TECHNICAL FIELD

The present invention relates to a fluid circuit configured such that pressure fluid flows into an actuator from a pressure fluid source to drive a load.

BACKGROUND ART

Typically, a fluid circuit configured such that pressure fluid such as oil flows into an actuator from a pressure fluid source to drive a load has been used for driving a vehicle, a construction machine, an industrial machine, etc. For example, a hydraulic shovel supplies pressure fluid from a hydraulic pump to multiple actuators connected in parallel with a hydraulic circuit as a fluid circuit in a fluid manner, such as a bucket cylinder and an arm cylinder, thereby simultaneously driving and operating multiple loads. Various modification have been made considering improvement of operability, energy saving, acceleration, and safety.

In a typical example of the fluid circuit, a hydraulic circuit of an open center system applied to, e.g., a hydraulic shovel is configured such that at a neutral position of a direction switching valve connected to an actuator and an operation lever, pressure fluid from a hydraulic pump as a pressure fluid source is discharged to a tank by way of a bypass flow path and a pilot pressure based on an operation amount of an operation lever strokes a spool of the direction switching valve to obtain the actuation speed of the actuator according to the operation amount of the operation lever. However, in this system, in a case where a high load pressure is on the actuator, the operation lever needs to be operated to a high output side.

A fluid circuit of a load sensing system configured to make such control that the supply pressure of a hydraulic pump is constantly higher than the maximum load pressure of multiple actuators by a target pressure difference has been known as a fluid circuit solving the above-described problem (see Patent Citation 1). As an example of the fluid circuit of the load sensing system as described above, a fluid circuit illustrated inFIG. 7mainly includes a swash plate type variable capacity hydraulic pump102to be driven by a drive mechanism such as an engine or an electric motor, two actuators108,109connected in parallel with the hydraulic pump102in a fluid manner, two direction switching valves106,107connected to each of the actuators108,109and operation levers110,111to switch a supply destination of pressure fluid supplied from the hydraulic pump102, pressure compensation valves104,105provided at pressure-fluid-source-side flow paths of the direction switching valves106,107, and a load sensing valve141and a swash plate control unit142as a discharge amount control mechanism configured to control a pressure fluid discharge amount (the output) of the hydraulic pump102. The actuator maximum load pressure, which is selected by a shuttle valve116for the load sensing valve141, as a higher one of the load pressures of two actuators108,109through a pilot pipe line120and the supply pressure of the hydraulic pump102from the pressure-fluid-source-side flow paths of the direction switching valves106,107are guided to the load sensing valve141. Then, the degree of opening of the load sensing valve141is adjusted such that a difference between the supply pressure of the hydraulic pump102and the actuator maximum load pressure, i.e., a pressure difference (also referred to as a direction switching valve pressure difference) between a pressure fluid source side of the direction switching valve106,107and an actuator108,109side, reaches a target value (e.g., a constant value). Inclination of a swash plate143is increased/decreased by the swash plate control unit142, and in this manner, the output of the hydraulic pump102is controlled. Thus, in the fluid circuit of the load sensing system, in a case where a high load pressure is on the actuators108,109, the fluid circuit can respond to fluctuation in the load pressures of the actuators108,109by control by the discharge amount control mechanism.

CITATION LIST

Patent Literature

Patent Citation 1: JP 3-74605 A (Page 28, FIG. 1)

SUMMARY OF INVENTION

Technical Problem

However, in the fluid circuit of the load sensing system ofFIG. 7, in a case where a high load acts on two actuators, an appropriate hydraulic pump for such a load may be used. However, a large hydraulic pump needs to be provided, leading to a problem that an energy efficiency is degraded.

The present invention has been made in view of such a problem, and is intended to provide a high-energy-efficiency fluid circuit using a load sensing system.

Solution to Problem

For solving the above-described problem, a fluid circuit according to the present invention is a fluid circuit including a pressure fluid source configured to supply pressure fluid, multiple actuators connected to the pressure fluid source, a direction switching valve configured to switch a supply destination of the pressure fluid supplied from the pressure fluid source, and a discharge amount control mechanism configured to control output pressure of the pressure fluid source such that a pressure difference between the output pressure of the pressure fluid source and the maximum load pressure of the load pressures of the multiple actuators reaches a target value, which includes an accumulator configured to accumulate part of return fluid from the actuators, the accumulator being able to discharge the pressure fluid from the accumulator to a pressure-fluid-source-side flow path of the direction switching valve, and adjustment means configured to adjust a control amount of the pressure fluid source based on the pressure of the accumulator being further provided. According to the aforementioned feature of the present invention, the fluid circuit configured to make such control that the supply pressure of the pressure fluid source is constantly higher than the maximum load pressure of the multiple actuators by the target pressure difference can compensate for the output pressure of the pressure fluid source according to the pressure of the accumulator capable of discharging the pressure fluid to the pressure-fluid-source-side flow path of the direction switching valve. Thus, a high-energy-efficiency fluid circuit can be provided.

It may be preferable that the control amount is adjusted by the adjustment means when the pressure fluid is discharged from the accumulator to the pressure-fluid-source-side flow path of the direction switching valve. According to this preferable configuration, the output pressure of the pressure fluid source can be adjusted at proper timing, and therefore, a favorable energy efficiency is provided.

It may be preferable that the fluid circuit further includes pressure detection means configured to detect the pressure of the accumulator, and a control unit having an arithmetic circuit, the adjustment means being actuated by an electric signal output from the control unit based on the pressure detected by the pressure detection means. According to this preferable configuration, favorable adjustment means responsiveness is provided.

It may be preferable that the discharge amount control mechanism includes a load sensing valve configured to adjust an opening degree of the load sensing valve based on a difference between pressure-fluid-source-side pressure and actuator-side pressure of the direction switching valve guided by a pilot pipe line, and a pressure reduction valve as the adjustment means is provided at the pilot pipe line guiding the actuator-side pressure of the direction switching valve. According to this preferable configuration, the degree of opening of the load sensing valve can be adjusted based on the value obtained from the actuator maximum load pressure and the accumulator pressure, and a control amount by the discharge amount control mechanism can be adjusted by a simple circuit.

It may be preferable that a pressure reduction amount in the pressure reduction valve is adjusted based at least on the pressure-fluid-source-side pressure and the actuator-side pressure of the direction switching valve and the pressure of the accumulator. According to this preferable configuration, the pressure reduction amount in the pressure reduction valve can be adjusted based on the pressure-fluid-source-side pressure and the actuator-side pressure of the direction switching valve and the pressure of the accumulator, and therefore, the direction switching valve pressure difference can be quickly controlled to the target value.

