Hydraulic system of construction machine

A hydraulic system includes: a travel motor; travel pump connected to the travel motor, the travel pump driven by an engine; a work pump that sucks hydraulic oil from a tank through a suction line, and supplies the oil to a work hydraulic actuator through a delivery line, the work pump driven by the engine; a switching valve connected to the delivery line, and connected to a part of the suction line downstream of the check valve; and an accumulator connected to the switching valve. The switching valve switches between a neutral position in which the switching valve blocks a pressure accumulation line, a pressure release line, and the relay line, the pressure accumulation position in which the switching valve wherein the pressure accumulation line communicates with the relay line, a pressure release position in which the switching valve brings the relay line into communication with the pressure release line.

TECHNICAL FIELD

The present invention relates to a hydraulic system of a construction machine.

BACKGROUND ART

Among construction machines, such as wheel loaders and forklifts, there are those mounted with a hydraulic system that includes: a travel circuit that forms a hydraulic static transmission (HST) between an engine and traveling means, such as wheels or crawlers; and a work circuit for performing work, such as scooping work or lifting work.

For example, Patent Literature 1 discloses a hydraulic system100of a construction machine, which includes a travel circuit110as shown inFIG. 6. The travel circuit110includes: a travel pump120, which is driven by an engine101; and a travel motor130, which rotates a travel drive shaft102. The travel pump120is connected to the travel motor130by a pair of supply/discharge lines141and142, such that a closed loop is formed. The supply/discharge lines141and142are connected to a charge pump150.

The travel pump120is an over-center pump. When the construction machine travels forward, hydraulic oil is supplied to the travel motor130through one of the supply/discharge lines141and142, and when the construction machine travels backward, the hydraulic oil is supplied to the travel motor130through the other one of the supply/discharge lines141and142. The travel drive shaft102transmits torque from the travel motor130to a wheel104. The travel drive shaft102is provided with a mechanical brake103.

In addition, the hydraulic system100is configured to be able to regenerate kinetic energy during decelerating travel. Specifically, accumulators170are connected to the pair of supply/discharge lines141and142, respectively, via switching valves160. Pressurized oil discharged from the travel motor130during decelerating forward travel, and pressurized oil discharged from the travel motor130during decelerating backward travel, are separately accumulated in these accumulators170, respectively. The pressurized oil accumulated in the accumulator170during decelerating forward travel is utilized when the construction machine travels backward next time, and the pressurized oil accumulated in the accumulator170during decelerating backward travel is utilized when the construction machine travels forward next time.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

However, the hydraulic system100shown inFIG. 6requires two accumulators170for regenerating kinetic energy during decelerating travel. This causes high cost. Moreover, when accumulating the pressurized oil from one of the supply/discharge lines141and142into the accumulator170, the hydraulic oil in a corresponding amount needs to be charged from the charge pump150to the other one of the supply/discharge lines141and142.

Furthermore, in the hydraulic system100shown inFIG. 6, since the accumulator170is connected to each of the supply/discharge lines141and142, braking force is limited by the setting pressure of the accumulator170, and there is a case where only by the accumulation of the pressurized oil in the accumulator170, sufficient braking force cannot be obtained. Also, the pressurized oil accumulated in the accumulator170cannot be utilized in a situation where the delivery pressure of the travel pump120becomes higher than the setting pressure of the accumulator170(e.g., during rapid acceleration).

In view of the above, an object of the present invention is to provide a hydraulic system of a construction machine, the hydraulic system being capable of regenerating kinetic energy during decelerating travel without using an accumulator in a travel circuit.

Solution to Problem

In order to solve the above-described problems, a hydraulic system of a construction machine according to the present invention includes: a travel motor that rotates a travel drive shaft; a travel pump that is connected to the travel motor such that a closed loop is formed, the travel pump being driven by an engine; a work pump that sucks hydraulic oil from a tank through a suction line provided with a check valve, and supplies the hydraulic oil to a work hydraulic actuator through a delivery line, the work pump being driven by the engine; a switching valve that is connected to the delivery line by a pressure accumulation line, and connected to a part of the suction line downstream of the check valve by a pressure release line; and an accumulator that is connected to the switching valve by a relay line. The switching valve is switched between a neutral position, a pressure accumulation position, and a pressure release position, the neutral position being a position in which the switching valve blocks the pressure accumulation line, the pressure release line, and the relay line, the pressure accumulation position being a position in which the switching valve brings the pressure accumulation line into communication with the relay line, the pressure release position being a position in which the switching valve brings the relay line into communication with the pressure release line.

