Pressure medium driven device performing linear motion

A pressure medium operated device comprising:

• a body forming an enclosed space, said body having an inlet port, an outlet port, and a land having two ends;
• a pressure medium that enters said enclosed space via said inlet port and exits said enclosed space via said outlet port, wherein an operating pressure is supplied to said device by said pressure medium entering said input port;
• a primary piston alternately driven in a backward and a forward direction within said enclosed space by the operating pressure of said pressure medium, said primary piston comprising
• a first driving surface,
• a second driving surface located opposite said first driving surface and having a surface area greater that of said first driving surface,
• a built-in control valve system disposed within said primary piston, and
• a discharge channel having a first opening into said built-in control valve system and a second opening that is temporarily sealed when said discharge channel is positioned between said two ends of said land,
• wherein said built-in control valve system and said discharge channel together control the operation of the pressure of said pressure medium to the primary piston and thus the direction of movement of the primary piston,
• said built-in control valve system comprising:
• a first facing surface and a second oppositely facing surface disposed on said primary piston, and
• a valve member disposed within said primary piston, said valve member including a first ring-shaped surface and a second ring-shaped surface, each of said first and second ring-shaped surfaces respectively cooperating with said first and said second oppositely facing surfaces in an alternating fashion such that when said first ring-shaped surface abuts against said first facing surface, said pressure medium is directed to supply the operating pressure against said first driving surface to move said primary piston and said discharge channel in the forward direction until said discharge channel reaches one end of said land of said body to open said discharge channel to allow said pressure medium to shift said built-in control valve system such that said second ring-shaped surface abuts against said second facing surface which equates the pressure on each of said driving surfaces and thereby drives said primary piston in said backward direction due to the equal pressure upon the greater surface area of said second driving surface until said discharge channel reaches the other end of said land which enables said discharge channel to vent said pressure by allowing said pressure medium to exit said outlet port and thereby allow said pressure to shift said built-in control valve system so that said first ring-shaped surface and said first facing surface are once again engaged by the operating pressure of said pressure medium entering said input port which once again drives said piston in said forward direction.