation.

165
However, the required size of particle for detection may reduce

the capability of this technique to detect engine problems early

in their growth progression.

BORESCOPE INSPECTION - A variety of optical devices are available for the visual inspection of the internal components of

aircraft turbine engines. Either a glass optics system housed

in a rigid cylindrical barrel, or a flexible fiber optics bundle

is used to bring the optical image external to the engine.

Illumination means include an external light source through a

fiber bundle, internal incandescent lamps, and internal gas

discharge arc lamps. An integrated system comprised of a borescope

and illumination means, a low light level television camera, and

a video tape recorder has recently been assembled and used

successfully. The value of this system will be determined in

time

Monitoring Rationale and Interface Requirements - Many of

the symptoms of deterioration of engine internal components

can be readily assessed by visual inspection. The presence

and magnitude of nicks, tears, rips, rubs, erosions, burns,

cracks, buckles, distortions, coating removal, deposits, etc.

in the compressor, combustion, and turbine areas of the engine

provide very significant and accurate bases for maintenance

decisions. The basic requirements of a borescope inspection system

is to provide ari acceptable external image of the internal engine

parts of the engine. The quality of the image is determined by its

magnification, resolution, and brightness; it defines the scope of

l6k
the manual observation and photographic documentation of the

borescope inspection.

The photographic capability of a borescope inspection

system is an important aspect of its application. The old

adage that "a picture is worth a thousand words" is certainly

true in this case. The analysis of successive borescope

photographs of the same area of the engine is an effective

method for trend monitoring distress progression.

The effectiveness of borescope inspection of an aircraft

engine depends upon the capability of the basic borescope

system to extract a useful image from the engine, and the access

provisions for inserting the borescope into the areas to be

Inspected. These two criteria are related, since the engine

access ports set the maximum diameter of the borescope, thus

establishing its resolution, illumination, light gathering power

and viewing distance. In a typical large turbofan engine,

the access ports are sized to accept a 0.400 inch diameter borescope.

This is the maximum diameter that can be reasonably accommodated

between stator vanes at the high pressure stages of the compressor,

through the cooling holes of the outer combustion liner, and between

the airfoils of the first stage high pressure turbine nozzle diaphragm.

To meet the requirements for borescope Inspection, four

sets of access ports are incorporated into the turbofan engine.

Access provisions are Incorporated in the compressor case for

access between the stator vanes of each stage to allow the inspection

165
of the trailing edges of the upstream rotor blades, and the leading

edges of the downstream rotor blades as the core engine is slowly

turned over. In the combustor area, six access ports are approximately

equally spaced around the circumference of the engine. These ports

provide access for inspection of fuel nozzles, combustor swirl cups,

all areas (forward, middle, and aft) of the inner and outer combustor

liner, and the leading edges of the first stage, high-pressure turbine

nozzle. The two stages of high pressure turbine blades are inspected

through access ports in each of the two turbine nozzle diaphragms.

The low pressure turbine blades are inspected through access ports

in the respective nozzle diaphragms of each stage.

Historical Background - Borescope inspection of aircraft gas

turbine engines has continued to grow in effectiveness as

improved equipment has become available, and inspection

access has been designed into the newer engines . Early borescope

applications used the adaptation of medical diagnostic optical

instruments, and suffered accordingly from the dissimilar environments

of the operating room and the aircraft flight line.

One of the continuing problems of the older borescopes

was that of obtaining sufficient light inside the engine to

provide an adequately bright image. Among the methods used were

miniature "grain of wheat" incandescent bulbs operated over-voltage

to get maximum light, with a resulting short bulb life, and the

possibility of bulbs shattering inside the engine. Another method

166
utilized a special gas discharge lamp at the tip of the borescope.

This provided adequate illumination for visual and photographic

inspection, but presented its own set of problems, such as a complex

and expensive power supply, high voltages in the system, and a hot

exposed bulb.

Continued progress has been achieved by working with

borescope manufacturers to design special equipment tailored

for aircraft engine application, with simplicity and ruggedness

high on the list of design priorities.

Borescope Equipment and Applications - Periodic borescope

inspection is a generally accepted standard operating procedure.

Borescope inspection data provides early warning of distress in

the engine and allows a scheduling of maintenance before the

distress grows to the point where unscheduled shutdown occurs.

The engine inspection requirements specify such borescope features

as depth of penetration, viewing angle, field of view, illumination intensity, magnification, and object distance.

A current borescope system being applied to a large

turbofan engine uses a rigid optics train for image transmission, and a glass fiber bundle for illuminating light

transmission. The basic system consists of a number of

different probes, each having a selected angle of view and

magnification, a light source, and the interconnecting glass

fiber light bundle. The power supply provides two light sources.

167
one an incandescent projector bulb for continuous operation,

and a 1000-watt gas discharge lamp for intermittant use in

photography. The light bundle is protected by an armored sheath

and an outer plastic coating.

An important aspect of borescope inspection is its capacity

for pictorial trend monitoring, in which the progressive growth

of distress is observed, and predictions are made as to when

distress has progressed to its allowable limit. This requires

storage, and comparison of sequential borescope inspection

data. In one method of pictorial trend monitoring, a 35mm SLR

camera is used to obtain black-and-white or color photographs through

the borescope. For obtaining quick turn-around on the availability

of borescope photographs, a Polaroid pack attachment is fitted

to the back of the 35mm camera body. These two photographic

attachments are shown with the borescope probe in Figure 7.

Excellent borescope photographs have been obtained of many areas

of different engines. Figure 8 and Figure 9 illustrate some of

the typical areas of the engine which can be photographed successfu

through the borescope.

Since the exposure time for a given photograph is determined

by the combined effects of film type, borescope optical system

losses, engine area reflectivity, object distance, and illumination intensity, borescope photography is often an art, rather

than a science. However, recent equipment developments and the

availability of such equipment have removed the guesswork, and

168
made possible the obtaining of properly exposed borescope photographs

under the diverse combinations of variables encountered in the

application. The device which accomplishes this feat is the

through- the-lens light meter with booster amplifier, which automatically

adjusts the shutter speed for actual light reaching the film.

With such a camera system, the inspector need only select the ASA

setting for the film being used, and the camera will automatically

expose the film correctly, covering a range of a few milliseconds

to up to 70 seconds.

The application of a lightweight, low-light-level, television camera to the borescope has opened new avenues of borescope inspection capability. A prototype system used in the

application evaluation of the concept is shown in Figure 10.

The borescope is coupled to the vidicon TV camera through a

two-stage image intensifier. The output of the system is

displayed on a conventional video monitor and can also be recorded

on video tape.

The borescope-video system provides a real-time image

of the internal area of the engine which can be viewed simultaneously by a number of people. In addition, it brings the image

to a more comfortable area for study, since many borescope

inspections require awkward and tiring positions for manual,

direct viewing. The use of a video recorder can provide a record

of each blade of a turbine stage having up to 100 blades with

169
an elapsed time of several minutes. Still photographs of

each blade would require up to four hours. Another very

important feature of the borescope-video-recorder system is its

use as a training aid. Through the generation of typical

sequences of engine images it is possible to train inspectors

in the implementation of standardized inspection techniques.

RADIOGRAPHY - The application of radiographic inspection

techniques to condition monitoring of the internal components

of assembled, on-wing engines has increased considerably in

recent years. In this technique, a gamma ray source is used

to expose industrial x-ray film placed on the appropriate area

of the engine. Three different techniques are applied in

obtaining radiographic data. In the major number of radiographic

inspections, a radial technique is applied in which the source

is placed along the engine centerline, and the film is placed

on the surface of the engine. A reverse radial technique is

sometimes used in which the film is placed at the engine

centerline, and the source is placed on the surface of the

engine. In the third technique, chordal shots are obtained

by placing the film on one side of the engine and the source

on the other.

Monitoring Rationale and Interface Requirements - The engine

areas amenable to radiographic inspection include compressor

stator vanes, the combustor and fuel nozzles, the high pressure

turbine nozzle diaphragms, the transition ducts, and the low

170
pressure turbine nozzle diaphragms. Excellent correlation has

been obtained between radiographic inspection data and teardown

inspection

.

Radiographic inspection is applied to aircraft engines

during periodic inspections , or whenever there is reason to

suspect engine distress amenable to detection by this technique.

The results of radiographic inspection are used as the basis for

maintenance action decisions. Limits are established for

different types of distress, such as length, location and

number of cracks in the combustor, and amount of bowing of

first stage high pressure turbine nozzle diaphragm airfoil

trailing edges . Trend monitoring of the results of sequential

radiographs of a particular area of the engine allows determination

of the deterioration rate, and prediction of the remaining

operating time before the distress will reach the limit value.

Radiographic inspection of aircraft gas turbine engines

requires design and configuration features compatable with

radiographic inspection technology. First, convenient access

to the centerline of the engine is required. Second, the surface

of the engine cases and frames should be free of obstruction to

allow optimum film placement. And third, the basic design of

the engine must be amenable to radiographic inspection, with

areas of interest capable of clear radiographic projection on

film fan visualization.

171
Historical Background - X-ray inspection techniques have been

used in the routine, non-destructive testing of airframe and

aircraft engine components for many years. Portable x-ray

equipment and gamma ray sources have been applied to the inspection of airframe structures and equipment to obtain information

on the condition of these structures and components without

removal and disassembly. However, in the case of aircraft

turbine engines, the application of radiographic Inspection

techniques has been a more recent development.

Among the technical advances which have contributed to the

development of radiographic inspection technology was the advent

of the depleted uranium shielded source container which provided

up to 100 curies of Iridium - 192 in an easily portable 40-pound

container. The current generation of large turbofan engines with

their easily accessible hollow shafts and "clean" engine surfaces

have made the application of radiography practical. The commitment by General Electric of several man-years of effort exclusively

to the development, refinement, and application evaluation of

radiographic Inspection has provided the basic knowledge for

establishing a firm Inspection technique.

From this development program, a system of Radiographic

Inspection Procedures has been produced, which are documented

Ln the format of a Non-Destructive Inspection Manual. Also,

a kit of special support equipment has been designed for the

application of this technique.

172
Applications - Radiography has become an accepted tool in the

maintenance plans of many aircraft engine applications. It

is being used routinely to monitor the condition of development

and cyclic endurance engines in test cell operations, as well as

of flight test engines. The same techniques and equipment are

being applied on-wing by airline personnel to monitor the condition

of selected engine areas on revenue aircraft during periodic

inspections

In a typical radiographic inspection of a large turbofan

engine, the source tube is inserted along the centerline of

the engine either at the fan spanner, or through the center vent

tube at the rear. Figure 11 shows the source tube being installed

in the engine. The film cassettes are held on the surface of

the engine by an elastic rubber strap, as shown "in Figure 12,

and provide total circumferential coverage of the area of interest

except where access is prevented by engine structures.

Figure 13 shows a typical radiograph of the first stage,

high pressure turbine nozzle diaphragm area. With an ideally

exposed film read on a high intensity viewer, considerable data

on the condition of this area of the engine can be obtained.

These include the presence and size of platform cracks, trailing

edge distortion or bowing, burning or erosion of the platform,

and airfoil segment weld integrity.

173
In the chordal radiographic inspection technique, the film

cassette is placed against the engine surface at an angle, and

the source is placed at the opposite side of the engine. The

gamma rays pass through the area of interest and impinge on

the cassette perpendicular to the plane of the cassette. Figure

14 shows a chordal radiograph of the interface penetration

at the aft end of the combustor and the 1st stage high pressure

turbine nozzle diaphragm.

OTHER MONITORING TECHNIQUES - Several monitoring techniques

will now be discussed which are at various stages of development

and which show potential for inclusion in the list of useful

condition monitors.

Infrared Turbine Blade Pyrometer - One of these advanced techniques

,

which is rapidly maturing into operational status, is the infra-red

turbine blade pyrometer. In the pyrometer system, an infrared

detector is mounted on the turbine case, and oriented to allow

the sensor to scan along the blade as it passes under the pyrometer.

The transient response of the pyrometer system is sufficiently high

to provide the temperature profile along each individual blade for

a blade passing frequency of up to 20,000 per second. The

simplest form of data interpretation is the gross average temperature

of the turbine blades. Pyrometers are currently being used in

development test engines to evaluate blade cooling design

effectiveness. A pyrometer measurement system is being designed

into the control system of an advanced engine.

l-jk
Dynamic Pressure Monitoring

Dynamic pressure monitoring techniques are being developed

to utilize the high frequency components of gas and liquid

streams of the engine. Special sensors and diagnostic methods

are being applied to the assessment of pump condition, valve

operation, and gas path flow characteristics.

Maintenance Recorder

The maintenance recorder concept is being pursued in an

effort to obtain a system of monitoring methods which will

provide an indication or measure of the consumed life of the

engine. By the integration of such concepts as operating severity

modeling, cycle counting, and time-at-temperature , a measure

of the Influence of thermal cycling and low cycle fatigue upon

the expected life of engine parts can be estimated. The goal

of the maintenance recorder is to provide a maintenance factor,

made up of the combined, weighted factors of engine duty which

can be used as a guide in scheduling detailed inspections of the

engine, or estimating remaining service life before risk of engine

problems become significant.

175
CONCLUSIONS AND RECOMMENDATIONS

This paper has been restricted to a review of the basic

condition monitoring techniques, their design interface with

the engine, and their current application status. The achievement of an effective operational condition monitoring capability

requires additional interface considerations, which are beyond

the scope of this paper. These areas include coordination with the

" airframe design for the transmission of data from the engine,

data processing, data recording and data display. Other important

interfaces are the integration of condition monitoring into

aircraft operations, and integration into the engine maintenance

plan.