According to the above configuration, during decelerating travel, since the travel pump functions as a motor, the driving of the work pump is assisted thereby. Accordingly, when the switching valve is switched to the pressure accumulation position, kinetic energy during decelerating travel can be converted into pressurized oil delivered from the work pump, and the pressurized oil can be accumulated in the accumulator. On the other hand, when the switching valve is switched to the pressure release position, the pressure at the suction side of the work pump increases. As a result, motive power required for driving the work pump decreases, and thereby energy consumption is reduced. Through such a cycle, kinetic energy during decelerating travel is regenerated. Since the accumulator is provided in a work circuit including the work pump, kinetic energy during decelerating travel can be regenerated without using an accumulator in a travel circuit.

The construction machine may be a wheel loader. The above hydraulic system may further include: a controller that controls the switching valve; an accelerator pedal that receives an acceleration command; a mechanical brake provided on the travel drive shaft; a brake pedal that receives a brake operation for the mechanical brake; a vehicle speed detector that detects a vehicle speed; and a delivery pressure detector that detects a delivery pressure of the work pump. The controller may: when a pressure accumulation condition is satisfied, switch the switching valve to the pressure accumulation position, the pressure accumulation condition being a condition that the accelerator pedal and the brake pedal are depressed concurrently and the vehicle speed detected by the vehicle speed detector is higher than a first threshold; and when the pressure accumulation condition is not satisfied, if the delivery pressure of the work pump, which is detected by the delivery pressure detector, is higher than a second threshold, switch the switching valve to the pressure release position, whereas if the delivery pressure of the work pump is lower than the second threshold, switch the switching valve to the neutral position. When the wheel loader performs the work of scooping a material to be carried, the wheel loader first travels to the vicinity of the material to be carried, and stops. Then, the wheel loader performs the scooping work with a bucket and a boom. When the wheel loader stops traveling, the brake pedal is depressed with the accelerator pedal kept depressed in order not to decrease the engine rotation speed in preparation for the scooping work performed thereafter. In light of this, if the pressure accumulation condition is defined as a condition that the accelerator pedal and the brake pedal are depressed concurrently and the vehicle speed is higher than the first threshold, it becomes possible to accumulate pressurized oil in the accumulator by efficiently utilizing a situation where the work pump is driven at a high engine rotation speed.

The above hydraulic system may further include a boom operation device and a bucket operation device, each of which includes an operating lever. The second threshold may be a value that is higher than the delivery pressure of the work pump when the operating lever of the boom operation device and the operating lever of the bucket operation device are in a neutral state. According to this configuration, when the work pump is in a standby state, the accumulator is in a pressure-releasing state. This consequently makes it possible to prevent the engine rotation speed from becoming excessively high.

The work pump may be a variable displacement pump. The above hydraulic system may further include a regulator that adjusts a tilting angle of the work pump. When the pressure accumulation condition is satisfied, the controller may switch the switching valve to the pressure accumulation position, and control the regulator such that the tilting angle of the work pump increases. According to this configuration, the braking force when the pressure accumulation condition is satisfied can be increased.

The above hydraulic system may further include a solenoid proportional valve provided on a signal pressure line that leads a signal pressure corresponding to a depression amount of the brake pedal to the mechanical brake from a brake operation device including the brake pedal, the solenoid proportional valve being capable of decreasing the signal pressure. The controller may control the solenoid proportional valve such that a braking force of the mechanical brake when the pressure accumulation condition is satisfied is less than the braking force of the mechanical brake when the pressure accumulation condition is not satisfied. This configuration makes it possible to suppress the overall braking force from becoming excessively high due to the pressurized oil being accumulated in the accumulator.

Advantageous Effects of Invention

The present invention makes it possible to regenerate kinetic energy during decelerating travel without using an accumulator in a travel circuit.

DESCRIPTION OF EMBODIMENTS

FIG. 1shows a hydraulic system1of a construction machine according to one embodiment of the present invention. In the present embodiment, the construction machine is a wheel loader including a wheel15as traveling means. Alternatively, the construction machine may be, for example, a forklift, a compact truck loader, or a crawler carrier. In a case where the construction machine is a compact truck loader or a crawler carrier, the traveling means is a crawler.

The hydraulic system1installed in the construction machine includes: an engine11; a travel circuit2, which forms a hydraulic static transmission (HST) between the engine11and the wheel15; and a work circuit4for performing, for example, scooping work.

