JPH1182427A - Hydraulic circuit of construction machine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the damage and an operation failure of hydraulic equipment by the low temperature of an operating oil by providing a bypass line branched from a return line, connected to a tank, and allowed to communicate with the tank when the temperature of the operating oil is a prescribed value or lower. SOLUTION: When the temperature of an operating oil is a prescribed value or lower, a switching means 1 is situated in a close position [position (a)] to allow a return pipe line 14 to communicate with a first bypass line 15. Thus, the operating oil having a low temperature is directly drained to a tank 3 through the first bypass line 15 without passing an oil cooler 2. When the temperature of the operating oil is higher than the prescribed value, the switching means 1 is situated in an open position [position (b)] to interrupt the return pipe line 14 from the first bypass line 15. The operating oil is thus passed from a cooler pipe line 17 through the oil cooler 2. When the temperature of the operation oil is the prescribed value or lower, or the operating oil has a low temperature, the operating oil is never over cooled by the operation of the switching means 1. When the temperature of the operating oil is higher than the prescribed value, the operating oil can be cooled through the oil cooler 2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hydraulic circuit for a construction machine such as a hydraulic excavator, and more particularly to a hydraulic circuit for preventing hydraulic oil from being overcooled at a low temperature, appropriately cooling the hydraulic oil, and durability of hydraulic equipment. The present invention relates to a hydraulic circuit for a construction machine for improving the hydraulic circuit.

[0002]

2. Description of the Related Art As shown in FIG. 7, a conventional cooling device is provided with a radiator 35 and a fan 33 in front of an engine 32.
And a fan 33 attached to the engine 32 can be rotated by a part of the power of the engine 32 via a pulley 34. Engine cooling water and hydraulic oil are circulated in the radiator 35 and the oil cooler 31, respectively, and a cooling air path is set by the rotation of the fan 33. When the engine 32 is rotating, the fan 33 always rotates. In addition, the temperatures of the engine cooling water and the hydraulic oil are maintained at a specified temperature (heat balance temperature) or lower.

However, in the above-described cooling device, the fan 3 always rotates even if the temperature of the engine cooling water or the working oil circulating in the radiator 35 or the oil cooler 31 is low. Therefore, when the engine is started in the early morning of winter, for example, the hydraulic oil becomes overcooled, which causes problems such as damage to hydraulic equipment or malfunction, and a long warm-up operation time.

[0004] In order to solve the above-mentioned problem, for example, Japanese Utility Model Application Laid-Open No. 64-41622 has been filed. In this application, as shown in FIG. 8, a radiator 50, an oil cooler 41, and a fan 43 are arranged near an engine 42, and engine cooling water and hydraulic oil are circulated in the radiator 50 and the oil cooler 41, respectively. In a cooling device in which a cooling air path is set by rotation of the fan 43, an electric motor 40 for driving the fan 43
On the other hand, temperature sensors 44 and 45 are provided in the return passages 49 and 41a for the engine cooling water and the working oil, respectively.
The temperature sensors 44 and 45 and the switch 46 of the electric motor 40 are connected via a controller 48, and the switch 46 of the electric motor 40 can be opened and closed by an operation signal output from the controller 48. The technology describes that the fan 40 is stopped when the temperatures of the engine cooling water and the working oil are lower than a set temperature, and the fan 43 is automatically rotated when the temperature is higher than the set temperature.

As another prior art, Japanese Utility Model Laid-Open No. 4-1 is disclosed.
No. 34565 has been filed. The contents of this application are shown in FIG.
As shown in the figure, the oil cooler 51 and the radiator 53 are disposed separately, and the oil cooler 51 is
At 2, the radiator 53 is connected to the engine fan 54.
The cooling fan 5
A switching valve 58 is provided in the middle of a pipe 57 that communicates a hydraulic motor 56 that drives the hydraulic motor 2 and a hydraulic pump 55 that supplies pressure oil to the hydraulic motor 56. A technique is described in which the switching valve 58 is switched automatically or manually to stop or rotate the cooling fan 52.

[0006]

However, in the cooling device described in Japanese Utility Model Laid-Open Publication No. Sho 64-41622, the radiator 50 and the oil cooler 41 are cooled by one fan 43. Therefore, matching between the operating oil temperature and the engine cooling water temperature is not good.
For this reason, when starting the engine early in the winter,
Conventional problems such as overcooling of hydraulic oil, damage or malfunction of hydraulic equipment, and prolonged warm-up operation time have not been solved. Further, since the electric motor 40 and the power supply 47 are installed to drive the fan 43, there is a problem that the cost is increased.

