Abstract: An automatically coupling fan for the cooling system of automobiles, comprising a thermostat, in the form of a gas-tight container with a solid and a flexible wall, containing a liquid whose steam temperature within a predetermined temperature range couples the fan to the drive shaft of the motor by means of a friction clutch, characterized in that the thermostat has the form of a metal bellows which turns along with the water pump shaft. The bellows dimensions are such that the water ring generated during rotation of the thermostat by to the centrifugal forces exerted on the thermostat filler largely remains outside of the effective bellows cross-section (a.sub.