A broad overview examination of condition monitoring shows

that it has come a long way in approaching its true potential,

with demonstrated condition monitoring techniques directly

applicable to gas turbine engines. However, there still is

much to be done. The major thrust of basic technology develop-

- ment has been completed. The time has now arrived for the

hardnosed evaluation of condition monitoring in the real world

of aircraft operation. The techniques, equipment, and systems

which survive will be those which demonstrate their worth as

cost-effective additions to aircraft operating and

maintenance plans

176

1
From a review of the status of the individual condition

monitoring techniques and overall system concepts, the

following recommendations are made:

o Maximum effectiveness of a condition monitoring

system demands its integration with the basic engine design

and installation.

o Condition monitoring must become a viable element

of the engine maintenance plan.

o A condition monitoring system cannot remain a static

concept, but must exhibit flexibility for change and growth to

meet the challenges of engine design improvements, technique

changes, and changes in engine distress patterns.

177
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Figure 1. CATT Output Trend Plot of Normalized Exhaust

Gas Temperat-ure

Fig\:ire 2. Trend Analysis Digital System (TADS) Equipment.

178
Figure h. Airborne and Test Cell Models of 6 - Channel

Bearing Monitor.

179
Figure 5* In-Line Light Scatter/Attenuation Monitor.

Figure 6. Conducting Debris Monitoring Oil Screen.

180

Figure 9« Borescope Photograph of High Pressure Turbine

Blades.
185
Figure 15 . Copy of Radiograph of High Pressure Turbine

Nozzle Area.

Fig\xre 1*+. Copy of Radiograph of Combuster/HPT Nozzle

Interface

l8i+
PANEL DISCUSSION

Panel Members : Nharray Hoffinan, Joel F. Kuhlberg, George Staton,

David W. Leiby

Panel Moderator : Keith Hamilton, Wright Patterson Air Force Base

Ephraim Regelson^ NAVAIR: This question is to Mr. Kuhlberg. You

mentioned conditioning monitoring with respect to a Navy program.

What aircraft did this involve and can you elaborate on the results

o£ the program?

Joel Kuhlberg : The program is called ICEMS, "In Flight Condition

Engine Nfonitoring System". It's sponsored by NASC. It involved

the TF-30 engine in the A7 aircraft. There is a complimentary program using the A7 aircraft with the Allison TF-41 engine. Our

particular phase has ended. We demonstrated the feasibility of

the kind of hardware I showed you. Allison has a follow-on where

they are going to use their hardware in a ten aircraft squadron test

to shake the bugs out and to qualify it for fleet incorporation.

Charles Jackson, Monsanto : Would you mdnd commenting on your tape

recorder facility that you showed which seemed to be portable? Is

it an FM unit, how portable is it? I presume you use this so that

you can be very portable about getting data. It looked like you

were using about a half inch tape.

Joel Kidilberg : I believe that was a regular one inch digital tape.

I hear Hamilton makes it.

Mirray Hoffinan : On that ICEMS program the hardware that was provided

is similar to the hardware that is now flying in commercial aircraft

in the U.S. and in Europe and is available right now. It's all commer- cial hardware including the tape recorder, the signal conditioning

equipment, and the IMJ. The ICEMS program was using that hardware,

but of course a different set of condition monitoring done through

really the soft ware program, which was different than the commercial

program that is now in the commercial airlines.

Lee Doubleday, Naval Air Systems Command: As you know, the cost of

engines is high these days and our support philosophy, from the logistics

view point, has turned toward modularization, that is supporting the

engine by conponent. A number of the systems now in existence, and

planned, depend on the trends and the inter- relationships within a given engine. I wonder how the panel plans to handle modular engines.

185
David Leiby : We're recognizing this as a problem on the modular

engine; what do you do with an engine that has some parts that last

for 30,000 hours, some parts for 20,000 hours, some for 10,000, some for 5,000, and on down the line. In several years there is a mix of

different age components in the engine. This is going to conplicate

the bookkeeping in the software programs for trend monitoring, and

there will have to be changes made in the memory bank for a particular

engine when it comes back with parts of different ages. We don't

know exactly how we're going to do it. But we recognize the problem.

Joel Kuhlberg : I would like to comment further on the 9D in the 747.

To get back to a nominal baseline how do you know for instance that

your overhauled compressor is operating properly? The compressor is

being put into another engine that happens to be on base which is

perhaps comprised of test parts, and run as a complete engine. If

the engine performs properly with this compressor, then the conpressor

is ready as a module to go back in service. Modules aren't tested as modules, they're tested as parts of an engine.

Murray Hoffman : Let me add a little postscript to that, not so much

for the modular engines but how engines that have changed their baseline characteristics are handled at least in the industrial area. If

you interchange the modular pieces, you're going to change the baseline.

The system I described actually interrogates the engine for its baseline

characteristics, and then instructs the user to run through a calibration

test, running the engine at various power levels while the computer mak:es

the measurements and establishes a new baseline. This occurs all the

time.

Lee Doubleday : Do you use the same device for measuring both bearing

condition and unbalance, with perhaps a filter to get down to low

frequencies for the unbalance measurement?

David Leiby : We use the same accelerometer. The one internal accelerometer on the main bearing is the one we tie into the mass unbalance

monitor, the FOD detector, and the bearing monitor. The bearing monitor

is usually used in a multiplex type of operation where one monitor will

scan several of the accelerometers , but the FOD detector is committed

all the time to that accelerometer.

Lee Doubleday : You spoke of how the engine manufacturer does a good

job of putting monitoring ports in the engine. Then an air framer wraps

his QEC or airconditioning ducts all around it and blocks these ports

which blows the whole thing as far as monitoring the engine on a wing.

186
I would like to stress to the industry that when you write specifications, try to give yourself an envelope that would prevent the airframer from wandering into the horoscope areas so that we could

possibly use some of these good things that are provided in the first

cut of the engine world, which starts about 3 years before the airframe. You didn't mention anything about radiography while running,

but I understand Rolls Royce in England has done some work with

running engines, and we had experience with a TF-41 engine where we

actually saw a fit loosen up with the use of running radiography.

David Leiby : We have not done any per se but we've been thinking

about what can be done, and what the gains might be. If you take an engine apart you can look at the seals and you can see that the engine

will wander back and forth up to an inch, during transients, relative

to actual positions of rotors, etc. It would be nice to know how far

the parts do move with respect to each other so you can then keep the

clearances of seals down to a bare minimum and improve the performance

of the engine. One of the ways of doing this is to use very high power,

high pulse energy x-ray machines. You're talking about 8 million volts

and using the best high speed film you can get so that you essentially

freeze the motion of the engine by shooting across it, and getting a pulse of 20-30 nano- seconds with enough energy to expose double royal

blue film with accentuating screens on it. You can get a useful image

and measure the actual clearances of the parts of the engine and tak:e a series of radiographs with an automatic film transport tied onto the

engine so that as you go through a throttle burst you can see the

relative positions of the parts as they move around inside the engine.

These are things we are going to be looking at in the future.

Lee Doubleday : From our viewpoint, this is a laboratory tool at present.

As a development tool, I think it's real worth while.

Jim Reis, Northrop, Corp . : Were you able to detect worn main bearings

in the dies el engine?

George Staton : On a laboratory basis, yes. On an online basis, the

diagnostics are not fully developed. We're still working on it. Up

until now it has taken some manual interpretation of the data in order

to get a positive diagnosis.

Jim Reis : Are there any particular techniques that have been more

successful than others in doing this?

187
George Staton : The technique we're using which we think is the

most successful (obviously we're windowing) is looking for that

portion of the cycle where a given connecting rod is down at the

lower end or reversing directions so that we can see what the

effect on the conrod at that point would be. You see more of the

vibration of the main bearings. Right now I am running about 12

passes through a digital filter at that window and I'm having to

bring the data off line and analyze it with an experimental cor- relation program in order to fully define the malfunction.

Jim Reis : Have you been able to detect defects in main bearings in

the turbine with the Hamilton Standard unit?

Murray Hoffman : No. Not that we couldn't, but we haven't had that

problem come up in the tests that we've run. However, the ability

to detect main bearing problems with that system is rather low

because we are depending primarily on case mounted accelerometers

and the attenuation through the unit is going to be pretty heavy.

Those engines do not have any kind of internal instrumentation.

Jim Reis : Would you then anticipate when the manufacturers start

manufacturing internal accelerometers to include them into your

basic system?

IVftgrray Hoffman : Yes, in fact I try every chance I get to encourage

the manufacturers to include them internally. I have even asked the

manufacturers if they would code the metal that they put in the

bearings so that when deposits are found in the oil, you can identify

the source.

Cedric Beachem, Naval Research Lab : Mr. Kuhlberg mentioned that there

are some A7's that have been instrumented, I suppose for flight operations. Are the Naval Air rework facilities encouraged to become

equipped to do the more extensive follow-up on the ground?

Lee Doubleday : The A7 program that was mentioned is a 10 engine

development program or a 10 aircraft development program. There's

been a first phase development program to determine the winner, and

DDAD happened to win. I should say that Teledyne won. They were

affiliated with the Allison engine, where Hamilton was affiliated with

Pratt and Whitney. There is nothing in place yet at any rework facility

to do this. At our depots, this is a flight line type of approach and

the depot is many miles away so we don't really get a close tie-in witn

188
the squadrons other than a gripe about an engine as it comes back.

The whole approach to this has been on a squadron level or IMA. level

as far as the diagnostics go, and then if the maintenance task is

beyond their capability, it gets sent back as it would be now. A .P. Brackney, Monsanto : First I would like to make some observations

on the session this morning and then direct a discussion to the panel.

I thought the papers were very excellent. Yesterday, Charlie Jackson

observed that equipment is not fickle, like people. Years ago before

we had the scientific way, we used to have a lot of conversations with

people. A conversation today is two guys, one talking and one waiting

to talk, and nobody's listening. We have conversations with our equipment, and if you don't listen, there's no conversation. We listen to

equipment but we don't listen to people. For the user of an engine,

be it an aircraft engine, or a pump in an Monsanto plant, you've got

to talk to the maintenance man and tell him some of the things that

you have observed. I would like to ask the panel and the group to

tell your compatriots on the other side of the fence what information

you need to make a job easier, and let's define easier in terms of

cost, and being right the first time 100% of the time. There's much

information that a user can tell a maintenance man and if he would be

smart enough to tell him, the end result would help him. He would get

better engines back and he would know what to do and what not to do

while he's running them. It's the little details that make a plane go

down, or a tank stop in the middle of a battle, or a pump fail.

David Leiby : At General Electric we have an organization called Field

Service Engineering. They are our ears and eyes out in the field with

our customer. We have a pretty good run down on all the problems the

maintenance people have had. We have very good rapport between our

field service engineering and the maintenance sheds at all our customer's

overhaul bases and operating bases, so we know where the weak spots are. We know what monitoring techniques need to be employed, we know which

radiographic procedure needs a few changes, we know that the borescopes

aren't working in a certain port because something is interfering. So

we get very good feedback from our customers.

A. P. Brackney : That tells me that you are getting feedback from your

people that you sent out in the field. The customers are telling your

man out there, and your man is listening, but you had to take that overt

step, you felt that was necessary. That proves the point that usually

there is not much communication between the user and the guy that fixes or makes the engine. I think what you're doing is good.

189
David Leiby : This has been our operating procedure for years. We

always have this liason between the customer and our own engineering

and manufacturing operations.

George Staton : We have a little bit different problem than most

people in that in the Army, our maintenance staff and our users don't

remain fixed for any given length of time. The typical government way

to do things is to fill out forms which end up as a pile of unusable

data. What a driver of a tank or a truck is going to say when he

brings it in for maintenance is that it doesn't run right, or it makes

a noise. All he wants to do is get rid of that engine, or that vehicle,

and get a new one that runs better. So we have a severe communication

problem. We do all the typical kinds of things like having civilian

maintenance, both government people and contractor people, out on site

trying to help our own maintenance people and our users to communicate

with each other. For us, it's almost an insurmountable task because

you have no continuity from year to year in the staff. Therefore, in

our diagnostic analyses, we're looking at the equipment for the first

time and we can't use a trending approach.

Joel Kuhlberg : Pratt 5 Whitney also has field engineers all over the

world watching our engines. Generally they are assigned to an over- haul base or an operational base. At an overhaul base when there is a repair situation, care must be taken to observe the evidence as it is

exposed on disassembly. Operationally, you've got to get a good story

from the flight crew that flew the airplane that that engine was in.

Sometimes that's a problem. It depends on the airplane, the kind of

mission flown, and the crew. If you've got a fighter with a couple of

guys flying a mission and they get an engine problem, they are probably

pretty busy and they may not get a very good story of what is happening

to all those dials in front of them. Of course, that's where condition

monitoring is supposed to help, to get some of that story. On the other

hand on a commercial transport with a flight engineer, you probably can

get a better story of the event. When we do have an unusual problem,

generally we request that the hardware be sent back to the plant and

engineering tries to figure out what's going on and how to fix it. The

communication gets to be quite direct when you have the harc3ware in

front of you.

A. P. Brackney : When you are getting information back from a flight

engineer, is this contact face to face? Are these what the observations

were when the airplane was doing thus and so?

Joel Kuhlberg : It can be face to face between the pilot and our field

engineer on a debriefing. Then our field engineer gets back to us.

190
Murray Hoffman : I'm between the user and the supplier in condition

monitoring. On the part of the manufacturer, there's a tendency to

play down the biggest problems and to indicate that these problems

are just normal routine maintenance problems. On the part of the

user, I find myself overwhelmed by the requirements that he throws

at me and I can't build a house big enough to hold the equipment to

satisfy those requirements. The user is so amazed and flattered at

being asked to contribute to the situation that he goes so conpletely

overboard that you drown. You go back to the manufacturer and ask

"What problems can we help you with?" You get back the answer,

"What problems?" I don't think there is a good answer for it now,

but I think that there is an awareness of the problem.

