Die cushion equipment for press

Abstract: Instead of the conventional type equipment, which consists of an air cylinder unit (101) opened to atmospheric air, the present invention proposes a closed type hydraulic cylinder (1) unit. The lower (1L) chamber and the upper chamber (1U) of the closed type cylinder unit are communicated with each other, and the differential pressure is controlled. Therefore, if the capability diagram of the press is kept in memory, the differential pressure between two chambers can be automatically con­ trolled according to the desired curve. Because die cushion capability during press operation can be changed and adjusted to the value suitable for the product to be processed, a wide variety of products can be produced in high quality and with high efficiency.

Description: A conventional type die cushion equipment is shown in Fig. 5. A piston 102 is incorporated in an air cylinder 101 so that it can freely go up and down, and a wear plate 104 is fixed on upper portion of a piston rod 103. The wear plate 104 receives the lower surface of a cushion pin (not shown), which is inserted into female mold, and the blank holder pressure is transmitted to the cushion pin by the air cylinder 101 and the piston 102. An upper chamber 101U and a lower chamber 101L are formed by air cylinder 101 and piston 102, and the upper chamber 101U is communicated with atmospheric air through an opening 105. The lower chamber 101L is communicated with an air tank 106 through a connection pipe 107. The air tank 106 is connected to an air supply circuit 108, and compressed air of the predetermined pressure is supplied. When press operation (e.g. drawing) is performed by this die cushion equipment, the compressed air corresponding to the blank holder pressure required is supplied to the air tank 106. The compressed air with lower chamber pressure PL satisfying the condition: Blank holder pressure F = (Effective pressed area A1 of piston 102) × (Pressure in lower chamber PL) is supplied. As the slide of the press goes down, the piston 102 goes down through cushion pin, wear plate, etc., and blank holder pressure is generated. When the piston 102 comes to the lower limit (at the lowest position), the drawing process is completed. Then, the slide of the press goes up, and wear plate 104, piston 102, etc. also go up. (Problems to be solved by the invention) In the above die cushion equipment of conventional type, it is discussed here how the blank holder pressure is changed when the piston 102 goes down. Fig. 4 shows the operation of the equipment according to this invention and that of the conventional type equipment. The stroke of the piston 102 is given on the abscissa, and the blank holder pressure is shown on the ordinates. Namely, the abscissa shows the position when the piston 102 moves from the upper limit (the highest position) to the lower limit LL, and the ordinates give the blank holder pressure generated by the piston 102 (such as F, Fa, Fae, etc.). When it is supposed that the necessary blank holder pressure is Fa, the blank holder pressure is Fae when the piston 102 reaches the lower limit. The blank holder pressure at the lower limit is increased by Fae-Fa compared with the value at the upper limit because the capacity of the lower chamber 101L is decreased as the piston 102 goes down. It is not desirable that the blank holder pressure increases in the drawing process. Therefore, an air tank with large capacity air tank 106 is furnished in the past. The capacity should be as large as possible. It is about 5 - 8 times as high as that of the air cylinder 101 in normal case because of the space of installation. Accordingly, there have been the following problems with the die cushion equipment of the conventional type: (1) A large capacity air tank 106 is required, and it is not very easy to keep the space of installation. Especially, serious problem arises in case of transfer press because a large number of die cushion equipment are to be installed. (2) Even when the large capacity air tank 106 is furnished, it is impossible to turn the increase of the blank holder pressure to zero. (3) Air pressure in the air tank 106 must be adjusted when die is replaced. This requires long time because of large capacity. When it is adjusted by decreasing the air pressure, compressed air must be discharged and this is not very economical. (4) When the blank holder pressure is to be increased or decreased as the piston 102 goes down in the drawing process, it cannot be freely increased or decreased. DE-A-32 30 673 discloses a controllable die cushion for press having at least one piston-cylinder unit operatively connected to a gas-loaded compressed oil reservoir. The cushion plate is drawn further downwards by a specific amount without additional energy consumption when the lower end position of the press ram has been reached, thereby allowing the energy stored in the cushion to be used to generate a prestressing force counteracting the cushion force. SUMMARY OF THE INVENTION The object of the present invention is to offer a small compact die cushion equipment for press, which is easy to handle and can be produced at low cost and which can maintain die cushion capability at constant level and can change the settings during operation. Thus, the disadvantages