DESIGN OF BEARINGS MISCELLANEOUS ELEMENTS
Sliding Contact Bearings
A bearing is a machine element which support another moving machine element (known as journal). It permits a relative motion between the contact surfaces of the members, while carrying the load. A little consideration will show that due to the relative motion between the contact surfaces, a certain amount of power is wasted in overcoming 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 (known as lubricant) may be provided. The lubricant used to separate the journal and bearing is usually a mineral oil refined from petroleum, but vegetable oils, silicon oils, greases etc., may be used.
1 Classification of Bearings
Though the bearings may be classified in many ways, yet the following are important from the subject point of View:
1.1 Depending upon the direction of load to be supported.
The bearings under this group are classified as:
Radial bearings, and
In radial bearings, the load acts perpendicular to the direction of motion of the moving element as shown in Fig (a) and (b). In thrust bearings, the load acts along the axis of rotation as shown in Fig (c).
1.2 Depending upon the nature of contact. The bearings under this group are classified as:
Sliding contact bearings, and
Rolling contact bearings.
In sliding contact bearings, as shown in Fig (a), the sliding takes place along the surfaces of contact between the moving element and the fixed element. The sliding contact bearings are also known as plain bearings.
In rolling contact bearings, as shown in Fig (b), the steel balls or rollers, are interposed between the moving and fixed elements. The balls offer rolling friction at two points for each ball or roller.
2 Types of Sliding Contact Bearings
The sliding contact bearings in which the sliding action is guided in a straight line and carrying radial loads, as shown in Fig (a), may be called slipper or guide bearings. Such type of bearings are usually found in cross-head of steam engines. The sliding contact bearings in which the sliding action is along the circumference of a circle or an arc 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° as shown in Fig (a), 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°, as shown in Fig (b), 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 in 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 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, as shown in Fig (c). The sliding contact bearings, according to the thickness of layer of the lubricant between the bearing and the journal, may also be classified as follows:
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.
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.
Zero film bearings. The zero film bearings are those which operate without any lubricant present.
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.
Rolling Contact Bearings
In rolling contact bearings, the contact between the bearing surfaces is rolling instead of sliding as in sliding contact bearings. We have already discussed that the ordinary sliding bearing starts from rest with practically metal-to-metal contact and has a high coefficient of friction. It is an outstanding advantage of a rolling contact bearing over a sliding bearing that it has a low starting friction. Due to this low friction offered by rolling contact bearings, these are called antifriction
1 Advantages and Disadvantages of Rolling Contact Bearings over Sliding Contact Bearings
The following are some advantages and disadvantages of rolling contact bearings over sliding contact bearings
Low starting and running friction except at very high speeds.
Ability to withstand momentary shock loads.
Accuracy of shaft alignment.
Low cost of maintenance, as no lubrication is required while in service.
Small overall dimensions.
Reliability of service.
Easy to mount and erect.
More noisy at very high speeds.
Low resistance to shock loading.
More initial cost.
Design of bearing housing complicated.
2 Types of Rolling Contact Bearings
Following are the two types of rolling contact bearings:
Ball bearings; and
The ball and roller bearings consist of an inner race which is mounted on the shaft or journal and an outer race which is carried by the housing or casing. In between the inner and outer race, there are balls or rollers as shown in Fig.
A number of balls or rollers are used and these are held at proper distances by retainers so that they do not touch each other. The retainers are thin strips and is usually in two parts which are assembled after the balls have been properly spaced. The ball bearings are used for light loads and the roller bearings are used for heavier loads. The rolling contact bearings, depending upon the load to be carried, are classified as:
Radial bearings, and
The radial and thrust ball bearings are shown in Fig (a) and (b) respectively. When a ball bearing supports only a radial load (WR), the plane of rotation of the ball is normal to the centre line of the bearing, as shown in Fig (a). The action of thrust load (WA) is to shift the plane of rotation of the balls, as shown in Fig (b). The radial and thrust loads both may be carried simultaneously.