Hydraulic control systems

Claim 1

In an hydraulic system having a source for supplying fluid under pressure to a cylinder or other work performing means actuated by fluid under pressure, a supply line for fluid under pressure between the source and the work performing means, a pressure control valve connected by a control line to the source and presenting a variable restriction, a fixed restriction in said control line, a normally closed pilot-operated primary flow control valve of the spool type interposed in the supply line between the source and the work performing means, a first pilot line connected at one end to the control line between said fixed restriction and said pressure control valve and connected at its other end to the flow control valve for delivering a valve-open demand signal to the flow control valve represented by fluid under pressure to move the spool to a valve open position in the supply line placing the source in communication with the work performing means, a second pilot line connected to the opposite end of the flow control valve to deliver fluid under pressure thereto tending to close the flow control valve, said second pilot line being connected to the supply line between the primary flow control valve and the work performing means for receiving fluid under pressure proportional to the pressure in the supply line, the proportioning being such that when the pressure in the second pilot line reaches a value determined by the pressure in the first pilot line the spool is shifted toward its closed position, a relief valve in the supply line between the work performing means and said primary flow control valve, the relief valve being normally closed and in its normally closed position allowing flow of fluid under pressure from the source to the work performing means, a pilot line between the pressure control valve and the relief valve modulating the pressure at which the fluid in the supply line, delivered to the work performing means, is vented to tank and by which the relief valve is allowed to open to reduce the pressure between the primary flow control valve and the work performing means when the primary flow control valve closes.