Paula J. Dempsey and James J. Zakrajsek

Glenn Research Center, Cleveland, Ohio

Minimizing Load Effects on NA4 Gear

Vibration Diagnostic Parameter

NASA/TM—2001-210671

February 2001
The NASA STI Program Office . . . in Profile

Since its founding, NASA has been dedicated to

the advancement of aeronautics and space

science. The NASA Scientific and Technical

Information (STI) Program Office plays a key part

in helping NASA maintain this important role.

The NASA STI Program Office is operated by

Langley Research Center, the Lead Center for

NASA’s scientific and technical information. The

NASA STI Program Office provides access to the

NASA STI Database, the largest collection of

aeronautical and space science STI in the world.

The Program Office is also NASA’s institutional

mechanism for disseminating the results of its

research and development activities. These results

are published by NASA in the NASA STI Report

Series, which includes the following report types:

• TECHNICAL PUBLICATION. Reports of

completed research or a major significant

phase of research that present the results of

NASA programs and include extensive data

or theoretical analysis. Includes compilations

of significant scientific and technical data and

information deemed to be of continuing

reference value. NASA’s counterpart of peerreviewed formal professional papers but

has less stringent limitations on manuscript

length and extent of graphic presentations.

• TECHNICAL MEMORANDUM. Scientific

and technical findings that are preliminary or

of specialized interest, e.g., quick release

reports, working papers, and bibliographies

that contain minimal annotation. Does not

contain extensive analysis.

• CONTRACTOR REPORT. Scientific and

technical findings by NASA-sponsored

contractors and grantees.

• CONFERENCE PUBLICATION. Collected

papers from scientific and technical

conferences, symposia, seminars, or other

meetings sponsored or cosponsored by

NASA.

• SPECIAL PUBLICATION. Scientific,

technical, or historical information from

NASA programs, projects, and missions,

often concerned with subjects having

substantial public interest.

• TECHNICAL TRANSLATION. Englishlanguage translations of foreign scientific

and technical material pertinent to NASA’s

mission.

Specialized services that complement the STI

Program Office’s diverse offerings include

creating custom thesauri, building customized

data bases, organizing and publishing research

results . . . even providing videos.

For more information about the NASA STI

Program Office, see the following:

• Access the NASA STI Program Home Page

at http://www.sti.nasa.gov

• E-mail your question via the Internet to

help@sti.nasa.gov

• Fax your question to the NASA Access

Help Desk at 301–621–0134

• Telephone the NASA Access Help Desk at

301–621–0390

• Write to:

 NASA Access Help Desk

 NASA Center for AeroSpace Information

 7121 Standard Drive

 Hanover, MD 21076
February 2001

National Aeronautics and

Space Administration

Glenn Research Center

Prepared for the

55th Meeting

sponsored by the Society for Machinery Failure Prevention Technology

Virginia Beach, Virginia, April 2–5, 2001

Paula J. Dempsey and James J. Zakrajsek

Glenn Research Center, Cleveland, Ohio

NASA/TM—2001-210671

Minimizing Load Effects on NA4 Gear

Vibration Diagnostic Parameter
Available from

NASA Center for Aerospace Information

7121 Standard Drive

Hanover, MD 21076

Price Code: A03

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22100

Price Code: A03

Available electronically at http://gltrs.grc.nasa.gov/GLTRS
NASA/TM—2001-210671 1

MINIMIZING LOAD EFFECTS ON NA4 GEAR VIBRATION

DIAGNOSTIC PARAMETER

Paula J. Dempsey and James J. Zakrajsek

National Aeronautic and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Abstract: NA4 is a vibration diagnostic parameter, developed by researchers at NASA

Glenn Research Center, for health monitoring of gears in helicopter transmissions. NA4

reacts to the onset of gear pitting damage and continues to react to the damage as it spreads.

