Differential torque multiplying device

A differential torque transmitting device comprising first and second rotary slip couplings, the second being of a type whereof the torque-transmitting capacity increases as the rotational speed of the coupling increases, each of which couplings has an input member and an output member, the output member of the first coupling being directly connected to the input member of the second coupling and the output member of the second coupling being connected to the input of a driven machine, an epicyclic reduction gear train having an input element connected to the output member of the first coupling at a speed which bears a constant relationship to the speed of the input member of the second coupling, a reaction element, associated with said gear train, and an output element drivingly connected to the output element of the second coupling, and means acting to hold said reaction element of said gear train fixed against rotation in one direction only and adapted automatically to release said reaction element for rotation in the opposite direction to discontinue the transmission of power through the epicyclic gear train to the output element of the second coupling and to permit the speed of the output member of the second coupling to rise to a value equal to that of the input member of the second coupling, said second coupling being designed so that, whenever the input shaft of the driven machine slows down as a result of an external cause, the torque transmitted backwards by the output member of the second coupling to the first coupling is always lower than the torque transmitted by the output member of the said first coupling to the second coupling.