Bi-rotational, two-stage hydraulic system

摘要

A hydraulic system for an actuator provides staged operation during extend and retract. A pair of reversible pumps are conjointly driven by a reversible motor. A relief valve in the retract circuit directs fluid to tank when fluid pressure increases above a predetermined value. An unloader valve in the extend circuit is responsive to pressure at a first stage pump to direct flow to tank when the pressure increases above a second predetermined value. A non-return check valve connected between the pumps in the extend circuit closes the flow circuit to the second stage pump during high loads, so that the flow from the second stage pump goes to tank, and the return flow from the actuator goes only to the first stage pump. The non-return check valve is responsive (via a signal line to the retract circuit portion) to the pressure in the extend and retract circuit portions.