#Viscous coupling: what is it and how it works

12 / July 2022
Viscous coupling: what is it and how it works

Seletron Performance

What is viscous coupling, and what is it used for?


Viscous coupling is a mechanical component that is used in some passenger car transmissions. Normally there are two uses: one in which it is employed as an additional organ for self-locking differentials (on both 2WD and 4WD transmissions) and another when it acts as an auxiliary element in non-permanent all-wheel drives. It is also sometimes employed for connecting the radiator cooling fan to the engine, although electric fans are currently preferred for this function.


How Viscous Coupling Works


The viscous joint (or viscostatic joint) is used to connect two rotating shafts loosely. It features packets of lamellae between which a fluid is applied that varies its viscosity according to temperature. The lamellae are very close together (although not in a frictional condition) and are connected alternately to one of the two rotating shafts. In the presence of synchronous (or at least similar) rotational speeds, the lamellae rotate virtually at the same speed and thus are almost static in relation to each other. The fluid (usually silicone-based) maintains a low temperature (a few degrees more than the ambient temperature) that ensures a certain fluidity that allows the two sets of lamellae and, thus, the two rotating shafts to remain mechanically unattached.


In the presence of slip (thus asynchronous speed between the two shafts), the blades begin to "rub" through the fluid, which abruptly increases its temperature and, consequently, its viscosity, creating a momentary mechanical blockage between the two sets of blades and thus between the two shafts in engagement. The moment the speed differential is drastically reduced or nullified, the fluid returns to lower temperatures and thus to a more liquid state, allowing some degree of smoothness between the alternating blades connected to the two shafts.


Use of viscous coupling as a self-locking differential


In order to achieve greater traction on the same axle, controlled locking or self-locking differentials are installed on some vehicles. Those of the second type may employ a viscous coupling connected between the two axle shafts. In normal motion, when turning, the differential allows the difference in rotational speed between the wheel on the inside of the curve (lower speed) and the wheel on the outside of the curve (higher speed) to be compensated for. A possible viscous coupling connected between the two axles would leave the two shafts practically untethered under normal driving conditions.


In the event of wheel slippage (skidding), the speed difference between the two axles reaches a value high enough to immediately raise the temperature of the viscous coupling fluid, which would momentarily lock the two axles together, transferring a large part of the torque to the tire gripping the road surface, allowing greater traction. As soon as the vehicle begins to accelerate, the wheel with poor grip begins to rotate at a speed similar to or equal to the wheel gripping the road surface, the viscous coupling plates reduce their friction, and the fluid resumes the physical conditions of normal low-temperature operation. The two shafts are disengaged from one another and managed by the normal operation of the differential until either wheel loses grip again, restarting the cycle.


Use of viscous coupling in non-permanent all-wheel drives


Some cars use non-permanent all-wheel-drive systems, meaning torque is typically transferred to just one axle of the vehicle (e.g., in the Seat Alhambra 4drive, traction is normally front). Only when the wheels of the axle typically in traction skid does the viscous coupling (locking up as in the case of the self-locking differential) transfer drive torque to the other axle. In the Seat Alhambra 4drive example, it transfers torque to the rear axle, freeing the vehicle from the difficulty. As soon as the speed difference between the two axles is restored to normal values (i.e., when traction is regained), the viscous coupling disengages the two drive shafts, effectively returning 2WD traction. This is a simpler and less expensive solution than permanent all-wheel drive, which requires 3 differentials (one per axle + 1 center differential for cornering speed compensation that is different between the 2 axles) but is the best solution in terms of drivability.



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