Bearing

Friction & Wear
When a lubricant film cannot completely separate the moving parts of bearing, friction and wear increase. The resulting frictional heat combined with high pressure promotes localized welding of the two rubbing surfaces. These welded contact points break apart with relative motion and metal is pulled from one or both surfaces decreasing the life of the bearing. This friction and welding is most common when like metals, such as steel or cast iron, are used as bearings - they easily weld together. Therefore, compatibility of bearing materials and absorption of lubricants upon the bearing surface, are necessary to reduce metallic contact and extend bearing life.

Babbitt
In 1839, Isaac Babbitt received the first patent for a white metal alloy that showed excellent bearing properties. Since then, the name babbitt has been used for other alloys involving similar ingredients. Babbitts offer an almost unsurpassed combination of compatibility, conformability, and embedability. They easily adapt their shapes to conform to the bearing shaft and will hold a lubricant film. Foreign matter not carried away by the lubrication is embedded below the surface and rendered harmless. These characteristics are due to babbitt’s hard/soft composition. High-tin babbitts, for example, consist of a relatively soft, solid matrix of tin in which are distributed hard copper-tin needles and tin-antimony cuboids. This provides for “good run-in”, which means the bearing will absorb a lubricant on the surface and hold the lubricant film. Even under severe operating conditions where high loads, fatigue problems, or high temperatures dictate the use of other stronger materials, babbitts are often employed as a thin surface coating to obtain the advantages of their good rubbing characteristics.

Uses of Tin and Lead-Based Babbitt
Babbitt metal is used as the lining for bearing shells of cast iron, steel, and bronze. Fry manufactures two basic types of babbitt: (1) high-tin alloys and (2) high-lead alloys. Both are relatively low-melting materials consisting of hard compounds in a soft matrix. The compounds found in each group are similar; it is in the composition and properties of the matrix that they differ. High-tin babbitt is used for high unit load and high operating temperatures. They display excellent corrosion resistance, easy bonding, and less tendency for segregation and welding. They are preferred for use under steady load conditions in steam and gas turbines, electric motors, blowers and pumps.

Lead-based babbitts are not the equal of tin-based but are adequate for lower loads and moderate temperatures. Lead-based babbitt is prone to separate into elemental lead and tin and has a lower thermal conductivity.

Users of babbitt ingot melt the metal in iron kettles from which they can ladle or pump the molten alloy. The bearing shell or backing is pre-coated with tin for tin-base bearings or with lead/tin for lead-base bearings. While the coating is still molten, the white metal is cast onto the backing and allowed to solidify from the bond inward. This prevents contraction cavities at the bond and restricts the growth of intermetallic compounds at the interface between the bearing shell and the babbitt. The lining is then machined to a mirror-bright finish and specified thickness. Babbitt can also be sprayed onto the bearing shell with the use of a flame arc gun and Fry’s babbitt wire.

Changing To Lead-free Babbitt
With increasing legislative concern over the hazards of lead in the workplace, more companies are interested in lead-free alternatives. There are many alloys of tin and/or lead that have been used to resist wear and support a load between contacting metallic surfaces. The selection of the appropriate babbitt has usually been a cost first, application second decision. Frequently, a babbitt user does not know the composition of the metal they are using. Confusion abounds because the same babbitt alloy may have various trade names in different parts of the country or from one manufacturer to the next. The American Society for Testing and Materials (ASTM) has produced a specification with only 8 alloys, 4 of which are lead-free. To help end the confusion, Fry uses the ASTM B-23 to describe babbitt alloys. The only exception is the ASTM B-23 Grade 2 Babbitt with the addition of Nickel - commonly known as ‘XXXX ® Nickel Babbitt’.

Tin-based babbitts, with exception of Grade 1, are superior to all lead-based alloys. The mechanical properties of the tin base increases steadily for Grade 11, Grade 2, and Grade 3. Grade 2 or Grade 3 can replace any lead-based alloy and show improved wear characteristics. Grade 2 is also available in wire form for flame spray metallization. Grade 11 is similar to Grade 1, but the higher copper makes alloy segregation a problem. It is much more difficult to produce a uniform alloy. This leaves Grade 2 as the preferred alternative and Grade 3 as a higher strength option.

BABBITT ALLOYS

ASTM B-23-83 Specifications ^A,B

Tin Base

Lead Base

A – All values not given as ranges are maximum unless shown otherwise
B – Alloy number 9 was discontinued in 1946 and numbers 4, 5, 6, 10, 11, 12, 16 and 19, were discontinued in 1959. A new number 11, similar to SAE Grade 11, was added in1966.
C - To be determined by difference