Other Nonmetallic Bearings

Nov. 15, 2002
Nonmetallic materials offer self-lubricating properties, high-temperature stability, solvent resistance, or low cost.

Nonmetallic materials offer self-lubricating properties, high-temperature stability, solvent resistance, or low cost. The most common of these are carbon graphite, wood, rubber, and ceramics.

Carbon graphite: The self-lubricating properties of carbon bearings, their stability at temperatures up to 750°F, and their resistance to attack by chemicals and solvents, give them important advantages where other bearing materials are unsatisfactory. Carbon-graphite bearings are used where contamination by oil or grease is undesirable, as in textile machinery, food-handling machinery, and pharmaceutical processing equipment. They are used as bearings in and around ovens, furnaces, boilers, and jet engines where temperatures are too high for conventional lubricants. They are also used with low-viscosity and corrosive liquids in such applications as metering devices or pumps for gasoline, kerosene, water, seawater, chemical process streams, acids, alkalis, and solvents.

The composition and processing used with carbon bearings can be varied to provide characteristics required for particular applications. Carbon graphite has from 5 to 20% porosity. These pores can be filled with a phenolic or epoxy resin for improved strength and hardness, or with oil or metals (such as silver, copper, bronze, cadmium, or babbitt) to improve compatibility properties.

A PV limit of 15,000 ordinarily can be used for dry operation of carbon bearings. This should be reduced for continuous running with a steady load over a long period of time to avoid excessive wear. When operating with liquids which permit the development of a supporting fluid film, much higher PV values can be used.

A hard, rust-resistant shaft with at least a 10-∝in. finish should be used. Hardened tool steel or chrome plate is recommended for heavy loads and high-speed applications. Steel having a hardness over Rockwell C50, bronzes, 18-8 stainless steels, and various carbides and ceramics also can be used.

Certain precautions should be observed in applying carbon graphite. Since this material is brittle, it is chipped or cracked easily if struck on an edge or a corner or if subjected to high thermal, tensile, or bending stresses. Edges should be relieved with a chamfer. Sharp corners, thin sections, keyways, and blind holes should be avoided wherever possible. Because of brittleness and low coefficient of expansion (about one-fourth that of steel), carbon-graphite bearings are often shrunk into a steel sleeve. This minimizes changes in shaft clearance with temperature variations and provides mechanical support for the carbon-graphite elements.

Rubber: Elastomeric materials give excellent performance in bearings on propeller shafts and rudders of ships, and in other industrial equipment handling water or slurries. The resilience of rubber helps isolate vibration to provide quiet operation, allows running with relatively large clearances, and helps compensate for misalignment.

Commercial rubber bearings consist of a fluted structure supported in a solid metal shell. This allows the shaft to be carried on a series of rubber strips running the length of the bearing. A water flow 2 gpm/in. of diameter is normally provided to cool the bearing and to flush through any dirt collecting in the channels between the rubber strips. Maximum load should be limited to 35 to 50 psi. There is no limit on speed, as long as operating temperature remains below 150°F.

Wood: Lignum vitae and oil-impregnated maple and oak offer self-lubricating properties, low cost, and clean operation. Wood bearings are useful at temperatures up to 150°F and at speeds up to several hundred rpm.

Ceramics: Silicon nitride has been developed as a high-performance bearing material exhibiting fatigue properties equaling or exceeding that of high-quality bearing steels. The primary use for these materials is in such rolling elements as balls and rollers for integration in bearings made with steel races. Such bearings are hybrid ceramic and are available in most sizes with standard offerings available in high-performance super-precision ball bearings. All-ceramic bearings are available in limited quantities upon special order for special applications.

The hardness, rigidity, corrosion resistance, and fine finish of various ceramics, carbides, and cermets have made them of great interest as bearing materials. Except for silicon nitride, there is only limited success in applying these materials as bearings.

Sapphire and glass: Synthetic industrial sapphire (100% aluminum oxide) is widely used for jewel bearings in low-torque instrument applications. Borosilicate glass has been substituted for sapphire in some applications. Jewel bearings are available in several configurations, including vee, ring, endstones, cup, and orifice.

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