Page "Clutch" Paragraph 41

There are other clutches found in a car.

For example, a belt-driven engine cooling fan may have a clutch that is heat-activated.

The driving and driven members are separated by a silicone-based fluid and a valve controlled by a bimetallic spring.

When the temperature is low, the spring winds and closes the valve, which allows the fan to spin at about 20 % to 30 % of the shaft speed.

As the temperature of the spring rises, it unwinds and opens the valve, allowing fluid past the valve which allows the fan to spin at about 60 % to 90 % of shaft speed.