CONSTRUCTION MACHINE

A construction machine includes a relief valve that opens to limit a pressure of a pump discharge line that is a line between a main pump and a direction switching valve, a back pressure generation mechanism that is disposed on a return line that is a line connected to a tank and generates a back pressure in the return line, and an air bleed line that connects the relief valve and a portion downstream from the back pressure generation mechanism in the return line.

TECHNICAL FIELD

The present disclosure relates to a construction machine.

BACKGROUND ART

Patent Literature 1 discloses a hydraulic drive device of a construction machine for bleeding air from a pilot pump and a connection pipe thereof. The hydraulic drive device includes an air bleed oil passage connected between a discharge oil passage of the pilot pump and a hydraulic oil tank, and an air bleed valve interposed in the air bleed oil passage. The air bleed valve blocks the air bleed oil passage in a case where a lock lever is at an unlocked position, and makes communication of the air bleed oil passage in a case where the lock lever is at a locked position.

In the hydraulic drive device of Patent Literature 1, even if air bleed can be performed on the pilot pump and the connection pipe thereof, air building up in a relief valve disposed in a hydraulic circuit cannot be effectively discharged. Specifically, in general, the hydraulic circuit in the construction machine includes a relief valve that opens to limit a pressure of a pump discharge line that is a line between a hydraulic pump and a direction switching valve. This relief valve has a structure where air easily builds up. Therefore, it is desirable to effectively discharge the air building up in the relief valve of the hydraulic circuit from the relief valve.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

The present disclosure has been made in view of the above problems, and an object of the present disclosure is to provide a construction machine capable of effectively discharging air building up in a relief valve of a hydraulic circuit from the relief valve.

A construction machine to be provided includes a tank that stores hydraulic oil, a main pump that is a hydraulic pump that discharges the hydraulic oil sucked from the tank, a hydraulic actuator that receives supply of the hydraulic oil from the main pump to operate, a direction switching valve that controls supply and discharge of the hydraulic oil between the main pump and the hydraulic actuator, a relief valve that opens to limit a pressure of a pump discharge line that is a line between the main pump and the direction switching valve, a back pressure generation mechanism that is disposed on a return line as a line connected to the tank and generates a back pressure in the return line, and an air bleed line that connects the relief valve and a portion downstream from the back pressure generation mechanism in the return line.

DESCRIPTION OF EMBODIMENT

An embodiment of the present disclosure will be described with reference to the drawings.FIG.1is a side view illustrating a hydraulic excavator100according to the embodiment. The hydraulic excavator100is an example of a construction machine.

As illustrated inFIG.1, the hydraulic excavator100includes a lower travelling body1capable of traveling on a ground, an upper slewing body2supported to the lower travelling body1to be slewable about a slewing center axis X facing a vertical direction, and a work device3supported to the upper slewing body2. The lower travelling body1and the upper slewing body2are examples of a machine body.

The lower travelling body1includes a pair of crawler travelling devices and a lower frame connecting these travelling devices. The upper slewing body2includes an upper frame slewably supported to the lower frame, a cabin supported to a front portion of the upper frame, and a counterweight supported to a rear portion of the upper frame. In the present embodiment, the work device3includes a boom4, an arm5, and a bucket6.

The boom4is supported to the upper frame to be raised and lowered with respect to the upper frame of the upper slewing body2. The arm5is supported to the boom4to be rotatable with respect to the boom4. The bucket6is supported to the arm5to rotationally move with respect to the arm5.

The hydraulic excavator100further includes a plurality of hydraulic actuators for hydraulically moving the work device3and the upper slewing body2. The plurality of hydraulic actuators includes a boom cylinder7, an arm cylinder8, a bucket cylinder9, and a slewing motor11.

FIG.2is a diagram illustrating a hydraulic circuit of the hydraulic excavator100according to the embodiment and devices related thereto. As illustrated inFIG.2, the hydraulic excavator100further includes a tank21, a plurality of main pumps including a main pump22, a pilot pump23, a control valve unit30, a boost check valve67(check valve), a bypass check valve68(check valve), a filter62, a plurality of operation levers69, a plurality of pilot valves65, a lever lock81, a lever lock valve64, an unloading electromagnetic proportional valve51, an air bleed switching valve63, and a controller70.

