David G. Lewicki

Glenn Research Center, Cleveland, Ohio

Kelsen E. LaBerge

U.S. Army Research Laboratory, Glenn Research Center, Cleveland, Ohio

Ryan T. Ehinger

Bell Helicopter Textron Inc., Hurst, Texas

Jason Fetty

U.S. Army Aviation Applied Technology Directorate, Fort Eustis, Virginia 

Planetary Gearbox Fault Detection Using 

Vibration Separation Techniques

NASA/TM—2011-217127

November 2011

AHS2011–000134
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David G. Lewicki

Glenn Research Center, Cleveland, Ohio

Kelsen E. LaBerge

U.S. Army Research Laboratory, Glenn Research Center, Cleveland, Ohio

Ryan T. Ehinger

Bell Helicopter Textron Inc., Hurst, Texas

Jason Fetty

U.S. Army Aviation Applied Technology Directorate, Fort Eustis, Virginia 

Planetary Gearbox Fault Detection Using 

Vibration Separation Techniques

NASA/TM—2011-217127

November 2011

AHS2011–000134

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

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Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2011-217127 1

Planetary Gearbox Fault Detection Using 

Vibration Separation Techniques

David G. Lewicki

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Kelsen E. LaBerge

U.S. Army Research Laboratory

Glenn Research Center

Cleveland, Ohio 44135

Ryan T. Ehinger

Bell Helicopter Textron Inc.

Hurst, Texas 76053

Jason Fetty

U.S. Army Aviation Applied Technology Directorate

Fort Eustis, Virginia 23604

Abstract

Studies were performed to demonstrate the capability to 

detect planetary gear and bearing faults in helicopter mainrotor transmissions. The work supported the Operations 

Support and Sustainment (OSST) program with the U.S. Army 

Aviation Applied Technology Directorate (AATD) and Bell 

Helicopter Textron. Vibration data from the OH-58C 

planetary system were collected on a healthy transmission as 

well as with various seeded-fault components. Planetary fault 

detection algorithms were used with the collected data to 

evaluate fault detection effectiveness. Planet gear tooth cracks 

and spalls were detectable using the vibration separation 

techniques. Sun gear tooth cracks were not discernibly 

detectable from the vibration separation process. Sun gear 

tooth spall defects were detectable. Ring gear tooth cracks 

were only clearly detectable by accelerometers located near 

the crack location or directly across from the crack.

Enveloping provided an effective method for planet bearing 

inner- and outer-race spalling fault detection.

Introduction

The U.S. Army has the goal of transitioning from traditional 

maintenance practices into Condition Based Maintenance 

(CBM) for its fleet of vehicles. Rotorcraft plays an important 

role in the Army fleet. The Army Aviation Applied 

Technology Directorate (AATD) and Bell Helicopter Textron 

teamed in a joint program titled Operations Support and 

Sustainment Technologies (OSST) (Ref. 1). The goal of this 

program was to mature and demonstrate new technology 

solutions to reduce the maintenance and support burden on 

current and future Army rotorcraft.

The drive systems of current rotorcraft play a significant 

role in overall safety and maintenance costs. In recent years, 

significant research has been devoted to the development of 

health and usage monitoring systems (HUMS) for drive 

systems (Refs. 2 to 6). Much of this activity has concentrated 

on gear and bearing fault detection. Planetary, or epicyclic, 

transmission configurations are used on many rotorcraft in the 

final stage of main-rotor drive systems. Planetary systems 

provide an efficient and compact method to reduce speed and 

have been used successfully for years in aerospace and 

automotive applications. Planetary systems, however, exhibit 

unique challenges relating to gear and bearing fault detection.

First, multiple planets are contained in such systems. Since 

these planets all operate at the same speeds and loads, healthy 

components could potentially mask the vibration signatures of 

faulty components. Second, the planet gears exhibit epicyclic 

motion about a sun gear. Thus, the location of a faulty 

component, such as a planet tooth defect, changes position in 

time. This could provide a challenge for fixed-position

accelerometers (usually mounted on the transmission housing) 

which measure the vibration signals that monitor component 

health. There has been some recent work in the development 

of planetary fault detection (Refs. 7 to 15), but most of these 

efforts were not validated for helicopter applications.

In support of the OSST activities, work was performed to 

mature planetary system fault detection methodologies. The 

objective of this work was to demonstrate the capability to 

detect planetary system gear and bearing faults in helicopter 

main-rotor transmissions. Experiments were performed on an 

OH-58C helicopter main-rotor transmission in the NASA 

Glenn 500-hp Helicopter Transmission Test Facility. A total 

of 15 tests were performed. The first test was with a healthy 

transmission. The next 12 tests used seeded-fault components 

simulating a cracked planet gear tooth, a spalled planet gear 
NASA/TM—2011-217127 2

tooth, a cracked sun gear tooth, a spalled sun gear tooth, a 

cracked ring gear tooth, and spalled planet bearings. The last 

two tests were blind demonstration tests in which the 

conditions of the transmission were unknown to the analyst.

These consisted of a test with a simulated-cracked planet gear 

tooth and a test with all healthy components. Vibration data 

from the OH-58C transmission planetary system were 

collected and planetary fault detection algorithms were used to 

evaluate fault detection effectiveness.

Apparatus

Test Facility

The tests were performed in the NASA Glenn 500-hp 

helicopter transmission test facility (Fig. 1). The test stand 

operates on the closed-loop or torque-regenerative principle.

Mechanical power re-circulates through a closed loop of gears 

and shafting, part of which is the test transmission. The output 

of the test transmission attaches to the bevel gearbox. The 

output shaft of the bevel gearbox passes through a hollow 

shaft in the closing-end gearbox and connects to the 

differential gearbox. The output of the differential attaches to 

the hollow shaft in the closing-end gearbox. The output of the 

closing-end gearbox attaches to the input of the test 

transmission, thereby closing the loop.

A 200-hp variable-speed direct-current (DC) motor powers 

the test stand and controls the speed. The motor output 

attaches to the closing-end gearbox, and functions to control 

speed and replenish losses due to friction in the loop. A 15-hp 

DC motor provides the torque in the closed loop and drives a 

magnetic particle clutch. The clutch output does not turn but 

exerts a torque. This torque is transferred through a speedreducer gearbox and a chain drive to a large sprocket on the 

differential gearbox. The torque on the sprocket applies torque 

in the closed loop by displacing the gear attached to the output 

shaft of the bevel gearbox with respect to the gear connected 

to the input shaft of the closing-end gearbox. This is done 

within the differential gearbox through use of a compound 

planetary system where the planet carrier attaches to the 

sprocket housing. The magnitude of torque in the loop is 

adjusted by changing the electric field strength of the magnetic 

particle clutch.

