Machine Design

Bearing Design

Plain bearing design may be simple or complex. It may amount to picking a bearing from a catalog on the basis of PV factor or may require a sophisticated analysis of lubricant flow.

Most slow moving or self-lubricated bearings are boundary lubricated. These are generally simple cylinders or flat washers. Grooves may be added to feed lubricant to load-bearing surfaces in boundary-lubricated bearings. Design variables are material, lubricant, and bearing size. Lubricant and material determine the load limit. Size is based on keeping within the lubricant/material limitations. Bearing performance is difficult to predict and safety factors should be ample as overheating is the most common cause of failure.

With a constant coefficient of friction, the designer has control over two variables -- bearing pressure P (load/bearing area) and sliding velocity V (rpm bearing circumference) -- can be combined (PV) as a rough indicator of heat generation. This factor can be used as a starting point for determining materials and size.

Safe maximum PV values for self-lubricating materials have been determined experimentally. PV values should be used with caution because they are not constant for most self-lubricating materials over a range of speeds and loads. Thus, a bearing material used for a large bushing operating at low bearing pressure and moderate speed cannot be used for a small bushing operating at high bearing pressure and very low speed. The heat-dissipation characteristics of the large bearing are much different from that of the small bearing.

For many self-lubricating materials -- especially plastics -- friction varies with speed or pressure. For instance, PTFE shows a decrease in friction coefficient with increasing pressure.

For boundary-lubricated bearings using an oil or grease lubricant, a more reliable criterion is maximum bearing stress capacity. Most applications involve relatively slow speeds, making bearing stress the more important variable.

TAGS: Bearings
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