JP2001020903A - Hydraulic circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce cost and minimize energy loss by eliminating the necessity of a pilot pump in a pilot circuit. SOLUTION: A main circuit 35 for driving a by draulic actuator 34 by controlling the oil discharged from a pump and a pilot circuit 36, which uses the pressure reduced a part of the oil discharged from the pump as a pilot pressure source, commonly use a same pump 33. The pilot circuit 36 has a pressure reducing valve 52, which controllably reduce the pressure of a part of the hydraulic fluid discharged from the pump 33 to the set pressure, and a load holding check valve 54 is arranged downstream of the pressure reducing valve 52 through a passage 53. An accumulator 56 is arranged for a passage downstream of the load holding check valve 54, and an operation valve 58 is also arranged. This operation valve 58 receives pilot primary pressure, which is controlled to be reduced by the pressure reducing valve 52, and outputs pilot secondary pressure according to operation quantity to the pilot operation section of the control valve 46 of the main circuit 35.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hydraulic circuit having a pilot circuit.

[0002]

2. Description of the Related Art FIG. 4 shows a hydraulic circuit of a construction machine such as a hydraulic excavator. A main circuit 12 for driving and controlling a hydraulic actuator 11 such as a hydraulic cylinder or a hydraulic motor is provided with a high-pressure circuit driven by an engine 13. The working pressure oil is supplied from the main pump 14 having a large capacity.

On the other hand, the control valve 15 controls the flow rate and the direction of pressure oil supplied to the hydraulic actuator 11 and return oil from the hydraulic actuator 11 to the tank. In order to control the stroke of the spool of the control valve 15, a pilot valve is used. A circuit 16 is provided.

The pilot circuit 16 normally supplies pilot pressure oil discharged from a low-pressure and small-capacity pilot pump 18 whose drive shafts are coupled to a main pump 14 via a coupling 17 by a pilot relief valve 19 and a pilot relief valve 19. The hydraulic remote control valve 22 is supplied via a pilot primary pressure line 21 having a load hold check valve 20 to a hydraulic remote control valve 22. The hydraulic remote control valve 22 is connected to a pilot operating section of the control valve 15 of the main circuit 12 via a pilot secondary pressure line 23. Is controlled according to the operation amount of an operation lever or the like. An accumulator 24 is provided on the primary side of the hydraulic remote control valve 22.

[0005]

However, while the operator is at rest, while the engine 13 is rotating,
Although the hydraulic remote control valve 22 of the pilot circuit 16 does not require any pilot pressure oil, the pilot pump 18 continues to discharge the pilot pressure oil and the pilot relief valve 19
The oil bleeds into the tank line at the set pressure, so that there is a problem that wasteful energy is wasted. In addition, the cost for installing the pilot pump 18 other than the main pump 14 is incurred.

As described above, conventionally, a pilot pump source for hydraulic equipment control requires a pilot pump 18 having a small capacity but relatively high cost, and related parts such as a coupling 17 thereof, and an engine. During the rotation of 13, there is a problem of energy loss in which energy is constantly consumed by the pilot pump 18.

[0007] The present invention has been made in view of such a point, and by eliminating the need for the conventional pilot pump and its related parts, the cost is reduced, and the pump is constantly consumed during rotation of the engine. The purpose is to eliminate energy loss.

[0008]

According to a first aspect of the present invention, there is provided a main circuit for driving a hydraulic actuator by controlling a working fluid discharged from a pump, and a pump for discharging a working fluid to the main circuit. A pilot circuit for supplying a pressure obtained by controlling the pressure of a part of the discharged working fluid to a main circuit as a pilot pressure source.

Further, since the same pump is used for the main circuit and the pilot circuit, the conventional pilot pump and its related parts are not required, the cost can be reduced, and the pilot pump is constantly consumed during engine rotation. Energy loss can be prevented, and energy can be saved.

According to a second aspect of the present invention, there is provided a pressure reducing valve in the fluid pressure circuit according to the first aspect, wherein the pilot circuit controls a pressure of a part of the working fluid discharged from the pump common to the main circuit to a set pressure. It is a fluid pressure circuit provided with.

The pressure reducing valve costs about the same as the pilot relief valve used in the conventional pilot circuit, and is incorporated in the pilot circuit in a manner to replace the pilot relief valve. The cost can be reduced by reducing the number of pilot pumps.

