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CATEGORIES (articles) > Engine, Gearbox > Technical > White metal coating

White metal coating

The white metals are any of several light-colored alloys used as a base for plated silverware, ornaments or novelties, as well as any of several lead-base or tin-base alloys used for things like bearings, jewellery, miniature figures, fusible plugs and metal type.

Some of the metals that make up a white metal alloy are antimony, tin, lead, cadmium, bismuth, and zinc. Not all of these metals are found in all white metal alloys but are mixed to achieve a desired goal or need. An example would be for jewelry. You would need it to be castable, polishable, have good flow characteristics, have the ability to cast fine detail without an excessive amount of porosity and cast at between 230 °C and 300 °C (450 °F and 575 °F).

Tin-lead and tin-copper alloys

Tin-lead & tin-copper alloys have a low melting point that is ideal for use as solder, but these alloys also have ideal characteristics for plain bearings. Most importantly for bearings, the material should be hard and wear resistant and have a low coefficient of friction. It must also be shock resistant, tough and sufficiently ductile to allow for slight misalignment prior to running in.

Pure metals are soft, tough and ductile with a high coefficient of friction. Intermetallic compounds are hard and wear resistant but brittle. By themselves, these do not make ideal bearing materials.

Alloys consist of small particles of a hard compound embedded in the tough, ductile background of a solid solution. In service the latter can wear away slightly leaving the hard compound to carry the load. This wear also provides channels to allow in lubricant (oils). All bearing metals contain antimony (Sb) which forms hard cubic crystals.
% Sn % Sb % Cu % Pb Applications
93 3.5 3.5 - Light and medium IC engine big end bearings
86 10.5 3.5 - Light and medium IC engine main bearings
80 11 3.0 6 General purpose heavy bearings (lead increases plasticity)
60 10 28.5 1.5 Heavy duty marine engine bearings, electrical machines
40 10 1.5 48.5 Low cost, general purpose, medium duty bearings

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