A mast shaft loading system in the test stand simulates rotor 

loads imposed on the OH-58C transmission output mast shaft.

The OH-58C transmission output mast shaft connects to a 

loading yoke. Two vertical load cylinders connected to the 

yoke produce lift loads. A single horizontal load cylinder 

connected to the yoke produces a bending load. A 2000-psig 

nitrogen gas system powers the cylinders. Pressure regulators 

connected to the nitrogen supply of each of the load cylinders 

adjust the magnitude of lift and bending.

OH-58C Test Transmission

Tests were performed using an OH-58C helicopter mainrotor transmission (Fig. 2). The OH-58C transmission is rated 

at maximum continuous power of 335 hp at 6180 rpm input 

speed. The main-rotor transmission is a two-stage reduction 

gearbox with an overall reduction ratio of 17.44:1. The first 

stage is a spiral-bevel gear set with a 19-tooth pinion that 

meshes with a 71-tooth gear. Triplex ball bearings and one 

roller bearing support the bevel-pinion shaft. Duplex ball 

bearings and one roller bearing support the bevel-gear shaft.

Fig. 1. NASA Glenn 500-hp helicopter transmission test facility.

200-hp dc motor

15-hp

dc motor

Magnetic

particle

clutch

Torque increaser

gearbox

Differential gearbox

Closing-end gearbox

Bevel gearbox

Mast load

cylinders

Test transmission

Slip rings

Speed decreaser

gearbox

Fig. 2. OH-58C helicopter main-rotor transmission.

Output

Input

Spiral

bevel

gear

Sun gear

Ring gear

Planet gear

Spiral bevel pinion

Planet bearing
NASA/TM—2011-217127 3

The pinion is straddle mounted and the gear is overhung. A 

planetary mesh provides the second reduction stage. The 

bevel-gear shaft is connected through a spline to a sun gear 

shaft. The 27-tooth sun gear meshes with four 35-tooth planet 

gears, each supported with cylindrical roller bearings. The 

planet gears mesh with a 99-tooth fixed ring gear. The ring 

gear is connected to the transmission housing through a spline 

on its outer diameter. Power is taken out through the planet 

carrier which is connected to the output mast shaft through a 

spline. The output shaft is supported on top by a split-innerrace ball bearing and on the bottom by a roller bearing.

The OH-58C transmission is lubricated and cooled with its 

dedicated lubrication system. The 71-tooth bevel gear of the 

first reduction stage for the OH-58C transmission drives a 27-

tooth accessory gear. The accessory gear runs an internal oil 

pump, which supplies pressurized oil. After passing through 

the standard OH-58C 10-μm filter, the oil is ported to an 

external facility heater and heat exchanger, allowing precise 

control of oil inlet temperature. The oil is then routed back 

into the OH-58C transmission, providing lubrication for the 

gears and bearing through jets and passageways located in the 

transmission housing. The lubricant used in the OH-58C 

transmission was a synthetic base helicopter transmission oil 

conforming to the DOD-L-85734 specification. The nominal 

oil outlet pressure was 80 psig and regulated to 45 psig at the 

oil jets. The oil inlet temperature was 180 °F.

Test Components

A total of 15 tests were performed, as shown in Table I.

Only healthy components were used for the baseline test (Test 

1). All components for the baseline test were standard OH58C production hardware except for the ring gear. The ring 

gear was modified to allow placement of a specially-designed 

fiber-optics strain sensors band (not described in this report), 

where portions of the ring gear outer-spline were removed to 

accommodate the band and cabling.

For Tests 2 and 3, one tooth of a planet gear was modified 

using the electrical discharge machining (EDM) method to 

simulate a cracked tooth (Table I). The crack/notch was placed 

in the tooth fillet region (where the largest tensile stress 

occurs) along the complete width of the tooth. The crack/notch 

had a crack depth of about 25 percent of the total tooth crosssection length and a circular path similar to that which would 

naturally occur (Ref. 16). Note that the planet gear is 

symmetric and can be installed “face-up” or “face-down” in 

the assembly. For Test 2, the planet was installed face-up so 

that the crack/notch tooth opened in tension when in mesh 

with the sun gear. Since the planet acts similar to an idler gear, 

the crack/notch tooth closed when in mesh with the ring gear. 

For Test 3, the planet was installed face-down so that the 

crack/notch tooth opened in tension when in mesh with the 

ring gear.

For Tests 4 and 5, one tooth of a planet gear was modified 

similar to that for Tests 2 and 3, but with a crack depth of 

about 50 percent of the total tooth cross-section length

(Table I). For Tests 6 and 7, one tooth of a planet gear was

modified using the EDM method to simulate a tooth spall 

defect (Table I). Here, an elliptical section of the meshing 

tooth surface was machined with a defect.

For Test 8, one tooth of a sun gear was modified using the 

EDM method to simulate a cracked tooth (Table I). The 

crack/notch was placed in the tooth fillet region along the 

complete width of the tooth and with a crack depth of about 

25 percent of the total tooth cross-section length. For Test 9, a 

sun gear from a previous endurance test (Ref. 17) was used

(Table I). The gear had a naturally generated tooth spall on 

one of its teeth.

For Tests 10 and 11, two additional ring gears were 

modified. One was modified to simulate a cracked tooth with a 

crack depth of about 25 percent of the total tooth cross-section 

length for Test 10, and the other was modified to simulate a 

cracked tooth with a crack depth of about 50 percent of the 

total tooth cross-section length for Test 11 (Table I). As with 

the healthy ring gear, both ring gears were modified to allow 

placement of the fiber-optics strain sensors band.

For Test 12, the outer race of a planet bearing was modified 

using the EDM method to simulate a bearing spall defect

(Table I). The outer races of both rows of the two-row bearing 

were modified with a defect of about 0.10 in. length and 

0.020 in. depth and along the complete race widths. For 

Test 13, the inner race of a planet bearing was modified using 

the EDM method to simulate a bearing spall defect (Table I). 

The inner race was modified with a defect of about 0.10 in. 

length and 0.020 in. depth and along the complete race width.

Test 14 was a blind repeat of Test 4. Here, one tooth of an 

additional planet gear was modified using the EDM method to 

simulate a cracked tooth with crack depth of about 50 percent

of the total tooth cross-section length (Table I). Lastly, Test 15 

was a blind repeat of the baseline Test 1 and consisted of all 

the same healthy components.

