Automatically actuatable fan clutch, in particular for cooling systems for internal combustion engines

In an automatically actuatable fan clutch, in particular for cooling systems of internal combustion engines, the improvement comprising a thermostat secured to said fan and rotatable therewith, said thermostat being positioned so that it is swept over by the moving cooling air, said thermostat comprising a gas-tight annular container having a solid metallic wall and an axially flexible wall, said annular container having a liquid therein, the boiling point of which is approximately in the temperature range at which said fan clutch is supposed to be engaged, means for guiding said liquid to the radially outer periphery in response to a rotation of said annular container, said flexible wall expanding into engagement with a friction clutch member in said fan clutch to couple the fan to a drive shaft in response to a gas pressure occurring inside of said container due to said liquid having reached the boiling temperature, said liquid being contained in such amounts in said container that a part thereof remains still liquidy even in the case of the highest occurring cooling air temperature and is rotated with said container so that this liquid is moved by centrifugal force to said outer periphery of said container to effect the application of an additional axially acting pressure onto said flexible wall and resilient means normally urging said friction clutch member out of clutch engaging relation, the force of said resilient means being greater than the amount of force generated by said non-boiling liquid on said flexible wall during rotation of said annular container and less than the force generated by said gas from said boiling liquid.