A.P. Brackney : I can see disciplines forming within this group.

Maybe we need an inter- discipline committee within this group just to

make sure we don't go building walls around original equipment manu- facturers and repair shops and users.

Ron Phillips, U.S. Naval Aviation Integrated Logistics Support Center : What reliability have you gotten from your transducers and what kind

of print-outs have you gotten that said there is a fault, where in

actuality no fault existed either due to a bad transducer or programming error or whatever?

Murray Hoffman : It's a little early to draw any real conclusions on reliability, but to date, our experience with the transducers has been

very, very good. We haven't had a single transducer failure in a year

and a half, nor have we had a single transducer that has gotten out

of calibration. There is a good reason for that. We are very careful

in our selection of transducers. For example, we put 11 pressure transducers on an engine and the raw cost is around $6,000. In terms of

the rest of the equipment we had some laboratory conglomerations that

had seen better times and I wouldn't really want to integrate their

performance into the reliability of the system. I think that the

electronics probably have as good a reliability as the transducers

have experienced. The only wrong messages that we have generated so far have been those where the test engineers, while working with an experimental engine had double instrumented, and so overloaded the

lines, that the results were erroneous and the computer read these as changes in performance. We work to very small tolerances. Data is

accumulated for 150 hours and, on the basis of this, decisions are made.

This is not an alarm system. This is a trend system. I know what you

are really looking for but we haven't had enough field experience to

give those answers.

191
Joel Kuhlberg: I guess I brought up the point a little bit when

I spoke of reliability. I don't have a number for you in terms of

transducers but I have an idea that it's probably in the 100 's of

hours domain. The kind of reliability that we hope to achieve with

electronics, sensors, etc. is the same kind of reliability we now

have with the hydra-mechanical control, and those numbers are like

20,000 hours. I don't think we have that kind of reliability in

most of the sensors that we have. Another thing in^jortant to a

flight engine as opposed to an industrial installation is that the

range of operation has considerably increased and the environment

that it has to live in is a lot more severe, and that's part of the

problem of accuracy. We talk about half a percent of full scale

but if you talk about a pressure transducer that has a turn down

ratio of about 70-1, you find out a half percent full scale down

at the one is an awful lot of PSI, and that's not good enough for

controls or condition monitoring.

George Staton : A major portion of our problems with transducers

has been due to the fact, that in our system the transducers are removed after each test. We have had failures which are not con- sidered reliability failures but are due mainly to two things. The

first is handling. The second is e:xposure to severe environments

like in the exhaust manifold. We knew when we designed the system

that the transducer would have a finite life, so we have been seeing

failures where we expected to see them, and they are occurring at

roughly 1300°F in a high impact environment.

David Leiby : We have found that the sensors and transducers are very reliable. The problems are in the connectors. I think we need

some improvements in reliability in the electronics.

William Glew, Naval Eng. Test Establishment : Mr. Staton, why do you

rely on obtaining cylinder pressure and injector efficiency measurements

with an external accelerometer rather than on pressure transducers

mounted on the head of the engine to give you a P-V diagram?

George Staton : The major problem is that we have to disassemble to

get those pressure measurements. This is a 12 cylinder aircooled

engine, which means we have to have adapters that would go into the

injector hole of every one of those cylinders because it's the only

access port we have. We don't think that we need to get the com- pression pressure directly if we can read the relative compression

pressure from the start of the current wave form, which we do, and

look at the injector performance through the pressure timewave form

of the injector.

192
William Glew: Vly only thought is that what you're doing is a very rough class measurement of the effectiveness, and in fact

the cylinder pressure tenperature conditions are very important

to the effectiveness or to the efficiency of a diesel engine.

I would have thought that once you go to the trouble of having

a condition monitoring system, the extra cost of the pressure

transducer tappings would be justified in the long run.

George Staton : We are not doing trend monitoring with that system.

We are doing diagnostic monitoring in the as received condition.

We get the engine in an unknown condition. We make an assessment

of it, and based on that one look at the engine, decide what has

to be done to fix: it. When it's fixed the instrumentation is re- moved and it's shipped back to the field. We can't change the

basic design of the engine, so we have to use only those access ports that are available. We try to minimize the amount of labor

needed to mount our instrumentation. Unless it's absolutely critical

to our diagnosis, we don't like to break into the lines, we don't

like to go into major disassemblies of the engine because if you

get to that point, you might as well tear the engine down and do a

full rebuild.

William Glew : I guess the trend monitoring is the next step. It seems to me that the only weakness at present with this system is

that you're not getting a good bearing deterioration picture. Statistically, how significant are failures of bearings in engine failures?

Is it a significant overall failure mechanism?

Murray Hoffman : Yes, bearing failures appear to be a significant

failure mode in terms of gas turbine engines. However, inside cor- rosion from poor fuel is a very common problem with industrial engines.

Problems like this tend to necessitate that the engine be torn down

more frequently than a bearing problem would require, and at that

time often the bearings are replaced.

William Glew : Do you think the trend system in its present form is

suitable for the shipboard installation on the Marine FT- 4?

Mjrray Hoffman : Oh yes. As a matter of fact it's being applied to

other engines besides Pratt ^ Whitney's intermarine applications. It can be applied to any gas turbine.

195
William Glew: How much do you charge per system?

Murray Hoffman : How much have you got? This is a commercial system

available to the commercial market at a fixed price. Installation

costs vary, however, and can run the total cost up significantly.

In round terms, it's about $65,000 for the first engine, and about

$130,000 for 8 engines. You can estimate from there for whatever

number you want using a single central unit. On a ship, you have

a different problem. You can't go back to the beach although you

could if you wanted to use your radio length.

William Glew : If I was running a ship and you installed one of the

GE bearing monitors I would like to have your GE bearing monitor

system with me, I would like to comment philosophically that we

installed a very simple vibration monitoring system in the Navy, and

what has struck me is that quite often there is an element of re- luctance on the part of the maintainers to draw the deductions that

can be obtained from this analytical equipment. Instead of regarding

it as an extra screwdriver or right arm, they tend to say when something

goes wrong, "That's your black box again". It seems to me that we, 'as engineers, must all be very careful not to depersonalize the maintenance

operation and thus make the maintainers feel that they are completely

out of the picture and just automations doing what the computer and

the computer technicians tell them they have to do.

George Staton: If we had or could retain personnel with the trained

skills, then they could do much better than any machine that we can

build. But unfortunately, in our particular environment, if we got

a skilled diagnostician we would promote him to sergeant so he can do

paper work. So we're stuck with the 18-20 year old recruit who gets

about 12 weeks training if he's lucky. If he's not, when he gets to

the unit somebody hits him on the head and says you're now a 63B

automotive mechanic. The guy who was in school could also type, so he's now a company clerk. We have to somehow stop a very significant

traffic in serviceable parts being removed from engines and being

returned to the supply system. By significant I mean 60 million dollars

a year that we can identify, and lots more that we can't. Despite the

fact that there may be some few people out there who would get depersonalized by us putting a machine there, we can hardly justify not

putting it in on a cost effective basis.
Robert Boole, General Radio Co. : There have been some comments about

the frequency response of sensors or transducers that are used in these

programs, but no one has really mentioned the dynamic range that might

be required. I would be interested in comments on any of the transducers that are used and what sort of dynamic range is required?

Murray Hoffman : In terms of the range of instrumentation that I am

applying in the TREND system, there's a very small dynamic range. We

work over very small ranges because we're concerned with the engine

only when it's in a steady state condition. We define steady state

condition as the rotor speed being plus or minus 0.1% for 10 minutes,

and then at that point we take a reading.

Robert Boole : Are you talking about typical ranges of less than 10-1?

The gentleman from l&iited Aircraft mentioned a 70-1 reduction from a full scale reading to where he was operating.

Mjrray Hoffman : In terms of aircraft engine condition monitoring,

you're monitoring from start up to shut down. In the system I'm

looking at, I'm only monitoring during the steady state condition, so

I don't need the large dynamic range. I have a little simpler problem.

The primary thing that I'm concerned with is repeatability. If I get

good, solid substantial repeatability, I'm not overly concerned with

absolute value or any of these other things or the large dynamic range.

Joel Kuhlberg: 70-1 is the pressure range that the pressure transducer

has got to be exposed to and work over in an advanced supersonic engine.

Obviously, the temperature ranges and speed ranges aren't anything like

that. Are you interested in particular parameters?

Robert Boole : There have been plots shown here, particularly of vibration,

which probably have the widest requirements for dynamic range, and I have seen shifts on some of the charts with as much as 40 dB, in some

cases more, between various points on the spectrum. If you do a spectrum

analysis, you might see 2 levels plotted on that spectrogram which differ

by 40 dB which means that the sensor has to be able to respond in amplitude over that range. That's what I was trying to get at for vibration

or any of these other parameters. Over what sort of an amplitude range

must the device be capable of giving the useable output signal?

Charles Jackson : I think 40 dB is about right. That's 100/1, that's

about what we find.

195
S. Grant, Carolina Power ^ Light Co . : I understand that biological

contamination of diesel fuel oil and perhaps other oils can be a problem and I wonder if any of you have any experience with this

and ways of checking for it?

Lee Brock, Eastern Airlines : We occasionly get this micro-organism

growth, and I assume that is what you are speaking of, in the aircraft fuel tanks. You can usually detect this by the pick-up in the

fuel filter. We have a fungicidal treatment that we put into the fuel

tanks to kill this growth. I don't know who the manufacturer is, but

it is a commercially available product. It's a one time treatment.

You put a certain amount in the fuel tank and run a tank full of fuel

through the engine. We don't use it continuously and there is no noticable effect on our engines.

S. Grant : How did the biological contamination affect your engine?

Lee Brock : It caused control malfunctions to the engine. One of the

main concerns to an airline is the fact that it eats holes in the wings

of the airplane. This micro-organism causes severe aluminum corrosion

in wet fuel tanks. It looks almost like cotton. The first few times

we saw it, we thought the tanks were contaminated with solid material.

P. Chiarito, NASA : Getting back to specific needs for a detection

system, has anyone seen a need for a system that would detect crack

growth in the rim of a turbine disk during flight operation? Is anyone

here aware that turbine disks do burst occasionally? If we have a detector for crack growth in the rim, we hopefully will have an indication

of incipient structural failure.

David Leiby : We have been looking in the general development field of

acoustical emission for detecting cracks, not necessarily in disks, but

in any internal parts of engines.

M. Rosen, GE : How do you go about establishing the threshold level

that triggers an alert or alarm signal especially using gas path

analysis or oil analysis on both new and old machines?

Murray Hoffman : The way I approach the problem is to look for a change in the performance of the unit over a long period of time. I'm

looking for small changes, I need certain parameters to establish when

a change is significant. I have to know the intimate performance limits

196

It
of the engine in a general sense. Then I relate this to the baseline performance and then establish quantitative values which then

represent the limits that I am establishing for whatever flags I might bring up. There are probably a family of limits that are established here. There is one that is just short of catastrophic

or maybe there's one that will say the engine just left the airplane.

Way down on the other side is the limit, that 100 hours from now

there will be deterioration to the point of non-acceptance. We

know the percentage change in the various performance parameters,

particularly with gas path analysis, which define the loss of efficiency. This is based on performance history, of which there

are many thousands of hours on 100 's of 1000 's of engines. It's

a very dynamic thing, and I wouldn't pretend that we have all the

right limits now. As you gain experience, your engines tend to

change too. I think the precise point where you should put those

limits will probably never be found, but a good working number based

on history and a knowledge of engine performance can be available

right now.

David Leiby : In many of the applications, for example the exhaust

gas temperature, you have a hard solid limit that is a red line

saying thou shalt not go above this or you're going to damage the engine.

Some of the limits are established by other limits. In condition

monitoring, you can evaluate the operating condition of the engine

and come back and put in hardline limits, of course we also have

soft limits. The fact that you're now departing from the normal indicates that you're approaching the hard limit at a certain rate.

Therefore in 20 hours, 40 hours, 50 hours, some period of time you're

going to get to this limit, so you better start scheduling this engine

to be taken off at the most convenient base the craft is going to

be entering.

197

SESSION IV

NEW APPROACHES

IN SENSING

AND PROCESSING

Chairman: Henry R. Hegner

IIT Research Institute

APPLICATION OF ACOUSTICAL HOLOGRAPHY

TO INDUSTRIAL TESTING

Byron B. Brenden

Holosonics, Inc.

Richland, Wa.

The application of the techniques of holography

to acoustical imaging has been rather recent, certainly not predating 196 4. These techniques have

had a great impact upon the development of acoustical imaging not only by making testing techniques

depending upon imaging more useful but also stimulating improvements in acoustical imaging in general.

From the beginning, systems employing a scanned

receiver, as schematically shown in Figure 1, have

been popular. The most effective scanned system

is one conceived of by K. A. Haines and B. P. Hildebrand and described in U. S. Patent No. 3,632,183.

In this system the source and receiver are scanned

over the hologram plane together.

In a system perfected by H. D. Collins and

R. P. Gribble of Holosonics , and shown schematically

in Figure 2, a single transducer serves as both the

source and receiver on a time shared basis. This

201
system has been demonstrated to be very effective in

imaging, locating and sizing flaws in large rotor

shafts and reactor pressure vessels. Figure 3 is

a picture of the system showing the mechanical

scanner held to a simulated reactor pressure vessel

wall with riabber suction cups. Figure 4 shows a

reactor pressure vessel under test using the

Holosonics HolScan equipment. This equipment forms

a hologram on polaroid transparency film. Subsequent imaging by illuminating the hologram in an

optical processor enables one to see the defect or

feature in its true configuration and permits one

to measure its size and actual location.

