Most bearings operate with a fluid film -- oil, another liquid, or a gas. By far the largest number of bearings are oil lubricated. The oil film can be maintained through pumping by a pressurization system -- in which case the lubrication is termed hydrostatic. Or it can be maintained by a squeezing or wedging of lubricant produced by the rolling action of the bearing itself -- termed hydrodynamic lubrication. If loads are too high or speeds too slow, the hydrodynamic action begins to break down a condition referred to as boundary lubrication.
Hydrostatic lubrication: The main virtue of hydrostatic lubrication is that it can accommodate heavy loads at low speeds because it does not depend upon relative motion to maintain the lubrication film. Instead, lubricant is supplied from a special pump and feed lines to the bearing. The oil is fed through flow restrictors, which generally are stationary. The flow restrictors automatically adjust the oil flow for the applied load. Another advantage of this lubrication system is low deflection in certain load ranges, making it preferred for many high-precision machine tools. The disadvantage of hydrostatic lubrication is its high cost and complexity.
Hydrodynamic lubrication: This form of lubrication occurs more or less naturally in properly finished, sized, and lubricated holes and shafts. Essentially, rotation of the journal causes it to drag lubricant into a wedged-shaped channel generating a load-carrying pressure. The lubricant in this wedge creates sufficient pressure to keep the journal riding on the oil film. This form of lubrication is generally preferred because it is simple and dependable. Also, the lubrication action improves as speed increases, which in most applications goes hand-in-hand with an increase in loads experienced as speed increases. Its main drawback is an inability to carry heavy loads at low speed and appreciable wear under frequent stops or starts, or motion reversals.
The oil for hydrodynamic lubrication can be fed from an oil reservoir. Or the bearing can be made of a porous metal impregnated with oil that "bleeds" to the bearing surface as the shaft rotates. Most porous-metal bearings, however, operate under boundary or mixed-film conditions.
Boundary lubrication: This form of lubrication is essentially a breakdown of hydrodynamic action. At high loads or low speeds, the pressure of the hydrodynamic film cannot prevent metal-to-metal contact. So the opposing surfaces partially ride on an oil film and partially rub together as surface high points come in contact. Lubrication is provided by lubricant decomposition products or surface-active additives which form a thin, soft, solid film on the metal surfaces and prevent metal-junction adhesion.
Boundary lubrication is not the most desirable operating mode, yet at times it is completely unavoidable. It is found mainly with slow-moving loads where the cost and expense of a hydrostatic system is not warranted. Hinge bearings in aircraft landing gears, for example, do not move fast enough to develop hydrodynamic films, yet hydrostatic systems would be too heavy, costly, and cumbersome.