Failure Analysis of OH-58 Kiowa Warrior 

Drive Gear Shafts 

by Scott Grendahl, Victor Champagne, and Dan Snoha 

ARL-TR-3827 June 2006 

prepared by 

U.S. Army Research Laboratory 

Weapons and Materials Research Directorate 

Aberdeen Proving Ground, MD 21005-5069 

for 

U.S. Army Research, Development, and Engineering Command 

Aviation Engineering Directorate 

Propulsion Division 

Aberdeen Proving Ground, MD 21010-5424 

Approved for public release; distribution is unlimited. 
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Destroy this report when it is no longer needed. Do not return it to the originator. 
Army Research Laboratory 

Aberdeen Proving Ground, MD 21005-5069 

ARL-TR-3827 June 2006 

Failure Analysis of OH-58 Kiowa Warrior 

Drive Gear Shafts 

Scott Grendahl, Victor Champagne, and Dan Snoha 

Weapons and Materials Research Directorate, ARL 

prepared by 

U.S. Army Research Laboratory 

Weapons and Materials Research Directorate 

Aberdeen Proving Ground, MD 21005-5069 

for 

U.S. Army Research, Development, and Engineering Command 

Aviation Engineering Directorate 

Propulsion Division 

Aberdeen Proving Ground, MD 21010-5424 

Approved for public release; distribution is unlimited.
ii

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Failure Analysis of OH-58 Kiowa Warrior Drive Gear Shafts 

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M042-489Y31 

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Scott Grendahl, Victor Champagne, and Dan Snoha 

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U.S. Army Research Laboratory 

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Aberdeen Proving Ground, MD 21005-5069 

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U.S. Army Research, Develoment, and Engineering Command 

Aviation Engineering Directorate 

Propulsion Division 

Aberdeen Proving Ground, MD 21010-5424 

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13. SUPPLEMENTARY NOTES 

14. ABSTRACT 

A 15-hp spare drive gear from a 250-C30/R3 engine on an OH-58 Kiowa Warrior failed in service, causing loss of AC power. 

The failed gear, all recovered teeth, and the driving pinion were sent to the U.S. Army Research Laboratory for investigation. 

This investigation focused on determining the mode of failure and any possible metallurgical or manufacturing contribution to 

this failure. Analysis included optical and scanning electron microscopy, chemical analysis, and measurements of hardness and 

surface finish. 

15. SUBJECT TERMS

failure analysis, 9310 steel, fatigue, helicopter gear 

16. SECURITY CLASSIFICATION OF: 

19a. NAME OF RESPONSIBLE PERSON

Scott Grendahl 

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UNCLASSIFIED 

b. ABSTRACT 

UNCLASSIFIED

c. THIS PAGE 

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OF ABSTRACT 

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410-306-0819 

Standard Form 298 (Rev. 8/98) 

Prescribed by ANSI Std. Z39.18
iii

Contents 

List of Figures iv

List of Tables vii

1. Introduction 1

2. Discussion 1

3. Optical Microscopy 2

4. Scanning Electron Microscopy 20

5. Hardness 41

6. Metallography 44

7. Edge Break 44

8. Chemical Analysis 46

9. Surface Finish 46

10. Residual Stress 48

11. Recommendations 48

12. Conclusions 48

Distribution List 50 
iv

List of Figures 

Figure 1. Location of recovered teeth on spare drive gear. (Gear is driven 

counterclockwise.) .....................................................................................................................1 

Figure 2. Tooth 1, tooth fracture half..............................................................................................3 

Figure 3. Tooth 1, gear fracture half...............................................................................................3 

Figure 4. Primary origin, drive side of tooth 1, tooth fracture half.................................................4 

Figure 5. Coast side of tooth 1, tooth fracture half. ........................................................................4

Figure 6. Tooth 2, tooth fracture half..............................................................................................5 

Figure 7. Tooth 2, gear fracture half...............................................................................................5 

Figure 8. Tooth 3, tooth fracture half..............................................................................................6 

Figure 9. Tooth 3, gear fracture half...............................................................................................6 

