Machinery's Handbook, 31st Edition
2402 Bearing Lubricants nature of the load and the intermittent oil supply in splash fed applications requires the designer to use experience and judgment when determining the probable load capacity of bearings lubricated in this way. Oil bath lubrication, in which the bushing is submerged in oil, is the most reliable of all methods except pressure lubrication. It is practical if the housing can be made oil tight, and if the shaft speed is not so great as to cause excessive churning of the oil. Oil ring lubrication, in which oil is supplied to the bearing by a ring in contact with the shaft, will, within reasonable limits, bring enough oil to the bearing to maintain hydrody namic lubrication. If the shaft speed is too low, little oil will follow the ring to the bearing; and, if the speed is too high, the ring speed will not keep pace with the shaft. Also, a ring revolving at high speed will lose oil by centrifugal force. For best results, the peripheral speed of the shaft should be between 200 and 2000 fpm (61 and 610 m/min). Safe load to achieve hydrodynamic lubrication should be one-half of that for pressure fed bearings. Unless the load is light, hydrodynamic lubrication is doubtful. The safe load, then, to achieve hydrodynamic lubrication, should be one-quarter of that of pressure fed bearings. Wick or waste pack lubrication delivers oil to a bushing by the capillary action of a wick or waste pack; the amount delivered is proportional to the size of the wick or pack. Lubricants: The value of an oil as a lubricant depends mainly on its film-forming capac ity, that is, its capability to maintain a film of oil between the bearing surfaces. The film- forming capacity depends to a large extent on the viscosity of the oil, but this should not be understood to mean that oil of the highest viscosity is always the most suitable lubricant. For practical reasons, an oil of the lowest viscosity that will retain an unbroken oil film between the bearing surfaces is the most suitable for purposes of lubrication. A higher viscosity than that necessary to maintain the oil film results in a waste of power due to the expenditure of energy necessary to overcome the internal friction of the oil itself. Fig. 6 provides representative values of viscosity in centipoises for SAE mineral oils. Table 56a on page 2873 is provided as a means of converting viscosities of other units to centipoises. Grease packed in a cavity surrounding the bushing is less adequate than an oil system, but it has the advantage of being more or less permanent. Although hydrodynamic lubrication is possible under certain very favorable circumstances, boundary lubrication is the usual state. Lubricant Selection.— In selecting lubricants for journal bearing operation, several fac tors must be considered: 1) type of operation (full, mixed, or boundary film) anticipated; 2) surface speed; 3) and bearing loading. Fig. 7 combines these factors and facilitates general selection of the proper lubricant viscosity range. As an example of using these curves, consider a lightly loaded bearing operating at 2000 rpm. At the bottom of the figure, locate 2000 rpm and move vertically to intersect the light-load full-film lubrication curve, which indicates an SAE 5 oil. As a general rule-of-thumb, heavier oils are recommended for high loads and lighter oils for high speeds. In addition, other than using conventional lubrication oils, journal bearings may be lubricated with greases or solid lubricants. Some of the reasons for use of these lubricants are to: 1) Lengthen the period between relubrication: 2) Avoid contaminating surrounding equipment or material with “leaking” lubricat ing oil; 3) Provide effective lubrication under extreme temperature ranges; 4) Provide effective lubrication in the presence of contaminating atmospheres; and 5) Prevent intimate metal-to-metal contact under conditions of high unit pressure which might destroy boundary lubricating films.
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