Instrumentation

Eight piezoelectric accelerometers were installed on the 

housing of the OH-58C transmission (Fig. 3). Six 

accelerometers (1, 2, 3, 4, 5, and 7) were directly mounted to 

the transmission housing adjacent to the ring gear through 

threaded holes. The holes were tapped in the housing at 

various positions along the circumference of the ring gear to 

measure vibration in the radial direction relative to the 

planetary. Two accelerometers (6 and 8) were mounted on 

brackets installed on the transmission top cover housing to 

measure vibration in the tangential direction relative to the 

planetary. For accelerometer 8, the bracket was mounted 

directly to the transmission housing. For accelerometer 6, the 
NASA/TM—2011-217127 4

TABLE I.—LIST OF TESTS PERFORMED.

Test Description Test component photo

1 Baseline test, all healthy components.

2 Planet gear tooth crack, 25% depth, opened when in mesh with sun gear.

3 Planet gear tooth crack, 25% depth, opened when in mesh with ring gear.

4 Planet gear tooth crack, 50% depth, opened when in mesh with sun gear.

5 Planet gear tooth crack, 50% depth, opened when in mesh with ring gear.

6 Planet gear tooth spall at pitch line, in mesh with sun gear.

7 Planet gear tooth spall at pitch line, in mesh with ring gear.

8 Sun gear tooth crack, 25% depth.

9 Sun gear tooth spall at pitch line.

10 Ring gear tooth crack, 25% depth.

11 Ring gear tooth crack, 50% depth.

12 Planet bearing outer-race defect.

13 Planet bearing inner-race defect.

14 Demo test 1, Planet gear tooth crack, 50% depth, opened when in mesh with sun gear.

15 Demo test 2, Baseline test, all healthy components.
NASA/TM—2011-217127 5

bracket was mounted on a top cover mounting stud. The 

accelerometers were commercially available and had a flat 

high-frequency response up to about 40 kHz and a resonant 

frequency of about 90 kHz. Coaxial fiber optic reflective 

scanners were mounted on both the OH-58C transmission input 

and output shafts to produce a once-per-rev tachometer pulse of 

each shaft. The OH-58 output shaft tachometer pulse was also 

routed to a facility data acquisition system totalizer and logic 

cards. The output of the logic card was programmed to produce 

a tachometer pulse for the planetary hunting tooth period of 

every 105 output shaft rotations of the OH-58C transmission.

The outputs of the accelerometers and tachometer pulses 

were routed to anti-aliasing filters and a PC-based data 

acquisition system. For all test cases and test conditions, data 

were acquired at 50 kHz sampling rate with a 25 kHz aliasing 

filter cut-off frequency. The data were acquired for 40 sec per 

set, with a total of ten sets for each test condition. The start of 

each data acquisition for each set was triggered by the 

planetary hunting tooth pulse. For the full operating speed of 

6180 rpm OH-58 transmission input speed, the planetary 

hunting tooth period was 17.8 sec. Therefore, two complete and 

continuous planetary hunting tooth cycles were collected for 

each set and a total of 20 hunting tooth cycles were collected 

per test condition.

Assembly and Test Procedure

The planetary fault detection algorithms required precise 

knowledge as to which sun, planet, and ring gear teeth were in 

mesh at a given time. To do this, special indexing and a 

specific transmission assembly procedure were required.

The OH-58C planetary assembly is shown in Figure 4. First, 

the ring gear was positioned in the OH-58 transmission housing 

top cover so that a defined “tooth 1” was aligned with a fixed 

mark on the housing. Ring gear teeth were then numbered 

clockwise (as viewed from the bottom of the housing cover 

looking up, as in Fig. 4). Next, the OH-58 planets and sun gear 

were installed in the top cover housing. All tooth numbering 

was incremented in a clockwise manor. Tooth 1 of planet 1 was 

installed between teeth 1 and 2 of the ring gear, tooth 1 of 

planet 2 was installed between teeth 75 and 76 of the ring gear, 

and so on. Tooth 1 of the sun gear was positioned between 

teeth 18 and 19 of planet 1, tooth 22 of the sun gear was 

positioned between teeth 18 and 19 of planet 2, and so on. A 

special sun gear locking tool was then used to secure the 

planetary components. This kept the components from rotating, 

locking them in place, and allowed the complete assembly to 

flip for installation in the bottom case of the transmission. Once 

installed in the bottom case, the sun gear locking tool was 

removed without disturbing the planetary components and the 

transmission output shaft was installed. In this position, the 

output shaft tachometer was aligned so that the leading edge of 

the pulse was engaged. This procedure allowed precise position 

of the planetary tooth number indexing with respect to the 

transmission output tachometer and planetary hunting tooth 

tachometer.

The test procedure for each of the 15 tests of Table I was the 

same. For each test, the OH-58C transmission was run at 16 

different operating conditions consisting of four different 

torques (25, 50, 75, and 100 percent of design), two mast load 

conditions (full and off), and two speed conditions (6180 and 

2060 rpm transmission input speed). For each test condition, 

the transmission was run until steady state was reached and 

then the vibration data were taken. As previously stated, 10 sets 

Fig. 3. Accelerometer locations on OH-58C transmission top cover 

housing, top view of cover housing.

1

2

Accelerometer

3

4

5

6

7

8

Input

Output

91.43°

91.43°

88.57°

88.57°

Planet #1

Planet #2

Planet #3

Planet #4 Sun

Ring

Fig. 4. OH-58C planetary assembly indexing definition at assembly, 

as viewed from the bottom of the housing cover looking up.
NASA/TM—2011-217127 6

of two complete and continuous planetary hunting tooth cycles 

were collected for a total of 20 hunting tooth cycles per test 

condition.

All data were digitized and recorded on a disk for later 

processing. At the end of each test, the transmission was 

stopped at the beginning of a hunting tooth cycle. The 

transmission was then removed from the facility and 

disassembled. The top cover housing containing the planetary 

was removed using the reverse of the previously described 

procedure. Again, the special locking tool was used to secure 

the planetary during disassembly. Using this disassembly 

procedure, the teeth positions for all planetary gears were 

oriented as in Figure 4 upon disassembly.

Fault Detection Methodology

Planet Bearing Fault Detection

Enveloping is a common method of detecting the health of 

stationary (non-orbiting) bearings. This method also proved to 

work well for detecting planet bearing faults, despite the 

orbiting planets. This method is well defined in Howard’s 

review of rolling element bearing vibration (Ref. 18). The 

method is based on the knowledge that every time a rolling 

element passes by some raceway imperfection a short duration 

impulse excites bearing and structural natural frequencies.

Enveloping analysis allows the extraction of the frequencies at 

which these impulses occur.

This particular project focused on inner- and outer-raceway 

defects. Equations for the inner-ball pass frequency and the 

outer-ball pass frequency are shown in Equations (1) and (2).

