JP2008169981A - Hydraulic circuit of construction machine with boom - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a boom-fitted construction machine equipped with a safety measure circuit to be shut off by throttling a return oil in a boom-down line when a hydraulic hose is broken and an energy collecting circuit to make energy collection from the return oil in the boom-down line, whereby an opening in the safety measure circuit is widened to ensure energy collection as long as the frame holding pressure keeps a proper level in case the frame operating amount is small to make the opening in the safety measure circuit remain throttled. <P>SOLUTION: The arrangement of this hydraulic circuit includes a pressure sensor 50 to sense the frame holding pressure and a function releasing circuit 40 to work complying with the boom-down pilot pressure P and the boom holding pressure and lead out the operating pressure to the safety measure circuit 30 so as to open the throttle, and in case the boom holding pressure keeps a prescribed level or above when the boom-down operation takes place, the operating pressure from the function releasing means 40 is heightened so as to widen the throttle opening of the safety measure circuit 30. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a hydraulic circuit for a construction machine with a boom, and particularly, in a construction machine having a safety measure circuit that squeezes and shuts off return oil in a boom lowering line when a hydraulic hose is broken, when the boom holding pressure is appropriate. Further, the present invention relates to a hydraulic circuit for a construction machine with a boom that can regenerate energy by opening a throttle of a safety countermeasure circuit even when the amount of boom lowering operation is small.

  Conventionally, in a construction machine with a boom such as a hydraulic excavator or a hydraulic crane, the discharge side when the hydraulic cylinder (hydraulic cylinder) falls freely as a safety measure circuit to squeeze and shut off the return oil of the boom lowering line when the hydraulic hose breaks A switching valve with a detent and a throttle are arranged in series, and a hydraulic cylinder using a switching valve that is normally in the operating position and switches to the stop position when the differential pressure across the throttle exceeds a predetermined value. A fall prevention circuit is known (see, for example, Patent Document 1).

Further, the regenerative circuit that regenerates energy from the return oil of the boom lowering line includes a rotating machine (pump motor) that collects return oil from a plurality of hydraulic actuators and is driven to rotate by the recovered return oil. A hydraulic regenerative circuit, a guide oil passage for guiding return oil from the meter-out side of each hydraulic actuator to the hydraulic regeneration circuit, a switching means (switching valve) for switching between opening and closing of the guide oil passage, and a back pressure of each hydraulic actuator. There is known a hydraulic circuit of a construction machine provided with a pressure detecting means (pressure sensor) for detecting and a generator for generating electric power by the rotational force from the rotating machine (see, for example, Patent Document 2).
Japanese Patent Laid-Open No. 57-15104 JP 2004-11168 A

  The invention described in Patent Document 1 is a free fall prevention circuit for a hydraulic cylinder when a hydraulic hose is broken, and there is no description about energy regeneration.

  In the invention described in Patent Document 2, return oil from a plurality of actuators is efficiently recovered by two sets of hydraulic energy recovery means depending on the back pressure level, and energy recovery is performed. There is no description of the safety circuit in case of damage.

  Even if a regenerative circuit is provided in the discharge side oil passage of the hydraulic cylinder according to claim 1, the flow rate can be suppressed by the throttle provided on the discharge side. Therefore, when the operation amount of the hydraulic cylinder is small, the boom The potential energy is lost as a pressure loss of the throttle, and energy regeneration cannot be performed.

  Therefore, in a construction machine with a boom equipped with a safety measure circuit that squeezes and shuts off the return oil of the boom lowering line when the hydraulic hose is broken, the boom operation amount is small in a construction machine with a boom equipped with a regenerative circuit that regenerates the energy of the boom lowering line return energy. If the boom opening pressure is appropriate and the boom holding pressure is appropriate, there is a technical problem to be solved in order to increase the safety circuit opening to enable energy regeneration. Therefore, an object of the present invention is to solve this problem.

  The present invention has been proposed in order to achieve the above object, and the invention according to claim 1 is provided with a safety countermeasure circuit that squeezes and shuts off the return oil of the boom lowering line when the hydraulic hose is broken, and the boom lowering line. In a boom-equipped construction machine equipped with a regenerative circuit that regenerates the return oil of the boom, the boom holding pressure detecting means for detecting the boom holding pressure, and the operation to the safety countermeasure circuit corresponding to the boom lowering pilot pressure and the boom holding pressure And a function release circuit that opens the throttle by deriving the pressure, and when the boom holding pressure is equal to or higher than a predetermined value during the boom lowering operation, the operation pressure from the function release means is increased to reduce the throttle of the safety countermeasure circuit A hydraulic circuit for a construction machine with a boom, characterized in that the opening of the boom is enlarged.