The controller70includes a central processing unit (CPU), a memory, and the like. The controller70includes a command output unit that outputs command signals to the lever lock valve64, the unloading electromagnetic proportional valve51, and the like, and the command output unit is achieved by the CPU executing a control program.

The control valve unit30includes a control valve main body, a relief valve40, and an unloading valve50. The control valve main body includes a plurality of direction switching valves. The plurality of direction switching valves includes a boom cylinder direction switching valve31, an arm cylinder direction switching valve, a bucket cylinder direction switching valve, and a slowing motor direction switching valve. The control valve unit30includes the plurality of direction switching valves and various functional components including the relief valve40and the unloading valve50, and is configured so that these valves are integrated. That is, the relief valve40and the unloading valve50are mounted on the control valve main body.

The relief valve40is an electromagnetic relief valve capable of changing a set pressure. The relief valve40is interposed between the main pump22and the tank21. The relief valve40closes until a pressure of a pump discharge line91, which is a line between the main pump22and the control valve main body including the boom cylinder direction switching valve31, rises to the set pressure, and opens when the pressure reaches the set pressure to limit the pressure of the pump discharge line91to the set pressure or less. The relief valve40is disposed on a relief line95branched from the pump discharge line91. The relief line95is connected to a return line92. The return line92is a line connected to the tank21.

The relief valve40includes a relief valve main body41and a pressure adjustment mechanism unit42(pressure adjustment unit42) having a solenoid. The set pressure of the relief valve main body41changes in response to a set pressure command signal input from the controller70to the solenoid of the pressure adjustment mechanism unit42. Therefore, the upper limit value of the pressure of the pump discharge line91can be varied by changing the set pressure command signal (changing the set pressure). The relief valve40has an air bleed port43. The air bleed port43is formed in, for example, the pressure adjustment mechanism unit42.

In a method for mounting the relief valve40on the control valve main body, a mounting position and a mounting orientation are restricted by a position and an orientation of the control valve main body when the control valve unit30is mounted on the upper slewing body2. Therefore, a degree of freedom of disposition of the relief valve40is small when the control valve unit30is mounted on the upper slewing body2. Further, the relief valve40has a structure where air is likely to build up therein. In particular, the pressure adjustment mechanism unit42of the relief valve40has a structure where air is likely to build up. Therefore, in a case where the relief valve40is disposed in the upper slewing body2at an attitude such that the pressure adjustment mechanism unit42is positioned above the relief valve main body41, air is particularly likely to build up in the pressure adjustment mechanism unit42of the relief valve40.

The unloading valve50is a valve for running hydraulic oil discharged from the main pump22to the return line92without supplying the hydraulic oil to the plurality of hydraulic actuators including the boom cylinder7. The unloading valve50is disposed on an unloading line94branched from the pump discharge line91and connected to the return line92.

The unloading valve50has a pilot port. The unloading valve50opens so as to communicate the main pump22and the return line92via the unloading line94in a state where the pilot pressure is not applied to the pilot port, and prevents the communication between the main pump22and the return line92via the unloading line94in a state where a predetermined pilot pressure is applied to the pilot port. The unloading valve50changes the opening degree based on the pilot pressure input to the pilot port. The controller70outputs a command signal to the unloading electromagnetic proportional valve51, and the unloading electromagnetic proportional valve51outputs the pilot pressure corresponding to the command signal to the pilot port of the unloading valve50. As a result, the opening degree of the unloading valve50is adjusted to a magnitude based on the pilot pressure.

InFIG.2, only the main pump22among the plurality of main pumps is illustrated, and illustration of the other main pumps is omitted. Further, inFIG.2, only the boom cylinder7among the plurality of hydraulic actuators is illustrated, and illustration of the other hydraulic actuators is omitted. Further, inFIG.2, only the boom cylinder direction switching valve31among the plurality of direction switching valves is illustrated, and illustration of the other direction switching valves is omitted.