These are the frequencies at which rolling elements pass an 

imperfection on the inner or outer race. Increases in the 

magnitude of the envelope spectrum at these frequencies are an 

indication of damage to the inner and outer races.

( )

2

1 cos 



 



 − + α

= D

d Z f f

f

o i

bpfi (1)

( )

2

1 cos 



 



 − − α

= D

d Z f f

f

o i

bpfo (2)

where fbpfi is the inner-race defect frequency (Hz), fbpfo is the 

outer-race defect frequency (Hz), Z is the number of balls or 

rollers, fo is the bearing outer-race speed (Hz), fi is the bearing 

inner-race speed (Hz), d is the ball or roller diameter (in.), D is

the bearing pitch diameter (in.), and α is the contact angle (°).

For the OH-58C planetary, Z = 13 rollers, d = 0.4331 in, 

D = 2.264 in, and α = 0°. At 100 percent input speed of 

6180 rpm, the relative bearing speed is (fo – fi) = 16.7 Hz. Thus, 

the inner-race defect frequency is 129.4 Hz and the outer-race 

defect frequency is 87.8 Hz. Note that the faults induced in the 

OH-58C planetary bearing were along both rows of the two 

row bearing. Since the rollers in the bearing rows are 

positioned offset from each other, the bearing acts like a 26-

roller bearing instead of a 13-roller bearing. The pertinent 

inner- and outer-race defect frequencies were twice that of 

above, or 258.7 and 175.6 Hz, respectively.

A diagram for the calculation of the bearing condition

indicators (CI's) is shown in Figure 5. The first 524,288 (=219) 

points of the raw data sampled at 50 kHz were used. The first 

step in enveloping was to run the raw data through a band pass 

filter with a pass band around structural frequencies, usually 

between 20 and 40 kHz. Through trial and error, a high-pass 

filter with a cutoff frequency of 18 kHz was found to work well 

for this application, giving usable band pass data from 18 to 

25 kHz. After filtering, a Hilbert transform was computed. The 

Hilbert transform, H(n), is effectively a 90° phase shift in the 

time domain. The Hilbert transform data, H(n), and the filtered 

data, A(n), were combined into what is known as the analytic 

signal, A'(n), defined as follows:

A′(n) = A(n) + iH(n) (3)

The magnitude of this analytic signal gives the envelope 

signal. The health of the bearing raceways can be determined 

by examining the values of vibration at the bearing inner- and 

outer-race defects from the frequency spectrum of this analytic 

signal.

Read raw data (first 219 pts)

Compute bearing condition indicators (CI's):

Find vibration peak magnitudes of the enveloped

signal at the inner- and outer-race ball pass 

frequencies (±5 Kz)

ENVELOPING TECHNIQUE

Apply high-pass filter

(cutoff frequency 18 kHz),

A[n]

Compute Hilbert Transform,

H[n]

Compute FFT of 

A2[n] + H2[n]

Fig. 5. Bearing fault detection technique.
NASA/TM—2011-217127 7

Planet Gear Fault Detection

A flowchart of the planet gear fault detection methodology is 

shown in Figure 6. The methodology used was a modified 

version of McFadden’s method of separating planet vibrations 

(Refs. 8, 10, and 12). The first step was to take the raw data and 

separate it into hunting tooth cycles. The hunting tooth cycles 

were then re-sampled (using linear interpolation) with respect 

to each carrier cycle, so that every carrier cycle contained the 

same number of points. This accounted for any speed variations 

within the hunting tooth cycle. The number of points in each 

carrier cycle were set to be an integer multiple of the number of 

ring gear teeth. A total of 8415 points (99 ring gear teeth x 85 

points per tooth) were used for each carrier cycle. This was the 

greatest number of ring gear multiples that was still less than 

the original number of points from the raw data. For each test 

condition, data from 20 hunting tooth cycles for a given 

accelerometer were averaged together using this procedure to 

produce one average hunting tooth cycle, which will hereon be 

defined as a hunting tooth average (HTA). As previously 

stated, a hunting cycle was 105 carrier cycles, or 17.8 sec in 

duration for tests at 6180 rpm input speed. This procedure was 

repeated for each accelerometer.

The next step was to separate out the data for each planet 

into tooth vectors. Separation involved grabbing sections of 

data as each planet passed a particular accelerometer, 

multiplying it by some windowing function and then adding it 

into a tooth vector for that planet according to the planet tooth 

that was in mesh at the sensor location. The planet positions at 

the output tachometer pulse and the sequence of the teeth that 

mesh with each sensor were known beforehand. At the start of 

a given test, the initial planet positions were known from the 

planet assembly procedure (Fig. 4). The sequence of teeth in 

mesh were determined from the number of teeth on the ring 

gear and planet gears (Ref. 12). For the OH-58C application, 

all planet teeth pass by a particular sensor position due to the 

hunting tooth relationship between the planets (35 teeth) and 

ring gear (99 teeth). A window with tapered edges was selected 

that satisfied the criteria set forth in (Ref. 10). A Tukey 

window was chosen for this application. A Tukey window is 

similar to a raised cosine (Hanning) window, with a hold at a 

gain of one in the middle. After evaluating several window 

width designs, a Tukey window of five-teeth wide with a hold 

for one tooth in the center was chosen.

Every time a planet passed an accelerometer, a five-tooth 

wide section of data was sectioned and multiplied by the

Fig. 6. Planet gear tooth vibration separation fault detection technique.

0 45 90 135 180 225 270 315 360 45 90 135 180 225 270 315 360 45 90 135 180 225 270 315 360 -5

0

5 a) Hunting tooth average of raw data.

Acceleration (V)

34 35 1 2 3 -2

0

2

b) Windowed data, planet 1 

carrier cycle 1.

Acceleration (V)

Tooth number

28 29 30 31 32 -2

0

2

Tooth number

22 23 24 25 26 -2

0

2

Tooth number

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 1 -2

0

2

Planet gear tooth number

Acceleration (V)

e) Completed planet 1 tooth vector.

Planet 1 first pass Planet 1 second pass Planet 1 third pass

c) Windowed data, planet 1 

carrier cycle 2.

d) Windowed data, planet 1 

carrier cycle 3.

Carrier rotation, deg
NASA/TM—2011-217127 8

selected Tukey window. The window was then added into a 

tooth vector according to planet centered on the tooth in mesh 

at that point (Fig. 6). Once the windows were assembled, the 

values of the tooth vectors were divided by a factor of 9. This

was due to a window gain factor of 3 for a Tukey window of 

five-teeth wide and a hold for one tooth, and a factor of 3 due 

to three planet cycles in a hunting tooth cycle. Each planet 

vector had 2975 number of points (35 planet teeth x 85 points 

per tooth). Each planet vector was then resampled down to 

2048 points (even power of two) for ease in Fourier Transform 

calculations. This process was performed for all planets and all 

accelerometers. Thus, for each test condition, 32 planet vectors 

(4 planets x 8 accelerometer) were assembled. Condition 

indicators (CI's) were then applied to each planet vector.