The HolScan system is a very effective tool

for studying volumes on the order of one cubic foot

and looking for features as small as an eighth

inch in diameter. Furthermore the parameters of

the test may be varied to look at larger volumes

or smaller defect features.

Acoustical holography may also be used for

real time acoustical imaging. In this case the

202
system takes on the configuration shown schematically in Figure 5. Normally this system is used in a through-transmission, focused-image mode but it can

be used effectively in a reflection mode and without acoustical lenses. Holograms are formed at a 60Hz rate and read out instantaneously permitting

one to inspect moving objects and also permitting

one to move the object about to achieve the optimum

angle of view. This system is available in several

configurations, one of which is shown in Figure 6.

All of the configurations are based upon the

principles illustrated in Figure 5. A wave train

several hundred wavelengths long and about 5 inches

long in water is generated by the object transducer

at a typical frequency of 5MHz . Energy scattered

by the object passes through the acoustic lenses

along with unscattered energy. Acoustic lenses

focus the object into the hologram surface. A

reference beam is mixed with the object beam at

this surface. The resulting interference pattern

is impressed upon the liquid surface as variations

in elevation which affect the phase distribution

in the reflected light beam. The liquid surface,

which is fully stabilized against unwanted disturbance, is capable of forming one hundred holo205
grams per second and thus of imaging in real time.

Figure 6 illustrates a typical nondestructive

test situation involving a laminated strip of metal

made up by bonding a strip of aluminum to a base

strip of steel. The laminated strip is produced

in thicknesses ranging from 0.065 inches to 0.110

inches with about 0.010 inches being aluminum.

Figure 6 is the acoustical image of four inch

wide strip showing both edges and a long, narrow,

poorly bonded area. The handle of a hemostat is

also imaged through the strip in order to demonstrate the transparency of the strip to 5MHz ultrasound.

De laminations and anomalous areas in plate

like materials are readily imaged by this technique.

Cracks, slag and nonfused areas have been imaged

in steel pieces more than 8 inches thick.

Irregular shaped solid objects are more difficult to test but many of them can be inspected by

using off-axis insonification and special distortion

correction lenses. However, the correction lenses

are quite specific to the application and further

research is needed in this area.

20U

-s
MODULATING

SIGNAL

OBJECT

R. F. SOURCE

Figure 1

ACOUSTIC IMAGER

SERIES 200

GATE FILTER

Schematic diagram of Holosonics, Inc. Pulse Echo Acoustical Holography Systems and Holographic Reconstruction Unit.

Figure 2

205
Figure h

206
ACOUSTIC IMAGER

SERIES 100

SPATIAL FILTER LENS LASER

<r 8 >

LIQUID SURFACE HOLOGRAM

REFLECTOR

OBJECT u

n

OBJECT ACOUSTIC LENSES TRANSDUCER

REFERENCE TRANSDUCER

SCHEMATIC DIAGRAM OF HOLOSONICS, INC. LIQUID SURFACE

Figure 5

Figure 6

207
Figixre 7

208
DISCUSSION

Jerry Forest, Ontario Hydro : As I understand it, in the system you

are using, the liquid surface method produces an acoustical field

within the volume of water and the image is produced on the surface.

I take it that this is an interference pattern of the two acoustical

wave patterns producing a deflection at the surface of the water.

Do you use an optical holographic method to produce the picture of

the disturbed surface?

B. Brenden: That's correct. A complex grating is impressed upon

the liquid surface. This grating, which is a hologram, is formed

in terms of relative elevation on the surface. The fact that the

energy from the object beam is mixed with the energy from the reference beam causes this grating to form. Then that is read out in

the way you normally read out a hologram.

Jerry Forest : As the block was moved around in the liquid, the whole

liquid surface level moved. For this type of diffraction pattern,

in conventional optical holography one must keep motion to an absolute

minimum. How was that accomplished?

B. Brenden: The liquid surface has a dynamic response and the hologram

is formed in a matter of about 100 microseconds. The object then

must not move appreciably in terms of acoustical wave lengths over

that 100 microseconds. In effect you're taking a rapid sequence of

pictures. Your object can move around by a matter of 0.020 inch or

so during that 100 microseconds and you still have your hologram.

Cox, Harry Diamond Labs : I'm in the field of fluidics. In fluidics

we chemically etch passages in thin metal sheets and then diffusion

bond them together. Can acoustic holography detect unbonded sections

in such a structure, and if so how small a defect can we observe?

B. Brenden : Plate like structures and delaminations in plate like

structures are just ideal for examination by this method and the disbond

may have almost zero actual separation and be detected, but the areal

extent should be on the order of a couple of wavelengths of sound.

In that case if you're operating at 5 megahertz, there would be

approximately 0.040 inch in diameter to be detected.

209
APPLICATIONS OF TIME -LAPSE INTERFEROMETRY

AND CONTOURING USING TELEVISION SYSTEMS

A. Macovski , S. D. Ramsey, Jr., and L. F. Schaefer

^ One method of preventing mechanical failures is to non-des tractive ly

test manufactured parts for hidden defects or out-of -tolerance dimensions. Holographic interferometric techniques have been used for nondestructive testing but not on a mass scale due to the time and expense

involved. However, when real-time television techniques are used mass

testing becomes possible since no photographic processing is necessary

and no expendable materials are used.

Two modes of operation are used for testing depending on the

sensitivity of the measurement required. In the holographic interferometry mode the sensitivity of measurement is one half of the laser

wavelength used. Figure 1 shows a block diagram of the interferometric

system. An image hologram is formed on the television storage tube and

scanned out to form a video signal. For the object in one state of

stress, the video signal e^(t) is stored electronically on a video

recorder. After the state of stress is changed a second hologram is

scanned out as ®2^*^ ' stored signal is read out in synchronism.

The two signals are subtracted and envelope detected before being

displayed on a television monitor. The output is an image of an object

with a set of fringes superimposed indicating the change in surface

210

•I
configuration due to the difference in stress applied to the object.

A hidden defect manifests itself as an anomalous fringe pattern on

the object with each fringe indicating one half of a wavelength change

in surface position.

The second mode of operation is a moire technique and is shown in

Fig. 2. Two plane waves from a laser cause a grating to be formed in

space such that the object contour information Z(x,y) is contained

in the phase of the grating on the surface of the object. As in the

previous system two frames of information are required, the first being

the reference frame. The reference video signal is stored and then

read out in synchronism with the second frame containing the contour

information of the object. The two signals are product detected and

added to a low-pass filtered version of the object signal before being

displayed. Again, the output picture is a set of contour fringes

superimposed on an image of the object. If a contour map of the object

height is desired, then the reference is a flat plane and the fringes

represent equidistant altitude contours. For information about the

change in the object surface configuration due to an applied stress,

the unstressed object is the reference and the fringes indicate the

change in the surface. For gauging parts against a master, the master

part is the reference, and the fringes indicate the gauging difference.

The measurement sensitivity of the moire technique is dependent

on the laser wavelength and the geometry of illumination and viewing.

211
By varying the angles c and y the sensitivity may be changed in

a continuous manner. The range of sensitivities that are useful

depends on the size of the object but is generally in the range of ten

to several hundred wavelengths of measurement for every fringe produced.

212
Figure 1

REFERENCE

PLANE

TA-7605a3-a

Figure 2

213
DISCUSSION

Jerry Forest : I wasn't quite sure how you got away from the stability

o£ something like a granite table.

S. Ramsey : The way we get away from the granite table, as Prof. Leith

explained last night, is to use a pulsed laser, with a high energy,

but a short pulse life. Then you don't need the stability. We don't

go quite to that extreme. We use a continuous laser, although in

theory you can use a pulsed laser. The effective exposure time with

a television camera is short compared to the normal fluctuations of

the object. The only reason we can get around that particular problem

is that television cameras are in general much more sensitive than

the photographic films which are normally used for holography. So

we trade off resolution for speed in some sense.

211+
Application of Exoelectron Emission for Detection of

Fatigue Damage and Plastic Deformation

WILLIAM J. BAXTER

Research Laboratories, General Motors Corp., Warren, Michigan 48090

The photoelectron emission from a metal is temporarily

enhanced by abrasion or deformation of the surface, a phenomenon

commonly referred to as exoelectron emission. This paper describes

a scanning apparatus which displays the distribution of plastic

deformation by measuring the localized exoelectron emission.

Our results provide for the first time the early detection of

fatigue damage (in <1% of the fatigue life) , a quantitative assessment of the extent of the damage, and thus an inherent predictive

capability. Some additional experiments are also described which

show that the emission process is closely related to the development of microcracks in the surface oxide.

APPARATUS

The experimental arrangement is shown in Fig. 1. The speci-

_ g men is mounted in a vacuum chamber (10 torr) and scanned along

its length by a small spot (15-70 ym diameter) of ultraviolet

light. The electrons are detected by an electron multiplier, and

the emission rate is recorded as a function of the position of the

light spot. The specimens have a gauge length 0.1 in. long and

0.1 in. wide and are deformed or fatigued in_ situ by bending.

The results reported here were obtained from 1100 aluminum

and 1018 steel sheet stock. The samples were in the as-received

condition with an average grain size of "^^30 ym, and simply degreased

prior to insertion in the vacuum chamber.

215
RESULTS

Over 40 samples of aluminum and steel have been fatigued in

the bending mode with similar results. An example of the exoelectron emission produced by fatigue-cycling 1100 aluminum is

shown in Fig. 2. The two traces were obtained after 1000 cycles

and 4000 cycles. The initial emission before cycling is omitted

for clarity but is typified by the essentially constant noise

level on either side of the central peaks. These peaks are very

stable over a period of hours. Additional cycling caused these

emission peaks to grow and fresh peaks also emerged. The magnitude of the peaks A and B are plotted as a function of the number of cycles in Fig. 3. Failure finally occurred at the location

of the largest peak (A) after 140,000 cycles.

Since the location of failure always corresponds to the

largest emission peak, it is clearly important to ensure that

the scan path intercepts the region of maximum emission. Therefore in our more recent experiments the sample is scanned along

five parallel paths separated by ^300 y. Along each path several

emission peaks are observed, but it is only the height of the

largest emission peak, regardless of its location, which is important in assessing the accumulation of fatigue damage. This is

demonstrated by the results for such multiple scan measurements

on several steel samples covering a range of fatigue lives (Fig.

X

4). Only the height (y— - 1) of the largest emission peak observed

on each sample is plotted; I is the measured intensity of emission

and is the initial intensity at that location. This nomalization compensates for any experimental variations from one experiment

2l6
to another. The results all fall in a quite narrow band and

provide a good predictive capability. For example, we know

that when the maximum intensity of localized emission is 10 times

the initial background intensity, the sample is at 2.2+0.7% of

its ultimate fatigue life.

The onset of electron emission occurs long before the

development of fatigue cracks, the initial emission peaks corresponding to regions where slip lines have formed on the surface.

However slip lines produced by unidirectional defoiTnation do not

necessarily cause exoelectron emission. For example in the case

of aluminum, emission was observed after a tensile strain of 1%,

but a compressive strain of 5% produced no emission.

DISCUSSION

This scanning exoelectron emission technique clearly detects

localized fatigue damage earlier than 1% of fatigue life, provides a measure of the accumulated damage, and may be used to

predict the number of cycles to failure.

Since the intensity of the localized exoelectron emission is

very stable, it clearly results from a permanent change in the

surface, e.g. slip traces rather than surface adsorption or the

diffusion of lattice vacancies. The striking contrast between

tensile and compressive deformation points very clearly to the

important role of the development of microcracks in the surface

oxide

The continual increase of emission produced by fatigue

cycling may be understood in the following way. The initial surface deformation, consisting simply of slip lines, cracks open

217
the surface oxide permitting photoelectron emission from the underlying metal. Continued fatigue cycling enhances some of these slip

lines forming so-called persistent slip bands, wherein the deformation now accumulates producing extrusion and intrusion of material

and ultimately developing into fatigue cracks. Throughout this

process the surface deformation increases in severity, i.e., the

surface area of bare metal increases and hence the emission. Finally failure occurs in the location of most intense emission, i.e.,

the largest fatigue crack.

218
PREAMP R^IMETER

LIGHT SOURCE

VACUl

CHAMBER

ELECTRON

MULTIPLIERLATERAL SCAN

it.

SPECIMEN

RECORDER

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LONGITUDINAL SCAN

TRANSDUCER

Figure 1. Schematic diagram of experimental arrangement.

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1000 CYCLES

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6

POSITION OF SCANNING LIGHT SPOT (MM)

Figure 2. Exoelectron emission from aluminum after 1000

and kOOO fatigue cycles. Fatigue life = 1^0,000

cycles. Spot diameter = 70 l^ni*

219
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220
DISCUSSION

P. Chiarito , NASA, Lewis Research Center : Are you able to calibrate

these peaks to strain; does this really become an optical strain gauge?

W. J. Baxter : Yes, it does in a way constitute a strain gauge. The

emission comes from the deformed regions and it's a micro-strain

gauge, if you like.

P. Chiarito : But have you really calibrated this to strain? You said

you had a strain gauge in conpression. In the elastic range that would

give you equal and opposites. Certainly in tension, the plastic strain

on the upper surface would not be measureable by the gauge on the

compression surface.

W. J. Baxter : That's right, the strain gauge breaks after a few percent

of strain. ^Is that what you mean?

P. Chiarito : No, that isn't what I mean. You said you put a gauge on a compression side.

W. J. Baxter : That's right, and that measures the macroscopic strain

that you are putting in.