  According to this configuration, when the boom lowering pilot pressure is generated, the operation pressure is derived from the function release circuit to open the throttle of the safety countermeasure circuit. At this time, if the boom holding pressure is greater than or equal to the predetermined value, the flow of return oil in the boom lowering line is increased by deriving a high operating pressure from the function release circuit and increasing the throttle opening of the safety countermeasure circuit. Then, energy regeneration is performed in the regeneration circuit.

  According to a second aspect of the present invention, the function release circuit responds to a first switching valve that introduces return oil of the boom lowering line by adding a boom lowering pilot pressure, and a boom holding pressure introduced from the first switching valve. The boom lowering pilot pressure or a second switching valve for deriving the operating pressure to the safety countermeasure circuit by passing either the pressure oil higher than the boom lowering pilot pressure or the oil pressure passage by the poppet 2. A construction with a boom according to claim 1, further comprising: a holding valve that restricts and shuts off the valve, and a switching valve that opens the throttle of the holding valve as the operating pressure introduced from the function release circuit increases. Provide the hydraulic circuit of the machine.

  According to this configuration, during the boom lowering operation, the first switching valve of the function release circuit is switched by applying the boom lowering pilot pressure, and the return oil of the boom lowering line is introduced into the function release circuit. When the boom holding pressure of the boom lowering line introduced from the first switching valve is a low pressure that is less than a predetermined value, the boom lowering pilot pressure passes through the second switching valve of the function release circuit and is used as an operating pressure. When the boom holding pressure is equal to or higher than a predetermined value, the second switching valve is switched, and pressure oil higher than the boom lowering pilot pressure is led to the safety countermeasure circuit as the operating pressure.

  The safety control circuit switching valve opens the throttle of the holding valve. When the operating pressure of the function release circuit is a low pressure lower than the specified value, the holding valve is closed to shut off the return oil from the boom lowering line. When the operating pressure of the function release circuit becomes higher than a predetermined value, the holding valve is opened and the return oil of the boom lowering line is allowed to pass.

  The present invention provides a function release circuit that opens the throttle by deriving the operation pressure to the safety measure circuit, and if the boom holding pressure is greater than or equal to a predetermined value during the boom lowering operation, the operation pressure from the function release means is increased. Since the throttle of the safety countermeasure circuit is configured to open, the flow rate of return oil in the boom lowering line increases. Accordingly, even when the boom lowering operation amount is small, the pressure loss at the throttle is reduced, and the energy regeneration can be performed efficiently by the regeneration circuit.

  Hereinafter, the hydraulic circuit of the construction machine with a boom according to the present invention will be described with reference to preferred embodiments. In order to achieve the purpose of enlarging the safety measure circuit opening to enable energy regeneration if the boom operation amount is small and the safety measure circuit opening is restricted and the boom holding pressure is appropriate Further, the present invention provides a boom holding pressure detecting means for detecting a boom holding pressure, and a function release circuit for deriving an operating pressure to the safety countermeasure circuit and opening the throttle corresponding to the boom lowering pilot pressure and the boom holding pressure. When the boom holding pressure is greater than or equal to a predetermined value during the boom lowering operation, the operating pressure from the function release means is increased to increase the aperture of the throttle of the safety countermeasure circuit.

  FIG. 1 shows a hydraulic excavator 10 as an example of a construction machine with a boom, and an upper swing body 13 is mounted on a lower traveling body 11 via a swing mechanism 12 so as to be rotatable. The upper swing body 13 is provided with a cab 14 on one front side thereof, and a boom 15 is attached to the front center portion so as to be able to be raised and lowered. Further, an arm 16 is attached to the tip of the boom 15 so as to be rotatable up and down, and a bucket 17 is attached to the tip of the arm 16.

  Further, an intermediate portion between the upper swing body 13 and the boom 15 is connected by a boom cylinder 18, and when the boom cylinder 18 is extended when pressure oil is supplied from a hydraulic circuit to be described later, the boom 15 is raised and the boom cylinder 18 is contracted. The boom 15 is lowered.

