Abstract
Introduction: A renewal of interest in large metal-on-metal bearings has been seen due to the introduction of resurfacing prostheses. According to lubrication theory, large metal-on-metal bearings may obtain a film fluid lubrication. The mode of lubrication may be described by the lambda coefficient λ, which is the ratio between the thickness of the lubricant hc and the root mean square roughness of the bearing Rq. If this coefficient λ is higher than 3, a fluid film lubrication is expected. To have this situation, the following parameters must be optimized: diametral clearance and roughness. This presentation investigates the role of these two parameters, based on two commercially available products.
Methods: To determine the λ coefficient, the thickness hc of the lubricant must be determined, as well as the roughness of the bearing Rq. The Hamrock – Dawson equation (1) allows the determination of the thickness hc as a function of the bearing parameters. The roughness Rq is measured by a stylus profilometer.
Results: With a typical load of 3000 N, an angular velocity of 1 rad/s, and a viscosity of 0.005 Pas, the Hamrock – Dawson equation gives the following film thickness hc for a 50 mm metal-on-metal bearing with different diametral clearances:
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Diametral clearance [μm] 100 150 200 250 300<
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Minimum thickness hc [nm] 64.9 47.5 38.1 32.1 27.9
The following roughnesses Rq were measured for two types of resurfacing prosthesis:
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As cast CoCr alloy (BHR by MMT): 23 ± 6 nm
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Wrought-forged CoCr alloy (DUROM by Zimmer): 5 ± 2 nmThe as cast resurfacing prosthesis has a 250 μm diametral clearance and the wrought-forged resurfacing prosthesis has a 150 μm diametral clearance. Therefore, the following λ coefficients for a 50 mm metal-on-metal bearing are obtained:
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As cast CoCr alloy: 0.99
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Wrought-forged CoCr alloy: 6.72
This large difference in the λ coefficients indicates that the lubrication mode of these two different prostheses is probably different. Based on this analysis, the wrought-forged component has ideal lubrication (λ > 3) whereas the as cast does not reach ideal lubrication (λ < 3).
Conclusions: This investigation shows that minute differences in the geometry and in the roughness of a metal-on-metal prosthesis may significantly influence their lubrication behaviour as well as the wear resistance.
Theses abstracts were prepared by Professor Roger Lemaire. Correspondence should be addressed to EFORT Central Office, Freihofstrasse 22, CH-8700 Küsnacht, Switzerland.
References
1 Z. Jin et al; Proc Instn Mech Engrs, 211, part H, 1997, p. 247 Google Scholar
