Natural Fatigue Crack Initiation and Detection in High 

Quality Spur Gears

by David “Blake” Stringer, Ph.D., Kelsen E. LaBerge, Ph.D., 

Cory J. Burdick, and Brendan A. Fields

ARL-RP-0383 June 2012

A reprint from the 68th American Helicopter Society Annual Form, FT Worth, TX, May 2012.

Approved for public release; distribution unlimited. 
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Army Research Laboratory 

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ARL-RP-0383 June 2012 

Natural Fatigue Crack Initiation and Detection in High 

Quality Spur Gears 

David “Blake” Stringer and Ph.D., Kelsen E. LaBerge, Ph.D. 

Vehicle Technology Directorate, ARL 

Cory J. Burdick, and Brendan A. Fields 

United States Military Academy,

West Point, New York 

A reprint from the 68th American Helicopter Society Annual Form, FT Worth, TX, May 2012. 

Approved for public release; distribution unlimited. 
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Natural Fatigue Crack Initiation and Detection in High Quality Spur Gears

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David “Blake” Stringer, Ph.D., Kelsen E. LaBerge, Ph.D., Cory J. Burdick, and 

Brendan A. Fields

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14. ABSTRACT

There is a desire in the rotorcraft community to transition to an “on condition” maintenance program. This requires the ability 

to detect the presence of faults before failure. Gear-tooth root-cracks are of particular concern in the drive system, for they are 

generally difficult to detect until a crack has progressed to the point where catastrophic failure is eminent. Many diagnostics 

are developed using experimental data generated from specimens with machined, seeded faults rather than naturally developed 

cracks. The study presented here develops a methodology for seeding natural tooth root-cracks in gears for use in diagnostic 

experiments. Fatigue cracks are generated on a load frame and then test gears are run on a rotating fatigue rig instrumented 

with accelerometers. Data presented shows that by comparing baseline healthy vibration data to cracked gear data, damage can

be detected with commonly used condition indicators. However, these indicators were not able to show propagation of the 

crack because the load capability of the contact fatigue rig was insufficient and propagation did not occur.

15. SUBJECT TERMS

Gears, fatigue cracks, condition based maintenance

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David B. Stringer

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Standard Form 298 (Rev. 8/98) 

Prescribed by ANSI Std. Z39.18 
Natural Fatigue Crack Initiation and Detection in High Quality Spur Gears 

David “Blake” Stringer, Ph.D. and Kelsen E. LaBerge, Ph.D.

US Army Research Laboratory 

Cleveland, Ohio 

david.b.stringer@nasa.gov, kelsen.laberge@nasa.gov

Cory J. Burdick and Brendan A. Fields 

United States Military Academy, 

West Point, New York 

ABSTRACT 

There is a desire in the rotorcraft community to transition to an “on condition” maintenance program. This requires the 

ability to detect the presence of faults before failure. Gear-tooth root-cracks are of particular concern in the drive system, for 

they are generally difficult to detect until a crack has progressed to the point where catastrophic failure is eminent. Many 

diagnostics are developed using experimental data generated from specimens with machined, seeded faults rather than 

naturally developed cracks. The study presented here develops a methodology for seeding natural tooth root-cracks in gears 

for use in diagnostic experiments. Fatigue cracks are generated on a load frame and then test gears are run on a rotating 

fatigue rig instrumented with accelerometers. Data presented shows that by comparing baseline healthy vibration data to 

cracked gear data, damage can be detected with commonly used condition indicators. However, these indicators were not 

able to show propagation of the crack because the load capability of the contact fatigue rig was insufficient and propagation 

did not occur. 

INTRODUCTION 

The rotorcraft community is transitioning toward a 

condition based maintenance (CBM) approach where 

diagnostic equipment provides real-time component health 

information on vital drive components such as gears and 

bearings. Current technology in drive system diagnostics is 

not reliable in identifying fatigue cracks in drive gears 

before catastrophic failure occurs. In order to facilitate a 

CBM program without compromising the safety of the 

equipment and operators, the diagnostic technology must be 

able to provide early warning of fatigue failure using sensing 

equipment readily available to the aviation industry. 

