The present invention relates to a viscous fluid coupling adapted to drive an accessory device, such as a cooling fan, of an internal combustion engine. 
Japanese Utility Model Provisional Publications Nos.  57-204491 and 3-77825 disclose a viscous fluid coupling adapted to a cooling fan for an internal combustion engine.  As shown in FIGS.  21 and 22, this conventional viscous fluid coupling 100 includes a housing 104 relatively rotatable with a drive shaft 102, an operation plate 108 dividing a space in the housing 104 into a reservoir 105 and an operation chamber 106, a rotor 109 disposed in the operation chamber 106 and fixed to the drive shaft 102 and a valve mechanism 111 disposed at an outer peripheral portion of the rotor 109.  The housing 104 is constituted by a housing body member 121 supported by the drive shaft 102 and a cover member 122 fixed to the housing body member 121.  The cover member 122 includes a circular recess portion 123 for defining the operation chamber 105 and a ring-shaped flange portion 125 having a return passage 124.  The operation plate 108 is made of metal and formed into a disc shape.  The operation plate 108 is connected to the flange portion 124 of the cover member 122 by means of caulking so as to close an opening of the recess portion 123 of the cover member 122.  A torque transmitting section 110 is constituted by a plurality of annular projections 127 concentrically formed on the cover member 122 and a plurality of annular projections 128 concentrically formed on the rotor 109 so that the annular projections 127 and 128 are overlapped with each other. 
When the ambient temperature around the housing is low, the communication hole 107 formed on the operation plate 108 is closed by the valve mechanism 111 to stop the flow of the viscous fluid from the reservoir 105 to the operation chamber 106.  Therefore, the torque transmission amount from the rotor 109 to the housing 104 is lowered to stop or rotate a cooling fan in low speed.  When the ambient temperature around the housing is high, the communicated hole is opened by the operation of the valve mechanism 111 to allow the viscous fluid to flow from the reservoir 105 to the operation chamber 106.  Therefore, the torque transmission amount from the rotor 109 to the housing 104 is increased to rotate the cooling fan at high speed. 
However, this conventional viscous fluid coupling 101 encounters drawbacks.  For example, since the torque transmitting section 110 is formed by locating the annular projections 127 and the annular projections 128, adjacent to each other it is necessary to locate the torque transmitting section 110 on an outer peripheral portion of the cover member 122.  That is, because the operation plate 108 is installed at a central portion of the cover member 122, it is impossible to locate the annular projections 127 at an inner position corresponding to the operation plate 108. 
Further, since the torque transmitting section 110 is formed at an outer side as compared with an inner surface defining the reservoir 105, that it, since the reservoir 105 is located at a height which is (gravitationally) lower than that of the torque transmitting section 110, the torque transmitting section 110 becomes dipped in (viz, becomes at least partially immersed in or coated with) the viscous fluid when the engine stops.  This dipping of the torque transmitting section 110 generates a dragging-rotation phenomenon of the cooling fan when the engine is started.  Although Japanese Utility Model Provisional Publications Nos.  59-128933 and 1-83925 have proposed another conventional viscous fluid coupling arranged to prevent such a fan dragging-rotation phenomenon, this conventional viscous fluid coupling has generated another problem that the utility of the viscous fluid is degraded. 
A viscous fluid coupling according to the present invention is connected to an engine.  The viscous fluid coupling comprises a drive shaft connected to a rotation shaft of the engine.  A housing is rotatably supported to the drive shaft.  An operation plate is set in the housing so as to divide a space defined by the housing into a reservoir and an operation chamber.  The operation plate has a communication hole communicating the reservoir and the operation chamber.  Viscous fluid is stored in the reservoir and the operation chamber.  A rotor is disposed in the operation chamber and is fixed to the drive shaft.  A valve mechanism closes and opens the communication hole according to ambient temperature of the housing to control a flow rate of the viscous fluid from the reservoir to the operation chamber.  A driven wheel is fixed to the housing so as to be located between the operation plate and the rotor.  A torque transmitting section includes a plurality of first annular projections concentrically formed on an outer peripheral portion of the driven wheel and a plurality of second annular projections concentrically formed on an outer peripheral portion of the rotor.  The first annular projections are overlappedly adjacent to the second annular projections so as to be fluidly coupled with each other through the viscous fluid.