The tank21stores hydraulic oil. Each of the plurality of main pumps including the main pump22is a hydraulic pump that is driven by an engine, not illustrated, and discharges the hydraulic oil sucked from the tank21. Each of the plurality of main pumps supplies the hydraulic oil to at least one hydraulic actuator among the plurality of hydraulic actuators. In the present embodiment, the main pump22is a variable displacement type hydraulic pump, but may be a fixed displacement type hydraulic pump. The pilot pump23is a hydraulic pump that is driven by the engine, not illustrated, and discharges the hydraulic oil sucked from the tank21. The pilot pump23supplies the pilot pressure of the hydraulic oil to the pilot ports of the plurality of direction switching valves, the pilot port of the unloading valve50, the plurality of pilot valves, and the like. When the engine is started, each of the plurality of main pumps and the pilot pump23discharge hydraulic oil.

The boom cylinder7is a hydraulic cylinder that extends and contracts upon reception of supply of the hydraulic oil discharged from the main pump22. The boom cylinder7is attached to the upper slewing body2and the boom4such that the boom4is raised and lowered with respect to the upper slewing body2as the boom cylinder7extends and contracts.

The arm cylinder8is a hydraulic cylinder that extends and contracts upon reception of supply of the hydraulic oil discharged from any of the plurality of main pumps. The arm cylinder8is attached to the boom4and the arm5such that the arm5rotationally moves with respect to the boom4as the arm cylinder8extends and contracts.

The bucket cylinder9is a hydraulic cylinder that extends and contracts upon reception of supply of the hydraulic oil discharged from any of the plurality of main pumps. The bucket cylinder9is attached to the arm5and the bucket6such that the bucket6rotationally moves with respect to the arm5as the bucket cylinder9extends and contracts.

The slewing motor11is a hydraulic motor for hydraulically slewing the upper slewing body2with respect to the lower travelling body1. The slewing motor11has an output shaft, and the output shaft is connected to the upper frame of the upper slewing body2via a speed reducer, not illustrated. The slewing motor11operates so that the output shaft rotates in a direction corresponding to a direction of supply of the hydraulic oil discharged from any of the plurality of main pumps upon reception of the supply, and thus the upper slowing body2can be slewed in each of a left slewing direction and a right slewing direction.

The boom cylinder direction switching valve31controls supply and discharge of the hydraulic oil to and from the boom cylinder7. Specifically, the boom cylinder direction switching valve31has a pair of pilot ports. The boom cylinder direction switching valve31can switch among a neutral position, a boom raising position for guiding the hydraulic oil from the main pump22to a head-side chamber of the boom cylinder7, and a boom lowering position for guiding the hydraulic oil from the main pump22to a rod-side chamber of the boom cylinder7.

When the pilot pressure is applied to none of the pair of pilot ports, the boom cylinder direction switching valve31is maintained at the neutral position to shut off between the main pump22and the boom cylinder7. The boom cylinder direction switching valve31is switched to the boom raising position to allow the supply of the hydraulic oil from the main pump22to the head-side chamber of the boom cylinder7when the pilot pressure is applied to one of the pair of pilot ports, and is switched to the boom lowering position to allow the supply of the hydraulic oil from the main pump22to the rod-side chamber of the boom cylinder7when the pilot pressure is applied to the other one of the pair of pilot ports.

The hydraulic oil discharged from the boom cylinder7and passing through the boom cylinder direction switching valve31is discharged to a discharge line96. The discharge line96is connected to the return line92. Therefore, the hydraulic oil discharged from the boom cylinder7returns to the tank21through the return line92.

The arm cylinder direction switching valve controls supply and discharge of hydraulic oil to and from the arm cylinder8, the bucket cylinder direction switching valve controls supply and discharge of hydraulic oil to and from the bucket cylinder9, and the slewing motor direction switching valve controls supply and discharge of hydraulic oil to and from the slewing motor11. The basic structures and functions of the arm cylinder direction switching valve, the bucket cylinder direction switching valve, and the slewing motor direction switching valve are similar to those of the boom cylinder direction switching valve31, and thus detailed description thereof will be omitted.

The boost check valve67is a back pressure holding valve (back pressure valve) that generates a pressure (back pressure) set in advance in the return line92. The boost check valve67is disposed on the return line92, and opens when the pressure of the return line92is a predetermined pressure or more, and the hydraulic oil flows out to the tank21.

The filter62is for filtering the hydraulic oil before returning to the tank21in the return line92. The filter62is disposed, for example, upstream from the boost check valve67in the return line92.