Further, the above prior art, Japanese Utility Model Laid-Open No. 4-1
In the cooling device described in No. 34565, in addition to an engine fan 54 for cooling a radiator 53, a cooling fan 52 for cooling an oil cooler 51, and a hydraulic pump 55 and a hydraulic motor 56 for driving the same. There is a problem that the cost is increased due to the additional hydraulic device.

The present invention pays attention to the above problems and prevents overcooling of hydraulic oil at low temperatures to prevent damage to hydraulic equipment or malfunction of hydraulic equipment such as actuators and valves at low temperatures. It is an object of the present invention to provide a hydraulic circuit of a construction machine that can improve durability and reduce a warm-up operation time at the time of starting an engine.

[0009]

In order to achieve the above object, a first invention of a hydraulic circuit for a construction machine according to the present invention supplies a hydraulic pump and hydraulic oil discharged from the hydraulic pump to an actuator. And a return line connected to the tank at one end and an oil cooler disposed on a cooler line connected to the return line in the hydraulic circuit of the construction machine. And a bypass pipe 15 which branches off from the return pipe and is connected to the tank 3 and communicates with the tank 3 when the temperature of the hydraulic oil is equal to or lower than a predetermined value. According to the above configuration, since the bypass line communicating with the tank is provided when the oil temperature of the hydraulic oil is equal to or lower than the predetermined value, the low-temperature hydraulic oil is drained directly from the bypass line to the tank without passing through the oil cooler. It has become. As a result, when starting the engine in the early morning of winter, the operating oil does not become overcooled when the operating oil is at a low temperature. It can be prevented in a short time. Therefore, the durability of the hydraulic equipment is improved.

According to a second aspect of the present invention, in the configuration of the first aspect, when the oil temperature of the hydraulic oil is equal to or lower than a predetermined value, the bypass line 15 is communicated, the cooler line 17 is shut off, and the oil temperature of the hydraulic oil is reduced. Is equal to or more than a predetermined value, the bypass line 15 is cut off, and the switching means 1 and 1A provided on the bypass line 15 for connecting the cooler line 17 are provided. According to the above configuration, when the oil temperature of the hydraulic oil is equal to or lower than the predetermined value, the switching means is in the open position and the bypass pipe communicates with the tank. The tank is drained directly from the road. When the hydraulic oil temperature is higher than the specified value,
Since the switching means is in the closed position and shuts off the bypass line, the hydraulic oil passes through the oil cooler from the cooler line. Thereby, when the hydraulic oil is not more than a predetermined value due to the operation of the switching means, the hydraulic oil does not become overcooled, so that the warm-up operation time can be shortened, and when the oil temperature of the hydraulic oil is more than the predetermined value, Since the hydraulic oil can be cooled through the oil cooler, it is possible to prevent hydraulic equipment from being damaged or malfunctioning due to a decrease or increase in the temperature of the hydraulic oil. Therefore, the durability of the hydraulic equipment is improved.

According to a third aspect of the present invention, in the hydraulic circuit of a construction machine according to the second aspect of the present invention, the switching means 1 comprises a driving means 21 with oil temperature detection for detecting the oil temperature. According to the above configuration, since the switching means is provided with the driving means with oil temperature detection for detecting the oil temperature, when the oil temperature in the hydraulic circuit becomes a predetermined value or more, the driving means with oil temperature detection is opened. Then, the return oil from the actuator is circulated to the oil cooler. When the oil temperature is equal to or lower than a predetermined value, the drive means with oil temperature detection is in the closed position, and the return oil from the actuator is not circulated to the oil cooler. Therefore, damage to hydraulic equipment due to a decrease or increase in hydraulic oil temperature,
Alternatively, malfunctions can be prevented.

According to a fourth aspect of the present invention, in the configuration of the third aspect, the driving means 21 with oil temperature detection is provided with an expanding agent. According to the above configuration, when the operating oil has a high temperature equal to or higher than a predetermined value using an expanding agent made of CnH2n + 2 paraffin-based wax, the driving unit with oil temperature detection is in the open position, and the operating oil is in the predetermined position. When the temperature becomes lower than the value,
The drive means with oil temperature detection is set to the closed position.
Accordingly, the opening amount of the driving unit with oil temperature detection is appropriately set within the range from the minimum to the maximum depending on the oil temperature. Thus, even if the temperature of the hydraulic oil is equal to or higher than a predetermined value, the flow rate circulating to the oil cooler is adjusted according to the oil temperature. Therefore, problems such as overcooling of the working oil circulated to the oil cooler are eliminated, and damage or malfunction of the hydraulic equipment due to a decrease in the temperature of the working oil can be prevented.