The travel circuit2includes: a travel pump21, which is driven by the engine11; and a travel motor23, which rotates a travel drive shaft13. The travel pump21is connected to the travel motor23by a pair of supply/discharge lines31and32, such that a closed loop is formed.

In the present embodiment, the output shaft of the engine11is coupled to the rotating shaft of the travel pump21via a gearbox12. The output shaft of the engine11is also coupled to the rotating shaft of a work pump41via the gearbox12. The work pump41will be described below.

The travel pump21is an over-center pump. When the construction machine travels forward, hydraulic oil is supplied to the travel motor23through one of the supply/discharge lines31and32, and when the construction machine travels backward, the hydraulic oil is supplied to the travel motor23through the other one of the supply/discharge lines31and32. The travel drive shaft13transmits torque from the travel motor23to the wheel15. The travel drive shaft13is provided with a mechanical brake14.

The mechanical brake14is configured to press a wear component, such as a pad or lining, against a disc or drum that rotates together with the travel drive shaft13, thereby applying braking force to the travel drive shaft13. To be more specific, the mechanical brake14includes brake cylinders provided for respective wheels. When a brake pedal92, which will be described below, is depressed, pressurized oil is generated in a master cylinder. The generated pressurized oil is led to each brake cylinder, and consequently, the wear component is pressed against the disc or drum.

The supply/discharge lines31and32are connected to each other by a bridging passage33. The bridging passage33is provided with a pair of check valves34, which are directed opposite to each other. A part of the bridging passage33between the check valves34is connected to a charge pump25by a charge line35. Although not illustrated, a relief line is branched off from the charge line35, and the relief line is provided with a relief valve.

The rotating shaft of the charge pump25is coupled to the output shaft of the engine11via the rotating shaft of the travel pump21and the gearbox12. The charge pump25is driven by the engine11.

The travel pump21is a variable displacement pump whose tilting angle is changeable. In the present embodiment, the travel pump21is a swash plate pump whose tilting angle is defined by the angle of its swash plate. Alternatively, the travel pump21may be a bent axis pump whose tilting angle is defined by the angle of its tilted axis. The tilting angle of the travel pump21is adjusted by a regulator22.

In the present embodiment, the regulator22moves in accordance with an electrical signal. Alternatively, the regulator22may move in accordance with a hydraulic pilot pressure or a manual lever. For example, the regulator22may electrically change the hydraulic pressure applied to a servo piston coupled to the swash plate of the travel pump21, or may be an electric actuator coupled to the swash plate of the travel pump21.

The travel motor23is a variable displacement motor whose tilting angle is changeable. In the present embodiment, the travel motor23is a swash plate motor whose tilting angle is defined by the angle of its swash plate. Alternatively, the travel motor23may be a bent axis motor whose tilting angle is defined by the angle of its tilted axis. The tilting angle of the travel motor23is adjusted by a regulator24.

In the present embodiment, the regulator24moves in accordance with an electrical signal. Alternatively, the regulator24may move in accordance with a hydraulic pilot pressure. For example, the regulator24may electrically change the hydraulic pressure applied to a servo piston coupled to the swash plate of the travel motor23, or may be an electric actuator coupled to the swash plate of the travel motor23.

The regulators22and24are controlled by a controller7. It should be noted thatFIG. 1shows only part of signal lines for simplifying the drawing. For example, the controller7is a computer including a CPU and memories such as a ROM and RAM. The CPU executes a program stored in the ROM.

An accelerator pedal91and the brake pedal92are provided in an operator cab that is not shown. The accelerator pedal91receives an acceleration command regarding the engine rotation speed, and the brake pedal92receives a brake operation for the mechanical brake14.

The mechanical brake14is connected, by a signal pressure line93, to a brake operation device including the brake pedal92. The brake operation device outputs a signal pressure corresponding to a depression amount of the brake pedal92. That is, the signal pressure outputted from the brake operation device increases in accordance with increase in the depression amount of the brake pedal92. The signal pressure outputted from the brake operation device is led to the mechanical brake14through the signal pressure line93.

A depression amount θA of the accelerator pedal91is detected by a first depression amount detector74. A depression amount θB of the brake pedal92is detected by a second depression amount detector75. The first depression amount detector74and the second depression amount detector75are, for example, potentiometers.