This research also indicates NA4 reacts similarly to load variations. The sensitivity of NA4

to load changes will substantially affect its performance on a helicopter gearbox that experiences continuously changing load throughout its flight regimes. The parameter NA4 has

been used to monitor gear fatigue tests at constant load. At constant load, NA4 effectively

detects the onset of pitting damage and tracks damage severity. Previous

research also shows that NA4 reacts to changes in load applied to the gears in the same way

it reacts to the onset of pitting damage. The method used to calculate NA4 was modified to

minimize these load effects. The modified NA4 parameter was applied to four sets of

experimental data. Results indicate the modified NA4 is no longer sensitive to load changes,

but remains sensitive to pitting damage.

Key Words: Damage assessment; Damage detection; Gears; Health monitoring; Oil debris

monitor; Pitting fatigue; Transmissions; Vibration

Introduction: Although various techniques exist for diagnosing damage in helicopter transmissions, the method most widely used involves monitoring vibration. Numerous algorithms have been developed for the processing of vibration data collected from gearbox

accelerometers to detect when gear damage has occurred. One of these algorithms, NA4,

was developed to detect the onset of gear damage and to continue to react to the damage as

it spreads [1]. NA4 is a dimensionless parameter with a nominal magnitude of approximately 3. When pitting damage occurs, the magnitude of NA4 shows a significant increase

above 3. Unfortunately, NA4 responds similarly to load changes. The sensitivity of NA4 to

even minor changes in load has been documented in several research papers [2,3]. The

magnitude of NA4 reacts to changes in load since the load change affects the running average in the denominator of this algorithm. When using this algorithm to detect gear pitting

damage on helicopter gearboxes in different flight regimes, the load effect on this algorithm

must be minimized. The goal of this research was to minimize the effect of load on vibration diagnostic parameter NA4 while maintaining its sensitivity to pitting damage.

Apparatus and Test Procedure: Experimental data was recorded from tests performed in

the Spur Gear Fatigue Test Rig at NASA Glenn Research Center [4]. Figure 1 shows the test
NASA/TM—2001-210671 2

apparatus. Operating on a four-square principle,

the shafts are coupled together with torque applied by a hydraulic loading mechanism that

twists two shafts with respect to one

another. The power required to drive the system is that to overcome friction losses in the

system [5]. The test gears are standard spur

gears having 28 teeth, a 3.50-in.-pitch diameter, and a 0.25-in.-face width.

Data was collected using vibration, speed, and

pressure sensors installed on the test rig. Vibration was measured on the housing near a

shaft support bearing using a miniature, lightweight, piezoelectric accelerometer. The location of this sensor is shown in Fig. 2. This

location was chosen based on an analysis of

optimum accelerometer location for this test rig

[6]. Gear rotation and speed was measured by

an optical sensor that creates a pulse signal for

each revolution of the gear. Hydraulic pressure

to the loading device was measured using a

capacitance pressure transducer. Shaft torque

is proportional to the pressure. The measured

pressure will be referred to as load pressure in

this report.

Data was also collected from an oil debris monitor (ODM). The ODM is installed on the rig to

give another indication when pitting damage

occurs [7]. Oil debris data was collected using

a commercially available oil debris sensor that measures the change in a magnetic field

caused by passage of a metal particle. The amplitude of the sensor output signal is proportional to the particle mass. The sensor measures the number of particles, determines their

approximate size (125 to 1000 microns), and calculates an accumulated mass [8]. The ODM

was used to automatically shut down the rig when the accumulated mass measured by the

monitor exceeded a preset limit.

Speed, pressure, ODM, and raw vibration data were collected and processed in real time

using the program ALBERT, Ames-Lewis Basic Experimentation in Real Time, codeveloped

by NASA Glenn and NASA Ames. Pressure data was recorded once per minute. Vibration

and speed data was sampled at 200 kHz for a 1-sec duration every minute. Vibration algorithm NA4 was calculated from this data and recorded every minute. Vibration algorithm

FM4 was also calculated from this data. FM4 is a widely recognized vibration algorithm

developed to detect changes to the vibration pattern resulting from damage to a limited

number of teeth. FM4 is a nondimensional number independent of load and speed [7, 9,10].

