The use or application of a material (lubricant) to reduce friction and/or wear that can occur when two surfaces undergo relative motion while in contact under an applied load. Friction and lubrication performance depend upon the relative speed of the surfaces, the lubricant viscosity, and the applied load. There are four major lubrication regimes or types of behavior.
- Boundary Lubrication – The lubricant film is too thin to form a fluid layer that completely separates two surfaces. Asperities on the surfaces collide. Friction and wear depend upon the presence of chemical additives that adsorb and form molecular layers on the surfaces.
- Elastohydrodynamic Lubrication – Elastohydrodynamic or EHD occurs in rolling element bearings and certain gears when very high loads are concentrated on small surface areas. Under these conditions, the surfaces deform elastically or flatten to increase the surface area that bears the load. Lubricant is trapped between the surfaces, and its viscosity can increase under the pressure. As a result, the lubricant is able to form a hydrodynamic film and separate the surfaces.
- Hydrodynamic Lubrication – Under appropriate conditions, the relative motion of two sliding surfaces causes a continuous fluid film to form and completely separate the surfaces. This requires a balance between the sliding speed, the applied load, and the lubricant viscosity. Fluid Film Lubrication is another name for this type of lubrication.
- Mixed Lubrication – A lubrication regime where the load is supported partially by a fluid film and partially by the surfaces in contact
- Hydrostatic Lubrication – Lubricant is supplied under pressure to a plain bearing. This applied pressure forces the lubricant to form a continuous fluid film that completely separates the surfaces. Hydrostatic lubrication occurs typically during start-up of plain bearings. Hydrodynamic lubrication becomes effective when plain bearings are in motion.