The controller7is electrically connected to the first depression amount detector74and the second depression amount detector75. The controller7is also electrically connected to a rotation speed detector71, which detects the engine rotation speed. The controller7is further electrically connected to a vehicle speed detector72, which detects a vehicle speed V. In the present embodiment, the vehicle speed detector72detects the rotation speed of the travel drive shaft13as the vehicle speed V.

The controller7controls the engine11, such that the engine rotation speed detected by the rotation speed detector71changes in accordance with the depression amount θA of the accelerator pedal91, which is detected by the first depression amount detector74, and the vehicle speed V, which is detected by the vehicle speed detector72. For example, when the vehicle speed V is lower than a particular value, the engine rotation speed of the engine11is increased to a predetermined engine rotation speed in accordance with increase in the depression amount θA of the accelerator pedal91, and when the vehicle speed V is higher than or equal to the particular value, the engine rotation speed of the engine11is kept at a substantially constant rotation speed.

Further, in a case where only the accelerator pedal91is depressed, the controller7controls the regulator22for the travel pump21and the regulator24for the travel motor23based on the depression amount θA of the accelerator pedal91. For example, the controller7controls the regulator22for the travel pump21, such that the tilting angle of the travel pump21increases in accordance with increase in the depression amount θA of the accelerator pedal91.

On the other hand, when the brake pedal92is depressed with the accelerator pedal91kept depressed, the controller7controls the regulator22for the travel pump21and the regulator24for the travel motor23based on the depression amount θB of the brake pedal92. For example, the controller7controls the regulator22for the travel pump21, such that the tilting angle of the travel pump21decreases in accordance with increase in the depression amount θB of the brake pedal92.

The work circuit4includes: the work pump41, which is driven by the engine11; and a boom cylinder53and a bucket cylinder56, which serve as work hydraulic actuators. The boom cylinder53swings an unshown boom, and the bucket cylinder56swings an unshown bucket.

The work pump41is connected to a tank by a suction line43, and to a boom control valve51and a bucket control valve54by a delivery line45. The boom control valve51is connected to the boom cylinder53by a pair of supply/discharge lines52, and the bucket control valve54is connected to the bucket cylinder56by a pair of supply/discharge lines55. That is, the work pump41sucks the hydraulic oil from the tank through the suction line43, supplies the hydraulic oil to the boom cylinder53through the delivery line45, the boom control valve51, and one of the supply/discharge lines52, and supplies the hydraulic oil to the bucket cylinder56through the delivery line45, the bucket control valve54, and one of the supply/discharge lines55.

The work pump41is a variable displacement pump whose tilting angle is changeable. In the present embodiment, the work pump41is a swash plate pump. Alternatively, the work pump41may be a bent axis pump. The tilting angle of the work pump41is adjusted by a regulator42. It should be noted that the minimum delivery flow rate of the work pump41is set to be greater than zero.

A delivery pressure Pw of the work pump41is kept to be lower than or equal to a relief pressure by an unshown relief valve. An unloading line46is branched off from the delivery line45, and the unloading line46is provided with an unloading valve47.

In the present embodiment, the regulator42moves in accordance with an electrical signal. Alternatively, the regulator24may move in accordance with a hydraulic pilot pressure. For example, the regulator42may electrically change the hydraulic pressure applied to a servo piston coupled to the swash plate of the work pump41, or may be an electric actuator coupled to the swash plate of the work pump41.

A boom operation device81and a bucket operation device82are provided in the operator cab, which is not shown. The boom operation device81includes an operating lever that receives a boom operation, and outputs a boom operation signal corresponding to an inclination angle of the operating lever. That is, the boom operation signal outputted from the boom operation device81increases in accordance with increase in the inclination angle (i.e., operating amount) of the operating lever. Similarly, the bucket operation device82includes an operating lever that receives a bucket operation, and outputs a bucket operation signal corresponding to an inclination angle of the operating lever.

In the present embodiment, each of the boom operation device81and the bucket operation device82is an electrical joystick that outputs an electrical signal as an operation signal. The boom operation signal outputted from the boom operation device81, and the bucket operation signal outputted from the bucket operation device82, are inputted to the controller7. The controller7controls the boom control valve51via an unshown pair of solenoid proportional valves, such that the opening area of the boom control valve51is adjusted to an opening area corresponding to the boom operation signal. The controller7also controls the bucket control valve54via an unshown pair of solenoid proportional valves, such that the opening area of the bucket control valve54is adjusted to an opening area corresponding to the bucket operation signal.

