Hydraulic system

A hydraulic system includes: a cylinder in which an interior of a tube is divided by a piston into a first pressure chamber and a second pressure chamber; a first bidirectional pump connected to the first pressure chamber by a first supply/discharge line; a second bidirectional pump connected to the second pressure chamber by a second supply/discharge line and coupled to the first bidirectional pump in a manner enabling torque to be transmitted between the first and second bidirectional pumps; a relay line connecting the first and second bidirectional pumps such that a hydraulic liquid discharged from one of the first and second bidirectional pumps is introduced into the other of the first and second bidirectional pumps; and an electric motor that drives the first or second bidirectional pump. At least one of the first and second bidirectional pumps is a variable displacement pump whose delivery capacity per rotation is freely variable.

TECHNICAL FIELD

The present invention relates to a hydraulic system including a cylinder.

BACKGROUND ART

For example, a known hydraulic system for incorporation into a press machine or the like includes a single-rod cylinder that moves a moving object such as a movable die in the vertical direction and a bidirectional pump connected to the cylinder such that a closed circuit is formed. The bidirectional pump is typically driven by a servomotor.

For example, Patent Literature 1 discloses a hydraulic system100as shown inFIG. 5which is for incorporation into a press machine. This hydraulic system100includes a single-rod cylinder110disposed such that a rod112projects downward from a tube111closed at both ends. That is, a moving object (movable die)160is lowered by extension of the rod112and raised by retraction of the rod112.

A rod-side chamber113of the cylinder110is connected to a bidirectional pump140by a first supply/discharge line120, and a head-side chamber114of the cylinder110is connected to the bidirectional pump140by a second supply/discharge line130. The first supply/discharge line120is provided with a counterbalance valve121. Further, a bypass line122is connected to the first supply/discharge line120in such a manner as to bypass the counterbalance valve121, and the bypass line122is provided with a speed-switching valve123.

The lowering speed of the moving object160is switched by the speed-switching valve123between an approaching speed which is relatively high and a working speed which is relatively low. That is, during pressing, a reactive force is applied against extension of the rod by means of the counterbalance valve121.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

In the configuration like that of the hydraulic system100shown inFIG. 5, where during pressing a reactive force is applied against extension of the rod by means of the counterbalance valve, the speed, stroke, and thrust of the cylinder can be stably controlled (hereinafter, the speed, stroke, and thrust of a cylinder will be collectively referred to as “the speed etc.” of the cylinder). However, in this configuration, energy loss occurs due to passing of the hydraulic liquid through the counterbalance valve. In some cases, the counterbalance valve is used to apply a reactive force against retraction of the rod.

The counterbalance value can be used also when the rod projects in a direction opposite to the projecting direction inFIG. 5, namely when the rod projects upward from the tube or when the axial direction of the single-rod cylinder is horizontal, in order to apply a reactive force against extension or retraction of the rod and thus stably control the speed etc. of the cylinder. These configurations also suffer from energy loss occurring due to passing of the hydraulic liquid through the counterbalance valve. Further, the counterbalance valve can be used to stably control the speed etc. of a double-rod cylinder by applying a reactive force against the movement of the rods relative to the tube.

The present invention aims to provide a hydraulic system able to stably control the speed etc. of a cylinder without the use of any counterbalance valve.

Solution to Problem

In order to solve the problem described above, a hydraulic system of the present invention includes: a cylinder in which an interior of a tube is divided by a piston into a first pressure chamber and a second pressure chamber; a first bidirectional pump connected to the first pressure chamber by a first supply/discharge line; a second bidirectional pump connected to the second pressure chamber by a second supply/discharge line and coupled to the first bidirectional pump in a manner enabling torque to be transmitted between the first and second bidirectional pumps; a relay line connecting the first and second bidirectional pumps such that a hydraulic liquid discharged from one of the first and second bidirectional pumps is introduced into the other of the first and second bidirectional pumps; and an electric motor that drives the first or second bidirectional pump, wherein at least one of the first and second bidirectional pumps is a variable displacement pump whose delivery capacity per rotation is freely variable.