Sun Gear Fault Detection

The separation of time averages for the sun gear was slightly 

more complicated. Sun teeth are only “seen” by accelerometers 

through the passing planets where the accelerometers are 

mounted on the housing near the ring gear. McFadden presents 

his method as applicable for both sun and planet gears.

However, the sun gear brings up a different set of 

complications. If sun teeth are only viewed through the passing 

planets, all sun teeth aren’t necessarily seen. In the OH-58C 

main transmission application, three sun gear teeth are viewed 

by an accelerometer for a particular planet due to the nonhunting tooth virtual relationship between the sun gear (27 

teeth) and ring gear (99 teeth). The same three teeth are also 

viewed by the accelerometer for the planet directly opposite 

(180° apart). Three additional sun teeth are viewed by the 

accelerometer for the remaining two planets. Thus, for the OH58C, only six of the sun teeth are viewed by a particular 

accelerometer through the passing planets. In addition, these 

teeth are not evenly spaced around the sun. The OH-58C has 

neighboring planets that are either 91.4° or 88.6° apart, with 

even or odd planets 180° apart (see Fig. 4). This results in 

uneven spacing of the sun teeth viewed since the planet are not 

evenly spaced at 90° to each other.

There are two ways to deal with these issues. The first option 

is to average the data from all the passing planets into a single 

sun tooth vector. While this method allows the smallest 

window to be used, the windows must be asymmetric in order 

to account for uneven spacing between the teeth viewed. This 

also involves averaging data from different planets together, 

each of which have their own vibration signature. The second 

option is to generate four separate sun gear tooth vectors per 

accelerometer, each one being the sun vibration as seen through 

an individual planet. This method requires a wider window 

because only three sun teeth are viewed through each planet.

These teeth, however, are evenly spaced around the sun since a

given sun vector is seen through only one given planet. Several 

iterations of each option were tried, but the best results were 

obtained using the second option with a symmetric Tukey 

window that was 27-teeth wide with a hold of one for nine 

teeth in the center.

Ring Gear Fault Detection

The method used to obtain time averages of the ring gear 

was fairly simple. Taking the hunting tooth average, each 

carrier cycle was averaged. This data set was used as the ring 

gear tooth vector. One set of ring data then included the data of 

all four passing planets. Therefore, ring gear damage would be 

seen four times in the ring time average because it is loaded 

four times by the passing planets.

Condition Indicators

A total of nine condition indicators (CI's) were used to 

determine the health of the gears in the planetary system. All of 

these indicators were previously developed for single gear sets 

and were applied to the separated sun, planet, and ring gear 

tooth vectors discussed above. Indicators were also calculated 

directly from the hunting tooth average (HTA). This was done 

to gauge the benefit of performing the extra vibration 

separation processing. In calculating the indicators for the 

HTA, only the first 218 points were used because it was found 

that using the full HTA was time intensive and the results 

obtained using fewer points were nearly identical.

Before computing the CI’s, the bevel meshing frequency was 

removed from the sun, ring, and HTA signals. The assembly of 

planet tooth vectors was such that the bevel meshing frequency 

was averaged out with no further processing needed.

The nine CI's used were RMS, crest factor, energy ratio, 

FM0, kurtosis, energy operator, FM4, M6A and M8A. All of 

these indicators are explained in (Ref. 19). Condition indicator 

M8A performed the best in this study and will be the only one 

discussed in this report. The parameter M8A, proposed by 

Martin (Ref. 20), uses the eighth moment normalized by the 

variance to the fourth power and is given as

( )

( ) 4

1

2

1

8 3

8



  δ − δ

δ − δ =

∑

∑

=

=

N

i i

N

i N i

M A (4)

where N is the number of samples, i is the sample index, δ is 

the difference signal, and δ is the mean of the difference 

signal. The difference signal, δ, is defined as

x(t) y (t) δ = − d (5)

where x(t) is the original time synchronous signal and yd(t) is 

the signal containing the component shaft frequency, the gear 

mesh frequency, their harmonics, and their first-order 

sidebands. For this project, the best results were obtained by 

only removing the fundamental gear meshing frequency and 

the first ten harmonics. For planet and sun gears, the stop-band 

width used was three shaft orders. Ring gears were more 

complicated because the planets pass four times within the gear 

rotation. The difference signal was most sensitive to damage 

when a stop-band width of 16 times the output frequency was 
NASA/TM—2011-217127 9 

removed. The stop-band for the HTA data also included the 

planet pass envelope. From previous work, it was found that 

the first-order sidebands occur at a sideband width equal to the 

planet pass frequency. Therefore, the stop-band width used in 

calculating the HTA difference signal was slightly more than 

eight times the carrier frequency. 

Results and Discussion

Planet Gear Tooth Defects

Test 2 used a planet gear with an EDM-simulated tooth 

crack, 25 percent depth, in planet 1, tooth 17, installed face up 

such that the crack opened when in mesh with the sun gear.

Test 3 used the same cracked planet gear as Test 2, but 

installed face down such that the crack opened when in mesh 

with the ring gear. By definition, tooth 1 on the planet gear was 

the same physical tooth for Tests 2 and 3. For both tests, the 

planet teeth numbering on the gear faces was incremented in a 

counter clockwise direction as viewed from the bottom of the 

housing cover looking up. Thus, the cracked tooth was defined 

as tooth 17 for Test 2 and tooth 20 for Test 3. Test 4 and 5 were 

similar to Test 2 and 3, respectively, but used a planet gear with 

a simulated tooth crack of 50 percent depth.

Figure 7 shows the effect of a crack on planet 1 tooth vectors 

and difference signals for accelerometer 1 at 100 percent torque, 

speed, and mast loads. The planet gear tooth vectors and 

difference signals for Tests 1 and 4 are plotted against planet 

gear tooth number, which is defined as the planet tooth number 

in mesh with the ring gear at the location of the accelerometer. 