Historically, research conducted in gear tooth fatigue cracks 

has been done with artificially created machined notches at 

the root of the tooth. Combining a single gear-tooth 

bending-fatigue capability with a high-speed, gear mesh 

contact-fatigue rig allows for the initiation of naturally 

occurring fatigue cracks followed by propagation analysis in 

a rotating mesh apparatus. The objective of this experiment 

was to initiate a natural fatigue crack and detect its presence 

before a drive-system-compromising failure occurred by 

 

Presented at the American Helicopter Society 68th Annual Forum, 

Ft. Worth, Texas, May 1-3, 2012. This is a work of the U.S. 

Government and is not subject to copyright protection in the U.S. 

analyzing condition indicators within the vibration signals 

received by high speed accelerometers. 

Gears are designed to fail as a result of wear and pitting 

on the surface of the tooth before failure of the tooth or rim. 

This is because tooth and rim failure is often catastrophic 

and results in damage or destruction of the machine in which 

it operates. This study is focused specifically on tooth and 

rim failure. 

Significant research has been done regarding crack 

initiation and propagation; however, until recently this has 

been artificially induced by machining notches at the base of 

the tooth [1]. While this method allows for the measurement 

of crack propagation, it does not accurately simulate fatigue 

failure in a gear tooth, nor does the resulting vibration signal 

depict that of a true naturally-grown crack. 

Studies have been conducted at both the NASA Glenn 

Research Center and the Rochester Institute of Technology 

regarding natural fatigue-crack initiation using a high-cycle 

gear-tooth bending-fatigue apparatus and measuring the 

propagation of these cracks while being loaded in a gearmesh fatigue rig [2,3]. The method used in detection and 

analysis of the cracks uses condition indicators (CIs) derived 

from the vibration of the gearbox. These CIs are well 

documented in literature, and their methods are described in 

[4-6] including the process of obtaining signal averages and 
the process of filtering such averages. The FM4 and M8A 

CIs, discussed in this paper, are designed to indicate 

localized damage on a small number of teeth.

In this experiment, the gears are “broken in” on a gear 

contact-fatigue rig to obtain the gear-pair’s baseline 

vibration characteristics. The crack is then initiated on a 

high-cycle single gear-tooth bending rig. The cracked gear 

is then returned to the rotating contact-fatigue rig, where 

vibration signals are collected and then analyzed. This 

method of crack initiation and detection has the potential to 

provide insight into the shortcomings of the crack detection 

methods currently used in onboard health and usage 

monitoring systems (HUMS), as well as the timeframes 

associated between crack initiation and catastrophic failure 

of the gear tooth itself. 

TEST APPARATUS

The experiments for this paper were conducted using 

two test rigs at NASA Glenn Research Center. The highspeed Single Gear-Tooth Bending Test Facility was used in

order to quickly generate a naturally grown tooth root crack. 

This facility consists of a 1000 Hz High-Cycle Fatigue Test 

System produced by the MTS Systems Corporation with a 

special test head shown in Figure 1. The load arm applies a 

cyclic load to the test tooth at the highest point of single 

tooth contact. A reaction tooth, located 2 teeth from the test 

tooth, counteracts the applied load through the reaction 

anvil, which contacts the reaction tooth at its root [2]. For 

ease of discussion, this rig will be referred to as the tooth 

bending rig for the duration of this paper.

The NASA Glenn Research Center Spur-Gear Fatigue 

Test Rigs (referred to here as rotating fatigue rigs) have been 

used for more than 30 years to test new lubricants and loss 

of oil characteristics, with an emphasis on the study of gear 

contact-fatigue. These rigs, shown in Figure 2, use the foursquare or torque regenerative principle such that the drive 

motor need only overcome the frictional losses in the 

system. Torque is applied hydraulically as shown in Figure 

2 (b). Oil pressure and leakage replacement flow is supplied 

to the load vanes within the right side slave gear through a 

shaft seal. As the oil pressure is increased on the load vanes 

inside the slave gear, torque is applied to the shaft. This 

torque is then transmitted to the test gears. The load on the 

gear teeth can be changed by altering the flow rate on the 

pump. The loading system also allows the rig to be started 

under no load and gradually increase the load while running. 

