Abstract: Motor rotated cams each actuate a lever and each lever is connected by a flexible non-extensible member, cable or chain, to the element of the machine to be controlled. A spring loaded bell crank lever is provided for each flexible non-extensible member and the non-extensible member passes over a pulley mounted at one end of an arm of the bell crank lever. Each arm of the bell crank lever actuates an electric switch connected to a motor whereby if the tension in the flexible non-extensible member increases or decreases excessively the appropriate one of the switches is opened to stop the motor.