Failure Analysis of a CH-47 Horizontal Hinge Pin 

Assembly, P/N 114RS226 

by Marc Pepi and Gary Wechsler 

ARL-TR-4011 December 2006 

Approved for public release; distribution is unlimited. 
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Army Research Laboratory 

Aberdeen Proving Ground, MD 21005-5069 

ARL-TR-4011 December 2006 

Failure Analysis of a CH-47 Horizontal Hinge Pin 

Assembly, P/N 114RS226 

Marc Pepi 

Weapons and Materials Research Directorate, ARL 

Gary Wechsler 

U.S. Army Research, Development and Engineering Command 

Approved for public release; distribution is unlimited. 
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4. TITLE AND SUBTITLE 

Failure Analysis of a CH-47 Horizontal Hinge Pin Assembly, P/N 114RS226 

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680L31 

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6. AUTHOR(S) 

Marc Pepi and Gary Wechsler* 

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U.S. Army Research Laboratory 

ATTN: AMSRD-ARL-WM-MC 

Aberdeen Proving Ground, MD 21005-5069 

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AMCOM 

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U.S. Army Aviation and Missile Command 

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Approved for public release; distribution is unlimited. 

13. SUPPLEMENTARY NOTES 

*U.S. Army Research, Development and Engineering Command, Redstone Arsenal, AL 35898 

14. ABSTRACT 

A CH-47D horizontal hinge pin assembly was examined to determine the root cause of ring and roller fracture and spalling. 

Chemical analyses, metallography, and hardness testing indicated that the rings and rollers conformed to the material 

requirements of the governing drawing. Metallography and visual and scanning electron microscopy indicated the failure likely 

initiated in the lag ring due to rolling contact fatigue because of high-service loads. 

15. SUBJECT TERMS

failure analysis, parachute bag knife, metallography, horizontal hinge pin, rolling contact pin 

16. SECURITY CLASSIFICATION OF: 

19a. NAME OF RESPONSIBLE PERSON

Marc Pepi 

a. REPORT 

UNCLASSIFIED 

b. ABSTRACT 

UNCLASSIFIED

c. THIS PAGE 

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OF ABSTRACT 

UL 

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OF PAGES 

38 

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410-306-0848 

Standard Form 298 (Rev. 8/98) 

Prescribed by ANSI Std. Z39.18
Contents 

List of Figures iv

List of Tables vi

Acknowledgments vii

1. Summary 1

2. Background 1

3. Visual Examination 1

4. Chemical Analysis 6

4.1 Metallography .................................................................................................................7 

4.2 Hardness Testing ...........................................................................................................11 

4.2.1 Rings....................................................................................................................11 

4.2.2 Rollers .................................................................................................................14 

4.3 Scanning Electron Microscopy .....................................................................................20 

5. Discussion 20

6. Recommendations 24

7. Conclusions 25

8. References 26

Distribution List 27 

iii
List of Figures 

Figure 1. The rollers, cage, and sleeve of the lead roller bearing set. The scale is 6 inches in 

length..........................................................................................................................................2 

Figure 2. View of the lag ring showing that the fractured region encompassed an 

approximately 4-in arc length of the ring’s circumference........................................................2 

Figure 3. Three pieces of material that fractured from the lag ring depicted in figure 2................3 

Figure 4. The lag-ring raceway exhibited a spalled surface topography. Fracture and 

cracking were relegated to the portion of the lag ring exhibiting a spalled raceway 

surface........................................................................................................................................3 

Figure 5. Lag-ring fracture exhibiting beach marks (parallel with yellow lines), river patterns 

(parallel to black arrows), and multiple origins (white arrows). Scale demarcations at 

image bottom denote 1/64-in spacing........................................................................................3 

Figure 6. Typical surface damage and spalling exhibited on the lag rollers. The topography 

of the surface created by spalling exhibited the beach marks (parallel with the blue lines) 

and river patterns (parallel with the black arrows) indicative of subsurface crack initiation 

and propagation by RCF. Magnified ~5.7×..............................................................................4 

Figure 7. One of the eight damaged rollers was broken in the two pieces shown. Scale 

demarcations at image bottom denote 1/64-in spacing..............................................................4 

Figure 8. Inner surface of the lag retainer exhibiting wear damage caused by contact with the 

mating rotating roller—indicative of high contact loads between the two parts. 

Magnified 9.7×...........................................................................................................................5 

Figure 9. Lead-ring raceway spalling. ............................................................................................5 

Figure 10. Lead-ring raceway spalling likely began by the edge of a groove................................5 

Figure 11. The HHP in the as-received condition exhibited discolored lead-sleeve and lagsleeve raceways. The chrome-plated surface of the HHP between the sleeves exhibited 

wear induced by mating seals and fretting product created by the relative motion between 

the HHP and a mating pitch-shaft sleeve...................................................................................6 

Figure 12. Micrograph of an as-polished section cut from the spalled portion of the lag-ring 

raceway. No microstructural anomalies were observed. Magnified 200×...............................7 

Figure 13. Spalling and cracking on the raceway of the lead ring. Dashed line represents 

section made for metallographic examination. ..........................................................................8 

Figure 14. Detail of lead-ring raceway cracking depicted within red box in figure 13—likely 

the onset of spalling. ..................................................................................................................8 

