How a Manual Transmission Clutch Works
The clutch changes and regulates the power and torque being delivered from the engine to the wheels. These changes are referred to as shifting or changing gears. Most cars and trucks use a friction unit to engage and disengage the flow of power. When you engage the clutch, you allow power from the engine to go to the transmission to drive the wheels. When you disengage the clutch, you cut the power to the transmission and the vehicle coasts – but the engine continues turning.
The key parts of the clutch’s function are:
- Clutch Plate
- Pressure Plate
- Release Bearing
- Control Linkage
The flywheel is attached to the crankshaft and is the smooth surface that the clutch contacts to create the friction needed to accomplish its main job – transferring torque from the engine to the transmission. The flywheel also acts as a balancer for the engine and dampens vibrations. It also has teeth along its edge, around the circumference, that allows the starter motor to engage with the engine during starting.
Clutch Plate (AKA Clutch Disc)
The clutch plate is a disc-shaped steel plate attached to a splined hub. It is covered with material that is conducive to creating friction between the flywheel and the pressure plate. In the center of the disc is the hub which fits over the input shaft of the transmission. When the clutch is engaged, the clutch plate is gripped between the pressure plate and the flywheel, which then allows power from the engine to transfer to the transmission via the connection between disc hub and the input shaft.
The pressure plate is basically a spring-loaded clamp that is bolted to the flywheel. It is made of many individual parts, including a diaphragm spring, a pressure ring that provides a frictional surface for the clutch plate, a push ring for the release bearing, and release levers. The levers decrease the force of the holding springs when the clutch is disengaged.
The release bearing is the main part of the clutch operation. When the clutch pedal is pushed a master cylinder pushes the clutch fork to the flywheel. This presses the release bearing against the pressure plate, which pushes on the release levers, which in turn reduces pressure on the pressure plate’s diaphragm spring, causing it to move away from the clutch disc. This releases the clutch from the engine. When the clutch pedal is released the release bearing moves away from the pressure plate and engages the clutch. This allows the pressure plate’s springs to press against the clutch disc. This engages the clutch to the flywheel and resumes power transfer from the engine to the transmission.
A mechanical or hydraulic linkage operates the clutch in a manual transmission. A mechanical linkage consists of a cable or shaft and lever. A master cylinder is connected to the clutch pedal by a rod, which in turn is connected to a slave cylinder which moves the clutch release lever, also known as the release arm or fork. The linkage is basically what connects the driver to the overall clutch system.
Signs the Clutch is Going Out
There are a few tell-tale signs that your clutch is about to leave your engine spinning, but not your wheels.
When there is not enough friction to hold the flywheel and pressure plate, the clutch will not engage as readily and will disengage faster. There are a few symptoms of slipping that you will notice. There will be more “play” in the clutch pedal – you might not have to press the pedal in as far to get the clutch to disengage, but having to press it hard all the way to the floor is a bad sign, too. If the pedal moves easily, but the transmission will not go into gear, then the clutch linkage is probably loose or disconnected.
The engine will rev noticeably when the clutch is engaged, and the car will accelerate more slowly. This is another indication that the clutch is slipping.
If it is difficult to shift gears, or the clutch doesn’t disengage when the pedal is pushed, or the gears tend to resist or grind when changing (up, down, or reverse), then the clutch should be inspected. This could also be due to linkage problems – or just that the driver doesn’t know how to drive stick. In either case, it’s probably a good idea to have the clutch inspected.
Chatter or Burn
Clutch chatter is usually caused by overheating – or “burning out” the clutch. This can occur when starting on an incline and “teasing” or slipping the clutch so as not to stall, or when oil gets on the clutch plate. You will probably be able to smell the burning. When the clutch burns out, it needs to be replaced.
How to Test if Your Car’s Clutch is Slipping
Although many new cars have self-adjusting linkages, on older models the linkage might need to be checked and adjusted.
A simple road test can help you decide whether your car’s clutch needs to be adjusted or repaired.
- Start your car.
- Engage the clutch (push the clutch pedal down).
- Put the car into third gear.
- Release the parking/emergency brake and slowly release the clutch.
The engine should quickly stall. If it continues to run and struggle as you release the clutch, or if it tries to move forward, then stalls, the clutch is slipping.
Come to AAMCO Minnesota for Clutch Inspection and Repair
At AAMCO Minnesota our mechanics can diagnose your clutch problems and make the necessary adjustments or repairs. A failing or badly adjusted clutch can affect the performance and lifespan your car’s engine, and is a safety hazard – so be proactive and schedule an appointment at your local Minnesota AAMCO. There could be a lot more going on, so the AAMCO multi-point inspection is always a good idea.
See this excellent illustration of a manual transmission clutch.