The bypass check valve68is a bypass valve that is provided in parallel with the boost check valve67and opens when its pressure becomes higher than that of the boost check valve67, and bypasses and causes the hydraulic oil to flow out to the tank21in a case where clogging occurs in the filter62. The bypass check valve68is disposed on a bypass line93branching from the return line92.

The plurality of operation levers69includes a right operation lever and a left operation lever disposed on the left and right of a driver's seat80(seeFIG.2) on which an operator sits. For example, the right operation lever may function as a boom operation lever when being operated in a front-rear direction, and may function as a bucket operation lever when being operated in a left-right direction. The left operation lever may function as an arm operation lever when being operated in a front-rear direction, and may function as a slewing operation lever when being operated in a left-right direction. The functions of the left and right operation levers are not limited to the above specific examples, and may be configured to be arbitrarily changeable by an operator's instruction, for example.

An operation for operating the boom cylinder7is given to the boom operation lever by an operator. An operation for operating the arm cylinder8is given to the arm operation lever by an operator. An operation for operating the bucket cylinder9is given to the bucket operation lever by an operator. An operation for operating the slewing motor11is given to the slewing operation lever by an operator.

The plurality of pilot valves65includes a boom operation pilot valve, an arm operation pilot valve, a bucket operation pilot valve, and a slewing operation pilot valve. The boom operation pilot valve is interposed between the pilot pump23and the boom cylinder direction switching valve31, and controls the operation of the boom cylinder direction switching valve31. The boom operation pilot valve operates to supply the pilot pressure based on the operation amount of the boom operation lever to a pilot port corresponding to the operation direction of the boom operation lever among the pair of pilot ports of the boom cylinder direction switching valve31. As a result, a flow rate and a supply direction of hydraulic oil to be supplied to the boom cylinder7are adjusted.

The arm operation pilot valve is interposed between the pilot pump23and the arm cylinder direction switching valve, and controls the operation of the arm cylinder direction switching valve. The bucket operation pilot valve is interposed between the pilot pump23and the bucket cylinder direction switching valve, and controls the operation of the bucket cylinder direction switching valve. The slewing operation pilot valve is interposed between the pilot pump23and the slewing motor direction switching valve, and controls the operation of the slewing motor direction switching valve. The basic structures and functions of the arm operation pilot valve, the bucket operation pilot valve, and the slewing operation pilot valve are similar to those of the boom operation pilot valve, and thus detailed description thereof will be omitted.

The lever lock81includes an operation member capable of switching between a valid state (unlocked state) and invalid state (locked state) of the operations of the plurality of operation levers69. The lever lock81is an example of a locking mechanism (locking switch). The lever lock81receives an operation for switching the state of the hydraulic circuit between an unlocked state where the operations of the cylinders7,8, and9and the slewing motor11are permitted in response to the operations given to the plurality of operation levers69and a locked state where the operations of the cylinders7,8, and9and the slewing motor11are prevented in response to the operations given to the plurality of operation levers69.

The operation member of the lever lock81is disposed, for example, on the left side of the driver's seat80, and is configured to be capable of being raised and lowered by an operator, for example, when the operator enters or leaves the cabin. The lever lock81inputs a locking signal, which is an electric signal corresponding to the locked state, to the controller70when the operation member of the lever lock81is disposed at the locked position. As a result, the operations of the plurality of operation levers69is invalidated. On the other hand, when the operation member of the lever lock81is disposed at the unlocked position, the lever lock81inputs an unlocking signal, which is an electric signal corresponding to the unlocked state, to the controller70. As a result, the operations of the plurality of operation levers69is validated.

The lever lock valve64is an electromagnetic valve having a solenoid that receives a command signal output from the controller70, and is an electromagnetic valve that receives the command signal input from the controller70and opens so that hydraulic oil from the pilot pump23is supplied to the air bleed switching valve63and the plurality of pilot valves65. When the lever lock81inputs the unlocking signal to the controller70, the controller70inputs a command signal to the solenoid of the lever lock valve64. As a result, the lever lock valve64opens, and the hydraulic oil from the pilot pump23is supplied to the air bleed switching valve63and the plurality of pilot valves65.