Figure 15. Metallography of the lead-ring region depicted in figure 14 revealed a subsurface 

crack running parallel with the lead ring’s raceway surface......................................................9 

Figure 16. Metallography of the spalled and cracked area to the left of the area depicted in 

figure 13 revealed cracking parallel with the raceway surface and 45o

 to the raceway 

surface........................................................................................................................................9 

iv
Figure 17. Etching the microstructure of the subsurface crack depicted in figure 15 revealed 

the presence of microstructural alterations—a feature associated with RCF. Etchant: 

Vilella’s reagent.......................................................................................................................10 

Figure 18. Microstructure of the lead ring exhibiting the carbides (both acicular and 

globular) dispersed in a matrix of tempered martensite observed in all the metallographic 

cross sections produced from the M50 steel rings and rollers. Etchant: Vilella’s reagent. ...10 

Figure 19. Micrograph of the lag-ring microstructure showing a grain size that conformed to 

the required average ASTM grain size no. 8 or finer, with occasional grains as large as 

ASTM grain size no. 6. Etchant: Vilella’s reagent. Magnified 100×. ..................................11 

Figure 20. Schematic showing location of hardness readings on lead-ring sample, adjacent to 

wear (see table 3). ...................................................................................................................12 

Figure 21. Schematic showing location of hardness readings on lead-ring sample away from 

wear (see table 4). ....................................................................................................................13 

Figure 22. Schematic showing location of hardness readings on lag-ring sample, adjacent to 

wear (see table 5). ...................................................................................................................14 

Figure 23. Schematic showing location, of hardness readings on lag-ring sample away from 

wear (see table 6). ....................................................................................................................15 

Figure 24. One of the fracture origins on the surface of the lag ring (identified via lightoptical microscopy and labeled with the nomenclature A in figure 13)..................................20 

Figure 25. Detail view of the origin shown in figure 24 showing that the topography of the 

origin is smeared......................................................................................................................21 

Figure 26. The fracture morphology of the lag ring was transgranular—typical of a fatiguefracture mode. ..........................................................................................................................21 

Figure 27. Surface spalling (fatigue pit) exhibited by a lag roller—typical of the spalling 

observed on the other lag rollers..............................................................................................22 

Figure 28. Detail of the spall depicted in figure 27 showing the beach marks typical of RCFinduced spalling. ......................................................................................................................22 

Figure 29. Detail of figure 28 showing the transgranular fracture morphology of the spalled 

surface typical of RCF-induced spalling..................................................................................23 

v
List of Tables 

Table 1. Component identification. ................................................................................................1 

Table 2. Chemical composition (weight-percent)...........................................................................7 

Table 3. Lead-ring section hardness results adjacent to wear and spalling. HRC—150-kgf 

major load. ...............................................................................................................................12 

Table 4. Lead-ring section hardness results away from wear and spalling. HRC—150-kgf 

major load. ...............................................................................................................................13 

Table 5. Lag-ring section hardness results adjacent to wear and spalling. HRC—150-kgf 

major load. ...............................................................................................................................14 

Table 6. Lag-ring section hardness results away from wear and spalling. HRC—150-kgf 

major load. ...............................................................................................................................15 

Table 7. Microhardness of a lead-ring section from the raceway surface to the core—Knoop 

scale, 500-gmf load, 50× objective..........................................................................................16 

Table 8. Microhardness of a longitudinal section of the lag-ring from the raceway surface to 

the core—Knoop scale, 500-gmf load, 50× objective. ............................................................17 

Table 9. Nanohardness testing results, lead-ring section..............................................................17 

Table 10. Surface-hardness values of two lead rollers. HRC scale—150-kgf load.....................18 

Table 11. Cross-section hardness traverse of the lead roller (no. 1) that exhibited the lowerthan-average surface hardness—Knoop scale, 500-gmf load, 50× objective..........................18 

Table 12. Surface hardness of two lag rollers. HRC scale—150-kgf load. .................................19 

Table 13. Microhardness testing results, lag-ring roller no. 1—Knoop scale, 500-gmf load, 

50× objective............................................................................................................................19 

vi
Acknowledgments 

Thanks are extended to the following U.S. Army Research Laboratory personnel for their 

assistance during this investigation: Scott Grendahl, Dr. Andrew Davis, Benjamin Hardisky, 

Chris Norfolk, and Tom Juliano. The authors are also grateful for the manuscript review 

performed by Randy McFarland, U.S. Army Materiel Command.

vii
INTENTIONALLY LEFT BLANK.

viii
1. Summary 

A CH-47D horizontal hinge pin (HHP) assembly was examined to determine the root cause of 

ring and roller fracture and spalling. Chemical analyses, metallography, and hardness testing 

indicated that the rings and rollers conformed to the material requirements of the governing 

drawing. Metallography and visual and scanning electron microscopy indicated the failure likely 

initiated in the lag ring due to rolling contact fatigue (RCF) because of high-service loads. 

2. Background 

The HHP assembly failure was found during an inspection of a main rotor head assembly that 

had been removed from CH-47 S/N 81-23387 because of an oil leak. The HHP assembly had 

accumulated 637 hr of flight time when the failure was found and recorded as Category I quality 

deficiency report no. 019449-04240-001, U.S. Army Materiel Command control 

no. M24H90314. 

