Pneumatical piston-cylinder unit having a hydraulic control means

A pneumatic piston-cylinder unit comprising:

• an air cylinder;
• an air piston reciprocatable in said air cylinder and which divides said air cylinder into first and second sides;
• a tubular piston rod attached to said air piston including a hydraulic chamber which is filled with a hydraulic liquid;
• first and second air channels by which compressed air is selectively fed to the first and second sides of said air cylinder;
• an integrated hydraulic control system including
• (a) a hydraulic cylinder which is located within said hydraulic chamber of said tubular piston rod and which is movable a limited distance with respect to said air cylinder from a first position to a second position,
• (b) a hydraulic piston in said hydraulic cylinder which is stationary with respect to said air cylinder so that movement of said hydraulic cylinder moves said hydraulic cylinder relative to said hydraulic piston, and
• (c) a liquid flow path between said hydraulic cylinder and said hydraulic piston which is closed when said hydraulic cylinder is in the first position and which is opened when said hydraulic cylinder is in the second position;
• and
• an actuation means, which is fixedly mounted in said cylinder, (i) for moving said hydraulic cylinder to the second position and hence for opening the liquid flow path when compressed air is present in either of said first and second air channels whereby said air piston is movable in said air cylinder by the compressed air, and (ii) for moving said hydraulic cylinder to the first position and hence for closing the liquid flow path when compressed air is absent from both of said first and second air channels whereby any movement of said air piston relative to said air cylinder is prevented by the closing of said liquid flow path.