In the above configuration, since the second bidirectional pump is coupled to the first bidirectional pump in a manner enabling torque to be transmitted between the first and second bidirectional pumps, both the first and second bidirectional pumps are driven once one of the pumps is driven by the electric motor. Additionally, since at least one of the first and second bidirectional pumps is a variable displacement pump whose delivery capacity per rotation is freely variable, the delivery capacity ratio between the first and second bidirectional pumps can be appropriately set even if the rotational speed ratio between the first and second bidirectional pumps is constant. Thus, a reactive force can, without the use of any counterbalance valve, be applied against extension or retraction of the rod when the cylinder is a single-rod cylinder and against the movement of the rods relative to the tube when the cylinder is a double-rod cylinder. In consequence, the speed etc. of the cylinder can be stably controlled.

Further, special benefits are achieved by the fact that the second bidirectional pump is coupled to the first bidirectional pump in a manner enabling torque to be transmitted between the first and second bidirectional pumps. For example, when the cylinder is disposed to move a moving object in the vertical direction, the potential energy of the moving object can, during lowering of the moving object, be recovered in the form of rotational torque by one of the first and second bidirectional pumps (the pump into which the hydraulic liquid discharged from the cylinder flows). When the cylinder is disposed to move the moving object in the horizontal direction, the drive power of the one of the first and second bidirectional pumps can be recovered in the form of torque for generating a reactive force against extension or retraction of the rod. Thus, the driving of the other of the first and second bidirectional pumps can be assisted regardless of the movement direction of the moving object.

One of the first and second bidirectional pumps may be a variable displacement pump whose delivery capacity per rotation is freely variable, and the other of the first and second bidirectional pumps may be a fixed displacement pump whose delivery capacity per rotation is invariable or a variable displacement pump whose delivery capacity per rotation is selectively switchable between a first fixed value and a second fixed value. In this configuration, the cost can be reduced compared to that required when both the first and second bidirectional pumps are variable displacement pumps.

Alternatively, both the first and second bidirectional pumps may be variable displacement pumps whose delivery capacities per rotation are freely variable. In this configuration, the flow rate control can be performed more flexibly than when one of the first and second bidirectional pumps is a fixed displacement pump or a variable displacement pump whose delivery capacity is selectively switchable.

The first bidirectional pump may include a cylinder-side port (a pump port connected to the cylinder) and a cylinder-opposite port (a pump port connected to an element other than the cylinder) having a larger diameter than the cylinder-side port, and the second bidirectional pump may include a cylinder-side port and a cylinder-opposite port having a larger diameter than the cylinder-side port. In this configuration, since the internal passage of each of the first and second bidirectional pumps that communicates with the cylinder-opposite port is subjected to a lower pressure than the passage communicating with the cylinder-side port, the internal passage need not be strong enough to withstand high pressures and can have an increased passage area. This can reduce the pressure drop which occurs when the hydraulic liquid is passing through the passage.

For example, the cylinder may be a double-rod cylinder or a single-rod cylinder.

The hydraulic system may further include: an inlet line connecting the relay line and a tank; a check valve disposed in the inlet line to permit a flow from the tank toward the relay line and prohibit the opposite flow; an outlet line connecting the relay line and the tank; and an outlet valve disposed in the outlet line to permit a flow from the relay line toward the tank when a pressure in the relay line is higher than a preset value. In this configuration, insufficient flow rate of the hydraulic liquid sucked into the first or second bidirectional pump and excessive increase in pressure in the relay line can be prevented.

Advantageous Effects of Invention

According to the present invention, the speed etc. of a cylinder can be stably controlled without the use of any counterbalance valve.

DESCRIPTION OF EMBODIMENTS

FIG. 1shows a hydraulic system1A according to Embodiment 1 of the present invention. This hydraulic system1A is incorporated, for example, into a press machine. The hydraulic liquid used in the hydraulic system1A is typically an oil, and may be another liquid such as water.