Complete specs on this apparatus can be found in [7]. Both 

the rotating fatigue rigs and the tooth bending rig utilize the 

same test gears, the design specifications for which are 

shown in Table 1. The gears utilized in the experiments 

presented here were manufactured from 9310 gear steel. 

PCB accelerometers were mounted on the right side 

bearing housing between the test and slave gear. While data 

from 2 accelerometers (one top mounted and one side 

mounted) was obtained, only data from the top 

accelerometer is discussed here. Speed was also recorded 

using an optical tachometer aimed at the rear side of the belt 

pulley (see Figure 2 (b)) on which a single white line is 

painted. Data was collected from these sensors in 1 second 

acquisitions every 60 seconds at 200 kHz for the majority of 

the tests. Using NASA’s Mechanical Diagnostics System 

Software, data was both recorded and processed after each 

acquisition, and gear CIs were recorded in a database. Due 

to the close proximity of the slave gears on this particular 

rig, the slave gear meshing frequency and harmonics (at 

multiples of 35 shaft orders including 1st order sidebands) 

were removed in addition to the test gear meshing 

frequency, harmonics, and 1st order sidebands. Gear CIs

were calculated from the filtered signal average. 

Figure 1. NASA/ARL Signal Gear-Tooth Bending 

Test Facility [2].

Table 1. Spur gear specimen design parameters [8].

Characteristic Dimension

Number of Teeth 28

Diametral Pitch (1/in.) 8

Circular pitch (in.) 0.3927

Whole depth (in.) 0.3

Addendum (in.) 0.125

Chordal tooth thickness ref (in.) 0.191

Pressure angle (deg.) 20

Pitch diameter (in.) 3.5

Outside diameter (in.) 3.75

Root fillet (in.) 0.040 - 0.060

Measurement over pins (in.) 3.7867 - 3.7915

Pin diameter (in.) 0.216

Backlash reference (in.) 0.01

Tip relief (in.) 0.010 to 0.015
(a)

(b)

Figure 2. Schematic and cutaway views of the NASA 

Glenn Research Center gear fatigue test apparatus [8].

METHODOLOGY

Four crack initiation and propagation experiments were 

performed. The first experiment was used to establish the 

parameters for crack initiation, the second for method 

development, and the third and fourth for method validation. 

Before cracks were initiated, the gears were run on the 

rotating fatigue rig for up to 2 hours, running until CIs level 

off from the initial startup transient. 

For the first experiment, baseline data was taken at 50 

psig load pressure (6.2 ft-lb torque) and 7500 rpm. After the 

initial break-in period, the gear was placed in the bending 

fatigue rig and the first tooth was cracked. To crack the 

tooth, the load applied to the test tooth was cycled between 

200 and 3060 lb at 100 Hz. To prevent complete fracture of 

the tooth, the rig was set to shutdown when the maximum 

displacement of the tooth increased 2% from the initial 

maximum displacement. This initial test yielded a large 

crack measuring 2.4 mm in the tooth. The gear was then 

placed back into the rotating rig, but tooth fracture occurred 

before the rotating fatigue rig reached the set speed. The 

undamaged teeth on this original specimen were then used to 

determine a better method for initiating cracks on the tooth 

bending rig (see Figure 3). After several attempts, it was 

determined that the most reliable way to generate a tooth 

crack is to set the initial displacement limit at 2%, manually 

stopping the rig when a crack is visible. If the displacement 

limit is reached before crack initiation occurs, the 

displacement limit is increased and cyclic loading is 

continued until a crack is visible.