The controller7further controls the regulator42and the unloading valve47, such that the tilting angle of the work pump41increases, and the opening area of the unloading valve47decreases, in accordance with increase in the boom operation signal and/or the bucket operation signal.

It should be noted that each of the boom operation device81and the bucket operation device82may be a pilot operation valve that outputs a pilot pressure as an operation signal. In this case, pilot ports of the boom control valve51are connected by pilot lines to the boom operation device81, which is a pilot operation valve, and pilot ports of the bucket control valve54are connected by pilot lines to the bucket operation device82, which is a pilot operation valve. Further, in a case where the boom operation device81is a pilot operation valve, the pilot lines between the boom control valve51and the boom operation device81are each provided with a pressure detector, and a pilot pressure (a boom operation signal) detected by the pressure detector is inputted to the controller7. The same is true in a case where the bucket operation device82is a pilot operation valve. Alternatively, the boom control valve51and the bucket control valve54may be solenoid pilot valves.

The present embodiment further adopts a configuration for regenerating kinetic energy during decelerating travel by utilizing the work pump41.

Specifically, the suction line43of the work pump41is provided with a check valve44. A part of the suction line43downstream of the check valve44is connected to a switching valve61by a pressure release line63. Also, the switching valve61is connected to the delivery line45by a pressure accumulation line62, and to an accumulator65by a relay line64.

The switching valve61is switched between a neutral position, a pressure accumulation position (upper position inFIG. 1), and a pressure release position (lower position inFIG. 1). When the switching valve61is in the neutral position, the switching valve61blocks the pressure accumulation line62, the pressure release line63, and the relay line64. When the switching valve61is in the pressure accumulation position, the switching valve61blocks the pressure release line63, and brings the pressure accumulation line62into communication with the relay line64. When the switching valve61is in the pressure release position, the switching valve61blocks the pressure accumulation line62, and brings the relay line64into communication with the pressure release line63.

The switching valve61is controlled by the controller7. The controller7is also electrically connected to a delivery pressure detector73, which detects the delivery pressure Pw of the work pump41.FIG. 2is a flowchart of control of the switching valve61, which is performed by the controller7.

First, the controller7determines whether or not a pressure accumulation condition is satisfied (step S1). When the pressure accumulation condition is satisfied (YES in step S1), the controller7switches the switching valve61to the pressure accumulation position (step S2).

The pressure accumulation condition is a condition that the accelerator pedal91and the brake pedal92are depressed concurrently and the vehicle speed V detected by the vehicle speed detector72is higher than a first threshold α. The controller7determines whether or not the accelerator pedal91is depressed by comparing the depression amount θA of the accelerator pedal91, which is detected by the first depression amount detector74, with a first setting value θ1, and determines whether or not the brake pedal92is depressed by comparing the depression amount θB of the brake pedal92, which is detected by the second depression amount detector75, with a second setting value θ2. The first threshold α is an index for determining whether or not the wheel loader is traveling. The first threshold α is, for example, about 1 km/h.

When switching the switching valve61to the pressure accumulation position, the controller7controls the regulator42such that the tilting angle of the work pump41increases (step S3). It should be noted that step S3may be eliminated. Also, when switching the switching valve61to the pressure accumulation position, the controller7may control the regulator42such that the amount of increase in the tilting angle of the work pump41decreases in accordance with decrease in the vehicle speed V.

On the other hand, when the pressure accumulation condition is not satisfied (NO in step S1), the controller7switches the switching valve61to the neutral position or the pressure release position based on the delivery pressure Pw of the work pump41, which is detected by the delivery pressure detector73. To be more specific, in a case where the delivery pressure Pw of the work pump41is higher than a second threshold β, the controller7switches the switching valve61to the pressure release position, whereas in a case where the delivery pressure Pw of the work pump41is lower than the second threshold β, the controller7switches the switching valve61to the neutral position.

The second threshold β is an index for determining whether or not the hydraulic oil is being supplied from the work pump41to a work hydraulic actuator (i.e., whether or not work by the work circuit4is being performed). For example, the second threshold β is 0.1 to 10 MPa. Desirably, the second threshold β is a value (e.g., 2 to 10 MPa) that is higher than the delivery pressure Pw of the work pump41when the operating lever of the boom operation device81and the operating lever of the bucket operation device82are in a neutral state. The reason for setting the second threshold β to such a value is that if the second threshold β is set to a relatively high value, the accumulator65will be in a pressure-releasing state when the work pump41is in a standby state. This consequently makes it possible to prevent the engine rotation speed from becoming excessively high.

