Clutch disc
Components of the clutch disc
| Item | Description |
|---|---|
| 1 | Friction lining |
| 2 | Rivet |
| 3 | Multi-spline |
| 4 | Torsion spring |
| 5 | Carrier plate |
The function of the clutch disc
The clutch disc ensures judder-free engagement of the clutch, compensates for tolerances between the surfaces of the pressure plate and the flywheel, and reduces vibrations in the transmission.
Wear and generation of heat
Normal wear increases the actuation travel, and this can be compensated for via an automatic adjustment mechanism.
Overheating of the surface of the clutch disc will cause it to glaze. Here, glazing means a hardening and embrittlement of the friction lining surface. The friction lining surface may display cracks and start to become brittle. It is not a rare occurrence for individual segments of the friction lining surface to become detached. The coefficient of friction - and therefore the amount of torque which can be transmitted - is reduced as a result of glazing.
This damage is not undone by normal wear.
If the friction lining is connected to the carrier plate with adhesive, the latter can become soft if the components overheat, and as a result the friction lining can be displaced or detached over large areas. If this happens the clutch will no longer function as intended.
On wet clutches, the coolant can be chemically changed as a result of continuously high temperatures. If oil is used as the coolant, this can lead e.g. to increased oxidation or to the 'cracking' of oil molecules. 'Cracking' means that the structure of the oil molecule chain is changed in comparison to the original structure. Changes to the oil molecule chain can have a detrimental effect on the function of the clutch.
If there is a local build-up of heat between the friction lining and the pressing ring as a result of a localized concentration of heat, this is referred to as a 'hot spot'. The 'thermal peaks' created in the process occur as a result of uneven accumulation of material in the friction lining and in the pressing ring.
Function of the clutch disc
On the inside, the multi-splines on the clutch disc allow it to be moved axially in relation to the transmission input shaft whilst not permitting any relative rotation.
The splines allow the clutch disc to be detached from the flywheel after the pressing ring has been lifted off. If this axial freedom of movement is no longer given - e.g. due to rust - or the clutch disc has too much runout, then the clutch will no longer fully disengage. Torque is then still transmitted from the engine to the transaxle. This places correspondingly higher loads on the synchronizer assembly. This can result in difficult and noisy gearshifts.
During engagement of the clutch, the clutch disc is clamped between the pressing ring and the flywheel. If no dual-mass flywheel is fitted, coil springs which are arranged tangentially in the intermediate layers and which permit limited radial twisting provide damping for noises from the transaxle (rattling) and other vibrations.
Apart from the size, there are a number of other differences between the clutch disc of a car and that of a truck or commercial vehicle.
In the case of a truck or commercial vehicle, the ratio between the friction surface and the overall area is greater in order to allow the clutch to transmit very high torques. The lining itself also contains more stabilizing fibers, and the internal splines to the transmission input shaft are bigger.