The failed CH-47D HHP assembly was sent to the U.S. Army Research Laboratory from the 

U.S. Army Aviation and Missile Research, Development, and Engineering Center for failure 

analysis by the CH-47 Cargo Division, Aviation Engineering Directorate. Table 1 lists the part 

and serial number information marked on the lead-roller and lag-roller bearing sets—information 

which indicated each set had been manufactured by RBC Bearings in West Trenton, NJ. 

Table 1. Component identification. 

Nomenclature Drawing No. Part No. Serial No. 

Lag-roller bearing, HHP 114R2197 — 77272 

Lead ring 114RS225 TJ-75222-9D — 

Lag ring 114RS226 TJ-75223-9D — 

In service, only half of an HHP assembly is subjected to significant loading. After 1200 hr of 

service, each HHP is removed, rotated 180°, and reinstalled for an additional 1200 hr of service 

before it is retired. With only 637 hr of service, the subject HHP assembly did not reach the 

1200 hr point when the pin would have been removed, rotated, and reinstalled. 

3. Visual Examination 

The lead-roller and the lag-roller bearing sets were each comprised of 58 rollers, one cage, and 

one ring (the lead set is shown in figure 1). 

1
Figure 1. The rollers, cage, and sleeve of the lead roller bearing 

set. The scale is 6 inches in length. 

The lag-roller bearing set displayed much more damage than the lead roller bearing set. The lagroller bearing set consisted of a fractured lag ring with a spalled raceway and eight rollers that 

exhibited extensive surface damage and spalling—one of which fractured into two pieces. The 

lag ring’s fracture extended over an arc length of approximately 4 in (see figures 2 and 3). The 

lag ring’s fracture and all secondary cracking were relegated to portions of the ring that exhibited 

raceway spalling (see figure 4). Inspection of the lag ring’s fracture surfaces via light-optical 

microscopy revealed multiple thumbnail-shaped beach marks. These marks indicate fracture was 

likely initiated by fatigue from multiple sites along the spalled raceway surface and propagated 

by fatigue to the outer diameter of the ring (see figure 5). 

Figure 2. View of the lag ring showing that the fractured region 

encompassed an approximately 4-in arc length of the 

ring’s circumference. 

2
Figure 3. Three pieces of material that fractured from the lag ring depicted 

in figure 2. 

Figure 4. The lag-ring raceway exhibited a spalled surface topography. Fracture 

and cracking were relegated to the portion of the lag ring exhibiting a 

spalled raceway surface. 

Figure 5. Lag-ring fracture exhibiting beach marks (parallel with yellow lines), river patterns (parallel to 

black arrows), and multiple origins (white arrows). Scale demarcations at image bottom denote 

1/64-in spacing. 

3
Eight of the lag rollers exhibited surface damage and spalling (figure 6). Many of the spalled 

regions within the rollers exhibited river patterns and concentric beach marks, which indicate 

that spalling had initiated and propagated due to fatigue from a subsurface origin—a typical 

failure mode of RCF. One of the eight damaged lag rollers fractured into two pieces (figure 7). 

The lag-ring bearing cage exhibited wear damage likely due to high-contact loads with the flat 

ends (or sides) of the mating rollers (see figure 8). 

Figure 6. Typical surface damage and spalling exhibited on the lag rollers. The 

topography of the surface created by spalling exhibited the beach marks 

(parallel with the blue lines) and river patterns (parallel with the black 

arrows) indicative of subsurface crack initiation and propagation by RCF. 

Magnified ~5.7×. 

Figure 7. One of the eight damaged rollers was broken in the two pieces shown. 

Scale demarcations at image bottom denote 1/64-in spacing. 

The only damage exhibited by the lead-roller bearing set was the spalled raceway surface of the 

ring (see figures 9 and 10). The surfaces of the mating lead rollers and the cage did not exhibit 

surface damage. 

4
Figure 8. Inner surface of the lag retainer exhibiting wear damage caused by 

contact with the mating rotating roller—indicative of high contact 

loads between the two parts. Magnified 9.7×. 

A

Figure 9. Lead-ring raceway spalling. 

Figure 10. Lead-ring raceway spalling likely began by the edge of a groove. 

The lead-sleeve and lag-sleeve raceways of the HHP were discolored (see figure 11). The lag 

sleeve was darker than the lead sleeve. Neither of the two sleeves exhibited a spalled surface. It 

appeared that the raceway discoloration in each sleeve was due to contact with the mating rollers 

and the oil contaminated with roller spalling and/or ring-spalling debris. 

5
Lag sleeve surface 

discoloration 

Lead sleeve surface 

Fretting product wear and damage 

Wear induced by 

mating seals. 

Figure 11. The HHP in the as-received condition exhibited discolored lead-sleeve and 

lag-sleeve raceways. The chrome-plated surface of the HHP between the 

sleeves exhibited wear induced by mating seals and fretting product created 

by the relative motion between the HHP and a mating pitch-shaft sleeve. 

