JPS642801B2 - - Google Patents

Description

[Detailed Description of the Invention] This invention is suitable for use in hydraulic excavators, cranes, cargo handling machines, etc., and can control the speed of the first and second actuators in a manner that is optimal for these machines, while reducing power loss. Regarding a fluid circuit that avoids

For example, as shown in FIG. 1, a hydraulic shovel includes trucks 2a and 2b driven by left and right hydraulic motors 1a and 1b, an upper revolving body 4 rotated by a hydraulic motor 3, and an upper revolving body 4, which is rotated by a hydraulic motor 3. A boom 6 that is pivotally attached to the body 4 and tilted by a boom cylinder 5; and an arm 8 that is pivotally attached to the boom 6 and pivoted by an arm cylinder 7.
A bucket cylinder 9 is pivotally connected to the arm 8.
It is equipped with a bucket 10 driven by. The main operations of this hydraulic excavator include excavation operations and crane operations. This excavation work
It consists of four operations: excavation, lifting rotation, earth removal, and lowering rotation. The lifting and turning operation is an operation of lifting and turning the bucket 10 from the excavation position onto the loading platform of a dump truck (not shown) that is stopped near the hydraulic excavator.
This can be represented by an operation in which the upper rotating body 4 is rotated by a hydraulic motor while being raised by a hydraulic motor. At this time, from the viewpoint of work efficiency, there is a demand that the boom 6 be raised quickly to the height of the loading platform of the dump truck, and from the viewpoint of safety, the upper revolving body 4 should be started slowly to avoid shock. That is, under the condition that the load pressure on the hydraulic motor 3 side is larger than the load pressure on the boom cylinder 5 side due to the large inertia of the upper revolving structure 4, a large amount of fluid is supplied to the boom cylinder 5, and the hydraulic motor 3
There is a request to supply a small amount of fluid to a device.
Further, the crane work described above involves hooking an extremely large heavy object (not shown) onto the tip of the bucket 10 and rotating at a slow speed while adjusting its height. This can be represented by an operation in which the upper revolving body 4 is rotated by the hydraulic motor 3. At this time, there is a demand for accurate speed control of the boom cylinder 5 and hydraulic motor 3, regardless of the magnitude of their loads, in order to easily align the heavy objects and from a safety standpoint. That is, under the condition that the load pressure on the boom cylinder 5 side is larger than the load pressure on the hydraulic motor 3 side that rotates at a very slow speed due to a heavy object, the hydraulic motor 3
There is a demand for ensuring safety by enabling accurate speed control of both the hydraulic motor 3 and the boom cylinder 5, without causing the speed of the boom cylinder 5 to become abnormally high due to the influence of the load pressure on the boom cylinder 5 side.

As mentioned above, as a conventional circuit that controls multiple actuators regardless of the load conditions, there is, for example, Japanese Patent Application Laid-Open No. 52-57478. A plurality of parallel main stream circuits connected to the discharge side of one variable type fluid pump are each provided with an actuator, and both main stream circuits have a constant differential pressure between the fluid pump and the actuator. Through the first pressure control section of the normally open type of the pressure compensation valve and the variable flow rate adjustment orifice, both main circuits control the second pressure of the normally open type of the pressure compensation valve between the actuator and the tank. While passing through the control section and the back pressure valve,
In front of the first pressure control section, a branch line branched from the main circuit is connected to a cylinder that operates a variable discharge amount control element of the fluid pump via a normally closed pressure control valve, which is an example of a power match valve. , a feedback passage branched from the main stream circuit between both the first pressure control parts and the actuator is connected to the back pressure chamber of the power match valve via the shuttle valve, and further connected to the secondary side of the both first pressure control parts. , a liquid replenishment circuit with a self-priming function is provided in each main circuit between the primary side of the back pressure valve and the primary side of the back pressure valve.