of conventional type equipment can be eliminated, which requires large capacity buffer tank, large size compressor and quick-acting large size exhaust valve, etc. because air cylinder system is adopted. To solve these problems, the equipment according to the present invention consists of a closed type cylinder unit, which is to replace the open-to-atmosphere type cylinder of the conventional equipment. The new equipment is also based on the principle that the die cushion capability by the closed type cylinder is determined by the differential pressure between the lower chamber pressure and the upper chamber pressure, and the differential pressure is controlled by communicating the lower and the upper chambers with each other. Specifically, the equipment according to the present invention comprises: a first control valve furnished in a first oil passage, which communicates a lower chamber and an upper chamber on both sides of a piston in a hydraulic cylinder, a second control valve furnished in a second oil passage, which communicates the lower chamber and the upper chamber on both sides of the piston in the hydraulic cylinder, a hydraulic pressure supply means being connected between the lower chamber and the first control valve through a check valve, and supplying a preset oil pressure, a buffer oil tank being connected between the lower chamber and the first control valve and accommodating a part of the oil flowing out of the lower chamber when the piston goes down, a first control means to open or close the first control valve when the differential pressure between the lower chamber pressure and the upper chamber pressure exceeds the preset differential value when the piston goes down, and thereby allow communication between the lower chamber and the upper chamber, and a second control means to open or close the second control valve and thereby allow communication between the lower chamber and the upper chamber, to hinder the increase of the upper chamber pressure when the piston goes up. Therefore, the present hydraulic pressure is to be established in the lower chamber of the hydraulic cylinder by hydraulic pressure supply means according to this invention. When the piston goes down from the upper limit due to the press load, the lower chamber pressure is increased, and the differential pressure between the lower chamber pressure and the upper chamber pressure is rapidly increased. Thus, the die cushion capability corresponding to the preset hydraulic pressure value is established. Further, when the piston goes down, the differential pressure between two chambers exceeds the preset differential pressure value. Then, the first control valve is opened by the first control means, and the lower chamber is communicated with the upper chamber. Consequently, the lower chamber pressure is decreased, and the first control valve is closed. Next, the first control valve is controlled when the piston goes down, and the differential pressure between two chambers is controlled to the preset differential pressure as set by the first control means. Therefore, if the preset differential pressure is determined according to piston stroke to gradual increase, gradual decrease, etc., die cushion capability can be changed and adjusted even during the press operation. If the preset differential pressure is set to a constant level, die cushion capability can be set to the constant value for all strokes. Quantitative imbalance corresponding to the volume of piston rod occurs between oil quantity discharged from the lower chamber and the quantity supplied to the upper chamber when the piston goes down, but this is absorbed by the buffer oil tank. On the other hand, when the slide goes up, the piston goes up from the lower limit to the upper limit by the differential pressure between two chambers. Then, the pressure in the upper chamber gradually increases, and when the pressures in both chambers are approximately equal to each other, the shut-off valve is opened, and the upper and the lower chambers are communicated with each other. As the result, the excessive increase of the upper chamber pressure is hindered, and the pressures in both chambers become equal to each other. Under such conditions, the piston is pushed upward by the difference of the effective area due to the presence of the piston rod (sectional area), and the piston goes up relatively slowly. Because residual pressure exists within the upper chamber, the piston goes up to the upper limit without generating big impact force. By taking the proper timing to close the second control valve, it is possible to extensively increase the upper limit damper effect. Therefore, it is possible according to the present invention to eliminate large size buffer air tank, high pressure large capacity compressor, quick-acting large size exhaust valve, etc. in the conventional type air cylinder apparatus and to establish die cushion capability accurately and quickly with compact and lightweight equipment at low running cost. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a general schematical diagram of an embodiment of this invention; Fig. 2 is a general schematical diagram of a second embodiment of the invention; Fig. 3 is a general schematical diagram of the embodiment 3 of the invention; Fig. 4 is a diagram to explain the operation of the embodiments in comparison with the operation of conventional type die cushion equipment; Fig. 5 is a general schematical drawing of a die cushion equipment of conventional air cylinder type. In the figures, 1 refers to a hydraulic cylinder, 1U an upper chamber, 1L a lower chamber, 2 a piston, 10 a first oil passage, 11 (A, B and C) pipes, 13 a buffer oil tank, 14 hydraulic pressure supply means, 17 a check valve, 20 a first control valve, 21 a main unit, 28 an air pressure regulating valve, 30 a second oil passage, 40 a second control valve, 50 first control means, 51 a control unit, 52 capability signal generating means, 53 capability memorizing means, 54 selection means, 55 a standard capability setter, 57 a pressure detector, 60 second control means, 61 a controller, 62 a pressure setter, 65 a pressure detector, and 70 a control panel. DESCRIPTION OF THE PREFERRED EMBODIMENTS In the following, the embodiments of the present invention will be described in connection with the drawings. (Embodiment 1) As shown in Fig. 1, the embodiment 1 comprises hydraulic cylinder units (1, 2), a first control valve 20, a second control valve 40, hydraulic pressure supply means 14, a buffer oil tank 13, a first control means 50, a second control means, etc. and it is designed in such manner that die cushion capability can be changed or adjusted or maintained at constant level during the press operation. First, the hydraulic cylinder units are of closed type, consisting of hydraulic cylinder 1 (upper chamber 1U and lower chamber 1L), a piston 2 and a piston rod 3 (connected to a wear plate 4). Specifically, basic structure is the same as the air cylinder units (101, 102, 103 and 104) as shown in Fig. 5. According to the present invention, it is designed as hydraulic pressure type and can be used at high pressure. This leads to the compact design and high responsiveness. The lower chamber 1L and the upper chamber 1U having a piston 2 of the hydraulic cylinder 1 between them are communicated with each other by a first oil passage 10 (pipes 11A - 11C) and a second oil passage 30 (pipes 31 and 32). The first oil passage 10 is provided with a first control valve 20, and the second oil passage 30 with a second control valve 40. A hydraulic pressure supply means 14 (pump 15 and motor 16) for supplying the preset hydraulic pressure is furnished between the lower chamber 1L and the first control valve 20 through a check valve 17. The means 14 is to set hydraulic pressure in the lower chamber 1L in the so-called initial state (with the piston 2 at the upper limit UL). 18 represents an oil tank, and 19 an exhaust oil valve to move the cushion downward. Further, a buffer oil tank 13 is connected between the lower chamber 1L and the first control valve 20, and it it is to accommodate a part of oil flowing out of the lower chamber 1L when the piston 2 goes down. In other words, when the piston 2 goes down, the oil in the lower chamber 1L is supplied to the upper chamber 1U through the first control valve 20. In this case, actual volume of the upper chamber 1U is smaller by the volume of the piston rod 3, which is inserted into the oil cylinder 1. In this connection, the buffer oil tank 13 is furnished to temperarily accommodate the volume difference. In the present embodiment, it is formed as an accumulator having the capacity to receive the oil with the volume equal to the volume difference when the piston 2 goes down to the lower limit LL. Therefore, a buffer 13A with sealed-in nitrogen gas is incorporated in this buffer oil tank 13, and it accommodates the oil when the pressure exceeds the preset oil pressure. Here, the first control valve 20 consists of a main unit in hollow cylindrical shape and of a valve disc 26 in cylindrical shape slidably inserted into this main unit 21. It has the structure of a check valve permanently closed. On the front end of the main unit 21, oil inlet 22-I and oil outlet 22-O to be connected with the pipe 11B are furnished. On the rear end of the main unit, an inlet 23-I to apply the upper chamber pressure PU is provided through the pipe 11A. On the other hand, spring 25 is mounted in the hollow portion 27H at the rear end of the valve disc 26, and a valve unit 27V is furnished on the front end to close the oil inlet 22-I. Accordingly, when the lower chamber pressure PL is increased, valve disc 26 is moved toward the right in Fig. 1 against the force of the spring 25, and the lower chamber 1L and the upper chamber 1U of the hydraulic cylinder 1 are communicated with each otehr through the oil inlet 22-I and the outlet 22-O. When oil is released from the lower chamber 1L, the lower chamber pressure PL is decreased, and the first control valve 20 is again blocked by the force of the spring 25. In this case, the cracking pressure of the first control valve 20 functioning as check valve is primarily determined by the force of the spring 25. The features of the first control valve 20 according to the present invention is that cracking pressure is variable. Thus, pressurized air (or oil pressure) is supplied from the air inlet 23C at the intermediate portion of the main unit 21. The pressurized air as control signal further increases the cracking pressure in addition to the force of the spring 25. In other words, minimum cracking pressure is established by the force of the spring 25, and the cracking pressure higher than this is determined by the air pressure (or oil pressure) supplied to the inlet 23C. Oil outlet 22-O and oil inlet 23-I are communicated with each other for the power balance of the valve disc 26. 