Figure 1.—Spur gear fatigue test rig.

Figure 2.—Accelerometer location on spur gear

 fatigue test rig.

Shaft

Driver

Driven
NASA/TM—2001-210671 3

Gears are run until initial pitting occurs on two or more teeth. Pitting is a fatigue failure of

the gear material on or near the surface induced by repeated contacts. Pitting is documented

by a video inspection system installed on the rig capable of following the progression of

gear pitting while avoiding the need to remove the gearbox cover. The gears were inspected

periodically based on a limit set on the ODM. For the purpose of this paper, different levels

of pitting must be defined. Due to the limited resolution of the video camera, only wear and

two levels of pitting could be monitored; initial and destructive. Initial pitting could not be

verified until inspection at completion of the experiment. For the purpose of identifying the

damaged gear, the gears are referred to as “driver” and “driven” as shown in Fig. 2.

Vibration Diagnostic Parameter NA4: The method used to calculate NA4 is published in

several research papers and will be discussed in the following paragraphs [2,11]. The first

step in calculating NA4 is to calculate the time-synchronous average of the raw vibration

data. Signal time-synchronous averaging is used to extract waveforms synchronous with

gear rotation from the total vibration signal. Vibration data is sampled at 200 kHz for a

1-sec duration and is then averaged synchronous to gear rotation. The desired signal which

is synchronous with the gear rotation will intensify relative to the nonsynchronous signals.

This time synchronous average signal is used to calculate NA4.

Several statistical and filtering operations are used to calculate NA4. First, the regular gearmeshing components are filtered from the signal resulting in a residual signal. The regular

gear-meshing components are the shaft and gear-meshing frequencies and their harmonics.

Variance and kurtosis are then calculated from the residual signal. The numerator, kurtosis,

the fourth moment of a probability density function, is used to indicate when the distribution is more peaked than a normal distribution.

The denominator is the square of the average variance, the mean value of the variance of all

previous readings in the run ensemble [11]. The NA4 is calculated as follows:

NA M

N rr

M

r r

i

i

N

ij j

i

N

j

M

4

1

1

4

1

1

2

1

2 ( )

( )

( )

= ( )

−

− 



































=

= =

∑

∑ ∑

where

r = residual signal = shaft and meshing frequencies and their harmonics removed from Fast

Fourier Transform (FFT) of time-synchronous-averaged signal

r = mean value of residual signal

N = total number of interpolated data points per reading

i = interpolated data point number per reading

M = current reading number

j = reading number

A change to the calculation of NA4 is required to minimize the effect of a fluctuating load

on NA4. This change, NA4 reset, is made when the load increases or decreases by a given
NASA/TM—2001-210671 4

percentage. For this application, a 10 percent load change was used. For NA4 reset, when

the load changes by 10 percent, the denominator resets to the square of the variance of the

same reading, and a new average variance is calculated starting with the reading measured

when the load changed. Each time the load changes by 10 percent, the first reading in the

average variance resets to the first reading when the load changed. This first reading is

calculated as follows:

NA M

N rr

r r

i

i

N

i

i

N

4 2

4

1

2

1

2 ( )

( )

= ( )

−

( ) − 















=

=

∑

∑

where

r = residual signal = shaft and meshing frequencies and their harmonics removed from FFT

of time-synchronous-averaged signal

r = mean value of residual signal

N = total number of interpolated data points per reading

i = interpolated data point number per reading

This denominator for the readings that follow is calculated as the square of the average

variance, the mean value of the variance of all previous readings starting with the first

reading when the load changed. Each time the load changes ±10 percent, the denominator is

reset by using Eq. (2) for the initial reading.

