The viscous coupling
is a form of differential
sometimes found in the transmission
system of an all wheel drive vehicle
. The viscous coupling
links the drive shaft
s of the front and rear wheels of such a vehicle
and allows torque
to be transferred to one set of wheels if the other set starts spinning, (loses traction
The coupling comprises two sets of plates inside a sealed housing which is filled with a viscous liquid. One set of plates is attached to the end of the front half of the drive shaft, whilst the other set connects to the rear half. The basic principle of operation is that the viscous fluid within the housing resists strong differences between the rotational speeds of the two sets of plates.
During normal driving conditions, both sets of plates and the liquid 'connecting' them within the coupling are all rotating at the same speed. However; If one set of wheels loses traction, its set of plates will rotate faster than the other set. In this case, the liquid within the viscous coupling transfers torque from the faster spinning plates to the slower spinning plates, ie torque is transferred to the set of wheels with more traction.