According to a third aspect of the present invention, in the hydraulic circuit of the second aspect, the pilot circuit is provided with a check valve provided downstream of the pressure reducing valve and an accumulator provided downstream of the check valve. And a control valve for receiving a pilot primary pressure smoothed by the accumulator and outputting a pilot secondary pressure corresponding to an operation amount to a main circuit.

The accumulator of the accumulator on the operating valve side of the check valve absorbs pressure fluctuations generated on the pressure reducing valve side or the operating valve side, protects the piping, and smoothes the stable operation of the pilot valve by the operating valve. Supply the next pressure.

According to a fourth aspect of the present invention, the pressure reducing valve in the fluid pressure circuit according to the second or third aspect is a pilot operated type pressure reducing valve.

Since the pilot operated pressure reducing valve has a wide pressure adjustment range, the pilot primary pressure supplied to the operation valve can be adjusted over a wide range.

According to a fifth aspect of the present invention, the pressure reducing valve in the fluid pressure circuit according to the second or third aspect is a direct-acting pressure reducing valve.

The direct-acting pressure reducing valve has a simple structure and hardly generates a drain flow, so that further energy saving can be achieved.

According to a sixth aspect of the present invention, a plurality of pumps in the fluid pressure circuit according to any one of the second to fifth aspects are installed in parallel, and a plurality of pumps are disposed between the plurality of pumps and the pressure reducing valve. This is a fluid pressure circuit provided with selection means for selecting a high-pressure pump discharge pressure and supplying it to a pressure reducing valve.

Even if the discharge pressure of one pump becomes low in relation to the main circuit, the working fluid discharged at a higher pressure from the other pump is supplied to the pilot circuit via the selection means. A sufficient supply pressure to the pilot circuit can be secured.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an embodiment of the present invention will be described with reference to a hydraulic circuit of FIG. 1, and another embodiment of the present invention will be described with reference to a hydraulic circuit of FIG. Another embodiment will be described with reference to the hydraulic circuit of FIG.

The hydraulic circuit shown in FIG. 1 is a high-pressure, large-capacity pump driven by an engine 31 and having a variable capacity means 32 such as a swash plate. (This pump corresponds to a conventional main pump. A main circuit 33), a main circuit 35 for controlling a hydraulic oil as a working fluid discharged from the main pump 33 to drive a hydraulic actuator 34 such as a hydraulic cylinder or a hydraulic motor. The main circuit 35 uses the pressure obtained by controlling the pressure of a part of the hydraulic oil discharged from the main pump 33 as a pilot pressure source.
And a pilot circuit 36 for pilot-controlling the hydraulic circuit.

In the main circuit 35, a discharge passage 41 is connected to a discharge port of the main pump 33, and the discharge passage 41 is connected to a tank.
42, a main relief valve 43 for setting an upper limit of the supply pressure to the main circuit 35 and the pilot circuit 36, and a common bypass valve 44 for gradually restricting the passing flow rate in connection with the capacity increase control of the main pump 33. Are provided respectively.

The discharge passage 41 is connected to a supply port of a pilot-operated control valve 46 via a load hold check valve 45. The output port of the control valve 46 is connected to the fluid pressure actuator 34, and the discharge port is connected to a tank.
42 connected to each other.

On the other hand, in the pilot circuit 36, a discharge passage 51 different from the discharge passage 41 to the main circuit 35 is connected to a discharge port of the main pump 33 common to the main circuit 35. Is connected to a pilot-operated pressure reducing valve 52 that controls the pressure of a part of the hydraulic oil discharged from the main pump 33 to a set pressure, and loads the valve as a check valve downstream from the pressure reducing valve 52 via a passage 53. A hold check valve 54 is provided, an accumulator (pressure accumulator) 56 is provided for a passage 55 downstream of the load hold check valve 54, and the passage 55 is a passage for an external pipe.
In a construction machine such as a hydraulic excavator through 57, the hydraulic control valve is connected to an operation valve 58 called a hydraulic remote control valve.

The operation valve 58 receives the pilot primary pressure smoothed by the accumulator 56 and outputs a pilot secondary pressure corresponding to an operation amount to a pilot operation section of the control valve 46 of the main circuit 35 through a pilot passage 59. I do.