However, in this circuit, when driving multiple actuators simultaneously, for example, when driving one actuator at low pressure and large flow rate and the other actuator at high pressure and small flow rate, the spring chamber of the power match valve has the power selected by the shuttle valve. Maximum load pressure is applied. Therefore, the pump will be operated by the power match valve at the maximum load pressure and the total flow rate of both actuators, and if the power of this pump exceeds the capacity of the prime mover such as the engine, the prime mover will stop and multiple actuators will be operated. There are problems that are not only out of control, but also dangerous.

Therefore, an object of the present invention is to: When the load pressure of the first flow rate directional control valve that supplies fluid to the first actuator is greater than the load pressure of the second flow rate directional control valve that supplies fluid to the second actuator, By supplying a large amount of fluid to the second actuator and a small amount of fluid to the first actuator, the second actuator is operated quickly and the first actuator is operated slowly to improve work efficiency and safety, while the second actuator is operated quickly and the first actuator is operated slowly. The load pressure of the first flow rate directional control valve is
If the flow rate is less than the load pressure of the directional control valve,
pressure-compensating both the first and second flow rate directional control valves;
The speed of each of the first and second actuators can be accurately controlled without being affected by other load pressures, improving workability and safety. Then,
The variable pump that can change the control pattern and supplies fluid to the first and second actuators does not discharge wasteful fluid, causing no power loss, and furthermore, the required horsepower of the variable pump can be set. To provide a fluid circuit that can ensure safety without exceeding the horsepower, and therefore preventing the prime mover driving the variable pump from stopping and causing multiple actuators to become uncontrollable. There is a particular thing.

In order to achieve the above object, in the fluid circuit of the present invention, as illustrated in FIG. A second flow rate directional control valve with a load pressure sensing port, which is connected to the pump for pressure compensation and controls a second actuator, is connected between the variable pump and the pressure reduction type pressure compensation valve, and further, the pressure reduction type pressure compensation valve is connected to the variable pump and the pressure reduction type pressure compensation valve. The maximum pressure between the fluid pressure in the spring chamber of the valve and the fluid pressure in the load pressure sensing port of the second flow directional control valve is selected by the shuttle valve and transmitted to the spring chamber of the power match valve, while the pilot chamber of the power match valve is selected. The power match valve receives a signal indicating the discharge amount of the variable pump and a signal indicating the discharge pressure of the variable pump, and sets the required horsepower of the variable pump to a constant value. A constant horsepower control valve that controls the following:
connected in series between the discharge side of the variable pump and its discharge rate control section, so that the discharge rate control section can be switched and connected to the discharge side of the variable pump and the tank by switching the power match valve; The present invention is characterized in that the discharge amount control section can be switched and connected to the discharge side of the variable pump and the tank by switching the constant horsepower control valve.

The effect is described below.

When the load pressure of the first flow direction control valve is greater than the load pressure of the second flow direction control valve, the load pressure of the larger first flow direction control valve is applied to the spring chamber of the power match valve through the shuttle valve. Reportedly. At this time, the variable pump has the ability to drive multiple actuators. Since the second flow rate directional control valve has flow resistance, the pressure on the primary side (main line side) of the second flow rate directional control valve is higher than the pressure on the secondary side (load side) of the second flow rate directional control valve. Therefore,
Even if the load pressure of the first flow directional control valve is higher than the load pressure of the second flow directional control valve, the main line pressure is higher than the load pressure of the second flow directional control valve and the first flow directional control valve.
Since it is higher than the load pressure of the flow direction control valve,
A small amount of fluid is supplied to the first actuator and a large amount of fluid is supplied to the second actuator, so that the first actuator is operated slowly and the second actuator is operated quickly, thereby improving safety and work efficiency. When the load pressure of the first flow directional control valve is lower than the load pressure of the second flow directional control valve, the load pressure of the second flow directional control valve, which is higher, is applied through the shuttle valve in the spring chamber of the power match valve. Therefore, the second flow directional control valve is pressure compensated with a variable pump, and the first flow directional control valve is pressure compensated with a pressure reducing type pressure compensation valve. In this way, both the first and second flow direction control valves are pressure compensated,
The speeds of the first and second actuators are accurately controlled without being influenced by other load pressures, improving workability and safety, and the variable pump does not discharge wasteful fluid. No power loss. In this way, the control pattern is changed depending on the magnitude of the load pressure of the first and second flow direction control valves.

