The Diaphragm Clutch

Diaphragm Springs have an original application, the Diaphragm Clutch. So what is Diaphragm clutch and what/how does it work, and what function must the diapragm spring perform?

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Basic Clutch Assembly

The diaphragm clutch is a conventional friction mechanism and consists of a clutch, the thrust plate, a diaphragm spring and the release sleeve. The diaphragm is held between the inner end of the main bearing and its outer circumstance fits into the counter bore of the thrust plate. The distinctive radial geometry of a diaphragm spring plays an esential role in it achieving its design function. These radial slots terminating with ovals/holes are the spring against which the required thrust on the pressure plate must be provided (your foot depressing a pedal traditionally).
This design arrangement eliminates the need to provide separate release levers.

Engaged

In the engaged position the spring pivots on the inner pivot rings as it is held on the clutch cover so that its outer rings contacts with the pressure plate. Again in this conical position the spring exerts through pressure to keep the pressure plate in firm contact with the clutch plate and flywheel.

Fig 1.1 - The Clutch is engaged allowing the machine to work.

Disengaged

When the pedal is pressed the linkage moves to release the bearing towards the flywheel and disengages the clutch. As the bearing makes contact with inner part of the spring it drives this position forward causing the link to move backwards. This releases load on the pressure plate and disengages the clutch plate from contact with other working components.

Fig 1.2 - The Clutch in its Dis-Engaged position.

Design Features of a Diaphragm Clutch

The release levers one might expect from a design incorporating coil springs are not required in this assembly. This is because the diaphragm spring itself performs the function of the release levers. Unlike coil springs, diaphragm springs do not have constant rate load characteristic instead it is designed to have a linear load curve, and thus the pressure on the diaphragm spring must increases until it reaches flat position, whereafter it decreases rapidly once passing position. This has the delightful result that the driver need not exert sustained pedal pressure to hold the clutch in a disengaged state as happens in those designs where a coil spring is used.

Fig 1.3 A comparitive example - The design expected from a Harley-Davidson, simple, not too refined, built to last .

Advantages of Using a Diaphragm Spring

Allows for a more compact design, and saves space. The Load Characteruistic curve is more fit for purpose.

Fig 1.4 A comparitive example - The basic coneptuals design elements are present in all Diaphragm Springs, but the same difference are important .