Introduction

The satisfactory operation of both hydrostatic and hydrodynamic bearings requires that the solid surfaces which constitute the bearing faces are completely separated by the intervening fluid film. Since the bearing surfaces are then not physically touching, the resistance to their tangential motion, that is, the force of friction, is directly attributable to viscous losses in the lubricant. If the lubricant (whether liquid or gas) exhibits Newtonian rheological behaviour with constant viscosity then the value of this frictional force, and the associated coefficient of friction, will increase with the value of the tangential sliding velocity. We have seen (eqn (7.27)) that the coefficient of friction within a hydrodynamically lubricated bearing is generally dependent on the square root of the group ULη/W where U is the relative sliding speed of the surfaces, W/L the normal load supported per unit length, and η the Newtonian viscosity. A reduction in speed, or an increase in the specific load on the bearing, leads to a fall in the friction coefficient. However, there is a limit to this process: when the specific load is very high, or the relative sliding speed small, it is difficult to build up a sufficiently thick film to entirely separate the bearing faces, and so there will be some mechanical interaction between opposing surface asperities. This is inevitable, even allowing for the large increase in effective lubricant viscosity and the elastic flattening of the surface profiles that can occur in the elasto-hydrodynamic regime.