The hydraulic circuit of the hydraulic excavator100according to the present embodiment includes an air bleed line90that connects the pressure adjustment mechanism unit42of the relief valve40and the return line92. The air bleed line90is a pipe for collecting air building up in the relief valve40together with hydraulic oil to the tank21through the air bleed line90.

An upstream end of the air bleed line90is connected to the air bleed port43of the relief valve40, and a downstream end of the air bleed line90is connected to a portion of the return line92between the boost check valve67and the tank21. Further, the downstream end of the air bleed line90is connected to a portion of the return line92, the portion being on the tank21side relative to the bypass line93.

When the main pump22is discharging the hydraulic oil, the pressure of a portion92aupstream from the boost check valve67in the return line92is higher than the pressure of a portion92bof the return line92downstream from the boost check valve67. The relief line95is connected to the portion92aupstream on the return line92. Therefore, when the main pump22is discharging the hydraulic oil, the pressure in the relief valve40disposed on the relief line95is higher than the pressure at the portion92bdownstream on the return line92. By using such a pressure difference between the pressures in the relief valve40and the pressure at the portion92bdownstream on the return line92, the hydraulic oil containing air building up in the relief valve40flows out from the air bleed port43of the relief valve40to the air bleed line90, flows toward the tank21through the air bleed line90, and is collected in the tank21. As a result, the air building up in the relief valve40can be bled.

The air bleed switching valve63is a valve capable of switching between an allowed state where the hydraulic oil from the relief valve40is allowed to flow to the tank21through the air bleed line90and a blocked state where the hydraulic oil from the relief valve40is blocked from flowing to the tank21through the air bleed line90. The air bleed switching valve63is disposed on the air bleed line90.

The air bleed switching valve63has a pilot port. In a state where a pilot pressure is not applied to the pilot port, the air bleed switching valve63opens to allow the hydraulic oil from the relief valve40to flow to the tank21through the air bleed line90, and enters the allowed state. On the other hand, the air bleed switching valve63is brought into the blocked state with a predetermined pilot pressure being applied to the pilot port, and blocks the hydraulic oil from the relief valve40from flowing to the tank21through the air bleed line90. The air bleed switching valve63changes the opening degree based on the pilot pressure input to the pilot port.

When the lever lock81inputs the unlocking signal to the controller70, the controller70inputs a command signal to the solenoid of the lever lock valve64. As a result, the lever lock valve64opens, and the hydraulic oil from the pilot pump23is supplied to the pilot port of the air bleed switching valve63and the air bleed switching valve63is switched to the blocked state. On the other hand, when the lever lock81inputs the locking signal to the controller70, the controller70does not input, to the solenoid of the lever lock valve64, a command signal for communicating the line between the pilot pump23and the pilot port of the air bleed switching valve63. In this case, since the hydraulic oil from the pilot pump23is not supplied to the pilot port of the air bleed switching valve63, the air bleed switching valve63enters the allowed state. When the main pump22discharges the hydraulic oil, the above-described pressure difference is generated between the pressure in the relief valve40and the pressure at the portion92bdownstream on the return line92. As a result, the hydraulic oil containing air building up in the relief valve40flows out from the air bleed port43of the relief valve40to the air bleed line90, flows toward the tank21through the air bleed line90, and is collected in the tank21.

In the hydraulic excavator100according to the present embodiment, the air building up in the relief valve40of the hydraulic circuit can be effectively discharged from the relief valve40by using the pressure difference between the pressures of the pump discharge line91and the portion92bdownstream from the boost check valve67on the return line92. Therefore, regardless of the disposing orientation of the relief valve40in the upper slewing body2, the air building up in the relief valve40, particularly, the air building up in the pressure adjustment mechanism unit42of the relief valve40can be effectively discharged from the relief valve. As a result, even in a case where the orientation of the relief valve40is restricted by the disposition of the control valve main body, the risk of air building up in the relief valve40can be reduced. This heightens the degree of disposition freedom of the control valve unit30when the control valve unit30is mounted on the upper slewing body2, thus leading to cost reduction.

Further, in the present embodiment, by setting the air bleed switching valve63to the blocked state, the relief valve40can be caused to exhibit the function of limiting the pressure of the pump discharge line91, the function being the original function of the relief valve40. On the other hand, by setting the air bleed switching valve63to the allowed state, the air can be bled from the relief valve40at the timing when the air bleed is required. The air bleed of the relief valve40is performed, for example, immediately before an operator starts work with the hydraulic excavator100.

