Semi centrifugal clutch :-
For small torque transmission the clutch springs may be designed so that they have sufficient strength for applying the required amount of force and at the same time are not so stiff as to cause any strain to the driver while disengaging.
However, for high powered engines, the clutch spring pressure required may be considerable and thus the action of disengaging the clutch becomes fatiguing to the driver.
However, for high powered engines, the clutch spring pressure required may be considerable and thus the action of disengaging the clutch becomes fatiguing to the driver.
To obviate this trouble, the help is taken of the centrifugal force.
The clutch springs are designed to transmit the torque at normal speeds, while for higher speeds, centrifugal force assist in torque transmission.
Such type of clutches are called semi centrifugal clutch.
Figure shows a semi centrifugal clutch.
Three hinged and weighted levers are arranged at equal intervals. One of these is shown in figure on enlarged scale.
This lever is having fulcrum A and is hinged to pressure plate at B.
The upper end of the lever is weighted at C.
D is the adjusting screw, by means of which the maximum centrifugal force on the pressure plate can be adjusted.
To reduce friction, the levers are mounted on needle roller bearings on the pressure plate.
At moderate speeds the pressure of the spring is sufficient to transmit the required torque.
However at high speeds, the weight C, due to the centrifugal force moves about A as fulcrum thereby pressing the pressure plate.
The centrifugal force is proportional to the square of the speed so that adequate pressure level is attained. Figure shows the variation of force on the pressure plate at various speeds.
Three hinged and weighted levers are arranged at equal intervals. One of these is shown in figure on enlarged scale.
This lever is having fulcrum A and is hinged to pressure plate at B.
The upper end of the lever is weighted at C.
D is the adjusting screw, by means of which the maximum centrifugal force on the pressure plate can be adjusted.
To reduce friction, the levers are mounted on needle roller bearings on the pressure plate.
At moderate speeds the pressure of the spring is sufficient to transmit the required torque.
However at high speeds, the weight C, due to the centrifugal force moves about A as fulcrum thereby pressing the pressure plate.
The centrifugal force is proportional to the square of the speed so that adequate pressure level is attained. Figure shows the variation of force on the pressure plate at various speeds.
Centrifugal clutch :-
In the fully centrifugal type of clutches, the springs are eliminated altogether and only the centrifugal force is used to apply the required pressure for keeping the clutch in engaged position.
The advantage of the centrifugal clutch is that no separate clutch pedal is required.
The clutch is operated automatically depending upon the engine speed.
This means that car can be stopped in gear without stalling the engine.
Similarly while starting, the driver can first select the gear, put the car into the gear and simply press the accelerator pedal.
This makes the driving operation very easy.
Figure shows a schematic diagram of a centrifugal clutch.
As the speed increases, the weight A fies, thereby operating the bell crank Lever B which presses the plate C.
This force is transmitted to the plate D by means of springs E.
The plate D containing friction lining is thus pressed against the flywheel F thereby engaging the clutch.
Spring G serves to keep the clutch disengaged at low speed, say, 500 RPM. The stop H limits the amount of centrifugal force.
The operating characteristics of this type of clutch will then be as shown in figure.
Force P is proportional to the centrifugal force at a particular speed, while force Q exerted by spring G is constant at all speeds.
The firm line in the figure shows the net force on the plate D for various engine speeds.
At the upper end the curve is made flat by means of stop H.
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