The single-plate dry clutch is mainly installed in motor vehicles to separate and close the power flow between an engine and transmission when starting or changing gears and to adjust the speed between engine output and transmission drive. It consists of a clutch disc that is pressed against the engine flywheel by one or more springs with a pressure plate. When the clutch is actuated, the spring pressure is overcome by the release lever, the disc can rotate freely - the clutch disengages. Since the late 1960s, clutches have generally been designed with a diaphragm spring.
The single-plate dry clutch is a friction clutch that can be switched during operation, ie it uses the frictional resistance between two plates to transfer forces from one plate to another. The transmittable torque is dependent on
- the effective radius of the friction surface,
- the coefficient of static friction of the material pairing of the friction surfaces and
- the force with which the discs (flywheel, clutch disc with lining / so-called drive plate and pressure plate) are pressed against each other.
In order to minimize the diameter of the clutch, the lining must have a high coefficient of friction, ie the torque of the transition from static friction to sliding friction must be significantly higher than the torque of the motor. The clutch must separate the frictional connection between the engine and the transmission with every shift, since shifting under load (while the transmission is transmitting the engine's torque) can considerably shorten the service life of the synchronizers in gear transmissions. When starting, the clutch must enable the possibility of a smooth starting up to static friction of the clutch (pedal released) due to decreasing sliding friction (the drive plate between the flywheel and the pressure plate) between the engine and the gearbox. When starting up, the clutch does a lot of friction work (comparable to starting up aFlywheel ); When shifting between two gears, the friction work theoretically approaches zero if the engine and transmission input shaft are at the same speed before the clutch is engaged. Therefore, the pedal can be released much faster here than when moving off.
The way in which the coupling works results in severe mechanical wear. With frequent operation in the area of sliding friction, the thermal load increases enormously. Therefore, the clutch disc must have high wear resistance and high temperature resistance. In practice, materials are used that are similar to those used in brake pads . The transmission of torque in the fully engaged state takes place with an efficiency of almost 100 percent. In addition, the possibility of overload protection is required (e.g. blockages in the drive train, sudden engagement). Modern single-disc dry clutches meet these requirements.
Layout and function
The unit consisting of the pressure plate (white; colors see illustration) and disc spring (red) is non-rotatably connected to the flywheel (turquoise). The clutch disc (green) with the friction linings (orange) is slidably attached to the gear shaft by means of a toothing (dark green).
The release lever (gray) acts on the thrust bearing (blue) and presses against the disc spring in the direction of the flywheel . As a result, the pressure plate is moved in the direction of the gearbox by a lever system. The clutch disc is relieved and does not transmit any rotary motion to the gear shaft. If there is no force acting on the release lever, the thrust bearing does not exert any pressure on the disc spring. The diaphragm spring presses the pressure plate onto the clutch plate and pushes it against the flywheel. The frictional connection is established, the gear shaft rotates at full engine speed.
If the release lever is only pressed halfway (e.g. when starting up) the disc spring can only transfer part of its force to the pressure plate. As a result, the clutch disc is not clamped properly and slips continuously. One speaks of grinding. The transmitted force and speed depend on the engine speed, the position of the release lever and the coefficient of friction of the friction linings.
Instead of the disc springs listed here, edge spring clutches were also used in the past. Several levers with coil springs are distributed radially on the clutch disc. The way it works is like a disc spring. Disadvantages of an edge spring clutch compared to a diaphragm spring clutch are:
- more moving parts
- therefore more expensive and heavier
- higher unbalance or concentricity is more difficult to handle
- sensitive at higher speeds
- In contrast to the disc spring, the operating force does not decrease if the release lever moves further
- the individual parts can make noises (rattle, rattle)
The clutch disks are often equipped with torsional vibration dampers or torsional vibration springs. If no two-mass flywheel is installed, these helical springs, which are tangentially attached in the intermediate layers and allow limited radial rotation, dampen gear noises (rattling) and vibrations. The resilient, sometimes slightly corrugated intermediate layers can be seen between the coverings and the carrier plate. The front coverings are riveted to these. They allow the friction linings to rest evenly when the clutch is engaged and thus enable a smooth start.
The thrust bearing - a radial bearing - consists of three parts. The first is fixed; it is attached to the release lever with a clamp. The second can be turned independently of the first and presses against the rotating disc spring. The third part consists of a guide bushing (or a thrust bearing), which ensures that it fits perfectly on the gear shaft.
