Flow sharing priority circuit for open circuit systems with several actuators per pump

A priority circuit is provided that is in communication with an open circuit pump and priority and auxiliary systems. The circuit comprises a first valve with an inlet in communication with the pump, an outlet in communication the priority system, and a load sensing inlet in communication with the priority system, wherein the first valve modulates to maintain a predetermined pressure differential between pressure at the load sensing inlet and pressure at the outlet. The circuit further comprises a second valve with an inlet in communication with the pump, an outlet in communication with the auxiliary system, and a load sensing inlet in communication with the priority system, wherein the second valve is normally closed and opens when pressure at the load sensing inlet of the second valve is less than pressure at the inlet of the second valve minus the predetermined pressure differential of the first valve.

BACKGROUND OF THE INVENTION

The present invention relates to an open circuit hydraulic system and, more specifically, a flow sharing priority circuit for an open circuit hydraulic system.

Open circuit hydraulic systems are commonly used to drive a plurality of subsystems attached thereto. For example, an open circuit hydraulic system provided on heavy machinery may be used to drive the steering system, the fan motor, and a plurality of other components of the heavy machinery.

Open circuit hydraulic systems typically use priority valves to ensure that certain subsystems receive adequate hydraulic flow from the open circuit hydraulic pump. For instance, in an open circuit hydraulic system driving a critical system, such as a steering system, and a plurality of non-critical auxiliary systems, a priority valve may be used to ensure that the flow requirements of the critical system are satisfied before supplying hydraulic flow to the auxiliary systems. As such, priority valves ensure that the auxiliary systems do not starve off hydraulic flow from a critical system should the open circuit pump not be able to meet all of the demands of the system.

One disadvantage of conventional priority valves is that they tend to experience some undesired shift with changes in flow between the priority and auxiliary systems. Typical priority valve designs comprise a single spool or valve actuating element. It is inherent in this design that the valve spool will tend to shift when excess flow is provided from the priority system to the auxiliary system, resulting in undesired changes in the pressure and flow to the priority system. For instance, in an open circuit system driving a priority steering system and an auxiliary fan motor, a user will notice a small undesired jerk in the machine's steering wheel as the fan motor turns on or off.

It is therefore a principal object of this invention to provide a priority valve that prevents undesired changes in the pressure and flow of the priority system upon activation or deactivation of the auxiliary system.

A further object of this invention is to provide a flow sharing priority circuit that utilizes two independent priority valves.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed towards a flow sharing priority circuit in fluid communication with an open circuit hydraulic pump, a priority system, and auxiliary systems. The priority circuit comprises a first valve with an inlet in fluid communication with the open circuit hydraulic pump, an outlet in fluid communication with the priority system, and a load sensing inlet in fluid communication with the priority system, wherein the first valve is normally open and modulates between open and closed positions to maintain a predetermined pressure differential between pressure at the load sensing inlet and pressure at the outlet.

The priority circuit further comprises a second valve with an inlet in fluid communication with the open circuit hydraulic pump, an outlet in fluid communication with the auxiliary systems, and a load sensing inlet in fluid communication with the priority system, wherein the second valve is normally closed and opens when pressure at the load sensing inlet of the second valve is less than pressure at the inlet of the second valve minus the predetermined pressure differential of the first valve.

Because this flow sharing priority circuit uses two independent priority valves, pressure and flow provided to the priority system does not fluctuate as excess flow is directed to the auxiliary system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

With reference toFIG. 1, a flow sharing priority circuit10is shown with an open circuit pump port12, a priority system port14, an auxiliary system port16, a first priority valve18, and a second priority valve20.

Pump port12is connected to the outlet of an open circuit pump (not shown), such as a pressure-compensated, load-sensing axial piston pump. Priority system port14is connected to a priority system (not shown), such as a steering system or other critical subsystem driven by the open circuit pump. Auxiliary system port16is connected to an auxiliary system (not shown), such as a fan motor or other non-critical subsystem driven by the open circuit pump.