Furthermore, when the required horsepower of the variable pump exceeds a certain level, the constant horsepower control valve operates to guide fluid to the discharge flow control section of the variable pump to control the required horsepower of the variable pump so that it does not exceed the constant level. Therefore, the prime mover that drives the variable pump will not stop due to overload, and the actuator will not become uncontrollable, thus ensuring safety.

Hereinafter, an embodiment shown in FIGS. 2 and 3 in which the present invention is applied to a hydraulic excavator will be described in detail.

In FIG. 2, 21 is a boom pivotally connected to the revolving structure 22, 23 is an arm pivotally connected to the boom 21, 24 is a bucket pivotally connected to the arm 23,
25 is a boom cylinder as a second actuator that tilts the boom 21; 26 is the arm 2;
3, an arm cylinder as a second actuator for swinging the rotating body 22; 27, a bucket cylinder for driving the bucket 24; 28, a hydraulic motor as a first actuator for rotating the rotating structure 22;
29 is the hydraulic motor for left travel, 30 is the first one
Hydraulic motor for right travel as actuator, 3
Numerals 1 and 32 are valve units that control fluid to actuators such as the boom cylinders, and 33 is a pump unit that supplies fluid to the valve units 31 and 32.

The valve unit 31 includes a pressure reducing type pressure compensating valve 36.
, first flow rate directional control valves 37a, 37b, and second flow rate directional control valves 38a, 38b.

The first and second flow rate directional control valves 37a, 37
b, 38a, and 38b each have the functions of a directional control valve and a flow rate control valve, and furthermore, as shown in FIG. 3, they detect the pressure behind the flow rate adjustment section (not shown) via the feedback passage 41 at the switching position. Equipped with a load pressure sensing port d that can be used.

A first flow direction control valve 37 is connected to the secondary port 45 of the pressure reducing type pressure compensation valve 36 via a line 46.
The pump ports a and 37b are connected in parallel, and a main line 47 connected to the primary port 40 of the pressure reducing pressure compensation valve 36 is connected to each of the second flow rate directional control valves 38a and 38b via a line 48. Pump ports are connected in parallel.

The spring chamber of the pressure reducing type pressure compensation valve 36 includes a first
The maximum pressure among the detected pressures of each load pressure sensing port d of the flow direction control valves 37a and 37b is set to the shuttle valve 5.
0 is selected and transmitted, and the pressure compensation valve 36 is actuated to compensate for the pressure of the first flow direction control valve 37a or 37b having the maximum load pressure. I try to set it to a constant value that corresponds to the force.

On the other hand, the maximum pressure among the detected pressures of the load pressure sensing ports d and d of the second flow rate directional control valves 38a and 38b is selected by the shuttle valve 51 and transmitted to the port at one end of the shuttle valve 52. The pressure in the spring chamber of the pressure reducing type pressure compensating valve 36 is transmitted to the port at the other end of the valve 52, and the maximum pressure selected by the shuttle valve 52 is transferred from the central port to the line 5.
3 to the pump unit 33. In addition, the first and second flow rate directional control valves 37a, 37
b; When 38a and 38b are all in the neutral position, via the vent line 54 that passes through the vent passage 42 and load pressure sensing port d provided respectively,
The spring chamber of the pressure reducing type pressure compensating valve 36 and ports at both ends of the shuttle valve 52 communicate with the tank 55.