In addition to load changes, NA4 was also sensitive to restarts after the test rig was shut

down. The shutdowns are logged automatically in the data acquisition system during each

experiment. This information was used to calculate NA4 reset when the rig was restarted

after a shutdown.

Discussion of Results: The analysis discussed in this section is based on data collected

during four experiments, three of which pitting damage occurred. The first experiment was

to verify the effect of load on the NA4 parameter. The load was increased and decreased

with NA4 calculated from the vibration data. The gear set had no evidence of pitting before

or after the test. A plot of load pressure, NA4, and NA4 reset for the first experiment is

shown in Fig. 3. Data was collected every minute; therefore, the reading number is equivalent to minutes. Since the shaft speed is 10 000 revolutions per minute, the reading number

can also be interpreted as mesh cycles equal to the reading number times 104.

As discussed previously, NA4 reset is the same as NA4 except the average variance in the

denominator is reset each time the load fluctuated by 10 percent. From this plot, the sensitivity of NA4 to changes in load can be easily observed. NA4 appeared to track load pressure. The plot of NA4 reset shows that applying this technique minimizes the sensitivity of

NA4 to load.

Although the sensitivity of NA4 to load changes can be corrected by resetting the denominator, one must verify that applying this technique does not significantly decrease the
NASA/TM—2001-210671 5

sensitivity of NA4 to pitting damage. Data from three experiments when pitting damage

occurred and the load fluctuated was used to verify resetting the denominator of NA4 did

not decrease its sensitivity to pitting damage. Descriptions of the pitting damage that

occurred during these three experiments are listed in Tables I to III. Photos of damage

progression on a selected tooth during each experiment are shown in Figs. 4 to 6. The test

gears are run offset to provide a narrow effective face width to maximize gear contact stress.

Damage levels are described as follows:

(1) Wear—Layers of metal uniformly removed from the surface

(2) Initial Pitting—Pits of the initial type are less than 1/64 in. in diameter and cover less

than 25 percent of the tooth contact area

(3) Destructive Pitting—Destructive pitting is more severe with pits greater than 1/64 in. in

diameter and cover greater than 25 percent of the tooth contact area

Figure 3.—Data from experiment 1 illustrating load effects.

0

5

10

15

20

25

0 25 50 75 100 125 150 175 200 225

Reading number = run time, min

NA4

0

50

100

150

200

250

300

Load pressure, psi

NA4

NA4 Reset

Pressure

TABLE I.—DAMAGE DESCRIPTION FOR EXPERIMENT 2

Reading number

run time (min)

Damage description Teeth damaged on

driver gear

Teeth damaged on

driven gear

60 Run-in wear All All

120 Run-in wear All All

1581 Run-in wear All All

10622 Run-in wear All All

14369 Wear

Destructive pitting

All

6

All

6

14430 Wear

Destructive pitting

All

6

All

6

14512 Wear

Destructive pitting

All

6, 7

All

6, 7

14688 Wear

Destructive pitting

All

6, 7

All

6, 7

14846 Wear

Destructive pitting

All

6, 7

All

6, 7

15136 Wear

Initial pitting

Destructive pitting

All teeth

All teeth

6, 7, 8

All teeth

6, 7, 8
NASA/TM—2001-210671 6

Initial pitting on specific teeth will only be discussed in reference to test completion.

Although initial pitting most likely occurred prior to test completion, a detailed analysis of

the inspection images is required to verify when it occurred and is outside the scope of this

paper.

Plots of the data measured during these three experiments are shown in Figs. 7 to 13. Two

different plots are shown for each experiment. The first plot is of load pressure, NA4, and

NA4 reset for each experiment. The diamonds indicate when the rig was restarted after a

shutdown. The second is a plot of FM4, NA4 reset, and the accumulated mass from the

ODM. The triangles on the x-axis indicate the reading number that the rig was shut down

for inspection. These reading numbers are listed in Tables I to III. Each experiment will be

discussed in turn.