P. Chiarito : On the compression side, not on the tension side.

W. J. Baxter : Yes, but it's equal and opposite to ^^^at's going on in

the tensile s ide

P. Chiarito : In the plastic region?

W. J. Baxter : Macroscopically it is. Microscopically the strain gauge

doesn't tell you what is going on at all. Because the deformation isn't

uniform.

P. Chiarito : Let me ask again, can you relate these peaks to strain

so that you know, is it really a strain gauge?

W. J. Baxter : You are asking me a question which I can't answer

because how can you measure the strain any other way? As far as I can see, exo-electron emission is the only way of measuring it properly.

So I have no way of calibrating it.

P. Chiarito : Pardon my persistence, but can you relate the peak heights

to strain?

W. J. Baxter : Yes, I can take another aluminum sample and I can deform

It m tension, the same amount and I will get peaks the same height.

If that's what you mean.

221
p. Chiarito : When you get a peak height, does this represent a certain amount of strain?

W. J. Baxter : Yes it does. I don't know how much it is though.

I have no independent way of measuring it on that scale. I am measuring essentially the strain in a region 100 microns big.

I have no way of telling you what the strain is in that region

by any other method.

J. Bennett, NBS : Have you run stronger materials which presumably

would not show the damage so soon, and also have you run specimens

at stress levels where they will not break to see what the effect is?

W. J. Baxter : I haven't looked at stronger materials yet because

my present system is built for little samples and it doesn't have

the strength to do much more than that. I have not run tests below

the fatigue limit yet. I have run tests which have lasted 7 million

cycles and seen the emission in the normal way. I have also run one

test for 3 million cycles and saw nothing and decided I didn't have

enough time to carry on. I pulled it out and didn't continue the

test. I think in that case I was below the fatigue limit.

J. Bennett : That's vAiat I was wondering, whether you would get the

increase m emission below the fatigue limit?

W. J. Baxter : I don't really know. I haven't done enough experiments

to define the fatigue limit well enough to know which side of it I am on.

Sam Grant, Carolina Light and Power Co. : Do you know the source of the

electrons? Are these conduction electrons?

W. J. Baxter : Out of the metal, yes.

Sam Grant : You don't get any bound electrons, they are just the

free electrons?

W. J. Baxter : I believe so. I have seen no evidence whatever to

suggest that they are anything other than normal electrons coming out

of a piece of metal.

Sam Grant : Could you repeat for me again how you detect this? You

seem to be using some other source of light and a vacuum.

W. J. Baxter : Yes, I am using ultra-violet light and that's what

kicks the electrons out of the metal.

222
Sam Grant : The ultra-violet source is outside your vacuum?

W. J. Baxter: Yes, I just send the light beam through a window

into the vacuum.

Sam Grant : Then you detect it with an electron multiplier?

W. J. Baxter : An electron multiplier, yes.

R. Hohenberg^ AVCO Lycoming Division : I understand vdiy you have to

put the specimen into vacuum to measure the exo- electrons. Is it

also necessary to have the specimen in vacuum during the application

of load?

W. J. Baxter : I have done the following. In the case of aluminum,

I've fatigued under atmospheric pressure and then punned down to

make the measurement. The effect is still there but reduced in

magnitude. I've tried the same thing with steel and the effect is

reduced in magnitude so much you can barely see it.

R. Hohenberg : This effect may be due to the oxidation which occurs

as the fissures are created.

W. J. Baxter : That's right.

R. Hohenberg : So you would expect the tenperature in an ambient

atmosphere to be a very strong biasing effect?

W. J. Baxter : Yes, I think it would. The important thing is that

the damage produced by the fatigue in breaking up the oxide has got

to keep ahead of the reoxidation process.

Gerald Gardner, Southwest Research Institute : I would like to know

precisely how you prepared the surface prior to the time it went

into the vacuum chamber?

W. J. Baxter : I didn't. All I did was wash the grease off with acetone.

Gerald Gardner : You didn't go through any abrasion process originally?

W. J. Baxter : No, I did nothing.

Gerald Gardner : You evacuate the chamber and then perform the experiment?

W. J. Baxter : That's right.

Gerald Gardner : Did you atten5)t the experiment without the vacuum

chamber at all?

W. J. Baxter: No, I didn't.

223
Gerald Gardner: Are you familiar with the work conducted at

North Merican Rockwell in which they perform the experiment in air?

W. J. Baxter : I think I have seen one report on their work.

Gerald Gardner : The presence of the vacuum does two things, presumably.

One is that it protects the surface from additional oxidation. In

your hypothesis, this is important because that's where the exo- electrons are coming from. Another excuse one might have for the

vacuum is that one needs a vacuum in order to get the electrons over

to the counter. The latter is not so.

W. J. Baxter : It depends on your emission levels. If you have a high

current coming out, it's okay. You will measure the ion current

- of the electrons which come out and probably attach to oxygen molecules.

Gerald Gardner : You get virtually 100% efficiency in electron collection

that way.

W. J. Baxter : But at this emission level you won't be able to do it.

Gerald Gardner : You're satisfied you can't, you have objective evidence

that you can not. . . ?

W. J. Baxter : Well, you tell me how you can.

Gerald Gardner : Just run the experiment with the air in it.

W. J. Baxter : How are you going to measure it at 10 anps?

Gerald Gardner: With a picoammeter. I won't pursue this any further.

However, it has been done and it's not all that difficult. There is one other question which I would like to ask you. I can't recollect

the individual who performed the experiment but about 7 years ago a very similar experiment was performed in France in which a specimen

was illuminated with a flying spot derived from a laser. They used

a double arrangement to create a raster and actually looked at a fatigue crack as it developed.

W. J. Baxter : Were you thinking of the University of Delph? They used

a mercury arc, I believe, and a geiger counter as the detection system.

Gerald Gardner: I just want to know what relation you think your work

bears to theirs?

W. J. Baxter: I think it bears a very close relationship and I think

the reason that their results apparently didn't lead to anything was

that the gas in the geiger counter interfered with this whole business.

They couldn't get systematic data. All they showed, if you remember,

was just a picture of a crack.

22k
Gerald Gardner

Did you examine the specimen afterwards to

determine whether the site at which the failure crack ultimately

did initiate was in any way associated with any metallurgical

defect that was there originally?

W. J. Baxter : You mean fatigue it, take it out, look at it, and

then put it back in and finish it off?

Gerald Gardner : No, just when it fractured were you able to look

back at the fracture surface and see if there was something -that

should have picked that point out?

W. J. Baxter : No.

Bruce Christ, NBS : What precautions did you take with the vacuum

system and Wiat vacuum pressures did you have to operate at to

prevent reoxidation or other contamination of the surface?

W. J. Baxter: Well, as one usually does, you use what's on hand.

I happen to have quite a decent vacuum system and it normally

runs at 10"° torr. I am looking into the question of how bad

a vacuum I can put up with. I don't really have a firm answer yet.

Charles Jackson, ^4onsanto : I was trying to relate to the fatigue

gauge which has great application, at least from a practical standpoint, if the levels are like 1200 macrostrain or greater. Is your

objective to develop a practical application similar to or possibly

better than the fatigue gauge?

W. J. Baxter : By fatigue gauge, do you mean strain gauge?

Charles Jackson : Not strain gauge, strain hardening sensitive gauge

which changes resistance with fatigue. It doesn't measure direct

strain, but is strain sensitive, naturally, but it primarily changes

resistance with fatigue.

W. J. Baxter : That's right, objective is to develop a system where

one can have an abbreviated fatigue test without running to failure.

225
Qiarles Jackson : Suppose you put a fatigue gauge on the tension side

which does correlate not only to strain, but also to strain hardening

which is a measure of change of resistance to fatigue life. I am

curious if an experiment wouldn't be valid observing the surface of

the fatigue gauge. The S/N fatigue gauge is now marketed by

Micro Measurements.

W. J. Baxter : You're changing the whole experiment. You are asking

lire now to detect deformations in the strain gauge which is a different

material all together.

Qiarles Jackson: I guess I keep objecting to you using the term strain

gauge. I'm talking about a fatigue gauge.

W. J. Baxter: It is a thing that is bonded onto the surface, right?

Charles Jackson : True- -intended to measure fatigue, though.

W. J. Baxter : What is the thing made of?

Charles Jackson : I don't know. It's proprietary.

W. J. Baxter : It may behave like a thick plastic coating. Is it

encapsulated?

Charles Jackson: The aluminum and steel bars that I have broken have

behaved very consistently. That is, on steel, if you pick up 5 or 6 ohms

change in resistance, you've got a crack. I was curious if you've

correlated to this?

W. J. Baxter : No, I haven't tried using those things at all.

J. Thorp, U.S. Army Aviation Systems Command: Was this aluminum or

steel previous]^ cold worked or was it annealed?

W. J. Baxter : It's as received. The steel is cold rolled and I assume

that the aluminum is rolled.

J. Thorp : I was wondering if there should be a difference between the

annealed and the cold worked material. If you tested both conditions,

would you get a different rate of electron emission?

W. J. Baxter : You might.

226
APPLICATION OF AUTOMATED DIAGNOSTICS

IN MOTOR VEHICLE SAFETY INSPECTION

By

JOHN L. JACOBUS

Highway Safety Research Engineer

National Highway Traffic Safety Administration

Department of Transportation

400 - 7th Street, S.W.

Washington, D.C. 20590

Presented To:

18th Meeting

Mechanical Failures Prevention Group

Detection, Diagnosis, and Prognosis

November 9, 1972

At the

National Bureau of Standards

Gai thersburg , Maryland

227
APPLICATION OF AUTOMATED DIAGNOSTICS

IN MOTOR VEHICLE SAFETY INSPECTION

INTRODUCTION

^ In order to solve the problems of motor vehicle safety inspection, many new approaches in sensing and processing

will be required which will challenge all segments of the

scientific and engineering community. These new approaches

are embodied in an Automated Diagnostic Inspection System

concept

The National Highway Traffic Safety Administration considers

automated diagnostic inspection systems to mean the application of computerized inspection equipment and electronic

data processing techniques to first inspect or identify

those automobile systems which pose a safety hazard and,

second, to diagnose or determine those subsystems or components causing the hazardous condition.

We believe that automated diagnostic inspection systems

are an answer to upgrading and improving State inspection

programs so as to reduce the accident potential of defective

vehicles as well as to remove unsafe automobiles from our

roads and highways.

THE NEED FOR AUTOMATED DIAGNOSTIC INSPECTION SYSTEMS

The need for automated diagnostic inspection systems is

obvious when considering current motor vehicle inspection

programs which comply with the spirit of Federal recommendations and fail to objectively and accurately inspect

automobile systems.

Something must be done to upgrade the "safety quality" of

the vehicles on our roads while maintaining the ability to

cope with current and future numbers of automobiles.

228

4
As more safety critical areas are identified and confirmed

by research, and the consumer demand for in-depth automobile diagnostics increases, automated diagnostic inspection

techniques will have to be increasingly utilized.

EXISTING DIAGNOSTIC AND INSPECTION TECHNIQUES

AND UNIVERSAL DRIVE-ON AND ON-BOARD PLUG-IN METHODS

NHTSA feels that universal drive-on inspection equipment,

coupled with passive on-board sensors and plug-in umbilical

cord, are a means to upgrading all inspection programs.

In terms of dynamically checking brakes, steering and suspension, and tire systems: visual inspection methods,

active on-board transducer methods, and self-diagnosing

component techniques do not seem comprehensively applicable.

Universal drive-on inspection equipment is needed that can

accommodate any vehicle wheel base configuration and on-board

transducers which will allow the usage of stationary electronic data processing (EDP) equipment and computer techniques.

SPECIFIC PERFORMANCE CHECKS AND METHODS

Drive-on equipment is necessary to perform five primary

brake system performance checks which consist of (1) system

gain, (2) system response, (3) fade, (4) front-to-rear

proportioning, and (5) side-to-side balance.

Areas requiring plug-in techniques and on-board sensors

would include:

1. Brake shoe/pad thickness

2. Brake shoe/pad temperature

3. Brake line pressure

4. Master cylinder fluid level

5. Brake pedal reserve

6. Ant i -skid device

229
Three primary performance checks are required to inspect

automobile steering and suspension systems. These checks

include the following:

1. Steering response or steering wheel play

- 2. Front-end parameters and ball joints

3. Shock absorber/spring subsystem response

Areas requiring plug-in techniques include a steering wheel

servo unit to measure steering wheel displacement as well

as torque transducers to relate front wheel brake imbalance

to steering wheel inputs. It may also be desirable to incorporate transducers to indicate tire/wheel imbalances.

Tire pressures and tread depths are considered safety

critical parameters and also require drive-on equipment.

TECHNOLOGY FOR AUTOMATED DIAGNOSTIC

INSPECTION SYSTEMS

Commercially available chassis-type dynamometers, chassistype brake analyzers, front-end analyzers, and shock absorber

testers are all readily adaptable to computerization.

An Indiana manufacturer has developed, for its rent-a-car

division, an automatic electro-mechanical device to measure

tire tread depth and tire pressure.

A Minnesota manufacturer has developed and is offering for

sale, a computerized vehicle diagnostic system applied

specifically to engine analysis.

Volkswagen is in the process of serving their new vehicle

purchasers with a plug-in diagnostic package facilitating

the use of a computer which is plugged into a central socket

in the engine compartment.

ADVANCED INSPECTION SYSTEMS

Several advanced inspection concepts have been proposed which

230
show great potential as inspection tools. These include:

1. Gas chromatography

2. Fiberoptics

3. Holography

The use of gas chromatography techniques has been proposed

for indicating shock absorber and brake fluid leakage. A

probe or "sniffing device" would sample and analyze the

air around the shock absorbers, brake line/hose connections,

and brake hose/wheel cylinder connections. These "sniffers"

could also be used to detect significant exhaust, fuel line

and fuel tank leakages.