During the second experiment, the gear was run on the 

rotating fatigue rig at 50 psig load pressure (6.2 ft-lb torque) 

and 7500 rpm for break-in followed by crack initiation on 

the tooth bending rig. The crack initiation methodology 

discussed above was used to generate a 2.4 mm root crack. 

The gear was then returned to the rotating fatigue rig. 

Several tests were run on the gear at increasing speeds and 

loads. Little or no propagation of the crack was recorded. 

For the third series, it was determined that in order to 

get results in a timely manner the mesh fatigue rig must be 

run at maximum load. Thus the break-in for this gear was 

run at 400 psig (58 ft-lb torque) and 10,000 rpm. After the 

break-in period, the crack was initiated in the same manner 

as the previous test. The gear was then run in the rotating 

fatigue rig. This test was run with the gears offset such that 

the contact area was cut in half. No visible crack 

propagation was recorded. However, significant pitting was 

experienced after running for approximately 12 hours.

Similarly, experiment 4 was run at 400 psi (58 ft-lb 

torque) at 7600 RPM for 2 hours with no offset. A crack

was generated with a depth of 1.5 mm. The gear was 

returned to the rotating fatigue rig and continued to run for 

over 70 hours with no increase in crack length. 

During each of these experiments, the test gear 

containing the crack was installed on the right side as 

viewed from the front (see Figure 2 (a)). A new mating (left 

side) gear was also installed at the beginning of each 

baseline experiment. It is also important to note that the 

crack generated in each of these experiments was visible on 

both sides of the gear.

RESULTS

In experiment 1, the initial crack was too large and 

resulted in tooth failure during the start-up of the rotating 

fatigue rig. Figure 3 shows the gear after validating the 

crack-initiation procedure. While no statistical data could be 

obtained from this experiment, it did yield the methodology 

for further crack initiation.
Figure 3. 1st Gear used in the development of a crack 

initiation methodology. Photos (a-g) represent test teeth 

1-7.

The crack generated in experiment 2 is shown in Figure 

4. Vibration data, taken during the rotating fatigue rig 

portion of this test, was processed to obtain the filtered 

signal averages both before and after crack initiation (Figure 

5 (a) and (b)). Filtered signals are created by removing the 

mesh frequencies and harmonics as well as their first order 

sidebands from the signal average. The presence of the 

crack can be seen in Figure 5 (b) at approximately 210 

degrees where there is a sudden spike. The frequency 

spectrum of the signal average before filtering is shown in 

Figure 5 (c) and (d). The meshing frequency of the test 

gears can be seen at 28 shaft orders with the first harmonic 

at 56 shaft orders. Note the increase in the higher order 

sidebands in the cracked frequency spectrum. These 

sidebands contribute to the increase in the cracked FM4 CI 

to 4.5 from a 2.9 baseline and an increase in the M8A CI 

from 128 to 935. 

Figure 4. Cracked tooth after experiment 2.

Figure 5. Experiment 2 results at 7500 RPM and 50 psi.

The cracked gear in experiment 2 was run for 

approximately 14.5 hours at increasing speeds and loads and 

experienced a crack depth increase of only 0.5 mm. While 

the crack may have been detectable in the data taken during

these higher loads and speeds, baseline data was not 

recorded at these conditions for comparison. After 8 hours, 

the gear was also offset, changing the contact area to half the 

facewidth. Visible surface fatigue damage was generated 

while running at higher loads toward the end of experiment 

2. Pitting on this gear set is shown in Figure 6. 

Figure 6. Pitting damage on test gear after experiment 2.

