Aluminum-Alloy Bearings

Nov. 15, 2002
Aluminum bearing alloys have wear resistance, high load-carrying capacity, fatigue strength, thermal conductivity, and excellent corrosion resistance, and low cost.

Aluminum bearing alloys have wear resistance, high load-carrying capacity, fatigue strength, thermal conductivity, and excellent corrosion resistance, and low cost. They are used extensively in connecting-rod and main bearings in internal combustion engines, hydraulic gear pumps, oil-well pumping equipment, roll-neck bearings in steel mills, reciprocating compressors, and aircraft equipment. Aluminum alloys require sufficient lubrication, good surface finish, and shafts hardened to about Rockwell B85. They have relatively poor compatibility characteristics and lack embeddability and conformability. For automotive use, considerable improvement in antiscoring characteristics and embeddability is obtained by using a thin-lead babbitt or electrodeposited lead-tin overlay.

Aluminum bearing alloys are used either as solid cast material or as aluminum strips (normally from 0.02 to 0.125 in. thick) applied to steel backing. Cast alloys generally are specified by designations of The Aluminum Assoc., while the steel-backed bearings are usually specified by SAE designations.

An alloy widely used in crank and piston-rod bearings is a cast material, 850.0-T5 (equivalent to SAE 770). This alloy is the most ductile of the cast alloys and can withstand heavy loads without cracking or fretting. It also has good compatibility. An easier-casting version is A850.0-T5. In hardness, this alloy stands between the bronzes and the babbitts.

The most widely used cast aluminum-tin bearing alloy is B850.0-T5. Its properties are similar to those of general-purpose bronze. Alloy B850.0-T5 is the strongest of the aluminum alloys and can be used with moderate shock loads. But being harder, it does not conform to minor misalignments as readily as A850.0.

The most common steel-backed alloys are SAE 780 and 781. Alloy 780 is similar to SAE 770 (or 850.0-T5) except for having 1.5% more silicon. SAE 781 contains 1 to 3% cadmium for improved bearing properties. This alloy is used in making steel-back bearings with a 0.0005-in. overlay containing approximately 90% lead and 10% tin. The layer of babbitt improves surface properties during break-in.

A third type has been developed to provide steel-backed aluminum automotive bearings with no need for a babbitt overlay. It is used in most English automobiles and diesel engines. Another class, which has been used in U.S. automobile engines, uses a dispersion of 5% lead in an 0.018-in.-thick aluminum overlay.

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