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.

Abstract: A fan for a cooling system in an internal combustion engine. The fan has a friction clutch therein which is engageable in response to a predetermined elevated temperature of air passing over a thermostat. The thermostat rotates with the fan. The thermostat is composed of an annular container filled with a liquid having a boiling point which is approximately in the temperature range at which the fan is to become engaged. One wall of the container is movable and forms a part of the friction clutch. The pressure of the liquid in the container is insufficient to effect an engagement of the friction clutch. However, when the temperature is elevated to a point whereat the liquid begins to boil, a gas pressure develops in the container and the gas pressure is sufficient to move the wall of the container to effect an engagement of the friction clutch and, as a result, a driving of the fan.