Thin, bimetal, half-round bearings that fit in the journal box around a shaft to provide a smooth, hard surface. One-half of the insert (in cross section, a semicircle) fits into the journal box, the other half into the journal box cap. Insert bearings era designated bimetal because although the bearing surface is made of babbitt (q.v.) it is backed with a layer of bronze, brass, or steel. There are also trimetal insert bearings. They are made with steel backing, a "soft" alloy middle layer, and a babbitt outer layer. Babbitt metal, an alloy of tin, copper, and antimony. was invented by Isaac Babbitt in 1862.
