Machinery's Handbook, 31st Edition
2400 PLAIN BEARINGS and stationary members, are throttling bushings and labyrinths. Both types operate by fluid-throttling action in narrow annular or radial passages. Clearance seals are frictionless and very insensitive to temperature and speed. They are chiefly effective as devices for limiting leakage rather than stopping it completely. Although they are employed as primary seals in many applications, the clearance seal also finds use as auxiliary protection in contact-seal applications. These seals are usually designed into the equipment by the designer himself, and they can take on many different forms. Advantages of this seal are that friction is kept to an absolute minimum and there is no wear or distortion during the life of the equipment. However, there are two significant dis advantages: The seal has limited use when leakage rates are critical; and it becomes quite costly as the configuration becomes elaborate. Static Seals: Static seals such as gaskets, “O” rings, and molded packings cover very broad ranges of both design and materials. Some of the typical types are as follows: 1) Molded packings: a) lip type, and b) squeeze- molded; 2) simple compression packings; 3) diaphragm seals; 4) nonmetallic gaskets; 5) “O” rings; and 6) metallic gaskets and “O” rings. Data on “O” rings are found starting on page 2666. Detailed design information for specific products should be obtained directly from manufacturers. Hardness and Surface Finish.— Even in well-lubricated full-film sleeve bearings, momentary contact between journal and bearing may occur under such conditions as starting, stopping, or overloading. In mixed-film and boundary-film lubricated sleeve bearings, continuous metal-to-metal contact occurs. Hence, to allow for any necessary wearing-in, the journal is usually made harder than the bearing material. This arrange- ment allows the effects of scoring or wearing to take place on the bearing, which is more easily replaced, rather than on the more expensive shaft. As a general rule, recommended Brinell Hardness* of the journal is at least 100 points harder than the bearing material. The softer cast bronzes used for bearings are those with high lead content and very little tin. Such bronzes give adequate service in boundary- and mixed-film applications where full advantage is taken of their excellent “bearing” characteristics. High-tin, low-lead content cast bronzes are the harder bronzes and these have high ulti mate load-carrying capacity: higher journal hardnesses are required with these bearing bronzes. Aluminum bronze, for example, requires a journal hardness in the range of 550 to 600 BHN (Brinell Hardness Number). In general, harder bearing materials require better alignment and more reliable lubrica tion to minimize local heat generation if and when the journal touches the shaft. Also, abrasives that find their way into the bearing are a problem for the harder bearing materials and greater care should be taken to exclude them. Surface Finish: Whether bearing operation is complete boundary, mixed film, or fluid film, surface finishes of the journal and bearing must receive careful attention. In applica tions where operation is hydrodynamic or full-film, peak surface variations should be less than the expected minimum film thickness; otherwise, peaks on the journal surface will contact peaks on the bearing surface, with resulting high friction and temperature rise. Ranges of surface roughness obtained by various finishing methods are: boring, broaching, and reaming, 32 to 64 m in (0.813–1.626 μ m), rms; grinding, 16 to 64 m in (0.406–1.626 μ m), rms; and fine grinding, 4 to 16 m in (0.102–0.406 μ m), rms. In general, the better surface finishes are required for full-film bearings operating at high eccentricity ratios because full-film lubrication must be maintained with small clearances, and metal-to-metal contact must be avoided. Also, the harder the material, the better the surface finish required. For boundary- and mixed-film applications, surface finish requirements may be somewhat relaxed because bearing wear-in will in time smooth the surfaces. * Brinell Hardness Number (BHN) also may be identified with the letters HB, HBS for hardness measured with a steel indenter, and HBW for hardness measured with a carbide indenter.
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