Types of operation
A clutch cable ( Bowden cable ) transmits (e.g. on a motorcycle or car) the force acting on the clutch lever to the release lever , which actuates the release bearing (thrust bearing) and thus the clutch. It is exposed to high mechanical tensile stress and can tear if it is too worn.
With a slightly worn single-plate dry clutch, the clutch play is reduced compared to a clutch that is as good as new . However, in order to ensure proper function, the coupling play must be increased again by adjusting the length of the coupling cable. This is not necessary with automatically adjusting clutch cables or with hydraulically operated clutches.
The force for disengaging (disengaging the clutch) is transmitted to the disengagement lever via a linkage with several deflection points. This variant is only rarely used (e.g. Rasentrac) and requires a coupling aid for powerful engines because of the higher pressure forces. This can be a dead center spring or a semi-centrifugal clutch. In the past, the mechanics were also supported by hydraulics or pneumatics .
With hydraulic actuation, there are no mechanical components between the clutch lever (pedal) and the release lever; the forces are transmitted purely hydraulically. The basic structure is very similar to a hydraulic brake system. The clutch lever actuates a hydraulic cylinder, whereby the working cylinder actuates the release lever via a hydraulic line. When the operator is released, the system is depressurized. The working cylinder is pushed back by the clutch spring. With this type of actuation, the thrust bearing remains in constant contact with the rotating clutch spring. It must therefore be dimensioned differently (permanently speed-resistant). Due to this characteristic, this clutch is self-adjusting. The brake fluid is usually used as the hydraulic fluid .
One speaks of an automatically operated clutch when a normal clutch is automatically externally switched. External switching means that the actuating force is operated from outside. To control the clutch, certain operating conditions must be measured. Detect sensors and switches :
- Engine speed
- Transmission speed
- the selected gear
- Accelerator pedal position or position of the throttle valve
- The desire to switch. z. B. by touching the shift lever.
The data is either converted via a system of relays, switches and hydraulic circuits or processed by electronics, with a map being processed. As a result, a hydraulic cylinder actuates the release lever at exactly the right moment at the right speed. A magnetic coupling or an electric motor can also be controlled electrically directly.
In contrast, there is the self-shifting automatic clutch. Due to its design, this switches itself without any external influence. This category includes centrifugal clutches , freewheel clutches , slip clutches and hydrodynamic clutches .
- Example vehicles with automatic clutch
The required actuation force increases with the size of the clutch and the amount of torque transmitted. Therefore, operating aids are used for easier operation. In addition to normal actuation (e.g. mechanically), a hydraulic, pneumatic or electrical support mechanism acts. However, even if this technology fails, the clutch can be operated with increased effort.
- Dead center spring
- Centrifugal assistance
- pneumatic support
- hydraulic support
- Dead center spring
The simplest form of support is the dead center spring (also known as the over center spring). A pretensioned spring is attached to the clutch pedal in a slightly offset position to the pedal shaft. If the dead center is overcome when the pedal is pressed, the spring relaxes in the actuating direction. Thus the force required to operate is greatest at the beginning of the pedal travel and then becomes smaller.
- Semi-centrifugal clutch
The semi-centrifugal clutch is an edge spring clutch in which the operating levers are equipped with weights. At increased speed, the weights are pushed outwards and the levers are relieved. The actuation force for the clutch becomes smaller.
- Pneumatic support
With pneumatic assistance, when the pedal is depressed, compressed air is fed into a working cylinder via a valve that is operated at the same time. This presses on the release lever. Instead of compressed air, negative pressure (as in a brake system) can also be used.
- Hydraulic support
The structure of hydraulic support is similar to hydraulic actuation. However, it is installed in addition to mechanical actuation.
- Hans Jörg Leyhausen: The master craftsman's examination in the automotive trade part 1 . 12 edition, Vogel Buchverlag, Würzburg, 1991, ISBN 3-8023-0857-3
- Hans-Hermann Braess, Ulrich Seiffert: Vieweg handbook automotive technology. 2nd edition, Friedrich Vieweg & Sohn Verlagsgesellschaft mbH, Braunschweig / Wiesbaden, 2001, ISBN 3-528-13114-4