Abstract: Magnetic flux produced by an electrically energized coil threads radially between a housing and a flange on a rotatable shaft, threads axially between the flange and a control collar for the clutch, threads radially between the control collar and an input hub via a helical spring contracted around the hub, and threads axially between a flange on the hub and the housing to complete the flux circuit. Flux cutting axially between the flange on the shaft and the control collar causes the control collar to shift axially into engagement with the flange so as to effect turning of the collar relative to the input hub and contraction of the spring around an output hub.

Abstract: The electrical brake coils of a trailer are energized when the stoplight switch of the towing vehicle is closed and the trailer surges forwardly with respect to the towing vehicle to cause the wiper of a rheostat to move relatively along the resistor of the rheostat. A hazard switch for causing the stoplights of the towing vehicle to flash is connected in parallel with the stoplight switch in the stoplight circuit of the towing vehicle. The resistor of the rheostat is formed with a normally non-conductive portion which prevents the brake coils from being repetitively pulsed with current when the hazard switch is actuated. Once surge braking has been initiated, however, the non-conductive portion of the resistor is rendered conductive so that current will be supplied continuously to the brake coils over the full range of movement which the trailer undertakes during surge braking.