THE MATERIALS COMMONLY USED FOR BEARINGS

The materials commonly used for bearings are classified below.

1. Babbit metal

The tin based and lead based babbits are widely used as a bearing material, because they satisfy most requirements for general applications. The babbits are recommended where the maximum bearing pressure is not over 70 kg/ cm² to 140 kg/ cm². When applied in automobiles, the babbit is generally used as a thin layer, 0.05 mm to 0.15 mm thick, bonded to an insert or steel shell.

The composition of babbit metals is as follow,

Tin base babbits :  Tin 90%, Copper 4.5%, Antimony 5%, Lead 0.5%.

Lead base babbits :  Lead 84%, Tin 6%,  Antimony 9.5%, Copper 0.5%.

2. Bronzes

The bronzes (alloys of copper, tin and zinc) are generally used in the form of machined bushes pressed into the shell. The bush may be one or two pieces. The bronzes commonly used for bearing material are gun metal and phosphor bronzes.

The gun metal (Copper 88%, Tin 10%, Zinc 2%) is used for high grade bearings subjected to high pressure (not more than 100 kg/ cm²) and high speeds.

The phosphor bronze (Copper 80%, Tin 10%, Lead 9%, phosphorus 1%) is used for bearings subjected to very high pressure (not more than 140 kg/ cm²) and high speeds.

3. Cast iron

The cast iron bearings are usually used with steel journals. Such types of bearings are fairly successful where lubrication is adequate and the pressure is limited to 35 kg/ cm² and  speed to 40 meters per minute.

4. Silver

The silver and silver lead bearings are mostly used in air craft engines where the fatigue strength is the most important consideration.

5. Non metallic bearings

The carbon graphite bearings are self lubricating, dimensionally stable over a wide range operating conditions, chemically inert and can operate at high temperatures than other bearings. Such types of bearings are used in food processing and other equipment where contamination by oil or grease must be prohibited. These bearings are also used in applications where the shaft speed is too low to maintain a hydrodynamic oil film.

The soft rubber bearings are used with water or other low viscosity lubricants, particularly where sand or other large particles are present. In addition to the high degree of embed ability and conformability the rubber bearing are excellent for absorbing shock loads and vibrations. The rubber bearing are used mainly on marine propeller shafts, hydraulic turbines and pumps.

       The wood bearing is used in main application where low cost, cleanliness in attention to lubrication anti-seizing is important.

        The commonly used plastic material for bearing is Nylon and Teflon. This material has main characteristics desirable in bearing material and both can be used dry i.e. as a zero film bearing. The nylon is stronger, harder and more resistant to abrasive wear. It is used for application in which these properties are important e.g. elevator bearings, cams in telephone dials etc. The Teflon is rapidly replacing as a wear surface or liner for journal and other sliding bearings because of the following properties.

1) It has lower coefficient of friction, about 0.04 (dry) as compared to 0.15 for nylon.

2) It can be used at higher temperature up to about 315⁰ c as compare to 120⁰c for nylon.

3) It is dimensionally stable because it does not absorb moisture, and

4) It is practically chemically inert.