Hydraulic system combining open center and closed center hydraulic circuits

摘要

A hydraulic system is provided in which a single variable displacement pump supplies the hydraulic fluid requirements of both an open center hydraulic circuit and a closed center hydraulic circuit. The hydraulic output from the variable displacement pump is delivered to a pressure compensated flow divider which at all times delivers a priority minimum and constant flow to the open center hydraulic circuit despite variations in pump output pressures. Unless there is a demand for hydraulic flow by a device or devices in the closed center hydraulic circuit, there is no hydraulic flow delivery from the flow divider to the closed center hydraulic circuit, and the variable displacement pump operates at relatively low power input required to supply the demands of the open center hydraulic circuit. Particularly when the only hydraulic demand on the pump is to satisfy the requirements of the open center hydraulic circuit in neutral position, the pump can operate on a "standby" basis at a relatively low output pressure, such as 300 pounds per square inch, with consequent low input power to the pump, and with relatively low "wear and tear" on the pump. The variable displacement pump assembly comprises a hydraulically actuated stroke control means including a tiltably adjustable swash plate, and a hydraulic compensator for controlling the tilt of the swash plate. A first pilot circuit is permanently connected between the open center hydraulic circuit and the control input point of the hydraulic compensator which controls the angular position of the tiltably adjustable swash plate, whereby the output pressure of the pump may respond to changes in hydraulic pressure requirements in the open center hydraulic circuit.PPA second pilot circuit is connected at one of its ends to the hydraulic conduit which connects the flow divider to the closed center hydraulic circuitry, the opposite end of the second pilot circuit being connected to the control input point of the aforementioned compensator which controls the angular position of the tiltably adjustable swash plate. The second pilot circuit is completed to communicate a hydraulic input signal to the compensator from the closed center hydraulic circuit only in response to the actuation of at least one closed center device requiring hydraulic flow. Completion of the second pilot circuit as just mentioned will supersede the first pilot circuit whereby to cause the compensator to readjust the tiltably adjustable swash plate to cause the pump to supply the substantially higher hydraulic pressure requirements, such as 3,000 pounds per square inch, of the closed center hydraulic system.