Viscous coupling

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The viscous coupling is used in the drive train of motor vehicles. In principle, it transmits a rotary movement inside via a circular disc (lamella) on the inlet side to a viscous (highly viscous) liquid, which in turn releases it to the lamellae on the outlet side. The viscous coupling transmits a torque without the shafts being rigidly coupled to one another and thus enables speed compensation. The greater the speed difference between the input and output lamellas, the greater the torque transmitted.

Applications in vehicle construction

  1. With a clutch - controlled all-wheel drive , one axle is permanently driven by the engine. The second axle is driven if necessary, but without a central differential. To enable speed compensation between the axles, they are connected via a viscous coupling. Examples:
    1. Subaru Justy JMA / MS (viscous coupling connects the rear axle),
    2. VW T3 Syncro (viscous coupling connects the front axle),
    3. VW Golf 2 Country (viscous coupling connects the rear axle),
    4. Porsche 996 (911) Carrera 4 (viscous coupling connects the front axle),
    5. Volvo XC70 (viscous coupling connects the rear axle),
    6. Opel Calibra 4x4 (viscous coupling connects the rear axle),
    7. Audi R8 (viscous coupling connects the front axle),
    8. Volvo 850 AWD / V70I AWD
  2. The viscous coupling is combined with a differential ( differential gear ) (as axle or central differential ) or planetary gear (as central differential ) . There it brakes the compensatory movement between the output shafts or (more often) between the differential carrier / planet carrier on the one hand and an output shaft on the other.
  3. The fan in front of the engine is occasionally driven by a viscous coupling. There, as the temperature rises, a bimetal spring presses liquid from a storage space into the actual coupling on the fan wheel, so that it rotates faster.

Re 1: As soon as the permanently driven axle gets too much power, the slip on its wheels increases. As a result, the axle rotates faster than the vehicle speed corresponds to and it also rotates faster than the other axle. This creates a differential speed between the axles, which also leads to a differential speed in the viscous coupling. The higher this differential speed, the more the second axle is driven via the viscous coupling. If the first, directly driven axle is on ice and spins, part of the power is transferred to the second axle and the rest is converted into heat in the viscous coupling.

Re 2: The viscous coupling brakes the speed differences that occur on a differential. If the speed difference on an axle becomes too great (for example a wheel on ice), the viscous coupling brakes this compensating movement and distributes the drive torque to the slower wheel. With central differentials, the speed difference between the axles is reduced if, for example, an axle is on ice stands or hangs in the air.

Re 3: The torque of the drive shaft is transmitted to the fan side via a defined amount of silicone oil in the area between the rotor and the housing. A valve control (usually coupled with a bimetallic spring) regulates the amount of oil that is used to transmit the torque. The remaining amount is in a reservoir. Within the coupling, the oil moves in a circuit that is driven by a pump system. The pumping effect depends on the speed difference between the drive side and the fan side. A large difference in speed creates a high pumping effect. Nowadays, new fan clutches are controlled electrically via the vehicle electronics. This primarily reduces fuel consumption, since the cooling is demand-controlled and no longer depends on the engine speed in difficult situations (traffic jams in summer).

technology

Depending on the design, an internal pressure of up to 150 bar can arise in a viscous coupling. In addition to speed compensation, the viscous coupling also acts as a vibration damper in the drive train.

construction

Viscous coupling, schematic diagram

The graphic shows a viscous coupling which transfers power and torque via the drive side (1) to the differential (D) of an axle. The viscous coupling itself consists of a housing (2), the inside of which also serves as a carrier for the outer plates (3). A fluid (5) is located between the outer discs (3) and the inner discs (4). The inner plates (4) themselves are held by an inner plate carrier (6), which is also the drive shaft. A seal (7) protects the housing against leakage of the fluid (5) and the ingress of dust.

Silicone oil is often used as the fluid (5) for viscous clutches, as its high viscosity is less dependent on temperature than that of mineral oils.

The characteristics of the torque and power transmission depend on the number of lamellas (3 + 4, mostly made of steel), their inside and outside diameter and the viscosity of the fluid (5). The fluid (5) is sheared by the input and output disks at different speeds and thereby transmits the torque.

Both outer disks (3) and inner disks (4) can move axially and are set, for example by means of spring washers, so that they do not touch without a speed difference. Mechanical contact between the lamellas should therefore only occur as an exception and only if the hump effect was provided for in the design.

Hump ​​effect

The lamellas are usually slotted and the viscous coupling contains a certain amount of residual air. When moving, this air is distributed in the slits of the slats. As a result of heating, the silicone oil can expand and first compress and then absorb the air. When the air has been completely dissolved in the silicone oil and some of the slats (outer or inner slats) have a wing profile, they are now pressed against the other slats at a differential speed. This creates a mechanical frictional contact, which allows the transmittable torque to increase sharply and at the same time reduces the friction in the fluid.

The clutch thus offers higher torque capacity and self-protection against overheating at the same time under heavy loads.

The hump effect only occurs when the clutch has reached a certain temperature; it cannot be planned for normal driving dynamics applications. Since the hump effect is a hydrodynamic effect, it only works as long as the differential speed is applied to the clutch. The hump does not allow a rigid through drive.

Compatibility with electronic control systems

A viscous coupling transmits torque depending on the differential speed - even if the axles / wheels have to be largely decoupled, for example when braking with an anti-lock braking system (ABS) or interventions by the electronic stability program (ESP). These control systems are (according to the current design) dependent on wheels that can be controlled as independently of one another as possible. Therefore, the necessary adjustments to the viscous coupling are relocated, whereby a freewheel or a separating clutch can be used.

Due to the lack of adaptation of the ESP systems up to now, viscous clutches have largely been displaced by other solutions, such as the Haldex clutch and purely electronically controlled multi-plate clutches .

literature

  • Andreas Kern: The end of the viscous coupling as an all-wheel drive concept: Investigation into the declining use of viscous transmission in new vehicles. Vienna, 2014, ISBN 3-958-20253-5
  • Wilfried Staudt: Vehicle technology manual. Volume 2: Energy supply and starting systems, engine mechanics, engine management systems, exhaust systems. Bildungsverlag EINS, Troisdorf 2005, ISBN 3-427-04522-6 .
  • Max Bohner, Richard Fischer, Rolf Gscheidle: Expertise in automotive technology. 27th edition, Verlag Europa-Lehrmittel, Haan-Gruiten, 2001, ISBN 3-8085-2067-1
  • Peter A. Wellers, Hermann Strobel, Erich Auch-Schwelk: Vehicle technology expertise. 5th edition, Holland + Josenhans Verlag, Stuttgart, 1997, ISBN 3-7782-3520-6

See also

Individual evidence

  1. ↑ "Visco" is a word mark from Behr GmbH, Stuttgart. DPMA registration number 1130963 ; Nice class 12, 7: Fluid friction clutches for machines and land vehicles ; However, Behr does not offer any products that correspond to the viscous couplings in the drive train described here.

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