Fiberoptic techniques have been suggested as a means of

indicating roughness and scoring of brake drums and discs.

For drums, a fiberoptic probe would be inserted through an access hole in the backing plate of each wheel which would

then scan the width of the drum. Reflected light intensity

would be compared to a standard to indicate outages.

Holography techniques have been proposed as a means of inspecting tires and brake hoses. Defects such as ply separations and structural discontinuities would show up as

areas of high stress concentration in the holographic print.

APPEARANCE & OPERATION OF AN

AUTOMATED DIAGNOSTIC INSPECTION SYSTEM

A hypothetical automated diagnostic inspection system will

be discussed employing modified commercial and prototype

equipment with advanced inspection concepts. This system

has been designed to demonstrate the compatibility of engine

and emissions inspection with brakes, steering and suspension

and tires inspection.

The first element of this system would be an automobile

equipped or manufactured with various on-board transducers

connected to one central plug on the firewall or in the

engine compartment.

The second element of this automated diagnostic inspection

system is "slave" actuation equipment for the brake and

accelerator pedals as well as steering wheel.

251
The third element of this inspection system consists of integrated, universal drive-on equipment.

The fourth element of this inspection system consists of

computer hardware and software with an appropriate time

sharing system which would be connected to a national

computer network.

Inspection for this proposed automated diagnostic inspection

system would consist of the nine system performance tests

previously discussed, plus an exhaust emissions performance

test. If the vehicle fails to pass any one of these ten

performance checks, the computer logic will utilize the

additional information obtained through the on-board sensors

to diagnose the cause of failure.

CONCLUSIONS

In order to accurately and quickly inspect exhaust, brakes,

steering and suspension, and tire systems, automobiles will

have to be manufactured with passive on-board transducers.

In order to facilitate speed and repeatability, "slave"

actuation devices and electronic data processing equipment

must be interfaced with on-board sensors and universal

drive-on equipment.

Integrated, universal drive-on inspection equipment will

have to be developed so that multiple inspection functions

can be performed simultaneously by one machine.

NHTSA feels that automated diagnostic inspection systems

will provide a means of objectively and accurately inspecting

hundreds of thousands of cars quickly and inexpensively.

232
233
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AUTOMATED DIAGNOSTIC INSPECTION SYSTEM COMPUTER PRINT OUT

1 <uo +-» •H nj (/) o t—

n3 I—

P

uS 0) <D m •H

CD e m 03 (D CO <

BRAKE SYSTEMS INSPECTION

1. Brake System Gain

2. Brake System Response

a. Activation

b. De -Activation

3. Braking Ratio

4. Brake Balance

a. Front

b. Rear

c. Front Drag Imbalance

d. Rear Drag Imbalance

5. Brake System Fade

BRAKE SYSTEM DIAGNOSTICS

A. Brake Pedal Force

B. Brake Pedal Reserve

C. Brake Pedal Travel

D. Brake Shoe/Pad Thickness

a. RF

b. LF

c. LR

d. RR

E. Brake Line Pressure Hold Checks

a. RF

b. LF

c. LR

d. RR

F. Brake Hose Chafing

a. RF

b. LF

c. LR

d. RR

G. Brake Drum/ Disc Roughness

a. RF

b. LF

H. Brake Drum Roughness

a. RR

b. LR

r. Master Cylinder Fluid Check

J. Vapor Lock Check

K. Anti-Skid Check

2hl
AUTOMATED DIAGNOSTIC INSPECTION SYSTEM COMPUTER PRINT OUT

STEERING a SUSPENSION SYSTEM INSPECTION

6. Steering Response

7. Front End Parameters § Ball Joints

a. Toe „ b. Camber (RF)

c . Camber (LF)

d. Caster (RF)

e. Upper/Low Ball Joints (RF)

£. Upper/Low Ball Joints (LF)

g. Caster (LF)

- 8. Shock Absorber Response

a. RF (High/Low Frequency)

b. LF (High/Low Frequency)

c. RR (High/Low Frequency)

d. LR (High/Low Frequency)

STEERING § SUSPENSION SYSTEM DIAGNOSTICS

A. Maximum Steering Angle

B. Tracking Check

C. Independent Rear Camber

a. RR

b. LR

D. Independent Rear Toe

a. RR

b. LR

E. Tie Rod End Wear

F. Idler Arm Wear

G. Steering Arm Joint Wear

H. Shock Absorber Leakage

a. RF

b. LF

c . RR

d. LR

TIRE SYSTEMS INSPECTION

9. Tire Pressure ^ Tread Depth

a. RF

b. LF

c. RR

d . LR

TIRE SYSTEM DIAGNOSTICS

A. Tire Mix Check

B. Wheel/Tire Balance

a. RF

2U2
AUTOMATED DIAGNOSTIC INSPECTION SYSTEM COMPUTER PRINT OUT

b . LF

c. RR

d. LF

C. Tire Condition

a. RF

b. LF

c. RR

d. LR

ENGINE 5 POWER TRAIN INSPECTION

10. Emissions

A. Hydrocarbons

B. Nitrous Oxides

C. Carbon Monoxide

MISCELLANEOUS SYSTEMS INSPECTION

11. Fuel Systems

A. Fuel Tank Leakages

a . Front

b. Rear

B. Fuel Line Leakages

a. Front

b. Rear

12. Exhaust Systems

A. Front Leakage

B. Rear Leakage

13. Headlight Systems

A. Aim

B. Intensity

14. Air Conditioning System

15. Heating System

16. Cooling System

2^3
APPLICATION OF OPTICAL PROCESSING TO FLAW DETECTION

Bill Baker, Hugh Brown, Bob Markevitch, Dave Rodal

Ampex Corporation , Redwood City, California 94 063

Telephone (415) 367-3135

Optical processing is an improved method of spectral analysis which can be

used to monitor the noise or vibrations of rotating machinery or to analyze

the response of a structure subjected to a known dynamic loading. A

change in the noise spectrum or signature of a device can sometimes be

interpreted as an indicator of a potential failure.

Important characteristics of a spectrum analyzer are its resolution and its

ability to extract the significant characteristic frequencies of a device from

extraneous background noise. The capability of the spectral analyzer can

be described in terms of its time bandwidth product or the number of data

samples which are stored and, later, treated in the process of analysis.

For conventional digital or analog analyzers, the number of samples may

be only of the order of 10^ digital words or analog values. On the other

hand, 10^ to 10^ samples may be stored on a square centimeter of film

which is utilized in the optical processor. As in most systems, it is

possible to trade off bandwidth versus the recording time interval. The

resolution of the optical system is always of the order of 10"^ to 10"^

times the bandwidth.

An application to the analysis of a simple structural panel is indicated

in Fig. 1. The specimen is stimulated by a transducer driven by a signal

generator. A second transducer provides the output signal which describes

the response of the plate at a given point. The optical recording is simply

a photograph of the face of a CRT which is scanned in a manner similar to

that of a TV tube with the intensity of the beam being modulated by the

incoming signal

The film is chemically processed in a few seconds with a Bimat technique

and is then ready for spectral analysis in the optical processor in the lower

portion of Fig. 1. The spectrum is focused on the face of a TV camera which

is used to produce displays on an orthographic TV screen and on a 3D or isometric screen at the right.

A minicomputer has also been tied into the system to perform certain

calculations, to display alphanumeric data on the TV screens and to

produce hard copy of special data.

21+1+
A schematic presentation of the optical principles governing the formation

of the spectrum or Fourier transform is shown in Fig. 2. This includes

more details of the optical processor in the lower part of the system diagram

of Fig . 1

.

At the upper left portion of Fig. 2 is the previously recorded and developed

film which contains the information of the input signal stored as variations

in optical transmittance . The square of film in the X-Y plane is a time

exposure photograph made as the beam scanned the face of the CRT as

indicated by the parallel equally-spaced lines.

Of course, several hundred lines are scanned in the usual square centimeter

format, and the line transmittance varies as the strength of the signal at the

recording time corresponding to each point on the lines. However, let's

temporarily consider the uniformly spaced lines to be of constant transmittance

so they form a simple grating.

If the grating is illuminated by a coherent monochromatic light source,

such as a laser beam, then at great distances diffraction produces bright

dots along a line such as the fj^ axis perpendicular to the lines of the grating.

There is a bright spot called the zero order on the line normal to the film plane.

The next bright spot, labeled "line rate", corresponds to fo the spatial

frequency of the grating. Since the transmittance grating is not perfectly

sinusoidal, there are also bright spots at the harmonics 2fQ, Sf^, etc. , and

because of symmetry, there are corresponding dots at -fo, -21^, -3fQ, etc. in

the opposite direction from the zero order. These line rate dots and a few

harmonics can even be observed by looking through a grating at a small,

distant light bulb in a darkroom.

A theoretical derivation of the amplitude of light in a plane far from the film

is presented by Goodman (1). The resulting simple form is given by the integral

at the bottom of the figure; That is, the amplitude Aq is directly proportional

to the Fourier transform of the transmittance t^ extended over the entire

uniformly illuminated aperture. The introduction of the transform lens

preserves the relation and allows the two planes to be brought conveniently

close together.

By further calculations, it can be shown that the Fourier transform of the

modulated raster scan recording has the following properties:

1. The presence of any coherent component in the recording will

produce a bright spot in the frequency plane.

2. That spot must lie on one of the frequency loci which form another

raster indicated by the inclined parallel lines passing through the

line rate spot and its harmonics. '

3. Each spot is repeated periodically in the fx direction with period fq .
4. In a manner similar to that of the time raster in the film plane,

motion along a locus corresponds to a continuous variation of

frequency, but motion in the positive fy direction to the next

locus corresponds to an increase in frequency by exactly the

line rate fQ.

Because of the periodicity in the frequency plane, the entire spectrum is

displayed in any vertical half strip of width fQ. It is convenient to use the

shaded strip of Fig. 2 bounded on the left by the f^ - axis, below by the i-^ - axis, and on the right by the vertical line fx =

^o*

Optical processing has been used to produce spectra of engines and other

machines. Early work included ONR sponsored spectral analysis of acoustic

and accelerometer data recorded from a Pratt and Whitney jet engine. Those

studies (2,3,4,5) clearly indicated many frequencies including vibrations

caused by the chopping of the air stream in the compressor and turbine wheels

or by the meshing of accessory drive gears. Many of these components had not

been identified by other methods of spectral analysis.

In order to compare the signatures of worn and new components, a jig equipped

with an accelerometer was constructed to test starter bearings from the jet

engine. Eastern Airlines supplied a worn bearing which was compared with

a new replacement bearing. In addition to the generally noisy spectrum due

to minute spalling of the contact surfaces, the worn bearing also showed

strong coherent content corresponding to the ball spin frequency. There was

also an unexpected component at a frequency considerably lower than the

theoretical spin frequency for the given bearing geometry. Closer inspection

of the bearing revealed that most of the balls were so badly worn that they

slipped and spun at the lower rate rather than at the theoretical one.

Spectra have also been run on reciprocating air compressors and internal

combustion engines. It has been found that improperly assembled bearings

produce distinct characteristic changes in the signature.

Current work includes a feasibility study of the use of optical processing in

the detection of structural flaws under NASA contract NASI- 11749 . Preliminary

experiments have been made on simple rectangular aluminum plates with

various combinations of holes or simulated flaws. The spectra change

noticably as holes are introduced or as cracks are lengthened. Studies of

jet engine spectra are also being continued.

Because of the large time bandwidth product of the optical processing system,

much finer frequency resolution and improved signal to noise ratio are obtained.

Use of this method of spectral analysis holds promise of providing diagnostic

information for routinely monitoring the condition of engines and critical

structures. Such techniques can increase the safety and service factors of

equipment over conventional time based maintenance methods.

21+6
Comparisons based upon high data rat^s (6) shows that optical spectrum

analyzers cost only about 10"^ to 10 times as much as hard wired fast

Fourier transformers.

2k'J
REFERENCES

Goodman, J. W. , Introduction to Fourier Optics, McGraw-Hill,

New York, 1968.

Markevitch , Bob V., and van Heeckeren, Jacob,

Research on Failure Indicator System, First Technical

Report, Office of Naval Research Contract No. NOOO 14-67-

C-0401.

Knight, Gordon R. , Research on Failure Indicator System,

Second Technical Report, Office of Naval Research

Contract No. N00014-67-C-0401 , July, 1969.

Knight, Gordon, R. , Research on Failure Indicator System,

Third Technical Report, Office of Naval Research Contract

No. 00014-67-C-0401, July 1970.

Elson, Benjamin M . , "Optical System Analyzes Engines",

Aviation Week & Space Technology, March 1, 1971.

Oliver, Bernard M. , Project Cyclops , NASA Rpt. OR 11444 5, 1971.

21^8


DISCUSSION

J. Sudey, NASA: You showed a coii5)arison between a conventional analysis

and your optical technique. Can you indicate what the band width

on a conventional analyzer was?

B, Baker : I believe that the coirparison was made by taking successive

segments in the spectrum and analyzing those. I don't know whether

the storage capacity is really available at any time to handle this

many sanple points, 300,000 data points. I'm not sure whether that

can even be done.

J. Sudey: The band width then is somewhat dependent upon or related

to the data points?

B. Baker: Yes. This is roughly equal to the time band width product.

If you want to record a certain frequency, a certain band width, then

you have to have san^les at roughly twice that rate. That is a minimum rate. Actually, since bands are usually not very sharp, you

usually want to process three times the highest frequency. You have

to sanple at that rate for a certain time to produce a certain number

of samples. That number of sauries is the number of data points that

we can store on this square centimeter of film. Any other system would

have to be able to take care of that many elements.

