Aluminum Alloys for Bearings

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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 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.

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).

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.

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. Variables 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.

Pumps

Among the main components of a gear-type pump for which aluminum may be used are the housing and end covers. In some instances, an aluminum design incorporates the housing and an end cover in a one-piece casting, maintaining a close fit between gears and housing for high efficiency. Using die castings in pumps frequently permits a design providing cast O-ring grooves and other close-tolerance cavities that otherwise would require machining.

Pump components made in large quantities usually are die castings employing alloy 13 or 380. Permanent mold castings in alloys 333-T6 and 356-T7 are recommended for such components as housings, covers, and adapters, when a specific design or production quantity does not justify the cost of die casting equipment. More stringent design or quantity limitations may call for sand cast components, generally in 319-T6 or 356-T7.

Compressors

Common types of compressors are single-acting piston, double-acting piston, and centrifugal. With single-acting enclosed compressors, such as used in automotive air-conditioning the chief concern is obtaining pressure tight crank-systems, cases. This is accomplished by using permanent mold castings or die castings impregnated with a sealant.

Die casting is employed for high-volume production of small-diameter trunk-type pistons of aluminum-silicon alloys. Permanent mold or sand casting is used for larger designs or lower production rates.

Aluminum connecting rods are made as 380 or X385 alloy die castings, 333 alloy permanent mold castings, or forgings, depending on size, loading, and economics. Connecting rods can be run directly on a steel crankshaft, if the projected bearing area is large enough and the oil supply adequate.

Alloy 380 generally is specified for die cast compressor parts, although alloy 13 is preferred when pressure tightness is a problem. Alloys 333-T5, 333-T6, and 356-T7 are used for permanent mold castings; alloys 319-T5, 319-T6, and 356-T7 are selected for sand castings; and forgings are fabricated in 2014-T6 and 4032-T6.

Crossheads generally are made of an aluminum bearing alloy. Impellers and diffusers used in centrifugal compressors require precise dimensions and usually are made as plaster or investment castings in alloys 355-T61 or 356-T7.

Fans and Blowers

Components of aluminum fans and blowers are either riveted or welded, using alloy 5154 or 6061 sheet for blades with hubs cast in 356-T6 or forged in 2014-T6. High-speed impellers for pumping corrosive gases in certain industrial processes are aluminum weldments.

Large propeller-type fans fabricated from sections extruded in alloy 6061-T6 are used for applications such as preventing frost in fruit orchards and moving air in industrial cooling towers. The extrusion process produces efficient airfoil sections.

Hydraulic Circuits

Hydraulic circuits normally consist of a pump, valves, filter, accumulator, and operating cylinder. Hydraulic valves made of aluminum cost less than brass valves. Because of intricate passageways throughout the valve body, sand casting and semi-permanent mold casting are the best fabrication methods. Aluminum-silicon alloys such as 355 and 356 are used for pressure tightness and resistance to corrosion. Successful application of expendable cores could permit die casting of high-production valve bodies.

Although forgings and bar stock sometimes are employed, considerable machining is required to obtain body cavities. If an aluminum valve spool is used, the outside diameter can be anodized for improved wear resistance. The spool can be a wrought screw-machine product or a casting. Alloys having approximately the same coefficient of thermal expansion can be chosen for the body and spool.

Filter components incorporate aluminum even on a high-pressure unit, where the head is made as an alloy 13 die casting. The tank (or element container) is either an impact extrusion or a forging of 6061 or 2014.

Accumulator heads and bodies have aluminum components. High-pressure systems utilize forged accumulator heads, and lower-pressure systems employ permanent mold or die cast heads. The accumulator body can be made of aluminum tube. A hard-anodized inside wall surface is required for good wear resistance. Pistons have Teflon backup rings.

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