Bearings

Bearings

Bearing refers to any kind of support in direct contact with a moving machine parts. Most of the bearing supports the rotating shafts or spindle. Mainly they are used to minimize the friction, wear and power loss.

Types of bearings:

1.      Plain or sleeve bearing

2.      Antifriction bearing

Plain bearing:

            Plain bearing is a simple cylindrical shape where shaft rotates through the journal area by the separation of a film of lubricating oil.

Antifriction bearing:

            The bearing where various rolling element like steel balls, rollers etc is used called antifriction bearing.

                               

                                    Figure: Antifriction bearings (rollers and balls).

Bearing selection: Bearing selection depends upon the factors –

Ø  Power consumption

Ø  Working life time

Ø  Types of load caring (axial or radial)

Ø  Tolerance and fitness

Ø  Shaft speed (rpm)

Ø  Operating temperature

Ø  Lubrication

Bearing materials:

Ø  Babbitt material

Ø  Bi-material

Ø  Bronze

Ø  Cast iron

Ø  Graphite

Ø  Jewels

Ø  Plastic etc

Among them the ‘Babbitt’ material is most widely used for bearings coating.

Babbitt material:

                        It is the alloys of Sn, Pb, Cu, Cd etc. In 1839 scientist ‘Isaac Babbitt’ was invented the materials. Hence it is called so. It has lower load capacity and fatigue strength than other materials. Babbitt is usually used in integral bearings. It is coated over the bore, usually to a thickness of 0.025 to 2.5 mm, depending on the diameter. Babbitt bearings are designed to not damage the journal during direct contact and to collect any contaminants in the lubrication.

Advantages of Babbitt material:

Ø  Better corrosion resistance

Ø  Higher score resistance

Ø  Better performance under higher temperature.

There are two most important bearings used in power plant.

1.      Journal bearing

2.      Thrust  bearing

Journal bearing:

            Journal bearing means plain journal bearing. It supports the radial load of a rotating shaft and prevents radial movement of a shaft. It is used mainly in pump.

           

Advantages of journal bearing:

§  Less radial space required

§  Quite operation

§  Better resistance to unexpected overload

§  Lower cost.

Thrust bearing:

                        A thrust bearing is a particular type of rotary bearing. It supports the axial load of a rotating shaft and prevents the axial movement of this shaft. It is used in pump and turbine shaft.

                                               

Main parts of thrust bearing:

1.      Thrust pad

2.      Pad stop

3.      Carrier ring

4.      Tilting pad

5.      Retaining ring

6.      Thrust collar

7.      Journal bearing

8.      Journal bearing housing

9.      Cooling pipe assembly or cooler.

Advantages:

Ø  It holds the journal bearing carefully.

Ø  No tilting occurs. Tilting is balanced by tilting pads.

Ø  Axial movement is prevented by keeping the pads stop.

Ø  Retaining ring gives the external supports of pads as well as axial loads.