Valve for hydraulic systems

Valve device for a booster steering hydraulic system having a main pump, a reserve pump, and a sump, for operating a pressure operated consumer;

• said valve device being for selectively controlling flow to said sump from said pums and comprising means effecting a pair of pressure chambers;
• a pressure operable valve member and bias means biasing said valve member to a neutral position relative to said pressure chambers;
• said valve member being actuatable by differential pressure between said pressure chambers wherein differential pressure is operative to shift said valve member from said neutral position in opposition to said bias means;
• said valve device having pump, sump, and consumer passages for connecting to respective pump outlets and to said sump and consumer;
• said valve device having a throttle;
• said valve device having passage means whereby flow from said main pump passes through said throttle to said consumer passage and whereby said throttle communicates on opposite sides thereof with respective pressure chambers to effect a differential pressure between said pressure chambers for actuating said valve member responsive to a predetermined pressure differential;
• said valve device having coacting flow control means whereby said valve member is operative to block flow from pump passages to said sump passage in said neutral position of said valve member;
• actuation of said valve member to a first position responsive to a predetermined degree of differential pressure between said pressure chambers effecting coaction of said flow control means to direct the flow from said reserve pump passage to said sump passage;
• increased pressure differential actuating said valve member to a second position to effect a coaction of said flow control means to direct a portion of the flow from the main pump passage to said sump passage;
• and flow rate responsive means operative to require a predetermined increase in the flow rate to effect a pressure differential for actuating said valve member to said second position disproportionately greater than the flow rate required for effecting actuation to said first position.