Analysis of failure mechanisms in a planetary gear

Katapadi Vadiraja Sudhakar*, Levi George, and Nathan Huft

Montana Tech of the University of Montana, Butte, MT 59701, USA

*Corresponding Author: Tel.: 406-496-4267; Fax: 406-496-4664 E-mail: kvsudhakar@mtech.edu

Abstract

A planetary gear of an off highway haul truck that failed suddenly during haulage was investigated to determine the possible mechanisms/causes of fracture. The test methods included visual examination, dye-penetrant, hardness, metallography and SEM analysis with EDX attachment. Based on a detailed analysis, it was determined that the planetary gear was properly manufactured and the failure was predominantly due to fatigue but the presence of non-metallic inclusions lead to the unexpected and sudden fracture of the gear causing the haul truck to come to a sudden stop.

Keywords: planetary gear, failure mechanisms, non-destructuvie testing, optical microscopy, scanning electron microscopy

1. Introduction

A planetary system is a gear reduction system in a transmission. This system consists of an engine driving

a sun gear, which in turn drives one or more planetary gears, then an outer ring which drives the off

highway haul truck. Planetary systems are used in a wide variety of applications that are as small as

fractions of an inch to several feet in diameter. The gear under consideration was approximately 12.6

inches in size from tooth to tooth. Planetary gear has about 23 teeth with an internal diameter of 7.5 inches

and weighs approximately 100 pounds. This particular system consists of a large sun gear, four planetary

gears, and an outer ring attached to the inside diameter of the wheels as shown in Figure 1(a). Each tooth

goes through cyclic loading, and each fillet radii goes through high tensile and compressive stresses. The

motor of the haul truck drives a shaft, which drives the sun gear. The sun gear drives all four planetary

gears simultaneously, while the planetary gears drive the outer ring. This outer ring is what drives the rear

wheels. Planetary gear creates different gear ratios for the transmission. This particular gear was forged

to obtain flow lines that are responsible for high fatigue strength. As is well known, forged gears have

higher fracture toughness in comparison to cast gears. The grain orientation/flow lines can be controlled

when forging a part. The grain flow or growth is parallel to the gear surface to resist crack propagation.

The teeth also go through a post machining process to obtain a smoother surface that improves fatigue

resistance. These gears are subjected to gas-carburizing process that increases the carbon content to about

1.0% by weight. The gear is heated to about 950˚C in a carburizing atmosphere. The time and depth of

carburization depends on the temperature, base metal carbon content, and diffusivity of carbon in

austenite. There are a few literatures [1-2] that are relevant to the current investigation, but are different

in terms of the material composition and also the application and circumstances related to failure. The

objective of this investigation is to determine the possible cause/s for the sudden fracture of the planetary

gear.

1.1 Failure observation

The gear of interest was part of the drive train in an off-highway haul truck, which was operated in an

open pit hard rock mine. Standard maintenance procedures at this mine specify that the drive train gears

be replaced after every 20,000 hours of operation. At the end of their service life, the gears are replaced,

regardless of their apparent condition. The failure of this planetary gear seized the truck’s rear axle,

causing it to rapidly come to a complete stop. Fortunately, no one was injured as a result of this unexpected

failure.

2. Experimental Procedure

2.1 Visual Examination

Visual examination of the failed gear revealed several fracture pieces/surfaces as shown in Fig. 1(b). A

smaller portion of the gear disintegrated upon failure. The gear showed evidences of macroscopic plastic

deformation that occurred after final fracture. Macroscopic beach marks were also identified on one of the

fracture surfaces, as shown in Fig. 2, indicating that this fracture surface was formed through fatigue

process.

Figure 1 (a) Location of the planetary gear in the system. Fig.1 (b) Fractured/disintegrated planetary gear.

Fig. 2 Macroscopic fatigue fracture surface

2.2 Material (gear) composition

The chemical composition of the gear metal was analyzed using direct reading (optical emission type)

vacuum spectrometer. Based on this analysis, the material was determined to be one of low carbon, low

alloy steel type as shown in Table 1.

Table 1 Chemical composition of the gear

Elements C Cr Mn Ni Mo N Fe

Wt. % 0.21 0.96 0.98 0.13 0.31 0.006 Balance

2.3 Nondestructive (dye-penetrant) test

Dye-penetrant testing revealed extensive cracking in the fillet radii on the outer gear surface, as well as

on the inner gear surface, as shown in Fig. 3.

