24.4.
Multi-coil Spring Clutch
A multi-coil spring clutch (Fig. 24.5) unit mainly uses a flywheel, a cover-pressing, a pressure-plate, a driven-plate, thrust springs, and a release-lever mechanism.
The flywheel, the input member of the clutch, provides a rigid base to which the clutch assembly is bolted, and also forms the frictional faceplate for one side of the driven centre-plate.
The cover-pressing is made of steel having ventilation holes for heat dissipation and is bolted to the flywheel. It houses the pressure-plate assembly forming one part of the driving member of the clutch. This cover takes the reaction of the thrust coil springs and also provides a pivot point for the release-levers.
The pressure-plate is an annular cast-iron disc, which provides a smooth rubbing surface for one side of the driven centre-plate. It resists distortion and dissipates the generated heat when the clutch is slipped. Four protruding lugs are cast and then machined on the back of the plate, which locate and support the release-lever and strut assemblies.
Four laminated spring steel straps, arranged tangential around the pressure plate, hold the cover pressing and the pressure-plate together. One end of the each strap is riveted to the cover pressing, while the other end is bolted to the pressure plate. These straps deflect during clutch
operation and transmit the drive from the cover pressing to the pressure plate eliminating friction between them and maintaining their concentricity.
Fig. 24.5. Multi-coil spring clutch assembly.
A ring of helical-coil thrust-springs, installed between the pressure-plate and the cover in a state of compression, loads the pressure-plate against the driven-plate when the clutch is engaged, so that the drive is transmitted from the cover-pressing to the driven friction-plate.
The release-lever plate in the form of an annular cast-iron disc is installed at the inner ends of the release-levers and is retained in position by specially formed springs. It acts as the frictional face plate for the release bearing.
The driven-plate is sandwiched between the flywheel and the pressure plate. It is simply a spring-steel disc installed on a splined hub. Moulded asbestos friction facing is riveted to each side of this disc. The drive is transmitted from the flywheel and pressure-plate assembly to the friction discs and then to the splined gearbox primary shaft through the splined hub.
Four release-levers of case-hardened pressed-steel release the clutch. A knife-edge strut is installed in a groove formed in the outer end of each lever and is held in position by two extended ears. The ears fit into grooves in the bosses of the pressure plate. The levers pivot on loose pins, which fit into recesses in the levers and are free to move in eyebolts. The release-levers are fitted with anti-rattle springs located between the levers and the clutch cover pressing.
For operation of the clutch, the clutch pedal is pressed and the clutch fork is moved through the push rod, which pivots on the cross-shaft. The forked end moves the withdrawal bearing along the gearbox primary-shaft bearing cover and tubular extension support until the bearing contacts the inner tips of the release-levers. Further movement of the fork pivots the release-levers on the eyebolt pins and moves the clutch pressure-plate through the strut. These cause the release of driving pressure from the clutch driven-plate friction-disc facings. Once the clutch pressure is released, the movements are reversed and the driving pressure is re-applied to the disc faces.
Multi-coil springs clutch units are not normally used for cars, but they are common on heavy commercial vehicles where a single diaphragm spring can not provide sufficient clamping thrust.
