Viscous coupling unit

The viscous coupling is used in the drive train of motor vehicles. In principle, it transmits a rotational movement in its interior a circular disc ( plate ) on the input side to a fluid, which in turn drives a further plate at the output side. By this design, the viscous coupling transmits a torque and a rotational speed allows compensation. The greater the speed difference between input and output plate, the greater will be the torque that can transfer the viscous coupling.

Automotive applications

To 1: As soon as the permanently driven axis gets too much power, the slip at the wheels increased. The shaft rotates more quickly than it responds to the vehicle speed and it also rotates faster than the other axis. This results in a differential speed between the axles, which also leads to a speed difference in the viscous coupling. The higher the rotational speed difference is, the more the second shaft is driven via the viscous coupling. Is the first direct drive shaft and rotates with ice, the whole power is transmitted to the second axis of the viscous coupling.

To 2: The viscous coupling slows down the differences in rotational speed, which occur in a differential. When the difference in rotational speed on an axle is too large ( such as a wheel on ice), then slows the viscous coupling this compensating movement, and distributes the drive torque to the slower wheel with central differentials the rotational speed difference is reduced between the axles, for example if an axle on ice cream stands or hangs in the air.

To 3: The torque of the drive shaft is transmitted to the fan side a defined quantity of oil in the area between the rotor and housing. A valve controller regulates the amount of oil that is used for transmission of the torque. The remaining amount is then in a reservoir. Within the coupling, the oil moves in a circuit which is driven by a pumping system. The pumping action depends on the rotational speed difference between the drive side and the fan section. A large difference in speed produces a high pumping efficiency. New fan clutches are now electrically actuated through the vehicle electronics. Thereby, the cooling effect of the engine cooling system is optimized.

Technology

In a viscous coupling depending on the design can provide up to 150 bar internal pressure. In addition to the speed balancing the viscous coupling also acts as a vibration in the drive train.

Construction

The graph shows a viscous coupling, the power and torque passes an axis on the input side ( 1) to the differential (D). The viscous coupling itself is made of a housing (2 ), the inside of which serves as a carrier for the outer disks (3). Between the outer plate (3) and inner disc (4) there is a fluid (5). The inner disc (4) itself are supported by an inner plate carrier (6), the drive shaft is at the same time. The housing is protected by a seal (7) against leakage of the fluid ( 5) and the entry of dust.

A fluid (5) silicone oil is often used, since it is characterized by a high viscosity which, in comparison to oil is less temperature dependency for viscous couplings.

The characteristic of the torque and power transfer depends on the number of blades ( 3 & 4, usually made of steel ), the inside and outside diameter and the viscosity of the fluid (5 ) from. The fluid ( 5) is sheared at different rotational speeds of the input and output discs and thereby transmits the torque.

Both outer discs ( 3) and the inner disk ( 4) can move axially and be set for example by spring washers so that they do not touch without speed difference. A mechanical contact of the blades should only in exceptional and only arise if the Hump effect was constructively provided.

Hump ​​effect

In general, the fins are slit and the viscous coupling comprises a certain amount of residual air. During the movement of this air is distributed to the slots of the lamellas. By heating it may now happen that expands the silicone oil and compressed air first and then picks up. When the air was completely dissolved in the silicone oil and a part of the blades ( outer or inner plate ) has a wing profile, they will be pressed with the differential rotational speed to the other lamellae. This creates a mechanical frictional contact which can greatly increase the transmittable torque, while reducing the friction loss in the fluid.

The coupling has thus under heavy load at the same time higher torque capacity and a self-protection against overheating.

The hump will only take effect when the clutch reaches a certain temperature, it can not be planned for normal driving dynamic applications. As the hump - effect is a hydrodynamic effect, it is effective only as long as the differential rotational speed is applied to the clutch. It is therefore not possible by the hump a rigid through-drive.

Compatibility with electronic control systems

A viscous coupling transmits torque depending on the speed difference - even if the axles / wheels must be largely decoupled, eg when braking with anti-lock brakes (ABS ), or intervention of the electronic stability program (ESP). These control systems are ( according to the current interpretation ) rely on as independently controllable wheels. Therefore, the necessary adjustments to the viscous coupling to be moved back, in which a free-wheeling or a disconnect clutch may be used.

Due to the still missing adaptation of the ESP systems viscous couplings have been largely replaced by other solutions, such as the Haldex coupling and purely electronically controlled multi-plate clutches.