Between the lead sleeves and the lag sleeves, the HHP exhibited circumferential lines of wear, 

likely created by contact with the mating rollers and the oil contaminated with roller-spalling 

debris and/or ring-spalling debris. These lines were induced by mating seals and the fretting 

product created by the relative motion between the HHP and a mating, 15-5PH SS, pitch-shaft 

sleeve (see figure 11). The level of fretting was greatest in the areas immediately adjacent to the 

seals. A similar pattern of fretting products was found on the opposite side of the pin. 

4. Chemical Analysis 

The chemical contents of a lead roller and a lag roller, along with a section from each mating 

ring, were shown to have met the requirements of AMS 6491 (1)—the specification for VIMVAR M50 required by each roller-bearing set drawing (see table 2). Although the phosphorus 

content for each part was at or near the maximum level allowed by AMS 6491, literature on M50 

steel and a conversation with a bearing expert (2) indicated these levels were not atypical for the 

alloy in bearing applications. Also, a conversation with RBC revealed that the company 

considered steel that met the chemistry requirements of the governing drawings to be suitable for 

service. The chemical content of each part listed in table 2, except for carbon and sulfur, was 

determined via direct-current plasma emission spectroscopy. The carbon and sulfur content of 

each part listed in table 2 was measured by combustion/infrared detection. 

6
Table 2. Chemical composition (weight-percent). 

Element Lead Ring Lag Ring 

Lead-Ring 

Roller 

Lag-Ring 

Roller AMS 6491 

Carbon 0.81 0.83 0.83 0.83 0.80–0.85 

Manganese 0.22 0.21 0.23 0.19 0.15–0.35 

Silicon 0.19 0.20 0.18 0.18 0.25 max. 

Phosphorus 0.013 0.015 0.015 0.014 0.015 max. 

Sulfur 0.002 0.004 0.003 0.003 0.008 max. 

Chromium 4.13 4.13 4.06 4.13 4.00–4.25 

Molybdenum 4.29 4.25 4.30 4.25 4.00–4.50 

Vanadium 1.06 1.04 1.07 1.02 0.90–1.10 

Nickel 0.058 0.10 0.055 0.099 0.15 max. 

Cobalt 0.013 0.016 0.013 0.020 0.25 max. 

Tungsten 0.010 0.015 0.027 0.028 0.25 max. 

Copper 0.060 0.062 0.057 0.077 0.10 max. 

4.1 Metallography 

Sections of rollers and rings from the lead-ring and lag-ring assemblies were mounted in 

phenolic resin and metallographically prepared using silicon carbide papers ranging in grit size 

from 240 to 2400. Final polishing was accomplished with a 0.05-μm alumina suspension. 

Because the level of spalling had progressed too far in the region subjected to contact with the 

mating rollers, regions of the lag-ring raceway exhibiting the onset of spalling could not be found 

and examined metallographically. A metallographic specimen cut from the spalled region and 

examined in the as-polished condition exhibited the carbide inclusions typical of this alloy, but 

not any microstructural defects that may have contributed to the raceway spalling (see figure 12). 

Figure 12. Micrograph of an as-polished section cut from the spalled portion of 

the lag-ring raceway. No microstructural anomalies were observed. 

Magnified 200×. 

A metallographic cross section was produced from a portion of the lead ring exhibiting the onset 

of spalling (see figures 13 and 14). This metallographic cross section exhibited subsurface 

cracks—some of which didn’t open to the raceway surface and some of which did. Figure 15 

shows a subsurface crack parallel with the raceway surface that appeared to be a subsurface 

crack that initiated RCF. Figure 16 shows a subsurface crack that opened to the raceway surface 

at a 45° angle—an angle associated with the shear-lip formation that occurs typically during 

7
Figure 13. Spalling and cracking on the raceway of the lead ring. Dashed line 

represents section made for metallographic examination. 

Figure 14. Detail of lead-ring raceway cracking depicted 

within red box in figure 13—likely the onset 

of spalling. 

the final stage of an overload fracture. Etching the subsurface crack (figure 15) with Vilella’s 

reagent (5 mL of hydrochloric acid, 1g of picric acid, and 100 ml of ethanol) revealed the 

presence of local microstructural alterations (see figure 17). Microstructural alterations are the 

localized plastic deformations created by the subsurface cyclic stresses of RCF. These features 

create planes of weakness on which cracks can propagate (3). 

All of the roller metallographic samples and the ring metallographic samples etched with 

Vilella’s reagent revealed a microstructure typical of M50 in the hardened condition—equiaxed 

and acicular carbides dispersed in a matrix of tempered martensite (figure 18). The 

8
Figure 15. Metallography of the lead-ring region depicted in figure 14 revealed a 

subsurface crack running parallel with the lead ring’s raceway surface. 

Figure 16. Metallography of the spalled and cracked area to the left of the area 

depicted in figure 13 revealed cracking parallel with the raceway 

surface and 45o

 to the raceway surface. 

9
Figure 17. Etching the microstructure of the subsurface crack depicted in figure 

15 revealed the presence of microstructural alterations—a feature 

associated with RCF. Etchant: Vilella’s reagent. 

Figure 18. Microstructure of the lead ring exhibiting the carbides (both acicular 

and globular) dispersed in a matrix of tempered martensite observed 

in all the metallographic cross sections produced from the M50 steel 

rings and rollers. Etchant: Vilella’s reagent. 

metallographic specimens did not exhibit the overtempered and/or untempered Martensitic 

surface microstructures that are typically created by excessive service temperatures when bearing 

failure is caused by a lubrication problem. 

