Pneumatic die cushion equipment

Abstract: Die-cushioning capability is provided by a closed type piston cylinder unit 1, 2, 3, 4, instead of the open-to-atmosphere type unit of the conventional equipment. The die cushion capability of this closed type cylinder unit is determined by the differential pressure between the lower chamber pressure and the upper chamber pressure. This differential pressure is controlled in such manner that the lower chamber and the upper chamber communicate with each other through (a) a check valve 10 which opens when the differential pressure value, and (b) through a shut-off valve 20, which hinders the excessive increase of the upper chamber pressure when the piston goes up. The form of the check valve can be changed to permit convenient changing and adjusting of the die cushion equipment even during press fabrication.

Description: BACKGROUND OF THE INVENTION The present invention relates to a pneumatic die cushion equipment. A pneumatic die cushion equipment of conventional type, as described in more detail below, has an air cylinder, which is separated by a piston into an upper chamber and a lower chamber. The upper chamber is open to the atmosphere. A wear plate 4 is fixed on the upper end of a piston rod receives a cushion pin (not shown). A buffer tank, communicates with the lower chamber through a connection pipe. In a die cushion equipment of such structure but without a buffer tank, the pressure in the upper chamber is equal to atmospheric pressure and is thus constant. Consequently, the die cushion capability is the product of the pressure in the lower chamber and the sectional area of the piston. However, because the pressure in the lower chamber increase when the piston moves down, i.e. when the volume of the lower chamber is decreased, the die cushion capability increases. On the other hand, die cushion capability, i.e. the proper blank holding pressure necessary for press fabrication is determined by the quality of the material, and is inappropriate for the capability to increase too much. For this reason, conventional equipment is provided with the buffer tank, which has a volume 5 - 8 times as large as the volume of the cylinder. This hinders the extreme increase of the die cushion capability by enlarging the apparent volume of the lower chamber. This will be emphasized in more detail below. With the introduction of large-size presses and automated equipment such as transfer presses, and with the increasing demand on higher product quality and productivity, the following problems arise: (1) The space required for the buffer tank is large and not very economical, and this also causes inconvenience in the the introduction of large-size press and in the layout of the other functional components. It causes particular problems in transfer presses equipped with a large number of dies. (2) Much time is required for establishing the initial pressure or for the adjustment to increase die cushion capability, and this causes such inconvenience that, even where the other initial conditions are met within a short time, the press cannot be operated quickly. On the other hand, to establish the initial pressure quickly, a high-pressure large-capacity compressor must be installed, and this is disadvantageous in terms of the prime cost and space requirements. Also, in an adjustment to increase the die cushion capability, a quick-acting large-size exhaust valve must be installed and this also leads to higher prime cost. Moreover, since a large quantity of high pressure air must be released to atmosphere and thereafter re-supplied for each adjustment, this leads to additional operating costs disadvantages. (3) Furthermore, it is impossible to increase the capacity of the buffer infinitely. Therefore, the die cushion capability always increases to a greater or less extent as the piston goes down. However, current high quality and cost reduction requirements militate against increase of the die cushion capability in many fields of fabrication. (4) Materials used for press fabrication have become increasingly complicated and sophisticated, to reduce costs and improve quality of the end products. In some cases, defective products may be produced (depending upon the form of the materials or nature of the fabrication) unless the die cushion for press fabrication is controlled according to the circumstances. In other words, if the die cushion capability can be properly changed and adjusted during the press fabrication, a wide variety of products can be fabricated at high efficiency, and extensive cost reduction and production of high quality products can be realised. However, this cannot be accomplished by the pneumatic die cushion equipment of conventional type. GB-A-2 140 871 discloses a control arrangement for a fluid powered piston and cylinder actuator. The arrangement comprises means for controlling inlet and outlet valves of the opposite ends of the cylinder to control the differential pressure between the opposite sides of the piston and hence move the piston to, or maintain the piston at, any desired position. SUMMARY OF THE INVENTION The object of the present invention is to offer a small and compact pneumatic die cushion equipment with simple structure, easy maneuverability and low cost, which can eliminate the large capacity buffer tank, the large-size compressor, the quick-acting large-size exhaust valve, etc., and can maintain the die cushion capability at constant level. 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 upper chambers with each other. In the actual detail, the lower chamber and the upper chamber having the cylinder of the piston therebetween are communicated with each other through a check valve, which is opened when the differential pressure between the lower chamber pressure and the upper chamber pressure, changing with the downward movement of the piston, exceeds the preset value. Also, both chambers are communicated with each other through a shut-off valve, which hinders the excessive increase of the upper chamber pressure when the piston goes up. Therefore, in the equipment according to this invention, the lower chamber pressure increases and the differential pressure between the lower chamber pressure and the upper chamber pressure rapidly increases as the piston goes down from the upper limit due to the loading, and the die cushion capability corresponding to the preset differential pressure value is established. Further, when the piston goes down, the differential pressure between two chambers exceeds the preset value. Then, the check valve is opened, and the upper and the lower chambers are communicated with each other. Consequently, the lower chamber pressure is decreased, and the differential pressure is decreased to less than the preset value. At the same time, the check valve is closed. Thereafter, the opening and the closing of the check valve are repeated as the piston goes down, and the differential pressure between two chambers is fluctuated within the narrow range of the tolerance of the proper blank holding pressure. The die cushion capability is maintained at constant level until the piston reaches the lower limit. 