HYDRAULISCHES STELLGLIED

A double-acting actuating device (1) has a cylinder (2) with a piston (3) connected to a rod (4). The cylinder is provided with ports (8, 9) at opposite ends for flow/return lines (10, 11) to provide flow of hydraulic liquid to whichever side of the piston is to be pressurised for its movement and to provide return of hydraulic liquid from the other side. The lines (10, 11) pass to opposite ends of a shuttle valve (20) having a body (21) with a bore (22) accommodating a double acting piston (23) having short rods (24, 25) extending on either side. Each rod carries a centring spring (26, 27) acting against its face of the shuttle piston and an end fitting (28, 29) of the body. The springs are relatively stiff and centralise the shuttle. The fittings seal the bore, except that each has a central drilling (30) which can freely accommodate the rod on its side of the piston and allow hydraulic liquid flow at the same time. Outwards of the drilling, each fitting has a tapered bore (31) for receiving a non-return valve ball (32) held against the bore (31) by a spring (33). The bore has two ports (35, 36) respectively close to the end fittings (28, 29). The body (21) of the shuttle valve has two ports (61, 62), opening into the bore (22) adjacent the end fittings (28, 29), to which ports are connected flow and return lines (63, 64) from a reversible gear pump (65), selectively driven in either direction by an electric motor (66). In normal operation, the pump is driven in the direction required for the desired movement of the actuator, as a whole. Flow of hydraulic liquid is to one end of the shuttle valve. The pressure of the liquid lifts the valve in the respective end fitting (28, 29). Pressure is applied to the actuating device and via its piston to the liquid in the respective line back to the other of the fittings. Until the valve in this fitting is opened no movement can occur. The pressure acts on the shuttle, moving it towards this valve, which it opens via its rod (24, 25), the effective area of the balls (33) seating in their tapers (31) being less than that of the shuttle piston. Movement can then occur with the return of liquid back to the pump and piston rod effect flow to or from the accumulator via the line (75).