10
The average grain size of the rings and rollers (examined at 100× magnification) was 8.5, with an 

occasional grain as large as ASTM no. 6 (see figure 19). This met the RBC Heat Treatment 

Specification HT-8 requirement for an average grain size of ASTM no. 8 or finer, with 

occasional grains as large as ASTM no. 6. The grain size was determined using transparency 

overlays of various grain sizes conforming to ASTM E112 (4). The microstructure of all other 

metallographic ring and roller specimens exhibited a similar grain size. 

.

Figure 19. Micrograph of the lag-ring microstructure showing a grain size that 

conformed to the required average ASTM grain size no. 8 or finer, 

with occasional grains as large as ASTM grain size no. 6. Etchant: 

Vilella’s reagent. Magnified 100×. 

4.2 Hardness Testing 

4.2.1 Rings 

Rockwell C scale hardness measurements performed directly on the concave and convex 

surfaces of the lead ring were within the 60–64 HRC range drawing requirement (see figure 20 

and table 3 and figure 21 and table 4). There was no significant difference in the lead-ring 

raceway hardness measured adjacent to and away from regions exhibiting wear and spalling. In 

addition, Rockwell C scale hardness measurements performed directly on the concave and 

convex surfaces of the lag ring were within the 60–64 HRC range drawing requirement (see 

figure 22 and table 5 and figure 23 and table 6). There was no significant difference in the leadring raceway and lag-ring raceway hardness measured adjacent to and away from regions 

exhibiting wear and spalling. 

11
Concave 

1 

12 

Convex 1-12 

on opposite side 

Wear 

Figure 20. Schematic showing location of 

hardness readings on lead-ring 

sample, adjacent to wear (see 

table 3). 

Table 3. Lead-ring section hardness results adjacent to wear 

and spalling. HRC—150-kgf major load. 

Concave Readings Convex Readings 

62.3 62.2 

62.5 62.4 

63.8 62.6 

62.5 62.8 

62.5 62.8 

62.7 62.7 

62.9 62.3 

62.9 62.9 

62.4 62.4 

62.6 62.5 

62.5 62.5 

62.4 62.5 

Average = 62.7 HRC Average = 62.6 HRC 

Knoop microhardness testing performed on a metallographic cross section of the lead ring, 

traversing from just beneath the ring’s raceway surface to the ring’s core, indicated the raceway 

surface was lower than the allowable drawing minimum (see table 7). Converted readings of 

55.2 and 47.4 HRC were obtained from two separate areas, both approximately 0.001 in beneath 

the raceway surface. These values were lower than the 60–64 HRC drawing requirement. 

Knoop microhardness measurements performed on a metallographic cross section of the lag ring 

(traversing from just beneath the ring’s raceway surface to the ring’s core) and converted to the 

Rockwell C scale all fell within the 60–64 HRC range drawing requirement (see table 8). 

12
Concave 

A

12 

Convex 1-12 

on opposite side 

Figure 21. Schematic showing location of 

hardness readings on lead-ring sample 

away from wear (see table 4).

Table 4. Lead-ring section hardness results away from 

wear and spalling. HRC—150-kgf major load. 

Concave Readings Convex Readings 

60.7 61.5 

61.7 61.8 

61.8 61.9 

61.7 61.9 

61.8 61.7 

61.9 62.0 

61.9 61.7 

61.9 62.0 

62.0 62.0 

62.2 61.8 

61.9 61.9 

61.9 61.6 

Average = 61.8 HRC Average = 61.8 HRC 

To determine if an edge effect was the cause of the low Knoop measurements, nine MTS 

nanoindenter XP hardness measurements were made adjacent to the lead ring’s raceway surface 

and compared with nine nanohardness measurements of the ring’s core. Table 9 shows that the 

average nanohardness values adjacent to the raceway surface and in the ring core were very 

similar (especially considering the standard deviation of each average). This data indicates that 

the low Knoop hardness measurements obtained adjacent to the raceway surface were likely 

inaccurate. 

13
Concave 

1 

12 

Convex 1-12 

on opposite side 

Wear

Figure 22. Schematic showing location of 

hardness readings on lag-ring 

sample, adjacent to wear (see 

table 5). 

Table 5. Lag-ring section hardness results adjacent to 

wear and spalling. HRC—150-kgf major 

load. 

Concave Readings Convex Readings 

62.4 62.9 

62.5 62.8 

62.8 63.1 

62.7 63.3 

63.1 63.1 

63.0 63.1 

63.2 63.3 

63.2 63.3 

63.2 63.4 

63.3 63.2 

63.3 63.3 

63.2 62.8 

Average = 63.0 HRC Average = 63.1 HRC 

4.2.2 Rollers 

Rockwell C scale hardness measurements were performed directly on the surfaces of two lead 

rollers. One lead roller exhibited an average hardness of 59.6 HRC—slightly below the drawing 

requirement of 60–64 HRC. The average hardness of the other lead roller met the drawing 

requirement of 60–64 HRC (see table 10). 

14
Concave 

1 

12 

Convex 1-12 

on opposite side 

Figure 23. Schematic showing location, of 

hardness readings on lag-ring sample 

away from wear (see table 6). 

