The pressure plate is an integral factor in the function of an automobile’s manual transmission. It pushes the clutch disc, sometimes called the clutch plate, against the constantly spinning engine flywheel. The clutch disc is either stationary or rotating, and friction material, similar to that found on brake pads and brake drums, causes the disc to spin at the same speed as the engine flywheel. It is this friction between the clutch disc and flywheel that allows the engine torque to drive the wheels.
Pressure plates are, as the name implies, round, metallic devices containing springs and fingers, or levers, and controlled by the release fork connected to the shifter. All of the clutch components are enclosed in the bell housing of the transmission, between the rear of the engine and the front of the gearbox.
When the driver steps on the clutch pedal, a number of springs in the pressure plate are compressed by multiple — most often three — fingers. This compression of the spring(s) pulls the plate and the clutch disc away from the flywheel and prevents the clutch disc from rotating. When the clutch disc is stationary, the driver can shift into the proper gear and release the clutch pedal. When the pedal is let up, the fingers release their grip and the spring(s) expand to push the plate into the clutch disc, thereby engaging the flywheel. This release process is often called the “clamp load.”
There are three major types of pressure plates:
- The Long style contains nine coil springs for pressure against the flywheel and three thin fingers for release. It is used mainly for drag racing.
- The Borg & Beck style also contains nine coil springs and three fingers. The fingers are wider, however, and the plate has the more robust materials and design necessary for street driving.
- The diaphragm pressure plate is best suited for street use and is, therefore, the most common type found on later-model automobiles. It contains a single Bellville-style spring that applies a more even load from clutch plate to flywheel. Because the single-spring diaphragm is more effective “over-center”, there is also less effort needed by the driver to hold the clutch pedal in the depressed position at a stop.