Power transmission system

Claim 1

A power transmission system comprising: a. a cylinder including end closures having means to receive pressure fluid at opposite ends thereof; b. a piston including opposed pressure responsive faces slidably disposed in said cylinder, having an internally threaded bore therethrough, and having a hollow piston rod extending axially from the cylinder through an end closure at one end thereof for connection at the free end of the rod to a load; c. a source of pressure fluid; d. selector valve means and flow control means interconnecting said source of pressure fluid with said pressure receiving means at each end of said cylinder and said opposed piston faces, said source and valve means and fluid control means having a flow rate capacity enabling said piston to be displaced by pressure fluid acting thereon at a given rate in either direction; e. a screw in said cylinder rotatably mounted in bearings in an axially fixed position with respect to said cylinder at the end opposite to said one end of said cylinder and extending into said cylinder from said bearings, said screw having external threads in engagement with said internal threads of said piston and extendible into said hollow piston rod, said threads having an axial play therebetween and having such pitch and friction that said screw is incapable of rotation in response to any axial force applied thereto by said piston in either axial direction; and f. means secured to said cylinder for rotating said screw in a selected direction, said means being continuously engaged with said screw for rotation thereof, said means having an effective size rendering it incapable of rotating the screw in either direction whenever any axial force is extend on the threads of said screw and having a turning rate capacity which is effectively greater than said flow rate capacity for axially displacing the piston, whereby said piston can be displaced when supporting the load in either direction solely by means of fluid pressure acting on said piston, whereas the screw threads follow the displacemEnt of the piston threads without assisting to displace said piston and the screw serving solely for preventing axial displacement of the piston except when such displacement is accomplished by means of said pressure source, valve means and flow control means.