Table 6. Lag-ring section hardness results away from 

wear and spalling. HRC—150-kgf major 

load. 

Concave Readings Convex Readings 

62.8 62.7 

63.0 63.0 

63.0 62.9 

63.2 63.0 

63.1 62.9 

63.2 63.0 

63.2 62.9 

63.1 63.1 

63.1 62.9 

63.3 63.2 

61.7 63.1 

62.7 63.1 

Average = 63.0 HRC Average = 63.0 HRC 

Microhardness measurements were performed on a cross section of the lead roller (from just 

beneath its surface to its core) that exhibited slightly lower-than-average surface hardness. The 

hardness, converted to the Rockwell C scale, varied from within to slightly below the 

specification requirement of 60–64 HRC (see table 11). Since this roller did not exhibit surface 

cracking and spalling, the slightly low hardness values were not considered to have been a 

contributing factor to the HHP assembly failure. 

15
Table 7. Microhardness of a lead-ring section from the raceway surface 

to the core—Knoop scale, 500-gmf load, 50× objective. 

Lead Ring Longitudinal Cross Section, Hardness Traverse No. 1 

Reading Knoop Approx. HRC 

Distance From 

Surface 

(in) 

1 634.3 55.2 0.0010 

2 731.7 59.9 0.0025 

3 793.3 62.7 0.0045 

4 783.3 62.3 0.0065 

5 781.7 62.2 0.0085 

6 763.9 61.4 0.0105 

7 775.1 61.9 0.0125 

8 748.3 60.7 0.0145 

9 746.8 60.6 0.0165 

10 757.6 61.1 0.0185 

11 780.0 62.1 0.0205 

12 788.2 62.5 0.0225 

13 756.1 61.0 0.0245 

14 759.2 61.2 0.0265 

15 767.1 61.5 0.0285 

16 789.9 62.6 0.0305 

Lead Ring Longitudinal Cross Section, Hardness Traverse No. 2 

Reading Knoop Approx. HRC 

Distance From 

Surface 

(in) 

1 501.5 47.4 0.001 

2 710.0 59.0 0.002 

3 756.1 61.0 0.003 

4 745.3 60.6 0.004 

5 751.4 60.8 0.005 

6 756.1 61.0 0.006 

7 754.5 61.0 0.007 

8 753.0 60.9 0.008 

9 767.1 61.5 0.009 

10 756.1 61.0 0.010 

11 759.2 61.2 0.011 

12 765.5 61.5 0.012 

13 760.8 61.3 0.013 

14 759.2 61.2 0.014 

15 756.1 61.0 0.015 

16 767.1 61.5 0.016 

16
Table 8. Microhardness of a longitudinal section of the lag-ring from the 

raceway surface to the core—Knoop scale, 500-gmf load, 50× 

objective. 

Reading Knoop Approx. HRC 

DistanceFrom 

Surface 

(in) 

1 739.2 60.3 0.0015 

2 767.1 61.5 0.0025 

3 762.4 61.3 0.0045 

4 801.7 63.1 0.0065 

5 767.1 61.5 0.0085 

6 784.9 62.3 0.0105 

7 773.5 61.8 0.0125 

8 775.1 61.9 0.0145 

9 763.9 61.4 0.0165 

10 788.2 62.5 0.0185 

11 781.7 62.2 0.0205 

12 783.3 62.3 0.0225 

13 784.9 62.3 0.0245 

14 786.6 62.4 0.0265 

15 788.2 62.5 0.0285 

16 803.4 63.1 0.0305 

Table 9. Nanohardness testing results, lead-ring section. 

Results 

Region Tested Average of Nine Readings 

(GPa) 

Standard Deviation 

Surface 11.5 ±1.3 

Bulk 11.4 ±0.9 

The average Rockwell C scale hardness measured directly on the surface of two lag rollers was, 

for each lag roller, within the drawing requirement of 60–64 HRC (see table 12). 

Microhardness measurements were performed on a cross section of a lag roller, from just 

beneath its surface to its core. The hardness values, converted to the Rockwell C scale, varied 

from within to slightly below the specification requirement of 60–64 HRC (see table 13). The 

two hardness values exhibited within the cross section, 59.1 and 59.9 HRC, were not so far 

below the drawing’s minimum hardness requirement as to have likely contributed to the 

premature failure of the HHP assembly. 

17
Table 10. Surface-hardness values of 

two lead rollers. HRC 

scale—150-kgf load. 

Lead Roller No. 1 

Reading HRC 

1 59.3 

2 60.4 

3 60.1 

4 60.9 

5 59.1 

6 59.1 

7 59.3 

8 58.5 

Average 59.6 

Lead Roller No. 2 

1 64.4 

2 63.8 

3 64.1 

4 63.4 

5 63.7 

6 66.3 

7 62.5 

8 63.1 

Average 63.9 

Table 11. Cross-section hardness traverse of the lead roller (no. 1) that 

exhibited the lower-than-average surface hardness—Knoop 

scale, 500-gmf load, 50× objective. 

