The primary purpose of the clutch cover is to connect and disconnect engine power. It must be well balanced while it is rotating, and radiate heat efficiently at the time of clutch engagement. The clutch cover has a spring to push the pressure plate against the clutch disc. These springs may be either coil springs or a diaphragm spring. The latter is used in most clutches today.
1. Diaphragm spring type clutch
The diaphragm spring is made of spring steel. It is riveted or bolted to the clutch cover. A pivot ring is located at each side of the diaphragm spring and functions as a pivot while the diaphragm spring is operating. The retracting springs are used to connect the diaphragm springs to the pressure plate.
Recent models have adopted a DST (Diaphragm Spring Turn-over) type clutch cover. In this type of clutch cover, the tips of the clutch cover are turned over to hold the diaphragm spring at proper position directly. The straps are connected in a chordal (tangential) direction to transmit the torque.
2. Characteristics of diaphragm springs
The graph on the left shows the movement of the pressure plate along the horizontal axis and the pressure plate along the vertical axis. The solid line indicates the characteristics of the diaphragm spring, and the dotted line indicates the characteristics of the coil spring.
(1) Normal condition (When the clutch disc is brand new)
When the pressure applied to the pressure plate (P0) is equal for both types: coil spring type and diaphragm spring type, each pressure becomes P2 and P’2 with the clutch pedal fully depressed.
This means that for the diaphragm spring type, the force needed to depress the clutch pedal is smaller than that for the coil spring type by the amount shown by “a”.
(2) When wear on the contact surface of clutch disc exceeds the allowable limit
The pressure applied to the pressure plate of the coil spring type clutch decreases to P’1.
On the other hand, the pressure applied to the pressure plate of the diaphragm spring type clutch is P1, and the same as P0. That is, the power transmission ability of the diaphragmspring type clutch does not decrease till thedisc wear limit.Conversely, the pressure applied to the pressure plate of the coil spring type clutch decreases to P’1. As a result, the transmission ability decreases, causing the clutch to slip.
Automotive Clutch Disc
The clutch disc contacts the friction surface of the pressure plate and the flywheel uniformly to transmit motive power smoothly. It also consists to soften the impact of clutch engagement.
1. Torsion rubber
The torsion rubber is incorporated in the clutch hub and softens the rotational impact of clutch engagement by moving slightly in the circumferential direction.
2. Cushion plate
The cushion plate is riveted sandwiched between the clutch facings.
When the clutch is engaged suddenly, the impact is absorbed by this curved section to soften the shock of gear changing and allow power to be transmitted smoothly.
Wear of the torsion rubber and breakage of the cushion plate causes a large amount of impact shock and noise when the clutch is engaged.