Machinery's Handbook, 31st Edition
2398 PLAIN BEARINGS the region of mixed-film lubrication. When design speeds and loads are reached, the hydrodynamic action in a properly designed bearing will promote full-film lubrication. Methods of Retaining Bearings.— Several methods are available to ensure that a bear- ing remains in place within a housing. Which method to use depends upon the particular application but requires first that the unit lends itself to convenient assembly and disas - sembly; additionally, the bearing wall should be of uniform thickness to avoid introduc- tion of weak points in the construction that may lead to elastic or thermal distortion. Press or Shrink Fit: One common and satisfactory technique for retaining the bearing is to press or shrink the bearing in the housing with an interference fit. This method permits the use of bearings having uniform wall thickness over the entire bearing length. Standard bushings with finished inside and outside diameters are available in sizes up to approximately 5 inches (127 mm) inside diameter. Stock bushings are commonly provided 0.002 to 0.003 inch (50.8–76.2 μ m) over nominal on outside diameter sizes of 3 inches (76.2 mm) or less. For diameters greater than 3 inches, outside diameters are 0.003 to 0.005 inch (76.2–127 μ m) over nominal. Because these tolerances are built into standard bushings, the amount of press fit is controlled by the housing-bore size. As a result of a press or shrink fit, the bore of the bearing material “closes in” by some amount. In general, this diameter decrease is approximately 70 to 100 percent of the amount of the interference fit. Any attempt to accurately predict the amount of reduction, in an effort to avoid final clearance machining, should be avoided. Shrink fits may be accomplished by chilling the bearing in a mixture of dry ice and alco hol, or in liquid air. These methods are easier than heating the housing and are preferred. Dry ice in alcohol has a temperature of − 110°F ( − 79°C) and liquid air boils at − 310°F ( − 190°C). When a bearing is pressed into the housing, the driving force should be uniformly applied to the end of the bearing to avoid upsetting or peening of the bearing. Of equal importance, the mating surfaces must be clean, smoothly finished, and free of machining imperfections. Keying Methods: A variety of methods can be used to fix the position of the bearing with respect to its housing by “keying” the two together. Possible keying methods are shown in Fig. 4a through Fig. 4f including: a) set screws; b) Woodruff keys; c) bolted bearing flanges; d) threaded bearings; e) dowel pins; and f) housing caps. Factors to be considered when selecting one of these methods are as follows: 1) Maintaining uniform wall thickness of the bearing material, if possible, especially in the load-carrying region of the bearing. 2) Providing as much contact area as possible between bearing and housing. Mating surfaces should be clean, smooth, and free from imperfections to facilitate heat transfer. 3) Preventing any local deformation of the bearing that might result from the keying method. Machining after keying is recommended. 4) Considering the possibility of bearing distortion resulting from the effect of tempera ture changes on the particular keying method. Methods of Sealing.— In applications where lubricants or process fluids are utilized in operation, provision must be made normally to prevent leakage to other areas. This provi sion is made by the use of static and dynamic type sealing devices. In general, three terms are used to describe the devices used for sealing: Seal: A means of preventing migration of fluids, gases, or particles across a joint or open ing in a container. Packing: A dynamic seal, used where some form of relative motion occurs between rigid members of an assembly. Gaskets: A static seal, used where there is no relative motion between joined parts.
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