Reading Knoop Approx. HRC 

Distance From 

Surface 

(in) 

1 760.8 61.3 0.0015 

2 767.1 61.5 0.0025 

3 737.7 60.2 0.0035 

4 721.5 59.5 0.0045 

5 770.3 61.7 0.0055 

6 740.7 60.3 0.0065 

7 725.8 59.7 0.0075 

8 749.9 60.8 0.0085 

9 730.3 59.9 0.0095 

10 707.1 58.8 0.0105 

11 733.2 60.0 0.0115 

12 718.6 59.3 0.0125 

13 717.1 59.3 0.0135 

14 720.0 59.4 0.0145 

15 759.2 61.2 0.0155 

16 748.3 60.7 0.0165 

18
Table 12. Surface hardness of two 

lag rollers. HRC scale—

150-kgf load. 

Lag Roller No. 1 

Reading HRC 

1 56.1 

2 61.7 

3 65.5 

4 62.2 

5 61.4 

6 62.1 

7 61.0 

8 60.3 

Average 61.3 

Lag Roller No. 3

1 60.3 

2 60.5 

3 60.3 

4 60.5 

5 60.6 

6 60.9 

7 60.9 

8 61.2 

Average 60.7 

Table 13. Microhardness testing results, lag-ring roller no. 1—Knoop 

scale, 500-gmf load, 50× objective. 

Reading Knoop Approx. HRC 

Distance From 

Surface 

(in) 

1 739.2 60.3 0.0015 

2 753.0 60.9 0.0025 

3 760.8 61.3 0.0035 

4 745.3 60.6 0.0045 

5 754.5 61.0 0.0055 

6 776.8 62.0 0.0065 

7 760.8 61.3 0.0075 

8 742.2 60.4 0.0085 

9 712.8 59.1 0.0095 

10 745.3 60.6 0.0105 

11 748.3 60.7 0.0115 

12 733.2 60.0 0.0125 

13 731.7 59.9 0.0135 

14 736.2 60.1 0.0145 

15 745.3 60.6 0.0155 

16 746.8 60.6 0.0165 

19
4.3 Scanning Electron Microscopy 

Scanning electron microscopy (SEM) revealed that a lag-ring fracture origin identified by lightoptical microscopy (see the origin adjacent to the white arrow with the letter A in figure 5) 

exhibited a smeared surface topography (see figures 24 and 25). It was not clear if the surface 

topography of the crack origin was smeared due to the repeated closure and rubbing of the 

initiating crack or to post-fracture handling. However, there was no indication that the fatigue 

crack initiated anywhere except the spalled raceway. Areas of the fracture face that were not 

smeared were predominantly transgranular—the expected fracture morphology for fatigue 

failures of this alloy (see figure 26). 

The spalling exhibited by several of the lag rollers was also examined via SEM (see figure 27). 

The fracture topography of the spalled surfaces exhibited beach marks and a transgranular 

surface morphology (see figures 28 and 29)—characteristics of an RCF mode of failure. 

Fracture 

surface 

Origin 

Spalled 

raceway

Figure 24. One of the fracture origins on the surface of the lag ring (identified via light-optical 

microscopy and labeled with the nomenclature A in figure 13). 

5. Discussion 

The failed HHP assembly was discovered on wing after approximately 637 flight hours. It is not 

known when the failure initiated. Laboratory inspection of the HHP assembly revealed a greater 

level of damage (a combination of cracks, fractures, and spalling) in the lag ring and lag rollers 

20
Figure 25. Detail view of the origin shown in figure 24 showing that the 

topography of the origin is smeared. 

Figure 26. The fracture morphology of the lag ring was transgranular—typical 

of a fatigue-fracture mode. 

21
Figure 27. Surface spalling (fatigue pit) exhibited by a lag roller—typical of the 

spalling observed on the other lag rollers. 

Figure 28. Detail of the spall depicted in figure 27 showing the beach marks 

typical of RCF-induced spalling. 

22
Figure 29. Detail of figure 28 showing the transgranular fracture morphology of 

the spalled surface typical of RCF-induced spalling. 

than in the lead ring and lead rollers. Assuming the level of damage exhibited by each part was 

proportionate to the level of loading to which it was subjected as well as the length of time since 

the onset of failure, the lag roller set was likely subjected to higher loads in service than the lead 

roller set, and the HHP assembly failure likely initiated either in the lag ring or lag rollers. 

Inspection of the lead-roller bearing set revealed that the lead ring exhibited spalling damage and 

the mating lead rollers did not. If the mechanism of failure in the lead-roller set was also the 

mechanism of failure in the lag-roller set, then it is likely that the lag ring failed before the lag 

rollers, and, therefore, the HHP assembly failure initiated in the lag ring. 

Visual and metallographic characterization of the lag-ring spalling was not possible because of 

its advanced state of damage. However, the spalling within the lead ring and lag rollers could be 

characterized. Arrowhead-shaped spalling—the result of surface-initiated cracking caused by 

the interaction of RCF with sliding—was not exhibited by the spalling in the lead ring and the 

lag rollers (5, 6). Instead, the lead ring and the lag rollers exhibited oval-shaped spalling with 

steep sides likely caused by a subsurface crack that initiated RCF (5, 6). Metallography also 

indicated that spall cracks within the lead-ring raceway were subsurface initiating. Again, if the 

mechanism of failure in the lead-roller set was operating in the failure of the lag-roller set, it is 

likely that lag-ring spalling may have been subsurface initiating (due to RCF). 

