How a clutch works

The vast majority of road vehicles have a transmission. The purpose of the transmission is to adapt the power performance of the internal combustion engine (or electric motor in case of an EV) to the road and traffic conditions.

There are several types of transmissions:

• MT (Manual Transmission)
• AMT (Automated Manual Transmissions)
• DCT (Double Clutch Transmissions)
• AT (Automatic Transmissions)
• CVT (Continuously Variable Transmissions)

Regardless of the type of transmission, the connection between the internal combustion engine and the gearbox is done through a coupling device. Depending on the transmission type the coupling device can be a clutch, two clutches or a torque converter.

1. front wheel
2. internal combustion engine
3. coupling device (clutch)
4. gearbox / transmission
5. longitudinal shaft (propeller shaft)
6. differential
7. planetary shaft
8. rear wheel

In the tables below there is a summary on the possible coupling devices for every type of transmission.

	Single disc dry clutch	Multi-disc wet clutch	Torque Converter
Manual Transmission	yes	no	no
Automated Manual Transmission	yes	yes	no
Double Clutch Transmission	yes (two clutches)	yes (two clutches)	no
Automatic Transmission	no	yes	yes
Continuously Variable Transmission	no	yes	yes

All manual transmissions are equipped with a single disc dry clutch. The clutch is positioned between the engine and the gearbox.

The main functions of the clutch on a vehicle with manual transmission are:

• allows power interruption between engine and gearbox (e.g. when vehicle is stationary, during gear shifts)
• performs progressive coupling of the engine to the gearbox (e.g. during vehicle launch or after gear shift)
• keeps the engine connected to the gearbox without any slip

The decoupling of the engine from the gearbox, when a gear is engaged, is necessary to prevent the engine speed to go below idle speed. If gearbox disconnection is not performed, the engine will stall.

Also, when performing a gear upshift (or downshift) on a manual transmission, there should be no torque transmitted to the wheels. This is achieved by disconnecting the engine from the gearbox through the clutch.

There are different types of clutches, we can classify them mainly function of:

• number of friction discs:
  • single disc
  • multi-disc
• type of friction:
  • dry
  • wet
• actuation type:
  • mechanical (cable or rod)
  • hydraulic

In order to understand how it woks, we are going to use the single disc dry clutch as an example. The multi-disc, wet clutch is going to be explained in details later on.

In the image below you can see a schematic of a single disc clutch. The engine crankshaft, flywheel, spring (coil or diaphragm) and pressure plate are all connected together, they are fixed to each other. On the other hand, the clutch disc is connected to the gearbox input shaft.

When the clutch pedal is released (as in the image below), the spring is pushing on the pressure plate which is pressing the clutch disc on the flywheel. This way the rotation of the crankshaft is transmitted to the gearbox input shaft. The springs are generating enough pressing force so that the clutch is not slipping.

When the clutch pedal is pressed, through a lever type mechanism, the spring action on the pressure plate is removed and the clutch disc breaks away from the flywheel. This way the crankshaft is disconnected from the gearbox input shaft.

For a better understanding of the clutch function, we are going to study the image below. In addition there is a release bearing, the spring is a diaphragm (not coil) and we have also the fixing elements of the diaphragm spring with the clutch cover.

1. crankshaft
2. flywheel
3. clutch (friction) disc
4. pressure plate
5. diaphragm spring
6. input shaft (gearbox)
7. clutch release bearing
8. clutch cover (case)
9. ring (diaphragm spring fulcrum)
10. locating pin
11. rivet

When the vehicle driver presses the clutch pedal, the clutch bearing (7) presses the interior part of the diaphragm spring (5). The pushing force of the diaphragm spring on the pressure plate (4) is removed an the clutch disc (3) is not pressed anymore on the flywheel.

If the clutch is opened: the crankshaft (1) + flywheel (2) + clutch cover (8) + diaphragm spring (5) + pressure plate (4) + release bearing (7, external ring) are rotating, while the clutch disc (3) +  release bearing (7, internal ring) + gearbox input shaft (6) are stationary (if a gear is engaged and the vehicle stopped).

When we slowly release the clutch pedal, the diaphragm spring start to push the pressure plate. By controlling the clutch pedal position we regulate how much force is applied on the friction disc by the pressure plate. The amount of spring force is directly related to the clutch torque capacity. When the spring push force is high enough, the clutch stops slipping and the engine is fully connected to the gearbox.

1. dual mass flywheel
2. clutch cover
3. mechanical releaser
4. pedal vibration damping device
5. master cylinder
6. plastic pedal
7. slave cylinder
8. clutch (friction) disc

Clutch bearing

1. thrust ring (outter/exterior ring)
2. inner ring
3. mount for release fork

The clutch release bearing has the role of connecting a fixed part (lever) to a mobile, rotating part (diaphragm spring). The inner ring is in contact with the push lever while the outer ring presses on the diaphragm spring. Through the clutch release bearing it is possible to actuate a rotating diaphragm spring with a fixed lever.

Diaphragm spring

The role of the spring is to keep the clutch closed (engine connected to the gearbox) when the clutch pedal is not pressed. Nowadays almost all of the MT clutches have diaphragm springs. Older versions of the clutches had several (6-8) coil springs around the pressure plate. The spring has to put enough pressure/force on the pressure plate so that the clutch will not slip even if the engine is outputting maximum torque.

Pressure plate

The pressure plate is connected to the clutch cover and it’s spinning together with the input shaft of the gearbox. The role of the pressure plate is to push on the clutch disc, against the flywheel, when the clutch pedal is released. The pressure plate is quite heavy, has a bit of volume. The reason is that during clutch slip, there is an amount of heat that needs to be dissipated. The heat is captured by both the pressure plate and the flywheel and then released into the atmosphere.

Friction disc

The friction disc is a critical component of the clutch. It has the role of connecting a rotating part (engine flywheel) to another part which could be stationary or rotating (pressure plate). Due to this, during the lifetime, the friction disc has to withstand high mechanical and thermal stress. Nevertheless the friction disc must fulfil the following requirements:

• have a friction coefficient between limits, for different torque, slip or temperature values
• be able to sustain high mechanical stress
• work under hight temperature situations

The level of friction disc wear depends mainly on the amount of heat released during coupling/decoupling of the engine. The amount of heat (energy) depends on the slip and torque transmitted. The clutch slip is the speed difference between the flywheel (engine) and pressure plate (gearbox input shaft).

For example if we need to launch the vehicle on a road with high slope (e.g. 10%) we need to rev up the engine to be able to generate also a higher torque necessary for launching. The combination between the high speed and torque will generate a lot of heat to be dissipated. These kind of events will accelerate the clutch friction disc wear.

On the other hand, if we release the clutch pedal too fast, in order to reduce the slipping phase, if the delta speed between engine and gearbox is high, then will induce oscillations in the driveline or even stall the engine.

The best scenario is to release the clutch pedal as smooth as possible, with the engine at a low speed (if permitted) in a short time. This can be achieved easily by an experienced driver but harder by a novice.

By the end of this article you should be able to:

• identify the components of a single disc dry clutch
• explain how a clutch works
• understand the impact of slip on the clutch wear

If any of the above is not clear enough use the contact form below to ask questions.

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Next article:
– How to calculate the torque capacity of a clutch
– Multi-disc wet clutch