The flow sharing circuit10uses a combination of valves18and20to replace a conventional priority valve of the single spool design. Valves18and20are hydraulic differential sensing valves. First valve18includes an inlet22in fluid communication with the open circuit pump port12and an outlet24in fluid communication with the priority system port14. First valve18further includes a load sensing inlet26in fluid communication with a load sensing orifice which is provided externally and connected to28of priority system14. First valve18is normally open such that pressure from the open circuit pump is supplied directly to the priority system. However, first valve18limits the flow provided to the priority system when the pressure demanded at the auxiliary system port16is greater than the pressure demanded at the priority system port14. As first valve18is a differential sensing valve, valve18modulates between an open and closed position in order to maintain a predetermined pressure differential between the pressure at the load sensing inlet26and the outlet24. Valve18is capable of providing a differential pressure setting within the range of 20 to 360 PSI. In most heavy machinery, a differential pressure setting of 80 PSI provides optimal performance conditions.

A check valve32is located between valves18and20. When the pressure at the load sensing inlet26of valve18exceeds the pressure at load sensing inlet30of the open circuit pump, check valve32opens to permit fluid communication with second valve20. Check valve32is configured to allow for the passage of small flows of load sensing fluid with minimal backpressure. This is accomplished by equipping check valve32with a low backpressure spring or by eliminating the spring entirely.

Second valve20is normally closed and includes an inlet34in fluid communication with the open circuit pump port12and an outlet36in fluid communication with the auxiliary system port16. Second valve20further includes a load sensing inlet38in fluid communication with load sensing inlet26of first valve18and, therefore, in fluid communication with the load sensing orifice28of priority system14. When the pressure at load sensing inlet38is less than the pressure at inlet34minus the differential pressure setting of first valve18, second valve20shifts to an open position to provide excess flow from the open circuit pump to the auxiliary system at port16. In this way, valves18and20function as a priority circuit.

As excess fluid is directed to the auxiliary system at port16, the fluid passes through valves40and42. Valve42, which is an electronic proportional solenoid valve, is used to regulate the flow of hydraulic fluid that is supplied to the auxiliary system. Valve40, which is a differential sensing valve similar to valves18and20, works to modulate the pressure drop across valve42. Valve40is capable of providing a differential pressure setting within the range of 20 to 360 PSI. In most heavy machinery, a differential pressure setting of 80 PSI provides optimal performance conditions. Excess flow that is supplied to the auxiliary system at port16is returned to the flow sharing priority circuit10at port44. This return flow is directed to a drain port46, which returns the hydraulic fluid to the open circuit pump or to the hydraulic reservoir.

Alternatively, the excess flow may be directed to a closed loop hydraulic system. As shown inFIG. 2, alternative flow sharing priority circuit48provides excess flow to the auxiliary system at port16as well as to the charge circuit of a closed loop hydraulic system (not shown) at port50. Flow to the charge circuit at port50is initially provided by the flow returning from the auxiliary system to the flow sharing circuit48at port44. When the pressure at port50is less than the requirements of the charge circuit, reducing valve52opens to allow excess flow to be provided directly to port50. Reducing valve52is in fluid communication with the outlet36of valve20and port50of the close loop hydraulic system. Reducing valve52also is in fluid communication with the drain46as a pressure reference. Alternative flow sharing circuit48is further provided with a pressure relief valve54. When the return pressure at port44exceeds the requirements of the charge circuit at port50, relief valve54opens to vent the surplus pressure to the drain at port46. Alternative flow sharing circuit48provides for greater efficiency as the return flow at port44is directed to the charge circuit of a closed loop hydraulic circuit at port50as opposed to being sent to the drain at port46and returned directly to the open circuit pump.

It is therefore seen that by the use of two independent priority valves in a flow sharing configuration, this invention permits the activation and deactivation of an auxiliary system without undesired changes in the pressure and flow of the priority system.