Clutch: Description and Operation
System overview
Power is transmitted from the engine to the gearbox via a clutch. This clutch consists of a dry plate and a pressure plate of diaphragm spring type. Clutch operation is hydraulic.
Clutch
Power is transmitted from the flywheel on the engine to the input shaft in the gearbox via a driven plate. This power is transmitted by the pressure plate, which is bolted to the flywheel, forcing the friction surface of the driven plate against the flywheel.
The driven plate is joined to the primary shaft with a splined joint. When the release bearing on the slave cylinder presses on the "fingers" of the diaphragm spring, the fingers will act like levers to release the clamping force of the pressure plate from the driven plate-flywheel. The engine is disengaged from the transmission in this way.
Together with the dual-mass flywheel there is also a new driven plate hub and a new pressure plate. The pressure plate is of a new fully self-adjusting type, which means that the pressure springs against which the release bearing presses are always in the same position relative the release bearing. The pressure springs (diaphragm spring) can move between an adjusting ring and wear ring. When wear occurs on the driven plate lining, the wear ring with pressure surface will move to the pressure plate. This movement reduces the load on the adjusting ring so that it can be turned by two small springs and self-adjust via a ratchet heel in the pressure plate housing.
This allows the pressure spring to retain its original position, providing several advantages such as
^ Constant pedal application force irrespective of driven plate wear
^ Longer service life of driven plate and pressure plate
^ Lower pedal force required
This new concept means that the pressure plate and driven plate hub must always be changed in one unit.
Dual-mass flywheel
A modern, highly-efficient engine with low weight is able to dampen vibration itself. To obtain lower noise levels and increase comfort, there is a dual mass flywheel.
The combustion process in reciprocating engines causes torsional vibration in the crankshaft and therefore also the flywheel. This torsional vibration is conveyed to the transmission and via engine mountings also to the subframe and the body. Combustion, especially at low engine speeds, causes varying torque on the crankshaft and flywheel. This means that gears that are not in mesh will start to vibrate and gearbox clatter will easily arise at idling speed, under high loads and during engine braking at low engine speeds. The solution to this problem is a dual-mass flywheel comprising two separate masses, a primary and a secondary, with a spring damping system connecting the two. This almost completely eliminates torsional vibration in gearbox and transmission. This means that the whole transmission chain is able to run more smoothly without causing clatter in gears not in mesh. Vibration in the body conveyed via the engine mountings is greatly reduced.
This design improves shifting comfort and reduces gearbox wear. Fuel consumption is also reduced thanks to being able to drive at lower engine speeds.
The primary mass in the dual-mass flywheel is mounted on the engine (crankshaft) and the secondary on the transmission side. They are flexibly linked with a spring assembly that absorbs torsional vibration from the engine.
High torsional vibration occurs above all in conjunction with load transitions in the lower engine speed range, when driving at very low engine speeds with high torque output and when the engine is started and stopped. These peaks in torque are absorbed by the bow-shaped springs. At high engine speeds, the springs are pressed out against the spring housing so that their spring action becomes limited. At high engine speeds on the other hand, the variation in torque are so small that the influence of the action of the outer springs is reduced. The spring normally mounted in the driven plate hub are not present in engines with dual-mass flywheels as the flywheel itself has taken on the task of these springs. A pulley with vibration damper is mounted on the belt circuit side of the crankshaft so that the torsional vibration does not spread to components driven by the belt.
Clutch operation
Clutch operation is hydraulic and fully self-adjusting. The unit comprises:
1. Master cylinder
2. Connecting pipes
3. Slave cylinder
The master cylinder is mounted in the bulkhead and is connected to the clutch pedal via a piston rod.
The slave cylinder is an integrated unit mounted in the clutch housing and comprises:
1. Cylinder housing
2. Split piston
3. Fixed release bearing
The slave cylinder cannot be disassembled. The hydraulic pressure from the master cylinder presses on the seal, which then pushed the piston and release bearing against the pressure plate. A spring is mounted between the cylinder housing and the release bearing, which means the release bearing is always pressed against the pressure plate and in this manner reduces the play in the clutch pedal.
In order to prevent dirt from reaching sensitive parts of pistons and seals, a rubber gaiter is mounted between the cylinder and release bearing.
A hydraulic line links the master cylinder and the slave cylinder damper pipe (to reduce pedal vibration) and has a quick-release coupling at each end. The lower quick-release coupling, to the slave cylinder, has a bleed nipple.