According to a fifth aspect of the present invention, in the configuration of the third aspect, the driving means 21 with oil temperature detection includes at least two types of expanding agents having different expansion coefficients. According to the above configuration, the opening amount of the drive means with oil temperature detection is appropriately adjusted within a range of minimum to maximum by using at least two types of expansion agents having different expansion coefficients using CnH2n + 2 paraffin wax or the like. Is set. Thus, even if the temperature of the hydraulic oil is equal to or higher than a predetermined value, the flow rate circulating to the oil cooler is adjusted according to the oil temperature. Therefore, problems such as overcooling of the working oil circulated to the oil cooler are eliminated, and damage or malfunction of the hydraulic equipment due to a decrease in the temperature of the working oil can be prevented.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of a hydraulic circuit for a construction machine according to the present invention will be described below with reference to FIGS.
FIG. 1 is a diagram showing a hydraulic circuit of a construction machine according to the present invention. This hydraulic circuit includes a drive system circuit (A) for supplying pressure oil to the actuator 4 and a return system circuit (B) for guiding return oil from the actuator 4 to the tank 3. The drive system circuit (A) receives a hydraulic pump 6 driven by an engine 6A, an operation valve 5 for controlling supply of discharge oil from the hydraulic pump 6 to an actuator 4, and a supply of pressure oil from the operation valve 5. Actuating actuator 4
And a tank 3. Discharge oil sucked from the tank 3 through the suction line 18 by the hydraulic pump 6 and discharged to the delivery line 11 enters the pump port 5 a of the operation valve 5. Here, for example, the operation lever 9
Is operated to the raising position, the pilot pressure is output from the pilot valve 9a connected to the operating lever 9 to the operating portion 5e of the operating valve 5, and the operating valve 5 switches from the neutral position n to the a position. As a result, the discharge oil enters the bottom side of the actuator 4 from the actuator port 5b through the pipe 12 to drive the actuator 4 to extend, and the return oil from the rod side passes through the pipe 13 to the actuator of the operation valve 5. It enters port 5c and drains from tank port 5d to return line 14.

A return system circuit (B) applies a back pressure to a return line 14 connecting the operation valve 5 and the tank 3.
Lift check valve 7 provided on return line 14, cooler line 17 connecting switching means 1 and oil cooler 2
An oil cooler 2 provided between the cooler line 17 and the tank 3, a first bypass line 15 branching from the return line 14 and communicating with the tank 3, and an oil temperature of the hydraulic oil of a predetermined value or less. Provided on the first bypass line 15 for shutting off the cooler line 17 when the oil temperature of the hydraulic oil is equal to or higher than a predetermined value. The switching means 1, the second bypass line 16 that branches off from the return line 14 and communicates with the tank 3 when the peak pressure of the hydraulic oil is equal to or higher than a predetermined value, and when the peak pressure of the hydraulic oil is equal to or lower than the predetermined value. And a check valve 8 provided on the second bypass line 16 for communicating the second bypass line 16. Here, the switching means 1 will be described. In the embodiment shown in FIGS. 1 and 2, the opening / closing operation of the switching means 1 is performed by the driving means 21 with oil temperature detection of the hydraulic oil. In addition, the switching means 1 shown in FIG. An operator operates a switch from the operator's cab instead of the control valve (for example, a three-stage signal is issued so that the lift amount of the valve 22 shown in FIG. 2 can be operated in three stages).
By doing so, the proportional electromagnetic control valve may be operated. Thus, when the operating oil is at a low temperature, the switch is operated to set the switching means 1 shown in FIG.
Since the hydraulic oil is drained directly from the bypass pipe 15 to the tank 3 without passing through, the hydraulic oil is prevented from being overcooled.

A tank port 5d of the operation valve 5 shown in FIG.
The return oil that has flowed into the return line 14 is given a predetermined back pressure by the lift check valve 7, passes through the check valve 7, and enters the first port 1 a of the switching means 1.
Here, when the oil temperature of the hydraulic oil is equal to or lower than a predetermined value, the switching means 1 is held at the position a.
From the third port 1c to the tank 3 through the first bypass line 15. When the oil temperature of the hydraulic oil is equal to or higher than a predetermined value, the switching unit 1 switches from the position a to the position b.
The return oil flows into the oil cooler 2 from the second port 1b of the switching means 1 through the cooler line 17, where it is cooled and returns to the tank 3. When the peak pressure of the hydraulic oil is equal to or higher than a predetermined value, the return oil opens the check valve 8 on the second bypass line 16 and returns to the tank 3.

