To achieve linear characteristics in the relationship between a transmissible torque and a speed difference between two rotating parts in a viscous coupling which contains a working chamber filled with viscous fluid and a first and second plurality of circular resistance plates which are connected firmly to the first and second rotating part respectively, at least one of the circular resistance plates is provided with a plurality of blades for the purpose of moving the viscous fluid in the working chamber. The blades can be formed by forming a plurality of apertures in a spiral arrangement in the circular resistance plate and then bending the rim of each cut-out in an appropriate manner.