Each load port of the first flow direction control valve 37a is connected to the right travel hydraulic motor 30 via lines 61 and 62, respectively, and the first flow direction control valve 37b
Each load port is connected to the swing hydraulic motor 28 via lines 63 and 64, respectively. Further, each load port of the second flow rate directional control valve 38a is connected to the arm cylinder 26 via lines 65 and 66, respectively. A flow rate regulating valve 67 with a check, which serves as a meter-out restriction, is provided in the middle of the line 66. The line 65 communicates with the tank 55 via a check valve 68, and the line 66 communicates with the tank 55 via a relief valve 69. Each load port of the second flow rate directional control valve 38b is connected to the boom cylinder 25 via lines 70 and 71, respectively. A check valve 72 is provided in the middle of the line 70. The line 70 is connected to the tank 55 via a check valve 73, and the line 71
is communicated with the tank 55 via a relief valve 74.

On the other hand, the pump unit 33 includes two variable pumps 80a and 80b driven by an engine 79.
, power match valves 81a, 81b, and constant horsepower control valves 82a, 82b.

In order to match the discharge flow rate and discharge pressure of the variable pump 80a with the requirements of the flow rate directional control valve having the maximum load pressure, the power match valve 81
The center port of the shuttle valve 52 is connected to the spring chamber a through a line 53 with a throttle 85 installed in the middle, while the pilot chamber of the power match valve 81a is connected to a variable pump 80a and a pressure reducing type pressure compensating valve 3.
The pressure of the main line 47 between the power match valve 81a and the power match valve 81a is transmitted to operate the power match valve 81a. The fluid pressure corresponding to the spring force of the spring 94 of the power match valve 81a is
The fluid pressure corresponding to the spring force of the spring 49 is greater than the fluid pressure corresponding to the spring force of the spring 49. Further, the constant horsepower control valve 82a
A pressure signal from the main line 47 is transmitted to the pilot chamber of the pump, and a feedback signal from the swash plate indicating the discharge amount of the variable pump 80a is transmitted to the constant horsepower control valve 82a via a signal transmission path 86 such as a link mechanism, The constant horsepower control valve 82a is operated so that the required horsepower of the variable pump 80a, that is, the product of the discharge flow rate and the discharge pressure, is below a certain level. Port m of the constant horsepower control valve 82a and port Y of the power match valve 81a are connected to the main line 47, port l of the constant horsepower control valve 82a is connected to the tank 87, and port n of the constant horsepower control valve 82a is connected to the main line 47. It communicates with the port X of the power match valve 81a, and the port Z of the power match valve 81a communicates with a discharge amount control section 88 consisting of a swash plate control cylinder of the variable pump 80a. There is a check valve 9 in the middle between port n and port
The fluid from the constant horsepower control valve 82a is directly connected to the discharge amount control section 88 through this line 92 regardless of the symbol position of the power match valve 81a. We are making it possible to supply the following. The spring chamber of the power match valve 81a is communicated with a tank 95 via a pilot relief valve 93.

Power match valve 81 on the variable pump 80b side
b and the constant horsepower control valve 82b are the variable pump 8
0a side power match valve 81a and constant horsepower control valve 8
It is connected in exactly the same way as 2a.

The fluid circuit used in the hydraulic excavator having the above configuration operates as follows.

This fluid circuit is now in the state shown in FIGS. 2 and 3, and in this state, the first and second flow direction control valves 37
b, 38b are both switched to symbol position S ₁ ,
The hydraulic motor 28 rotates the revolving body 22, and the boom cylinder 25 is extended to rotate the boom 21.
Suppose that the above-mentioned lifting and turning of the excavation work is performed by raising the excavator.

At this time, due to the large inertia of the rotating body 22,
At startup, the pressure of the load line 63 of the first flow direction control valve 37b, that is, the pressure of its load pressure sensing port d, is equal to the load line 7 of the second flow direction control valve 38b.
0, that is, it is higher than the pressure at the load pressure sensing port d. The pressure at the load pressure sensing port d of the first flow rate directional control valve 37b, which is the maximum pressure, is selected by the shuttle valve 50 and
It is transmitted to the spring chamber of the pump, and also selected by the shuttle valve 52 and transmitted to the pump unit 33.

The pump unit 33 operates as described later to control the fluid so that the pressure in the main line 47 becomes a set pressure higher than the maximum pressure by a pressure corresponding to the spring force of the spring 94 of the power match valve 81a. Trying to spit it out.