Figure 10. Tooth 4, tooth fracture half............................................................................................7 

Figure 11. Tooth 4, gear fracture half.............................................................................................7 

Figure 12. Tooth 28, tooth fracture half..........................................................................................8 

Figure 13. Tooth 28, gear fracture half...........................................................................................9 

Figure 14. Tooth 29, tooth fracture half..........................................................................................9 

Figure 15. Tooth 29, gear fracture half.........................................................................................10 

Figure 16. Tooth 30, tooth fracture half........................................................................................10 

Figure 17. Tooth 30, gear fracture half.........................................................................................11 

Figure 18. Tooth 31, tooth fracture half........................................................................................11 

Figure 19. Tooth 31, gear fracture half.........................................................................................12 

Figure 20. Drive side, tooth 1. ......................................................................................................12 

Figure 21. Coast side wear, tooth 1...............................................................................................13 

Figure 22. Top of tooth 1. .............................................................................................................13 

Figure 23. Drive side, tooth 18. ....................................................................................................14 

Figure 24. Coast side, tooth 18. ....................................................................................................14 

Figure 25. Top of tooth 18. ...........................................................................................................15 

Figure 26. Drive side, tooth 31. (Note: worn adjacent to fracture surface.) ...............................15 

Figure 27. Coast side, tooth 31. ....................................................................................................16 

Figure 28. Top of tooth 31. ...........................................................................................................16 

Figure 29. Typical tool marks, tooth root fillet, between teeth 15 and 16....................................17 

Figure 30. Typical tool marks, tooth root fillet, between teeth 16 and 17....................................17 
v

Figure 31. Typical tool mark depth, tooth root, measurement no. 1.............................................18 

Figure 32. Typical tool mark depth, tooth root, measurement no. 2.............................................18 

Figure 33. Drive side idler gear. ...................................................................................................19 

Figure 34. Coast side idler gear. ...................................................................................................19 

Figure 35. Tooth 1.........................................................................................................................21 

Figure 36. Tooth 1 drive side origin. ............................................................................................21 

Figure 37. Tooth 1 drive side pit origin site..................................................................................22 

Figure 38. Tooth 1 primary origin, drive side gear fracture half. .................................................22 

Figure 39. Transgranular morphology, tooth 1 drive side. ...........................................................23 

Figure 40. Transgranular morphology, tooth 1 coast side. ...........................................................23 

Figure 41. Overload transition, tooth 1.........................................................................................24 

Figure 42. Tooth 2 drive side primary pit origin, tooth half.........................................................24 

Figure 43. Drive side, tooth 2, gear half, origin location. (Note the damage.)............................25 

Figure 44. Drive side, tooth 3, primary origin gear half...............................................................25 

Figure 45. Tooth 4 drive side primary pit origin. .........................................................................26 

Figure 46. Tooth 4 drive side primary pit origin, gear fracture half.............................................26 

Figure 47. Tooth 4 drive side........................................................................................................27 

Figure 48. Tooth 28 fracture surface.............................................................................................27 

Figure 49. Tooth 28 drive side, primary origin.............................................................................28 

Figure 50. Tooth 28 drive side, primary pit origin. ......................................................................28 

Figure 51. Tooth 28 drive side origin site, gear fracture half. ......................................................29 

Figure 52. Tooth 28 fracture surface, transition to overload at top. .............................................29 

Figure 53. Tooth 29 drive side origin area....................................................................................30 

Figure 54. Tooth 29 drive side origin, site no. 1...........................................................................30

Figure 55. Tooth 29 drive side origin, site no. 2...........................................................................31

Figure 56. Tooth 29 drive side origin, site no. 3...........................................................................31

Figure 57. Tooth 29 drive side, primary pit origin. ......................................................................32 

Figure 58. Tooth 29 drive side origin, site no. 4...........................................................................32

Figure 59. SEM tooth 30 fracture surface.....................................................................................33 

Figure 60. Tooth 30 origin, gear fracture half. .............................................................................33

Figure 61. Tooth 30 pit origin, gear fracture half. ........................................................................34 

