Aluminum bushings are usually utilized for relatively light, low-speed work compared to bearings, and are made from aluminum and alloy composites, depending on the frictional and mechanical properties required for their use.
Aluminum bearing alloys unite to a greater degree than any other single bearing material because of these desired characteristics: cost effectiveness, durability, high-resistance to corrosive agents in lubricants, high mechanical compatibility with steels (no damage to shaft), incredible heat conductivity, good compressive and fatigue strength, light weight, conformable, embeddable, high-speed capability, and a monometallic (solid) design.
Composition
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.
Design of Aluminum Bearings
Typical recommended bearing loads for cast and wrought monometallic aluminum bearings are:
1. Reciprocating load, pressure lubrication: 35 MPa (5000 psi )
2. Unidirectional load, pressure lubrication: 20 MPa (3000 psi)
3. Unidirectional load, doubtful lubrication: 7 MPa (1000 psi)
These values are guides only. Associated components and other factors specific to an application influence allowable bearing loads.
Aluminum bearings, depending on the application, run with pressure-times-velocity (pv) values from 10,000 to 750,000. They withstand very high speeds, as evidenced by successful tests conducted at a surface speed of 85 m/s (275 fps). Commercial applications include water-air type turbines operating at 2.1·105 degree/s (35,000 rpm).
Good lubrication is vital with aluminum bearings, as with bearings of other materials. Lubricants can be applied in several ways, including gravity, wick, and pressure methods. Best results are obtained with pressure lubrication, provided the design details are effective.
Factors that should be considered in the design include speed and load; misalignment; length-to-diameter ratio; grooving; size and quantity of dirt particles; operating conditions; viscosity, stability, pressure, and cooling properties of the lubricant; and finish of adjacent parts.
Factors governing the practical clearance of aluminum bearings include the journal, bearing, and housing materials; journal hardness and finish; type and degree of lubrication; bearing loading and type (unidirectional, rotating, or reciprocating); speed; average size of dirt particles; and quantity of dirt in circulation.
Clearance must be increased or decreased based on evaluation of all these factors and actual experience. In marry applications, aluminum monometallic bearings are operating with oil clearances appreciably below those of equivalent bronze or Babbitt bearings.