23-O is an opening opened to atmospheric air. Next, the first control means 50 is a means to set the cracking pressure of the first control valve 20, i.e. a control means to open or close the first control valve 20 when the differential pressure between the pressure PU in the lower chamber 1L and the pressure PL in the upper chamber 1U, changing with the downward movement of the piston 3, exceeds the preset differential pressure. In the present embodiment, it is to set the air pressure (or oil pressure) to supply to the air inlet 23C of the first control valve 20. Namely, the first control means 50 consists of a control unit 51 and a pressure regulating valve 28 serving as an electric converter and mounted on the pipe 29, which connects the air inlet 23C of the first control valve 20 with the air source (or oil source). Control unit 51 and others are stored in the control panel 7 together with the controller 61 and others. The control unit 51 in this embodiment issues electric signals to control the pressure regulating valve 28 in order to equalize the differential pressure, obtained through comparative calculation from lower chamber pressure PL and upper chamber pressure PU, to the differential pressure corresponding to the capability signal, using the lower chamber pressure PL from pressure detector 57, the upper chamber pressure PU from pressure detector 65 and the crackshaft angle from angle detector 58 as input factors. In other words, it is to control the cracking pressure of the first control valve 20 by closed loop. This capability signal may be formed to be memorized in the control unit 51 itself. In this embodiment, it is outputted from the capability signal generating means 52. The capability signal generating means 52 specifies the die cushion capability to obtain the blank holder pressure necessary for press operation in relation to the stroke of the piston 2, and it comprises a capability memorizing means 53 to memorize a plurality of capability diagrams, i.e. die cushion capability-piston stroke curves. Capability diagram is a diagram of the curves (2) - (5) as shown in Fig. 4, and the capability is changed during the change of piston stroke, i.e. during press operation. For wider usability, the curve (1) with constant capability is also memorized. The selection means 54 selects the curve from the capability diagram. Further, the capability signal generating means 52 in the embodiment 1 is designed in such manner that it can generate the capability signal, rapidly rising up to the standard capability F as set by the standard capability setter 55 up to the piston stroke ST1 in Fig. 4. It goes without saying that the diagram for the range from the upper limit UL to the lower limit LL or from the lower limit LL to the upper limit UL may be memorized by the capability signal generating means 52 and the capability signal corresponding to the diagram selected by the selection means 54 may be outputted to the control unit 51. In this case, the standard capability setter 55 may not be provided. The piston stroke is specified by the crankshaft angle inputted from the angle detector 58. Also, because the capability memorizing means 53 is formed from reloadable ROM, the capability diagram can be changed, added or deleted as appropriate. The second oil passage 30 communicates the upper chamber 1U with the lower chamber 1L at proper timing when the piston 2 is going up, and it consists of pipes 31 and 32, which connect the two chambers 1U and 1L. The second control valve 40 is composed of an electromagnetic valve or a servo-valve, which is installed in the pipes 31 and 32. The second control means 60 to control the second control valve 40 comprises a pressure setter 61 and a controller 62, and the controller 62 compares the upper chamber pressure PU detected by the pressure detector 65 and the preset value of the pressure setter 61 and issues the signal to excite solenoid when these two values are equalized to each other. When solenoid is excited, the second control valve 40 is opened. The controller 62 turn off the signals and closes the second control valve 40 again when the piston 2 moves most closely to the upper limit UL. In the embodiment 1 with the above arrangement, the operation is performed as follows: First, initial pressure in the lower chamber 1L is set by the hydraulic pressure supply means 14. Then, the standard capability F to be established up to the piston stroke ST1 is set by the standard capability setter 55, and the capability from the stroke ST1 and after is selected from the curve suitable for the desired product type (e.g. the curve (5) of Fig. 4) by the selection means 54. Also, the upper chamber pressure PU to close the second control valve 40 is set by the pressure setter 62. In Fig. 4, the curves (2) to (5) are drawn by simplified expression to the curve (1) (average capability is shown), and the data for the upward movement of piston are not given in the diagram because they are easily imaginable from the curve (1). The die cushion