Babbitt (metal)

Babbitt, also called Babbitt metal or bearing metal, is any of several alloys used for the bearing surface in a plain bearing.

The original Babbitt alloy was invented in 1839 by Isaac Babbitt in Taunton, Massachusetts, USA. He disclosed one of his alloy recipes but kept others as trade secrets. Other formulations were later developed. Like other terms whose eponymous origin is long since deemphasized (such as diesel engine or eustachian tube), the term babbitt metal is frequently styled in lowercase. It is preferred over the term "white metal", because the latter term may refer to various bearing alloys, lead- or tin-based alloys, or zinc die-casting metal.

Babbitt metal is most commonly used as a thin surface layer in a complex, multi-metal structure, but its original use was as a cast-in-place bulk bearing material. Babbitt metal is characterized by its resistance to galling. Babbitt metal is soft and easily damaged, which suggests that it might be unsuitable for a bearing surface. However, its structure is made up of small hard crystals dispersed in a softer metal, which makes it a metal matrix composite. As the bearing wears, the softer metal erodes somewhat, which creates paths for lubricant between the hard high spots that provide the actual bearing surface. When tin is used as the softer metal, friction causes the tin to melt and function as a lubricant, which protects the bearing from wear when other lubricants are absent.

Internal combustion engines use Babbitt metal which is primarily tin-based because it can withstand cyclic loading. Lead-based Babbitt tends to work-harden and develop cracks but it is suitable for constant-turning tools such as sawblades.

In the traditional style of a babbitt metal bearing, a cast iron pillow block is assembled as a loose fit around the shaft, with the shaft in approximately its final position. The inner face of the cast iron pillow block is often drilled to form a key to locate the bearing metal as it is cast into place. The shaft is coated with soot as a release agent, the ends of the bearing are packed with clay, and molten metal poured into the cavity around the shaft, initially half filling the pillow block. The bearing is stripped, and the metal trimmed back to the top surface of the pillow block. Hardened babbitt metal is soft enough to be cut with a knife or sharp chisel.

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