Next, the structure of the switching means 1 will be described with reference to FIG. The switching means 1 is a driving means 21 with oil temperature detection.
The valve assembly 20 is attached to the first port chamber 10a of the manifold 10 by a flange 23. A first chamber 10 having a first port 1a is provided in the manifold 10.
a, a second chamber 10b having a second port 1b, a third chamber 10c having a third port 1c, and a first port 1a.
Is connected to the return line 14, the second port 1b is connected to the cooler line 17, and the third port 1c is connected to the first bypass line 15. Valve assembly 20
Is a drive unit 21 with oil temperature detection comprising a heat sensitive part 24, an expansion body 25, and a sleeve 26; a valve 22 integrated with a piston 27; Body 28 having return spring 29
And a flange 23 to which the heat-sensitive portion 24 is fixed. The expansion body 25 is made of one type of CnH2n + 2 paraffin-based wax or at least two types of expansion agents having different expansion coefficients using CnH2n + 2 paraffin-based wax or the like. The rate of change of oil temperature. Driving means 21 with oil temperature detection
When the hydraulic oil whose oil temperature is equal to or higher than a predetermined value comes into contact with the heat-sensitive portion 24, the expansion body 25 expands, the sleeve 26 is compressed, and the piston 27 is pushed upward together with the valve 22 against the return spring 29. Has become. In addition, the heat sensitive part 24
When the hydraulic oil whose oil temperature is lower than the predetermined value comes into contact with the
Is contracted, the sleeve 26 is released from the compressed state, and the piston 27 is pushed down together with the valve 22 by the return spring 29. The valve 22 has a thin cylindrical shape having an upper concave surface in which an oil passage hole 22a is drilled, and the cylindrical outer surface 22b comes into contact with a seal 10e fitted to the manifold 10 so as to be slidable up and down. , The second port chamber 10b of the manifold 10 and the third port chamber 10c are shut off via a seal 10e. The lower end 22c of the valve 22 contacts the seat surface 23a of the flange 23 to shut off the space between the first port chamber 10a and the second port chamber 10b of the manifold 10. Further, the valve 22 is at the upper limit, and the upper end surface 22d of the valve 22 is
The first port chamber 10a and the third port chamber 1 of the manifold 10 contact the seat surface 10d of the third port chamber 10c.
0b.

Next, the operation of the switching means 1 will be described with reference to FIGS. FIG. 2 shows a state where the oil temperature of the hydraulic oil is equal to or lower than a predetermined value and the switching means 1 is fully closed. Heat sensing part 24
The expansion body 25 inside is contracted and the sleeve 26 is contracted, and the piston 27 is pushed down together with the valve 22 to the lower limit by the return spring 29, and the switching means 1 is fully closed. As a result, the lower surface 22c of the valve 22 comes into contact with the seat surface 23a of the flange 23, and the manifold 1
The first port chamber 10a and the second port chamber 10b are shut off, and the first port chamber 10a and the third port chamber 10c of the manifold 10 are connected to the oil passage hole 23b of the flange 23 and the valve 2
The return line 14 and the cooler line 17 are shut off because they communicate with each other through the second oil passage hole 22a.
And the first bypass line 15 communicate with each other. Therefore, the return oil flows from the return line 14 to the tank 3.

FIG. 3 shows a state in which the switching means 1 is fully opened when the oil temperature of the hydraulic oil is equal to or higher than a predetermined value. The expansion body 25 inside the heat sensing part 24 expands, the sleeve 26 is compressed, the piston 27 is pushed up to the ascending limit position together with the valve 22 against the return spring 29, and the switching means 1 is in the fully opened state. . Thereby, the upper surface 22d of the valve 22 abuts on the seat surface 10d of the third port chamber 10c of the manifold 10, and the first port chamber 10a and the third port chamber 10c of the manifold 10 are shut off. Since the first port chamber 10a and the second port chamber 10b of the manifold 10 communicate with each other through the oil passage hole 23b of the flange 23, the return line 14 and the first bypass line 15 are cut off, and the return line 14 and the cooler line 17 communicate with each other. Therefore, the return oil flows from the return line 14 to the oil cooler 2.