In the present embodiment, in a case where the state of the hydraulic circuit is switched from the locked state to the unlocked state, the controller70outputs a command signal for switching the air bleed switching valve63from the allowed state to the blocked state. Therefore, the air bleed of the relief valve40can be performed in the locked state, and the pressure of the pump discharge line91can be limited by the relief valve40in response to the switching from the locked state to the unlocked state.

In the present embodiment, the controller70may output a tilt indicator current which is a command signal for increasing the capacity of the main pump22when the air bleed switching valve63is in the allowed state. Specifically, when the lever lock81inputs the locking signal to the controller70(YES in step S11inFIG.4), the controller70does not input, to the solenoid of the lever lock valve64, a command signal for communicating the line between the pilot pump23and the pilot port of the air bleed switching valve63, thereby bringing the air bleed switching valve63into the allowed state (step S12inFIG.4). Further, the controller70outputs, for example, a preset tilt indicator current to the main pump22until a preset time elapses, thereby maintaining a state where the capacity of the main pump22increases (step S13inFIG.4). The preset time is set to, for example, a time longer than a time required for completing the air bleed in the relief valve40. Since the pressure difference is increased by increasing the capacity of the main pump22when the air bleed in the relief valve40is performed in this manner, the air bleed in the relief valve40can be performed in a shorter time.

When the hydraulic circuit is in the locked state, even if the air of the relief valve40is not sufficiently bled, for example, an automatic load operation, described later, may be automatically started. In each automatic load operation, the controller70performs control so that the hydraulic oil is discharged from the main pump22and the unloading valve50closes when the hydraulic circuit is in the locked state. Normally, the unloading valve50opens during non-operation and closes during the automatic load operation. In this case, when the automatic load operation is started in a state where the air bleed in the relief valve40is insufficient, a defect such as generation of an abnormal noise may occur in the relief valve40.

In the present embodiment, when the hydraulic circuit is in the locked state, the controller70outputs a command signal for opening the unloading valve50to the unloading electromagnetic proportional valve51. As a result, since the unloading valve50opens when the hydraulic circuit is in the locked state, the generation of an abnormal noise in the relief valve40can be avoided even if the automatic load operation is automatically started. Examples of the automatic load control include a warming-up operation at start of the engine, a deposited soot burning operation for burning deposited soot of a diesel particulate filter (DPF), and the like.

Specifically, when the lever lock81inputs a locking signal to the controller70after the start of the engine (specifically, for example, immediately after the start of the engine) (YES in step S21inFIG.5), the controller70does not input, to the solenoid of the lever lock valve64, the command signal for communicating the line between the pilot pump23and the pilot port of the air bleed switching valve63, thereby causing the air bleed switching valve63to be in the allowed state (step S22inFIG.5), and outputs the indicator current for opening the unloading valve50to the unloading electromagnetic proportional valve51until a preset time elapses (step S23inFIG.5). The preset time is set in advance to a time longer than a time required for completing the air bleed in the relief valve. The indicator current may be, for example, a value at which the opening of the unloading valve50is maximum. In such a manner, since the unloading valve50opens when the hydraulic circuit is in the locked state, the generation of an abnormal noise in the relief valve40can be avoided even if the automatic load operation is automatically started.

FIG.3is a diagram illustrating the hydraulic circuit of the hydraulic excavator100according to a first modification of the present embodiment and devices related thereto. The hydraulic circuit of the hydraulic excavator100according to the first modification is different from the hydraulic circuit illustrated inFIG.2in further including a pressure sensor61and in that the air bleed switching valve63is an electromagnetic switching valve, and the other parts are the same as those of the hydraulic circuit illustrated inFIG.2.

The pressure sensor61detects a discharge pressure of the main pump22and inputs a detection signal corresponding to the detected pressure to the controller70. The pressure sensor61is disposed, for example, on the pump discharge line91between the main pump22and the control valve main body.