Figures 7a and 7c are for the baseline Test 1 of all healthy 

components. Repeated vibration patterns for each tooth for both 

the planet gear tooth vector and difference signal plots are 

evident. By comparison, Figure 7b and 7d are for Test 4 with a 

planet gear tooth crack of 50 percent depth. At planet gear tooth 

number 17 in Figure 7, the crack was closed and in mesh with 

the ring gear. At tooth number 35, the crack was opened and in 

mesh with the sun gear. The effect of the crack was clearly seen 

in Figure 7d as a disturbance in the difference signal at tooth 18 

and at tooth 1 (or end of tooth 35 since the plot is periodic). The 

crack was more detectable when forced closed at the ring rather 

than when opened at the sun. It was believed that the crack 

closing behavior was more discernable because the planet cracks 

were machined notches with a gap at the crack region. Without 

this removal of material, the dominant crack closing behavior at 

tooth 18 is expected to be greatly reduced if not eliminated. This 

would probably reduce the fault detection effectiveness of a CI, 

but how much is not presently known. Note that planet faults 

were seen in tooth vectors one tooth after going through mesh 

with the ring. This is probably due to backlash and windup of the 

complete system (OH-58C test transmission and facility 

gearboxes) under torque.

Figure 8 shows the effect of a planet tooth crack on M8A for 

all accelerometers at 100 percent speed, torque, and mast loads 

for Tests 1 and 4. Again, M8A responds to the spikiness of a 

signal, so that disturbances in the difference signal, such as in 

Figure 7d, can be identified. From Figure 8, strong indications 

of a defect are given for accelerometers 1, 2, 3, and 8, while 

small to moderate indications are given for accelerometers 4, 5, 

6, and 7. Acceleration, g's

-15

-10

-5

0

5

10

15

Acceleration, g's

-15

-10

-5

0

5

10

15

Acceleration, g's

-10

-5

0

5

10

Planet gear tooth number

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35

Acceleration, g's

-10

-5

0

5

10

Fig. 7. Effect of crack on planet 1 tooth vectors and difference signals, 

for accelerometer 1 at 100% speed, torque, and mast loads.

a) Tooth vector for Test 1 (baseline, healthy components) .

b) Tooth vector for Test 4 (planet tooth crack, 50% 

depth, opened when in mesh with sun gear).

c) Difference signal for Test 1 (baseline, healthy components) .

d) Difference signal for Test 4 (planet tooth crack, 50% 

depth, opened when in mesh with sun gear).

Crack closed at

planet/ring mesh

Crack opened at

sun/planet mesh

Accelerometer number

A1 A2 A3 A4 A5 A6 A7 A8

M8A

0

500

1000

1500

2000

15000

20000

Fig. 8. Effect of crack on planet 1 M8A for all accelerometers, at 

100% speed, torque, and mast loads. Test 1: baseline, healthy 

components; Test 4: planet tooth crack, 50% depth, opened 

when in mesh with sun gear.

Test 1

Test 4
NASA/TM—2011-217127 10

Figure 9 shows the effect of a crack on the planet 1 

difference signal for Test 5 for accelerometer 1 and at 

100 percent torque, speed, and mast loads. Here, a strong 

disturbance is seen at tooth 4 due to the crack being closed at 

the sun-planet mesh. Also, a disturbance is seen at tooth 21 for 

the planet-ring mesh (crack being opened). Figure 10 gives the 

M8A results for all accelerometers of Test 5. All 

accelerometers give a strong indication of the defect as 

compared to the baseline test. Figures 7 through 10 show the 

fault detectability for the crack with 50 percent depth. Similar 

results for the 25 percent depth crack tests were seen although 

not as pronounced.

It was observed from many of the seeded-fault tests that the 

vibration irregularities that occurred during the passing of a 

fault through the sun-planet mesh bled through to all the 

planets. This was observed from the difference signals of the

planet 1 through 4 tooth vectors and resulting M8A indications.

This was thought to be a result of the sun gear, which has a 

floating center, to displace slightly and affect the meshing 

action of all the planets during passing of a defect. Due to the 

bleed through, it may be difficult, in some cases, to distinguish 

a fault to a particular individual planet. This, however, should 

not be an issue in actual helicopter applications since it would 

be more than adequate to identify a faulty planetary system and 

not an individual planet gear.

Figure 11 shows the effect of planet gear tooth spalling on 

the difference signals for planet 1 of Tests 6 and 7, 

accelerometer 1, and at 100 percent torque, speed, and mast 

loads. The spall was an EDM-machined defect, elliptical in 

shape, approximately 0.020-in. in depth, and centered along the 

gear tooth face width and the pitch line. The spall was placed 

on tooth 17 for Test 6 (planet tooth spall in mesh with the sun 

gear), and also defined as tooth 20 for Test 7 (planet tooth spall 

in mesh with the ring gear). For Figure 11a, the defect is faintly 

seen at planet tooth number 1 (planet-ring mesh), which is 

diametrically opposite tooth 17 (sun-planet mesh). For 

Figure 11b, the defect is very noticeable at planet tooth 21. The 

effect of planet gear spalling on M8A is shown in Figure 12 for 

all accelerometers at 100 percent speed, torque, and mast loads.

Many of the accelerometers gave an indication of the defect.

As with the planet gear tooth crack test, some leakage of the 

defect signals to the neighboring planets occurred when the 

planet fault went through mesh at the sun gear.

Lastly, it should be noted that the planet tooth defects were 

not clearly distinguishable in the HTA signals. Thus, the planet 

gear tooth separation technique described in this study provided 

an effective means to detect planet tooth defects.

Sun Gear Tooth Defects

Test 8 used a sun gear with an EDM-simulated tooth crack, 

25 percent depth, on tooth 8, oriented such that the crack 

opened when in mesh with the planet gears. Test 9 used a sun 

Planet gear tooth number

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35

Acceleration, g's

-10

-5

0

5

10

Fig. 9. Planet 1 difference signal for Test 5 (planet tooth crack, 50% 

depth, opened when in mesh with ring gear), for accelerometer 

1 at 100% speed, torque, and mast loads.

Crack opened at

planet/ring mesh

Crack closed at

sun/planet mesh

Accelerometer number

A1 A2 A3 A4 A5 A6 A7 A8

M8A

0

500

1000

100000

200000

300000

Fig. 10. Effect of crack on planet 1 M8A for all accelerometers, at 

100% speed, torque, and mast loads. Test 1: baseline, healthy 

components; Test 5: planet tooth crack, 50% depth, opened 

when in mesh with ring gear.

Test 1 

Test 5

Acceleration, g's

-10

-5

0

5

10

Planet gear tooth number

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35

Acceleration, g's

-10

-5

0

5

10

Fig. 11. Effect of meshing gear on planet 1 difference signals for 

planet gear tooth spall tests, for accelerometer 1 at 100% speed, 

torque, and mast loads.

a) Test 6 (planet gear tooth spall, in mesh with sun gear).

b) Test 7 (planet gear tooth spall, in mesh with ring gear).
NASA/TM—2011-217127 11

gear with a natural occurring spall defined on tooth 8, also 

oriented such that the spall was in mesh with the planet gears. 

