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.

CLASSIFICATION OF BEARINGS

Bearings can be classified in the following ways,

1. Direction of load to be supported

 a) Radial bearings and

 b) Thrust bearings.

In radial bearings, the load acts perpendicular to the direction of motion of the moving element.

In thrust bearings, the load acts along the axis of rotation.

2. The nature of contact

1. Sliding contact bearings and

2. Rolling contact bearings.

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In sliding contact bearings, the sliding takes place along the surfaces of contact between the moving & fixed elements. The sliding contact bearing are known as plain bearings.

In rolling contact bearings, the steel balls & rollers are interposed between the moving and fixed elements. The balls offer rolling friction at two points for each ball or roller

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3. Sliding contact bearings

The sliding contact bearing in which the sliding action is guided in a straight line & carrying radial loads may be called slipper or guide bearings. Such types of bearing are found in crosshead of steam engines.

The sliding bearing in which the sliding action is along the circumference or an arc of a circle or an arc of a circle and carrying radial loads are known as journal or sleeve bearings.

The sliding bearings, according to the thickness of layer of the lubricant between the journals, may also classified as follow.

1. Thick film bearings are those in which the working surfaces are completely separated from each other by the lubricant. Such bearings are also called hydrodynamic lubricated bearings.

2. Thin film bearings are those in which although lubricant is present, the working surfaces partially contact at least part of the time. Such type of bearings are also called boundary lubricated bearings.

3. Zero film bearings are those which operate without any lubricant present.

4. Hydrostatic or externally pressurized lubricated bearing are those which can support steady loads without any relative motion between the journal and bearing. This is achieved by forcing externally pressurized lubricant between the members.

BEARING

A bearing is a machine element, which supports other moving machine elements (known as journal). It permits a relative motion between contact surfaces of members, while carrying the load. Due to the relative motion between the contact surfaces, a certain amount of power is wasted in overcoming the frictional resistance and if the rubbing surfaces are in direct contact, there will be rapid wear. In order to reduce frictional resistance and wear and in some cases to carry away the heat generated, a layer of fluid (lubricant) may be provided. The lubricant used to separate the journal and bearing is usually mineral oil refined from petroleum, but vegetable oils, silicon oils, greases etc. may be used.