Metallic Contact and Friction between Sliding Surfaces

Abstract

A new device has been developed and used to study metallic contact and friction between sliding, lubricated surfaces. The system consists basically of a fixed metal ball loaded against a rotating cylinder. The extent of metallic contact is determined by measuring both the instantaneous and average electrical resistance between the two surfaces. Friction between the ball and cylinder is recorded simultaneously with contact. In general, the electrical resistance was found to oscillate rapidly between an extremely low value and infinity, suggesting that metallic contact is discontinuous. The average resistance of an oil film is therefore a time-average, that is, a measure of the per cent of the time that metallic contact occurs. The results indicate that metallic contact is much more prevalent than would have been expected from other published studies in which electrical resistance or discharge methods were employed. Using this apparatus, the entire region from hydrodynamic (no metallic contact) to pure “boundary” lubrication (continuous metallic contact) can be readily investigated. Load, speed, mineral oil viscosity, the presence of additives, and operating time were found to be important variables influencing metallic contact. The apparatus is particularly useful in studying the action of antiwear and “extreme pressure” additives. It allows one to measure not only the effectiveness of these compounds in reducing metallic contact, but also the rate at which they act and the durability of protective films which may form.