Improvements in and relating to hydraulic jacks

724,062. Hydraulic jacks. STAMFORD METALLICS, Ltd. Dec. 9, 1953 [Dec. 11, 1952], No. 31445/52. Class 78(3 ). An hydraulic jack is provided with means to effect rapid movement of the lifting column under light load. When the jack is under a heavy load and fluid is pumped through the port 6 the pressure in chamber 30 rises sufficiently to move plunger 8, 9 to the right against the action of adjustable spring 14, so allowing the fluid in chamber 31 to exhaust into the reservoir 20 via conduit 21, plunger chamber 10, port 12, and conduit 19. When the jack is under light load the pressure in chamber 30 is insufficient to move the plunger 8, 9 and so exhaust the fluid from chamber 31, and since the pressure in chamber 31 is greater than the pressure in chamber 30 (due to the different effective areas above and below the piston 4) the valve 27 opens and allows fluid to pass from chamber 31 to chamber 30 so increasing the rate of operation of the piston. On the down stroke of the piston the chamber 31 is refilled from the reservoir via valve 25 and conduit 24. According to a modification (not shown) the valve 27 in the piston may be replaced by a conduit arranged externally of the cylinder; or formed in the wall thereof, and provided with a non-return valve. In a modification described in the Provisional Specification only the reservoir 20 may form an extension of the upper part of the cylinder.