The structure of the pressure reducing valve 52 will be described.
A spool fitting hole 62 is formed in 61, and this spool fitting hole
A pressure reducing spool 63 is fitted to 62 so as to be slidable in the axial direction. On the inner peripheral surface of the spool fitting hole 62, an inlet annular groove 64 communicated with the discharge passage 51 on the hydraulic oil supply side, and a load hold check valve below the inlet annular groove 64 in the drawing.
An outlet annular groove 65 communicated with the passage 53 on the side of the 54 is formed.

At a relatively upper part of the pressure reducing spool 63, one land portion 66 is formed with a large diameter, and a spring chamber 67 is defined above the land portion 66.
A compression coil-shaped main spring 68 is provided in 67,
The main spool 68 biases the pressure reducing spool 63 downward in the figure.

A communication groove 69 having a small diameter is formed on the lower side of one land 66 over the entire circumference of the decompression spool 63. Below the communication groove 69, the inlet annular groove 64 is formed. The other land portion 72 for controlling the opening degree of the communicating restriction portion 71 is formed with a large diameter.

Further, the pressure reducing spool 63 has a path for guiding the pump discharge pressure introduced into the outlet annular groove 65 from the inlet annular groove 64 through the communication groove 69 to the spring chamber 67. Is provided.

That is, the axial groove 73 formed in the lower part of the land 72, the radial groove 75 formed in the disc 74 at the lower end of the land 72, and the substantially entire length of the pressure reducing spool 63. A hole 76 formed at the center thereof, and an orifice 77 formed at the upper end of the hole 76 and opened to the spring chamber 67 are continuously provided.

The spring chamber 67 can communicate with the tank 42 via a drain chamber 81 formed on the upper side in the figure and a passage 83 by a drain hole 81 formed in the valve body 61. A pilot poppet 84 is disposed so as to be in contact with the opening edge of the upper end of the drain hole 81 in the figure so as to be able to freely contact and separate.

An adjuster screw 85 is screwed into the valve body 61 above the drain chamber 82 in the figure, and the adjuster screw 85 is fixed by tightening a nut 86 screwed with the adjuster screw 85. A compression coil-shaped pilot spring 89 is fitted between a protrusion 87 projecting from the lower side of the adjuster screw 85 and a protrusion 88 projecting from the upper side of the pilot poppet 84 in the figure. The pilot poppet 84 is pressed against the opening edge at the upper end of the drain hole 81 in the figure by the pilot spring 89 whose compression length, that is, the initial load is adjusted by the screw 85.

As described above, the pressure reducing valve 52 is a pressure reducing spool that controls the pressure of the high pressure oil supplied from the main pump 33.
A pressure reduction controller 91 composed of 63 and a main spring 68, a pilot poppet 84, an adjuster screw
A pilot pressure regulator 92 comprising a pilot spring 85 and a pilot spring 89 for pilot-controlling the pressure-reducing spool 63 to reduce the secondary pressure of the pressure-reducing valve in the passage 53 to a constant value (set pressure value). It is a type pressure reducing valve and a secondary pressure constant type pressure reducing valve.

The structure of the load hold check valve 54 will be described. A check valve poppet 94 is slidably fitted in a check valve fitting hole 93 formed in the valve body 61, and is fitted to the valve body 61. It is freely movable toward and away from the formed sheet portion 95.

A spring chamber 96 is formed on the right side of the check poppet 94 in the figure, and a compression coil spring 97 is provided in the spring chamber 96. One end of the compression coil spring 97 is fitted into the concave portion 98 of the check poppet 94, and the other end is the convex portion 10 of the plug 99 screwed to the valve body 61.
Fitted on 1.

The check poppet 94 of the load hold check valve 54 has a seat portion in close contact with the seat portion 95.
A front end closing portion 103 for closing the inlet opening 102 of the nozzle 95 is provided, and an oblique hole communicating the outlet opening 104 provided on the upper side of the check valve fitting hole 93 in the drawing with the spring chamber 96.
The oblique hole 105 guides the load pressure of the downstream passage 55 to the spring chamber 96.

