Babbitt (alloy)

Pigs and bars of Grade #2 Babbitt

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[1] in Taunton, Massachusetts, United States. He disclosed one of his alloy recipes but kept others as trade secrets.[2] Other formulations were developed later.[3] 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.[3][4] It is preferred over the term "white metal", because that term refers to zinc die-casting metal, to lead-based alloys, to tin-based alloys, and to the bearing metal.

Microstructure of babbitt

Babbitt metal is most commonly used as a thin surface layer in a complex, multi-metal assembly, 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, technically, a metal matrix composite. As the bearing wears, the softer metal erodes somewhat, creating 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, protecting 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.

  1. ^ Hellemans, Alexander; Bunch, Bryan (1988). The Timetables of Science. Simon & Schuster. p. 305. ISBN 0671621300.
  2. ^ Isaac Babbitt, "Mode of making boxes for axles and gudgeons," Archived 2020-07-16 at the Wayback Machine U.S. patent no. 1,252 (issued: July 17, 1839). Babbitt did not patent his alloy, although he does state its formulation: "The inner parts of the boxes are to be lined with any of the harder kinds of composition known under the names of britannia metal or pewter, of which block tin is the basis. An excellent compound for this purpose I have prepared by taking about 50 parts of tin, five of antimony, and one of copper, but I do not intend to confine myself to this particular composition."
  3. ^ a b Oberg & Green 1996, p. 2150.
  4. ^ American Heritage Dictionary, 4th ed, headword babbitt metal.