S. Ramsey, Stanford Research Institute : One point that should probably

be made is the time it takes to perform the Fourier transform optically,

as opposed to visually, is the time it takes to propagate from the film

to the transfom plane, which may be several nanoseconds. If you are going to do the transformation digitally, it may take up to several

hours. Conputer costs might be somewhat prohibitive.

B. Baker : That's right. The computer costs are so high because of the

amount of processing required. At that rate, it would require a tremendous amount of digital systems. I should have mentioned that

in the brainwave studies, we are doing some work for medical people

where they bring in tape recordings of data. They are studying different

things such as fetal response and response of subjects to drugs, etc.

We are interested in low frequencies, but they have a long time interval.

We are not interested in a very wide band width, so we can get a tremendous time coropressed into one centimeter of film. We have been

able to play the tape at high speeds onto the film and we can get

30 to 40 minutes worth of data. It might have taken the conputer

center a week or so to get that spectra back from that amount of data.

It is enormously fast, not just because of the propagation, but

because we can compact so much data in one small area.

251
MECHANICAL FAILURES PREVEICTIGN GROUP

l8th MEETIITG

Detection, Diagnosis, and Prognosis

Sponsored by: National Bureau of Standards

November 8, 9, 10, 19T2

Albert Aksomitas

Turbo Power & Marine Sys . Inc

,

Pratt & Whitney Aircraft Co.

70 Ox Yoke Drive

Wethersfield, Conn. O6IO9

D. P. Altholz

U. S. Army Aviation "-'^ .:>rC- Systems Command

P. 0. Box 209 - Main Office

St. Louis, Missouri 63I66

John J. Anderson

Federal Scientific.

15 Lake Drive

Boonton, Nev Jersey 07005

Alan E. Ankeny

Naval Air Development Center

Code 302119

Wariaihister , Pennsylvania 1897^

Cdr. F. A. H. Ashme^d RN

British Defence Staff

British Embassy

3100 Mass. Avenue, N. V.

•Washington, D. C. 20008

W. H. Austin

Naval Ship Engineering Center - SMMSO

Center Bldg, Prince George's Center

Hyattsville, Maryland 20782

Alexander S, Babkin

Korfund Dynamics

Cantiague Road

Westbury, Nev York 11590

Bruce C . Baird

The Boeing Company

P. 0. Box 587^+7

Houston, Texas 77058

Barry Basket

U. S. Army Aviation Sys. Command

12th & Spruce Streets

St.- Louis, Missouri 63166

LTC Richard B. Baxter

0PM Util Acft.

P. 0. Box 209

St. Louis, Missouri 63166

W. J. Baxter

General Motors Research

12 Mile 8c Mound Roads

Warren, Michigan kQ090

Cedric D. Beachem

Naval Research Lab.

Washington, D. C. 20390

William H. Belke

Caterpillar Tractor Co.

Technical Center

Peoria, Illinois 6l602

252
David H. Biggs

Bently Nevada Corp.

Box 157

Minden, Nevadd 89^^23

Norton N. Black

Bendix Field Engineering Corp.

9250 Rt. 108

Columbia, Maryland 210^3

David Board

Boeing Co.-Vertol Div.

P. 0. Box 16858

Philadelphia, Pa. 191^2

Robert A. Boole

General Radio Company

300 Baker Avenue

Concord, Massachusetts '017^2

Charles A. Bowes

ENDEVCO Corp.

801 S. Arroyo Parkway

Pasadena, Calif. 9IIO9

A. P. Brackney

Monsanto Co.

730 Worcester Street

Indian Orchard, Mass. 01051

R. E. Braland

ARINC Res. Corp.

2551 Riva Road

Annapolis, Md. 2li+01

Robert F. Breese

Assoc. of American Railroads

Research Center

31UO 3. Federal Street

Chicago, Illinois 60616

B. L. Brock

Eastern Airlines

International Airport

Miami, Florida 331^8

H. C. Biirnett

Metallurgy Division

National Bureau of Standards

Washington, D. C. 2023^

255

Hubert L. Bull

LJ. S. Army Aviation Systems Command

NAS

Corpus Christi, Texas 78iil9

E. T. Caldwell

Fluor Engineers & Const . Inc

5559 E. Ferguson Dr.

City of Commerce, California 90022

Clarence Cantor

ENSCO

Springfield, Virginia 22151

Charles H. Castle

TRW Inc.

23555 Euclid

Cleveland, Ohio kkllj

Robert Y. Chapman

Naval Submarine Base New London

Box 300 Code 78O

Groton, Conn. 063^0

Stewart M; Chen

Army Aviation System Command

12 & Spruce

St.- Louis, Missouri 63033

Patrick T. Chiarito

NASA Lewis Research Center

21000 Brookpark Road

Cleveland, Ohio i+Ul35

CDR William E. Clark, Jr.

Naval Air Systems Command

8905 Bluegate Drive

Fairfax, Virginia 22030

R. A. Colton

NSRDC

Annapolis Lab,

Annapolis, Md. 2lU02

Jack Conway

Oil International Labs . Inc

1703 Terminal Tower Bldg.

Cleveland, Ohio 44113
Jack P. Conway

Computor Coordinator

1801 E. 1+Oth Str€.'et Cleveland, Ohio iiUlOS

R. A. Coulombe

Naval Ship Eng. Center

Hyattsville, Md. 20782

P. E. Craft

Hughes Aircraft Co.

P. 0. Box 929OT

Los Angeles, Calif. 90009

Arnold R. Daddario

Pratt & ^Thitney Aircraft

EB-3S, 1+00 Main Street

East Hartford, Conn. O6IO8

Millard K. Darnall '

' Caterpillar Tractor Co.

100 NE Adams

Peoria, Illinois 6l602

John V. Deller

Naval Ship Engineering Center

3700 East West Highway

Hyattsville, Maryland 20782

Stanley W. Doroff

Office of Naval Research

Code 1+38

Arlington, Virginia 22217

Lee Doubleday

Naval Air Systems Command HQ

5600 Colvimbia Pike

Washington, D. C. 20360

James F. Dray

Naval Ship Engineering Center

3900 East-West Highway

Hyattsville, Maryland 20782

Michael J. Drosjack

Ohio State Univ.

730 K Thurber Dr.

Columbus, Ohio U3215

Bruce E. Duff

Supt. Mech. Refr.

Fruit Growers Express Co.

16 Roth St.

Alexandria, Va. 2231^+

H. Alan Duguid

Huntington Industries

66 Old Shelton Road

Huntington, Conn. 06i+8i|

John W. Eichhorn

Service Research General Motors

Corporation GMSRDC

Tech. Center

Warren, Michigan ii8090

Kjartan A. Eikrem

Atomic Energy of Canada Limited

Engineering Research Branch

Chalk River Nuclear Labs

Chalk River, Ontario, Canada

R. M. Evan-Iwanowski

Syracuse University

Dept. Mech. & Aerospace Eng.

Syracuse, New York 13210

Joseph W. Fedena

Naval Ship Eng. Center

Philadelpha, Pennsylvania 19112

Harry Feingold

U. S. Naval Ship Research

And Development Center

Cajrderock, Maryland

John L. Fenton

NAVSEC Code 6153H2

P. G. Center, Room 321A

Hyattsville, Maryland 20782

Dr. Ferol F. Fish

Borg-Warner Research Center

Wolf & Algonq.uin Roads

Des Plaines, Illinois 6OOI8
John Fitzgerald

Army Material Command

AMCRDGP

Washington, D. C. 20315

LCDR Donald W. Flage, USN

Naval Ship Engr. Center

Centre Bldg.

Hyattsville, Md. 20782

Paul M. Fleming

Metallurgy Division

National Bureau of Standards

Washington, D. C. 2023^+

Jerry W. Forest

Ontario Hydro, Res. Division

800 Kipling Avenue

Toronto 18

Ontario , Canada

John L. Frarey

Mechanical Technolgy Inc.

968 Albany Shaker Road

Latham, New York 12110

Joseph G. Freeman

Grumman Aerospace

Bethpage

Long Island, New York 1171^

Dwayne N. Fry

Oak Ridge National Laboratory

Box X

Oak Ridge, Tenn. 37830

Harold F. Gallagher

Aerojet Nuclear Company

P. 0. Box 181+5

Idaho Falls, Idaho 83UOI

C. Gerald Gardner

Southwest Research Inst

8500 Culebra Road

San Antonio, Texas • 7828I+

John E. Gates

Battelle-Columbus

505 King Avenue

Columbus, Ohio 1+3201

Robert R. Gatts

Inst, for Applied Technology

National Bureau of Standards

•Washington, D. C. 20231+

Bernard K. Genetti

General Electric Co.

2901 East Lake Road

Erie, Penn. I65OI

Dr. John A. George

Parks College of St.

Louis Univ.

Cahokia, Illinois 62206

Charles Gerhardt

Ensco, Inc.

5I+O8A Port Royal Road

Springfield, Va. 22151

Thomas E. Gilder

Rocketynde Division

N. American Rockwell Corp.

6633 Canoga Avenue

Canoga Park, Calif. 9130I+

Nathan Glassman

Naval Ship Research and

Development Center

Annapolis, Maryland 211+02

W. Glew

Naval Eng. Test Establishment

161 Wanklyn St.

LaSalle, Quebec

Canada

William R. Godwin

Westinghouse Electric Corp.

Box 153 MS 71+73

Baltimore, Maryland 21203

255
Harvey J. Goodfriend

Hamilton Standard

Div. of United Aircraft Corp.

Airport Road, Bldg. 3

Windsor Locks, Connecticut O6096

S. P. Grant

Carolina Pover & Light Co.

5313 Dixon Drive

Raleigh, North Carolina 27609

W. W. Gunkel

Environmental Act. Staff (EAS)

General Motors Technical Center

Warren, Michigan 48090

Peter Hallick

FAA FS-303

800 Independence Avenue, S. W.

Washington, D. C. 20590

M. Hamberg

Atlantic Richfield Co.

3lkk Passyunk Ave.

Philadelphia, Pa. I9IOI ,

Keith R. Hamilton

USAF AFAPL/TBC

Wright -Patterson AFB, Ohio k^hSS

N. W. Harper

Naval Ship Engineering Center

SMMSO

Center Bldg. Prince George's Center

Hyatt sville, Md. 20782

John G. Hartwell

K West

9371 Kramer Avenue

Westminster, California 92683

Harold P. Hatch

U.S. Army Matls- & Mechanics

Research Center

NDTD Branch

Watertown, Mass. 02172

Merle W. Hauser

Beloit Corp.

1 'St. Lawrence Street

Beloit, Wisconsin 53511

Major Lawrence R. Hawkins

Combat Developments Command

Maintenance Agency

Aberdeen Proving Ground, Md. 21005

Keith L. Hawthorne

Assoc. of American Railroads

1920 "L" Street, N. W.

Washington, D. C. 20036

Henry R. Hegner

IIT Research Inst.

10 West 35th Street

Chicago, Illinois 60616

Anton H. Hehn

General American Transp. Corp.

Research Division

74^9 N. Natchez Avenue

Niles, Illinois 60648

E. D. Hietanen

Bendix Research Labs.

Bendix Center

Southfield, Michigan 48076

Michael R. Hoeprich

The Timken Company

1835 Dueber S. W.

Canton, Ohio 44706

Bernard Hoffman

U. S. Army, Frankford Arsenal

Bridge & Tacony Streets

Philadelphia, Pa. 19137

Dr. John D. Hoffman

Director

Institute for Matls Research

National Bureau of Standards

Washington, D. C. 20234

256
Murray Hoffman

Hamilton Standard Div.

United Aircraft Corp.

Windsor Locks, Conn. O6096

G. William Hogg

Eustis Directorate, USAAMRDL

Ft. Eustis, Virginia 23^90

R. Hohenberg

Avco Lycoming Division

550 S. Main Street

Stratford, Connecticut 06^97

Dr. Oscar J. Horger

Brenco, Inc.

P. 0. Box 389

Petersburg, Virginia 23803

Dr. Emanuel Horowitz

Institute for Materials Res.

National Bureau of Standards

Washington, D. C. 2023^

Donald R. Houser

Ohio State University

206 W. 18th Avenue

Columbus, Ohio 43710

Pa\il L. Howard, Jr.

SKF Industries Inc.

1100 First Avenue

King of Prussia, Pa. 19^06

L. R.- Hulls

RCA

Box 588

Burlington, Mass. OI8OI

Patrick W. Humphrey

JJASA/GSFC

Greenbelt, Maryland 20784

John P. Husko

Pacific. Airmotive Corporation

29^0 N. Hollywood Way

Burbank, California 91503

George Hurt

Detroit Diesel Allison Div.

Indianapolis, Indiana 46201

Dr. C. G. Interrante

Mechanical Properties Sect.

National Bureau of Standards

Washington, D. C. 20234

Charles Jackson

Monsanto Co.

Materials & Mechanical Tech. Dept.

P. 0. Box 1311

Texas City, Texas 77590

Martin Jacobs

Goodyear Atomic Corp.

P. 0. Box 628

Piketon, Ohio 4566I

John R. Jamieson

NASA-LV-GDC-28

Kennedy Space Center, Fla.

Richard Johnson

Gen. American Transport Corp.

7449 Natchez Avenue

Niles, Illinois 60035

Ronald B. Johnson

Institute for Matls Res

National Bureau of Standards

Washington, D. C. 20234

Frank C. Jones

K West

9371 Kramer Avenue

Westminster, California 92683

Peter W. Kamber

Boeing

P. 0. Box 3707, Stop 73-07

Seattle, Washington 98124

Robert L. Karlson

Endevco

IC Crystal Lake Plaza

P. 0. Box 444

Crystal Lake, Illinois 600l4

257
Capt, J, T, Kasemets

Canadian Armed Forces

AMDU CFB Trenton

Trenton, Ont^ario, Canada

A. C. Keller

Spectral Dynamics Corp. of San Diego

8911 Balboa Avenue, Box 67I

San Diego, California 92112

E. E. Klaus

Prof, of Chemical Eng.