The valve 22 is fully closed (lift amount =
0) and fully open state (lift amount = L2),
Because of the hysteresis of the driving means 21 with oil temperature detection, FIG.
As shown in (1), the oil temperature T is loop-controlled with respect to the lift amount L of the valve 22. The expansion member 25 has different expansion coefficients 2
The rate of change α of oil temperature T
Can be changed to α1 and α2. Thereby, required oil temperature characteristics can be easily obtained.

According to this embodiment, when the oil temperature of the hydraulic oil is equal to or lower than a predetermined value, the switching means 1 shown in FIG. The low-temperature hydraulic oil is connected to the oil cooler 2 because it communicates with the bypass line 15.
Drain from the first bypass line 15 directly to the tank 3 without passing through. When the oil temperature of the hydraulic oil is equal to or higher than a predetermined value, the switching means 1 is set to the open position (position b) and the return line 14 and the first bypass line 15 are cut off. The oil cooler 2 passes through the pipe 17. By the operation of the switching means 1, when the oil temperature of the hydraulic oil is equal to or lower than a predetermined value, the operating oil is not overcooled at a low temperature, so that the warm-up operation time can be shortened and the oil temperature of the hydraulic oil is equal to or higher than the predetermined value. In this case, the hydraulic oil can be cooled through the oil cooler 2, so that it is possible to prevent hydraulic equipment from being damaged or malfunctioning due to a decrease or increase in the temperature of the hydraulic oil. Therefore, the durability of the hydraulic equipment is improved. When the peak pressure of the hydraulic oil is equal to or higher than the predetermined value, the hydraulic oil is drained from the second bypass line 16 directly to the tank 3.

Since the switching means 1 uses the driving means 21 with oil temperature detection for detecting the oil temperature, when the oil temperature in the hydraulic circuit becomes equal to or higher than a predetermined value, the driving means 21 with oil temperature detection switches the switching means. 1 is in the open position (b position), and the return oil from the actuator 4 is circulated to the oil clutch 2. When the oil temperature in the hydraulic circuit is equal to or lower than a predetermined value, the driving means 2 with oil temperature detection
1, the switching means 1 is set to the closed position (a position), and the return oil from the actuator 4 is not circulated to the oil cooler 2. Therefore, it is possible to prevent the hydraulic equipment from being damaged or malfunctioning due to a decrease or increase in the temperature of the hydraulic oil. Since the switching means 1 incorporates the driving means 21 with oil temperature detection for detecting the oil temperature, the structure is simpler and more reliable than the switching means provided with an oil temperature detection sensor and a controller separately. Oil temperature control is performed reliably. In addition, the lift check valve 7 provided on the return line 14
Builds back pressure in the circuit. An appropriate flow rate is circulated to the oil cooler 2 according to the back pressure.

Inflatable body 25 of drive means 21 with oil temperature detection
Uses one kind of CnH2n + 2 paraffin-based wax or at least two kinds of expanding agents having different expansion coefficients using CnH2n + 2 paraffin-based wax or the like. The opening amount is appropriately set within a range from a minimum to a maximum. Thereby, even if the temperature of the hydraulic oil becomes equal to or higher than a predetermined value, the oil cooler 2
The flow rate circulating to the can be adjusted. Therefore,
Problems such as overcooling of the hydraulic oil circulated to the oil cooler 2 are eliminated, and damage to hydraulic equipment or malfunctions due to a decrease or increase in the temperature of the hydraulic oil can be prevented.

Next, a second embodiment of a hydraulic circuit for a construction machine according to the present invention will be described with reference to FIGS. 1 to 3 and FIGS. The components denoted by the same reference numerals as those in the first embodiment are the same components, and the description thereof will be omitted. The switching means, the controller, and the like, which are the main parts of the second embodiment, will be described. First, the switching means 1 and 1A shown in FIG. 5 have the function of the switching means 1 including the driving means 21 with oil temperature detection shown in FIGS. 1 and 2, and the switching means shown in FIG. It has a function of a valve 1A (hereinafter, referred to as a control valve 1A). By the operation of the control valve 1A, the valve 22 shown in FIG. 2 is lifted to open or close. As shown in FIG. 5, a signal from an oil temperature sensor that detects the oil temperature of the hydraulic oil is input to the controller 19. Further, a signal from a switch 19 b installed in a cab (not shown) is input to the controller 19. The switch 19 moves the valve 22 shown in FIG. 2 to the lift amount L1, L2, L3.
The three-stage switching signal can be input to the controller 19 so that the valve can be opened in three stages. The controller 19 receives and calculates a signal from the oil temperature sensor or the switch 19 and outputs a command to the control valve 1A.
Is issued to lift the valve 22 shown in FIG.

