30-05-2012, 12:22 PM
Bearings
Bearings .ppt (Size: 282.5 KB / Downloads: 277)
A Bearing is a machine element which support another moving machine element. It permits relative motion between the contact surfaces of the members, while carrying a load.
Most bearings support rotating shafts against either radial or axial loads.
To minimize friction, the contacting surfaces in a bearing may be partially or completely separated by a film of liquid (usually oil) .
If , the surfaces in a bearing are separated by balls , then they are known as Ball Bearings .
If , the surfaces in a bearing are separated by rollers, they are known as Roller Bearings.
Because there is line contact between a roller and the races, while in a ball bearing there is point contact, in a given space ,a roller bearing can carry a greater radial load than a ball bearing.
MAGNETIC BEARINGS
A Magnetic bearing is a bearing which supports a load using magnetic levitation .
A Magnetic bearing support moving machinery without physical contact, for example, they can levitate a rotating shaft and permit relative motion without friction or wear.
Basic Operation
An Active Magnetic Bearing (AMB) consists of an electromagnet assembly, a set of power amplifiers which supply current to the electromagnets, a controller, and gap sensors with associated electronics to provide the feedback required to control the position of the rotor within the gap.
The power amplifiers supply equal bias current to two pairs of electromagnets on opposite sides of a rotor.
ADVANTAGES
Magnetic bearing advantages include :----
Very low and predictable friction
Ability to run without lubrication and in a vacuum.
Free abrasion
Free of wear maintenance
Good thermal isolation of the rotor and stator And
Small bearing loss.
Disadvantages include High cost, and Relatively large size.
Applications
Magnetic bearings are increasingly used in industrial machines such as compressors, turbines, pumps, motors and generators
Magnetic bearings are commonly used in watt-hour meters by electric utilities to measure home power consumption.
Magnetic bearings are also used in high-precision instruments and to support equipment in a vacuum.
Magnetic bearings are also used to support Maglev trains in order to get low noise and smooth ride by eliminating physical contact surfaces.
A very interesting new application of magnetic bearings is their use in Artificial hearts.
Now-a-days, they are in service in such industrial applications as Electric power generation, Petroleum Refining, Machine tool operation and Natural gas pipelines.