Figure 13 gives the difference signals for accelerometer 1 of 

Tests 8 and 9 at 100 percent speed, torque, and mast loads. 

Figure 14 gives the M8A results.

The horizontal axis of Figure 13 is defined as the sun gear 

tooth number in mesh with planet 1 at the location of 

accelerometer 1. This, however, is fundamentally different than 

that for the planet gear analysis (Figs. 7, 9, and 11). For the 

planet gear analysis, every tooth of a planet gear was in mesh 

with the ring gear at a given location of an accelerometer due to 

the hunting tooth meshing action. Thus, the planet tooth vectors 

are made up of 35 distinct regions summed together with 

centers at each tooth number. For the sun analysis of the OH58C, only three sun gear teeth were in mesh with a given planet 

at a given accelerometer position. Figure 13, as an example, is 

made up of three distinct regions summed together with centers 

at sun gear tooth 3, 12, and 21 for accelerometer 1 and planet 1.

Thus, the sun gear results are less sensitized than the planet 

gears with respect to analysis of individual teeth. This made the 

sun gear tooth crack defect difficult to detect (Fig. 13a). Also, 

the difficulty to detect the sun gear tooth crack (as indicated by 

M8A in Fig. 14) was noticed for all accelerometers.

The sun gear spall defect was detectable, but not as apparent 

as the planet gear faults. A slight disturbance at tooth 7 in 

Figure 13b is seen (spalled sun in mesh with planet). Three 

more disturbances can be seen in the figure to correlate when 

the other planets were in mesh with the spalled sun tooth. This 

was due to the leakage effect as described in the planet defect 

section. Accelerometers 2 and 6 gave an indication of the sun 

tooth spall as indicated by M8A in Figure 14. It is not clear at 

this time as to why these accelerometers performed better than 

the others. The ability to detect the spall did not appear to 

correlate to when the spalled sun gear tooth was in direct 

alignment (through a planet) with an accelerometer.

Ring Gear Tooth Defects

Test 10 was run with a simulated ring gear tooth crack, 

25 percent in depth, on ring gear tooth 71, which was aligned 

with accelerometer 1. Test 11 was similar, but with a 

50 percent depth crack. Figure 15 gives the ring gear difference 

signals for accelerometer 1 at two different torque conditions, 

50 and 100 percent. The horizontal axis is defined as the ring 

gear tooth number in contact with planet 1. The data of the 

figure is time-averaged and the period is one-carrier rotation.

Due to the four passing planets within the carrier cycle, the 

fault was seen four times in the accelerometer 1 tooth vector.

This was seen when planet 1 was in mesh with ring teeth 22 

(planet 3 pass), 46 (planet 4 pass), 71 (planet 1 pass), and 96 

(planet 2 pass). The fault is more evident for the 50 percent

torque test conditions than the 100 percent torque condition.

Accelerometer number

A1 A2 A3 A4 A5 A6 A7 A8

M8A

0

500

1000

5000

10000

15000

20000

Fig. 12. Effect of spall and meshing gear on planet 1 M8A for all 

accelerometers, at 100% speed, torque, and mast loads. Test 1: 

baseline, healthy components; Test 6: Planet gear tooth spall at 

pitch line, in mesh with sun gear; Test 7: Planet gear tooth spall 

at pitch line, in mesh with ring gear.

Test 1 

Test 6

Test 7

Acceleration, g's

-10

-5

0

5

10

Sun gear tooth number

1 3 5 7 9 11 13 15 17 19 21 23 25 27

Acceleration, g's

-15

-10

-5

0

5

10

15

Fig. 13. Difference signals from sun gear tooth fault tests, for sun-toplanet 1 transfer path for accelerometer 1 at 100% speed, 

torque, and mast loads.

a) Test 8 (sun gear tooth crack, 25% depth).

b) Test 9 (sun gear tooth spall at pitch line).

Accelerometer number

A1 A2 A3 A4 A5 A6 A7 A8

M8A

0

500

1000

1500

2000

Fig. 14. Effect of sun gear tooth faults on M8A, for sun-to-planet 1 

transfer path, at 100% speed, torque, and mast loads. Test 1: 

baseline, healthy components; Test 8: sun gear tooth crack, 

25% depth; Test 9: sun gear tooth spall at pitch line.

Test 1

Test 8

Test 9
NASA/TM—2011-217127 12

For the other accelerometers, the ring gear crack was 

difficult to detect. Figure 16 gives the M8A results for all 

accelerometers at 100 percent speed, torque, and mast loads. As 

seen in the figure, accelerometer 1 shows the most detectability 

of the defect. For accelerometers not aligned with a cracked

ring gear tooth, fault detection was difficult. The only 

exception was accelerometer 7. This accelerometer gave a 

moderate indication of the defect. Accelerometer 7 was 

positioned 180° opposite accelerometer 1, and thus in direct 

contact with the defect through the ring, planets, and sun gears.

Planet Bearing Defects

Test 12 was run with a localized defect on the outer race of 

planet 1 that extended across both rows of the bearing race.

Test 13 was performed with a localized defect on the inner race 

of the planet 1 bearing that extended across both rows of the 

bearing race. Figure 17 shows the results from the enveloping 

technique for fault detection for the seeded-fault bearing tests. 

Figure 17a gives the vibration magnitude at the outer-race 

frequency, previously calculated as 175.6 Hz. A clear and 

distinct increase in vibration of the outer-race frequency is 

depicted for the planet bearing outer-race defect test (Test 12).

Figure 17b gives the vibration magnitude at the inner-race 

frequency, previously calculated as 258.7 Hz. A clear and 

distinct increase in vibration of the inner-race frequency is 

depicted for the planet bearing inner-race defect test (Test 13).

Therefore, for these tests, the enveloping technique provided an 

effective way to detect bearing race defects.

Figure 18 gives the results of the planet gear fault analysis 

for the data from the planet bearing fault tests. Shown is the 

M8A results of the planet tooth 1 vector difference signal, for 

all accelerometers at 100 percent, torque, and mast loads. No 

Acceleration, g's

-15

-10

-5

0

5

10

15

Ring gear tooth number in contact with planet 1

1 8 15 22 29 36 43 50 57 64 71 78 85 92 99

Acceleration, g's

-20

-10

0

10

20

Fig. 15. Effect of torque on difference signals for Test 11 (ring 

gear tooth crack, 50% depth), for accelerometer 1.

a) 50% torque, 100% speed and mast loads.

b) 100% torque, 100% speed and mast loads.

