How do viscous fans work?

[dnb]

Can anyone explain how a viscous fan works? I have never had a vehicle with one up to now.

Not sure if my wife's P38 4.6 vogue has a problem with the viscous fan. It seems to be running more than I remember it doing last time I drove it.

Thanks.

[Hammer67]

A fan clutch is a thermostatic device that exists as an integral component of certain automotive cooling systems. When the engine is cool or even at normal operating temperature, the fan clutch partially disengages the engine's mechanically-driven radiator cooling fan, generally located at the front of the water pump and driven by a belt and pulley connected to the engine's crankshaft. This saves power, since the engine does not have to fully drive the fan.

However, if engine temperature rises above the clutch's engagement temperature setting, the fan becomes fully engaged, thus drawing a higher volume of ambient air through the vehicle's radiator, which in turn serves to maintain or lower the engine coolant temperature to an acceptable level