BEARING

 A bearing is machine part, which support a moving element and confines its motion. The supporting member is usually designated as bearing and the supporting member may be journal. Since there is a relative motion between the bearing and the moving element, a certain amount of power must be absorbed in overcoming friction, and if the surface actually touches, there will be a rapid wear.

Classification:

Bearings are classified as follows: -

· Depending upon the direction of the load to be supported:

Radial bearing and Thrust bearing.

· Depending upon the nature of contact between the working surfaces:-

Sliding contact bearings and Rolling contact bearings.

Journal bearing:

It is one, which forms the sleeve around the shaft and supports a bearing at right angles to the axis of the bearing. The portion of the shaft resting on the sleeve is called the journal.

 Example of journal bearings are-

·        Solid bearing

·        Bushed bearing, and

·        Pedestal bearing.

Solid bearing:

A cylindrical hole formed in a cast iron machine member to receive the shaft which makes a running fit is the simplest type of solid journal bearing. Its rectangular base plate has two holes  drilled in it for bolting down the bearing in its position as shown in the figure. An oil hole is provided at the top to lubricate the bearing. There is no means of adjustment for wear and the shaft must be introduced into the bearing endwise. It is therefore used for shafts, which carry light loads and rotate at moderate speeds.

Bushed bearing:

It consists of mainly two parts, the cast iron block and bush; the bush is made of soft material such as brasss, bronze or gunmetal. The bush is pressed inside the bore in the cast iron block and is prevented from rotating or sliding by means of grub- screw as shown if the figure. When the bush gets worn out it can be easily replaced. Elongated holes in the base are provided for lateral adjustment.

Pedestal bearing:

It is also called Plummer block. Figure shows half sectional front view of the Plummer block. It consists of cast iron pedestal, phosphor bronze bushes or steps made in two halves and cast iron cap. A cap by means of two square headed bolts holds the halves of the steps together. The steps are provided with collars on either side in order to prevent its axial movement. The snug in the bottom step, which fits into the corresponding hole in the body, prevents the rotation of the steps along with the shaft. This type of bearing can be placed any where along the

shaft length.

Thrust bearing:

It is used to guide or support the shaft, which is subjected to a load along the axis of the shaft. Since a thrust bearing operates without a clearance between the conjugate parts, an adequate supply of oil to the rubbing surfaces is extremely important. Bearings designed to carry heavy thrust loads may be broadly classified in to two groups-

· Foot step bearing, and

· Collar bearing

Footstep bearing:

Footstep bearings are used to support the lower end of the vertical shafts. A simple form of such bearing is shown in fig. It consists of cast iron block into which a gunmetal bush is fitted. The bush is prevented from rotating by the snug provided at its neck. The shaft rests on a concave hardened steel disc. This disc is prevented from rotating along with the shaft by means of pin provided at the bottom.

Collar bearing:

The simple type of thrust bearing for horizontal shafts consists of one or more collars cut

integral with the shaft as shown in fig.15.5. These collars engage with corresponding bearing surfaces in the thrust block. This type of bearing is used if the load would be too great for a step bearing, or if a thrust must be taken at some distance from the end of the shaft. Such bearings may be oiled by reservoirs at the top of the bearings.

Advantages of sliding contact bearings:

·        They can be operated at high speeds.

·        They can carry heavy radial loads.

·        They have the ability to withstand shock and vibration loads.

·        Noiseless operation.

Disadvantages:

·        High friction losses during staring.

·        More length of the bearing.

·        Excessive consumption of the lubricant and high maintenance.

Rolling contact bearings:

The bearings in which the rolling elements are included are referred to as rolling contact bearings. Since the rolling friction is very less compare to the sliding friction, such friction is known as anti friction bearings.

Ball bearing:

It consists of an inner ring which is mounted on the shaft and an outer ring which is carried  by the housing. The inner ring is grooved on the outer surface called inner race and the outer ring  is grooved on its inner surface called outer race. In between the inner and outer race there are  number of steel balls. A cage pressed steel completes the assembly and provides the means of equally spacing and holding the balls in place as shown in the figure 15.6. Radial ball bearings are used to carry mainly radial loads, but they can also carry axial loads.

Cylindrical roller bearing:

The simplest form of a cylindrical roller bearing is shown in fig 15.7. It consists of an inner  race, an outer race, and set of roller with a retainer. Due to the line contact between the roller and the raceways, the roller bearing can carry heavy radial loads.

Tapered roller bearings:

In tapered roller bearings shown in the fig. 15.8., the rollers and the races are all truncated cones having a common apex on the shaft center to assure true rolling contact. The tapered roller bearing can carry heavy radial and axial loads. Such bearings are mounted in pairs so that the two bearings are opposing each others thrust.

Advantages rolling contact bearings:

·        Low starting and low running friction.

·        It can carry both radial as well as thrust loads.

·        Momentary over loads can be carried without failure.

·        Shaft alignment is more accurate than in the sliding bearings.

Disadvantages:

·        More noisy at high speeds.

·        Low resistance to shock loads.

·        High initial cost.

·        Finite life due to eventual failure by fatigue