Your questions answered: Clutch actuation systems


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Ed explains a clutch's mechanical advantage : credit: © Ed Evans
One reader wants to know more about how Clutches work...

Clutch’s mechanical advantage

Can you explain clutch actuation systems? I’m interested to know how they work with specific reference to the ‘fulcrum-load-effort’ system of the release fingers.


Ed Evans replies: The clutch system shown in the diagram above has the drive from engine to gearbox engaged. The diaphragm spring (blue), which keeps the clutch engaged, is a circular spring-steel disc with radial slots cut outward from the centre forming the ‘release fingers’.

This diaphragm spring is firmly located at pivot (or fulcrum) points close to its
perimeter (five are visible here). At its perimeter, the spring is attached to the circular pressure plate (red) which presses the fiction plate (green) against the engine flywheel (yellow) to transmit drive. (The friction plate is splined to the gearbox shaft running through the centre so as it revolves with the flywheel it transmits rotation to the gearbox.)

When the pedal is pressed, the clutch release bearing (brown) presses the ‘fingers’ inward at the centre, and they pivot on the fulcrum points so the outer perimeter of the spring (and thus the attached pressure plate) is levered outward away from the friction disc, releasing it from the flywheel and disconnecting the drive. Let’s say, just for example, those fulcrum points are three quarters of the way along the fingers. That would mean they provide a leverage of 3 to 1 along the fingers, so the effort at the release bearing is magnified three times at the perimeter. So a relatively light effort at the release bearing produces the greater force needed to disengage the clutch.

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In normal service, the clutch release bearing being held against the centre of the diaphragm spring will press the fingers, causing them to pivot about the fulcrum points (seen around the cover plate)

Here’s the reverse side showing the circular steel pressure plate attached to the diaphragm spring, and the friction plate which transmits drive from the engine flywheel

Also, because the diaphragm spring is slightly convex, less pedal effort is required to hold the clutch disengaged than is needed to initially disengage it. (It’s hard to squeeze a convex drinks can until you overcome the curvature, then it’s easy to squeeze it further).


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