Patent Publication Number: US-10780485-B2

Title: Die cushion device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. 2017-035058, filed on Feb. 27, 2017. The above application is hereby expressly incorporated by reference, in its entirety, into the present application. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a die cushion device, and more particularly to an inexpensive and functionally efficient die cushion device. 
     Description of the Related Art 
     Conventionally, as an inexpensive and functional die cushion device, a die cushion device has been proposed in Japanese Patent Application Laid-Open No. 2016-407 (hereinafter referred to as Patent Document 1). 
     The die cushion device includes a cushion pad, a hydraulic cylinder for moving the cushion pad up and down, and a hydraulic closed circuit connected to a die cushion pressure generation chamber of the hydraulic cylinder. The hydraulic closed circuit has: a pilot drive type logic valve operable as a main relief valve during die cushion process; and a pilot relief valve for generating pilot pressure for controlling the logic valve. In addition, hydraulic oil is filled in a pressurized manner in the hydraulic closed circuit. 
     Hydraulic oil filled in a pressurized manner in the hydraulic closed circuit is pressurized only by die cushion force applied from the cushion pad via the hydraulic cylinder during one cycle period of the cushion pad including a die cushion process and a knockout process, and is accumulated in an accumulator as low-pressure system pressure capable of the knockout process. The hydraulic oil accumulated in the accumulator is supplied to the die cushion pressure generation chamber of the hydraulic cylinder in the knockout process. 
     According to the die cushion device, the hydraulic oil is filled in a pressurized manner in the hydraulic closed circuit, and a hydraulic pump for pressurizing and supplying the hydraulic oil in one cycle period of the cushion pad, is not provided. Therefore, it is possible to make the die cushion device simple and inexpensive, and to save the power cost required for the die cushion process. 
     CITATION LIST 
     Patent Literature 
     Patent Document 1: Japanese Patent Application Laid-Open No. 2016-407 
     SUMMARY OF THE INVENTION 
     In the die cushion device described in Patent Document 1, as illustrated in  FIG. 8 , when a slide reaches a bottom dead center, the cushion pad is locked for a predetermined period at the bottom dead center. When the slide begins to rise (move upward) from the bottom dead center, the system pressure (around 40 kg/cm 2 ) applied to the die cushion pressure generation chamber of the hydraulic cylinder is released (the compressed volume of hydraulic oil is released) (Arrow A in  FIG. 8 ) and the cushion pad rises slightly (around 2 mm). 
     After the cushion pad slightly rises, since the hydraulic oil supplied to the die cushion pressure generation chamber of the hydraulic cylinder is shut off, the cushion pad is locked in the vicinity of the bottom dead center. 
     As described above, in the die cushion device described in Patent Document 1, because the cushion pad rises by around 2 mm from a position of the bottom dead center when the system pressure is released, there is a concern that troubles such as scratches and cracks might occur in a molded product when a thin plate is drawn. 
     The present invention has been made in view of the above circumstances, and aims to provide a die cushion device that does not require equipment such as a hydraulic pump that consumes electric power, is inexpensive and functionally efficient, and is capable of complete locking (below the slide bottom dead center). 
     In order to achieve the object above, a die cushion device according to one aspect of the present invention includes: a cushion pad; a fluid-pressure cylinder configured to move the cushion pad up and down; and a fluid-pressure closed circuit. The fluid-pressure closed circuit including: a die cushion pressure generation line connected to a die cushion pressure generation chamber of the fluid-pressure cylinder; a first system pressure line connected to a first accumulator which is configured to accumulate hydraulic fluid having first system pressure capable of lowering process of the fluid-pressure cylinder; a lowering pressure generation line connected to the cushion pad lowering pressure generation chamber of the fluid-pressure cylinder; a second system pressure line connected to a second accumulator which is configured to accumulate hydraulic fluid having second system pressure lower than the first system pressure, the second system pressure line capable of knockout process; a pilot drive type logic valve provided between the die cushion pressure generation line and the first system pressure line, and operable as a main relief valve when die cushion process is performed; and a pilot relief valve provided between the die cushion pressure generation line and the first system pressure line, and configured to generate pilot pressure for controlling the logic valve. In the fluid-pressure closed circuit, hydraulic fluid is filled in a pressurized manner, the fluid-pressure closed circuit does not include a fluid-pressure pump configured to pressurize and feed the hydraulic fluid. In the first system pressure line and the second system pressure line in the fluid-pressure closed circuit, the hydraulic fluid can be pressurized by using only die cushion force applied from the cushion pad through the fluid-pressure cylinder, in one cycle period of the cushion pad, including the die cushion process and the knockout process. 
     According to the one aspect of the present invention, in addition to the die cushion pressure generation line and the first system pressure line, the fluid-pressure closed circuit is provided with a cushion pad lowering pressure generation line and a second system pressure line. During locking at the bottom dead center of the cushion pad, the cushion pad lowering pressure generation chamber of the fluid-pressure cylinder can be connected to the first system pressure line via the cushion pad lowering pressure generation line, and the die cushion pressure generation chamber of the fluid-pressure cylinder can be connected to the second system pressure line via the die cushion pressure generation line. As a result, even if the slide begins to move upward from the bottom dead center, the fluid-pressure cylinder enables the lowering process of the cushion pad by differential pressure between the first system pressure and the second system pressure. Thereby, it is possible to prevent a slight rise of the cushion pad during locking, that is, it is possible to hold the cushion pad below the slide bottom dead center. 
     In addition, in the fluid-pressure closed circuit combining the logic valve and the pilot relief valve, hydraulic fluid is filled in a pressurized manner. The hydraulic fluid in the fluid-pressure closed circuit is pressurized only by die cushion force applied from the cushion pad via the fluid-pressure cylinder during one cycle period of the cushion pad including the die cushion process and the knockout process. The fluid-pressure pump is not provided. During the die cushion process, the logic valve operates as a main relief valve and generates die cushion pressure according to the pilot pressure generated by the pilot relief valve. Also, raising process of the cushion pad after the locking for a predetermined period (or fixed period) is performed by hydraulic fluid of second system pressure accumulated in the second accumulator. In this manner, during one cycle period of the cushion pad, the hydraulic fluid is pressurized only by the die cushion force applied from the cushion pad via the fluid-pressure cylinder. Because the fluid-pressure closed circuit is not provided with a fluid-pressure pump, the power cost can be saved. 
     In a die cushion device according to another aspect of the present invention, it is preferable to provide a first solenoid valve configured to switch pressure acting on a pilot port of the logic valve, to any one of the pilot pressure and the first system pressure during one cycle period of the cushion pad. When the first solenoid valve switches such that the pilot pressure acts on the pilot port of the logic valve, it is possible to generate die cushion pressure corresponding to the pilot pressure in the die cushion pressure generation line. In addition, when the first solenoid valve switches such that first system pressure acts on the pilot port of the logic valve, it is possible to release die cushion pressure generated in the die cushion pressure generation line. 
     In a die cushion device according to yet another aspect of the present invention, it is preferable that the first solenoid valve is a poppet type solenoid valve. This is because there is no leak of hydraulic fluid in the poppet type solenoid valve. 
     In a die cushion device according to yet another aspect of the present invention, it is preferable to provide a second solenoid valve that enables opening and closing between the die cushion pressure generation line and the second system pressure line. The second solenoid valve is controlled to enable the raising process of the cushion pad. 
     In a die cushion device according to yet another aspect of the present invention, it is preferable that the second solenoid valve is the poppet type solenoid valve. This is because there is no leak of hydraulic fluid in the poppet type solenoid valve. 
     In a die cushion device according to yet another aspect of the present invention, it is preferable to provide a third solenoid valve that enables opening and closing between the lowering pressure generation line and the first system pressure line, and a fourth solenoid valve that enables opening and closing between the lowering pressure crating line and the second system pressure line. The third solenoid valve is controlled to enable the lowering process of the fluid-pressure cylinder during the locking at the bottom dead center and to hold the cushion pad at the bottom dead center or below the bottom dead center. 
     In a die cushion device according to yet another aspect of the present invention, it is preferable that the third solenoid valve and the fourth solenoid valve are the poppet type solenoid valves. This is because there is no leak of hydraulic fluid in the poppet type solenoid valve. 
     In a die cushion device according to yet another aspect of the present invention, it is preferable that there is provided a controller configured to control the first solenoid valve and the second solenoid valve, and that the controller controls the first solenoid valve such that the pilot pressure is applied to the pilot port of the logic valve during the lowering period of the cushion pad, and controls the second solenoid valve during the raising period of the cushion pad. 
     In a die cushion device according to yet another aspect of the present invention, the first solenoid valve is controlled by the controller so as to apply the pilot pressure to the pilot port of the logic valve during the lowering period of the cushion pad to enable die cushion pressure corresponding to the pilot pressure to be generated in the die cushion pressure generation line, as well as to enable die cushion force to be generated in the fluid-pressure cylinder during the lowering period of the cushion pad. In addition, by closing the second solenoid valve, the supply of the hydraulic fluid of second system pressure to the die cushion pressure generation chamber of the fluid-pressure cylinder is shut off to enable the cushion pad to be locked. In this case, since the cushion pad is locked by preventing the hydraulic fluid of second system pressure from being supplied to the die cushion pressure generation chamber of the fluid-pressure cylinder, the cushion pad is moved upward slightly from the bottom dead center (no complete locking is done). However, it is applicable to a case where locking does not affect the press forming even when the cushion pad slightly moves upward. Further, during raising process of the cushion pad after locking for a predetermined period, the second solenoid valves are opened to enable the hydraulic fluid of second system pressure to be supplied to the die cushion pressure generation chamber and the cushion pad lowering pressure generation chamber of the fluid-pressure cylinder, respectively. An upward force acts on the fluid-pressure cylinder according to the difference in pressurized area (or pressure receiving area) between the die cushion pressure generation chamber and the cushion pad lowering pressure generation chamber, and the hydraulic fluid of second system pressure is supplied to the die cushion pressure generation chamber of the fluid-pressure cylinder, thereby enabling to raise (move upward) the cushion pad. In addition, since this controller performs only simple control of the first and second solenoid valves (because a special control device is unnecessary), a part of the press machine controller (PLC: programmable logic controller) and the like can be diverted and the device is inexpensive. 
     In a die cushion device according to yet another aspect of the present invention, it is preferable to include a controller configured to control the first solenoid valve, the second solenoid valve, the third solenoid valve and the fourth solenoid valve, wherein the controller controls the first solenoid valve such that the pilot pressure is applied to the pilot port of the logic valve during lowering period of the cushion pad, generates a die cushion force on the cushion pad by the fluid-pressure cylinder, controls the first solenoid valve, the second solenoid valve, the third solenoid valve and the fourth solenoid valve such that the cushion pad stops at or below a bottom dead center between the lowering period of the cushion pad and raising period of the cushion pad, and controls the second solenoid valve during the raising period of the cushion pad. 
     According to yet another aspect of the present invention, die cushion pressure corresponding to the pilot pressure is generated in the die cushion pressure generation line by controlling the first solenoid valve by the controller such that the pilot pressure is applied to the pilot port of the logic valve during the lowering period of the cushion pad and die cushion force can be generated in the fluid-pressure cylinder during the lowering period of the cushion pad. Further, during locking of the cushion pad at the bottom dead center, by opening the second solenoid valve and the third solenoid valve and closing the fourth solenoid valve, the first system pressure is applied to the cushion pad lowering pressure generation chamber of the fluid-pressure cylinder and the second system pressure is applied to the die cushion pressure generation chamber of the fluid-pressure cylinder. Thereby, even when the slide begins to move upward from the bottom dead center, downward pressing force can be applied to the fluid-pressure cylinder by differential pressure between first system pressure and second system pressure. Thus, it is possible to prevent the slight rise of the cushion pad during the locking, that is, to hold the cushion pad at or below the slide bottom dead center. Further, during the raising process of the cushion pad after locking for a predetermined period, the second solenoid valves are opened to enable the hydraulic fluid of second system pressure to be supplied to the die cushion pressure generation chamber and the cushion pad lowering pressure generation chamber of the fluid-pressure cylinder, respectively. An upward pressing force acts on the fluid-pressure cylinder according to the difference in pressurized area between the die cushion pressure generation chamber and the cushion pad lowering pressure generation chamber, and the hydraulic fluid having the second system pressure is supplied to the die cushion pressure generation chamber of the fluid-pressure cylinder, thereby enabling to raise (move upward) the cushion pad. In addition, since this controller performs only simple control of the first, second, third, and fourth solenoid valves, a part (PLC) of the controller of the press machine and the like can be diverted, and the device is inexpensive. 
     In a die cushion device according to yet another aspect of the present invention, it is preferable that a plurality of the second solenoid valves are provided in parallel between the die cushion pressure generation line and second system pressure line, and the controller individually controls opening and closing of the plurality of second solenoid valves during the raising period of the cushion pad to control a rising speed of the cushion pad. That is, by changing the number of the second solenoid valves to be opened and closed, a flow rate of hydraulic fluid supplied from the second accumulator to the die cushion pressure generation line can be changed gradually, as a result, the rising speed of the cushion pad can be controlled. 
     In a die cushion device according to yet another aspect of the present invention, it is preferable to dispose a throttle valve, or a throttle valve and a coupler for fluid feeding and system pressure sealing in the die cushion pressure generation line, the first system pressure line, the second system pressure line, and a pilot pressure generation line having the pilot relief valve. This is because when hydraulic fluid is filled in the fluid-pressure closed circuit in a pressurized manner by an external feeding fluid device, the valve or the valve and the coupler serve as a filler port and an exhaust port for the hydraulic fluid. 
     In a die cushion device according to yet another aspect of the present invention, it is preferable to include a feeding fluid device which includes: a tank configured to store the hydraulic fluid; a discharge port for feeding the hydraulic fluid into the fluid-pressure closed circuit; a return port for receiving the hydraulic fluid returned from the fluid-pressure closed circuit, the return port being connected to the tank; and a fluid-pressure pump configured to supply the hydraulic fluid from the tank to the fluid-pressure closed circuit through the discharge port. In the feeding fluid device, the fluid-pressure pump is driven only when the hydraulic fluid is filled in the fluid-pressure closed circuit in a pressurized manner. The feeding fluid device above is an external device that is attached to and detached from the die cushion device, and that is connected to be used only when the hydraulic fluid is filled in the fluid-pressure closed circuit in a pressurized manner. The feeding fluid device is not required to be accompanied for each of die cushion devices, but one feeding fluid device can be used for a plurality of controlled die cushion devices. 
     In a die cushion device according to yet another aspect of the present invention, it is preferable to accompany the feeding fluid device with an extension hose that is to be connected to at least one of the discharge port and the return port, and preferable that a coupler is provided at each of both ends of the extension hose. As a result, if the discharge port and the return port of the feeding fluid device cannot be directly connected to the fluid-pressure closed circuit, it is possible to be connected to the fluid-pressure closed circuit through the extension hose. 
     According to yet another aspect of the present invention, the first solenoid valve is controlled so as to apply the pilot pressure to the pilot port of the logic valve during the lowering (moving down) period of the cushion pad to enable die cushion pressure corresponding to the pilot pressure to be generated in the die cushion pressure generation line, as well as to enable die cushion force to be generated in the fluid-pressure cylinder during the lowering period of the cushion pad. In addition, the second solenoid valve is opened at an appropriate timing after the die cushion process to enable hydraulic fluid at system pressure accumulated in the accumulator to be supplied to the fluid-pressure cylinder through the die cushion pressure generation line. As a result, it is possible to raise the cushion pad to a standby position. 
     According to the present invention, in one process cycle (die cushion lowering process, locking process, raising process), a fluid-pressure pump or an air pressure source or the like that consumes electric power is unnecessary, and locking can be perfectly performed (at or below the slide bottom dead center) when the cushion pad is locked by an inexpensive and functional device without using a special (dedicated) controller. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a configuration diagram illustrating an embodiment of a die cushion device according to the present invention when applied to a press machine; 
         FIG. 2  is a configuration diagram illustrating an embodiment of an oil supply device; 
         FIG. 3  illustrates an extension hose that connects a hydraulic closed circuit and the oil supply device; 
         FIG. 4  illustrates a state where the hydraulic closed circuit and the oil supply device are connected through the extension hose; 
         FIG. 5  is a block diagram illustrating an embodiment of an automatic control unit of the oil supply device when oil supply and pressure release are performed automatically; 
         FIG. 6  is a block diagram illustrating an embodiment of a controller applied to the die cushion device; 
         FIG. 7  is a waveform diagram illustrating a slide position of a slide, a position of a cushion pad (die cushion position) and a die cushion pressure, and a diagram illustrating an ON/OFF state of a first solenoid valve, a second solenoid valve, a third solenoid valve and a fourth solenoid valve in one cycle period; and 
         FIG. 8  is a waveform diagram illustrating a slide position, a die cushion position and a die cushion force in a conventional slide. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     With reference to accompanying drawings, preferred embodiments of a die cushion device according to the present invention will be described in detail. 
     &lt;Configuration of Die Cushion Device&gt; 
       FIG. 1  is a configuration diagram illustrating an embodiment of a die cushion device according to the present invention when applied to a press machine. 
     In the press machine  10  illustrated in  FIG. 1 , a frame is composed of a bed  11 , a column  12  and a crown  13 , and a slide  14  is movably guided in a vertical direction by a guide section  15  provided in the column  12 . The slide  14  is moved in the vertical direction in  FIG. 1  by a servo motor (not illustrated), or a crank mechanism including a crankshaft  16  to which rotational driving force is transmitted by a flywheel (not illustrated). 
     It is preferable that the press machine  10  is provided, on its bed  11  side, with a slide position detector  17  that detects a position of the slide  14 , or that the crankshaft  16  is provided with a crankshaft encoder  18  that detects an angle of the crankshaft  16 . 
     An upper die  20  is mounted on the slide  14 , and a lower die  22  is mounted on a bolster  19  on the bed  11 . 
     A blank holder (blank holding plate)  102  is disposed between the upper die  20  and the lower die  22  such that a lower side of the blank holder is supported by a cushion pad  110  through a plurality of cushion pins  104  and a material  30  is set on (brought into contact with) an upper side of the blank holder. 
     &lt;Structure of the Die Cushion Device&gt; 
     A die cushion device  100  includes: the blank holder  102 ; the cushion pad  110  that supports the blank holder  102  through the plurality of cushion pins  104 ; a hydraulic cylinder (fluid-pressure cylinder)  120  that supports the cushion pad  110  and generates die cushion force for the cushion pad  110 ; and a hydraulic closed circuit (fluid-pressure closed circuit)  150  that is connected to a die cushion pressure generation chamber  120   a  and a cushion pad lowering pressure generation chamber (pressure generation chamber for lowering the cushion pad)  120   b  of the hydraulic cylinder  120 . 
     The hydraulic cylinder  120  and the hydraulic closed circuit  150  serve as a cushion pad lifting unit that moves (lifts) the cushion pad  110  up and down. Further, the hydraulic cylinder  120  and the hydraulic closed circuit  150  serve as a die cushion force generation unit that generates die cushion force for the cushion pad  110 . 
     In addition, the hydraulic cylinder  120  is provided with a die cushion position detector  124  that detects a position of a piston rod of the hydraulic cylinder  120  in an expanding direction (expanding/contracting direction) thereof as a position of the cushion pad  110  in an up-and-down direction thereof. The die cushion position detector  124  may be provided between the bed  11  and the cushion pad  110 . 
     Next, a configuration of the hydraulic closed circuit  150  that drives the hydraulic cylinder  120  will be described. 
     The hydraulic closed circuit  150  includes: a die cushion pressure generation line  152  that is connected to the die cushion pressure generation chamber  120   a  of the hydraulic cylinder  120 ; a first system pressure line  156  which is connected to a first accumulator  154  that accumulates hydraulic oil (hydraulic fluid) having first system pressure capable of moving the hydraulic cylinder  120  downward; a lowering pressure generation line  153  that is connected to the cushion pad lowering pressure generation chamber  120   b  of the hydraulic cylinder  120 ; a second system pressure line  159  which is connected to a second accumulator  155  that accumulates hydraulic oil having second system pressure lower than the first system pressure, and that is capable of knockout process; a pilot drive type logic valve  158  that is disposed between the die cushion pressure generation line  152  and the first system pressure line  156 , and that is operable as a main relief valve at the time of die cushion process; and a pilot relief valve  160  that is disposed between the die cushion pressure generation line  152  and the first system pressure line  156 , and that generates pilot pressure for controlling the logic valve  158 . Here, it is preferable that the logic valve  158  and the pilot relief valve  160  are a direct acting type in which there is little leak (no leak). 
     The first accumulator  154  is filled with a gas having a pressure of about 40 kg/cm 2  to 120 kg/cm 2 . The first system pressure line  156  to which the first accumulator  154  is connected, is filled with hydraulic oil having an approximately constant pressure (the first system pressure) of about 60 kg/cm 2  to 140 kg/cm 2  in advance (before machine operation). The pressure of the hydraulic oil is higher than the pressure of the second system pressure line  159 . 
     The first system pressure line  156  including the first accumulator  154  plays a role of a power source that mainly lowers the hydraulic cylinder  120 . The first system pressure line  156  also plays a role of a preliminary pressure source having a preliminary pressure that accelerates the response of the die cushion pressure, and a role of a compensating element by a boosting action, that cancels out an override characteristic (as the position approaches the bottom dead center, the slide speed decreases and the pressure decreases) of the die cushion pressure by the logic valve  158  that functions as the main relief valve. Therefore, it is preferable that the first accumulator  154  has an appropriate volume (capacity) such that an increment of first system pressure caused by accumulating the hydraulic oil during the die cushion action cancels the override characteristic of the die cushion pressure by the logic valve  158 . 
     The second accumulator  155  is filled with a hydraulic oil having a pressure lower than the gas pressure of the first accumulator  154  by about 20 kg/cm 2  to 50 kg/cm 2 . The second system pressure line  159  to which the second accumulator  155  is connected, is filled with a hydraulic oil having an approximately constant pressure (the second system pressure) lower than the first system pressure by about 20 kg/cm 2  to 50 kg/cm 2 , in advance (before machine operation). The second system pressure line  159  including the second accumulator  155  mainly plays a role of a power source for knocking out (raising) the hydraulic cylinder  120 , and a role of a tank. 
     In addition, the hydraulic closed circuit  150  includes a first solenoid valve  164  that switches pressure to act on a pilot port of the logic valve  158 , to any one of the pilot pressure generated in the pilot pressure generation line  162  and the first system pressure generated in the first system pressure line  156 . It is preferable that the first solenoid valve  164  is a poppet type solenoid valve with a slight leak (non-leak) at a closed port. Further, the pilot pressure generation line  162  is provided with throttle valves (variable throttle valves)  166 ,  168 , and the flow rate is regulated here. In this example, the throttle valve  168  is fully opened. 
     Further, between the die cushion pressure generation line  152  and the second system pressure line  159 , a throttle valve  170  and a second solenoid valve  172  are disposed in parallel, similarly, a throttle valve  174  and a second solenoid valve  176  are disposed in parallel. The second solenoid valves  172  and  176  are ON/OFF controlled, respectively. The second solenoid valves  172  and  176  are solenoid valves that enable opening and closing between the die cushion pressure generation line  152  and the second system pressure line  159 . It is preferable that the second solenoid valves  172  and  176  are poppet type solenoid valves with little leakage when fully closed. 
     Further, between the lowering pressure generation line  153  and the first system pressure line  156 , a throttle valve  173  and a third solenoid valve  175  are disposed. Between the lowering pressure generation line  153  and the second system pressure line  159 , a fourth solenoid valve  171  is disposed. The third solenoid valve  175  and the fourth solenoid valve  171  are ON/OFF controlled, respectively. The third solenoid valve  175  and the fourth solenoid valve  171  are solenoid valves that enable opening and closing between the lowering pressure generation line  153  and the first system pressure line  156 , and between the lowering pressure generation line  153  and the second system pressure line  159 , respectively. It is preferable that the third solenoid valve  175  and the fourth solenoid valve  171  are poppet type solenoid valves with little leakage when fully closed. 
     The first accumulator  154  and the second accumulator  155  are provided with cooling devices  178  and  179  such that hydraulic oil can be cooled via the first accumulator  154  and the second accumulator  155  by the cooling devices  178  and  179 . Though these cooling devices  178  and  179  are air cooling type cooling devices using fans, the type of the cooling devices is not limited to this. The cooling devices  178  or  179  may be a water cooling type cooling device that cools hydraulic oil by circulating cooling water. When the use frequency of the die cushion device  100  is low, it is possible to cope with only natural heat radiation without providing a cooling device, and a more inexpensive device can be implemented. 
     In addition, the die cushion pressure generation line  152 , the lowering pressure generation line  153 , the first system pressure line  156 , the second system pressure line  159  and the pilot pressure generation line  162  are respectively provided with throttle valves (needle valves)  180 ,  181 ,  182 ,  183 ,  184  and couplers  186 ,  187 ,  188 ,  189 ,  190 , for fluid feeding and system pressure sealing. 
     Further, a pressure detector  192  for detecting the die cushion pressure and a pressure detector  194  for detecting the pilot pressure are provided on the die cushion pressure generation line  152  and the pilot pressure generation line  162 , respectively. 
     In  FIG. 1 , reference numerals  197 ,  198 , and  199  indicate relief valves functioning as safety valves. 
     &lt;Oil Supply Device (Feeding Fluid Device)&gt; 
     Next, an oil supply device will be described. 
       FIG. 2  is a configuration diagram illustrating an embodiment of the oil supply device. 
     The oil supply device  200  is used when oil is supplied and the system pressure is filled, or when the system pressure is released (at the time of setup preparation), but is not used when the die cushion device  100  performs its cyclic function (normal function). 
     Thus, the oil supply device  200  is not required to be accompanied for each of die cushion devices  100 . It is sufficient to prepare one fluid supply device for a plurality of die cushion devices  100  to be managed. 
     As illustrated in  FIG. 2 , the oil supply device  200  includes: a tank  202  that stores hydraulic oil; a hydraulic pump (fluid-pressure pump)  206  that is driven by an induction motor  204 ; a relief valve  208  that serves as a safety valve; couplers  210  and  212 ; a check valve  214 ; and filters  216  and  218 . 
     The two couplers  210  and  212  of the oil supply device  200  are connected to any two of the five respective couplers  186 ,  187 ,  188 ,  189 , and  190 , provided in the die cushion pressure generation line  152 , the lowering pressure generation line  153 , the first system pressure line  156 , the second system pressure line  159 , and the pilot pressure generation line  162 , in the hydraulic closed circuit  150 , respectively. 
     In a case where the couplers  210  and  212  of the oil supply device  200  cannot be connected to any two of the five respective couplers  186 ,  187 ,  188 ,  189 , and  190 , of the hydraulic closed circuit  150 , the couplers  210  and  212  are connected to any two of them through one extension hose  230  or two extension hoses  230  and  240  illustrated in  FIG. 3 . 
     The extension hose  230  ( 240 ) is provided at its both ends with respective couplers  232  ( 242 ) and  234  ( 244 ) such that the coupler  210  or  212  on the oil supply device side and the coupler  186 ,  187 ,  188 ,  189 , or  190  on hydraulic closed circuit side can be connected through the couplers. 
     When a switch  220  is turned ON, the induction motor  204  of the oil supply device  200  is driven by AC current (alternating-current) from an AC (alternating-current) power source  222  to operate the hydraulic pump  206 . Accordingly, it is possible to supply the hydraulic oil in the tank  202  to the hydraulic closed circuit  150  of the die cushion device  100  through the filters  216  and  218 , the check valve  214 , and the coupler  210  (or the coupler  210  and the extension hose  230 ). In addition, it is possible to return the hydraulic oil to the tank  202  from the hydraulic closed circuit  150  through the coupler  212  (or the coupler  212  and the extension hose  240 ). 
     Further, the oil supply device  200  is provided, in its lower surface, with casters  224  so as to make the oil supply device  200  easily movable. 
     &lt;Flushing/Oil Supply/Pressure Releasing&gt; 
     When the die cushion device  100  of the present embodiment is used, it is required to perform preparation and setup operation for filling hydraulic oil into the hydraulic closed circuit  150  in a pressurized manner. 
     With reference to  FIG. 4 , an example of the preparation and setup operation will be specifically described. 
     First, the coupler  210  at the discharge port of the oil supply device  200  (or the coupler  234  at one end of the extension hose  230  in which the coupler  232  at the other end is connected to the coupler  210 ), the coupler  212  at the return port of the oil supply device  200  (or the coupler  244  at one end of the extension hose  240  in which the coupler  242  at the other end is connected to the coupler  212 ), and any two couplers out of the five couplers  186 ,  187 ,  188 ,  189 , and  190  of the hydraulic closed circuit  150  are connected. Then, the hydraulic closed circuit  150  circulates the hydraulic oil to perform a flushing operation for contamination removal and air bleeding inside the hydraulic closed circuit  150 . The throttle on the flow path inside the hydraulic closed circuit  150  through which the hydraulic oil circulates is fully opened, the relief valve is set at the lowest pressure, and the solenoid valve is turned ON at the proper place at the appropriate time. Connection points between the coupler  210  at the discharge port of the oil supply device  200  (or the coupler  234  at the one end of the extension hose  230 ), the coupler  212  at the return port of the oil supply device  200  (or the coupler  244  at the one end of the extension hose  240 ), and any two couplers of the five couplers  186 ,  187 ,  188 ,  189 , and  190  of the hydraulic closed circuit  150  are changed in several ways. 
     For example, in  FIG. 4 , when flushing inside the hydraulic closed circuit  150 , particularly between the first system pressure line  156  and the pilot pressure generation line  162 , the coupler  210  at the discharge port on the discharge side of the oil supply device  200  (or the coupler  234  at the one end of the extension hose  230 ) and the coupler  188  of the first system pressure line  156  are connected, and the coupler  212  at the return port of the hydraulic closed circuit  150  (or the coupler  244  at the one end of the extension hose  240 ) and the coupler  190  of the pilot pressure generation line  162  are connected. Then, all the throttle valves  182 ,  166 ,  168 ,  184  therebetween are fully opened, and the first solenoid valve  164  is turned ON such that the hydraulic oil flows through the poppet portion of the logic valve  158 . 
     When the flushing is completed, contaminants are removed inside the hydraulic closed circuit  150 , and hydraulic oil at atmospheric pressure is filled. The flushing operation may be performed only once (at the time of starting up the device) after the device is manufactured. 
     Next, the hydraulic closed circuit  150  is supplied with oil. Basically, as for the oil supply method (path), one manner (one pattern) is determined for each device (for each closed circuit). In the case of  FIG. 4 , the coupler  210  of the discharge port on the discharge side of the oil supply device  200  (or the coupler  234  at the one end of the extension hose  230 ) is connected to the coupler  188  of the first system pressure line  156  (which accumulates hydraulic oil having the highest pressure in the closed circuit). The relief valves  198  and  199  are set to predetermined values (in this example, the relief valve  198  is set to 300 kg/cm 2  as a safety valve and the relief valve  199  is set to 120 kg/cm 2 ), and the second solenoid valve  172  is turned ON (all other solenoid valves are in the OFF state, the throttle valve  182  is fully opened, and the throttle valve in the hydraulic closed circuit  150  is set to a predetermined set value). In the present example, a setting relief pressure (pressure of the relief valve  208 ) on the pump discharge side of the oil supply device  200  is 120 kg/cm 2 . 
     When the hydraulic pump  206  of the oil supply device  200  is turned ON in this state, first, while accumulating pressure in the first accumulator  154 , the first system pressure line  156  is filled with hydraulic oil having a pressure of 120 kg/cm 2 . While accumulating, via the relief valve  199 , pressure in the second accumulator  155  that acts as the tank of the second system pressure line  159 , the surplus hydraulic oil pressurizes the lowering pressure generation line  153  from the second system pressure line  159  via the fourth solenoid valve  171 , and pressurizes the die cushion pressure generation line  152  via the second solenoid valve  172  in the ON state. At this point, the cushion pad  110  is raised to the upper limit position. Finally, when a pressure (pressure detector  192 ) of the die cushion pressure generation line  152  reaches 80 kg/cm 2 , oil supply is completed. 
     In the normal state, the first system pressure line  156  and the die cushion pressure generation line  152  are shut off. In order to prevent hydraulic oil from leaking from the first system pressure line  156  to the second system pressure line  159  via the die cushion pressure generation line  152  and the second solenoid valve  172 , when the first solenoid valve  164  is OFF, the first system pressure acts on the pilot port of the logic valve  158 . 
     Oil supply is performed every time when a die is exchanged. Every time when a die is exchanged, the pressure of the main part filled in the hydraulic closed circuit  150  is released, the cushion pad  110  is lowered temporarily, and the die attaching and detaching work is performed, and then, oiling is performed before the next production operation using the newly mounted die. 
     Similarly, as for a pressure release method, basically, one manner (one pattern) is determined for each device (hydraulic closed circuit). In the case of  FIG. 4 , the coupler  212  at the return port of the oil supply device  200  (or the coupler  244  at the one end of the extension hose  240  in which the coupler  242  at the other end is connected to the coupler  212 ) is connected to the coupler  189  of the second system pressure line  159 . The second solenoid valve  172  is turned ON in the same manner as in oil supply. When the throttle valve  183  is opened in this state, hydraulic oil which has filled the lowering pressure generation line  153 , the die cushion pressure generation line  152 , and the second system pressure line  159  is discharged to the tank  202  of the oil supply device  200 , and pressure of the lines is released. When the pressure of the die cushion pressure generation line  152  (pressure detector  192 ) has decreased to the atmospheric pressure, the pressure release is completed. At this point, the cushion pad  110  is lowered down to the lower limit position. At this time, hydraulic oil in the first system pressure line  156  remains with a predetermined pressure value. As a result, time needed for the next oil supply can be shortened. 
     Oil supply and pressure releasing can be automated when they are performed frequently for each die change operation. 
     As one example, the coupler  210  at the discharge port of the oil supply device  200  (or the coupler  234  at the one end of the extension hose  230  in which the coupler  232  at the other end is connected to the coupler  210 ) and the coupler  188  at the first system pressure line  156  are always connected to each other, the coupler  212  at the return port of the oil supply device  200  (or the coupler  244  at the one end of the extension hose  240  in which the coupler  242  at the other end is connected to the coupler  212 ) and the coupler  189  at the second system pressure line  159  are always connected to each other, and the throttle valve  182  and the throttle valve  183  are replaced with a pressure accumulation valve  252  and a pressure release valve  254  that are configured by poppet (non-leak) type solenoid valves respectively (see  FIG. 5 ). 
       FIG. 5  is a block diagram illustrating an embodiment of an automatic control unit of the oil supply device  200  when oil supply and pressure release are performed automatically. 
     The automatic control unit of the oil supply device  200  illustrated in  FIG. 5  includes: the pressure accumulation valve  252  and the pressure release valve  254  replacing the throttle valves  182  and  183  as described above; a pressure accumulation button  260  and a pressure release button  262  that are push button switches for selecting between pressure accumulation and pressure release; a pressure switch SW-A that is turned ON in the vicinity of the atmospheric pressure and a pressure switch SW-B which is turned ON in the vicinity of 80 kg/cm 2 , that are disposed in the die cushion pressure generation line  152 ; an oil supply controller  250 ; and relays  251 ,  253 , and  219  that respectively drive the pressure accumulation valve  252 , the pressure release valve  254 , and the switch  220  (the switch that operates the induction motor  204 ). 
     Then, when the pressure accumulation button  260  is depressed in the pressure released state, the oil supply controller  250  turns ON the pressure accumulation valve  252  via the relay  251  and turns ON switch  220  via the relay  219 , until the pressure switch SW-A is turned OFF and the pressure switch SW-B is turned ON (until the pressure accumulation is completed). As a result, the hydraulic pump  206  is driven (rotated) by the induction motor  204 , and hydraulic oil is supplied from the oil supply device  200  to the hydraulic closed circuit  150 . 
     When the pressure accumulation is completed (the pressure switch SW-A is turned OFF and the pressure switch SW-B is turned ON), the oil supply controller  250  turns OFF the pressure accumulation valve  252  and turns OFF the switch  220  to stop the hydraulic pump  206 . 
     On the other hand, when the pressure release button  262  is depressed in the pressure accumulated state, the oil supply controller  250  turns ON the pressure release valve  254  via the relay  253  until the pressure switch SW-B is turned OFF and the pressure switch SW-A is turned ON (until the pressure release is completed), and the hydraulic oil is discharged. When the pressure release is completed (the pressure switch SW-B is turned OFF and the pressure switch SW-A is turned ON), the pressure release valve  254  is turned OFF. 
     &lt;Controller&gt; 
       FIG. 6  is a block diagram illustrating an embodiment of the controller  130  applied to the die cushion device  100 . 
     The controller  130  illustrated in  FIG. 6  controls ON/OFF of the first solenoid valve  164 , the second solenoid valves  172 ,  176 , the third solenoid valve  175 , and the fourth solenoid valve  171  of the hydraulic closed circuit  150  illustrated in  FIG. 1 . The controller  130  controls ON/OFF of the relays  134 ,  136 ,  138 ,  140 , or  142  according to the slide position detector  17 , outputs a driving current to the first solenoid valve  164 , the second solenoid valves  172 ,  176 , the third solenoid valve  175 , or the fourth solenoid valve  171  via relays  134 ,  136 ,  138 ,  140 , or  142  that are ON/OFF controlled, and individually controls ON/OFF of the first solenoid valve  164 , the second solenoid valves  172 ,  176 , the third solenoid valve  175 , or the fourth solenoid valve  171 . 
     The controller  130  of the present embodiment provides a simple control for individually controlling ON/OFF of the first solenoid valve  164 , the second solenoid valves  172 ,  176 , the third solenoid valve  175 , or the fourth solenoid valve  171  and the special control device is unnecessary. Therefore, a part (PLC) of the controller of the press machine  10  can be diverted, which does not lead to an increase in the cost of the die cushion device  100 . 
     The ON/OFF control timing of the first solenoid valve  164 , the second solenoid valves  172 ,  176 , the third solenoid valve  175 , or the fourth solenoid valve  171  by the controller  130  will be specifically described later. In addition, the controller  130  may control ON/OFF of the first solenoid valve  164 , the second solenoid valves  172 ,  176 , the third solenoid valve  175 , or the fourth solenoid valve  171 , according to the angle of the crankshaft  16  detected by the crankshaft encoder  18 . 
     Hereinafter, one cycle process of the die cushion device  100  will be described with reference to waveform diagrams of each part of the die cushion device  100  shown in  FIG. 7 . In  FIG. 7 , the horizontal axis shows time (unit: second), the left vertical axis shows a die cushion position (unit: mm), and the right vertical axis shows a pressure (unit: kg/cm 2 ). 
     &lt;Standby Process&gt; 
     When at least the slide  14  is positioned at the top dead center, the controller  130  turns ON the second solenoid valve  172  (portion (B) in  FIG. 7 ), and turns OFF the other solenoid valves (portions (A), (C) and (E) in  FIG. 7 ) such that the die cushion pressure generation line  152  and the pressure of the second system pressure line  159  have the same pressure. Thereby, the second system pressure acts on the die cushion pressure generation chamber  120   a  and the cushion pad lowering pressure generation chamber  120   b  of the hydraulic cylinder  120 , and the hydraulic cylinder  120  stops (stands by) at a rising limit (maximum height limit) (the cushion pad  110  abuts against the upper limit stopper  111  of the bed  11 ). 
     &lt;Impact/Die Cushion Force Action Process&gt; 
     The slide  14  of the press machine  10  begins to move downward, and before the slide  14  “impacts” the cushion pad  110  via the upper die  20 , the material  30 , the blank holder  102 , and the cushion pin  104  (near the half stroke position (a crank angle of around 90 degrees) on the lowering side), the controller  130  turns OFF the second solenoid valve  172  (portion (B) in  FIG. 7 ) and turns ON the first solenoid valve  164  (portion (C) in  FIG. 7 ). As a result, the first system pressure of approximately 120 kg/cm 2  is applied to the die cushion pressure generation line  152 . 
     In that state, when the slide  14  impacts the cushion pad  110 , the die cushion pressure proportional to the die cushion force is generated in the die cushion pressure generation chamber  120   a  of the hydraulic cylinder  120  due to synergistic effect of the logic valve  158 , the throttle valve  166  (throttle valve  168 ), and the pilot relief valve  160 . That is, from the die cushion pressure generation line  152  till the first system pressure line  156 , a hydraulic flow (a flow rate of hydraulic oil flowing per unit time) that is sourced from the die cushion pressure generation chamber  120   a  and driven via the throttle valve  166 , the throttle valve  168  and the pilot relief valve  160 , is generated. Along with the hydraulic flow, the pilot pressure lower than the die cushion pressure is generated between the throttle valve  166  and the throttle valve  168  (the pilot pressure generation line  162 ). As a result, the following pressures acts on the poppet of the logic valve  158  to keep balance of force: the die cushion pressure acting mainly on pressurized area on a die cushion pressure acting side; the first system pressure acting on pressurized area on a first system pressure acting side; the pilot pressure acting on pressurized area on a pilot pressure acting side (pressurized area on an X port side) through the first solenoid valve  164 ; a spring force acting on the poppet inside the logic valve; and a fluid force acting on the logic valve  158  in a direction interfering with (direction closing the valve) the flow of hydraulic oil from the die cushion pressure generation line  152  till the first system pressure line  156 . Thus, a poppet position (opening degree) of the logic valve  158  is maintained according to a speed of the slide  14  (that is, the poppet position is almost constant if the speed is constant), and the die cushion pressure is generated during the above series of acts. 
     At this time, since the die cushion pressure is pressurized from an initial pressure of 120 kg/cm 2 , it is possible to shorten the pressurizing time required to increase the die cushion pressure up to a set pressure of 250 kg/cm 2 . 
     At this time, hydraulic oil flowing from the die cushion pressure generation line  152  to the first system pressure line  156  accumulates in the first accumulator  154 , first hydraulic oil pressurized with the first system pressure of approximately 120 kg/cm 2  and discharges the surplus oil from the relief valve  199  to the second system pressure line  159 . Here, the first accumulator  154  also plays a role of temporarily storing the hydraulic oil that cannot be instantaneously discharged from the relief valve  199 . Hydraulic oil in the first system pressure line  156  mainly plays a role of suppressing the rise of the cushion pad  110  (slightly lowering the cushion pad) at the time of locking, and also plays a role of maintaining the accuracy of the die cushion pressure. This will be described below. 
     When the slide  14  approaches the bottom dead center and the slide speed decreases, the die cushion pressure decreases accordingly, and the die cushion pressure is then affected by overriding characteristic (pressure reduction characteristic) peculiar to the pilot relief valve  160  functioning by the pilot pressure set in the pilot relief valve  160  acting on the logic valve  158 . 
     Further, according to a die cushion stroke (as the slide  14  approaches the bottom dead center), the hydraulic oil is fed to the first accumulator  154  such that the first system pressure is increased (pressurized). In particular, since the first accumulator  154  can have a relatively small capacity in order to accumulate the small power necessary mainly for the locking process, the pressure is more likely to increase due to the die cushion stroke. Then, the die cushion pressure is generated according to (by adding) the first system pressure having this strong pressure increasing characteristic. 
     As a result, since the pressure reduction characteristic of the pilot relief valve  160  and the pressure increasing characteristic of the first accumulator  154  simultaneously affect and cancel each other, the die cushion device  100  of this embodiment has an excellent accuracy in the die cushion pressure over the entire die cushion stroke (that is, high smoothness). 
     In this way, it is preferable that the first accumulator  154  has a capacity that can achieve pressure increasing characteristics suitable for canceling the pressure reduction characteristic of the logic valve  158 . 
     &lt;Pressure Release/Locking Characteristics&gt; 
     The controller  130  turns OFF the first solenoid valve  164  when the slide  14  of the press machine  10  is moved downward and reaches the bottom dead center or a position slightly higher than the bottom dead center (near the bottom dead center) (refer to portion (C) in  FIG. 7 ). Accordingly, the poppet of the logic valve  158  moves in an opening direction (because the pilot pressure acting (on the pressurized area on the pilot pressure acting side) in a direction of closing the poppet is released to the first system pressure line  156 ) such that the die cushion pressure is released. The first accumulator  154  accumulates the amount of oil in the die cushion pressure generation chamber  120   a  which has been pushed away due to the moving-down of the hydraulic cylinder  120 . Thereby, the die cushion pressure drops to a pressure of around 120 kg/cm 2  (that is, a pressure slightly higher than 120 kg/cm 2 ) (arrow P A  in  FIG. 7 ) that is equal to (close to) the sum of the first system pressure which has become higher than a pressure in the standby state and the cracking pressure corresponding to the spring force of the logic valve  158 . When the pressure release is completed, the poppet of the logic valve  158  is closed. 
     Here, the sectional areas of the die cushion pressure generation chamber  120   a  and the cushion pad lowering pressure generation chamber  120   b  of the hydraulic cylinder  120  are set to Sa and Sb, respectively. 
     In this embodiment, Sa=78.5 cm 2 , Sb=53.9 cm 2 . At this time, a force of Fa≈120× Sa=9420 kgf is applied to the die cushion pressure generation chamber  120   a  of the hydraulic cylinder  120 , and a force of Fb≈80×53.9=4312 kgf is applied to the cushion pad lowering pressure generation chamber  120   b . A force of Ft=Fa−Fb=5108 kgf acts on (the whole of) the hydraulic cylinder  120  in the upward direction. This force (reaction force) is indirectly supported by the slide  14  near the bottom dead center (via the upper die  20 , the material  30 , the blank holder  102 , the cushion pin  104 , and the cushion pad  110 ). 
     Assuming that the slide  14  does not support the reaction force in this state (according to the rising of the slide), the pressure in the die cushion pressure generation chamber  120   a  drops to approximately 54.9 kg/cm 2  until the resultant force of 5108 kgf in the upward direction becomes 0 (zero). At this time, the cushion pad  110  is moved upward by the amount corresponding to the elastic release of the hydraulic oil due to the pressure reduction from 120 kg/cm 2  to 54.9 kg/cm 2 . This is a problem in the die cushion device described in Patent Document 1, and the die cushion device  100  of the present invention improves this characteristic. 
     After the first solenoid valve  164  is turned OFF as described above, the controller  130  turns ON the fourth solenoid valve  171  through the delay time  1  (such that the fourth solenoid valve  171  is turned ON when the pressure in the die cushion pressure generation chamber  120   a  decreases to around 120 kg/cm 2  (slightly higher than 120 kg/cm 2 ) as described above) (portion (D) in  FIG. 7 ), and blocks the cushion pad lowering pressure generation chamber  120   b  of the hydraulic cylinder  120  from the second system pressure line  159 . Then, the third solenoid valve  175  is turned ON after a delay time  2  (such that the third solenoid valve  175  is turned ON after the fourth solenoid valve  171  is securely turned ON) (portion (E) in  FIG. 7 ). Further, after a delay time  3 , the second solenoid valve  172  is turned ON (portion (B) in  FIG. 7 ). 
     At this time, the force Fa≈180×Sa=6280 kgf is applied to the die cushion pressure generation chamber  120   a  of the hydraulic cylinder  120 , and the force Fb≈120×53.9=6468 kgf is applied to the cushion pad lowering pressure generation chamber  120   b , the force Ft=Fa−Fb=−188 kgf, that is, the downward force of 188 kgf is applied to (the whole of) the hydraulic cylinder  120 . Then, this force is applied during the locking time to move the hydraulic cylinder  120  downward to a position slightly (about 0.2 mm in this example) lower than the bottom dead center. At this point, the slide  14  is still near the bottom dead center. The locking position can be easily controlled (with a timer) without sudden lowering (all at once) because the slight downward force acts for a fixed time period of locking time. 
     After the lapse of the locking time, the second solenoid valve  172  is turned OFF (portion (B) in  FIG. 7 ), and then the third solenoid valve  175  is turned OFF (portion (E) in  FIG. 7 ). In the hydraulic cylinder  120 , the pressure of the die cushion pressure generation chamber  120   a  and the pressure of the cushion pad lowering pressure generation chamber  120   b  are stabilized at around 80 kg/cm 2  (slightly higher than 80 kg/cm 2 ) and around 120 kg/cm 2  (slightly lower than 120 kg/cm 2 ), respectively, such that maintain force balance is maintained in the vicinity of the position lower than the locking position by 0.2 mm. 
     &lt;Knockout Process&gt; 
     In the locking process, after the locking has been performed for the fixed time, the controller  130  turns OFF the fourth solenoid valve  171  when the slide  14  reaches 90 mm (in this example) (portion (D) in  FIG. 7 ). As a result, the pressure in the cushion pad lowering pressure generation chamber  120   b  of the hydraulic cylinder  120  drops to the second system pressure and the balance of the forces acting on the hydraulic cylinder  120  (the whole) collapses once. The hydraulic cylinder  120  slightly moves upward and the pressure in the die cushion pressure generation chamber  120   a  slightly decreases (is released) so as to maintain the force balance again (arrow P B  in  FIG. 7 ). 
     Thereafter, when the slide  14  reaches 100 mm (in this example), the controller  130  turns ON the second solenoid valve  172  and the second solenoid valve  176  (portions (A) and (B) in  FIG. 7 ), and performs the knockout process at a higher speed. At this time, both the die cushion pressure generation chamber  120   a  and the cushion pad lowering pressure generation chamber  120   b  of the hydraulic cylinder  120  communicate with the second system pressure of the second system pressure line  159  of approximately 80 kg/cm 2 . Therefore, the knockout force Fk≈80×(Sa−Sb)=1968 kgf can work. 
     Thereafter, when the slide  14  reaches 140 mm (in this example), the controller  130  turns OFF the second solenoid valve  176  (portion (A) in  FIG. 7 ) to reduce the knockout speed (slow down) such that the cushion pad  110  gradually reaches (impacts) the upper limit (standby position). This slow down action is effective in preventing the product from falling. When it is not necessary to change the rising speed of the cushion pad  110 , a plurality of second solenoid valves (the two second solenoid valves  172 ,  176 ) may be configured by one second solenoid valve. 
     The one cycle process of the die cushion device  100  is terminated after the standby process, the impact/die cushion process, the pressure releasing/locking process, and the knockout process, described above. 
     &lt;Others&gt; 
     In this embodiment, when the cushion pad  110  is locked in the vicinity of the bottom dead center, a downward pressure is applied to the hydraulic cylinder  120  due to the differential pressure between the first system pressure and the second system pressure, and the cushion pad  110  is held below the bottom dead center of the slide (that is, complete locking is performed). However, when the cushion pad  110  does not affect the press forming even if the cushion pad  110  is locked at a position slightly higher than the bottom dead center (that is, even if complete locking is not performed), the solenoid valve may be controlled so as not to apply the downward pressure to the hydraulic cylinder  120  during the locking. 
     That is, even when the slide  14  reaches the vicinity of the bottom dead center, the second solenoid valves  172  and  176  are kept in the closed state. Thereby, the supply of the hydraulic oil of second system pressure to the die cushion pressure generation chamber  120   a  of the hydraulic cylinder  120  is shut off and the cushion pad  110  is locked. In addition, after the locking for a fixed period, when the cushion pad  110  is moved upward, the second solenoid valves  172 ,  176  are opened such that the hydraulic oil of second system pressure can be supplied to the die cushion pressure generation chamber  120   a  and the cushion pad lowering pressure generation chamber  120   b  of the hydraulic cylinder  120  to move the cushion pad  110  upward. 
     Further, in this embodiment, oil is used as a hydraulic fluid for the die cushion device. However, the present invention is not limited to this. Water or other liquid may be used. That is, in the embodiment of the present application, the hydraulic cylinder and the hydraulic closed circuit are used, but the invention is not limited to these. Needless to say, hydraulic cylinders and hydraulic closed circuits using water or other liquids may be used in the present invention. Further, the die cushion device according to the present invention can be applied not only to the crank press, but also to any type of press machine including a mechanical press. 
     Further, the hydraulic cylinder disposed in the cushion pad is not limited to one place in the above embodiment. Hydraulic cylinders may be disposed, for example, at two places in front of and behind the cushion pad, or four places in front, back, left and right of the cushion pad. 
     Further, the present invention is not limited to the above examples. Needless to say, various improvements and modifications may be made without departing from the gist of the present invention.