  FIG. 2 shows a hydraulic circuit that drives the boom cylinder 18. The pressure oil discharged from the hydraulic pump 19 is controlled in flow rate and direction by the control valve 20, and passes through the oil passage L <b> 1 or the oil passage L <b> 2. It is supplied to the rod side 18a or the bottom side 18b. The oil passage L2 on the bottom side is provided with a safety countermeasure circuit 30, a function release circuit 40, a pressure sensor 50 as a boom holding pressure detecting means, and a pilot operated check valve 51. Further, the check valve 51 A regeneration circuit 60 is provided by branching a regeneration oil passage L3 from between the secondary ports of the control valve 20.

  The safety countermeasure circuit 30 includes a holding valve 32 that restricts and shuts off the oil passage L2 by a poppet 31, and a switching valve 33 that opens a throttle of the holding valve 32. ) Position. As will be described later, when the operation pressure from the function release circuit 40 is introduced into the safety countermeasure circuit 30 through the oil passage L4, the switching valve 33 is switched from the (d) position to the (e) position, When the operating pressure from the function release circuit 40 increases, the position is changed to the (f) position, and the throttle of the holding valve 32 is opened.

  On the other hand, the function release circuit 40 includes a first switching valve 41 for introducing return oil of the boom lowering line when the boom lowering pilot pressure P is applied, and a boom holding pressure introduced from the first switching valve 41. Accordingly, the boom lowering pilot pressure P or the pressure oil from the hydraulic pressure source 22 higher than the boom lowering pilot pressure P is passed through the oil passage L4 to derive the operating pressure to the safety countermeasure circuit 30. And a switching valve 42.

  The regenerative circuit 60 includes an energy regenerative hydraulic motor 61, a generator 62 driven by the rotation of the hydraulic motor 61, an on-off valve 63 that opens and closes an oil passage L3 to the regenerative circuit 60, and the open / close The solenoid valve 64 supplies or shuts off the pressure oil from the hydraulic source 22 to the pilot control unit of the valve 63.

  Next, the operation of the hydraulic circuit will be described. As shown in FIG. 2, when the control valve 20 is in the neutral position (A), the oil passages L <b> 1 and L <b> 2 are closed, and the oil discharged from the hydraulic pump 19 returns to the tank 21. Therefore, the boom cylinder 18 is in a state in which expansion and contraction is fixed.

  Since the boom lowering pilot pressure P is not generated when the control valve 20 is in the neutral position (A), the first switching valve 41 of the function release circuit 40 is in the (G) position and the second switching valve 42 is in the (R) position. Therefore, the operating pressure is not derived from the function release circuit 40 to the oil passage L4. Therefore, the switching valve 33 of the safety countermeasure circuit 30 is in the (d) position, and the holding valve 32 is closed by the bias of the spring 34. In addition, since the electromagnetic valve 64 of the regenerative circuit 60 is off and the on-off valve 63 is in the (le) position, the hydraulic motor 61 does not rotate and power generation by the generator 62 is not performed.

  At the time of boom raising operation, as shown in FIG. 3, the control valve 20 is switched to the (B) position by the boom raising pilot pressure, the oil discharged from the hydraulic pump 19 is led to the oil passage L2, and the check valve 51 and The holding valve 32 of the safety countermeasure circuit 30 is pushed open and supplied to the bottom side 18 b of the boom cylinder 18. The pressure oil on the rod side 18a of the boom cylinder 18 returns to the tank 21 from the (B) position of the control valve 20 through the oil passage L1. Thus, the boom cylinder 18 extends and the boom 15 rises.

  On the other hand, during the boom lowering operation, as shown in FIG. 4, the control valve 20 is switched to the position (c) by the boom lowering pilot pressure P, and the discharged oil of the hydraulic pump 19 passes through the oil passage L1. It is supplied to the rod side 18a of the boom cylinder 18. The pressure oil on the bottom side 18b of the boom cylinder 18 is squeezed by the holding valve 32 of the safety countermeasure circuit 30 and led to the oil passage L2, and passes through the check valve 51 opened by the action of the boom lowering pilot pressure P. The control valve 20 returns to the tank 21 from the (c) position. Thus, the boom cylinder 18 contracts and the boom 15 descends.

  Here, the operation of the safety countermeasure circuit 30 and the function release circuit 40 during the boom lowering operation will be further described.