The hydraulic system1A includes a cylinder5. In the present embodiment, the cylinder5is a single-rod cylinder5that moves a moving object10in the vertical direction. The axial direction of the cylinder5need not be exactly parallel to the vertical direction, and may be slightly inclined with respect to the vertical direction (for example, the angle of inclination with respect to the vertical direction is 10 degrees or less). Alternatively, the axial direction of the cylinder5may be horizontal or oblique.

The hydraulic system1A further includes a first bidirectional pump3and a second bidirectional pump4which are connected to the cylinder5such that a closed circuit is formed. The closed circuit is connected to a tank60by an inlet line64and an outlet line66.

The cylinder5includes: a tube55closed at both ends by a head cover and a rod cover; a piston56dividing the interior of the tube55into a first pressure chamber51located on the head cover side and a second pressure chamber52located on the rod cover side; and a rod57extending from the piston56and penetrating through the rod cover. That is, in the present embodiment, the first pressure chamber51is a head-side chamber, and the second pressure chamber52is a rod-side chamber. The moving object10is mounted on the tip of the rod57.

In the present embodiment, the cylinder5is disposed such that the rod57projects downward from the tube55. That is, the first pressure chamber51is located on the upper side, the second pressure chamber52is located on the lower side, and the second pressure chamber is pressurized by the rod57and the weight of the moving object10. Alternatively, the cylinder5may be disposed such that the rod57projects upward from the tube55and that the second pressure chamber52is located on the upper side and the first pressure chamber51is located on the lower side.

The first bidirectional pump3includes a cylinder-side port31and a cylinder-opposite port32that switch between functioning as a suction port and functioning as a delivery port depending on the rotational direction of the pump. The cylinder-side port31is connected to the first pressure chamber51of the cylinder5by a first supply/discharge line61. The cylinder-side port31is designed to withstand high pressures, and the cylinder-opposite port32is held at a low pressure. Thus, the cylinder-opposite port32has a larger diameter than the cylinder-side port31.

The second bidirectional pump4includes a cylinder-side port41and a cylinder-opposite port42that switch between functioning as a suction port and functioning as a delivery port depending on the rotational direction of the pump. The cylinder-side port41is connected to the second pressure chamber52of the cylinder5by a second supply/discharge line62. The cylinder-side port41is designed to withstand high pressures, and the cylinder-opposite port42is held at a low pressure. Thus, the cylinder-opposite port42has a larger diameter than the cylinder-side port41.

The cylinder-opposite port42of the second bidirectional pump4is connected to the cylinder-opposite port32of the first bidirectional pump3by a relay line63. Thus, the hydraulic liquid discharged from one of the first and second bidirectional pumps3and4is introduced into the other of the first and second bidirectional pumps3and4through the relay line63.

The inlet and outlet lines64and66mentioned above connect the relay line63and the tank60. The inlet line64is provided with a check valve65, and the outlet line66is provided with an outlet valve67. The check valve65permits a flow from the tank60toward the relay line63and prohibits the opposite flow.

The outlet valve67permits a flow from the relay line63toward the tank60when the pressure in the relay line63is higher than a preset value (e.g., 0.1 to 2 MPa), and otherwise prohibits the flow between the relay line63and the tank60. In the present embodiment, the outlet valve67is a check valve whose cracking pressure is set to a somewhat high value. Alternatively, the outlet valve67may be a relief valve.

The first and second bidirectional pumps3and4are coupled together in a manner enabling torque to be transmitted between them. In the present embodiment, the first and second bidirectional pumps3and4are coaxially arranged. For example, the rotating shafts of the first and second bidirectional pumps3and4are coupled directly by means such as a coupling.

Alternatively, a plurality of gears may be disposed between the rotating shafts of the first and second bidirectional pumps3and4, and the first and second bidirectional pumps3and4may be arranged in parallel. In this case, the rotational speeds of the first and second bidirectional pumps3and4may be different.