Results from experiment 3 are questionable in that the 

gears were mounted incorrectly for the first 10 hours on the 

rotating fatigue rig (including baseline). The gear-pair was 

mounted such that the bending-fatigue crack was on the 

opposite side of the tooth, and therefore the load transfer 

between gears was closing the crack versus continuing to a) b) c) d)

e) f) g)

0 4 5 9 0 135 180 225 270 315 360

-10

0

1 0

 

0 4 5 9 0 135 180 225 270 315 360

-10

0

1 0

 

0 2 0 4 0 6 0 8 0 100

0

2

4

 

0 2 0 4 0 6 0 8 0 100

0

2

4

 

a) Filtered Signal Average - Baseline

b) Filtered Signal Average - Cracked

c) Spectrum of Signal Average - Baseline

d) Spectrum of Signal Average - Cracked

Rotation (deg)

Rotation (deg)

Frequency (Shaft Orders)

Frequency (Shaft Orders)

Acceleration

(g’s)

Magnitude

(g’s)

Acceleration

(g’s)

Magnitude

(g’s)
force it open. Upon discovery, the gear was reversed for the 

last 120 minutes of testing. However, by that time contact 

fatigue damage was already evident on the “wrong” side of 

the test gear tooth and had spread to the meshing gear (left 

side gear). Since the mating gear was not flipped, the test 

gear was reinstalled in mesh with an already pitted gear, 

making it nearly impossible to discern between damage due 

to pitting and damage due to cracking. 

Figure 7 shows the filtered signal averages at 4 separate 

points during testing. Figure 7 (a) and (b) show the baseline 

data and data taken soon after the gear was installed 

(incorrectly) after crack initiation respectively. Data taken 

just before flipping the test gear over is shown in Figure 7 

(c), with pitting damage expected between 120 and 270 

degrees. The mating gear had already incurred pitting 

damage at this point and was not flipped. Therefore, the 

pitting damage on the mating gear is visible in the vibration 

signal taken shortly after reinstalling the test gear in the 

correct orientation (Figure 7 (d)). This makes crack 

identification difficult. A CI comparison between data 

shown in (a) and (d) shows an obvious increase, but would 

be an indicator of pitting and not the presence of a crack. 

Figure 7. Filtered signal averages for experiment 3 

(10,000 RPM, 400 psi) at several points during the 

experiment with the crack forced open and closed.

Filtered signal averages are shown in Figure 8 (a) and 

(b) for experiment 4. This experiment utilized a different 

PCB accelerometer with a higher frequency range (up to 60 

kHz). It was installed, as in the previous experiments, on the 

bearing housing, however, a different housing was used, 

which could account for the different vibration 

characteristics. The presence of the crack is visible in Figure 

8 (b) in the first 45 degrees of rotation. The frequency 

spectrum of the averaged signal for both the baseline and 

cracked cases indicates a dominating vibration at 

approximately 35 shaft orders. This indicates a strong slave 

gear meshing vibration. Regardless of this resonance, CIs 

for this particular experiment show a definite increase: 2.2 

to 4.2 for FM4 and 23 to 419 for M8A.

After over 70 hours of runtime 7600 RPM and 400 psi 

load, there was no visible increase in crack depth. This can 

be seen in Figure 9, which shows the baseline tooth root 

along with the initialized crack before installation into the 

rotating rig and crack after 70 hours of runtime.

Figure 8. Experiment 4 results at 7600 RPM and 400 psi 

load.

Figure 9. Test tooth root from experiment 4 before 

testing (a), after crack initiation (b), and after 70 hours 

of testing (c).

0 45 90 135 180 225 270 315 360

-100

0

100

 

0 45 90 135 180 225 270 315 360

-100

0

100

 

0 45 90 135 180 225 270 315 360

-100

0

100

0 45 90 135 180 225 270 315 360

-100

0

100

 

a) Baseline

b) Cracked (Closed) 

c) Cracked (Closed) – Pitting

d) Cracked (Open) - Pitting

Rotation (deg)

Rotation (deg)

Rotation (deg)

Acceleration

(g’s)

Acceleration

(g’s)

Rotation (deg)

Acceleration

(g’s)

Acceleration

(g’s)

0 4 5 9 0 135 180 225 270 315 360

-50

0

5 0

 

0 4 5 9 0 135 180 225 270 315 360

-50

0

5 0

 

0 2 0 4 0 6 0 8 0 100

0

1 0

2 0

 

