Abstract: A control coupling in which an enclosure houses first and second sets of mutually interleaved elements in a viscous fluid. The first and second sets of elements are arranged for rotation about a common axis and for driving connection with first and second rotatable members respectively. The quantity of liquid within the enclosure and the construction of the enclosure are such that, at a predetermined working temperature of at least 100.degree. C, the pressure of the liquid in the enclosure rises substantially above ambient. At least one set of the elements is free to move in directions parallel to the common axis of rotation and at least one set of the elements is provided with facings of friction material.