Fluid film or hydrodynamic lubrication occurs when a shaft rotating in a bearing is supported by a layer of oil and the shaft is not in contact with the bearing. Shaft must be rotating at a certain speed before hydrodynamic lubrication takes place.
Before the rotation commences the shaft rests on the bearing surface. When the rotation commences the shaft moves up the bore until an equilibrium condition is reached when the shaft is supported on a wedge of lubricant. The moving surfaces are then held apart by the pressure generated within the fluid film. Journal bearings are designed such that at normal operating conditions the continuously generated fluid pressure supports the load with no contact between the bearing surfaces. This operating condition is known as thick film lubrication and results in a very low operating friction and extremely low bearing load
The rotating shaft drags a wedge of oil beneath it that develops a pressure great enough to support the shaft and eliminate contact friction between the shaft and bearing
Viscosity of the lubricant is an important feature. The higher the viscosity, the higher the friction between oil and shaft, but the thicker the hydrodynamic film. However friction generates heat, which will reduce the viscosity, the thickness of the film and may result in metal to metal contact. Using an oil with a low initial viscosity will also result in a reduced oil film thickness. We have to be very careful that the distance between the two surfaces is greater than the largest surface defect. The distance between the two surfaces decreases with higher loads on the bearing, less viscous fluids, and lower speeds. Hydrodynamic lubrication is an excellent method of lubrication since it is possible to achieve coefficients of friction as low as 0.001 (m=0.001), and there is no wear between the moving parts. However because the lubricant is heated by the frictional force and since viscosity is temperature dependent, additives to decrease the viscosity's temperature dependence are used.
