A coupling for use in a transmission system and between rotary members which are arranged for driving engagement through axially engaging teeth thereon, the teeth being shaped to tend to disengage when normally driving in one direction, one of said members having two components connected by helical splines, the components being prevented from moving axially to disengage the teeth by a fusible element which will melt when temperature rises above a predetermined value, melting of said element permitting movement of the components axially to disengage the teeth and the helical splines permitting axial relative movement between the two components, when reverse rotation is taking place, to permit ratcheting of the teeth.