This is a modern clutch and it is increasingly being used in heavy duty applications eg trucks.
It may be in the form of a single or double clutch package.
Figure shows a double hydraulic clutch incorporating hydraulic balance, internal oil transfer and internal pressure modulation.
With the oil transfer system which enables oil to be transferred from one force chamber to the other without passing through the hydraulic pump or external oil supply system, a much smaller hydraulic pump is needed and the clutch engagement is also faster.
Starting with both clutches released the following sequence of operations takes place when one clutch is engaged.
(i) Oil under pressure enters the accelerator Piston cavity A, which causes the corresponding accelerator Piston to move towards the separator plate. This further results in the sealing of the disc valve assembly near cavity B against the separator and opening of the disc valve cavity C.
(ii) The main force Piston, then, moves towards right, into the engaged position. Simultaneously, oil is also being forced from chamber C through the pressure plate opening, into chamber B by opening one way valve adjacent to chamber B. The oil pressure in chamber C being higher than that in the chamber B, because of the movement of the force Piston, causes oil transfer to take place.
(iii) With the force Piston in the engaged position, the engagement is completed by pressurizing chamber B from chamber A at a controlled rate through an orifice in the accelerator Piston.
The clutch engagement rate can be controlled by controlling the pressure built up in the force cavity of the clutch, which can be done either externally or internally. Internal pressure modulation is found to be better than the external system because there the modulation is controlled by metering a much smaller quantity of oil.
The hydraulic clutch shown in figure here utilizes internal pressure modulation.
This is achieved with the use of an orifice between the accelerator cavity between the accelerator cavity and the force cavity passing through the accelerator Piston.
With the clutch in the engaged position, oil under pressure enters the accelerator position cavity closing the disc valve and moving the Piston into engaged position.
Since the displacement of the accelerator piston is small, the pressure drop in the accelerator cavity is only Instantaneous, thereby generating a portion of the clamping force in an extremely short time.
The remaining clamping force is then generated by a controlled pressure built up in the major force cavity created by metering the small amount of oil required through the orifice in the accelerator system.
Figure illustrates the effect of internal pressure modulation as compared to the unmodulated clutch.
It is seen that the modulated clutch begins the engagement much quicker than the unmodulated clutch.
This is due to oil transfer system.
The engagement increases steadily and smoothly till the lock up occurs.
The pressure in the clutch then continues to rise till maximum torque capacity of the clutch has been reached. On the other hand, in case of unmodulated clutch, the time required to get the position in the engaged position is considerably longer.
At the point of piston engagement the clutch torque rises suddenly from zero to maximum, resulting in very harsh engagement and very high rate of rate generation.
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