In the present embodiment, the first bidirectional pump3is a variable displacement pump (a swash plate pump or bent axis pump) whose delivery capacity per rotation is freely variable, and the second bidirectional pump4is a fixed displacement pump whose delivery capacity per rotation is invariable. The tilt angle of the first bidirectional pump3, which defines the delivery capacity, is regulated by a regulator35. For example, when the first bidirectional pump3is a swash plate pump, the regulator35may be a regulator that electrically varies the hydraulic pressure acting on a servo piston coupled to the swash plate of the first bidirectional pump3, or may be an electric actuator coupled to the swash plate of the first bidirectional pump3.

It should be noted that the second bidirectional pump4may, as shown inFIG. 2, be a variable displacement pump (a swash plate pump or bent axis pump) whose delivery capacity per rotation is selectively switchable between a first fixed value q1and a second fixed value q2greater than the first fixed value q1. In this configuration, the speed of the cylinder5can be switched between a low speed and a high speed. In this case, the tilt angle of the second bidirectional pump4, which defines the delivery capacity, is regulated by a regulator45. For example, when the second bidirectional pump4is a swash plate pump, the regulator45may be a regulator that electrically varies the hydraulic pressure acting on a servo piston coupled to the swash plate of the second bidirectional pump4or may be an electric actuator coupled to the swash plate of the second bidirectional pump4.

Referring back toFIG. 1, in the present embodiment, the first bidirectional pump3is driven by an electric motor2. For example, the rotating shafts of the first bidirectional pump3and electric motor2are coupled directly by means such as a coupling. Alternatively, the rotating shaft of the electric motor2may be coupled to the rotating shaft of the second bidirectional pump4, and the second bidirectional pump4may be driven by the electric motor2. It is desirable to use a servomotor as the electric motor2. However, a common motor may be used as the electric motor2.

In the hydraulic system1A of the present embodiment, as described above, the second bidirectional pump4is coupled to the first bidirectional pump3in a manner enabling torque to be transmitted between the first and second bidirectional pumps3and4, and thus the second bidirectional pump4is driven together with the first bidirectional pump3once the first bidirectional pump3is driven by the electric motor2. Additionally, since the first bidirectional pump3is a variable displacement pump whose delivery capacity per rotation is freely variable, the delivery capacity ratio between the first and second bidirectional pumps3and4can be appropriately set according to the difference in area between the first and second pressure chambers51and52of the cylinder5even if the rotational speed ratio between the first and second bidirectional pumps3and4is constant. The fact that the first bidirectional pump3is a variable displacement pump further makes it possible to more appropriately control the pressures in the two supply/discharge lines61and62despite the influence of factors such as the compressibility in the supply/discharge lines61and62. Thus, a reactive force can be applied against extension of the cylinder5without the use of any counterbalance valve. In consequence, the speed etc. of the cylinder5can be stably controlled.

Further, in the present embodiment, the potential energy of the moving object10can, during lowering of the moving object10, be recovered in the form of rotational torque by the second bidirectional pump4. Additionally, since the second bidirectional pump4is coupled to the first bidirectional pump3in a manner enabling torque to be transmitted between the first and second bidirectional pumps3and4, the driving of the first bidirectional pump3can be assisted by the potential energy of the moving object10. This can prevent the potential energy of the moving object10from being lost as heat, thus leading to energy saving. Further, since the amount of heat generated in the hydraulic liquid is reduced, the hydraulic liquid is less likely to be degraded when the hydraulic liquid is an oil.

It should be noted that the above-mentioned benefit of enabling assistance for the driving of the first bidirectional pump3can be obtained also when the cylinder5is disposed to move the moving object10in the horizontal direction. The reason for this is that the drive power of the first bidirectional pump3can be recovered in the form of torque for generating a reactive force against extension of the rod57.

In the conventional hydraulic system100as shown inFIG. 5, the two ports of the bidirectional pump140could be subjected to a high pressure, albeit not simultaneously. As such, the system100needs to use a special pump as the bidirectional pump140and requires high cost.

In contrast, in the present embodiment, the cylinder-opposite ports32and42of the first and second bidirectional pumps3and4are always held at low pressures. Thus, common pumps can be used as the first and second bidirectional pumps3and4. With the use of two common pumps, the cost can be reduced compared to that required by the hydraulic system100using a special pump and a counterbalance valve.