In a case where executing the automatic load operation involving discharging the hydraulic oil from the main pump22when the hydraulic circuit is in the locked state (YES in step S31inFIG.6), the controller70outputs a blocking signal, which is a command signal for switching the air bleed switching valve63to the blocked state, to the air bleed switching valve63(step S32inFIG.6). In the automatic load operation, it is necessary to cause the relief valve40to exhibit a function of limiting the pressure of the pump discharge line91, the function being an original function of the relief valve40. In the present embodiment, in a case where executing the automatic load operation, the controller70outputs, to the air bleed switching valve, the blocking signal which is the command signal for switching the air bleed switching valve63to the blocked state. Therefore, even in a case where the hydraulic circuit is in the locked state, the automatic load operation can be securely executed.

Specifically, in the present embodiment, the controller70outputs the blocking signal to the air bleed switching valve63in a case where the pressure detected by the pressure sensor61exceeds a predetermined threshold. When the automatic load operation is started, the hydraulic oil is discharged from the main pump22, and thus the pressure of the pump discharge line91rises. Therefore, the increase in the pressure of the pump discharge line91at the time when the hydraulic circuit is in the locked state can be an index indicating that the automatic load operation has been started. Therefore, in the present embodiment, the controller70outputs the blocking signal to the air bleed switching valve63when the pressure of the pump discharge line91, the pressure being detected by the pressure sensor61, exceeds the threshold, and thus the relief valve can exhibit the function of limiting the pressure of the pump discharge line91in the automatic load operation.

Modifications

The construction machine according to the embodiment of the present disclosure has been described above, but the present disclosure is not limited to the embodiment, and includes the following modifications, for example.

(A) Regarding Construction Machine

In the above embodiment, the construction machine is the hydraulic excavator100, but may be another construction machine such as a crane or a bulldozer.

(B) Regarding Control Valve Unit

The control valve unit30includes the control valve main body, the relief valve40, and the unloading valve50, but at least one of the relief valve40and the unloading valve50may not be mounted on the control valve main body.

In the above embodiment, the locking mechanism (locking switch) is the lever lock81, but may be another mechanism (another switch) to which an operation performed by an operator can be input. An example of another mechanism (another switch) includes a switch that can receive an operator's input. In this case, the air bleed switching valve is configured to switch between the allowed state and the blocked state in response to an input operation given by an operator to the switch as the locking mechanism (locking switch).

(D) Regarding Back Pressure Generation Mechanism (Back Pressure Generator)

In the above embodiment, the check valve is used as the back pressure generation mechanism (back pressure generator), but the back pressure generation mechanism (back pressure generator) may be a relief valve or a throttle.

According to the present disclosure, provided is a construction machine capable of effectively discharging air building up in a relief valve of a hydraulic circuit from the relief valve.

A construction machine to be provided includes a tank that stores hydraulic oil, a main pump that is a hydraulic pump that discharges the hydraulic oil sucked from the tank, a hydraulic actuator that receives supply of the hydraulic oil from the main pump to operate, a direction switching valve that controls supply and discharge of the hydraulic oil between the main pump and the hydraulic actuator, a relief valve that opens to limit a pressure of a pump discharge line that is a line between the main pump and the direction switching valve, a back pressure generation mechanism that is disposed on a return line as a line connected to the tank and generates a back pressure in the return line, and an air bleed line that connects the relief valve and a portion downstream from the back pressure generation mechanism in the return line.

In this construction machine, air building up in the relief valve of the hydraulic circuit can be effectively discharged from the relief valve by using the pressure difference between the pressures of the pump discharge line and the portion downstream from the back pressure generation mechanism in the return line. Specifically, air is likely to build up in the relief valve depending on the mounting orientation of the relief valve onto a machine body such as the upper slewing body. In this configuration, the air building up in the relief valve of the hydraulic circuit can be effectively discharged from the relief valve regardless of the orientation of the relief valve. Therefore, even in a case where the orientation of the relief valve is determined by a factor other than the relief valve, the risk of air building up in the relief valve can be reduced.

The construction machine preferably further includes an air bleed switching valve capable of switching between an allowed state where the hydraulic oil from the relief valve is allowed to flow to the tank through the air bleed line and a blocked state where the hydraulic oil from the relief valve is blocked from flowing to the tank through the air bleed line. In this configuration, by setting the air bleed switching valve to the blocked state, the relief valve can be caused to exhibit the function of limiting the pressure of the pump discharge line, the function being the original function of the relief valve. On the other hand, by setting the air bleed switching valve to the allowed state, the air can be bled from the relief valve at the timing when the air bleed is required. Specifically, air bleed of the relief valve is preferably performed, for example, immediately before the operator starts work using the construction machine.

