in varying embodiments , the fault tolerant linear actuator of the present invention is a new and improved linear actuator with fault tolerance and positional control that may incorporate velocity summing , force summing , or a combination of the two . in one embodiment , the invention offers a velocity summing arrangement with a differential gear between two prime movers driving a cage , which then drives a linear spindle screw transmission . other embodiments feature two prime movers driving separate linear spindle screw transmissions , one internal and one external , in a totally concentric and compact integrated module .