Development of a simplified test method for compliant layered bearings

Abstract

Compliant layered bearings could improve the useful life of replacement load-bearing joints. By incorporating an elastic bearing material, fluid film separation of the articulating surfaces can be maintained and so low friction and negligible wear should occur. Compliant bearings are still under development and so require extensive testing before they can be implanted. To give a realistic evaluation of their performance, the test conditions must be representative of the in vivo situation. To date this has meant using joint simulators. A simplified test method has been developed to measure the friction of compliant layered bearings. It uses a reciprocating materials-screening apparatus adapted to include a dynamic load, very low friction bearings, and a curved counterface. It has been validated by comparison with simulator tests and predictions made from Hertzian contact and elastohydrodynamic theories. An alternative to the Sommerfeld parameter been defined to allow comparison of the different test methods. Tests have shown that of the parameters which affect friction, the predicted Hertzian contact area was the most important. Similar predicted areas gave rise to similar coefficients of friction in both mixed and fluid film lubrication regimes. The simplified method showed improved repeatability and lower systematic errors than the simulator. The method has been used to examine the effect of design factors on the friction generated in compliant layered bearings, comparing the results obtained with those of the simulator. Increased load, decreased counterface roughness, increased entraining velocity and the use of a compliant layer over UHMWPE all reduced the coefficient of friction. Bearing conformity had a mixed effect on friction. Point contacts and line contacts showed similar trends. The mechanical properties of the compliant materials have also been considered including hardness, hysteresis in compression, and creep.