Transmissions: Clutches & Manual Transmissions

Clutches	Clutch principles	The clutch used in most light vehicle applications is a single plate friction type. The clutch transmits torque from the engine to the transmission and a release mechanism allows the driver to control the flow of torque between them.
	Single-plate clutches	A single-plate clutch unit consists of a friction type disc, a pressure plate assembly, and a release bearing and operating fork.
	Multi-plate clutches	Increasing the number of plates in a friction clutch unit, increases its torque capacity.
	Dual mass flywheels	The function of the Dual Mass Flywheels or DMF is to isolate torsion crankshaft spikes created by diesel engines with high compression ratios.
	Operating mechanisms	The clutch operating mechanism may be mechanical or hydraulic. When the clutch pedal is depressed, the movement is relayed to a release fork which carries the release bearing.

Clutch components	Pressure plate	In light vehicle applications, the pressure plate is normally a diaphragm type and is serviced as an assembly.
	Driven center-plate	The driven plate is a friction type disc with two friction facings carried on waved spring steel segments and riveted to a steel disc. Drive is transmitted from the disc to a central splined hub, through a number of torsional coil springs.
	Throw-out bearing	The clutch release bearing can be a thrust-type angular contact ball bearing. The bearing requires no periodic maintenance during its service life.

Manual transmissions	Gear ratios	The gear ratio can be calculated by dividing the number of teeth on the driven gear, by the number of teeth on the driver gear.
	Compound gear trains	Compound gear trains have two or more pairs of gears in mesh, so that they rotate together.
	Gear selection	In manual transmissions, an interlock mechanism prevents the engagement of more than one gear at any one time and a detent mechanism holds the gear, in detention, in the selected position.
	Bearings	In a manual transmission, the gears and shafts are supported on bearings or bearing surfaces, which allow for rotation and also maintain the alignment of the components.
	Oil seals & gaskets	Oil seals and gaskets are fitted to manual transmissions to prevent oil leakage and the ingress of dirt and moisture.

Gearbox layout & operation	Gearbox layouts	In a manual transmission for a rear-wheel drive vehicle, the gear train is built up on three shafts: the input shaft, the countershaft, and the main shaft.
	Transaxle designs	In transaxle designs, the drive is transferred through the clutch unit to a primary shaft. The primary shaft carries gears of different sizes which mesh with gears on a secondary shaft.
	Gearbox operation	The speed ratio and the torque transferred depend on which gear is selected.
	Baulk-ring synchromesh unit	A baulk ring synchromesh unit prevents gear engagement until the speeds of the rotating components are synchronized.
	Transaxle synchromesh unit	In transaxle designs, the synchromesh unit is located between the end of each engagement sleeve and the gear. The engagement hubs and sleeves may be located on the primary or on the secondary shaft, depending on design.

Clutches & manual transmissions procedures	Checking gearbox oil	Air can enter the system, and heat generation can result in the burning of vital internal components.
	Changing manual gearbox oil	If the vehicle has been driven just prior to checking the fluid level, the fluid could be very hot.
	Checking & adjusting a clutch	Some clutches are self-compensating for wear, while others require checking and adjusting.