Fan Clutch: Description and Operation

Fig. 1 Typical Variable Speed Cooling Fan:

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

Fig. 3 Variable-Speed Fan W/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; the less silicone oil, the slower the fan speed.

Two types of fan drive clutches are in use. On one, Fig.2 , a bi-metallic strip and control piston on the front of the fluid coupling regulates the amount of silicone oil entering the coupling. The bi-metallic strip flexes 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, bi-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.