Figure 62. Tooth 30 fracture surface, drive side...........................................................................34 

Figure 63. Tooth 30 fracture surface, coast side...........................................................................35 

Figure 64. SEM tooth 31 fracture surface.....................................................................................35 
vi

Figure 65. Tooth 31 origin............................................................................................................36 

Figure 66. Tooth 31 fracture surface.............................................................................................36 

Figure 67. Tooth 31 fracture surface, showing cyclic overload fracture......................................37 

Figure 68. Tooth 31 fracture surface, ductile overload.................................................................37 

Figure 69. Tool marks, tooth 28 root, gear half............................................................................38 

Figure 70. Coast side tool marks, tooth 28 root, gear half. (Note the undercut parallel to tool 

marks.) .....................................................................................................................................38 

Figure 71. Tooth 28 root, gear half, coast side cracking along tool marks...................................39 

Figure 72. Tooth 29 root, gear half, coast side tool marks. ..........................................................39 

Figure 73. Tooth 29 root, gear half, coast side tool marks. ..........................................................40 

Figure 74. Tooth 29 root, gear half, coast side tool marks, cracking along tool marks................40 

Figure 75. Tooth 29 root, gear half, coast side, cracking along tool marks..................................41 

Figure 76. Hardness data tooth and root profiles..........................................................................43 

Figure 77. Metallographic cross section demonstrating absence of continuous carbide 

networks...................................................................................................................................44 

Figure 78. Edge break measurement, tooth 28. ............................................................................45 

Figure 79. Edge break measurement, tooth 29. ............................................................................45 

Figure 80. Surface roughness measurement. ................................................................................47 

Figure 81. Optical photograph of shot peened surface finish. ......................................................47 
vii

List of Tables 

Table 1. Hardness measurements, tooth 3, pitch line. ..................................................................42 

Table 2. Hardness measurements, tooth 17, pitch line. ................................................................42 

Table 3. Hardness measurements tooth root.................................................................................43 

Table 4. Chemical composition of 15-hp spare drive gear. ..........................................................46 
viii

INTENTIONALLY LEFT BLANK. 
1

1. Introduction 

A 15-hp spare drive gear from a 250-C30/R3 engine on an OH-58 Kiowa Warrior failed in 

service, causing loss of AC power. The failed gear, all recovered teeth, and the driving pinion 

were sent to the U.S. Army Research Laboratory for investigation. This investigation focused on 

determining the mode of failure and any possible metallurgical or manufacturing contribution to 

this failure. Analysis included optical and scanning electron microscopy, chemical analysis, and 

measurements of hardness and surface finish. 

The objective of this report is root cause determination for the 15-hp spare drive gear failure. 

2. Discussion 

All teeth were stripped or separated from the failed gear; many fracture surfaces were too marred 

to yield useful information. Thirteen teeth were recovered essentially intact. The recovered 

teeth were successfully matched to their corresponding location on the gear by examination of 

the tooth geometry and fracture topography. The location of each tooth relative to the others is 

shown in figure 1. 

Figure 1. Location of recovered teeth on spare drive gear. (Gear is 

driven counterclockwise.)
2

Failure occurs due to very high cycle bending fatigue of the initial tooth. The next tooth in the 

meshing sequence experiences impact loading due to the momentary loss of contact with the 

driving pinion. Bending fatigue failure occurs progressively faster for each subsequent tooth. 

The initial tooth fails in very high cycle fatigue while the last tooth fails in low cycle fatigue. 

After loss of sufficient teeth, transmission error becomes so large that the gears do not mesh 

properly, causing rapid failure of the remaining gear teeth and damage to the pinion teeth as well. 

Failure order was established by fractographic examination. Optical and scanning electron 

microscope (SEM) examination of the fracture surfaces support the failure scenario presented.

Two groups of teeth failed sequentially in bending fatigue, teeth 1–4 and teeth 28–31. While it 

was not possible to determine with absolute certainty which group began to fail first, it is clear 

that within each group failure occurred in the order stated. Based on the extremely small 

overload region on tooth 1 (much smaller than that of tooth 28) and fractography it is suspected 

that group 1–4 began to fail first. It is also clear based on fractography that the two groups were 

failing simultaneously. 