Preferably, the construction machine further includes an operation lever to which an operation for operating the hydraulic actuator is given, a locking mechanism that receives an operation for switching a state of a hydraulic circuit between an unlocked state where the hydraulic actuator is permitted to operate in response to the operation given to the operation lever and a locked state where the hydraulic actuator is prevented from operating in response to the operation given to the operation lever, and a controller. The controller outputs a command signal for switching the air bleed switching valve from the allowed state to the blocked state in a case where the state of the hydraulic circuit is switched from the locked state to the unlocked state. In this configuration, the air bleed in the relief valve can be performed in the locked state, and the pressure of the pump discharge line can be limited by the relief valve in response to the switching from the locked state to the unlocked state.

The main pump is a variable displacement type hydraulic pump, and the controller outputs a command signal for increasing a capacity of the main pump when the air bleed switching valve is in the allowed state. In this configuration, since the pressure difference is increased by increasing the capacity of the main pump when the air bleed of the relief valve is performed, the air bleed in the relief valve can be performed in a shorter time.

The air bleed switching valve is an electromagnetic switching valve, and the controller outputs, to the air bleed switching valve, a blocking signal that is a command signal for switching the air bleed switching valve to the blocked state, in a case where an automatic load operation involving discharging the hydraulic oil from the main pump is executed when the hydraulic circuit is in the locked state. In the construction machine, the controller may execute the automatic load operation such as a warming-up operation at start of an engine, and a deposited soot burning operation for burning deposited soot of a diesel particulate filter (DPF). In each automatic load operation, the controller performs control so that the hydraulic oil is discharged from the main pump when the hydraulic circuit is in the locked state. In the automatic load operation, it is necessary to cause the relief valve to exhibit a function of limiting the pressure of the pump discharge line, the function being an original function of the relief valve. In this configuration, in a case where executing the automatic load operation, the controller outputs, to the air bleed switching valve, the blocking signal that is the command signal for switching the air bleed switching valve to the blocked state. Therefore, even in a case where the hydraulic circuit is in the locked state, the automatic load operation can be securely executed.

Preferably, the construction machine further includes a pressure sensor that detects a pressure of the pump discharge line, and the controller outputs the blocking signal to the air bleed switching valve in a case where the pressure detected by the pressure sensor exceeds a predetermined threshold. When the automatic load operation is started, the hydraulic oil is discharged from the main pump, and thus the pressure of the pump discharge line rises. Therefore, the increase in the pressure of the pump discharge line at the time when the hydraulic circuit is in the locked state can be an index indicating that the automatic load operation has been started. Therefore, in the this configuration, the controller outputs the blocking signal to the air bleed switching valve when the pressure of the pump discharge line, the pressure being detected by the pressure sensor, exceeds the threshold, thereby causing the relief valve to exhibit the function of limiting the pressure of the pump discharge line in the automatic load operation.

The construction machine further includes an unloading valve that opens to allow the hydraulic oil discharged from the main pump to flow to the return line without supply to the hydraulic actuator, and the controller outputs a command signal for opening the unloading valve when the hydraulic circuit is in the locked state. When the hydraulic circuit is in the locked state, even if the air in the relief valve is not sufficiently bled, for example, the automatic load operation may be automatically started. Normally, the unloading valve opens during non-operation, that is, when the operation lever is not operated, and closes during the automatic load operation. In this case, when the automatic load operation is started in a state where the air bleed in the relief valve is insufficient, a defect such as generation of an abnormal noise may occur in the relief valve. In this configuration, even if the automatic load operation is automatically started when the hydraulic circuit is in the locked state, the opening of the unloading valve is forcibly held, thus making it possible to avoid generation of an abnormal noise in the relief valve. More preferably, when the hydraulic circuit is in the locked state, the controller outputs a command signal for maintaining the unloading valve in the opened state until a predetermined time elapses. In this case, the predetermined time is preferably set to be longer than the time required for completing the air bleed in the relief valve.