When the force for pressing the check poppet 94 with the pressure generated in the upstream passage 53 exceeds the repulsive force of the compression coil spring 97 and the drag force due to the load pressure of the downstream passage 55, the check is stopped. The poppet 94 is lifted from the seat portion 95, and the inlet opening 102 of the seat portion 95 is moved to the outlet opening 104.
Communicate with

As described above, in this circuit configuration, the pilot pump 18 (FIG. 4) required in the conventional circuit is omitted, and the discharge passage 51 of the high-pressure large-capacity main pump 33 is omitted.
A pressure reducing valve 52 capable of adjusting the secondary pressure is installed in the
Thus, the low-pressure pilot pressure oil controlled to be reduced in pressure can be supplied to the operation valve 58 via the load hold check valve 54 and the accumulator 56.

Next, the operation of the embodiment shown in FIG. 1 will be described.

In the circuit example shown in FIG. 1, the conventional low pressure pilot pump is omitted, and a pressure reducing valve 52 capable of adjusting the secondary pressure is provided in the discharge passage 51 of the high pressure and large capacity main pump 33. Then, a part of the pump discharge pressure oil discharged from the main pump 33 is pressure-reduced by the pressure-reducing valve 52, and a substantially constant pressure-reducing valve secondary pressure adjusted by the adjuster screw 85 is output to the passage 53, and further loaded. The required pilot primary pressure is supplied to the operation valve 58 by the passages 55 and 57 via the hold check valve 54.

At this time, the pressure fluctuation generated on the pressure reducing valve 52 side and the pressure fluctuation generated by the operation of the operation valve 58, etc., due to the pressure accumulating action of the accumulator 56 installed on the operation valve 58 side from the load hold check valve 54 are reduced. The absorption protects the piping and supplies a smooth and stable pilot primary pressure to the operation valve 58.

The pressure reducing function of the pressure reducing valve 52 is performed by a main pump.
33, the pressure in the discharge passage 51 is reduced to
4, through the communication groove 69, the outlet annular groove 65, the axial groove 73, the radial groove 75, the central hole 76 and the orifice 77 at the lower part of the pressure reducing spool 63, and the drain hole from the spring chamber 67. Acting on the pressure receiving part of the pilot poppet 84 fitted to the tip 81, the force generated by this pressure
When the biasing force of the pilot spring 89 adjusted by 85 is overcome, the pilot poppet 84 opens, and a small amount of oil flows through the orifice 77 of the pressure reducing spool 63, and a differential pressure is generated around the orifice 77. I do.

That is, the pressure in the spring chamber 67 is lower than the pressure acting on the lower surface of the pressure reducing spool 63. Due to this pressure difference, the pressure reducing spool 63 overcomes the urging force of the main spring 68 and slides upward in the figure, so that the area of the restriction portion 71 decreases, and the high pressure from the discharge passage 51 is reduced to the passage 53. Supplied.

As described above, when the pressure in the passage 53 is reduced by the lifting operation of the pressure reducing spool 63 upward in the figure, the pressure is reduced to the lower grooves 73 and 75 and the central hole 76 of the pressure reducing spool 63.
Is fed back to the spring chamber 67 through the orifice 77 and acts on the pressure receiving portion at the tip of the pilot poppet 84.Since the pressure is reduced, the pilot poppet 84 closes and there is no flow through the orifice 77. The pressure of the pressure reducing spool 63 rises and returns again, and the pressure reducing spool 63 returns slightly downward in the figure due to the urging force of the main spring 68 and the pressure of the spring chamber 67, and the opening of the restriction portion 71 is just balanced, that is The equilibrium point at which the pressure in the passage 53 becomes a value adjusted by the adjuster screw 85 is maintained.

Therefore, the conventional pilot pump 18 (FIG. 4)
Thus, an alternative pilot hydraulic power source substantially equivalent to the above can be provided, and energy loss constantly consumed by the pilot pump 18 can be prevented.

As described above, since the same main pump 33 is shared by the main circuit 35 and the pilot circuit 36, the conventional pilot pump 18 and its related parts are not required.
The cost can be reduced, and the energy loss that is always consumed by the pilot pump 18 during the rotation of the engine can be prevented, and the energy can be saved.

The pressure reducing valve 52 is a pilot relief valve used in the conventional pilot circuit 16 (FIG. 4).
The cost is almost the same as that of the pilot relief valve 19, and the pilot relief valve 19 is incorporated in the pilot circuit 36 in such a manner that the pilot relief valve 19 is replaced.

