When two surfaces are in relative sliding motion, a normal load may be transmitted between them. Such a system is referred to as a sliding bearing. Under nominally dry conditions, this transmitted load is supported by small irregularities on the bearing surfaces, called asperities. These asperities are in direct contact. Under limited conditions of load and speed, such a bearing will work quite well and experience moderate wear. Materials such as high-leaded tin bronzes have self-lubricating properties that permit fairly high loads to be supported this way
If a lubricant in the form of a fluid or grease is present between the sliding surfaces, a pressure may develop such that a significant portion of the load is transmitted by fluid pressure rather than metal to metal contact. When all the load is supported by fluid pressure, this becomes a full-film or hydrodynamic bearing. In a truly hydrodynamic bearing, the two surfaces are are not in contact. Since fluids are easily sheared without damage, and generally have a lower shear resistance than most solids, relative motion between the two bearing surfaces takes place in the fluid film. This results in a lowering of friction and a reduction or elimination of wear. To achieve hydrodynamic or full-film lubrication in bearings requires special attention to several key variables that influence film formation. Most important among these are the geometry and motion of the bearing surfaces, the nature and consistency of the fluid employed, and the continual supply of this fluid.
The benefit of having the load supported by fluid pressure is very important. It reduces wear and friction by several orders of magnitude. Unfortunately you cannot ensure that the correct conditions exist at all times. Contact between two bearing surfaces will invariably occur at some point during operation. When contact occurs, behavior of the bearing material is very important in the reliable operation of the bearing. I addition to the bearing geometry, bearing kinematics and lubricant, the bearing materials are of crucial importance. It is not difficult to see that the design of lubricated sliding bearings can be quite complex.
The challenge of proper bearing design is a thoughtful process that requires proper alloy selection and bearing tolerances. Well designed bearings often have a very long life and are very low cost from an operational and maintenance point of view.
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