On the other hand, the load pressure of the second flow direction control valve 38b is lower than the load pressure of the first flow direction control valve 37b, but the variable pump has the ability to drive multiple actuators, and the second flow direction control valve has the ability to drive multiple actuators. Because there is flow resistance, the pressure on the primary side (main line side) is lower than the pressure on the secondary side (load side) of the second flow rate directional control valve.
pressure increases. Therefore, even if the load pressure of the first flow directional control valve is higher than the load pressure of the second flow directional control valve, the main line pressure is higher than the load pressure of the second flow directional control valve and even the first flow directional control valve. higher than the load pressure. Therefore, a large amount of fluid in the main line 47 flows to the boom cylinder 25 via the line 48 and the second flow direction control valve 38b, causing the boom 21 to rise rapidly.
During that time, the pressure in the main line 47 is affected by the large amount of fluid flowing into the boom cylinder 25 and becomes below the set pressure.
6 is unable to compensate the pressure of the first flow rate directional control valve 37b, the flow rate supplied to the first flow rate directional control valve 37b and the hydraulic motor 28 decreases, the hydraulic motor 28 starts slowly, and the rotating body 22 The startup is smooth and there are no shocks. After that, when the boom 21 finishes rising to a predetermined position, the second flow direction control valve 38b is placed in the neutral position to cut off the fluid supplied to the boom cylinder 25, and the pressure in the main line 47 corresponds to the maximum load pressure. The set pressure is reached, and the first flow rate directional control valve 37b is pressure-compensated by the pressure reducing type pressure compensation valve 36 to flow fluid at a flow rate proportional to the opening degree of the flow rate adjustment section, thereby controlling the proportional speed of the hydraulic motor 28. Let's do it.

Therefore, in this lifting and turning operation, the boom 21 is raised quickly at the time of startup, and at the same time, the rotating structure 22 is operated slowly and without shock.
After that, the rotating body 22 is connected to the first flow direction control valve 37b.
Since the proportional speed can be controlled more accurately, a hydraulic excavator using this fluid circuit has excellent work efficiency and is safe.

Next, a heavy object (not shown) is hooked onto the bucket 24, and the first and second flow direction control valves 37b, 38 are
b is located at the symbol position _S1 , and while the height of the boom 21 is adjusted by the boom cylinder 25, the revolving structure 22 is rotated at a slow speed by the hydraulic motor 28 to perform the cracker work.

By the way, since the revolving body 22 rotates in the horizontal direction around the vertical axis, its own weight hardly affects its rotation. Therefore, the rotating body 22 is an inertial load, and when it starts, the first flow direction control valve 37b
However, as long as the rotating body 22 is driven at a slow speed after starting the rotating body 22, even if the inertia of the rotating body 22 is large, the load pressure of the first flow rate directional control valve 37b, that is, its load pressure sensing port d The pressure is not that great. On the other hand, the boom cylinder 25
is attached with an arm 23 and a bucket 24, and also hooks a heavy object and moves upward, so even if the weight of the heavy object is small, it will be affected by the weight of the boom 21, etc., and the second cylinder on the boom cylinder 25 side will The load pressure of the flow direction control valve 38b, that is, the pressure at its load pressure sensing port d, is increasing. in this way,
Since the rotating body 22 rotates horizontally even if its inertia is large, unless its acceleration is small or acceleration is not performed, the first flow rate directional control valve 37 for the rotating body 22
The pressure at the load pressure sensing port d of b is the second for the boom that moves upward and is always affected by its weight.
The pressure is lower than the pressure at the load pressure sensing port d of the flow direction control valve 38b. That is, in this operating state, the pressure at the load pressure sensing port d of the second flow rate directional control valve 38b is always higher than the pressure at the load pressure sensing port d of the first flow rate directional control valve 37b.

