BEARINGS (1st unit of DMM-2 mech jntuk R16)

BEARINGS
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 nature of contact between the working surfaces:- __Sliding contact bearings and __Rolling contact bearings. SLIDING BEARINGS: Hydrodynamically lubricated bearings Bearings with boundary lubrication Bearings with Extreme boundary lubrication. Bearings with Hydrostatic lubrication. Rolling element bearings: 1,Ball bearings 2,Roller bearings 3,Needle roller bearings Based on the nature of the load supported: • Radial bearings - Journal bearings • Thrust bearings - Plane thrust bearings - Thrust bearings with fixed shoes - Thrust bearings with Pivoted shoes • Bearings for combined Axial and Radial loads.
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 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- 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(or)video 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.