2017-05-15 12:00:35

Aluminum Bearing Materials

Aluminum alloys with desirable bearing properties are used in a wide variety of applications. Steel-backed and solid aluminum bearings are employed as connecting rod and main bearings in internal combustion engines and industrial compressors. Other aluminum bearing applications are in heavy tooling, such as boring mills, presses, lathes, milling machines, and grinding mills, and as hydraulic pump bushings. Aircraft landing gear assemblies, power shovels, and track rollers utilize solid aluminum bearings to withstand high-shock loads. Rolling mill bearings are cast of aluminum alloys to increase load and speed capability.

Aluminum bushings are normally employed for relatively light, low-speed duty, compared to bearings, and they are made from aluminum bearing or other alloys, depending on the frictional and mechanical properties required for the application.

Aluminum bearing alloys combine to a greater degree than any other single bearing material these desired characteristics: low cost, long life, high resistance to corrosive agents in lubricants, high mechanical compatibility with steels (no damage to shaft), high heat conductivity, good compressive and fatigue strength, light weight, conformability, embeddability, high speed capability, and monometallic (solid) design.

Cast or wrought monometallic (solid) aluminum bearings have high load-carrying ability, and can withstand very high speeds. They serve in engines and machinery as heavy-duty bearings under loads as high as 69 MPa (10,000 psi) on projected areas of the bearing half shell, and at surface speeds up to 84 m/s (275 fps). In many laboratory tests, bearings have completed thousands of hours of successful operation at 83 MPa (12,000 psi) loading. With proper shaft preparation, modern lubricants, and excellent oil filtration, even higher load and speed levels can be tolerated.

Monometallic aluminum bearings give excellent service on either hard or soft steel shafts, which is an advantage when shaft cost must be low. However, under identical conditions, hard shafts show less wear than soft shafts.

Alloys 750, A750 and B750 can be cast in sand or permanent molds, but not as die castings. Alloy X385 is preferred for die cast bearings; although not equal in bearing characteristics to the 750-type alloys, it is considered to have good machining and bearing properties.

Alloys 750 and A750 have similar mechanical properties, but A750 is easier to cast and better adapted to the production of complicated parts. Cast bearings of alloys 750 and A750 are supplied in the T5 or T101 temper, the latter attained by cold working after a T5 heat treatment. The T101 temper substantially increases compressive yield strength, improving the ability of a bearing housed in a material of lower thermal expansion to maintain an interference fit through cycles of heating and cooling. The cold working has little influence on hardness or tensile strength.

Such parts as gear housings and pump bodies may have both structural and bearing functions, and a bearing alloy in the T101 temper provides the additional strength needed above bearing requirements. For more highly loaded parts, such as wrist-pin bushings, tractor-track roller bushings, and connecting rods, the still-higher-strength casting alloy B750-T5 is preferred. Die casting alloy X385 also has sufficient strength for use in parts designed for structural loads.

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