capability F is determined by the following equation: F = PL · A1 - PU · (A1 - A2) Because the upper chamber 1U is opened to the atmospheric air in the conventional air cylinder types as given in Fig. 5, the predetermined capability Fa is established as soon as the piston rod 103 is displaced downward. Then, the die cushion capability gradually increases up to Fae at the lower limit LL as shown by dashed line in Fig. 4. Because the volume of the buffer air tank 106 is larger by 5 - 8 times, it is impossible to maintain the necessary die cushion capability Fa at constant level. At the lower limit LL, the blank holder pressure becomes excessive by 20 - 25%. According to the present invention, when the piston 2 moves downward from the upper UL, the lower chamber pressure PL is raised, and the differential pressure from the upper chamber pressure PU is increased, and the standard capability F can be quickly generated. The higher the initial pressure to the lower chamber 1L is, the earlier this rise-up occurs. When the piston 2 tends to go down after the standard capability F is established, the control unit 51, constituting the first control means 50, controls the pressure regulating valve 28 so that the differential pressure obtained from the input from the pressure detectors 57 and 65 becomes equal to the differential value corresponding to the capability signal outputted from the capability signal generating means 52. As the result, the cracking pressure is regulated, and the first control valve 20 is opened by this cracking pressure. The excess oil is received by the buffer oil tank 13. Then, the lower chamber 1L is communicated with the upper chamber 1U, the lower chamber pressure PL decreases and the second control valve 40 is closed again. Therefore, the differential pressure is changed, repeating small fluctuations within the allowable range of necessary blank holder pressure. Thus, the standard capability F can be substantially maintained at constant level up to the stroke ST1. Here, if the curve (1) in Fig. 4 is selected by the selection means 54, the capability signal generating means 52 reads out said curve (1) from the diagram memorized by the capability memorizing means 53 and outputs it to the control unit 51. In this case, the first control valve 20 is controlled by the first control means (51, 28, etc.) to maintain the capability F at constant level until the piston 2 reaches the lower limit LL. When the curve (5) is selected, for example, the control unit 51 controls the pressure regulating valve 28 to change the die cushion capability according to the curve (5) based on the input from the capability signal generating means 52, the input from two pressure detectors 57 and 65, and the input from the angle detector 58, and it controls the first control valve 20. The die cushion capability is decreased stepwise after the piston stroke during press operation passes ST1, and it is maintained at constant level up to the lower limit LL. In case of the curve (4), it is gradually decreased. It is increased stepwise in case of the curve (3) and is gradually increased in case of the curve (2). On the other hand, the upward movement of the piston 2 from the lower limit LL complies with the upward movement of the slide in the initial stage according to the differential pressure between two chamber pressures PL and PU, and it is smoothly performed thereafter in no-load state. Because the lower chamber pressure PL becomes lower and the upper chamber pressure PU becomes higher, the differential pressure is rapidly decreased. When the upper chamber pressure PU exceeds the preset value set by the pressure setter 62, the second control valve 40 is opened by the signal from the controller 61, and two chambers 1U and 1L are communicated with each other. Accordingly, the pressure values in two chambers PL and PU are equalized to each other. The piston 2 goes up further by the pushing force generated by the difference of effective area due to the presence or absence of cross-sectional area A2 of the piston rod 3. This may be left to reach the upper limit UL. In the present embodiment, the second control valve 40 is blocked again immediately before the upper limit UL. Because the upper chamber pressure PU is silghtly increased, and the pushing force of the piston 2 is rapidly decreased, the damping effect at the upper limit UL can be extensively increased. When the piston 2 moves most closely to the upper limit UL, it is effective to open and close the first control valve 20 momentarily again. According to this embodiment, a first oil passage 10 and a second oil passage 30 are provided to communicate the upper chamber 1U and the lower chamber 1L of hydraulic cylinder 1 with each other. When the piston goes down, the first control valve 20 is controlled by the first control means 50 in order to control the differential pressure between the lower chamber pressure PL and the upper chamber pressure PU, and the desired die cushion capability can be established. Through the control of the second control valve 40 by the second control means 60 at proper timing during upward movement, adequate cushion damper is obtained by hindering the increase of the upper chamber pressure PU. Thus, there is no need to provide extra-large buffer air tank 106 or large size compressor