Fig. 3. Penetrant test shows cracks on inner surface (left) and outer surface (right).

2.4 Microstructural examination

The optical microscopy of the gear showed a tempered martensite structure at the core surrounded by a

carburized case, as shown in Fig. 4. A higher magnification view of the tempered martensite core is shown

in Fig. 5. This tempered martensite is responsible for the toughness of the core. The uniform structure

demonstrates that the gear was properly heat treated [3-5].

Fig. 4. Micrograph showing case (dark layer) and core

(light area)

Fig. 5. Tempered martensite in gear core.

2.5 Microindentation hardness testing

A Vickers microindentation hardness traverse was performed on a section of a gear tooth. As expected,

the hardness was maximum (HV 760) near the surface and progressively decreased as the distance from

the surface increased until the hardness plateaued approximately 0.1” into the gear. A plot of the hardness

profile is shown in Fig. 6.

Fig. 6. Micro hardness Traverse.

2.6 Electron (SEM) fractography

The fracture surfaces were analyzed using the SEM. The fracture surfaces showed the presence of non-

metallic inclusions, especially the sulphide inclusions (shown in EDX analysis of Fig.7) that were

presumably introduced during steel production and/or subsequent thermo-mechanical processing. The

overall fracture was a ductile fracture as can be clearly seen in Fig. 7 [6-8].

0.000 0.050 0.100 0.150 0.200

350

400

450

500

550

600

650

700

750

800

Depth (inches)

Ha

rdn

ess

(V

ick

ers

)

Microhardness Traverse

Fig. 7. Scanning electron micrograph and EDX (of the inclusion) of the fracture surface.

2.7 Corrosion test

Basic corrosion test was performed to check the possibility of any corrosion or corrosion fatigue

mechanisms. With gear design, lubrication is necessary to protect both gears from excessive wear. Figure

8 shows how oil is used in this gear system.

Fig. 8: Hydrodynamic layer between two gears shown on the left. The boundary layer of oil that forms on the

gear surface is shown on the right.3

A boundary layer of oil is formed on the gear surface to protect against wear. This boundary layer is

critical for low gear RPM for protection. At higher RPM’s a Hydrodynamic layer (figure on the right) is

formed. This layer thickness always exceeds the surface roughness of both gears meaning lubrication will

always be between the two surfaces. Higher temperatures and excessive rpm can cause gear contact

through both of these layers. In this particular case, lubrication did not cause any corrosion and the correct

60W oil was used.

3. Summary

� The gear was uniformly carburized/heat treated showing a ductile fracture surface which is

expected from a tougher tempered martensite structure.

� The gear had sufficient surface hardness (primarily due to a high carbon martensite structure) with

a tough inner core.

� The composition of the gear was determined as low alloy steel typically used as a carburizing grade

steel.

� No evidence of corrosion and/or corrosion fatigue was observed.

� The presence of non-metallic inclusions was largely responsible for the sudden fracture after

relatively a longer service life as evidenced by the fatigue striation marks on the planetary gear’s

surface.

Acknowledgements

The authors would like to thank Gary Wyss (CAMP-Lab/Equipment specialist) and Bill Gleason,

Associate Professor for their support in performing the SEM work.

References

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(2006).

[2] Sunyoung Park, Jongmin Lee, Uijun Moon and Deugjo Kim, Failure analysis of a planetary gear

carrier of 1200 HP transmission, Engineering Failure Analysis, 17(2), 521–529, (2010).

[3] Yue-Chao Zhao and Ying Fu, Material selection and heat treatment of gears, Mechanical Research and

Application, 20(5), 70–71, (2010).

[4] W.F. Smith, Foundations of Materials Science and Engineering, McGraw-Hill, (2004).

[5] Heat Treatment, ASM handbook, American Society of Metals, Metals Park, OH, (1991).

[6] C.R. Brooks, Failure analysis of engineering materials, McGraw-Hill, 4, (2002).

[7] Failure analysis and prevention, ASM handbook, Metals Park (OH), 11, (1986).

[8] Fatigue and fracture, ASM handbook. Metals Park (OH), 19, (1996).