Pit origins were observed at teeth 1, 2, 4, 28, and 30. Prominent tool marks were observed at the 

origins of teeth 3, 29, and 31. 

3. Optical Microscopy 

Components were examined optically using a Nikon SMZ-1500 zoom stereo microscope. 

Particular attention was given to fracture morphology and wear on both the failed gear and 

mating pinion. 

Facture surfaces for tooth 1 are shown in figures 2–5. The primary origin for this tooth is shown 

in figure 4. Propagation was by fatigue with a secondary crack initiating on the coast side. A 

very small overload region was present, as depicted in figure 5. 

Figures 6 (tooth half) and 7 (gear half) show the fracture surfaces observed on tooth 2. Note the 

more pronounced beach marks relative to tooth 1. 

Figures 8 and 9, respectively, present the tooth and gear fracture halves of tooth 3. The overload 

region was significantly larger than on the preceding teeth. 

The fracture halves of tooth 4 are presented in figure 10 (tooth half) and figure 11 (gear half). 

The beachmarks exhibited a “U” or bathtub shape. A large mound can be seen in figure 10; the 

mating cavity can be seen in figure 11. 
3

 Primary Origin

Coast 

Side 

Drive 

Side 

Figure 2. Tooth 1, tooth fracture half. 

 Primary Origin 

Coast 

Side 

Drive 

Side 

Figure 3. Tooth 1, gear fracture half.
4

Figure 4. Primary origin, drive side of tooth 1, tooth fracture half. 

Coast 

Side 

Fatigue 

Overload 

Drive 

Side 

Fatigue 

Figure 5. Coast side of tooth 1, tooth fracture half. 
5

Origin

Coast 

Side 

Drive 

Side 

Figure 6. Tooth 2, tooth fracture half. 

Drive 

Side 

Coast 

Side 

Figure 7. Tooth 2, gear fracture half. 
6

Coast 

Side 

Drive 

Side 

Figure 8. Tooth 3, tooth fracture half. 

Coast 

Side 

Drive 

Side 

Figure 9. Tooth 3, gear fracture half. 
7

Coast 

Side 

Drive 

Side 

Figure 10. Tooth 4, tooth fracture half. 

Coast 

Side 

Drive 

Side 

Figure 11. Tooth 4, gear fracture half. 
8

The second group, teeth 28–31, failed in a similar manner. All teeth failed in bending fatigue, 

originating in the drive side root. 

The primary failure for this group was tooth 28. The fracture halves are shown in figures 12 and 

13. As in tooth 1, a secondary crack originated on the coast side. A small overload region was 

present. 

Tooth 29 fracture halves are shown in figures 14 and 15. Fracture halves for tooth 30 are shown 

in figures 16 and 17. 

Figures 18 and 19 present the fracture halves for tooth 31. Wear was present at the drive side 

origin for this tooth (figures 26–28). Note that this was the last tooth in this sequence to fail by 

bending fatigue. 

Figures 20–28 are representative of the wear pattern observed on most of the teeth recovered. 

Prominent tool marks were observed in the roots of the gear teeth, as shown in figures 29–30. 

Depth of the typical tool marks measured was 0.0001–0.0002 in (figures 31 and 32). 

Figures 33 and 34 show the drive and coast sides of the mating pinion. Note that the wear 

pattern is offset, as the spare drive gear is not centered with respect to the pinion. 

 Origin 

Coast 

Side 

Drive 

Side 

Coast 

Side 

Fatigue 

Overload

Drive 

Side 

Fatigue 

Figure 12. Tooth 28, tooth fracture half.
9

 Origin 

Coast 

Side 

Drive 

Side 

Coast 

Side 

Fatigue 

Drive 

Side 

Fatigue 

Overload

Figure 13. Tooth 28, gear fracture half. 

Coast 

Side 

Drive 

Side 

Figure 14. Tooth 29, tooth fracture half. 
10

Coast 

Side 

Drive 

Side 

Figure 15. Tooth 29, gear fracture half. 

