Monday, December 20, 2010

PLAIN BEARING MATERIALS

Materials used for sliding bearings cover a wide range of metals and nonmetals. To make the optimum selection requires a complete analysis of the specific application. The important general categories are: Babbitts, alkali-hardened lead, cadmium alloys, copper lead,aluminum bronze, silver, sintered metals, plastics, wood, rubber, and carbon graphite.Properties of Bearing Materials.—For a material to be used as a plain bearing, it must possess certain physical and chemical properties that permit it to operate properly. If a material does not possess all of these characteristics to some degree, it will not function long as a bearing. It should be noted, however, that few, if any, materials are outstanding in all these characteristics. Therefore, the selection of the optimum bearing material for a given application is at best a compromise to secure the most desirable combination of properties required for that particular usage.
The seven properties generally acknowledged to be the most significant are:
1) Fatigue resistance;
2) Embeddability;
3) Compatibility;
4) Conformability;
5) Thermal conductivity;
6) Corrosion resistance; and
7) Load capacity.

These properties are described as follows:

1) Fatigue resistance is the ability of the bearing lining material to withstand repeated applications of stress and strain without cracking, flaking, or being destroyed by some other means.

2) Embeddability is the ability of the bearing lining material to absorb or embed within itself any of the larger of the small dirt particles present in a lubrication system. Poor embeddability permits particles circulating around the bearing to score both the bearing surface and the journal or shaft. Good embeddability will permit these particles to be trapped and forced into the bearing surface and out of the way where they can do no harm.

3) Compatibility or antiscoring tendencies permit the shaft and bearing to “get along” with each other. It is the ability to resist galling or seizing under conditions of metal-tometal contact such as at startup. This characteristic is most truly a bearing property, because contact between the bearing and shaft in good designs occurs only at startup.

4) Conformability is defined as malleability or as the ability of the bearing material to creep or flow slightly under load, as in the initial stages of running, to permit the shaft and bearing contours to conform with each other or to compensate for nonuniform loading
caused by misalignment.

5) High thermal conductivity is required to absorb and carry away the heat generated in the bearing. This conductivity is most important, not in removing frictional heat generated in the oil film, but in preventing seizures due to hot spots caused by local asperity breakthroughs or foreign particles.

6) Corrosion resistance is required to resist attack by organic acids that are sometimes formed in oils at operating conditions.

7) Load capacity or strength is the ability of the material to withstand the hydrodynamic pressures exerted upon it during operation.
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