Next, a control flowchart of the controller 19 when receiving a signal from the oil temperature sensor 19a shown in FIG. 5 will be described with reference to FIG. 6 with reference to FIG. S
It is determined at 1 whether the oil temperature of the hydraulic oil is the first predetermined temperature T1. If NO, an output is made to close the valve 22 shown in FIG. 2, and if YES, the first temperature is determined at S2. The control valve 1 is opened so that the valve 22 is opened up to the lift amount L1 of the valve 22 at the rate of change α1.
A command is output to A. In S3, it is determined whether the temperature is the second predetermined temperature T2. If the answer is NO, the process returns to S2.
A command is output to the control valve 1A so as to open the valve up to. S5
To determine whether the temperature is the third predetermined temperature T3 to T4. If the answer is NO, the process returns to S4.
A command is issued to keep the valve open with the lift amount L2. In S7, it is determined whether the temperature is the fourth predetermined temperature T5, and NO
Returns to S6, and if YES, returns to S8
A command is issued to keep the valve open with the second lift amount L3. In S9, it is determined whether the temperature is equal to or lower than the predetermined temperature T1. If NO, the process returns to S8, and if YES, the process returns to S1. By controlling the control valve 1A based on the above control flowchart, the lift amount of the valve 22 shown in FIG. 2 is controlled. When the temperature is equal to or lower than a predetermined temperature T, the valve 22 is closed and the cooler 2 is activated. By preventing oil from circulating, overcooling can be prevented.

[Brief description of the drawings]

FIG. 1 is a diagram showing a hydraulic circuit of a construction machine according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a fully closed state of the switching means.

FIG. 3 is a view showing a fully opened state of the switching means.

FIG. 4 is a diagram showing an oil temperature characteristic of the switching means.

FIG. 5 is a diagram showing a hydraulic circuit of the construction machine according to the second embodiment.

FIG. 6 is a control flowchart of the second embodiment.

FIG. 7 is a diagram showing a conventional cooling device.

FIG. 8 is a diagram showing an example of a conventional cooling device of the related art.

FIG. 9 is a diagram showing another example of a conventional cooling device of the related art.

[Explanation of symbols]

1, 1A: switching means (proportional electromagnetic control valve), 1a, 1
b, 1c port, 2 oil cooler, 3 tank, 4
... actuator, 5 ... operation valve, 6 ... hydraulic pump, 7 ...
Lift check valve, 8 ... check valve, 9 ... operation lever,
9a: pilot valve, 10: manifold, 10a, 1
0b, 10c: port chamber, 11: delivery line, 12,
13 ... actuator line, 14 ... return line, 15
... bypass line (first bypass line), 16 ... second bypass line, 17 ... cooler line, 18 ... suction line,
20: Valve assembly 20, 21: Drive means with oil temperature detection, 22: Valve, 23: Flange, 24: Heat sensitive part, 2
5 ... inflatable body, 26 ... sleeve, 27 ... piston.

Claims (5)

[Claims] A hydraulic pump, an operating valve for supplying hydraulic oil discharged from the hydraulic pump to an actuator, one end connected to the operating valve, the other end connected to a tank, and a return pipe connected to the tank. In a hydraulic circuit of a construction machine including an oil cooler disposed on a cooler pipe to be connected, the hydraulic circuit branches off from a return pipe and connects to a tank, and communicates with the tank when the oil temperature of hydraulic oil is equal to or lower than a predetermined value. A hydraulic circuit for a construction machine, comprising a bypass pipe. 2. When the oil temperature of hydraulic oil is lower than a predetermined value, the bypass line is communicated, the cooler line is shut off, and when the oil temperature of hydraulic oil is higher than the predetermined value, the bypass line is shut off. 2. The hydraulic circuit for a construction machine according to claim 1, further comprising switching means disposed on a bypass pipe for connecting the cooler pipe. 3. The hydraulic circuit for a construction machine according to claim 2, wherein said switching means comprises a driving means with oil temperature detection for detecting an oil temperature. 4. The hydraulic circuit for a construction machine according to claim 3, wherein the drive unit with oil temperature detection includes an expanding agent. 5. The hydraulic circuit for a construction machine according to claim 3, wherein the drive means with oil temperature detection includes at least two types of expansion agents having different expansion coefficients.