A Multi-Scale System Analysis and Verification for Improved Contact Fatigue Life Cycle of a Cam-Roller System

[+] Author and Article Information
D. Y. Hua, L. E. Seitzman

Surface Engineering and Tribology, Advanced Materials Technology, Caterpillar Inc., Peoria, IL 61656-1875

K. Farhang

Department of Mechanical Engineering and Energy Processes,Southern Illinois University at Carbondale, Carbondale, IL 62901-6603

J. Tribol 129(2), 321-325 (Jan 09, 2007) (5 pages) doi:10.1115/1.2540572 History: Received May 02, 2006; Revised January 09, 2007

Surface distress in the form of contact fatigue is encountered in cam-roller systems. The contact fatigue appears to be initiated at micrometer-scale subsurface region. High stress is a result of the macro-scale requirement on the cam-roller motion event that produces high contact loads due to inertia of the roller and its follower link. Sliding of the roller and its impact onto the cam surface further compounds the detrimental effect of contact load. While conventionally a Hertz contact stress analysis can be used in ascertaining contact stress and maximum subsurface von Mises stress, it generally underestimates the stress when compared to the micrometer-scale subsurface stresses due to the presence of surface roughness. Contact analyses of cam and roller with rough surfaces are performed to examine the effects of two surface treatments. These involve surface finishing process in which a surface is rendered smooth, and the addition of a coating to the roller surface. Measurements of such cam and roller surfaces are used in micro-contact analysis module of a Surface Distress Analytical Toolkit to examine the effect of surface finish and coating on maximum subsurface stress. It is found that smooth surface provides a 53% reduction in maximum subsurface stress. The analysis also shows that the addition of coating further reduces subsurface stress nearly 7%. The impact of the combined treatment of the surface is an increase in contact fatigue life of the cam-roller system by nearly two orders of magnitude. The above findings are confirmed by laboratory tests using six rollers with various degrees of finishing processes, and with and without addition of coating to the surfaces. Examination of the rollers indicates a general improvement in roller performance due to addition of coating. Most notably, the combination of finishing process and coating was found to provide the best contact fatigue life since the corresponding rollers showed no observable wear even after testing for 2161h, or the same number of cycles accumulated over about 500,000 truck miles.

Copyright © 2007 by American Society of Mechanical Engineers
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Figure 1

Camshaft and rocker arm assembly

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Figure 2

Surface distress regions on a roller

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Figure 3

Schematic illustration of cam-roller contact

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Figure 4

Surface measurement

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Figure 5

Contact patch and measured surface patch

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Figure 6

Iterative process to determine the corresponding applied force to the measured surface patch

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Figure 7

Contact pressure over the cam surface-used roller (smoother surface) giving a max pressure of 6.75GPa (std=0.34GPa)

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Figure 8

Contact pressure over the cam surface-unused roller (rougher surface) giving a max pressure of 13.7GPa (std=0.68GPa)

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Figure 9

Maximum von Mises stress in subsurface

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Figure 10

Contact fatigue life cycle predictions—high pressure region is on the right in each pair: Contact fatigue life is defined as the stress cycles to initiate crack at 36.8% of probability of survival.

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Figure 11

Surfaces after 100h of bench test: (a) ASF surface and (b) AG surface

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Figure 12

Coated ASF and AG rollers after 100h of bench test

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Figure 13

Laboratory tests showing coating thickness across the face of each roller. Arms 1 through 4 correspond to ASF processed coated rollers and Arms 5 and 6 are the AG coated rollers.




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