Coast 

Side 

Drive 

Side 

Figure 16. Tooth 30, tooth fracture half. 
11

Coast 

Side 

Drive 

Side 

Figure 17. Tooth 30, gear fracture half. 

Coast 

Side 

Drive 

Side 

Figure 18. Tooth 31, tooth fracture half. 
12

Coast 

Side 

Drive 

Side 

Figure 19. Tooth 31, gear fracture half. 

Figure 20. Drive side, tooth 1. 
13

Figure 21. Coast side wear, tooth 1. 

Coast 

Side 

Drive 

Side 

Figure 22. Top of tooth 1. 
14

Figure 23. Drive side, tooth 18. 

Figure 24. Coast side, tooth 18. 
15

Coast 

Side 

Drive 

Side 

Figure 25. Top of tooth 18. 

Figure 26. Drive side, tooth 31. (Note: worn adjacent to fracture surface.) 
16

Figure 27. Coast side, tooth 31. 

Coast 

Side 

Drive 

Side 

Figure 28. Top of tooth 31. 
17

Figure 29. Typical tool marks, tooth root fillet, between teeth 15 and 16. 

Figure 30. Typical tool marks, tooth root fillet, between teeth 16 and 17. 
18

Figure 31. Typical tool mark depth, tooth root, measurement no. 1. 

Figure 32. Typical tool mark depth, tooth root, measurement no. 2. 
19

Figure 33. Drive side idler gear. 

Figure 34. Coast side idler gear. 
20

4. Scanning Electron Microscopy 

Scanning electron microscopy was performed on all 13 teeth in a JEOL 6460LV SEM. The 

SEM investigation focused on fracture surface morphology and on features of interest at the 

origin. This work confirmed observations made in the optical examination. 

Figures 35–41 show SEM micrographs of tooth 1. Figures 36–38 present the origin for this 

primary failure. The pit origin can be clearly seen in figure 38. A separate crack originated from 

the coast side of the tooth and can be observed in figures 35 and 41. The two fractures connected 

through a small (100–150 μm) overload region as shown in figure 41. 

The pit origin for tooth 2 can be clearly seen in figure 42, which shows the tooth half of the 

fracture. Post fracture damage destroyed the gear half origin, depicted in figure 43. 

The origin for tooth 3 is shown in figure 44. Initiation appears to occur along a prominent tool 

mark. 

Tooth 4 had multiple origin sites (both pits and tool marks), which are shown in figures 45–47. 

There was a pit that led to cracking that can be observed on both the tooth and gear fracture 

halves (figures 45 and 46). 

Tooth 28 is shown in figures 48–52. The fracture was remarkably similar to tooth 1; both 

fractures possessed a primary pit origin (drive side), initiation and propagation from the coast 

side, and a very small overload region. Figures 49–51 show the origin of fatigue on this primary 

failure that can be traced to a line of pits including the pit which initiated failure. 

Figures 53–58 show the fracture surfaces associated with tooth 29. Failure originated at multiple 

points along the drive side, which are seen on both the tooth and gear fracture halves, including 

both pits and tool marks. 

Tooth 30 is shown in figures 59–63. Multiple origins were found on the drive side, shown in 

figures 60–61. 

The fracture surface generated by the low cycle fatigue failure of tooth 31 is presented in 

figure 64. The drive side origin is shown in figure 65. The fracture morphology includes a small 

amount of mixed mode fracture (figure 66), but is dominated by cyclic overload in the middle of 

the tooth (figure 67) and near the coast side (figure 68). 

Cracking originating from coast side tool marks is shown in figures 69–75. 
21

 Origin 

Overload

Drive 

Side 

Fatigue 

Coast 

Side 

Drive 

Side 

Coast Side 

Fatigue 

Figure 35. Tooth 1. 

Figure 38 Figure 37 

Figure 36. Tooth 1 drive side origin. 
22

Figure 37. Tooth 1 drive side pit origin site. 