Metallography of the lead ring’s lag ring, lead rollers, and lag rollers indicated that the parts were 

manufactured in accordance with the governing roller-bearing-set drawing requirements— 

minimizing the possibility that failure initiation was due to a manufacturing defect. 

23
Metallography and hardness testing indicated that the microstructures of the lead rings, lag rings, 

lead rollers, and lag rollers were relatively uniform from surface to core. The absence of a 

surface layer exhibiting a relatively soft, over-tempered or relatively hard, untempered 

Martensitic structure indicated that the parts had not been adversely heated during service (and 

during the failure)—a characteristic typical of failures caused by lubrication problems. 

6. Recommendations 

• Verify that bearing misalignment is not a contributor to premature HHP assembly failures 

by reviewing applicable assembly limits and then checking a sample of HHP assemblies 

with a variety of service hours on wing. 

• Verify that lubrication is not a contributor to premature HHP assembly failure by 

establishing the baseline oil level, quality, cleanliness, and water content for a sampling of 

HHP assemblies successfully achieving approximately 1200 hr of service (or, if possible, 

approximately 2400 total hr). Quality analysis should include tests for viscosity, additive 

content, and water content. Cleanliness analysis should characterize the content of 

particles in accordance with the ISO Method 4406 (7). Oil sampling should be performed 

at the time of each HHP oil change and/or at a time of service just short of the average 

HHP failure time. Sample analyses of assemblies that fail prior to reaching 1200 hr of 

service should indicate whether the measured factors are significant and/or whether the 

interval of oil sampling needs to be changed to obtain viable data. 

• If the level of the service-load spectrum cannot be reduced, consider utilizing material 

alternatives that may provide a greater service life than the M50 steel used in the current 

roller and ring designs. Literature indicates that M50NiL (a carburizing tool steel) and 

Pyrowear 675 (a carburizing stainless steel) may provide an increase in RCF life (8, 9). 

• If early detection of HHP assembly failures is required to mitigate air-worthiness risks, the 

feasibility of a nondestructive vibration-analysis method should be examined. Portable 

accelerometers and data-acquisition devices may provide a level of signal fidelity sufficient 

for an operator to discern the frequency spectrum that defines a properly functioning HHP 

assembly and one that is in the early stages of failure (from aircraft rotating their head 

assemblies while on the ground). Baseline vibration spectra of intact HHP assemblies 

could be obtained from aircraft with newly installed HHP assemblies. The vibration 

spectrum of HHP assemblies in the first stages of failure may have to be obtained through 

an iterative process, where a sampling of aircraft are inspected at predetermined intervals, 

and their HHP assemblies are torn down for analysis with each significant change in 

spectra recorded, until the onset of a sufficient number of failures are actually recorded 

(10). 

24
7. Conclusions 

• High-service loads likely caused the premature failure of the subject HHP assembly, 

starting in the lag ring by RCF-induced spalling. 

• The damage caused by the RCF-initiated spalling, combined with the service loads, led to 

the subsequent fracture of the lag ring and one of the lag rollers. 

• It could not be determined at what number of service hours spalling and fracture initiated in 

the lag ring and one of the lag rollers. 

• The lead roller set and lag roller set did not exhibit the heat-related microstructural damage 

typically generated in failures caused by lubrication problems. 

• Metallography and chemical analyses revealed the lead-roller set and lag-roller set were 

free from manufacturing anomalies known to contribute to RCF failures. 

25
8. References 

1. AMS 6491. Steel Bars, Forgings, and Tubing 4.1cr 4.2mo 1.0V (0.80–0.85c) Premium 

Aircraft – Quality for Bearing Applications, Double Vacuum Melted. 2005. 

2. Huang, P. U.S. Army Research Laboratory, Aberdeen Proving Ground, MD. Electronic 

correspondence, 14 April 2006. 

3. Failure Analysis and Prevention. ASM Handbook; Vol. 11. ASM International: Metals 

Park, OH, 1986; pp 504–505. 

4. ASTM E112. Standard Test Methods for Determining Average Grain Size. Annu. Book 

ASTM Stand. 2004.

5. Metal Suppliers Online. http://www.suppliersonline.com/propertypages/M50.asp (accessed 

9 May 2006). 

6. Wulpi, D. J. Understanding How Components Fail. American Society for Metals 1985, 

188. 

7. ISO Method 4406. Hydraulic Fluid Power – Fluids-Method for Coding the Level of 

Contamination by Solid Particles, International Organization for Standardization, 1999. 

8. Wells, M. G. H.; Beck, J. C.; Middleton, R. M.; Huang, P. J.; Wert, D. E. Rolling Contact 

Fatigue Behaviour of Pyrowear 675. Surface Engineering 1999, 15 (4), 321–323. 

9. Averbach, B. L.; Van Pelt, S. G.; Pearson, P. K.; Bamberger, E. N. Surface-Initiated 

Spalling Fatigue in M-50 and M-50Nil Bearings. Journal of the Society of Tribologists and 

Lubrication Engineers 1991, 47, 10, 843. 

10. Practicing Oil Analysis. http://www.practicingoilanalysis.com/article_detail.asp?articleid= 

603&relatedbookgroup=OilAnalysis (accessed 16 May 2006). 

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