A viscous fan clutch (10) for the drive of a fan in the cooling system of an internal combustion engine, comprises a driven housing (11) which is rotatably mounted through an anti-friction bearing on a driving shaft and is divided in the axial direction by a partition (17) into a working chamber (20) and a reserve chamber (19). The two chambers are at least partially filled with a viscous fluid. In the working chamber a rotor (18) connected fast in rotation with the driving shaft is so arranged that a first working gap (27) is provided between it and the outer wall (29) of the housing and a second working gap (28) is provided between it and the partition. The first working gap between the rotor and the outer housing wall is smaller than the second working gap between the rotor and the partition. <IMAGE>
