Clutch are used to obtained temporary connection between driver and driven shaft.
Coupling is used to obtained permanent connection between driver and driven shaft.
Clutches are used in application where driven machine required intermittent service (
Stop and start ). Coupling are used in application where driven machine required
continuous service.
Clutches are used to transmit power between two co-linear shafts. Coupling is used to
transmit power between two co-linear as well as non co-linear shafts.
In presence of clutches driven machine can run at variable speed. In presence of
coupling driven machine can run only speed equal to driver.
The clutch mechanism connects the gearbox to the wheels of a vehicle - with an
option to ‘engage’ or ‘disengage’ it. When engaged, the engine transfers power to the
wheels through the gearbox; when disengaged, the wheels are free to move.
Image courtesy of: CorvetteForum
Further reading: How does a car clutch work?
The coupling is a permanent connection - a mechanical device designed to transmit
torque between two shafts. It consists of two flexible joints, one fixed to each shaft. There
are several types of couplers - as in motor-to-pump, hose-coupling, and so on - but the
principle is the same.
Cams and linkages
The cam below would trigger a movement of three valleys and peaks per rotation. As the roller surface goes
along the edge of this three-lobed cam from peak to valley, there is a quick change of direction in the follower
arm, which may also create a noise.
Three lobed cam
The cam below would move the follower up and down five times per rotation.
5 lobed cam
Snail or drop cam
A snail (or drop) cam (above) has a slow rise and then a sudden drop. This is a one-event-per-turn cam with a
slow buildup to the peak and a rapidly decreasing edge. This cam has one direction only, which is counter
clockwise. You can think of this mechanism as a sound generating device with acoustics which are based on the
physics of the materials you select.
In the image below the turning snail cam moves a rod with a shoulder bolt at the tip.
Snail Cam and a vertical follower with a roller shoulder bolt at the end that makes contact with the spinning
cam
A shoulder bolt creates a rolling surface, so the follower arm will be able to roll against the cam with ease.
Shoulder bolt used to create rolling surface on rods or other objects coming into contact with the spinning cam.
Linkages
Linkages are connections that can be levers (but aren’t always) and function to transfer motion from one
component to another. The distance that the arm travels is determined by the size of the crank.1
The crank slider (below) is a type of linkage composed of three main components: the crank is the rotating
circular disc, the slider slides inside the slot or tube, and the pushrod connects the slider to the crank.
The pushrod traces a curved line as a mirror of the circulating crank. By changing the length of the slider or the
diameter of the crank you can change the shape of the circular line being traced.
Crank slider
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Before starting to study about different types of clutch, we must know about the meaning of
clutch. Clutch is defined as the device which is used in automobiles to transmit power from
one rotating shaft to another shaft. In cars it transmits power from the flywheel connected to
the engine shaft to the clutch shaft, and from clutch shaft it is transmitted to the rear wheels
through gear shaft, propeller shaft and differential.
Mainly clutches are divided into 2 parts:
1. Friction clutches and
2. Fluid flywheel
Friction clutches:
These clutches works on the principle of friction exist in between two rotating shaft when
they come in contact with each other.
Fluid flywheel:
Fluid flywheel clutches works on transfer of energy from one rotor to the other by means of
some fluid. for Example: Fluid coupling and torque converter.
Types of friction clutches:
1. Cone clutch
2. Single plate clutch
3. Multi-plate cutch
Semi-centrifugal clutch
Centrifugal clutch
Fluid flywheel or fluid coupling
A liquid coupling is used to transmit engine turning effort (torque) to a clutch and
transmission. The coupling is always a major part of the engine flywheel assembly. As
such it is sometime called a fluid flywheel.
One of the shells is fixed to the crankshaft of the engine and the other to the
clutch/gearbox shaft. The two shells are mounted very close, with their open ends facing
each other, so that they can be turned independently without touching. Housing
surrounds both units to make a closed assembly. About 80 percent of the interior of the
assembly is filled with oil.
Fluid coupling is also known as hydraulic coupling is a hydrodynamic device which is used to transfer
rotational power from one shaft to another by the use of transmission fluid. It is used in automotive
transmission system, marine propulsion system and in industries for power transmission. It is used
as an alternative for the mechanical clutch.
It was discovered by Dr. Hermann Fottinger. He patented his discovery of fluid coupling and torque
converter in the year 1950.
Main Parts
It consists of three main components
1. Housing: It is also known as the shell. It has oil-tight seal around the drive shaft. It also
protects the impeller and turbine from outside damage.
2. Impeller or pump: It is a turbine which is connected to the input shaft and called as impeller.
It is also known as pump because it acts as a centrifugal pump.
3. Turbine: It is connected to the output shaft to which the rotational power is to be
transmitted.
Also Read: Ignition Coil – Main Parts, Working Principle and Application
The impeller is connected to the prime mover (internal combustion engine) which is a power source.
The turbine is connected to the output shaft where rotation power is needed to be transmitted. The
impeller and turbine is enclosed in an oil-tight sealed housing. The housing consists of transmission
fluid.
Working Principle
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Image Source
The working principle of fluid can be easily explained by the taking two fans in which one is
connected to the power supply and other is not. When the power switch is ON, the air from the first
fan is starts to blow towards the second fan (which is not connected to the power source). Initially
when the first fan is blowing at lower speed, it does not able to drive the second fan. But as the
speed of the powered fan increases, the speed of air striking the blades of second fan also increases
and it starts to rotate. After some time it acquires the same velocity of that of the first fan.
On the same principle the fluid coupling works. In that the impeller act as first fan and the turbine
act as second fan. Both impeller and turbine enclosed in an oil tight housing. The impeller is
connected the input shaft of the prime mover and the turbine with the output shaft. When the
impeller is moved by the prime mover, the fluid in housing experiences centrifugal force and due to
curved vanes of the impeller the fluid directed towards the turbine blades. As the fluid strikes the
turbine blades it starts rotating. With the increase in the speed of impeller, the velocity of the
turbine increases and becomes approximately equal to the impeller speed. The fluid after passing
through the turbine blades again return to the impeller.
Also Read: How Ignition Distributor Works?
Working of Fluid Coupling
1. As the prime mover moves, it rotates the impeller of the coupling. The impeller acts as a
centrifugal pump and throws the fluid outward and directs it towards the turbine blade.
2. As the high moving fluid strikes the turbine blades, it also starts rotating, after striking on the
blades, the direction of the fluid is changed and it is directed towards the impeller again. The
blades of turbine are designed in such a way that it can easily change the direction of the
fluid. It is the changing of direction of the fluid that makes the turbine to rotate.
3. As the impeller speed increases, the speed of the turbine also increases. After sometime the
speed of both impeller and turbine becomes equal. In this way power is transmitted from
one shaft to another by the use of fluid coupling.
4. In same way torque converter works but the difference is that it has stator placed in
between impeller and turbine for torque multiplication.
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