For example,FIGS. 3A to 3Hare timing charts for a period from before to after the start of scooping work. When the wheel loader travels to the vicinity of a material to be carried, only the accelerator pedal91is depressed. Normally, while the wheel loader is traveling, the boom operation device81and the bucket operation device82are not operated. Accordingly, the switching valve61is kept in the neutral position until the brake pedal92is depressed.

When the wheel loader stops after arriving at the vicinity of the material to be carried, the brake pedal92is depressed with the accelerator pedal91kept depressed. It should be noted that some operator may depress the brake pedal92while slightly returning the accelerator pedal91. As a result, the tilting angle of the travel pump21decreases, and the mechanical brake14applies. Consequently, the vehicle speed V decreases. During decelerating travel, since the travel pump21functions as a motor, the driving of the work pump41is assisted thereby.

Since the brake pedal92is depressed with the accelerator pedal91kept depressed, the switching valve61is switched to the pressure accumulation position, and also, the tilting angle of the work pump41increases, causing increase in the delivery flow rate of the work pump41. Accordingly, the delivery pressure of the work pump41increases to the setting pressure of the accumulator65, and the oil delivered from the work pump41is accumulated in the accumulator65. That is, when the switching valve61is switched to the pressure accumulation position, kinetic energy during decelerating travel can be converted into pressurized oil delivered from the work pump41, and the pressurized oil can be accumulated in the accumulator65.

Thereafter, the scooping work is started, and when the delivery pressure Pw of the work pump41has become higher than the second threshold β, the switching valve61is switched to the pressure release position, and the pressure at the suction side of the work pump41increases. As a result, motive power required for driving the work pump41decreases, and thereby energy consumption is reduced.

As described above, in the hydraulic system1according to the present embodiment, kinetic energy during decelerating travel is regenerated through a cycle in which pressurized oil is accumulated in the accumulator65and then the accumulated pressurized oil is released from the accumulator65. Since the accumulator65is provided in the work circuit4, kinetic energy during decelerating travel can be regenerated without using an accumulator in the travel circuit2.

Further, in the present embodiment, the pressure accumulation condition is defined as a condition that the accelerator pedal91and the brake pedal92are depressed concurrently and the vehicle speed V is higher than the first threshold α. This makes it possible to accumulate pressurized oil in the accumulator65by efficiently utilizing a situation where the work pump41is driven at a high engine rotation speed immediately before the start of the scooping work.

Moreover, in the present embodiment, when the pressure accumulation condition is satisfied, the tilting angle of the work pump41increases. Accordingly, the torque generated by the work pump41(which is proportional to the product of the delivery pressure and the delivery capacity of the work pump41) increases, which makes it possible to increase the braking force.

Furthermore, in the present embodiment, when the switching valve61is switched to the pressure accumulation position, the amount of increase in the tilting angle of the work pump41(which is indicated by a hatched area inFIG. 3H) decreases in accordance with decrease in the vehicle speed V. Accordingly, the torque generated by the work pump41can be reduced in accordance with decrease in the vehicle speed V, i.e., in accordance with decrease in kinetic energy. This makes it possible to ease a sensation that the braking force is too strong in the latter half of the deceleration.

OTHER EMBODIMENTS

The present invention is not limited to the above-described embodiment. Various modifications can be made without departing from the scope of the present invention.

For example, as shown inFIG. 4, the signal pressure line93between the mechanical brake14and the brake operation device including the brake pedal92may be provided with a solenoid proportional valve94, which is capable of decreasing the signal pressure outputted from the brake operation device. In this case, the controller7controls the solenoid proportional valve94, such that the braking force of the mechanical brake14when the pressure accumulation condition is satisfied is less than the braking force of the mechanical brake14when the pressure accumulation condition is not satisfied. According to this configuration, when the load on the work pump41increases, braking force is applied to the travel drive shaft13via the travel pump21and the travel motor23. Accordingly, the amount of wear of the wear components, such as the pads or linings, of the mechanical brake14is reduced. Moreover, this configuration makes it possible to suppress the overall braking force from becoming excessively high due to the pressurized oil being accumulated in the accumulator65.

As shown inFIG. 5, the output shaft of the engine11may be directly coupled to the rotating shaft of the travel pump21. In this case, the rotating shaft of the work pump41may be directly coupled to the rotating shaft of the charge pump25.

REFERENCE SIGNS LIST