unlike the case of the conventional air cylinder type equipment. The space for installation is also small and economical. Because cylinder units (1, 2) are of hydraulic pressure type, each of the equipment components can be designed in compact form, and high responsiveness can be provided by increasing hydraulic pressure. The first control valve furnished in the first oil passage 10 has the function as a check valve, and the setting of the cracking pressure can be changed by the first control means 50 (51, 28, etc.), and it is very easy to set the die cushion capability. Moreover, initial idle twisting is reduced because it is of hydraulic pressure type, and the equipment is operated smoothly regardless of the volume of the upper chamber 1U of the cylinder 2. The first control means 50 is specified by the selection means 54 and capability memorizing means 53, and the first control valve 20 is controlled by differential pressure according to the capability signal issued from the capability generating means 52. Thus, the die cushion capability can be changed and adjusted according to the predetermined curve. Since die cushion capability can be changed during press operation, a wide variety of products can be produced in high quality and with high efficiency. The material costs can also be reduced, and there is no inconveniences such as the restriction in the form of materials. Quick start-up and stopping adjustment can be accomplished. Also, hydraulic cylinder units (1, 2) are designed in closed type and the first oil passage 10 and the second oil passage 30 are controlled by die cushion capability to communicate two chambers 1U and 1L with or to isolate them from each other. Accordingly, high pressure large capacity compressor or quick-acting large size exhaust valve can be eliminated, and the equipment is compact and economical. Because the adjustment of die cushion capability can be achieved simply by changing the setting of cracking pressure of the first control valve 20, it can be performed rapidly and accurately. Thus, waiting time is shorter, and press production efficiency can be increased. Moreover, there is no need to release oil during the adjustment of die cushion adjusting because buffer oil tank 13 is furnished. Therefore, the disadvantages caused by the release of the air in large quantity as seen in case of the conventional equipment can be eliminated, and operation economy is assured. In the first control valve 20, basic die cushion capability is restricted by the pushing force of the spring 25, and cracking pressure is set by changing the air pressure to the air inlet 23C through the balance system, in which the upper chamber pressure PU is applied to oil inlet 22-O and inlet 23-I. Thus, it is possible to reduce the size of hydraulic cylinder units (1, 2) by increasing the preset pressure of the oil in the lower chamber 1L, and smooth control can be accomplished by decreasing the control air pressure. Further, it is possible to leave the upper chamber pressure PU near the upper limit UL of the piston. Namely, the differential pressure can be reduced by taking proper timing to block the second control valve 40, and the impact at the upper limit UL can be extensively decreased. (Embodiment 2) Fig. 2 shows the embodiment 2 of this invention. In the embodiment 2, the equipment facilities are more simplified than in the embodiment 1. Specifically, when the form and the characteristics of hydraulic cylinder units (1, 2), the first control valve 20 and the second control valve 40 are defined, the lower chamber pressure PL and the upper chamber pressure PU during the upward and downward movement of the piston 2, the differential pressure between the pressure values PL and PU, and the relation between upward or downward movements of the slide and the blank holder pressure required are made clear if press arrangement and the products to be processed are specified. Therefore, the first control valve 20 is controlled by differential pressure, whereas the pressure values in two chambers and the differential pressure are not detected, and these are replaced by the crankshaft angle. Accordingly, the pressure detectors 57 and 65 in the embodiment 1 is not included in this arrangement. Also, the standard capability setter 55 is not provided, and die cushion capability for all strokes from the upper limit UL to the lower limit LL of the piston 2 is memorized by the capability memorizing means 53. Thus, the control unit 51 to form the first control means 50 specifies the capability signal inputted from the capability signal generating means 52 by the crankshaft angle from the angle detector 58, controls the pressure regulating valve 28 and sets the cracking pressure of the first control valve 20. That is, the embodiment 1 is designed in closed loop, while the embodiment 2 forms an open loop having a curve read from the capability memorizing means 53 as a preset differential pressure value. Further, the second control means 60 to control the second control valve 40 is constituted only from the controller 61, which is a program sequencer. In the upward movement of the piston 2, the second control valve 40 is controlled depending upon the crankshaft angle according to the predetermined procedure. However, opening and closing of the valve is controlled in the same timing as in the embodiment 1. The setting of the opening and closing procedure and the timing can be changed. Also, in the case of the embodiment 2, the same effects as in the embodiment 1 can be obtained (such as the elimination of conventional type large size buffer tank, high pressure large capacity compressor and quick-acting large size exhaust valve, change and adjustment of die cushion capability during press operation) by using the crankshaft angle as input. Moreover, simpler structure and low cost can be achieved compared with the case of the embodiment 1 through elimination of two pressure detectors (57 and 65) and the simplification of the first control means 50. Each operation is not defined by the differential pressure between the pressure values PL and PU in two chambers and it is indirectly defined by crankshaft angle. If the capability diagram to be memorized by the capability memorizing means 53 is clearly defined, automatic adjustment of die cushion capability during press operation can be freely and easily achieved. (Embodiment 3) The embodiment 3 is given in Fig. 3. In this embodiment, hydraulic cylinder units (1, 2), buffer oil tank 13, hydraulic pressure supply unit 14, the first control means 50, the second control valve 40, and the second control means 60 are the same as in the embodiment 1. The first control valve 20 consists of a servo valve directly controlled by differential pressure through electric signals. Thus, pneumatic equipment and devices (9, 28, 29) are eliminated to simplify the facilities. The same effects as in the embodiment 1 can be obtained in this embodiment 3, and the responsiveness is extensively increased because there is no need of electro-pneumatic conversion. In the above embodiments, the control unit 51, the capability signal generating means 52, the capability memorizing means 53, the controller 61, etc. are furnished separately, whereas these components may be organically integrated by computer or other devices including CPU, RAM, ROM, etc.

Claims: A die cushion equipment for press, comprising: a first control valve (20) furnished in a first oil passage (10), which communicates a lower chamber (1L) and an upper chamber (1U) on both sides of a piston (2) in a hydraulic cylinder (1), a second control valve (40) furnished in a second oil passage (30), which communicates the lower chamber (1L) and the upper chamber (1U) on both sides of the piston (2) in the hydraulic cylinder (1), a hydraulic pressure supply means (14) being connected between the lower chamber (1L) and the first control valve (20) through a check valve (17), and supplying a preset oil pressure, a buffer oil tank (13) being connected between the lower chamber (1L) and the first control valve (20) and accommodating a part of the oil flowing out of the lower chamber (1L) when the piston (2) goes down, a first control means (50) to open or close the first control valve (20) when the differential pressure between the lower chamber pressure and the upper chamber pressure exceeds the preset differential value when the piston (2) goes down, and thereby allow communication between the lower chamber (1L) and the upper chamber (1U), and a second control means (60) to open or close the second control valve (40) and thereby allow communication between the lower chamber (1L) and the upper chamber (1U), to hinder the increase of the upper chamber pressure when the piston (2) goes up. A die cushion equipment for press according to claim 1, wherein said first control valve (20) comprises a main unit in hollow cylindrical shape provided with an inlet (22-I) connected to the lower chamber (1L) of said hydraulic cylinder (1) and an outlet (22-O) connected to the upper chamber (1U), and a valve disc (26) slidably engaged in said main unit, and said valve disc (26) closes said inlet (22-I) when it is pushed by spring force, determining minimum cracking pressure, and air pressure or oil pressure to determine the cracking pressure higher than said minimum cracking pressure. A die cushion equipment for press according to Claims 1 or 2, wherein said first control means (50) comprises a pressure regulating valve (28) and a control unit (51), said control unit (51) issuing an electric signal for the opening control of said pressure regulating valve (28) so that the differential pressure between lower chamber pressure and upper chamber pressure of said hydraulic cylinder (1) is equalized to the differential pressure corresponding to the die cushion capability inputted from capability signal generating means (52). A die cushion equipment for press according to claim 3, wherein said capability signal generating means (52) comprises capability memorizing means (53) to memorize a plurality of capability diagrams and outputs the capability signal based on the capability diagram selected by selection means (54). A die cushion equipment for press according to claim 4, wherein said capability signal generating means (52) issues the capability signal, which rapidly rises up to the standard capability as set by a standard capability setter (55) until said piston (2) reaches the specified stroke.

Inventors: IMANISHI SHOZO, IMANISHI, SHOZO
Applicants: AIDA ENG LTD, AIDA ENGINEERING CO., LTD.
Application year: 1990
Main classification: B21D24
IPC codes: B21D24, B30B15