October 21, 2010

Babbitt was first created by Isaac Babbitt in the early 19th century.  Babbitt is a relatively soft metal that is used principally as a liner for oil-lubricated sliding bearings, both journal bearings and thrust bearings.   There are two characteristics that make Babbitt an extremely useful material for sliding bearing applications.  The first is “lubricity”, or the ability to slide surface to surface without galling, generally in the presence of a lubrication such as a mineral oil.  The second is embedability, that is, the characteristic whereby hard particulate matter in lube oil embeds in the layer of Babbitt and thereby minimizing scratching or other damage to the surface of the journal or thrust runner.

There are many combinations of constituents that are used in manufacturing Babbitt.  The first distinction is whether the Babbitt is “tin-based” or “lead-based”.  When tin was difficult to obtain during WW 2, some equipment manufacturers added portions of lead to the Babbitt used in bearings in order to stretch the supply of tin. Many of these bearings failed because lead made the tin-lead Babbitt brittle. This experience was a good justification for using only tin-based Babbitt for rotating equipment where ductility and endurance are always important.

Two widely used Babbitt compositions are known as ASTM B23 Grades 2 and 3.  There are other grades, however, for rotating machinery, Grades 2 and 3 are very common.  Grade 3 has higher strength than Grade 2, but Grade 2 is easier for Mechanical Technicians to use in the refurbishment of existing bearings, and for this reason, it is more commonly used. Other compositions may be used.  TRI typically uses a proprietary Babbitt material that has a unique combination of constituents and special methods of manufacture, with the result of significantly higher strength at higher temperatures.

One of the most important issues that affects the success of the performance of a bearing is the attachment of the Babbitt layer to the backing of the bearing. Carbon Steel is an excellent backing material.  The surface of the steel can be machined and then tinned, avoiding the use of mechanical Babbitt anchors, or dovetails.  Suitable tinning compounds are readily available in the commercial market.

Babbitt thickness is also an important factor in the ability of oil-lubricated babbitted bearings to take abusive pounding.  Thinner layers can survive higher levels of pounding forces, yet thin layers cannot permit large particulate matter to embed without damage to journal or runner surfaces.  Consequently, a compromise is required, usually in the range between 0.030 inches to 0.125 inches, depending upon the application and cleanliness of the lube oil.

Copper based backing materials that are not properly coated before tinning can be expected to develop a brittle coating at the boundary between the tin-based Babbitt and the copper-based backing material.  This phenomenon is called “copper-tin embrittlement and debonding”, and was discovered several years ago by another well-known Babbitt bearing manufacturer. This phenomenon can cause Babbitt layers of bearings with copper-based backing materials to fall off in the storeroom even if they have never been used.  They can also fall off in service, which obviously can damage a machine.  With proper surface preparation and coating, a copper-based backing can have a tin-based Babbitt layer attached, and this bearing can be expected to have excellent performance and service life.

For more information about Babbitt, Babbitted Bearings and Babbitted Bearing Repair, please contact an engineer at TRI Transmission & Bearing Corp.

One comment

  1. […] the shaft is typically around 1.5 mills per inch of journal diameter. TRI uses a layer of tin-based babbitt on the inisde surface of our bearings.  Babbitt that is relatively soft metal alloy that has […]

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