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
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:
Reciprocating load, pressure lubrication — 35 MPa (5000 psi )
- Unidirectional load, pressure
lubrication — 20 MPa (3000 psi)
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
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.
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
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
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
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 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.