09-11-2012, 12:49 PM
ELECTROMAGNETIC CLUTCH SYSTEM
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INTRODCTION
A clutch is a mechanism for transmitting rotation, which can be engaged and disengaged. Clutches are useful in devices that have two rotating shafts. In these devices, one shaft is typically driven by motor or pulley, and other shaft drives another device. The clutch
connects the two shafts so that they can either be locked together and spin at the same speed (engaged), or be decoupled and spin at different speeds (disengaged).
The clutch disc (centre) spins with the flywheel (left). To disengage, the lever is pulled (black arrow), causing a white pressure plate (right) to disengage the green clutch disc from turning the drive shaft, which turns within the thrust-bearing ring of the lever. Never will all 3 rings connect, with any gaps.
TYPES OF CLUTCH
DOG CLUTCH
A dog clutch is a type of clutch that couples two rotating shafts or other rotating components not by friction but by interference. The two parts of the clutch are designed such that one will push the other, causing both to rotate at the same speed and will never slip.
Dog clutches are used where slip is undesirable and/or the clutch is not used to control torque. Without slippage, dog clutches are not affected by wear in the same way that friction clutches are.
Dog clutches are used inside manual automotive transmissions to lock different gears to the rotating input and output shafts. A synchromesh arrangement ensures smooth engagement by matching the shaft speeds before the dog clutch is allowed to engage.
Wet and dry
A 'wet clutch' is immersed in a cooling lubricating fluid, which also keeps the surfaces clean and gives smoother performance and longer life. Wet clutches; however, tend to lose some energy to the liquid. A 'dry clutch', as the name implies, is not bathed in fluid. Since the surfaces of a wet clutch can be slippery (as with a motorcycle clutch bathed in engine oil), stacking multiple clutch disks can compensate for the lower coefficient of friction and so eliminate slippage under power when fully engaged.
Cone clutch
A cone clutch serves the same purpose as a disk or plate clutch.
However, instead of mating two spinning disks, the cone clutch uses two conical surfaces to transmit torque by friction.
The cone clutch transfers a higher torque than plate or disk clutches of the same size due to the wedging action and increased surface area.
Cone clutches are generally now only used in low peripheral speed applications although they were once common in automobiles and other combustion engine transmissions.
They are usually now confined to very specialist transmissions in racing, rallying, or in extreme off-road vehicles, although they are common in power boats. This is because the clutch doesn't have to be pushed in all the way and the gears will be changed quicker.
Centrifugal clutch
A centrifugal clutch is a clutch that uses centrifugal force to connect two concentric shafts, with the driving shaft nested inside the driven shaft.
Centrifugal clutches are often used in mopeds, lawnmowers, go-karts, chainsaws, and mini bikes.
Fluid coupling
A fluid coupling is a hydrodynamic device used to transmit rotating mechanical power. It has been used in automobile transmissions as an alternative to a mechanical clutch. It also has widespread application in marine and industrial machine drives, where variable speed operation and/or controlled start-up without shock loading of the power transmission system is essential.
Torque
It is also known as safety clutch. A torque limiter is an automatic device that protects mechanical equipment, or its work, from damage by mechanical overload. A torque limiter may limit the torque by slipping (as in a friction plate slip-clutch), or uncouple the load entirely (as in a shear pin). The action of a torque limiter is especially useful to limit any damage due to crash stops and jams.
Torque limiters may be packaged as a shaft coupling or as a hub for sprocket or sheave. A torque limiting device is also known as an overload clutch.
Electromagnetic clutch
Electromagnetic clutches operate electrically, but transmit torque mechanically. This is why they used to be referred to as electro-mechanical clutches.
CONSTRUCTION
A horseshoe magnet has a north and south pole. If a piece of carbon steel contacts both poles, a magnetic circuit is created. In an electromagnetic clutch, the north and south pole is created by a coil shell and a wound coil. In a clutch, when power is applied, a magnetic field is created in the coil. This field (flux) overcomes an air gap between the clutch rotor and the armature. This magnetic attraction, pulls the armature in contact with the rotor face. The frictional contact, which is being controlled by the strength of the magnetic field, is what causes the rotational motion to start. The torque comes from the magnetic attraction, of the coil and the friction between the steel of the armature and the steel of the clutch rotor. For many industrial clutches, friction material is used between the poles. The material is mainly used to help decrease the wear rate, but different types of material can also be used to change the coefficient of friction (torque for special applications). For example, if the clutch is required to have an extended time to speed or slip time, a low coefficient friction material can be used and if a clutch is required to have a slightly higher torque (mostly for low rpm applications), a high coefficient friction material can be used.
In a clutch, the electromagnetic lines of flux have to pass into the rotor, and in turn, attract and pull the armature in contact with it to complete clutch engagement. Most industrial clutches use what is called a single flux, two pole design. Mobile clutches of other specialty electromagnetic clutches can use a double or triple flux rotor. The double or trip flux refers to the number of north/south flux paths, in the rotor and armature.