Machinery's Handbook, 31st Edition
Machine Elements
2393
PLAIN BEARINGS Introduction
On the following pages are given data and procedures for designing full-film or hydrody namically lubricated bearings of the journal and thrust types. However, before proceed- ing to these design methods, it is useful to first review those bearing aspects concerning the types of bearings available; lubricants and lubrication methods; hardness and sur face finish; machining methods; seals; retainers; and typical length-to-diameter ratios for various applications. The following paragraphs preceding the design sections provide guidance in these mat ters and suggest modifications in allowable loads when other than full-film operating conditions exist in a bearing. Classes of Plain Bearings.— Bearings that provide sliding contact between mating sur faces fall into three general classes: radial bearings that support rotating shafts or jour- nals; thrust bearings that support axial loads on rotating members; and guide or slipper bearings that guide moving parts in a straight line. Radial sliding bearings, more com- monly called sleeve bearings, may be of several types, the most usual being the plain full journal bearing, which has 360-degree contact with its mating journal, and the partial journal bearing, which has less than 180-degree contact. This latter type is used when the load direction is constant and has the advantages of simplicity, ease of lubrication, and reduced frictional loss. The relative motions between the parts of plain bearings may take place: 1) as pure slid ing without the benefit of a liquid or gaseous lubricating medium between the moving sur faces such as with the dry operation of nylon or Teflon; 2) with hydrodynamic lubrication in which a wedge or film buildup of lubricating medium is produced, with either whole or partial separation of the bearing surfaces; 3) with hydrostatic lubrication in which a lubricating medium is introduced under pressure between the mating surfaces causing a force opposite to the applied load and a lifting or separation of these surfaces; and 4) with a hybrid form or combination of hydrodynamic and hydrostatic lubrication. Listed below are some of the advantages and disadvantages of sliding contact (plain) bearings as compared with rolling contact (antifriction) bearings. Advantages: 1) require less space; 2) are quieter in operation; 3) are lower in cost, par ticularly in high-volume production; 4) have greater rigidity; and 5) their life is generally not limited by fatigue. Disadvantages: 1) have higher frictional properties resulting in higher power consump tion; 2) are more susceptible to damage from foreign material in lubrication system; 3) have more stringent lubrication requirements; and 4) are more susceptible to damage from interrupted lubrication supply. Types of Journal Bearings.— Many types of journal bearing configurations have been developed; some of these are shown in Fig. 1. Circumferential-groove bearings, Fig. 1(a), have an oil groove extending circumferen tially around the bearing. The oil is maintained under pressure in the groove. The groove divides the bearing into two shorter bearings that tend to run at a slightly greater eccentricity. However, the advantage in terms of stability is slight, and this design is most commonly used in reciprocating-load main and connecting-rod bearings because of the uniformity of oil distribution. Short cylindrical bearings are a better solution than the circumferential-groove bearing for high-speed, low-load service. Often the bearing can be shortened enough to increase the unit loading to a substantial value, causing the shaft to ride at a position of substantial eccentricity in the bearing. Experience has shown that instability rarely results when the shaft eccentricity is greater that 0.6. Very short bearings are not often used for this type of application, because they do not provide a high temporary rotating-load capacity in the event some unbalance should be created in the rotor during service.
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