Viscous unit engaged (hot running temperature) (Cooling system)

When this clearance is filled with viscous fluid, a shearing action, caused by the speed differential between the two drive components, transmits torque to the fan. The fluid is thrown to the outside of the unit by centrifugal force from where it is recirculated to the reservoir (10) via the pump plate (4) adjacent to the drive member.

If the engine speed is increased the amount of slip will also increase to limit the maximum fan speed.

When the air temperature from the radiator drops sufficiently, the bi-metal coil closes the valve and prevents fluid entering the drive area. The fluid that is in the drive area will gradually pump out into the reservoir (10) and the fan will return to an idle condition.

Viscous unit engaged (hot running temperature)

Bi-metal coil (6) expanded, valve (5) open.