Clutch: Description and Operation
DESCRIPTION
General
The clutch system is a conventional diaphragm type clutch operated by a hydraulic cylinder. The clutch requires no adjustment to compensate for wear.
Hydraulic Clutch
The hydraulic clutch comprises a master cylinder, slave cylinder and a hydraulic reservoir, which is also shared with the braking system. The master and slave cylinders are connected to each other hydraulically by plastic and metal pipes. The plastic section of the pipe allows ease of pipe routing and also absorbs engine movements and vibrations
The master cylinder comprises a body with a central bore, Two ports in the body connect the bore to the hydraulic feed pipe to the slave cylinder and the brake/clutch fluid reservoir. A piston is fitted in the bore and has an external rod which is attached to the clutch pedal with a pin. Two coiled springs on the clutch pedal reduce the effort required to depress the pedal.
The master cylinder is mounted on the bulkhead in the engine compartment and secured with two bolts. The cylinder is connected to the shared brake/clutch reservoir on the brake servo by a braided connecting hose.
The slave cylinder is located on the left hand side of the gearbox housing and secured with two bolts. A heat shield protects the underside of the cylinder from heat generated from the exhaust system. The slave cylinder comprises a cylinder with a piston and a rod. A port in the cylinder body provides the attachment for the hydraulic feed pipe from the master cylinder. A second port is fitted with a bleed nipple for removing air from the hydraulic system after servicing. The piston rod locates on a clutch release lever located in the gearbox housing. The rod is positively retained on the release lever with a clip.
Clutch Mechanism
The clutch mechanism comprises a flywheel, drive plate, pressure plate, release lever and a release bearing. The clutch mechanism is fully enclosed at the rear of the engine by the gearbox housing.
A clutch release bearing sleeve is attached in the gearbox housing with two bolts and located on two dowels. A spigot with a ball end is formed on the release bearing sleeve and provides a mounting and pivot point for the clutch release lever. A dished pivot washer is located on the ball of the spigot. When the release lever is located on the ball, the pivot washer seats against the rear face of the release lever. A spring clip is located on the lever and the pivot washer and secures the lever on the spigot. A small bolt retains the spring clip in position.
The release lever is forked at its inner end and locates on the clutch release bearing carrier. The outer end of the release lever has a nylon seat which locates the slave cylinder piston rod. A second nylon seat, positioned centrally on the release lever, locates on the ball spigot of the release bearing sleeve and allows the release lever to pivot freely around the ball.
The clutch release bearing locates on the clutch release lever and the release bearing sleeve. The bearing is retained on a carrier which has two flats to prevent the carrier rotating on the release lever. A clip retains the release lever on the carrier. The bearing and carrier are not serviceable individually.
Flywheel
The flywheel is bolted to a flange on the rear of the crankshaft with six bolts. A dowel on the crankshaft flange ensures that the flywheel is correctly located. A ring gear is fitted on the outside diameter of the flywheel and seats against a flange. The ring gear is an interference fit on the flywheel and is installed by heating the ring and cooling the flywheel. The ring gear is a serviceable item and can be replaced if damaged or worn.
The operating face of the flywheel is machined to provide a smooth surface for the drive plate to engage on. Three dowels and six threaded holes provide for the location and attachment of the pressure plate. The flywheel is balanced to ensure that it does not produce vibration when rotating. A machined slot, with a series of holes within the slot, is located on the engine side of the flywheel. The slot accommodates the tip of the crankshaft position sensor which is used by the Engine Control Module (ECM) for engine management.
Pressure Plate
The pressure plate assembly comprises a pressure plate, cover and diaphragm and is mounted on and rotates with the flywheel.
The pressure plate is forged from cast iron and machined to provide a smooth surface for the drive plate to engage on. Three lugs on the outer diameter of the pressure plate connect it via three leaf springs to the cover. The leaf springs have two tempered steel leaves which assist in pulling the pressure plate away from the drive plate when the clutch pedal is depressed.
The cover is made from pressed steel and houses and locates all pressure plate components. Shouldered rivets support the diaphragm and fulcrum rings inside the cover. The cover also provides attachment for balance weights when the pressure plate assembly is balanced. Three holes in the cover locate on the dowels on the flywheel and six further holes provide for the attachment of the cover to the flywheel with six bolts and spring washers. Larger holes in the cover provide ventilation for the drive plate and pressure plate and flywheel contact surfaces.