Further, the load hold check valve 54
By accumulating the pressure of the accumulator 56 provided on the operation valve 58 side, the pressure fluctuation generated on the pressure reducing valve 52 side or the operation valve 58 side is absorbed, thereby protecting the piping and smoothing the operation valve 58. A stable pilot primary pressure can be supplied.

In addition, the pilot operated pressure reducing valve 52 having the pressure adjusting section 92 composed of the pilot poppet 84, the adjuster screw 85, the pilot spring 89 and the like has a wide pressure adjustment range. The pilot primary pressure to be controlled can be adjusted over a wide range.

FIG. 2 shows a hydraulic circuit using a direct-acting pressure reducing valve 52a. The structure of the pressure regulating section is different from that of the pilot operated pressure reducing valve 52 shown in FIG. Poppet 84 and said pilot spring 89
This is a full-pressure type pressure reducing valve that adjusts the secondary pressure of the pressure reducing valve in the passage 53 by directly adjusting the load of the main spring 68 that urges the pressure reducing spool 63 by the adjuster screw 85 while omitting the same. The spring chamber 67 is communicated with the tank 42 by a passage 83 so that pressure other than that of the main spring 68 does not act. The same parts as those of the pressure reducing valve 52 shown in FIG. 1 are denoted by the same reference numerals, and description thereof will be omitted.

The pressure reducing function of the pressure reducing valve 52a is such that the pressure in the discharge passage 51 of the main pump 33 is reduced by the inlet annular groove 64, the communication groove 69, the outlet annular groove 65, and the groove 73 of the pressure reducing valve 52a. And acts on the lower surface of the decompression spool 63, and by this pressure, the decompression spool 63 slides upward in the drawing against the urging force of the main spring 68.
The area of 71 decreases, and the high pressure from the discharge passage 51 is reduced and supplied to the passage 53.

As described above, when the pressure in the passage 53 decreases due to the lifting operation of the pressure reducing spool 63 upward in the figure, the pressure reduction is fed back to the lower surface of the pressure reducing spool 63 through the groove 73, and the pressure decreases. As a result, the pressure reducing spool 63 slightly returns downward in the figure, and the position where the opening of the restriction portion 71 is exactly balanced, that is, the pressure of the passage 53 is reduced by the biasing pressure of the main spring 68 adjusted by the adjuster screw 85. The equilibrium point at which the value is balanced is maintained.

The direct-acting pressure reducing valve 52a has a simple structure and does not generate a drain flow, so that energy can be saved. That is, the embodiment shown in FIG. 2 is different from the pilot operated pressure reducing valve 52 shown in FIG.
Since the pressure reducing valve 52a of the full-pressure type in which the pilot stage is omitted as in the pressure regulating section 92 is used, the drain at the pilot poppet 84 can be eliminated, and the energy can be further saved as compared with the embodiment shown in FIG. Can be achieved.

Next, FIG. 3 shows the pressure reducing valve 52a shown in FIG.
In contrast, a plurality of high-pressure and large-capacity main pumps 33a,
33b shows a hydraulic circuit of a type provided in parallel, and a selecting means for selecting a high-pressure side pump discharge pressure and supplying it to the pressure reducing valve 52a between these main pumps 33a, 33b and one pressure reducing valve 52a. This is an example in which check valves 111 and 112 are provided for each of the main pumps 33a and 33b.

That is, even if the discharge pressure of one of the main pumps 33a, 33b is low, if the discharge pressure of the other pump is high, the discharge passage 51 is passed through the check valve 111 or 112 of the high pressure side pump discharge section. High pressure oil is supplied to the pressure reducing valve 52a, the primary pressure of the pressure reducing valve 52a becomes lower than the secondary pressure (set pressure) of the pressure reducing valve 52a and the supply pressure to the operation valve 58 cannot be sufficiently secured. Circuit frequency. The main relief valve 43
Is provided in a passage 113 branched from the secondary side of these check valves 111 and 112 to the tank 42, and is not installed one by one in each of the main pumps 33a and 33b, but one main relief valve 43. Is shared.

For example, even if the discharge pressure of one main pump 33b, which is normally high pressure and large capacity, becomes low in relation to the operating condition of the fluid pressure actuator of the main circuit 35, the other main pump 33a Therefore, the hydraulic oil discharged at a higher pressure is selected by the check valve 111 and supplied to the discharge passage 51, so that a sufficient supply pressure to the pilot circuit 36 can be secured.

