Overload protection for DC motor-driven linear actuator

A DC motor-driven linear actuator comprising a permanent magnet DC motor having a pair of electrical terminals for connection to an electrical power source, a screw shaft connected to be rotated by said motor, a nut threaded on the screw shaft, an operating rod connected to the nut, bearing means for supporting the screw shaft for rotation on an axis, spring means held under a predetermined stress for normally maintaining the bearing means and screw shaft in a fixed axial position, and allowing the bearing means and screw shaft to move axially in either direction when the thrust of the operating rod exceeds the predetermined stress of the spring means, first and second self-resetting switches having normally closed contacts connected in series with each other and with the electrical terminals of the motor to conduct DC current from an electrical power source to the motor, actuator means for the switches, means connected to the screw shaft for operating the actuator means so that normally closed contacts of the first switch are opened when the screw shaft moves in a first axial direction and so that the normally closed contacts of the second switch open when the screw shaft moves in the opposite axial direction, a first diode connected across the normally closed contacts of the first switch in a direction to block current from the supply toward the motor when the motor is operating in a direction such that an excessive thrust on the operating rod would cause the normally closed contact of the first switch to open, and a second diode connected across the normally closed contacts of the second switch in a direction to block current from the supply toward the motor when the motor is operating in a direction such that an excessive thrust on the operating rod would cause the normally closed contacts of the second switch to open.