Pit Origin 

Figure 38. Tooth 1 primary origin, drive side gear fracture half. 
23

Figure 39. Transgranular morphology, tooth 1 drive side. 

Figure 40. Transgranular morphology, tooth 1 coast side. 
24

 

Overload 

Region 

Coast 

Side 

Fatigue 

Drive 

Side 

Fatigue 

Figure 41. Overload transition, tooth 1. 

Figure 42. Tooth 2 drive side primary pit origin, tooth half. 
25

Figure 43. Drive side, tooth 2, gear half, origin location. (Note the damage.) 

Tool 

Marks 

Figure 44. Drive side, tooth 3, primary origin gear half. 
26

Figure 45. Tooth 4 drive side primary pit origin. 

Figure 46. Tooth 4 drive side primary pit origin, gear fracture half. 
27

Tool 

Mark 

Figure 47. Tooth 4 drive side. 

Coast 

Side 

Drive 

Side 

Figure 48. Tooth 28 fracture surface. 
28

Figure 49. Tooth 28 drive side, primary origin. 

Figure 50. Tooth 28 drive side, primary pit origin. 
29

Continuation 

of line of pits 

which initiated 

failure 

Figure 51. Tooth 28 drive side origin site, gear fracture half. 

Overload 

Drive 

Side 

Fatigue 

Figure 52. Tooth 28 fracture surface, transition to overload at top. 
30

Figure 54 Figure 55 Figure 56 

Figure 53. Tooth 29 drive side origin area. 

Tool 

Marks 

Figure 54. Tooth 29 drive side origin, site no. 1. 
31

Tool 

Marks 

Figure 55. Tooth 29 drive side origin, site no. 2. 

Tool 

Marks 

Figure 56. Tooth 29 drive side origin, site no. 3. 
32

Figure 57. Tooth 29 drive side, primary pit origin. 

Tool 

Marks 

Figure 58. Tooth 29 drive side origin, site no. 4. 
33

Coast 

Side 

Drive 

Side 

Figure 59. SEM tooth 30 fracture surface. 

Tool 

Marks 

Figure 60. Tooth 30 origin, gear fracture half. 
34

Figure 61. Tooth 30 pit origin, gear fracture half. 

Figure 62. Tooth 30 fracture surface, drive side. 
35

Figure 63. Tooth 30 fracture surface, coast side. 

Coast 

Side 

Drive 

Side 

Figure 65 Figure 66 

Figure 67 

Figure 68 

Figure 64. SEM tooth 31 fracture surface. 
36

Figure 65. Tooth 31 origin. 

Figure 66. Tooth 31 fracture surface. 
37

Figure 67. Tooth 31 fracture surface, showing cyclic overload fracture. 

Figure 68. Tooth 31 fracture surface, ductile overload. 
38

Figure 70 

Figure 69. Tool marks, tooth 28 root, gear half. 

Figure 70. Coast side tool marks, tooth 28 root, gear half. (Note the undercut 

parallel to tool marks.) 
39

Figure 71. Tooth 28 root, gear half, coast side cracking along tool marks. 

Figure 72. Tooth 29 root, gear half, coast side tool marks. 
40

Figure 73. Tooth 29 root, gear half, coast side tool marks. 

Figure 74. Tooth 29 root, gear half, coast side tool marks, cracking along tool 

marks. 
41

Figure 75. Tooth 29 root, gear half, coast side, cracking along tool marks. 

5. Hardness 

Hardness measurements were provided by Redstone Technical Test Center. They were made at 

the tooth root and at the face (near the pitch line) using an Instron Model Tukon 2100 

microhardness tester. Effective case depth (EFD) (defined as the depth at which hardness drops 

to 50 Hardness Rockwell C-scale [HRC]) was found to be approximately 0.026 in at the pitch 

line and 0.018 in at the root. Hardness data for the profile and root are given in tables 1–3 and 

shown in figure 76. 

Core hardness was measured to be HRC 39.5 using a Wilson Rockwell indenter (Model C524-T). 

Required minimum core hardness was HRC 30. 
42

Table 1. Hardness measurements, tooth 3, pitch line. 