0 2 0 4 0 6 0 8 0 100

0

1 0

2 0

 

a) Filtered Signal Average - Baseline

b) Filtered Signal Average - Cracked

c) Spectrum of Signal Average - Baseline

d) Spectrum of Signal Average - Cracked

Rotation (deg)

Rotation (deg)

Frequency (Shaft Orders)

Acceleration

(g’s)

Magnitude

(g’s)

Acceleration

(g’s)

Magnitude

(g’s)

Frequency (Shaft Orders)

a) b) c)
CONCLUSIONS 

A methodology for initiating a natural gear-tooth 

bending fatigue crack using a new high-cycle, single geartooth bending fatigue rig is defined, efficiently creating a 

naturally initiated crack without the removal of material. 

However, the rotating fatigue apparatus utilized in this study 

is not ideal for this particular application. The drawbacks of 

this facility include the close proximity of the slave gears as 

well as limited loading capabilities. The close proximity of 

the slave gears in conjunction with the number of gear teeth 

on the slave gears being similar to that of the test gears 

complicates fault detection. However, in rotorcraft 

applications, clean vibration signals are not typical and in 

this application, the CIs were still able to identify the 

damage. 

Additionally, the loading capabilities of this facility are 

not sufficient to propagate a crack in a reasonable amount of 

time. New experimental facilities are available for future 

tests that both increase the loading capability and separate 

the slave and test sections. Running comparable 

experiments on such equipment will allow engineers to gain 

insight into the vibration characteristics associated with 

natural tooth root cracks and the timeframe between crack 

initiation and detection and catastrophic failure of the gear 

tooth. 

REFERENCES 

1. Lewicki, D.G., Ballarini, R. 1996, “Gear Crack 

Propagation Investigations,” NASA TM 107147.

2. Stringer, D. B., Dykas, B. D., LaBerge, K. E., 

Zakrajsek, A.J., Handschuh, R. F., 2011, “A New HighSpeed, High-Cycle, Gear-Tooth Bending Fatigue Test 

Capability,” Proc. of the 67th American Helicopter 

Society Forum, May 3-5, 2011, Virginia Beach, VA. 

3. Nenadic, N. G., Wodenscheck, J. A., Thurston, M. G., 

Lewicki, D.G., 2011, “Seeding Cracks Using a Fatigue 

Tester for Accelerated Gear Tooth Breaking,” NASA 

TM 2011-216983. 

4. Zakrajsek, J.J., and Lewicki, D.G., 1998, “Detecting 

Gear Tooth Fatigue Cracks in Advance of Complete 

Fracture,” Tribotest, 4(4), pp. 407–422.

5. Lebold, M., Mcclintic, K., Campbell, R., Byington, C., 

and Maynard, K., 2000, “Review of Vibration Analysis 

Methods for Gearbox Diagnostics and Prognostics,” 

Proc. 54th Meeting of the Society for Machinery Failure 

Prevention Technology, Virginia Beach VA, pp. 623–

634.

6. Samuel, P.D., and Pines, D.J., 2005, “A Review of 

Vibration-Based Techniques for Helicopter 

Transmission Diagnostics,” Journal of Sound and 

Vibration, 282(1–2), pp. 475–508.

7. Krantz, T.L., and Handschuh, R.F., “A Study of Spur 

Gears Lubricated With Grease-Observations From 

Seven Experiments,” Proceedings of the 58th Meeting 

of the Society for Machinery Failure Prevention 

Technology, Virginia Beach, VA, April 2004. 

8. Zakrajsek, J.J., Townsend, D.P., Decker, H.J., “An 

Analysis of Gear Fault Detection Methods as Applied to 

Pitting Fatigue Failure Data,” Proceedings of the 47th

meeting of the Mechanical Failure Prevention Group, 

Virginia Beach, Virginia, 1993. 

9. Townsend, D., Chevalier, J., Zaretsky, E., 1973, “Pitting 

Fatigue Characteristics of AISI M 50 and Super 

Nitralloy Spur Gears,” NASA TN D–7261. 
26

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