Accelerometer number

A1 A2 A3 A4 A5 A6 A7 A8

M8A

0

500

1000

1500

2000

Fig. 16. Effect of ring gear tooth defect on M8A for all 

accelerometers, at 100% speed, torque, and mast loads. Test 1: 

baseline, healthy components; Test 10: ring gear tooth crack, 

25% depth; Test 11: ring gear tooth crack, 50% depth.

Test 1

Test 10

Test 11

Accelerometer number

A1 A2 A3 A4 A5 A6 A7 A8

M8A

0

100

200

300

Fig. 18. Effect of planet bearing defect on M8A (relative to planet 1 

gear tooth fault) for all accelerometers, at 100% speed, torque, 

and mast loads. Test 1: baseline, healthy components; Test 12: 

planet bearing outer-race defect; Test 13: planet bearing innerrace defect.

Test 1

Test 12

Test 13

Magnitude, V

0

5000

10000

15000

Accelerometer number

A1 A2 A3 A4 A5 A6 A7 A8

Magnitude, V

0

5000

10000

15000

20000

Fig. 17. Enveloped spectrum vibration at bearing race frequencies 

for all accelerometers, at 100% speed, torque, and mast loads. 

Test 1: baseline, healthy components; Test 12: planet bearing 

outer-race defect; Test 13: planet bearing inner-race defect.

Test 1

Test 12

Test 13

a) At planet bearing outer-race frequency.

b) At planet bearing inner-race frequency.
NASA/TM—2011-217127 13

apparent trend is observed, which implies that the planet 

bearing fault does not give rise to a faulty indication of a planet 

gear tooth fault.

Blind Tests

In an effort to demonstrate the diagnostic capabilities 

developed, two blind demonstration tests were run. During 

these demonstration tests, the researcher processing the data 

had no prior knowledge as to the health of the transmission. It 

was established beforehand that the faults used in 

demonstration tests were to be limited to those previously run 

in seeded-fault Tests 1 through 13.

A fault detection procedure was developed prior to 

processing demonstration test results. This procedure used an 

“expert system” type methodology, where trends for each 

seeded-fault test were assembled and rules based on CI 

exceedance were compiled. The primary contribution for gear 

tooth fault detection was the M8A condition indicator based on 

the planet, sun, and ring gear tooth vectors. This was collected 

for all accelerometers and run conditions from each test. In 

addition, the vibration levels of the bearing race frequencies 

from the enveloped spectrums were used for bearing fault 

detection. Unique rules and thresholds for fault detection of 

each test case were developed.

Tests 14 and 15 were the two blind tests which were 

performed and analyzed using this methodology to predict 

which gearbox components were faulty. Test 14 used a planet 

gear with an EDM-simulated tooth crack, 50 percent depth, in 

planet 3, tooth 9, installed face up such that the crack opened 

when in mesh with the sun gear. This defect was correctly 

detected from the strong indications in the planet 3 tooth 

vectors and corresponding M8A results. Test 15 was the second 

blind test and consisted of the same all healthy components as 

the baseline Test 1. Unfortunately, a ring gear tooth crack 

defect was predicted from the analysis rather than a healthy 

transmission. The results were not distinctly indicative of a ring 

gear fault, but varied slightly enough from the baseline to give 

an erroneous false alarm prediction.

Conclusions

Studies were performed to demonstrate the capability to 

detect planetary gear and bearing faults in helicopter main-rotor 

transmissions. The work supported the Operations Support and 

Sustainment Technologies (OSST) program with the U.S. 

Army Aviation Applied Technology Directorate (AATD) and 

Bell Helicopter Textron. To support the OSST goal of maturing 

technology to reduce maintenance costs, experiments were 

performed on an OH-58C helicopter main-rotor transmission in 

the NASA Glenn 500-hp Helicopter Transmission Test 

Facility. Vibration data from the OH-58C planetary system 

were collected on a healthy transmission as well as with 

various seeded-fault components. Planetary fault detection 

algorithms were used with the collected data to evaluate fault 

detection effectiveness. A total of 15 tests were performed in 

the project. These consisted of tests on a healthy transmission 

and tests on different seeded-fault components such as planet 

tooth cracking, planet tooth spalling, sun tooth cracking, sun 

tooth spalling, ring tooth cracking, planet bearing spalling, and 

two blind demonstration tests. The following conclusion were 

derived:

1. Planet gear tooth cracks and spalls were detectable using 

the vibration separation techniques described in the 

paper.

2. Vibration during passing of a planet gear tooth defect 

bled through to other planets through the sun gear 

movement and was discernable by observation of the 

condition indicators from all planet tooth vectors.

3. A greater change in vibration was noticed when a 

machined crack was being closed during mesh rather than 

being opened during mesh. This was probably caused by 

the finite width of the machined crack/notch defect.

4. Sun gear tooth root cracks were not discernibly 

detectable from the vibration separation process.

5. Sun gear tooth spall defects were detectable.

6. Ring gear tooth cracks were only clearly detectable by 

accelerometers located near the crack location or directly 

across from the crack.

7. Enveloping provided an effective method for planet 

bearing inner- and outer-race spalling fault detection.

8. A planet tooth crack was correctly identified for the first 

blind demonstration test. Some scattered results for the 

second blind demonstration test prevented the correct 

identification of a healthy transmission.

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01-11-2011 

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Technical Memorandum 

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4. TITLE AND SUBTITLE

Planetary Gearbox Fault Detection Using Vibration Separation Techniques 

5a. CONTRACT NUMBER

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6. AUTHOR(S)

Lewicki, David, G.; LaBerge, Kelsen, E.; Ehinger, Ryan, T.; Fetty, Jason 

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WBS 877868.02.07.03.01.01.01 

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National Aeronautics and Space Administration 

John H. Glenn Research Center at Lewis Field 

Cleveland, Ohio 44135-3191 

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14. ABSTRACT

Studies were performed to demonstrate the capability to detect planetary gear and bearing faults in helicopter main-rotor transmissions. The 

work supported the Operations Support and Sustainment (OSST) program with the U.S. Army Aviation Applied Technology Directorate

(AATD) and Bell Helicopter Textron. Vibration data from the OH-58C planetary system were collected on a healthy transmission as well as 

with various seeded-fault components. Planetary fault detection algorithms were used with the collected data to evaluate fault detection 

effectiveness. Planet gear tooth cracks and spalls were detectable using the vibration separation techniques. Sun gear tooth cracks were not 

discernibly detectable from the vibration separation process. Sun gear tooth spall defects were detectable. Ring gear tooth cracks were only 

clearly detectable by accelerometers located near the crack location or directly across from the crack. Enveloping provided an effective 

method for planet bearing inner- and outer-race spalling fault detection.

15. SUBJECT TERMS

Gear teeth; Cracks; Transmissions (machine elements); Defects; Vibration 

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