Uncertainty of pin height measurement for the determination of wear in pin-on-plate test

The paper concerns measurement of pin height for the determination of wear in a pin-on-plate (POP) or pin-on-disc (POD) test, where a pin is mounted on a holder that can be fixed on the test rig and removed for measurements. The amount of wear is assessed as difference of pin height before and after the test, using the distance between holder plane and pin friction plane as measurand. A series of measurements were performed in connection with POP testing of different friction material pins mounted on an aluminium holder. Pin height measurements were carried out on a coordinate measuring machine (CMM), achieving an expanded measurement uncertainty (k = 2) better than 1 μm. A simple dedicated fixture adaptable to workshop environment was developed and its metrological capability investigated, estimating an average uncertainty of measurement in the order of 5 μm (k = 2). Fixture measurements were compared with CMM measurements using the normalised En value. Acceptable En values (En < 1) were obtained in 58% of the measurements, showing a discrepancy which was explained in terms of different probing patterns on CMM and fixture.