Penn State University

108 Chem. Eng. Bldg. ' University Park, Pa. l6802

Anthony J. Koury - .

• NAVAIR SYS CO. MHW

Washington, D. C.

Joel F. Kuhlberg

Pratt & Whitney Aircraft

Main Street

East Hartford, Conn. O61O8

Thomas A. Kunik

Naval Ship Engr. Center

Philadelphia-, Pa. 19112

S. N. Kutufaris

U. S. Navy

NAVSEC PHILA DIV

Philadelphia, Pa. 19112

Normand L. Lagasse

Avco Lycoming

550 Main Street

Stratford, Connecticut 06497

W. D. Laingor

1221 Redhud

Hurst, Texas T6053 :

W. W. Lake

U. S. Army Aviation Sys. Command

P. 0. Box 209 - Main Office

St. Louis, Misso\iri 63166

Allan H. Layman

U. S. Army Combat Developments

Command Maintenance Agency

Aberdeen Proving Ground, Md.

Peter J. Leibert

Northrop Corp.

Electro-Mechanical Division

500 E. Orangethrope Ave.

Anaheim, California 928OI

Leonard E. Leopold

ENDEVCO - Dynamic Instl. Div.

6 S. Haddon Avenue

Haddonfield, New Jersey 08033

Robert M. Lerner

Naval Air Systems Command

Washington, D. C. 20360

W. D. Lewis

Dupont Engr . Dept

Wilmington, Delaware I9898

W. Lichodziejewski

Gen. American Trans. Corp.

7hh9 N. Natchez

Niles, Illinois 6o6hQ

»Prof. Frederick F. Ling

Rensselaer Polytechnic Inst.

Troy, New York 12l8l

C . S . Lockman

Douglas Aircraft Co.

Code kl-32 C1-2T0

3855 Lakewood Blvd.

Long Beach, California 9O8OI

John W. Lyons

U. S. Dept. of Transportation

55 Broadway

Cambridge, Mass. 02lU2

James W. Mason

Hydrofoil Special Trials Unit.

NSRDC, Pier 3, Bldg. 58O PSNS

Bremerton, Washington 9831^

258
p. A. Madden

SKF Industries

1100 First Avenue

King of Prussia, Pa. I9U06

Rollert Maxtin

Institute for Materials Res.

National Bureau of Standards

Washington, D. C. 2023^

David McCullough

Naval Air Eng. Center-GSED

Bldg. 76-1

Philadelphia, Pa. 19112

Mauro E. Messina

Section B, Data Acq.uisition Branch

Technical Support Div.

Naval Air Test Center

Patiaxent River, Maryland 206T0

Raymond Misialek

Naval Ship Engineering Center

Philadelphia, Division, Code 6t62C

Philadelphia, Pennsylvania 19112

Jan K. Miska

Naval Ship Eng. Center

NAVSEC, Prince George's Center

Hyattsville, Md. '20782

Richard S. Miller

ON'R, Office of Naval Research

Code kSS

Arlington, Va. 2221?

M. Dave Monoghan

Foster-Miller Assoc., Inc.

135 Second Avenue

Waltham, Massachusetts 0215^

Elmer R. Mokros

Kimberly-Clark Corp.

Research & Engineering

2100 Winchester Road

Neenah, Wisconsin 5^956

Charles H. Morris

Hamilton Standard

375 Benedict Drive

South Windsor, Conn. 0607^

George J. Morris

Air Force Matls Lab.

SYMBOL : MBT

Wright-Patterson AFB, Ohio

Gerald J. Moyar

Brenco Inc.

Petersburg, Virginia 23803

Jack R. Nicholas

Naval Ship Engineering Center

SMMSO

Center Building

Prince George's Center

Hyattsville, Md. 20782

Chester R. Oberg

U. S. Atomic Energy Commission

970 Broad Street

Newark, New Jersey 07102

Miss Beatrice S. Orleans

Naval Ship Systems Cornmand

JSHIPS 0311) Dept. of the Navy

Washington, D. C. 2036O

Dr. Elio Passaglia

Chief, Metallurgy Division

National Bureau of Standards

Washington, D. C. 2023^

Leonidas Payne

U. S. Army, Utility Aircraft Project

Manager's Office

12th and Spruce Street

St. Louis, Missouri 63033

E. E. Pfaffenberger

Supervisor of R & D

F M C Corp., Link-Belt Bearing Div.

7601 Rockville Road, P. 0. Box 85

Indianapolis, Indiana i+6206

259
Roger W. Pfeil

Lockheed Missiles and Space Co,

P. 0. Box 50U, Bldg. 10i+, 0/75-7^^

Sunnjrvale, California 9U088

Ronald N. Phillips

U. S. ^Naval Aviation

Logistics Support Center

Patiixent River, Maryland 20670

Lamoine A. Plog

U. S. Army Aviations Sys . Command

Attn: AMSAV-ERS , 12th & Spruce

St. Louis, Missouri 63166

C. W. Postlethwaite

ARINC Research Corp.

2551 Riva Road

Annapolis, Maryland 21^01

S. Blair Poteate, Jr.

USA AMRDL

Ft. Eustis, Virginia 236oU

James W. Potter

EKDEVCO

801 S. Arroyo Pkwy

Pasadena, California 91109

J. E. Pratt

American Telephone and

Telegraph Co.

P. 0. Box 665

Herndon, Virginia 22070

Gene Procasky

U. S. Army Aviation Systems

Command

P. 0. Box 209

AMSAV-LEM

St. Louis, Missouri 63166

Leo R. Raisheck

U. S. Army Weapons Command

AMSWE-TM

Rock Island, Illinois 61201

CDR Glen P. Ray

Naval Air Systems Command

253J+ Oak Jalley Drive

Vienna, Virginia 22l80

Ephraim Regelson

MVAIR

Washington, D. C. 20360

James J. Reis

Northrop Corporation

Northrop Research and

Technology Center

3^01 West Broadway

Hawthorne, California 90250

C. H. Roos

General Electric

831 Broad Street

Utica, New York 13501

M. M. Rosen

General Electric Co.

1000 Western Avenue

Lynn, Mass. 01905

LCDR S. M. Ross

Canadian Armed Forces

2^50 Massachusetts Avenue, N. W.

Washington, D. C. 20008

T. James Rudd

ENSCO, Inc.

5ii08A Port Royal Road ' Springfield, Virginia 22151

W. T. Sawyer

Washington College

Chest ertown, Maryland 21620

Georges Sapy

Electricite De France

Direction Des Etudes Et Recherches

17 Avenue du General de Gaulle

92 Clamart

France

260

A
Dr. John J. Scialdone

NASA, Goddard Space Flight Center

Code 322

Greenhelt, I^ryland 20771

R. F. Scott, Jr.

Travelers Insurance Co.

1st National Bank Bldg.

Dallas, Texas 75202

CDR Richard W. Shafer, USN

Naval Ship Engineering Center

SMMSO

Center BuildingPrince George's Center

Hyatt sville, Md. 20782

Richard A. Shat

U. S. Navy Dept. WSAO

MCB Quanti CO

Quantico, Virginia 2213^

Charles W. Shattuck

The Torrington Co.

59 Field Street

Torrington, Connecticut 06790

Charles P. Shaw, Jr.

E. I. duPont de Nemours & Co. Inc.

Engineering Dept.

Wilmington, Delaware I9898

Leonard F. Shaw

U. S. Army Corps of Engineers

Portland District

P. 0. Box 29^+6

Portland, Oregon 97208

Terry V. Sherhurn

Jeffrey Mining Machinery Co.

274 E. First Avenue

Columhus, Ohio 1+3201

A. Smith

Naval Eng. Test Establishment

161 Wanklyn Street

LaSalle, Quebec, Canada

E. D. Smith

American Telephone and

Telegraph Co.

P. 0. Box 665

Herndon, Virginia 22070

Harvey E. Solomon

Naval Ship Eng. Center

Philadelphia, Pennsylvania 19112

David C. Stanley

Naval Air Rework Facility

N A S North Island

Code 3i+3 Bldg. 3^+1

San Diego, California 92135

Jesse E. Stern

NASA Goddard Space Flight Center

Greenbelt, Maryland 20771

J. W. Strosnider

Spectral Dynamics Corp. of San Diego

8911 Balboa Avenue, Box 671

San Diego, California 92112

Lloyd F. Sturgeon

General Electric Co.

P. 0. Box 8555 Mail Stop M9517

Philadelphia, Pa. I9IOI

John Sudey, Jr.

NASA GSFC

202 Sudbury Ct.

Timoniimi, Maryland 21093

Dr. Thomas Tauber

Technical Development Co.

305 S. Chester Pike

Glenolden, Pennsylvania I9036

D. Thomson

Columbia Gas Systems

1600 Dublin Road

Columbus, Ohio 1+3215

261
Robb Thomson

Inst, for Applied Technology

National Bureau of Standards

Washington, D. C. 2023^+

V. E. Thornburg

Engineering 2-B-2

Pratt & Whitney Aircraft Div.

1+00 Main Street

East Hartford, Connecticut 06IO8

John M. Thorp

U. S. Army Aviation Systems. Command

AMSAU-LIEP

12th and Spruce Streets

St. Louis, Missouri 63166

John Torgesen

Institute for Materials Res

National Bureau of Standards

Washington, D. C. 2023i|

Tony Tornatore

Army Aviation Systems Command

99 1+8 East Concord Road

St. Louis, Missouri 63128

William D. Tracy

U. S. Army Aviation Systems Command

12 & Spruce Streets

St. Louis, Missouri 63166

Francis S. Tse

U. of Cincinnati

M. L. 72 U. of Cincinnati

Cincinnati, Ohio 1+5221

George Uriano

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20231+

Angelo C. Veca

Sikorsky Aircraft Div of UAC

Stratford, Connecticut 061+97

William Vesser

Tektronix, Inc.

P-. 0. Box 500

Beauerton, Oregon 97005

Bill Verhoef

Tektronix, Inc.

P. 0. Box 500

Beauerton, Oregon 97005

Frank J. Vicki

Pratt & Whitney Aircraft

2I+I Cedar Ridge

Glastonbiary , Conn. 06033

Robert 0. Walker

Teledyne

200 Avaiation

El Segundo, California 90721+

D. Watson

Naval Eng. Test Establishment

161 Wanklyn Street

LaSalle, Quebec

Canada

John M. Ward

Naval Air Systems Command

Washington, D. C. 20360

M. Weber

Hughes Aircraft Co.

P. 0. Box 92907

Los Angeles, California 90009

Robert E. Weiller

Pacific Airmotive Corporation

29I+O North Hollywood. Way

B\n-bank, California 91503

Verne B. Whitehead

Hydrofoil Special Traisl Unit

NSRDC, Pier 3, Bldg. 58O PSNS

Bremerton, Washington 9831I+

262
Robert M. Whittier

Endevco Corp.

801 S. Arroyo Parkway

Pasadena, California 91109

CDR David R. Womack

Naval Air Systems Command

Washington, D. C. 20360

L. A. Wood

Code 36-9 i+ Douglas Aircraft Co.

3855 Lakevood Blvd.

Long Beach, California 908i+6

Wicc Woolman

Boeing-Vertol

Philadelphia, Pa. 191^2

James L. Wotipka

IBM Corporation

2682 3rd Place, N. E.

Rochester, Minn. 55901

Donald V. Woytowitz Sr.

Naval Ship Engineering

Center (NAVSEC)

Center Bldg, Prince Georges

Hyattsville, Maryland 20782

John H. Zirnhelt

Royal Military College

Dept. of Mechanical Eng.

Kingston, Ontario KTL 2W3

Ceinada

* U. S. GOVERNMENT PRINTING OFFICE : 1973—542-650/57

265
FORM N3S-1 14A (1-71) U.S. DEPT. OF COMM. 1. PUBLICATION OR REPORT NO. 2. Gov'r Accession

BIBLIOGRAPHIC DATA vtocTo -7-3 oqo SHEET NBSIR 73-252

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Proceedings of the 18th Meeting of the Mechanical

Failures Prevention Group, National Bureau of

Standards, Gaithersburg , Md., November 8-10, 1972.

5. Publication Date

6. Performing Organisation Code

7. Editors T. R. Shives and W. A. Willard8. Performing Organization

NBSIR 73-252

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 3120402

I 1 i C\T\tT ^ ft / C^TCK ti t Mr\ V^i..' 11 LI ct^ L/ VjlallL

12. Sponsoring Organiration Name and Address 13. Type of Report & Period

Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

These proceedings consist of a group of sixteen submitted papers and

discussions from the 18th meeting of the Mechanical Failures Prevention

Group which was held at the National Bureau of Standards on November

8-10, 1972. Failure detection, diagnosis, and prognosis represent the

central theme of the proceedings. Bearing condition monitoring,

diagnostic systems technology and applications, and new approaches in

sensing and processing are discussed.

17. KEY WORDS (Alphabetical order, separated by semicolons)

Condition monitoring; Failure detection; Failure diagnosis; Failure

prevention; Failure prognosis; Diagnostic systems.

18. AVAILABILITY STATEMENT

CS UNLIMITED.

n FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE

TO NTIS.

19. SECURITY CLASS

(THIS REPORT)

UNCL ASSIFIED

21. NO. OF PAGES

20. SECURITY CLASS

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UNCLASSIFIED

22. Price

- USCOMM-OC 6e.2'14-r7!