The load pressure of the second flow rate directional control valve 38b, which is the maximum pressure, is selected by the shuttle valves 51 and 52 and transmitted to the pump unit 33, which operates as described later to connect the main line 47 and The fluid is discharged so that the pressure in the line 48 becomes a set pressure higher than the maximum pressure by a pressure corresponding to the spring force of the spring 94 of the power match valve 81a.

Therefore, the differential pressure before and after the flow rate adjustment section of the second flow rate directional control valve 38b is equal to the pressure difference between the power match valve 81a
In other words, the second flow rate directional control valve 38b is pressure-compensated, flows a flow rate proportional to the opening degree, and operates the boom cylinder 25 accurately regardless of the magnitude of the load. Speed can be controlled.

On the other hand, since the load pressure of the first flow rate directional control valve 37b selected by the shuttle valve 50 is transmitted to the spring chamber of the pressure reducing type pressure compensating valve 36, the pressure reducing type compensating valve 36 The first flow direction control valve 37b is operated to compensate for the pressure of the first flow direction control valve 37b by making the spring force of the spring 49 correspond to the differential pressure before and after the flow rate adjustment section of the first flow direction control valve 37b. Therefore, the first flow rate directional control valve 37b accurately controls the proportional speed of the swing hydraulic motor 28, regardless of the magnitude of the load.

Therefore, in this crane work, both the first and second flow rate directional control valves 37b and 38b are pressure compensated, so that the boom cylinder 25 and the swing hydraulic motor 28 are not affected by other load pressures. Since the speed is accurately controlled without any movement, positioning can be performed accurately, easily, and safely.

During the above operation, the pump unit 33 operates as follows.

The constant horsepower control valve 82a uses a feedback signal indicating the flow rate transmitted from the swash plate of the variable pump 80a via the signal transmission path 86, and a pressure signal transmitted to the main line 47 to the pilot chamber.
When the required horsepower of the variable pump 80a is less than the set horsepower, the variable pump 80a is located at the symbol position _V2 , and the port X of the power match valve 81a is communicated with the tank 87. While controlling the discharge amount, if the required horsepower of the variable pump 80a is about to exceed the set horsepower, the symbol position
V ₁ , the fluid in the main line 47 is guided through the constant horsepower control valve 82a, and then directly through the check valve 91 and line 92 to the discharge amount control section 88, regardless of the symbol position of the power match valve 81a. The discharge amount of the variable pump 80a is reduced to prevent the required horsepower of the variable pump 80a from exceeding a set horsepower. Therefore, the engine 79 is not overloaded and the engine does not stall.

Note that the check valve 91 and line 92 may be removed.

On the other hand, the constant horsepower control valve 82a is at the symbol position.
V ₂ , the differential pressure between the maximum load pressure transmitted to the spring chamber of the power match valve 81a and the fluid pressure of the main line 47 causes the spring 9 of the spring chamber to
When the pressure exceeds the spring force of 4, the power match valve 81a is located at the symbol position _V1 ,
The fluid in the main line 47 is transferred to the power match valve 81.
a to the discharge rate control unit 88 through ports Y and Z of the variable pump 80a to reduce the discharge rate of the variable pump 80a and lower the pressure of the main line 47.
7 and the maximum load pressure becomes equal to or less than the pressure corresponding to the spring force of the spring 94, the power match valve 81a is located at the symbol position _V2 ,
The discharge amount control section 88 is communicated with the tank 87 via the power match valve 81a and the constant horsepower control valve 82a to increase the discharge amount of the variable pump 80a and increase the pressure of the main line 47.

Therefore, this pump unit 33 uses the constant horsepower control valve 82a to control the required horsepower of the variable pump to below the set horsepower, and uses the power match valve 81a to match the discharge flow rate and discharge pressure of the variable pump 80a to the maximum load, thereby eliminating waste. Since no fluid is discharged, there is no power loss and there is an energy saving effect. In other words, the heat balance is improved and the fuel consumption of the engine 79 is saved.