[A1] When the boom lowering pilot pressure P is less than the predetermined value Even if the boom lowering operation amount is small and the boom lowering pilot pressure P is less than the predetermined value, as shown in FIG. The first switching valve 41 of the release circuit 40 is switched to the (h) position, and the return oil of the boom lowering line is introduced into the function release circuit 40 from the oil passages L2 and L5.

  (A1-1) At this time, when the hydraulic pressures of the return oil passages L2 and L5 are low pressures less than a predetermined value, the pilot control unit of the second switching valve 42 passes through the (H) position of the first switching valve 41. Since the pilot pressure acting on the second valve 42 is also low, the second switching valve 42 maintains the (re) position. Therefore, the boom lowering pilot pressure P is led out to the safety countermeasure circuit 30 as the operating pressure via the (re) position of the second switching valve 42 and the oil passage L4, and the pilot control unit of the switching valve 33 of the safety countermeasure circuit 30 And the switching valve 33 is switched to the (e) position.

  However, in this case as well, the holding valve 32 maintains the closed state, and the pressure oil on the bottom side 18b of the boom cylinder 18 has an annular groove 35 provided in the small diameter portion of the poppet 31 of the holding valve 32 and It is led out to the oil passage L2 through the (e) position of the switching valve 33, but because of the pressure loss at the throttle provided at the (e) position of the switching valve 33, the boom lowering line flowing in the oil passage L2 Energy regeneration cannot be performed by the regenerative circuit 60 with the return oil.

  (A1-2) On the other hand, when the hydraulic pressure in the return oil passages L2 and L5 becomes a high pressure equal to or higher than a predetermined value, the second switching valve 41 passes through the (h) position of the first switching valve 41 as shown in FIG. The pilot pressure acting on the pilot control unit of the switching valve 42 is increased, and the second switching valve 42 is switched to the (n) position. Therefore, the pressure oil of the hydraulic pressure source 22 that is higher than the boom lowering pilot pressure P is led to the safety countermeasure circuit 30 as the operating pressure via the (nu) position of the second switching valve 42 and the oil passage L4. It acts on the pilot control section of the switching valve 33 of the countermeasure circuit 30 to switch the switching valve 33 to the (f) position. Thus, the pressure on the spring 34 side of the holding valve 32 decreases, the poppet 31 moves to the left in the figure by the hydraulic pressure of the return oil passage L2, and the throttle of the holding valve 32 is opened.

  Thus, even when the boom lowering operation amount is small and the boom lowering pilot pressure P is less than the predetermined value, the hydraulic pressure in the return oil passages L2 and L5 is a high pressure that is equal to or higher than the predetermined value, that is, the boom holding pressure is When the value is equal to or greater than the predetermined value, the function of the safety countermeasure circuit 30 is canceled and the pressure loss at the throttle is reduced.

  Accordingly, the flow rate of the return oil in the boom lowering line increases, and as shown in FIG. 5, if the solenoid valve 64 of the regenerative circuit 60 is turned on and the on-off valve 63 is switched to the (ヲ) position, the return of the oil path L2 Oil is introduced from the oil passage L3 to the regenerative circuit 60, the hydraulic motor 61 rotates to generate power by the generator 62, and even when the boom lowering operation amount is small, the return oil of the boom lowering line can be regenerated. .

[A2] When the boom lowering pilot pressure P is equal to or greater than a predetermined value On the other hand, when the boom lowering operation amount is large and the boom lowering pilot pressure P is equal to or higher than the predetermined value, as shown in FIG. The first switching valve 41 of the function release circuit 40 is switched to the (h) position by the pilot pressure P, and the return oil of the boom lowering line is introduced into the function release circuit 40 from the oil passages L2 and L5.

  (A2-1) At this time, when the oil pressure in the return oil passages L2 and L5 is a low pressure less than a predetermined value, the pilot control unit of the second switching valve 42 passes through the position (H) of the first switching valve 41. Since the pilot pressure acting on the second valve 42 is also low, the second switching valve 42 maintains the (re) position. In this case, the boom lowering pilot pressure P is led out to the safety countermeasure circuit 30 as the operating pressure via the (re) position of the second switching valve 42 and the oil passage L4, but the boom lowering pilot pressure P is not less than a predetermined value. Therefore, the high pressure acts on the pilot control unit of the switching valve 33 of the safety circuit 30 to switch the switching valve 33 to the (f) position. Thus, the throttle of the holding valve 32 is opened.

