Babbitt metal

Babbitt metal, also called white metal, is an alloy used to provide the bearing surface in a plain bearing. It was invented in 1839 by Isaac Babbitt [The Timetables of Science|pages=305] in Taunton, Massachusetts, USA. The term is used today to describe a series of alloys used as a bearing metal. Babbit metal is characterized by its resistance to galling.

Common compositions for Babbitt alloys:
* 90% tin 10% copper
* 89% tin 7% antimony 4% copper
* 80% lead 15% antimony 5% tin

Originally used as a cast in place bulk bearing material, it is now more commonly used as a thin surface layer in a complex, multi metal structure.

Babbitt metal is soft and easily damaged, and seems at first sight an unlikely candidate for a bearing surface, but this appearance is deceptive. The structure of the alloy is made up of small hard crystals dispersed in a matrix of softer alloy. As the bearing wears the harder crystal is exposed, with the matrix eroding somewhat to provide a path for the lubricant between the high spots that provide the actual bearing surface.

Traditional Babbitt bearings

In the traditional style of white 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 ends of the bearing are packed with clay and molten white metal poured into the cavity around the shaft, initially half filling the pillow block. The bearing is stripped, and the white metal trimmed back to the top surface of the pillow block. Hardened white metal is soft enough to be cut with a knife or sharp chisel.

A steel shim is inserted to protect the face of the lower bearing and to space the cap of the pillow block away from the shaft. After resealing the ends with clay, more white metal is then poured to fill the cap of the pillow block through the hole in the top of the pillow block cap that will eventually be a lubrication hole.

The two halves of the bearing are then split at the shim, the oil holes cleared of white metal and oil ways are cut into the surface of the new bearing. The shaft is smeared with engineer's blue and rotated in the bearing. When the bearing is disassembled the blue fills the hollows and is rubbed off the high spots. The high spots are scraped down, and the process repeated, until a uniform and evenly distributed pattern of blue shows when the shaft is removed. The bearing is then cleaned and lubricated, and shimmed up such that the shaft is held firmly but not binding in the bearing. The bearing is then "run in" by being run heavily lubricated at low load and revolution, completing the process of exposing the hard bearing surface. After final adjustment of the shimming, a very reliable and high load capability bearing results.

Before the advent of low cost electric motors, power was distributed through factories from a central engine via overhead shafts running in hundreds of Babbitt bearings.

The expression a "run bearing" also derives from this style of bearing, since failure of lubrication will lead to heat build up due to friction in the bearing, eventually leading to the white metal liquifying and literally running out of the pillow block.

Modern Babbitt bearings

In more modern practice, the crankshaft and connecting rod big end bearings in a modern automobile engine have bearings made of a replaceable steel shell, keyed to the bearing caps. The inner surface of the steel shell is plated with a coating of bronze which is in turn coated with a thin layer of Babbitt metal as the bearing surface.

The process of laying down this layer of white metal is known as Babbitting.

Alternative Bearings

In many applications, rolling-element bearings, such as ball or roller bearings, have replaced Babbitt bearings. Though such bearings can offer a lower coefficient of friction than plain bearings, their key advantage is that they can operate reliably without a continuous pressurised supply of lubricant. Ball and roller bearings can also be used in configurations that are required to carry both radial and axial thrusts. However, rolling-element bearings lack the beneficial damping and shock-load capability provided by fluid-film bearings.

References