Experiment 2 is plotted in Figs. 7 to 9. Figure 7 shows the effect of the rig restarts after

shutdowns on NA4 by the NA4 magnitude spikes that occur after shutdowns. Figures 8 and

9 indicate damage occurred just prior to inspection at reading 14369. Inspection at reading

14369 indicated destructive pitting first occurred on driver and driven tooth 6. The progression of damage is detailed in Table I and Fig. 4. Both NA4 and FM4 indicate an increase in

magnitude when it appears destructive pitting occurred. The NA4 reset, like FM4, is less

sensitive to damage as it progresses to a number of teeth and becomes more severe.

TABLE II.—DAMAGE DESCRIPTION FOR EXPERIMENT 3

Reading number

run time (min)

Damage description Teeth damaged on

driver gear

Teeth damaged on

driven gear

0

1573 Run-in wear All All

2199 Wear

Destructive pitting

All All

 11

2296 Wear

Destructive pitting

All All

 10, 11

2444 Wear

Initial pitting

Destructive pitting

All

 All

10, 11

All

10, 11, 14

10, 11, 14

TABLE III.—DAMAGE DESCRIPTION FOR EXPERIMENT 4

Reading number

run time (min)

Damage description Teeth damaged

on driver gear

Teeth damaged

on driven gear

0

58 Run-in wear All All

2669 Wear

Destructive pitting

All

1, 28

All

 1, 28

2857 Wear

Destructive pitting

All

1, 6, 28

All

 1, 6, 28

3029 Wear

Initial pitting

Destructive pitting

All

All

1, 6, 28

All

1, 6, 28

1, 6, 28
NASA/TM—2001-210671 7

Figure 4.—Damage progression of driver/driven tooth 6 for experiment 2.

Experiment 3 is plotted in Figs. 10 to 11. Damage progression is shown in Table II and

Fig. 5. Destructive pitting occurred on driven tooth 11 prior to inspection at reading 2199.

From Fig. 11, FM4 and NA4 both indicate an increase in magnitude at approximately reading 1700. As seen previously, both become less sensitive to damage as it progresses.

Experiment 4 is plotted in Figs. 12 to 13. Damage progression is shown in Table III and

Fig. 6. Destructive pitting occurred on driver and driven teeth 1 and 28 prior to inspection at

reading 2669. From Fig. 13, FM4 and NA4 both indicate an increase in magnitude prior to

inspection at reading 2669 and become less sensitive to damage as it progresses.

As seen in Figs. 7 to 13, NA4 does react to pitting damage. However, some of the response

magnitude is lost with the reset operation. The NA4 reset does increase the stability of the
NASA/TM—2001-210671 8

Figure 5.—Damage progression of driver/driven tooth 11 for experiment 3.

Rdg 0 Rdg 1573 Rdg 2296

NA4 parameter enabling it to have a more consistent threshold limit. This is a key critical

factor in reducing false alarm rates.

Conclusions: Operational effects, such as load and speed fluctuations, can adversely impact vibration diagnostic parameters and result in an unacceptable level of false alarms. To

minimize this, current practice is to reduce the sensitivity of the vibration-based-diagnostic

Figure 6.—Damage progression of driver/driven tooth 28 for experiment 4.
NASA/TM—2001-210671 9

techniques. However, this also results in a decreased sensitivity of these techniques to actual damage.

The goal of this research was to minimize the effect of load on the vibration-diagnosticparameter NA4 while maintaining its sensitivity to pitting damage. Results indicate the

NA4 reset is no longer sensitive to load changes but is still sensitive to pitting damage. Both

NA4 reset and FM4 indicate when destructive pitting occurs on one gear tooth. The NA4

reset, like the FM4, is less sensitive to damage as it progresses to a number of teeth and

increases in severity. The magnitude of NA4 reset is less than NA4 when pitting damage

occurs requiring a smaller threshold limit to indicate pitting damage. However, the magnitude of NA4 reset is significantly larger than FM4 when pitting damage begins to occur. It

should be noted that successful implementation of NA4 reset requires a signal that can be

Figure 7.—Data from experiment 2 illustrating load and shutdown effects.

