A study of Parched Lubrication
The elastohydrodynamic regime of lubrication is now quite well understood, to the extent that theoretically derived equations for lubricant film thickness are used routinely in engineering design. The boundary lubrication regime, which occurs at slow rubbing speeds and in which the surfaces are separated only by a chemically formed layer, is less clearly understood but many of the underlying concepts, such as the formation of a monomolecular adsorbed film or a thicker, reacted, glass-like layer, have been both demonstrated and modelled.
Between these two regimes there remains, however, a territory about which very little is known. This is the regime where a concentrated contact is heavily starved, permitting no conventional elastohydrodynamic film, and yet there remains between the surfaces a film of lubricant whose presence is governed by its rheology rather than its ability to bond to the rubbing surfaces. Such films exist in applications such as gyroscope bearings and probably in many grease-lubricated systems. This regime has been called “parched lubrication” (E. Kingsbury, Trans. ASME J. Tribol., 107 (1985) 229).
Up until the present it has not been possible to do much more than speculate upon the behaviour of this lubricant regime because such very thin films could not be directly measured. It was possible to deposit lubricant films of known thickness onto surfaces and to measure properties such as friction and wear but it was not possible to relate this to the thickness of such films under operating conditions.
This paper describes an experimental study of parched lubrication which addresses this problem. A recently developed technique for measuring very thin films in concentrated contacts is employed to monitor the occurrence and persistence of thin lubricant films within highly starved, concentrated contacts under a range of conditions. This is combined with IR reflection-absorption spectroscopy to measure the oil film thickness on the out-of-contact surfaces.
The main factors which determine the behaviour and performance of this type of starved film are discussed.