Friction clutches multiply the torque
Stepless torque transmission
BY ANDREAS LERCH
Even the early automobile designers had to deal with these two problems of power transmission and the peculiarities of internal combustion engines. For this coupling work, there were solutions with flat belts, which were tensioned when starting and relieved when stopping. Cone couplings were also known, but had some serious disadvantages. After the turn of the century, the first developments with multi-plate clutches were made in England. These are considered to be the forerunners of the single-plate dry clutch known today.
The main task of the clutch is to transfer the engine torque to the gearbox. During the start-up process, it must be possible to change the torque continuously between 0 Nm and the current engine torque so that the vehicle can start up smoothly. To change gears, on the other hand, the power flow between the engine and transmission must be quickly interrupted and just as quickly switched on again.
Since the four-cylinder internal combustion engine works twice per revolution of the crankshaft, there are shocks on the crankshaft and vibrations on the drive train. These vibrations can excite the unswitched transmission gears to make noises, which are transmitted into the vehicle interior through mechanical circuit connections.
Clutches also serve to protect against overloads: If the clutch is supplied with more torque than it can handle in the event of an engine failure or when shifting, the clutch disc will slip. The safety figure is given as a ratio or in percent and calculated as the quotient between the maximum transferable and the maximum motor torque (multiplied by 100%). The security is often between 1.5 and 2.
Before driving the car, the engine must be started. To do this, the clutch is pressed and the following conditions prevail when a gear is engaged: The crankshaft rotates at engine idling speed, but all transmission shafts stand still. This means that the clutch shaft and the clutch disc also come to a standstill.
The slip torque (Fig. 2: t0 to t1) causes a certain torque to flow through the clutch to the gearbox, the clutch point is reached. The torque braces the transmission tooth edges and the drive train up to the wheels; however, it is not enough to get the car going. The rest of the torque is converted into heat in the clutch.
After remaining at the coupling point, the clutch pedal is released and the entire torque flows to the transmission and the wheels. It is now big enough and the vehicle starts moving.
In the picture, the greenish area below the curve represents the transmitted torque, while the yellow area above the curve indicates that part of the torque that is converted into heat in the clutch.
Correctly, it must be stated that during the routine start-up between t0 to t3, the entire torque MK is not yet applied. During this time, the driver will of course do something not only with his left but also with his right foot.
Friction and surface pressure
The clutch works according to Newton's law of friction. This means that the contact pressure multiplied by the coefficient of friction between the clutch lining and the flywheel or between the clutch lining and the pressure plate results in the friction force of the clutch. Accordingly, with the tuned engine, the pressure spring of the clutch could simply be reinforced and the clutch would be ready for the new, increased torque. The clutch linings are sensitive to the surface pressure and can be destroyed if the contact pressure is too high. It is therefore the case that the contact force is increased with a higher engine torque, but that in this case the clutch lining must also be increased so that the surface pressure does not exceed the limit value.
Force curve of the driving force
The torque flows from the crankshaft via the flywheel to the clutch unit (clutch cover), from there via leaf springs to the clutch pressure plate. The clutch pressure plate is axially displaceable and can release the driver or clutch disc or press it against the flywheel. When the clutch is engaged, the drive plate is pressed against the flywheel and 50% of the torque comes from the flywheel to the drive plate, the other 50% travel over the pressure plate to the clutch plate.
The leaf springs connect the clutch cover and the clutch pressure plate in a rotationally fixed but axially movable manner. They enable a ventilation game to be built up when the clutch is disengaged. The spring action helps pull the clutch pressure plate away from the drive plate. The drive plate then moves away from the flywheel due to the lack of normal force and due to the small sliding friction in the serration of the clutch shaft.
The clutch pressure plate has the task of transmitting 50% of the torque to the drive plate, but it must also have enough mass to absorb the frictional heat generated when starting up. On the opposite side, the flywheel takes on the same tasks.
The drive plate takes on various tasks and fulfills different requirements. On the one hand, the friction lining must be wear-resistant and heat-resistant, but must also have the highest possible coefficient of friction and withstand high surface pressure. Its tensile strength must be so high that it cannot be ruptured by excessive revs and for reasons of comfort, the pads must not squeak when the clutch is engaged. There is also organic material: wound fibers soaked in resin with added fine metal wires now offer a balanced compromise for most applications. The surface structure can be divided into a strength-optimized lower layer and a friction coefficient-optimized upper layer. Sintered coatings or even ceramic coatings are rare and are mainly used in racing vehicles.
For a smoother coupling, resilient sheet metal plates are inserted between the two clutch linings of the drive plate. These lamellas are set and help that the coverings put on earlier, that they wear out more regularly and that they allow the frictional connection to be finely regulated.