As a selecting means for selecting a high-pressure pump discharge pressure and supplying it to the pressure reducing valve 52a, a single shuttle valve may be used instead of the two check valves 111 and 112. Further, a pilot operated pressure reducing valve 52 may be used instead of the directly operated pressure reducing valve 52a.

Finally, each of the embodiments shown in FIGS. 1 to 3 is preferably applied to a pilot hydraulic source for controlling hydraulic equipment in a hydraulic circuit of a construction machine such as a hydraulic shovel. The present invention is not limited to this, and can be applied to a general pilot pressure source.

[0059]

According to the first aspect of the present invention, the same pump is shared by the main circuit and the pilot circuit, so that the conventional pilot pump and its related parts are not required, and the cost can be reduced. In addition, it is possible to prevent energy loss that has been constantly consumed by the pilot pump while the engine is rotating, thereby achieving energy saving.

According to the second aspect of the present invention, the pressure reducing valve provided in the pilot circuit costs almost as much as the pilot relief valve used in the conventional pilot circuit. Therefore, the cost can be reduced by reducing the number of conventional pilot pumps.

According to the third aspect of the present invention, by accumulating the pressure of the accumulator on the operation valve side of the check valve, the pressure fluctuation generated on the pressure reducing valve side or the operation valve side can be absorbed, thereby protecting the piping and operating the pipe. A stable pilot primary pressure can be supplied to the valve by smoothing.

According to the fourth aspect of the present invention, since the pilot operated pressure reducing valve has a wide pressure adjustment range, the pilot primary pressure supplied to the operation valve can be adjusted over a wide range.

According to the fifth aspect of the present invention, the direct-acting type pressure reducing valve has a simple structure and does not generate a drain flow, so that further energy saving can be achieved.

According to the sixth aspect of the present invention, since the selecting means for selecting the highest pump discharge pressure and supplying it to the pressure reducing valve is provided between the plurality of pumps and the pressure reducing valve, the discharge of one pump is reduced. Even if the pressure becomes low in relation to the main circuit,
The working fluid discharged from another pump at a higher pressure can be supplied to the pilot circuit via the selection means, and a sufficient supply pressure to the pilot circuit can be secured.

[Brief description of the drawings]

FIG. 1 is a hydraulic circuit diagram and a sectional view showing an embodiment of a fluid pressure circuit according to the present invention.

FIG. 2 is a hydraulic circuit diagram and a sectional view showing another embodiment of the fluid pressure circuit according to the present invention.

FIG. 3 is a hydraulic circuit diagram and a sectional view showing still another embodiment of the fluid pressure circuit according to the present invention.

FIG. 4 is a hydraulic circuit diagram showing a conventional fluid pressure circuit.

[Explanation of symbols]

 33, 33a, 33b Pump 34 Fluid pressure actuator 35 Main circuit 36 Pilot circuit 52, 52a Pressure reducing valve 54 Check valve 56 Accumulator 58 Operating valves 111, 112 Check valve as selection means

Claims (6)

[Claims] 1. A main circuit for controlling a working fluid discharged from a pump to drive a fluid pressure actuator, and a pressure for controlling a pressure of a part of the working fluid discharged from the pump for discharging the working fluid to the main circuit. And a pilot circuit for supplying the main circuit as a pilot pressure source to the main circuit. 2. The fluid pressure circuit according to claim 1, wherein the pilot circuit includes a pressure reducing valve that controls a pressure of a part of the working fluid discharged from a pump common to the main circuit to a set pressure. 3. The pilot circuit is operated by receiving a check valve provided downstream of the pressure reducing valve, an accumulator provided downstream of the check valve, and a pilot primary pressure smoothed by the accumulator. 3. The fluid pressure circuit according to claim 2, further comprising an operation valve that outputs a pilot secondary pressure according to the amount to the main circuit. 4. The fluid pressure circuit according to claim 2, wherein the pressure reducing valve is a pilot operated type pressure reducing valve. 5. The fluid pressure circuit according to claim 2, wherein the pressure reducing valve is a directly operated pressure reducing valve. 6. A plurality of pumps are provided in parallel with each other, and a selecting means is provided between the plurality of pumps and the pressure reducing valve to select the highest pump discharge pressure and supply the selected pressure to the pressure reducing valve. The fluid pressure circuit according to any one of claims 2 to 5, wherein: