EP3885058A1

EP3885058A1 - Die-cushion device and press machine

Info

Publication number: EP3885058A1
Application number: EP19902428.2A
Authority: EP
Inventors: Atsuhiro KANBAYASHI; Hirokazu Nakamura
Original assignee: Daikin Industries Ltd
Current assignee: Daikin Industries Ltd
Priority date: 2018-12-26
Filing date: 2019-09-18
Publication date: 2021-09-29
Anticipated expiration: 2039-09-18
Also published as: EP3885058A4; JP2020104131A; EP3885058B1; WO2020137032A1; JP6662445B1

Abstract

A die cushion apparatus (200) includes: a double-acting hydraulic cylinder (210) configured to move a cushion pad (140) up and down; a first hydraulic pump (P1) configured to supply a hydraulic oil from an oil tank (T) to a first port (210a) of the hydraulic cylinder (210) and to drive a piston (212) of the hydraulic cylinder (210) upward; a second hydraulic pump (P2) configured to supply the hydraulic oil from the oil tank (T) to a second port (210b) of the hydraulic cylinder (210) and to drive the piston (212) of the hydraulic cylinder (210) downward; and a control unit (260) configured to control a rotation speed of the first hydraulic pump (P1) and a rotation speed of the second hydraulic pump (P2) and to control a flow rate and a pressure of the hydraulic oil which the first hydraulic pump (P1) supplies to the hydraulic cylinder (210) and a flow rate and a pressure of the hydraulic oil which the second hydraulic pump (P2) supplies to the hydraulic cylinder (210).

Description

TECHNICAL FIELD

The present disclosure relates to a die cushion apparatus and a press machine.

BACKGROUND ART

For example, JP 2007-075846 A (Patent Literature 1) discloses a die cushion apparatus in which a single-acting hydraulic cylinder is disposed below a cushion pad, and a die cushion pressure is controlled by a hydraulic oil discharged from a hydraulic pump.

CITATIONS LIST

PATENT LITERATURE

Patent Literature 1: JP 2007-075846 A

SUMMARY OF INVENTION

TECHNICAL PROBLEMS

In a press machine including the die cushion apparatus described above, the die cushion pad is supported at a low pressure in collision with a slide. This results in unsatisfactory reaction force to control a wrinkle in drawing and unsatisfactory pushing force for a molded product, failing to die cushion control with high accuracy and high rigidity.
The present disclosure proposes a die cushion apparatus capable of die cushion control with high accuracy and high rigidity, and a press machine including the die cushion apparatus.

SOLUTIONS TO PROBLEMS

The present disclosure is directed to a die cushion apparatus including:

a double-acting hydraulic cylinder configured to move a cushion pad up and down;
a first hydraulic pump configured to supply a hydraulic oil from an oil tank to a first port of the hydraulic cylinder and to drive a piston of the hydraulic cylinder upward;
a second hydraulic pump configured to supply the hydraulic oil from the oil tank to a second port of the hydraulic cylinder and to drive the piston of the hydraulic cylinder downward; and
a control unit configured to control a rotation speed of the first hydraulic pump and a rotation speed of the second hydraulic pump and to control a flow rate and a pressure of the hydraulic oil which the first hydraulic pump supplies to the hydraulic cylinder and a flow rate and a pressure of the hydraulic oil which the second hydraulic pump supplies to the hydraulic cylinder.

According to the present disclosure, the control unit controls the rotation speed of the first hydraulic pump and the rotation speed of the second hydraulic pump, thereby accurately and rapidly controlling the flow rate and pressure of the hydraulic oil which the first hydraulic pump supplies to the hydraulic cylinder and the flow rate and pressure of the hydraulic oil which the second hydraulic pump supplies to the hydraulic cylinder. The first and second hydraulic pumps therefore control the pressure near the first port in the hydraulic cylinder and the opposed pressure near the second port in the hydraulic cylinder. This configuration thus enables die cushion control with high accuracy and high rigidity.
An aspect of the present disclosure is directed to the die cushion apparatus, wherein
the first hydraulic pump is larger in amount of oil to be discharged per unit time than the second hydraulic pump.
According to the present disclosure, a hydraulic pump that is larger in amount of oil to be discharged per unit time than the second hydraulic pump is used as the first hydraulic pump. This configuration thus enables optimum selection of the first hydraulic pump and second hydraulic pump for the die cushion apparatus.
An aspect of the present disclosure is directed to the die cushion apparatus further including:

a first relief valve configured to return to the oil tank the hydraulic oil discharged from the hydraulic cylinder through the first port; and
a first pressure sensor configured to detect a pressure of the hydraulic oil as a first-port-side die cushion pressure of the hydraulic cylinder,
wherein
the control unit controls the rotation speeds of the first and second hydraulic pumps such that the pressure of the hydraulic oil detected by the first pressure sensor takes a die cushion pressure command value, and
the control unit rotates the first hydraulic pump reversely when the pressure of the hydraulic oil detected by the first pressure sensor is equal to or more than a set pressure for the first relief valve.

According to the present disclosure, in performing press molding using a press machine, the control unit controls the rotation speeds of the first and second hydraulic pumps in accordance with the pressure (i.e., a die cushion pressure) of the hydraulic oil near the first port in the hydraulic cylinder, the pressure being detected by the first pressure sensor. The control unit thus controls the flow rate and pressure of the hydraulic oil which the first hydraulic pump supplies to the hydraulic cylinder and the flow rate and pressure of the hydraulic oil which the second hydraulic pump supplies to the hydraulic cylinder. In die cushion control by the press machine, when a surge pressure is generated in the hydraulic cylinder in performing the press molding, so that the pressure of the hydraulic oil as the die cushion pressure of the hydraulic cylinder becomes equal to or more than the set pressure for the first relief valve, the first relief valve is operated to return to the oil tank the hydraulic oil discharged from the hydraulic cylinder. In addition, when the pressure of the hydraulic oil detected by the first pressure sensor becomes equal to or more than the set pressure for the first relief valve, the control unit rotates the first hydraulic pump reversely. This configuration thus suppresses the surge pressure generated from the hydraulic cylinder in the die cushion control.
An aspect of the present disclosure is directed to the die cushion apparatus further including:

a first check valve disposed closer to the first hydraulic pump than a joint is, the joint being located on a flow path between the hydraulic cylinder and the first hydraulic pump and connected to the first relief valve,
the first check valve being configured to regulate a flow of the hydraulic oil from the hydraulic cylinder to the first hydraulic pump,
wherein
the set pressure for the first relief valve is controlled by the pressure of the hydraulic oil discharged from the first hydraulic pump, and
the control unit rotates the first hydraulic pump reversely and opens the first relief valve when the pressure of the hydraulic oil detected by the first pressure sensor is equal to or more than the set pressure for the first relief valve.

According to the present disclosure, when the pressure of the hydraulic oil detected by the first pressure sensor is equal or more than the set pressure for the first relief valve, the control unit rotates the first hydraulic pump reversely. The first relief valve is thus operated to return to the oil tank the hydraulic oil discharged from the hydraulic cylinder through the first port.
The present disclosure is also directed to a press machine for subjecting a workpiece to press molding by pressurizing the workpiece between an upper die and a lower die,

the press machine including:
the die cushion apparatus described above;
an actuator configured to drive the upper die; and
a control device configured to control the die cushion apparatus and the actuator.

The present disclosure achieves a press machine that enables die cushion control with high accuracy and high rigidity.
An aspect of the present disclosure is directed to the press machine further including:

a first position sensor configured to detect a position of the upper die,
wherein
the control device controls the die cushion apparatus in accordance with the position of the upper die detected by the first position sensor, drives the cushion pad downward, and reduces a collision speed of the upper die with the lower die.

According to the present disclosure, the control device controls the die cushion apparatus, based on the position of the upper die detected by the first position sensor, drives the cushion pad downward, and reduces the collision speed of the upper die with respect to the lower die. This configuration therefore enables significant reduction in surge pressure to be generated from the hydraulic cylinder in performing the press molding.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a side view of a press machine including a die cushion apparatus according to a first embodiment of the present disclosure.
FIG. 2 is a schematic block diagram of the die cushion apparatus.

DESCRIPTION OF EMBODIMENTS

Embodiments will be described below. In the drawings, identical reference signs indicate identical or corresponding portions. The dimensions, such as a length, a width, a thickness, and a depth, illustrated in the drawings are appropriately changed from actual scales for making the drawings clear and simple; therefore, the actual relative dimensions are not illustrated in the drawings.

[First embodiment]

FIG. 1 is a side view of a press machine 100 including a die cushion apparatus 200 according to a first embodiment of the present disclosure. A servo motor 116a is used as a power source for the press machine 100.
The press machine 100 according to the first embodiment includes: a bed 110 disposed upright on a base 101; a bolster 111 disposed on the bed 110; four uprights 112 (FIG. 1 illustrates two of the four uprights 112) extending upward from the bed 110; a crown 113 supported by the uprights 112; and a slide 114 disposed below the crown 113 so as to be movable up and down along the uprights 112. The press machine 100 also includes a cushion pad 140 incorporated in the bed 110, and a double-acting hydraulic cylinder 210 disposed below the bed 110 to move the cushion pad 140 up and down.
The press machine 100 also includes a die 103 for forming a workpiece 102 (an object to be processed) by molding. The die 103 of the press machine 100 includes an upper die 120 attached to a lower face of the slide 114, and a lower die 130 mounted on an upper face of the bolster 111. The press machine 100 subjects the workpiece 102 to press molding by pressurizing the workpiece 102 between the upper die 120 and the lower die 130.
The press machine 100 also includes a die cushion apparatus 200 (see FIG. 2) including the hydraulic cylinder 210 configured to move the cushion pad 140 up and down.
The press machine 100 according to the first embodiment includes four drive mechanisms 116 (FIG. 1 illustrates two of the four drive mechanisms 116) configured to drive the slide 114. The four drive mechanisms 116 each include a servo motor 116a and a servo cylinder 116b configured to convert rotation of the servo motor 116a into linear motion with ball screws. The servo cylinders 116b each have a lower end connected to an upper face of the slide 114. The slide 114 moves up as the servo motors 116a rotate. Each of the drive mechanisms 116 is an example of an actuator.
The press machine 100 also includes: a first position sensor 115 configured to detect a position of the upper die 120; and a main machine controller 150 (see FIG. 1) configured to receive a first position signal indicating the position of the upper die 120 detected by the first position sensor 115, thereby controlling the die cushion apparatus 200, the drive mechanisms 116, and the like. The main machine controller 150 is an example of a control device.
FIG. 2 is a schematic block diagram of the die cushion apparatus 200. As illustrated in FIG. 2, the die cushion apparatus 200 includes the double-acting hydraulic cylinder 210. The hydraulic cylinder 210 includes: a cylinder tube 211; a piston 212 configured to reciprocate in the cylinder tube 211; and a piston rod 213 having a lower end connected to the piston 212. The piston rod 213 of the hydraulic cylinder 210 has an upper end to which the cushion pad 140 is coupled. The cushion pad 140 is provided with a second position sensor 215 (see FIG. 1) configured to detect a position of the cushion pad 140. The second position sensor 215 outputs a second position signal indicating a position of the cushion pad 140.
The die cushion apparatus 200 also includes: a first hydraulic pump P1 configured to supply a hydraulic oil from an oil tank T to a first port 210a provided on a lower side of the hydraulic cylinder 210 and to drive the piston 212 of the hydraulic cylinder 210 upward; a first motor M1 configured to drive the first hydraulic pump P1; a first relief valve 220 configured to return to the oil tank T the hydraulic oil discharged from the hydraulic cylinder 210; a first pressure sensor PS1 configured to detect a pressure of the hydraulic oil as a die cushion pressure of the hydraulic cylinder 210; and a first check valve 230 configured to regulate a flow of the hydraulic oil from the first port 210a of the hydraulic cylinder 210 to the first hydraulic pump P1.
The first check valve 230 is disposed closer to the first hydraulic pump P1 than a joint is, the joint being located on a flow path between the hydraulic cylinder 210 and the first hydraulic pump P1 and connected to the first relief valve 220. The first port 210a of the hydraulic cylinder 210 is connected to the discharge side of the first hydraulic pump P1 via the first check valve 230.
The first pressure sensor PS1 is connected to the first port 210a of the hydraulic cylinder 210.
The first relief valve 220 has an inlet port 221 connected to the first port 210a of the hydraulic cylinder 210, and an outlet port 222 connected to the oil tank T. The first relief valve 220 is a pilot-operated relief valve, and has a pilot port 223 to which the discharge side of the first hydraulic pump P1 is connected. The first relief valve 220 is thus controlled in such a manner that a discharge pressure (a pilot pressure) of the first hydraulic pump P1 is supplied to the pilot port 223 of the first relief valve 220.
The die cushion apparatus 200 also includes: a second hydraulic pump P2 configured to supply the hydraulic oil from the oil tank T to a second port 210b provided on an upper side of the hydraulic cylinder 210 and to drive the piston 212 of the hydraulic cylinder 210 downward; a second motor M2 configured to drive the second hydraulic pump P2; a second relief valve 240 configured to return to the oil tank T the hydraulic oil discharged from the hydraulic cylinder 210; a second pressure sensor PS2 configured to detect a pressure of the hydraulic oil near the second port 210b in the hydraulic cylinder 210; and a second check valve 250 configured to regulate a flow of the hydraulic oil from the second port 210b of the hydraulic cylinder 210 to the second hydraulic pump P2.
The second check valve 250 is disposed closer to the second hydraulic pump P2 than a joint is, the joint being located on a flow path between the hydraulic cylinder 210 and the second hydraulic pump P2 and connected to the second relief valve 240. The second port 210b of the hydraulic cylinder 210 is connected to the discharge side of the second hydraulic pump P2 via the second check valve 250.
The second pressure sensor PS2 is connected to the second port 210b of the hydraulic cylinder 210.
The second relief valve 240 has an inlet port 241 connected to the second port 210b of the hydraulic cylinder 210, and an outlet port 242 connected to the oil tank T. The second relief valve 240 is a pilot-operated relief valve, and has a pilot port 243 to which the discharge side of the second hydraulic pump P2 is connected. The second relief valve 240 is thus controlled in such a manner that a discharge pressure (a pilot pressure) of the second hydraulic pump P2 is supplied to the pilot port 243 of the second relief valve 240.
A hydraulic pump that is larger in amount of oil to be discharged per unit time than the second hydraulic pump P2 is used as the first hydraulic pump P1. This configuration thus enables optimum selection of the first hydraulic pump P1 and second hydraulic pump P2 for the die cushion apparatus 200.
The second hydraulic pump P2 drives the piston 212 of the hydraulic cylinder 210 downward to move the cushion pad 140 down. The second hydraulic pump P2 moves the cushion pad 140 down mainly using the load and gravity in performing press molding. Therefore, the amount of oil to be discharged from the second hydraulic pump P2 per unit time may be relatively smaller than the amount of oil to be discharged from the first hydraulic pump P1 per unit time. In contrast to this, the first hydraulic pump P1 drives the piston 212 of the hydraulic cylinder 210 upward to promptly move the cushion pad 140 up. Therefore, a hydraulic pump to be used as the first hydraulic pump P1 preferably discharges a large amount of oil per unit time.
The press machine 100 includes a control unit 260 configured to control a flow rate and a pressure of the hydraulic oil supplied from the first hydraulic pump P1 to the hydraulic cylinder 210 and a flow rate and a pressure of the hydraulic oil supplied from the second hydraulic pump P2 to the hydraulic cylinder 210.
The control unit 260 receives a command signal indicating a die cushion pressure command value from the main machine controller 150. The control unit 260 also receives a second position signal indicating a position of the cushion pad 140 from the second position sensor 215. The control unit 260 also receives a first pressure signal from the first pressure sensor PS1 and a second pressure signal from the second pressure sensor PS2. The control unit 260 then outputs a first drive signal for driving the first motor M1 and a second drive signal for driving the second motor M2. The control unit 260 thus controls a rotation speed of the first hydraulic pump P1 and a rotation speed of the second hydraulic pump P2.
In the press machine 100, any unit different from the main machine controller 150 may input the command signal to the control unit 260.
In the press machine 100 having the configuration described above, the main machine controller 150 controls the die cushion apparatus 200, based on the position of the upper die 120 detected by the first position sensor 115. In performing press molding using the press machine 100, the control unit 260 controls the rotation speed of the first hydraulic pump P1 and the rotation speed of the second hydraulic pump P2 in accordance with the pressure of the hydraulic oil detected by the first pressure sensor PS1 and the pressure of the hydraulic oil detected by the second pressure sensor PS2. The control unit 260 thus accurately and rapidly controls the flow rate and pressure of the hydraulic oil which the first hydraulic pump P1 supplies to the hydraulic cylinder 210 and the flow rate and pressure of the hydraulic oil which the second hydraulic pump P2 supplies to the hydraulic cylinder 210.
The control unit 260 thus controls the pressure near the first port 210a and the pressure near the second port 210b in the hydraulic cylinder 210, thereby controlling the position of the cushion pad 140 with high accuracy. In holding the cushion pad 140 at a predetermined position, the hydraulic cylinder 210 holds the cushion pad 140 at a higher pressure than a pressure which a conventional single-acting hydraulic cylinder applies to the cushion pad 140 from below, so as to adjust the balance between the pressure near the first port 210a and the pressure near the second port 210b in the hydraulic cylinder 210. This configuration thus enables die cushion control with high rigidity.
In holding the cushion pad 140 at a predetermined position while adjusting the balance between the pressure near the first port 210a and the pressure near the second port 210b in the hydraulic cylinder 210, the cushion pad 140 is supported at a lower pressure or is supported at a higher pressure to enhance the rigidity. This configuration thus extends a range of pressure level in die cushion control.
Immediately before generation of an external force at the action of the die cushion in the press molding using the press machine 100, the control unit 260 controls the rotation speeds of the first and second hydraulic pumps P1 and P2 to make the flow rates of the first and second hydraulic pumps P1 and P2 constant such that the pressure (die cushion pressure) of the hydraulic oil detected by the first pressure sensor PS1 takes a die cushion pressure command value. The control unit 260 thus sets the pressure of the pilot port 223 at the pressure corresponding to the die cushion pressure command value.
Next, when a surge pressure is generated in the hydraulic cylinder 210 by collision of the upper die 120 with the lower die 130 with the workpiece 102 interposed between the upper die 120 and the lower die 130, so that the pressure (die cushion pressure) of the hydraulic oil near the first port 210a in the hydraulic cylinder 210 becomes equal to or more than a set pressure Pp1 for the first relief valve 220, the first relief valve 220 is operated to return to the oil tank T the hydraulic oil discharged from the hydraulic cylinder 210. In addition, when the pressure (die cushion pressure) of the hydraulic oil detected by the first pressure sensor PS1 becomes equal to or more than the set pressure Pp1 for the first relief valve 220, the control unit 260 rotates the first hydraulic pump P1 reversely. This configuration thus suppresses the surge pressure generated from the hydraulic cylinder 210 in the die cushion control.
In returning the cushion pad 140 to an initial position after completion of one cycle of press molding, the control unit 260 rotates the first hydraulic pump P1 to drive the piston 212 of the hydraulic cylinder 210 upward. At this time, when the pressure of the hydraulic oil near the second port 210b in the hydraulic cylinder 210 becomes equal to or more than a set pressure Pp2 for the second relief valve 240, the second relief valve 240 is operated to return to the oil tank T the hydraulic oil discharged from the hydraulic cylinder 210. When the pressure (die cushion pressure) of the hydraulic oil detected by the second pressure sensor PS2 becomes equal to or more than the set pressure Pp2 for the second relief valve 240, the control unit 260 may rotate the second hydraulic pump P2 reversely.
In the first embodiment, the first and second hydraulic pumps P1 and P2 control the pressure near the first port 210a and the opposed pressure near the second port 210b in the hydraulic cylinder 210 to improve the degree of freedom as to how to set a pressure for supporting the cushion pad 140. This configuration thus achieves the die cushion apparatus 200 capable of die cushion control with high accuracy and high rigidity while suppressing a surge pressure, and the press machine 100 including the die cushion apparatus 200.
In the press machine 100, the die cushion pressure level is settable freely. A user therefore achieves die cushion control at arbitrary and optimum settings, which largely extend the degree of freedom in press molding.

[Second embodiment]

A press machine including a die cushion apparatus according to a second embodiment of the present disclosure is identical in configuration to the press machine 100 according to the first embodiment except for the operation of the main machine controller 150 (the control device), and is therefore described with reference to FIGS. 1 and 2.
In the press machine according to the second embodiment, a main machine controller 150 controls a die cushion apparatus, based on a position of an upper die 120 detected by a first position sensor 115. The press machine has a preliminary acceleration function of reducing a collision speed of the upper die 120 with respect to a lower die 130.
According to the preliminary acceleration function of the press machine, immediately before collision of the upper die 120 with the lower die 130 in press molding, the main machine controller 150 controls the die cushion apparatus. The main machine controller 150 controls rotation speeds of first and second hydraulic pumps P1 and P2 to move a cushion pad 140 downward. The main machine controller 150 thus reduces the collision speed of the upper die 120 with respect to the lower die 130. This configuration thus significantly suppresses a surge pressure generated from a hydraulic cylinder 210.
The second embodiment thus achieves the die cushion apparatus capable of die cushion control and preliminary acceleration with high accuracy and high rigidity while suppressing a surge pressure, and the press machine including the die cushion apparatus.
The first and second embodiments each describe the press machine 100 in which the drive mechanisms 116 including the servo motors 116a each serve as an actuator. Alternatively, this invention may be applied to a hydraulic press machine in which a hydraulic cylinder serves as an actuator.
The foregoing description concerns specific embodiments of the present disclosure; however, the present disclosure is not limited to the first and second embodiments, and various modifications and variations may be made within the scope of the present disclosure.
An aspect of the present disclosure is directed to the die cushion apparatus 200 including:

the second relief valve 240 configured to return to the oil tank T the hydraulic oil discharged from the hydraulic cylinder 210 through the second port 210b; and
the second pressure sensor PS2 configured to detect the pressure of the hydraulic oil near the second port 210b in the hydraulic cylinder 210,
wherein
the control unit 260 rotates the second hydraulic pump P2 reversely when the pressure of the hydraulic oil detected by the second pressure sensor PS2 is equal to or more than the set pressure for the second relief valve 240.

An aspect of the present disclosure is directed to the die cushion apparatus 200 further including:

the second check valve 250 disposed closer to the second hydraulic pump P2 than the joint is, the joint being located on the flow path between the hydraulic cylinder 210 and the second hydraulic pump P2 and connected to the second relief valve 240,
the second check valve 250 being configured to regulate the flow of the hydraulic oil from the hydraulic cylinder 210 to the second hydraulic pump P2,
wherein
the set pressure for the second relief valve 240 is controlled by the pressure of the hydraulic oil discharged from the second hydraulic pump P2, and
the control unit 260 rotates the second hydraulic pump P2 reversely and opens the second relief valve 240 when the pressure of the hydraulic oil detected by the second pressure sensor PS2 is equal to or more than the set pressure for the second relief valve 240.

REFERENCE SIGNS LIST

100: press machine
101: base
102: workpiece
103: die
110: bed
111: bolster
112: upright
113: crown
114: slide
115: first position sensor
116: drive mechanism (actuator)
116a: servo motor
116b: servo cylinder
120: upper die
130: lower die
140: cushion pad
150: main machine controller (control device)
200: die cushion apparatus
210: hydraulic cylinder
211: cylinder tube
212: piston
213: piston rod
215: second position sensor
220: first relief valve
230: first check valve
240: second relief valve
250: second check valve
260: control unit
M1: first motor
M2: second motor
P1: first hydraulic pump
P2: second hydraulic pump
PS1: first pressure sensor
PS2: second pressure sensor
T: oil tank

Claims

A die cushion apparatus (200) comprising:
a double-acting hydraulic cylinder (210) configured to move a cushion pad (140) up and down;

a first hydraulic pump (P1) configured to supply a hydraulic oil from an oil tank (T) to a first port (210a) of the hydraulic cylinder (210) and to drive a piston (212) of the hydraulic cylinder (210) upward;

a second hydraulic pump (P2) configured to supply the hydraulic oil from the oil tank (T) to a second port (210b) of the hydraulic cylinder (210) and to drive the piston (212) of the hydraulic cylinder (210) downward; and

a control unit (260) configured to control a rotation speed of the first hydraulic pump (P1) and a rotation speed of the second hydraulic pump (P2) and to control a flow rate and a pressure of the hydraulic oil which the first hydraulic pump (P1) supplies to the hydraulic cylinder (210) and a flow rate and a pressure of the hydraulic oil which the second hydraulic pump (P2) supplies to the hydraulic cylinder (210).
The die cushion apparatus (200) according to claim 1, wherein
the first hydraulic pump (P1) is larger in amount of oil to be discharged per unit time than the second hydraulic pump (P2).
The die cushion apparatus (200) according to claim 1 or 2, further comprising:
a first relief valve (220) configured to return to the oil tank (T) the hydraulic oil discharged from the hydraulic cylinder (210) through the first port (210a); and

a first pressure sensor (PS1) configured to detect a pressure of the hydraulic oil as a first-port (210a)-side die cushion pressure of the hydraulic cylinder (210),

wherein

the control unit (260) controls the rotation speeds of the first and second hydraulic pumps (P1, P2) such that the pressure of the hydraulic oil detected by the first pressure sensor (PS1) takes a die cushion pressure command value, and

the control unit (260) rotates the first hydraulic pump (P1) reversely when the pressure of the hydraulic oil detected by the first pressure sensor (PS1) is equal to or more than a set pressure for the first relief valve (220).
The die cushion apparatus (200) according to claim 3, further comprising:
a first check valve (230) disposed closer to the first hydraulic pump (P1) than a joint is, the joint being located on a flow path between the hydraulic cylinder (210) and the first hydraulic pump (P1) and connected to the first relief valve (220),

the first check valve (230) being configured to regulate a flow of the hydraulic oil from the hydraulic cylinder (210) to the first hydraulic pump (PI),

wherein

the set pressure for the first relief valve (220) is controlled by the pressure of the hydraulic oil discharged from the first hydraulic pump (P1), and

the control unit (260) rotates the first hydraulic pump (P1) reversely and opens the first relief valve (220) when the pressure of the hydraulic oil detected by the first pressure sensor (PS1) is equal to or more than the set pressure for the first relief valve (220).
A press machine (100) for subjecting a workpiece (102) to press molding by pressurizing the workpiece (102) between an upper die (120) and a lower die (130),
the press machine (100) comprising:
the die cushion apparatus (200) according to any one of claims 1 to 4;

an actuator (116) configured to drive the upper die (120); and

a control device (150) configured to control the die cushion apparatus (200) and the actuator (116).
The press machine (100) according to claim 5, further comprising:
a first position sensor (115) configured to detect a position of the upper die (120),

wherein

the control device (150) controls the die cushion apparatus (200) in accordance with the position of the upper die (120) detected by the first position sensor (115), drives the cushion pad (140) downward, and reduces a collision speed of the upper die (120) with respect to the lower die (130).