A linear actuator comprising: a ball or lead screw coupled to a rotational drive source and comprising a region of weakness and first and second sections on axially opposite sides of the region of weakness , wherein the first section is a portion of an input journal of the ball or lead screw, and the second section is a portion of a threaded region of the ball or lead screw, the threaded region comprising a thread form ; characterized in that the input journal comprises a keyway and in that the linear actuator further comprises a first coupling member comprising a first mating portion configured to engage the keyway and substantially prevent rotation of the first coupling member about the ball or lead screw, the first coupling member comprising a first contact surface extending along a longitudinal axis of the ball or lead screw toward the region of weakness; and a second coupling member comprising a second mating portion configured to engage the thread form and substantially prevent rotation of the second coupling member about the ball or lead screw, the second coupling member comprising a second contact surface extending along the longitudinal axis of the ball or lead screw toward the region of weakness, extent of the first contact surface along the longitudinal axis of the ball or lead screw overlapping extent of the second contact surface along the longitudinal axis of the ball or lead screw, wherein relative rotational angular separation between the first contact surface and the second contact surface is such that less than a full rotation of the second section of the ball or lead screw independent of the first section of the ball or lead screw is permitted before the first contact surface and the second contact surface engage each other in the event of failure of the ball or lead screw within the region of weakness. a ball or lead screw coupled to a rotational drive source and comprising a region of weakness and first and second sections on axially opposite sides of the region of weakness , wherein the first section is a portion of an input journal of the ball or lead screw, and the second section is a portion of a threaded region of the ball or lead screw, the threaded region comprising a thread form ; characterized in that the input journal comprises a keyway and in that the linear actuator further comprises a first coupling member comprising a first mating portion configured to engage the keyway and substantially prevent rotation of the first coupling member about the ball or lead screw, the first coupling member comprising a first contact surface extending along a longitudinal axis of the ball or lead screw toward the region of weakness; and a second coupling member comprising a second mating portion configured to engage the thread form and substantially prevent rotation of the second coupling member about the ball or lead screw, the second coupling member comprising a second contact surface extending along the longitudinal axis of the ball or lead screw toward the region of weakness, extent of the first contact surface along the longitudinal axis of the ball or lead screw overlapping extent of the second contact surface along the longitudinal axis of the ball or lead screw, wherein relative rotational angular separation between the first contact surface and the second contact surface is such that less than a full rotation of the second section of the ball or lead screw independent of the first section of the ball or lead screw is permitted before the first contact surface and the second contact surface engage each other in the event of failure of the ball or lead screw within the region of weakness. <EOS>