When you buy a vehicle, most parts are under warranty for at least a year, with some parts having a warranty for up to 10 years for a couple of manufacturers. Items that are subject to normal wear or abuse, however, don't have warranty, and the clutch is one of those items. If a part of the clutch breaks, there is warranty, but if it is damaged from slippage or worn out, you pay.
The clutch is a multi-part assembly connecting the engine to the transmission. Today there are several vehicles that use computer-controlled clutches with computer-controlled manual gearboxes that drive just like an automatic transmission, but the majority of clutches still connect the engine to a manual transmission. The clutch assembly is made up of a flywheel, clutch disc and pressure plate. The flywheel and pressure place are bolted together onto the engine crankshaft, while the clutch disc is clamped between them and slips over a splined shaft going into the transmission.
When you step on the clutch pedal, you are applying force against springs in the pressure plate assembly, moving it away from the flywheel. This allows the clutch disc to rotate independently, disengaging the transmission from the engine. When you release the clutch pedal, the pressure plate now clamps the clutch disc between it and the flywheel, so power is transmitted into the transmission.
A release bearing pushes against levers or fingers on the pressure plate to release the clutch. The mechanism that moves the release bearing doesn't rotate, but the pressure plate does, so the bearing allows contact with the rotating part without damaging parts. The release bearing only has a load placed on it when your foot is pushing down on the clutch pedal.
A pilot bearing (sometimes a bushing is used) is installed at the back of the engine to support the front end of the transmission input shaft and the clutch disc when it is not engaged.
Finally, there is the mechanism that transfers the motion of the clutch pedal to release the clutch. Mechanical linkages used to be common, but today most vehicles use hydraulic cylinders to do the work. When you step on the pedal, you force fluid out of the clutch master cylinder through some pipes into the clutch slave cylinder. This slave cylinder moves the release bearing to engage or disengage the clutch.
Two driving methods can damage clutches. The shock of a "drag-strip" launch, where the engine is revved up and the clutch is quickly released can overload the clutch and break damping springs in the clutch disk. More frequently, damage is done by slipping the clutch excessively. Every time you release the clutch pedal slowly, the clutch disc slips against the pressure plate and flywheel surfaces. The slipping creates heat, which wears the disc and can create hot spots or hardened areas on the metal pressure plate and flywheel and can cause the clutch to not engage smoothly. Starting out smoothly and at low engine speeds will allow the clutch to last for a long time. Keep your foot on the pedal so the clutch is partially engaged with the engine revving high and you could burn one out in a few hours.
A good clutch will engage smoothly, although ceramic racing clutch discs do have a harsh engagement. If the clutch is grabby or jerky despite your best efforts at smooth throttle and clutch control, then the clutch may be at fault. Oil leaking onto the clutch disc from the engine or transmission will have the same effect. Replacing the clutch is the only thing that will help.
A worn clutch will show up first when the vehicle is accelerated in high gear on the highway. If the engine speed suddenly jumps as you accelerate, the clutch is slipping.
Jim Kerr is an experienced mechanic, instructor and member of the Automobile Journalists' Association of Canada.
kerr.jim@sasktel.net
