Analysis of failure mechanisms in a planetary gear

Transcription

1 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.: ; Fax: 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, dyepenetrant, 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

2 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.

3 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.

4 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. % 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.

5 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 Fig. 5. Tempered martensite in gear core. (light area)

6 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. Microhardness Traverse Hardness (Vickers) Depth (inches) 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 nonmetallic 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].

7 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

8 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.

9 References [1] Osman Asi, Fatigue failure of a helical gear in a gearbox, Engineering Failure Analysis, 13, , (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), , (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).