Machinery's Handbook, 31st Edition
Plain Bearing Materials 2441 with low load capacity cause Teflon to be selected mostly in modified form, where other less expensive materials have proved inadequate for design requirements. Bearings made of laminated phenolics, nylon, or Teflon are all unaffected by acids and alkalies except if highly concentrated and therefore can be used with lubricants contain- ing dilute acids or alkalies. Water is used to lubricate most phenolic laminate bearings but oil, grease, and emulsions of grease and water are also used. Water and oil are used as lubricants for nylon and Teflon bearings. Almost all types of plastic bearings absorb water and oil to some extent. In some the dimensional change caused by the absorption may be as much as three percent in one direction. This means that bearings have to be treated before use so that proper clearances will be kept. This may be done by boiling in water, for water lubricated bearings. Boiling in water makes bearings swell the maximum amount. Clearances for phenolic bearings are kept at about 0.001 inch per inch (or mm per mm) of diameter on treated bearings. Partially lubricated or dry nylon bearings are given a clearance of 0.004 to 0.006 inch (101.6–152.4 μm) for a one-inch (25.4 mm) diameter bearing. Rubber: Rubber bearings give excellent performance on propeller shafts and rudders of ships, hydraulic turbines, pumps, sand and gravel washers, dredges and other industrial equipment that handle water or slurries. The resilience of rubber helps to isolate vibration and provide quiet operation, allows running with relatively large clearances and helps to compensate for misalignment. In these bearings a fluted rubber structure is supported by a metal shell. The flutes or scallops in the rubber form a series of grooves through which lubricant or, as generally used, water and foreign material such as sand may pass through the bearing. Wood.— Bearings made from such woods as lignum vitae, rock maple, or oak offer self- lubricating properties, low cost, and clean operation. However, they have frequently been displaced in recent years by various plastics, rubber and sintered-metal bearings. General applications are shown in Table 4. Carbon-Graphite.— Bearings of molded and machined carbon-graphite are used where regular maintenance and lubrication cannot be given. They are dimensionally stable over a wide range of temperatures, may be lubricated if desired, and are not affected by chemi cals. These bearings may be used up to temperatures of 700 to 750 ° F (371–399 ° C) in air or 1200 ° F (649 ° C) in a non-oxidizing atmosphere, and generally are operated at a maximum load of 20 pounds per square inch. In some instances a metal or metal alloy is added to the carbon-graphite composition to improve such properties as compressive strength and density. The temperature limitation depends upon the melting point of the metal or alloy and the maximum load is generally 350 psi (2.4 MPa) when used with no lubrication or 600 psi (4.2 MPa) when used with lubrication. Normal running clearances for both types of carbon-graphite bearings used with steel shafts and operating at a temperature of less than 200 ° F (93 ° C) are as follows: 0.001 inch (0.0254 mm) for bearings of 0.187 to 0.500-inch (4.75–12.7 mm) inside diameter, 0.002 inch (0.0508 mm) for bearings of 0.501 to 1.000-inch (12.73–25.4 mm) inside diam- eter, 0.003 inch (0.0762 mm) for bearings of 1.001 to 1.250-inch (25.43–31.75 mm) inside diameter, 0.004 inch (0.1016 mm) for bearings of 1.251 to 1.500-inch (31.77–38.1 mm) diameter, and 0.005 inch (0.127 mm) for bearings of 1.501 to 2.000-inch (38.13–50.8 mm) inside diameter. Speeds depend upon too many variables to list specifically so it can only be stated here that high loads require a low number of rpm and low loads permit a high number of rpm. Smooth journals are necessary in these bearings as rough ones tend to abrade the bearings quickly. Cast iron and hard chromium-plate steel shafts of 400 BHN (Brinell Hardness Number) and over, and phosphor-bronze shafts over 135 BHN are recommended.
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