fan fluid coupling the numerous fins 16 or the recessed walls 16 39 are formed on the wall surface opposite to the driving disk 7 on the enclosed housing near the outer periphery of the disk 7 thus during rotation the fins serve as guide vanes acting on the oil also the oil is conveyed by centrifugal force in addition the fins or the recessed walls perform pumping action to force oil from the torque transmission gap to the dam 12 especially the pressure of oil collected in the dam is increased with certainty to prevent hunting the flow of oil through the gap is smoothened this stabilizes the control action furthermore the oil passes through the torque transmission gap in a shorter time as a result the time for which the mechanism undergoes shear and is heated is decreased this minimizes the temperature increase of the oil the oil is quickly and smoothly circulated through the whole internal structure leading to improved heat dissipation hence the viscosity of the oil is kept from falling consequently the ability to cool the engine does not drop during prolonged operation the fan fluid coupling performs more appropriate control function in response to the varying ambient temperature if necessary the idle oil reservoir chamber 14 is also formed the fins 16 or the recessed grooves 16 39 are formed from the oil reservoir chamber 6 to the idle oil reservoir chamber 14 this enables the oil to circulate rapidly whereby effectively suppressing the unwanted increase of the rotational speed of the driven fan.