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 shafts 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.