Depth 

Knoop 

Hardness 

Rockwell HRC 

Hardness, Converted Depth 

Knoop 

Hardness 

Rockwell HRC 

Hardness, Converted 

0.002 687 57.8 0.028 500 47.3 

0.004 663 56.6 0.030 490 46.7 

0.006 651 56.1 0.032 478 45.9 

0.008 639 55.4 0.034 468 45.2 

0.010 635 55.2 0.036 464 44.9 

0.012 628 54.9 0.038 455 44.2 

0.014 617 54.3 0.040 455 44.2 

0.016 610 53.9 0.042 455 44.2 

0.018 590 52.7 0.044 452 44.0 

0.020 576 52.0 0.046 446 43.6 

0.022 550 50.5 0.048 449 43.8 

0.024 542 50.0 0.050 451 43.9 

0.026 522 48.8 — — — 

Table 2. Hardness measurements, tooth 17, pitch line. 

Depth 

Knoop 

Hardness 

Rockwell HRC 

Hardness, Converted Depth 

Knoop 

Hardness 

Rockwell HRC 

Hardness, Converted 

0.002 675 57.2 0.028 529 49.2 

0.004 662 56.6 0.030 508 47.8 

0.006 667 56.8 0.032 483 46.2 

0.008 652 56.1 0.034 482 46.1 

0.010 633 55.2 0.036 476 45.7 

0.012 631 55.0 0.038 464 44.9 

0.014 630 55.0 0.040 541 43.9 

0.016 614 54.1 0.042 443 43.4 

0.018 598 53.2 0.044 457 44.3 

0.020 599 53.3 0.046 441 43.2 

0.022 587 52.6 0.048 446 43.6 

0.024 578 52.1 0.050 446 43.6 

0.026 559 51.1 — — — 
43

Table 3. Hardness measurements tooth root. 

Depth 

Knoop 

Hardness 

Rockwell HRC 

Hardness, Converted Depth 

Knoop 

Hardness 

Rockwell HRC 

Hardness, Converted 

0.002 668 56.9 0.028 466 45.0 

0.004 628 54.9 0.030 455 44.2 

0.006 627 54.8 0.032 451 43.9 

0.008 618 54.3 0.034 446 43.6 

0.010 613 54.0 0.036 437 42.9 

0.012 598 53.2 0.038 441 43.2 

0.014 577 52.1 0.040 442 43.3 

0.016 566 51.4 0.042 432 42.5 

0.018 549 50.4 0.044 435 42.7 

0.020 526 49.0 0.046 421 41.5 

0.022 503 47.5 0.048 423 41.7 

0.024 501 47.4 0.050 416 41.1 

0.026 473 45.5 — — — 

Hardess Data, 15 HP Gear

400

450

500

550

600

650

700

0 10 20 30 40 50

Depth (Mils)

Hardness, Knoop, HK

40

45

50

55

60

Hardness, HRC

Tooth 3 Data

Tooth 17 Data

Root Data

Figure 76. Hardness data tooth and root profiles. 
44

6. Metallography 

Cross sections of the gear and two fractured teeth (teeth 28 and 29) were prepared. These were 

ground and polished through 0.02-μm colloidal silica and subsequently etched with a 2% nital 

solution. No microstructural abnormalities were observed. Formation of continuous carbide 

networks was ruled out (figure 77).

7. Edge Break 

The edge break for tooth 28 was measured to be 0.0089 in (figure 78). For tooth 29, a value of 

0.0088 in (figure 79) was obtained. These values are less than the specified maximum (0.010 in). 

Figure 77. Metallographic cross section demonstrating absence of continuous 

carbide networks. 
45

Figure 78. Edge break measurement, tooth 28. 

Figure 79. Edge break measurement, tooth 29. 
46

8. Chemical Analysis 

Gear chemical composition was analyzed with direct current plasma emission spectroscopy 

(DC plasma) and combustion infrared detection (carbon and sulfur). The sample was machined 

prior to testing to remove the carburized case. The results are listed in table 4 along with the 

chemical composition requirements for the material specified, AMS 6260. The chemical 

constituency compared favorably with the specification. 