0

5

10

15

20

0 2000 4000 6000 8000 10000 12000 14000 16000

Reading number = run time, min

NA4 

0

100

200

300

400

 Load pressure, psi

NA4

NA4 Reset

Pressure

Figure 8.—Vibration, ODM, and damage data from experiment 2.

0

5

10

15

20

25

0 2000 4000 6000 8000 10000 12000 14000 16000

Reading number = run time, min.

FM4 and NA4

0

10

20

30

40

50

 ODM Mass, mg

FM4

NA4 Reset

ODM
NASA/TM—2001-210671 10

Figure 9.—Vibration, ODM, and damage data from experiment 2.

0

5

10

15

20

25

14000 14250 14500 14750 15000 15250

Reading number = run time, min

FM4 and NA4

0

10

20

30

40

50

 ODM mass, mg

FM4

NA4 Reset

ODM

Reading number = run time, min

Figure 10.—Data from experiment 3 illustrating shutdown and load effects.

0

10

20

30

40

0 500 1000 1500 2000 2500 3000

NA4

0

100

200

300

400

Load pressure, psi

NA4

NA4 Reset

Pressure
NASA/TM—2001-210671 11

Figure 11.—Vibration, ODM, and damage data from experiment 3.

0

5

10

15

20

0 500 1000 1500 2000 2500 3000

Reading number = run time, min

FM4 and NA4

10

20

30

40

50

ODM mass, mg

FM4

NA4 Reset

ODM

Figure 12.—Data from experiment 4 illustrating shutdown and load effects.

0

25

50

75

100

0 500 1000 1500 2000 2500 3000 3500

Reading number = run time, min

NA4

0

100

200

300

400

Load pressure, psi

NA4

NA4 Reset

Pressure
NASA/TM—2001-210671 12

directly correlated to torque load. Additional research is required to define alert and fault

threshold limits for vibration algorithm NA4 reset.

References

1. Zakrajsek, J.J.; Townsend, D.P.; and Decker, H.J.: An Analysis of Gear Fault Detection

Methods as Applied to Pitting Fatigue Failure Data. NASA TM–105950, 1993.

2. Zakrajsek, J.J.; Handschuh, R.F.; and Decker, H.J.: Application of Fault Detection Techniques to Spiral Bevel Gear Fatigue Data. NASA TM–106467, 1994.

3. Zakrajsek, J.J.; Decker, H.J.; Handschuh, R.F.; Lewicki, D.G.; and Decker, H.J.:

Detecting Gear Tooth Fracture in a High Contact Ratio Face Gear Mesh. NASA

TM–106822, 1995.

4. Lewicki, D.G., and Coy, J.J.: Helicopter Transmission Testing at NASA Lewis Research

Center. NASA TM–89912, 1987.

5. Lynwander, P.: Gear Drive Systems Design and Application. Marcel Dekker, Inc., New

York, NY, 1983.

6. Zakrajsek, J.J.; Townsend, D.P., Oswald, F.B., and Decker, H.J.: Analysis and Modification of a Single-Mesh Gear Fatigue Rig for Use in Diagnostic Studies. NASA

TM–105416, 1992.

7. Dempsey, P.J.: A Comparison of Vibration and Oil Debris Gear Damage Detection

Methods Applied to Pitting Damage. NASA TM–210371, 2000.

8. Howe, B.; and Muir, D.: In-Line Oil Debris Monitor (ODM) for Helicopter Gearbox

Condition Assessment, January 1, 1998.

9. Stewart, R.M.: Some Useful Data Analysis Techniques for Gearbox Diagnostics. Report

MHM/R/10/77, Machine Health Monitoring Group, Institute of Sound and Vibration

Research, University of Southhampton, 1977.

10. Rock, D., Malkoff, D.; and Stewart, R.: AI and Aircraft. AI Expert, Feb. 1993,

pp. 28–35.

