Aspects Of A Vehicle Clutch System

P1 & P2: Demonstrate the knowledge & understanding of the fundamental aspects of a vehicle clutch system.

A clutch connects & disconnects one rotating mechanical component from another: a clutch transmits torque from the engine to transmission.

Most light vehicle use a single plate clutch to transmit torque from the engine to the transmission input shaft, the fly wheel is the clutch driving member. The clutch unit consists of a friction type disc with two friction facings & a central splined hub. A pressure plate assembly consists of pressed steel cover with a segmented diaphragm spring & a release bearing with a selector fork. The amount of torque a clutch can transmit depends on the co-efficient of the friction between the friction facings & their mating surface, thus increasing the diameter of the clutch increases it torque capacity as does increasing the spring force.

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The transmission input shaft passes through the centre of the pressure plate. Its parallel spleens engage with the internal splines of the central hub, on the friction disc allowing torque to be transmitted from the flywheel, through the friction disc to the central hub & to the transmission. When the clutch pedal is depressed, the movement is transferred though the operating mechanism (this being cable, hydraulic or electronic system), to the operating fork & release bearing, the depressing movement acts on the releasing bearing moving it forward & pushes the centre of the diaphragm spring towards the flywheel, the pressure plate disengages, thus drive is no longer transmitted. Releasing the pedal allows the diaphragm to re-apply it clamping force thus engaging the clutch thus restoring drive.

This control is necessary when placing the transmission into gear; the transmission input shaft must be disconnected from the engine.

Types Of Clutches:

Single Plate Transmission

Most modern vehicles use single plate clutch system to transmit torque from the engine to the transmission input shaft, the clutch unit is mounted on the flywheel, allowing the unit to rotate with the flywheel.

A single plate clutch unit usually consists of a friction plate with 2 friction facing & a central splined hub, a pressure plate assembly which consists of a pressed steel cover, a pressure plate with a machined flat face, a diaphragm spring, release bearing & release fork.

Pressure Plate: It mounts on the flywheel. It consists of four main parts & is more correctly called a clutch cover assembly. These parts are the pressure plate itself, the springs (or spring, if a diaphragm type), the clutch cover, & the release arms. There are two basic designs of clutches usually referred to by the spring type. This diaphragm is located inside the clutch cover on 2 fulcrum rings, held by a number of rivets passing through the diaphragm. The diaphragm type clutch works well in lightweight, low geared vehicles. It is not the best clutch for high RPM use as the diaphragm spring will stay “flat” or released from the centrifugal force generated by the RPM.

Clutch Disc/Friction Plate: This is the “driven” part of the clutch. It has a friction material riveted to each side of a wavy spring. This is attached to a splined hub that the transmission input gear protrudes into. There are basically two common types of friction material used for clutch lining. These are organic & metallic. The organic is best for all around use. The metallic is preferred by some for severe duty applications but requires high spring pressures & is hard on the flywheel & pressure plate friction surfaces.

Clutch Release Bearing: As its name implies, this is the bearing that releases the clutch. Because the release bearing only works when the clutch is being released it usually lasts quite a long time. However, improper linkage adjustment can wear the bearing prematurely.

Normally there should be a minimum clearance of 1/16” between the face of the bearing & the three release fingers or diaphragm spring of the pressure plate when the clutch is engaged.

Clutch Release Fork: This is the arm or lever that the linkage operates that moves the release bearing. There are several different styles of release arm. The most common type the can be found in a vehicle is the fork type.

Bell housing: This provides a mounting place for the transmission, as well as a means of aligning the transmission to the engine. In some applications it also has a structural mounting function.

Operating Mechanism:

The movement at the pedal is transferred through an operating mechanism to the clutch assembly; the mechanism may be mechanical or hydraulic.

Mechanical system uses a combination of levers & cables thus offering more flexibility, thus making this system very common.

The cable operated control (Image) for a front wheel drive vehicle; the outer cable is fixed to the pedal to the pedal box inside the vehicle & to the clutch housing in the engine compartment, the inner cable connects between the upper end of the clutch pedal & an external lever on top of the clutch housing.

This lever is connected to a vertical shaft, supported in the housing & attached internally to a release fork, which carries the release bearing.

In hydraulic clutch control, the pedal acts on a master cylinder, connected by a hydraulic pipe and flexible hose, to a slave cylinder, mounted on the clutch housing. The slave cylinder operates the clutch release fork. This vehicle uses a centre valve master cylinder. With the pedal in the off position, the centre valve is clear of the inlet port and fluid can flow to or from the reservoir, into the cylinder.

Dual Clutch Transmission

There are two basic types of transmissions, manuals which require a driver to change gears by depressing a clutch pedal, automatics which do the shifting work for drivers using clutches, torque converter & sets of planetary gears.

But the dual clutch transmission brings the best of manual & automatic transmission. A dual clutch transmission offers the function of two manual gearboxes in one, a dual clutch gearbox, uses two clutches but has no clutch pedal, instead electronics & hydraulics controls the clutches.

Has in result one clutch controls the odd gears (First, Third, Fifth & Reverse), which the other controls the even gears (Second, Fourth & Sixth), although the gear controls may vary from designs to manufacturers. Gears can be changed without interrupting the power flow from the engine to the transmission.

A dual clutch unit usually consists of a friction plate with 2 friction facing & a central splined hub, a dual clutch case, pressure plate assembly which consists of a pressed steel cover, a pressure plate with a machined flat face, a diaphragm spring, release bearing & release fork.

Wet Clutch: Like torque converters, wet clutches use hydraulic pressure to drive the gears. The fluid does its work inside the clutch piston. When the clutch is engaged, hydraulic pressure inside the piston forces a set of coil springs part, which pushes a series of stacked clutch plates and friction discs against a fixed pressure plate.

Single Plate Clutch Vs. Dual Clutch Transmission:

Single plate clutch offers many advantages & drawback over its counterpart dual clutch transmission:

Advantages:

Low manufacturing costs

Lightweight

Spare part easily accessible

Easy maintenance comparing to dual clutch transmission

Reliable due to few components

Disadvantages:

Unreliable in heavy duty vehicles

High clutch wear (requires more slip to pull away from a dead stop)

Drop of RPM’s during quicker up shifts

Dual clutch transmission offers many advantages & drawback over its counterpart single plate, as listed:

Advantages:

Very reliable in heavy duty vehicles.

Robust system compared to the rack & pinion

Quick & precise gear change without loss of RPMs

Improved MPG (miles per gallon) range

Disadvantages:

Heavier in weight compared to single plate transmission

Large amount of components

High manufacturing costs

Complex in fitting in a vehicle

Maintenance required frequently

When comparing the two both systems, each system has its advantages & drawbacks, but in the end each system has certain properties which are suitable for the vehicle is intended for, but when putting in real world application dual clutch does have some advantages over single plate transmission, due to its ability to improve MPG range, which could be a big factor for manufacturers to opt with dual clutch system over single plate over for high end to sport car, but due to its high manufacturing costs & components weight , dual clutch system may take time to take over single plate system has the most common system, so in conclusion the single plate may be good system for possible next five years, but dual clutch may become common after, due to its single highest factor of improved MPG range.