Table 4. Chemical composition of 15-hp spare drive gear. 

Element Gear Composition

% (by Weight) 

AMS 6260 Specification 

% (by Weight) 

Carbon 0.110 0.07–0.13 

Manganese 0.61 0.40–0.70 

Silicon 0.27 0.15–0.35 

Phosphorous 0.023 0.025 maximum 

Sulfur 0.021 0.025 maximum 

Chromium 1.25 1.00–1.40 

Nickel 3.06 3.00–3.50 

Molybdenum 0.13 0.08–0.15 

Boron <0.0005 0.001 maximum 

Copper 0.32 0.35 maximum 

9. Surface Finish 

A Taylor-Hobson Form TalySurf Series 2 was utilized to perform laser surface profilometry in a 

non-worn area of a typical gear tooth. The root mean square (RMS) surface roughness was 

measured to be 2.85 μm-in (see figure 80). This is well below the specified value (32 μm-in). It 

must be noted that this measurement was on the surface of a tooth that may have been polished 

by the mechanical action of the meshing gear teeth during service, although no appreciable wear 

was observed in the area where the measurement was taken. Additionally, evidence of shot 

peening was observed (see figure 81). Measurements in the root of the teeth were unobtainable. 
47

Figure 80. Surface roughness measurement. 

Figure 81. Optical photograph of shot peened surface finish. 
48

10. Residual Stress 

The residual stress was measured in the hoop (or rotational) direction on the non-worn areas on 

the coast side of teeth 28 and 29 which were separated from the gear during fracture. 

Compressive stresses of 114.4 and 125.5 (+ 2.6 ksi) were obtained. The difference between the 

measurements is likely due to experimental errors caused by small sample size and the slight 

curvature of the measured surface. However, the reported stresses compare favorably with other 

shot peened helicopter gears, such as the Blackhawk spiral bevel gear. 

11. Recommendations 

• Identify and eliminate the source of the pitting. 

• Eliminate or reduce the magnitude of the tool marks present in the tooth root fillet. 

12. Conclusions 

Gear failure resulted from bending fatigue failure of two primary teeth. Failure of these teeth 

initiated a sequence (in two separate groups) of increasingly faster bending fatigue tooth failures. 

Failure progressed in the order of meshing for both groups. Ultimately, transmission error 

became so large that the gears did not mesh properly, causing rapid stripping of the remaining 

teeth and damage to the pinion teeth as well. 

The two groups of bending fatigue failure sequences were identified as teeth 1, 2, 3, and 4 and 

teeth 28, 29, 30, and 31. Identical four tooth failure sequences have been found on other failed 

spare drive gears. However, this is only gear where two such sequences have been identified. 

Primary origins for all bending fatigue failures were located at the drive side root near the area of 

highest stress. This high stress point had been previously established by finite-element analysis 

at U.S. Army Aviation and Missile Command. 

Pits were found at the primary origins of the initial teeth (tooth 1 and tooth 28) in each of the 

bending fatigue failure sequences. A pit of approximately 10 μm was found at the primary 

origin. 

The EFD was determined to be 0.018 in at the root and 0.026 in at the pitch diameter. The EFD 

was defined as the depth at which a value of 50 HRC is obtained. 
49

No inherent microstructural abnormalities were observed, such as continuous carbide networks, 

but significant tool marks and pits were found on the exterior surface at or near the fracture 

origins. 

The edge breaks were found to be within specification (0.010 in maximum) for tooth 28 

(0.0089 in) and tooth 29 (0.0088 in). 

Chemical testing confirmed that the gear was made from AMS 6260 alloy steel in accordance 

with drawing material requirements. 

The surface finish was estimated to be 2.85 μm-in, RMS. This is within the drawing 

specification (32 μm-in maximum). However, tool marks were observed near the roots of gear 

teeth. 

Residual stresses were measured on the coast side of two teeth that separated from the gear. The 

resulting residual stresses were comparable to other shot peened 6260 material. 
NO. OF 

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50

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