11. Zakrajsek, J.J.; Decker, H.J.; and Handschuh, R.F.: An Enhancement to the NA4 Gear

Vibration Diagnostic Parameter. NASA TM–106553, 1994.

Figure 13.—Vibration, ODM, and damage data from experiment 4.

0

6

12

18

24

30

0 500 1000 1500 2000 2500 3000 3500

Reading number = run time, min

FM4 and NA4 

0

5

10

15

20

25

ODM mass, mg

FM4

NA4 Reset

ODM
This publication is available from the NASA Center for AeroSpace Information, 301–621–0390.

REPORT DOCUMENTATION PAGE

2. REPORT DATE

19. SECURITY CLASSIFICATION

 OF ABSTRACT

18. SECURITY CLASSIFICATION

 OF THIS PAGE

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources,

gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this

collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson

Davis Highway, Suite 1204, Arlington, VA 22202-4302, and to the Office of Management and Budget, Paperwork Reduction Project (0704-0188), Washington, DC 20503.

NSN 7540-01-280-5500 Standard Form 298 (Rev. 2-89)

Prescribed by ANSI Std. Z39-18

298-102

Form Approved

OMB No. 0704-0188

12b. DISTRIBUTION CODE

8. PERFORMING ORGANIZATION

 REPORT NUMBER

5. FUNDING NUMBERS

3. REPORT TYPE AND DATES COVERED

4. TITLE AND SUBTITLE

6. AUTHOR(S)

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

11. SUPPLEMENTARY NOTES

12a. DISTRIBUTION/AVAILABILITY STATEMENT

13. ABSTRACT (Maximum 200 words)

14. SUBJECT TERMS

17. SECURITY CLASSIFICATION

 OF REPORT

16. PRICE CODE

15. NUMBER OF PAGES

20. LIMITATION OF ABSTRACT

Unclassified Unclassified

Technical Memorandum

Unclassified

National Aeronautics and Space Administration

John H. Glenn Research Center at Lewis Field

Cleveland, Ohio 44135–3191

1. AGENCY USE ONLY (Leave blank)

10. SPONSORING/MONITORING

 AGENCY REPORT NUMBER

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546–0001

Available electronically at http://gltrs.grc.nasa.gov/GLTRS

February 2001

NASA TM—2001-210671

E–12578

WU–712–30–13–00

18

A03

Paula J. Dempsey and James J. Zakrajsek

Gears; Transmissions; Oil debris monitor; Vibration; Damage assessment;

Damage detection; Health monitoring; Pitting fatigue

Unclassified -Unlimited

Subject Category: 01 Distribution: Nonstandard

Prepared for the 55th Meeting sponsored by the Society for Machinery Failure Prevention Technology, Virginia Beach,

Virginia, April 2–5, 2001. Responsible person, Paula J. Dempsey, organization code 5950, 216–433–3398.

Minimizing Load Effects on NA4 Gear Vibration Diagnostic Parameter

NA4 is a vibration diagnostic parameter, developed by researchers at NASA Glenn Research Center, for health monitoring of gears in helicopter transmissions. The NA4 reacts to the onset of gear pitting damage and continues to react to the

damage as it spreads. This research also indicates NA4 reacts similarly to load variations. The sensitivity of NA4 to load

changes will substantially affect its performance on a helicopter gearbox that experiences continuously changing load

throughout its flight regimes. The parameter NA4 has been used to monitor gear fatigue tests at constant load. At constant

load, NA4 effectively detects the onset of pitting damage and tracks damage severity. Previous research also shows that

NA4 reacts to changes in load applied to the gears in the same way it reacts to the onset of pitting damage. The method

used to calculate NA4 was modified to minimize these load effects. The modified NA4 parameter was applied to four sets

of experimental data. Results indicate the modified NA4 is no longer sensitive to load changes, but remains sensitive to

pitting damage.