A multi- disc clutch or friction-disc clutch is a machine element . Their characteristic feature in comparison with other clutches is the parallel arrangement of several friction linings. The same contact pressure acts on all friction pairs .
Multi-plate clutches can be switched under load. They are widely used because they are compact and inexpensive. These are mostly used continuously in oil and predominantly in automatic transmissions , in heavily loaded main or starting clutches or in limited slip differentials . In construction machinery axles , multi-plate clutches are also used as service and parking brakes, the latter often in the form of a spring-loaded brake . The advantages lie in the higher power and energy consumption in contrast to dry clutches , since here it is cooled with oil. Disadvantages are lower coefficients of friction (approx. Μ = 0.08-0.12) compared to dry clutches and a higher drag torque when idling .
A friction plate clutch consists of at least one inner and one outer plate. The inner discs are toothed with a shaft and the outer discs are held in an internally toothed tubular carrier. In order to increase the torque to be transmitted, several inner and outer disks are often arranged alternately so that higher torques can be transmitted with the same actuation force due to the larger covering surface.
In practice, the number of these friction disks is limited to 10 to 20 disks, since during dynamic coupling the friction disks are pushed together and each friction disk has to move relative to its guide. As a result, part of the actuating force is not passed on to the adjacent friction plate, but is supported on the plate carrier. The frictional torque that is transmitted by each individual lamella decreases degressively in the direction of force application from the first to the last friction lamella.
In oil-lubricated (wet-running) multi-plate clutches, if there is sufficient lubrication, steel plates (alloys with the addition of molybdenum ) can run against each other. In practice, however, either the slats are coated or slats made of other materials are used. This allows the coefficient of friction to be increased, the temperature resistance to be improved, as well as a smoother response behavior or a reduction in operating noise.
Common friction materials are:
- Paper coverings get their name from the fact that they are manufactured using the same process as paper. This type of friction lining running in oil is the most commonly used. If they are axially compressed, they allow surface pressures of up to about 2 N / mm² in dynamic operation. The value is significantly higher in static operation. Here the surface pressure can be nominally up to 7 N / mm², locally it can be up to twice as great.
- In the case of carbon fiber coverings ( carbon coverings ), woven carbon fiber is predominantly used. The mostly interwoven fibers increase the strength of the covering. In addition, carbon fibers are also used as a complete covering (similar to paper covering). In principle, carbon fiber coverings can also be produced using the paper process, but because of the high waste and the cost of the material, woven mats made of carbon fiber are currently punched out and glued to the carrier (usually a steel lamella). Woven carbon coverings allow surface pressures of up to approx. 6 N / mm², but are more expensive than paper coverings.
- The advantages of sintered bronze are its high thermal and mechanical strength. However, sintered bronze linings in the clutch pack show a lower power consumption than, for example, high-temperature-resistant paper friction linings. Disadvantages are lower dynamic coefficients of friction than with paper coverings.
- Pressed graphite offers similar advantages as sintered bronze, but is lighter.
Wear and damage patterns
Aging and overloading lead to various effects in multi-plate clutches:
- Normal wear increases the actuation travel. This can be compensated for by an automatic adjustment.
- Overheating on the surface of the covering causes it to glaze. The coefficients of friction decrease and with it the transmittable torque. Usually, this damage is no longer reversed by normal wear and tear.
- If the friction materials are connected with a steel plate, the adhesive can soften if overheated and the friction lining shifts or comes off. The function of the clutch is no longer guaranteed.
- With wet-running clutches, the coolant can be chemically changed due to high permanent temperatures. In the case of oil, for example, this can lead to increased oxidation or the cracking of oil molecules.
- Hot spot or heat spots: Here, local thermal concentration between the friction lining and the steel counter-disk creates an accumulation of heat and the associated expansion. The resulting “thermal mountains or hills” are called hot spots or heat spots. Occurrence due to uneven material accumulation and high power input in the clutch.
Development and research
One area of intensive development and adaptation is the avoidance of noise in friction plate clutches, which is caused by self-excited frictional vibrations . The tribological relationship between the friction pairing and - in the case of wet-running clutches - the lubricant must be considered as an overall system .
In addition, temperature and wear resistance is another area of research. Especially in the case of carbon fiber reinforced coverings, the aging resistance and the softening temperature of the matrix material must be as high as possible.
Advantages and disadvantages of multi-plate clutches
- Low mass moment of inertia
- Dissipation of the switching heat through internal oil cooling
- Remote controllability
- Stepless torque transmission possible
- Low maintenance
- Long lifetime
- Strong warming
- Idle and drag torques
- Low heat capacity
- Low coefficients of sliding friction
- High wear on dry running lamellas
- Clutch - controlled all-wheel drives and limited slip differentials in the automotive industry
- In the special case, torque limitation for cordless screwdrivers
- Shift elements in manual transmissions
- Starting clutch in vehicles with internal combustion engines
- Clutches in motorcycle engines with manual transmissions (advantage of saving space)
- Generally where the separation of shafts is necessary for a short time.