  (A2-2) On the other hand, when the hydraulic pressure in the return oil passages L2 and L5 becomes a high pressure equal to or higher than a predetermined value, the pilot control unit of the second switching valve 42 passes through the (H) position of the first switching valve 41. The pilot pressure acting on the second valve 42 is increased, and the second switching valve 42 is switched to the (n) position. Accordingly, similarly to the configuration shown in FIG. 5, the pressure oil of the high pressure hydraulic source 22 is led out to the safety countermeasure circuit 30 as the operating pressure via the (nu) position of the second switching valve 42 and the oil passage L4. Then, it acts on the pilot control section of the switching valve 33 of the safety countermeasure circuit 30 to switch the switching valve 33 to the (f) position. Thus, the throttle of the holding valve 32 is opened.

  Thus, when the boom lowering operation amount is large and the boom lowering pilot pressure P is greater than or equal to a predetermined value, the safety is required as long as the boom holding pressure does not drop rapidly regardless of the hydraulic pressure in the return oil passages L2 and L5. The function of the countermeasure circuit 30 is released, and the pressure loss at the throttle is reduced.

  Therefore, if the flow rate of the return oil in the boom lowering line increases, and the electromagnetic valve 64 of the regenerative circuit 60 is turned on and the on-off valve 63 is switched to the (開 閉) position as shown in FIG. The return oil of L2 is introduced from the oil passage L3 to the regeneration circuit 60, and the hydraulic motor 61 rotates to generate power by the generator 62, so that the return oil of the boom lowering line can be regenerated.

  Next, the operation of the hydraulic circuit when the hydraulic hose is damaged and pressure oil leaks will be described.

[B1] During Neutral or Boom Raising Operation As shown in FIG. 2, when the control valve 20 is in the neutral position (A), the boom lowering pilot pressure P is not generated. The pilot pressure does not act on the pilot control section, the switching valve 33 maintains the (d) position, and the control valve 20 is switched to the (b) position as shown in FIG. In the case of time, since the pressure is not generated in L2 due to breakage of the hydraulic hose, the holding valve 32 cannot be opened.

  Therefore, even if the boom holding side hose is damaged and pressure oil leaks from the oil passage L2, the holding valve 32 is closed, so that the pressure oil on the bottom side 18b of the boom cylinder 18 does not flow out, and the boom 15 drops can be prevented.

[B2] Case of Boom Lowering Operation As shown in FIG. 4, when the control valve 20 is switched to the position (C) and the boom lowering operation is performed, the first function release circuit 40 is operated by the boom lowering pilot pressure P. The switching valve 41 is switched to the (h) position, and the return oil of the boom lowering line is introduced into the function release circuit 40 from the oil passages L2 and L5.

  When the boom holding side hose is damaged in this boom lowering operation state, the hydraulic oil leaks from the oil passage L2 and the hydraulic pressure is drastically lowered, and passes through the (h) position of the first switching valve 41 to pass through the second switching valve. Since the pilot pressure acting on the pilot control unit 42 also decreases, the second switching valve 42 switches to the (re) position. Therefore, the boom lowering pilot pressure P is led out to the safety countermeasure circuit 30 as the operating pressure via the (re) position of the second switching valve 42 and the oil passage L4, and the pilot control unit of the switching valve 33 of the safety countermeasure circuit 30 Act directly on.

  FIG. 7 is a graph showing the relationship between the magnitude of the boom lowering pilot pressure P acting on the pilot control unit of the switching valve 33 and the change in the opening area S of the switching valve 33. The boom lowering pilot pressure P is within a predetermined range ( The operation of the safety countermeasure circuit 30 differs between the case of less than P0 to P1) and the case of exceeding the predetermined range (P0 to P1).

  (B2-1) When the boom lowering pilot pressure P is a low pressure less than the predetermined range (P0 to P1), the low boom lowering pilot pressure P is applied to the switching valve 33 of the safety countermeasure circuit 30 as shown in FIG. Directly acting on the pilot control unit, the switching valve 33 is switched to the (e) position and the holding valve 32 is closed. When the boom holding side hose is damaged in this state, the pressure oil on the bottom side 18b of the boom cylinder 18 is passed through the annular groove 35 provided in the poppet 31 of the holding valve 32 and the (e) position of the switching valve 33. Although it flows into the oil passage L2 and is ejected to the outside, since the flow rate is reduced by the restriction of the (e) position of the switching valve 33, the boom 15 is gradually lowered to prevent a sudden drop.