The diaphragm comprises a cast ring with eighteen fingers. The diaphragm is attached to the cover with nine shouldered rivets. Two circular steel fulcrum rings are also secured by the shouldered rivets on each side of the diaphragm. The fulcrum rings allow the diaphragm to pivot between them when the clutch is depressed or released. When pressure is applied to the diaphragm fingers by the release bearing, the diaphragm pivots between the fulcrum rings and moves away from the pressure plate. Retractor clips are secured to the pressure plate and are located on the outer diameter of the diaphragm. The retractor clips ensure that the diaphragm remains in contact with the pressure plate.
Drive Plate
The drive plate is of the spring centered type and is sandwiched between the pressure plate and the flywheel. The drive plate has a splined hub which engages with the splines on the primary drive shaft from the gearbox. The hub is located in an inner plate which contains six compression damper springs. A spring retainer plate and a disc adaptor are secured together with stop pins which limit the angular deflection of the disc adaptor. Engine power is transmitted from the disc adaptor to the damper springs. The damper springs then transfer the power to the retainer plate and the hub. Friction washers are located between the hub, retainer plate and disc adaptor and provide further damping.
A spring steel plate is riveted to the disc adaptor and provides the attachment surface for the drive plate friction material. The friction material comprises discs which are secured with rivets to each side of the plate. The rivets are installed through recessed holes in the disc and emerge in recessed holes in the opposite disc. The drive plate is 267 mm (10.5 in) diameter and has a friction material manufactured from APTEC T385.
OPERATION
Hydraulic Operation
Hydraulic Operation Part 1 of 2
Hydraulic Operation Part 2 of 2
When the clutch pedal is depressed, the master cylinder piston is pushed into the master cylinder. The movement of the piston pressurizes the fluid in the master cylinder, forcing the pressurized fluid into the hydraulic feed pipe to the slave cylinder. The hydraulic pressure is felt at the slave cylinder piston which moves under the hydraulic force applied, pushing the clutch release lever via the piston rod.
When the clutch pedal is released, the force applied to the release lever by the fingers of the diaphragm, moves the release lever, which pushes the slave cylinder piston into the cylinder. The displaced hydraulic fluid is pushed up the hydraulic feed pipe and returned to the master cylinder.
Mechanism Operation
When the clutch pedal is depressed, hydraulic pressure extends the piston and rod in the slave cylinder. The extension of the piston pushes the rod against the outer end of the release lever which pivots around the ball spigot.
The inner end of the release lever pivots towards the engine applying pressure to the release bearing. The release bearing slides along the release bearing sleeve and pushes on the fingers of the diaphragm. The diaphragm pivots about the fulcrum rings in the cover. As the diaphragm is deflected, it removes pressure from the pressure plate. The pressure plate moves away from the drive plate assisted by the three leaf springs and retractor clips.
The removal of force from the pressure plate on the drive plate reduces the friction between the flywheel, drive plate and pressure plate. The drive plate slips between the flywheel and the pressure plate preventing rotary movement being transferred from the flywheel and pressure plate to the primary driveshaft.
When the clutch pedal is released, hydraulic force is removed from the piston in the slave cylinder. This allows the fingers of the diaphragm to push the release bearing along the release bearing sleeve. The movement of the release bearing moves the release lever which pivots on the ball spigot, pushing the piston and rod back into the slave cylinder.
The removal of pressure from the release bearing on the diaphragm, causes the diaphragm to pivot around the fulcrum rings in the cover. The force applied to the pressure plate from the diaphragm overcomes the force of the leaf springs and the pressure plate moves towards the drive plate and flywheel.
The pressure plate applies pressure to the drive plate which is pushed against the flywheel. As the clutch pedal is progressively released, the friction between the drive plate, flywheel and pressure plate increases. The increase in friction transfers the rotary movement of the flywheel and pressure plate to the drive plate which in turn starts to rotate the primary driveshaft. When the clutch pedal is released fully, the force applied by the diaphragm to the pressure plate forces the drive plate onto the flywheel with no slippage.