Friction RandomPOD-A new method for friction measurement in noncyclic, multidirectional, dynamic pin-on-disk tests for orthopaedic bearing materials.

The 16-station, computer-controlled RandomPOD wear test device was re-designed into a friction measurement device, Friction RandomPOD. The motion was implemented by a servo-electric x-y-stage and the load was proportional-pneumatic. The direction of sliding, velocity (v), acceleration, and the magnitude of the load (L) varied randomly and continuously. The nominal contact pressure p varied between 0 and 2.4 MPa and v between 0 and 40 mm/s. In the first version of the device, the frictional force components were continuously measured by two miniature load cells in two perpendicular directions. In the second version, the measurement was done with a three-axial, commercial load cell. The resultant frictional force was divided by the instantaneous load in order to obtain the coefficient of friction (μ) at a frequency of 200 Hz. Due to the light and accurate design of the device, vibrations were absent in the measured signals although the measurements were most dynamic. Hence no filtering was needed. Serum lubricated polyethylene/CoCr tests revealed non-symmetric distributions of μ, friction power Pμ, and μ vs. pv.