Babbitt Metal

Babbitt metal, is an antifriction metal alloy first produced by Isaac Babbitt in 1839. In present-day usage the term is applied to a whole class of silver-white bearing metals, or “white metals.” These alloys usually consist of relatively hard crystals embedded in a softer matrix, a structure important for machine bearings. They are composed primarily of tin, copper, and antimony, with traces of other metals added in some cases and lead substituted for tin in others.

Bearings used in large marine diesel engines are tin based babbitt metals. Lead content is a minimum. Tin based white metals have 4 times the load bearing characteristics and two and a half times the maximum surface speed of lead based white metals.

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.

Tin Aluminium

Tin aluminium bearings were developed to provide bearings that carry high loads. As a bearing material, unalloyed aluminium has a tendency to seize to a steel mating surface. It was found that 20% of tin added to the aluminium improved seizure resistance and that cold working and annealing helped to prevent brittleness. Special features are their good resistance to corrosion, high thermal conductivity and high fatigue strength,  but they have the disadvantages of only moderate embedding properties, poor compatibility and high coefficients of thermal expansion. If used as solid unbacked bearings this type of alloy is usually too weak to maintain an interference fit and too hard to run satisfactorily against an unhardened shaft. Considerable improvement in antiscoring characteristics and embedability is obtained by using a thin-lead babbitt or electrodeposited lead-tin overlay.

Alloys containing 20 to 40% tin, remainder aluminium, show excellent resistance to corrosion by products of oil breakdown and good embeddability. The sliding properties of this composition are very similar to those of tin based white metal but the loading capacity of this material is higher than tin based white metals for the same working temperature; this is due to the ideal combination of tin and aluminium, where tin gives the good embedability and sliding properties, while the aluminium mesh functions as an effective load absorber. The higher-tin alloys (40%) have adequate strength and better surface properties, which make them useful for main and  crosshead bearings in high-power marine diesel engines.