A vibration damper always has two masses, a spring and a damping system. This is the case with vehicle suspension / damping, and this basic principle also applies to vibration damping in the drive train. The coil springs of the clutch disks, which act in the circumferential direction, absorb the impact forces of the crankshaft. This will compress them. If no cylinder is working in the next moment, the coil springs relax again and can be compressed again by the next work cycle. The drive plate has friction dampers so that this vibration energy is dissipated. To do this, however, the drive plate has to be divided: It needs a disc part which is connected to the linings and a second part which is connected to the hub.
The two parts are connected by the helical springs that transmit the torque; The friction linings are also clamped between these two disks and are pretensioned with disc springs. In this way, the energy from the vibrations is converted into heat and not transferred to the gearbox.
Figure 6 shows the function of the vibration damper. The weaker springs of the clutch disc serve to dampen idling vibrations. If larger torques are applied, these springs are immediately on the stop and the strong springs with a steeper characteristic curve take over the task (of course always in conjunction with the friction devices). The damping play of a partial load situation is shown under the magnifying glass. If the spring absorbs a burst of combustion, it is compressed, during the intermediate cycle it relaxes, and the circle closes again before the next work cycle.
Force curve of the actuation
At the end of the actuation chain, the diaphragm spring is located in the clutch and presses against the pressure plate with a few kilonewtons. In order to overcome this force and to release the clutch comfortably, some gear ratios have to be built into the power transmission of the clutch actuation.
In the case of the clutch pedal, a first gear ratio is shown; in the case of the transmission from the clutch pedal to the clutch release fork, a gear ratio can also be provided between the master cylinder and the slave cylinder in the hydraulic transmission. No translation is possible with the mechanical transmission by means of a cable pull. To do this, the force is again directed to the clutch pressure bearing (7) via a lever (release fork 8 in Fig. 4). The release fork often forms a two-armed lever, in Figure 4 it represents a one-armed lever.
These ratios are summarized as the “outer” ratios, while the diaphragm spring in the clutch forms the “inner” ratio. Here, too, a distinction can be made between one-armed and two-armed levers. While the lamella tongues are two-armed levers when the clutches are pressed (Fig. 7), in which the clutch pressure bearing moves towards the flywheel when the clutch is operated, they are single-armed levers when the clutches are pulled.
Hydraulic clutch boosters, which receive the hydraulic pressure from the power steering pump, for example, can additionally facilitate the clutch process as external support.
Centrifugal clutches, magnetic particle clutches and hydraulic clutches are hardly used in the large-scale production of automobiles. The two-disc friction clutch, like the one that Porsche installed in its Carrera GT a few years ago (Fig. 1), has also become very rare. Obviously, the designers of clutch linings in particular have succeeded in designing the linings for higher transmission capacities. Because the engines became more and more powerful and the two-plate clutches less and less common. Porsche justified the use of the two-disc ceramic clutch (PCCC - Porsche Ceramic Composite Clutch) with the low mass of the ceramic coating, the small diameter of the clutch of 169 mm and the resulting low center of gravity.
The double clutches, however, are spreading more and more. Thanks to the investments with which the double clutch transmissions (DKG) are promoted, these clutches are also becoming increasingly popular.
Mainly for thermal reasons, the first DKGs were designed with wet multi-plate clutches. The heat can be carried away by the oil and the clutch linings can on average be kept at approximately the same temperature as the transmission oil. For structural reasons, the two couplings are not placed one behind the other, but on top of one another. This means that one coupling has a larger effective diameter than the other and could consequently transmit more torque. For this reason, the inner clutch pack can have one more friction pair than the outer one.
In double clutch transmissions, one clutch is connected to a solid shaft (Fig. 8: blue) and drives the even gears, for example, and the other clutch (orange-red) drives the uneven gears via a hollow shaft.
Since the DKG are often controlled electrohydraulically, the actuators of the double clutch are also hydraulically operated in these cases. In Fig. 8 it can be clearly seen that the plate pack that is actuated has to be pressed together with pressure without a spring applying the contact force.
Dry double clutch
In 2007, Volkswagen, in cooperation with the clutch manufacturer LuK, succeeded in producing the corresponding double clutch in a dry design. This has a small advantage in terms of efficiency, since the oil pump does not have to supply the clutch with oil. For thermal reasons, the clutches are not yet approved for all engines, the limit today is around 250 Nm.
The torsion damper can be clearly seen in Figure 9. The torque is transmitted from the flywheel via a rigid connection to the traction sheave and from there - depending on the case - to the left or right drive plate. The clutches can also be actuated hydraulically here. As with the double clutch with lamellas, when the clutch is actuated, the actuating force presses the pressure plate onto the drive plate. The spring tongue therefore forms a single-armed lever for the left clutch and a two-armed lever for the right clutch.
For the future, developments in the area of vibration dampers and also in the area of the higher performance of dry double clutches are to be expected.
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