In particular, when the cylinder-opposite port (32or42) of each of the first and second bidirectional pumps3and4has a larger diameter than the cylinder-side port (31or41) as in the present embodiment, since the internal passage of each pump that communicates with the cylinder-opposite port is subjected to a lower pressure than the passage communicating with the cylinder-side port, the internal passage need not be strong enough to withstand high pressures and can have an increased passage area. This can reduce the pressure drop which occurs when the hydraulic liquid is passing through the passage.

Further, since the present embodiment employs the inlet line64provided with the check valve65and the outlet line66provided with the outlet valve67, insufficient flow rate of the hydraulic liquid sucked into the first or second bidirectional pump3or4and excessive increase in pressure in the relay line63can be prevented.

MODIFICATION EXAMPLE

As shown inFIG. 3, the second bidirectional pump4may be a variable displacement pump whose delivery capacity per rotation is freely variable, and the first bidirectional pump3may be a fixed displacement pump whose delivery capacity per rotation is invariable. Alternatively, when the second bidirectional pump4is a variable displacement pump whose delivery capacity per rotation is freely variable, the first bidirectional pump3may be a variable displacement pump whose delivery capacity per rotation is selectively switchable between a first fixed value q1and a second fixed value q2.

Alternatively, both the first and second bidirectional pumps3and4may be variable displacement pumps whose delivery capacities per rotation are freely variable. In this configuration, the flow rate control can be performed more flexibly than when one of the first and second bidirectional pumps3and4is a fixed displacement pump or a variable displacement pump whose delivery capacity is selectively switchable. It should be noted, however, that when one of the first and second bidirectional pumps3and4is a fixed displacement pump or a variable displacement pump whose delivery capacity is selectively switchable as shown inFIG. 1 or 3, the cost can be reduced compared to that required when both the first and second bidirectional pumps3and4are variable displacement pumps whose delivery capacities per rotation are freely variable.

FIG. 4shows a hydraulic system1B according to Embodiment 2 of the present invention. In the present embodiment, the elements which are the same as those of Embodiment 1 are denoted by the same reference signs, and repeated descriptions of these elements will not be given.

In the hydraulic system1B of the present embodiment, a plurality of cylinders5(two cylinders5in the illustrated example) are employed, and they are double-rod cylinders. That is, both ends of the tube55of each cylinder5are closed by two rod covers, and the two rods57penetrate through the rod covers, respectively.

In the present embodiment, all the rods57are fixed, and the tubes55of all the cylinders5are coupled together by a movable table15. The moving objects10are mounted on the upper and lower surfaces of the movable table15.

In such a configuration, when at least one of the first and bidirectional pumps3and4is a variable displacement pump whose delivery capacity per rotation is freely variable, as in the configuration of Embodiment 1, the delivery capacity ratio between the first and second bidirectional pumps3and4can be appropriately set even if the rotational speed ratio between the first and second bidirectional pumps3and4is constant (e.g., a ratio other than 1:1). Further, with at least one of the first and bidirectional pumps3and4being a variable displacement pump, the pressures in the two supply/discharge lines61and62can be more appropriately controlled despite the influence of factors such as the compressibility in the supply/discharge lines61and62, even if the amount of pump internal leakage varies due to a difference in pressure level. Thus, a reactive force can be applied against the movement of the rods57relative to the tubes55without the use of any counterbalance valve. In consequence, the speed etc. of the cylinders5can be stably controlled.

It should be noted that Embodiment 2 is identical to Embodiment 1 in that during lowering of the moving object10, the potential energy of the moving object10can be recovered in the form of rotational torque by the second bidirectional pump4to assist the driving of the first bidirectional pump3.

Other Embodiments

The present invention is not limited to the embodiments described above, and various modifications can be made without departing from the gist of the present invention.

REFERENCE SIGNS LIST

1A,1B hydraulic system

51first pressure chamber

52second pressure chamber