855,390. Control of motors. HOOVER, V. A. May 7, 1958 [Sept. 5, 1957], No. 14601/58. Class 38 (3). [Also in Group XXIV] An actuator comprises an electric motor 11, reduction gearing 12, and a magnetic clutch and brake unit 13 which allows two actuators, when driven as a pair, to be operated by one drivemotor should the other motor fail, the mechanical interconnection for such operation being effected by differential gearing attached to output shaft 16. A torque-limiting clutch 14 prevents damage in the event of jamming. Detachable limit-switch mechanism comprises camshaft 20 carrying cams 25, 26, for operating switches 27, 28, and mechanical stops are provided to prevent over-travel of the cam-shaft if a limit-switch fails to operate. A stop-arm 30 is adapted to engage screw-stops 31, 32, which are generally set at positions just beyond the points at which the limit-switches are operated. The cam-shaft 20 is coupled to the output shaft 16 through torque-limiting mechanism which protects the actuator from damage when the stops are engaged, such engagement causing slippage between worm-wheel 35 and flange 37 so allowing continued rotation of the wormwheel while collar 36 and cam-shaft 20 remain fixed. The motor may be reversed out of a stop-position, the urging of resilient washer 38 being sufficient to re-establish frictional engagement between the worm-wheel and flange. The cam-shaft carries a further cam 42 for operating switch 43 which is connected in a circuit for indicating the position of the cam-shaft. In another arrangement, the torque-limiting clutch 14 is dispensed with, and the cam-shaft is threaded. The cam assemblies for operating the limit switches each consist of a flanged nut which travels along the cam-shaft, and if either of the limit-switches fails to operate the camassemblies are carried into mechanical stops. The nuts are then forced to rotate with the camshaft and further axial movement is prevented. When the actuator is reversed the nuts are released and since brakes are provided to keep the nuts from further rotation, the cam assemblies are caused to travel along the cam-shaft, Figs. 6-8 (not shown). In a further embodiment, the actuator is protected against damage in case of failure in the operation of a limit-switch by the action of a torque-limiting clutch which slips when there is an over-load. The limit-switch mechanism is not provided with a separate torque-limiter in this embodiment, Figs. 9-11 (not shown). Specification 821,411 is referred to.