Fan Clutch: Description and Operation

Fig. 1 Typical Variable-Speed Fan Installed:

Fig. 2 Variable-Speed Fan With Flat Bi-Metal Thermostatic Spring:

Fig. 3 Variable-Speed Fan With Coiled Bi-Metal Thermostatic Spring:

The fan drive clutch, Fig. 1, is a fluid coupling containing silicone oil. Fan speed is regulated by the torque-carrying capacity of the silicone oil. The more silicone oil in the coupling the greater the fan speed, and the less silicone oil the slower the fan speed.
Two types of fan drive clutches are in use. On one, Fig. 2, a bimetallic-metallic strip and control piston on the front of the fluid coupling regulates the amount of silicone oil entering the coupling. The bimetallic-metallic strip bows outward with an increase in surrounding temperature and allows a piston to move outward. The piston opens a valve regulating the flow of silicone oil into the coupling from a reserve chamber. The silicone oil is returned to the reserve chamber through a bleed hole when the valve is closed.
On the other type of fan drive clutch, Fig. 3, a heat-sensitive, bimetallic-metal spring connected to an opening plate brings about a similar result. Both units cause the fan speed to increase with a rise in temperature and to decrease as the temperature goes down.
In some cases a Flex-Fan is used instead of a Fan Drive Clutch. Flexible blades vary the volume of air being drawn through the radiator, automatically increasing the pitch at low engine speeds.