Fan Clutch: Description and Operation

Variable Speed Fan With Coiled Bi-Metallic Thermostatic Spring:

The fan drive clutch 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 used, is the coiled bimetallic thermostatic spring type, Fig. 1.
The fan drive clutch uses a heat-sensitive, coiled bimetallic spring connected to an opening plate. This units causes fan speed to increase with a rise in temperature and to decrease as temperature decreases.