Hydraulic actuating system

A hydraulic actuating system, comprising

• a hydrulic actuator having a fluid intake port and a fluid discharge port;
• first means forming a fluid supply line which communicates with said intake port of said hydraulic actuator;
• second means forming a fluid discharge line which communicates with said discharge port of said hydraulic actuator;
• a fluid reservoir;
• pump means for supplying fluid under pressure from said reservoir to said fluid supply line;
• a closed center control valve interposed in said fluid supply and said fluid discharge lines;
• an adjustable inflow control valve in said fluid supply line upstream of said intake port for controlling the rate of fluid flow into said intake port;
• an adjustable outflow control valve in said fluid discharge line downstream of said discharge port for controlling the rate of fluid outflow from said outflow port;
• two position switching value means communicating with said fluid lines intermediate said hydraulic actuator and said control valves for detecting a pressure difference between said fluid lines which results from a difference between the fluid requirements of said hydraulic actuator and fluid flow rate permitted by said control valves, and for communicating that one of said fluid lines which has the lower detected pressure with said reservoir via a low pressure relief valve so as to prevent excessive pressure build-up in the respective lines;
• and
• an amplifier valve assembly intermediate said hydraulic actuator and said control valves and including a housing having two valve chambers each interposed in parallel with one of said fluid lines and having inner circumferential surfaces formed with respective inlet and exhaust annular grooves connected directly to swaid fluid reservoir and said pump means, port means each communicating with one of said fluid lines and one end of said valve chamber, a pair of hollow inlet and exhaust valve spools each slidable in one of said chabers between positions in which it respectively permits and prevents fluid flow through the other end of the respective chamber to the respective fluid line to join with the fluid flow in the respective fluid line from said closed center control valve, and wherein said control means form differential pressure-detecting pilot-flow circuit means for controlling the extent of movement of said valve spools between said positions thereof in dependence upon the detected pressure difference between said fluid lines.