Machinery's Handbook, 31st Edition
2396 PLAIN BEARINGS Hydrostatic Bearings.— Hydrostatic bearings are used when operating conditions re- quire full film lubrication that cannot be developed hydrodynamically. The hydrostati cally lubricated bearing, either thrust or radial, is supplied with lubricant under pres- sure from an external source. Some advantages of the hydrostatic bearing over bearings of other types are: low friction; high load capacity; high reliability; high stiffness; and long life. Hydrostatic bearings are used successfully in many applications including machine tools, rolling mills, and other heavily loaded slow-moving machinery. However, special ized techniques, including a thorough understanding of hydraulic components external to the bearing package is required. The designer is cautioned against use of this type of bear ing without a full knowledge of all aspects of the problem. Determination of the operating performance of hydrostatic bearings is a specialized area of the lubrication field and is described in specialized reference books. Guide Bearings.— This type of bearing is generally used as a positioning device or as a guide to linear motion such as in machine tools. Fig. 2 shows several examples of guide way bearing designs. It is normal for this type of bearing to operate in the boundary lubri cation region with either dry, dry film such as molybdenum disulfide (MoS 2 ) or tetrafluo - rethylene (TFE), grease, oil, or gaseous lubrication. Hydrostatic lubrication is often used to improve performance, reduce wear, and increase stability. This type of design uses pumps to supply air or gas under pressure to pockets designed to produce a bearing film and maintain complete separation of the sliding surfaces.
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Fig. 2. Types of Guide Bearings Design.— The design of a sliding bearing is generally accomplished in one of two ways: 1) a bearing operating under similar conditions is used as a model or basis from which the new bearing is designed; and 2) in the absence of any previous experience with simi- lar bearings in similar environments, certain assumptions concerning operating condi- tions and requirements are made and a tentative design prepared based on general design parameters or rules of thumb. Detailed lubrication analysis is then performed to establish design and operating details and requirements. Modes of Bearing Operation.— The load-carrying ability of a sliding bearing depends upon the kind of fluid film that is formed between its moving surfaces. The formation of this film is dependent, in part, on the design of the bearing and, in part, on the speed of rotation. The bearing has three modes or regions of operation designated as full-film, mixed-film, and boundary lubrication with effects on bearing friction, as shown in Fig. 3. In terms of physical bearing operation these three modes may be further described as follows:
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