As is clear from the above description, the fluid circuit according to the present invention connects the first flow rate directional control valve with a load sensing port that controls the first actuator to the variable pump via the pressure reducing type pressure compensating valve to compensate for pressure. Meanwhile, a second flow rate directional control valve with a load pressure sensing port for controlling a second actuator is connected between the variable pump and the pressure reducing pressure compensating valve, and further controlling the fluid pressure in the spring chamber of the pressure reducing pressure compensating valve. and the fluid pressure of the load pressure sensing port of the second flow rate directional control valve is selected by the shuttle valve and transmitted to the spring chamber of the power match valve. convey the pressure of
Since the power match valve is actuated to control the discharge amount control section of the variable pump, the control pattern can be changed depending on the load pressure of the first and second flow direction control valves. ,
Variable pumps do not discharge unnecessary fluid.
That is, the load pressure of the first flow direction control valve is
If it is greater than the load pressure of the flow direction control valve,
By supplying a large amount of fluid to the second actuator and a small amount of fluid to the first actuator, the second actuator can be operated quickly and the first actuator can be operated slowly, resulting in excellent work efficiency and safety. The load pressure of the first flow rate directional control valve is
If the flow rate is less than the load pressure of the directional control valve,
pressure-compensating both the first and second flow rate directional control valves;
The speed of the first and second actuators can be controlled accurately without being affected by other load pressures, resulting in excellent workability and safety.Moreover, the variable pump does not discharge unnecessary fluid, saving energy. have an effect.

Further, the fluid circuit of the present invention includes a power match valve, a constant horsepower that controls the required horsepower of the variable pump to a certain value or less in response to a signal indicating the discharge amount of the variable pump, and a signal indicating the discharge pressure of the variable pump. A control valve is connected in series between the discharge side of the variable pump and its discharge rate control section, and the discharge rate control section is switched and connected to the discharge side of the variable pump and the tank by switching the power match valve. Along with making it possible,
By switching the constant horsepower control valve, the discharge amount control section can be connected to the discharge side of the variable pump and the tank, so that the required horsepower of the variable pump does not exceed a certain value. Safety is ensured because the prime mover does not stop due to overload and the actuator does not become uncontrollable.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram of a hydraulic excavator, FIG. 2 is a fluid circuit according to an embodiment of the present invention, and FIG. 3 is a partially enlarged view of FIG. 2. 21...Boom, 22...Swivel body, 25, 26
... Second actuator, 28, 30 ... First actuator, 36 ... Pressure reducing type pressure compensation valve, 37
a, 37b...first flow rate directional control valve, 38a, 3
8b...Second flow rate directional control valve, 52...Shuttle valve, 80a, 80b...Variable pump, 81a, 8
1b...Power match valve.

Claims (1)

[Claims] 1. The first flow direction control valve 37 with a load pressure sensing port that controls the first actuators 28, 30 is connected to the variable pump 80a via the pressure reduction type pressure compensation valve 36 for pressure compensation, while the second actuator 2
5, 26 is connected between the variable pump 80a and the pressure reducing type pressure compensating valve 36, and the fluid in the spring chamber of the reducing type pressure compensating valve 36 is connected between the variable pump 80a and the pressure reducing type pressure compensating valve 36. The maximum pressure of the pressure and the fluid pressure of the load pressure sensing port of the second flow rate directional control valve 38 is selected by the shuttle valve 52 and transmitted to the spring chamber of the power match valve 81a, while the pressure is reduced in the pilot chamber of the power match valve 81a. The pressure in front of the pressure compensation valve 36 is transmitted to the power match valve 8.
1a and a constant horsepower control valve 82a that receives a signal indicating the discharge amount of the variable pump 80a and a signal indicating the discharge pressure of the variable pump 80a and controls the required horsepower of the variable pump 80a to a certain value or less. The power match valve 81a is connected between the discharge side of the variable pump 80a and its discharge amount control section.
By switching the discharge amount control section to the variable pump 80
In addition, the discharge amount control section can be switchably connected to the discharge side of the variable pump 80a and the tank by switching the constant horsepower control valve 82a. fluid circuit.