Babbitts

Nov. 15, 2002
Tin and lead-base babbitts are among the most widely used bearing materials.

Tin and lead-base babbitts are among the most widely used bearing materials. They have an ability to embed dirt and have excellent compatibility properties under boundary lubrication.

In small bushings for fractional-horsepower motors and in automotive engine bearings, babbitt is generally used as a thin coating over a steel strip. For larger bearings in heavy-duty equipment, thick babbitt is cast on a rigid backing of steel or cast iron. After machining, the babbitt layer is usually 1/81/4 in. thick.

Compared with other bearing materials, babbitts generally have lower load-carrying capacity and fatigue strength, are slightly more costly, and require a more complicated design. Also, their strength decreases rapidly with increasing temperature. These shortcomings can often be avoided by using an intermediate layer of high-strength, fatigue-resisting materials between the steel backing and the thin babbitt surface layer. Such composite bearings frequently eliminate any need for alternative materials with poorer compatibility characteristics.

Tin babbitt: These materials are composed of 80 to 90% tin, with about 3 to 8% copper and 4 to 14% antimony added. An increase in the copper or antimony increases hardness and tensile strength and decreases ductility. Increasing the percentage of these hardening alloys above this range decreases both cracking resistance and fatigue strength.

Of the three tin-base babbitts, SAE Grade 11 (or ASTM Grade 3 with 6 1/2% copper and 7 1/2% antimony) has extensive industrial use. SAE 12 alloy is most widely used in automotive applications. Tin babbitt is used in smaller volume than lead babbitt because of the higher cost of tin. However, it is frequently preferred for industrial applications because of its higher corrosion resistance, easier bonding, lower tendency for segregation, better high-temperature characteristics, and lower tendency to pickup on a shaft under conditions of poor lubrication.

Lead babbitt: Generally, these compositions range from 10 to 15% antimony plus up to 10% tin. Lead babbitts based on lead-antimony-tin alloys have a structure consisting of hard antimony-tin crystals in a relatively soft high-lead matrix. SAE 14 alloy, containing 10% tin for improved corrosion resistance, is popular for many industrial, railroad, and automotive applications. SAE 13 alloy is used where a softer babbitt is needed; but SAE 15 arsenical alloy, with its better high-temperature hardness, is the most widely used of the three because it withstands higher loads and provides longer fatigue life in the automotive and diesel field.

Compared to tin babbitts, lead-base materials are less costly and have less tendency to score a shaft. With quick chilling to give a fine microstructure, thin surface layers for improved fatigue strength, and careful attention to bonding, lead babbitt gives excellent service and is used in much greater volumes than tin babbitt.

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