  (B2-2) When the boom lowering pilot pressure P is higher than the predetermined range (P0 to P1), the high boom lowering pilot pressure P is changed to the switching valve of the safety countermeasure circuit 30 as shown in FIG. Directly acting on the pilot control unit 33, the switching valve 33 is switched to the (f) position, and the holding valve 32 is opened. When the boom holding side hose is damaged in this state, the pressure oil on the bottom side 18b of the boom cylinder 18 flows into the oil passage L2 and is ejected in large quantities to the outside, so that the boom 15 is rapidly lowered.

  Therefore, in this case, when the pressure of the return oil passage L2 is detected by the pressure sensor 50 and the detected oil pressure is lower than a predetermined value or rapidly decreases, the switching valve 33 of the safety countermeasure circuit 30 is forcibly ( The safety can be increased by switching to the position e) or (d) and controlling the holding valve 32 to be closed.

  (B2-3) When the boom lowering pilot pressure P is in the range of the predetermined range (P0 to P1), the switching valve 33 of the safety countermeasure circuit 30 is in the transitional stage between the (e) position and the (f) position. Yes, if the boom holding hose is damaged in this state, the intermediate operation of (B2-1) and (B2-2) is performed.

  It should be noted that the present invention can be variously modified without departing from the spirit of the present invention, and the present invention naturally extends to the modified one. It is also possible to directly control the switching valve 33 of the safety countermeasure circuit 30 with an electromagnetic proportional valve.

1 is a side view of a hydraulic excavator to which the present invention is applied. Explanatory drawing which shows a neutral state in the hydraulic circuit which concerns on this invention. Explanatory drawing which shows a boom raising operation state with the hydraulic circuit which concerns on this invention. Explanatory drawing which shows the state in which boom lowering pilot pressure is less than predetermined value and boom holding pressure is less than predetermined value at the time of boom lowering operation state in the hydraulic circuit according to the present invention. Explanatory drawing which shows the state in which boom holding pressure is more than predetermined value at the time of boom lowering operation in the hydraulic circuit which concerns on this invention. Explanatory drawing which shows the state in which the boom lowering pilot pressure is more than predetermined value and boom holding pressure is less than predetermined value at the time of boom lowering operation in the hydraulic circuit according to the present invention. The graph which shows the relationship between the boom lowering pilot pressure in the hydraulic circuit which concerns on this invention, and the opening area of the switching valve of a safety countermeasure circuit. Explanatory drawing which shows the state in which the boom lowering pilot pressure is more than predetermined value and boom holding pressure is more than predetermined value at the time of boom lowering operation in the hydraulic circuit according to the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Hydraulic excavator 15 Boom 18 Boom cylinder 18a Rod side 18b Bottom side 19 Hydraulic pump 20 Control valve 21 Tank 22 Hydraulic source 30 Safety countermeasure circuit 32 Holding valve 33 Switching valve 40 Function release circuit 41 1st switching valve 42 2nd switching valve 50 Pressure sensor (Boom holding pressure detection means)
51 Check valve 60 Regenerative circuit L1-L5 Oil passage

Claims (2)

In a construction machine with a boom having a safety circuit that squeezes and shuts off the return oil of the boom lowering line when the hydraulic hose is broken, and a regenerative circuit that regenerates energy of the return oil of the boom lowering line,
A boom holding pressure detecting means for detecting the boom holding pressure, and a function release circuit for deriving the operation pressure to the safety countermeasure circuit and opening the throttle corresponding to the boom lowering pilot pressure and the boom holding pressure;
When the boom holding pressure is greater than or equal to a predetermined value during the boom lowering operation, the operating pressure from the function release means is increased to increase the aperture of the throttle of the safety countermeasure circuit. Hydraulic circuit. The function release circuit includes a first switching valve that introduces return oil of the boom lowering line by adding a boom lowering pilot pressure, and the boom lowering pilot pressure or the boom lowering according to the boom holding pressure introduced from the first switching valve. A second switching valve that passes any pressure oil higher than the pilot pressure and derives the operating pressure to the safety countermeasure circuit;
The safety countermeasure circuit has a holding valve that throttles and shuts off an oil passage with a poppet, and a switching valve that opens the throttle of the holding valve as the operating pressure introduced from the function release circuit increases. A hydraulic circuit for a construction machine with a boom according to claim 1.

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