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Among the significant characteristics of plastic bearings, the following are noteworthy:
• Low wear rates
• Relatively high performance rating (PV) among sleeve bearing materials
• Bearing O.D.’s compatible with standard sintered bronze sizes for upgrading existing equipment
• Kinetic and static coefficient of friction virtually the same under heavy loads
• Extremely low coefficient of friction as shown in Figure 1
• Light weight
• Ability to conform under load
The design characteristics of plastic and non-metallic bearings bear both similarities and differences relative to those of porous-metal bearings. This will now be described in greater detail.

2.0 PROPERTIES OF PLASTIC AND NON-METALLIC BEARING MATERIALS

Plastics (such as acetyl, nylon, PTFE). carbon graphite and other non-metallic materials have been
increasingly used as self-lubricating bearings. Their composition has been refined over many years so as to obtain favorable bearing characteristics. These include low friction, corrosion resistance, ability to conform under load (plastic bearings), ability to function over substantial temperature ranges and
substantial load-carrying capability. Although temperature ranges, dimensional stability and load limitations of plastic gears are in general less than for metallic bearings, plastic bearings are remarkably versatile and economical.
    A summary of characteristics of representative plastic and non-metallic materials has been given by Machine Design Magazine (Vol. 54, #14, June17, 1982, p. 132) with whose permission the following material is reprinted:

    Phenolics: Composite materials consisting of cotton fabric, asbestos, or other fillers bonded with phenolic resin. The good compatibility of the phenolics makes them easily lubricated by various fluids.
    They have replaced wood bearings and metals in such applications as propeller and rubber-shaft bearings in ships, and electrical switch-gear, rolling-mill, and water-turbine bearings. In small instruments and clock motors, laminated phenolics serve as structural members as well as a bearing material. They have excellent strength and shock resistance, coupled with resistance to water, acid, and alkali solutions.
    Some precautions must be observed with phenolic bearings. Thermal conductivity is low, so heat generated by bearing friction cannot readily be transmitted through the bearing liner. Consequently, larger, heavily loaded bearings must have a generous feed of water or lubricating oil to carry away heat. Some swelling and warping of these bearings occurs in the larger sizes, so larger-than-normal shaft clearances are required.
   Nylon: Although the phenolics have predominated in heavy-duty applications, they are frequently
replaced by nylon, which has the widest use in bearings. Nylon bushings exhibit low friction and require no lubrication. Nylon is quiet in operation, resists abrasion, wears at a low rate, and is easily molded, cast, or machined to close tolerances. Possible problems with cold flow at high loads can be minimized by using a thin liner of the material in a well-supported metal sleeve.

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