Fan Clutch: Description and Operation

Fan Drive Clutch Assembly:

Variable Speed Fan W/Bi-Metallic Coiled 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.
The type of fan drive clutch in use is a coiled bimetallic thermostatic spring, Fig. 2.
The fan drive clutch uses a heat-sensitive, coiled bimetallic spring connected to an opening plate which regulates 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. This unit causes the fan speed to increase with a rise in temperature and to decrease as temperature decreases.