SLIDING CONTACT BEARINGS
A bearing is a machine element which supports another moving machine element (known as journal). It allows a relative motion between the contact surfaces of the members, while carrying the load. The bearings may be classified as follows
1. Radial Bearings :- In radial bearings, the load acts perpendicular to the direction of motion of the moving element.
2. Thrust Bearings :- In thrust bearings, the load acts along the axis of rotation.
3. Sliding Contact Bearings :- In sliding contact bearings, the sliding takes place along the surfaces of contact between the moving element and the fixed element. These are also known as plain bearings.
4. Rolling Contact Bearings :- In rolling contact bearings, the steel balls or rollers are inter- posed between the moving and fixed element.
The sliding contact bearings in which the sliding action is along the circumference of a circle or an are of a circle and carrying radial loads are known as journal or sleeve bearings. When the angle of contact of the bearing with the journal is 360°, then the bearing is called a full journal bearing. This type of bearing is commonly used in industrial machinery to accommodate bearing loads in any radial direction. When the angle of contact of the bearing with the journal is 120 °, then the bearing is said to be partial journal bearing. This type of bearing has less friction than full journal bearing, but it can be used only where the load is always in one direction. The most common application of the partial journal bearings is found is rail road car axles. The full and partial journal bearings may be called as clearance bearings, because the diameter of the journal is less than that of the bearing. When a partial journal bearing has no clearance i.e. The diameters of the journal and bearing are equal, then the bearing is called a fitted bearing.
The sliding contact bearings, according to the thickness of the layer of the lubricant between the bearing and the journal, may also be classified as follows.
1. Thick Film Bearings :- The thick film bearings are those in which the working surfaces are completely separated from each other by the lubricant. Such type of bearings are also called as hydrodynamic lubricated bearings.
2. Thin Film Bearings :- The thin film bearings are those in which, although lubricant is present, the working surfaces partially contact each other atleast part of the time. Such type of Bearings are also called boundary lubricated bearings.
3. Zero Film Bearings :- The zero film bearings are those which operate without any lubricant present.
4. Hydrostatic or externally pressurized lubricated bearings :- The hydrostatic bearings are those which can support steady loads without any relative motion between the journal and the bearing. This is achieved by forcing externally pressurized lubricant between the members.
Wedge Film Journal Bearings
The load carrying ability of a wedge film journal bearing results when the journal and the bearing rotates relative to the load. When the journal rotates slowly in the anticlockwise direction, it will move towards the left of the bearing making metal to metal contact. When the journal rotates at high speed in the anticlockwise direction, it will move towards the right of the bearing making no metal to metal contact.
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