Module 6. Power transmission

Lesson 29

29.1 Introduction

A bearing is a machine element which supports 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 also be used.

29.2 Journal Bearing

A solid bearing, as shown in Fig. is the simplest form of journal bearing. It is simply a block of cast iron with a hole for a shaft providing running fit. The lower portion of the block is extended to form a base plate or sole with two holes to receive bolts for fastening it to the frame. An oil hole is drilled at the top for lubrication. The main disadvantages of this bearing are

  1. There is no provision for adjustment in case of wear, and
  2. The shaft must be passed into the bearing axially,
i.e. endwise.

Since there is no provision for wear adjustment, therefore this type of bearing is used when the shaft speed is not very high and the shaft carries light loads only.

Fig. 29.1 Journal bearing

29.3 Anti Friction Bearing

In rolling contact bearings, the contact between the bearing surfaces is rolling instead of sliding as in sliding contact bearings. 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 bearings.

29.3.1 Advantages of anti friction bearing

  • Low cost of maintenance, as no lubrication is required while in service.
  • Small overall dimensions.
  • Reliability of service.
  • Easy to mount and erect.
  • Cleanliness.

29.3.2 Disadvantages of anti friction bearing

  • More noisy at very high speeds.
  • Low resistance to shock loading.
  • More initial cost.
  • Design of bearing housing complicated.

29.4 Ball Tapered Roller Bearing

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. 29.2. 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 are 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.

Fig. 29.2 Ball tapered roller bearing

Last modified: Thursday, 27 September 2012, 10:18 AM