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 shut-off valve, it is possible to extensively increase the upper limit damper effect. The equipment according to the present invention is of such structure that, the upper and lower chambers of the cylinder are communicated with other through a check valve, that the check valve is opened when the differential pressure between two chambers reaches the preset differential pressure value, and that the excessive increase of the upper chamber pressure is hindered by the shut-off valve when the piston goes up. Thus, the invention offers a small and compact pneumatic die cushion equipment, which can eliminate the large-size buffer tank, the high-pressure large-capacity compressor, the large-size quick-acting exhaust valve, etc. and which can maintain the die cushion capability at constant level and on low operating cost. Another object of this invention is to offer a pneumatic die cushion equipment, by which it is possible to change and adjust the die cushion capability during press fabrication. To attain these objects, the equipment according the present invention comprises a lower chamber and an upper chamber separated by a movable piston and means for selectively placing the said two chambers in communication characterised in that the means comprises: - (a) a check valve, which opens when the differential pressure between the lower chamber pressure and the upper chamber pressure, changing with the downward movement of the piston, exceeds a preset differential pressure value, and (b) at least one shut-off valve which opens to hinder excessive increase of the upper chamber pressure when the piston goes up. 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, it is possible according to this equipment to adjust the die cushion capability through the control of differential pressure during the press fabrication by communicating the lower and upper chambers with each other. Specifically, the equipment according to this invention is characterized in that the lower and the upper chambers having a piston of the cylinder therebetween are communicated with each other through a first air passage equipped with a first shut-off valve and a second air passage equipped with a shut-off valve, said first shut-off valve is constructed in such manner that it is opened when the differential pressure between the lower chamber pressure and the upper chamber pressure exceeds the present differential pressure valve inputted by the differential pressure setting means, a control means is provided to control the opening and the closing of said second shut-off valve when the piston goes up, a capability memorizing means to memorize two or more capability diagrams and a selection means to select one of the capability diagrams memorized by this capability memorizing means, and the die cushion capability can be changed and adjusted during press fabrication by changing the preset differential pressure value outputted by the differential pressure setting means according to the capability diagram selected by the selection means. Therefore, if the preset differential pressure value outputted by the differential pressure setting means is constant, the lower chamber pressure increases when the piston goes down from the upper limit due to the loading. Then, the differential pressure between the lower chamber pressure and the upper chamber pressure quickly increases, and the die cushion capability corresponding to the preset differential pressure value is thus established. Further, when the piston goes down, the differential pressure between two chambers exceeds the preset differential pressure value. Then, the first shut-off valve serving as a check valve is opened, and the lower and the upper chambers are communicated with each other. Consequently, the lower chamber pressure is reduced, and the differential pressure goes down to below the preset differential pressure value. At the same time, the first shut-off valve is closed. Thereafter, the opening and the closing of the first shut-off valve are repeated as the piston goes down, and the differential pressure between two chambers is changed within the narrow range of the tolerance of the necessary blank holding pressure. Thus, the die cushion capability can be substantially maintained at the constant level until the piston reaches the lower limit. If a capability diagram is selected by the selection means from the capability diagrams memorized by the capability memorizing means, the preset differential pressure value inputted to the first shut-off valve by the differential pressure setting means corresponds to the capability diagram. Therefore, if the capability diagram is prepared in advance to "gradual increase", "gradual decrease", "stepwise increase or decrease", etc. for the press fabrication, the die cushion capability can be automatically changed and adjusted during the press fabrication. On the other hand, when the slide begins to go up, the piston goes up from the lower limit to the upper limit by the differential pressure between two chambers. Accordingly, the upper chamber pressure is gradually increased, and when the pressures in two chambers become equal to each other, the second shut-off valve is opened by the control means and the upper and the lower chambers are communicated with each other, As the result, the excessive increase of the pressure in the upper chamber is hindered, and the pressures in two chambers are equalized. Under such conditions, the piston is pushed upward by the effective area difference due to the presence of piston rod (sectional area), and the piston goes up relatively slowly. Because residual pressure exists in the upper chamber, the piston reaches the upper limit without generating big impact force. By taking the proper timing to close the second shut-off valve, it is possible to extensively increase the effect for the upper limit damping. The equipment by this invention is constructed in such manner that the upper and the lower chambers of the cylinder are communicated with each other through the first and the second air passages, that the die cushion capability is adjusted through the control of differential pressure by the first shut-off valve, and that the second shut-off valve is properly controlled when the piston goes up. Thus, the equipment by this invention can be designed in small and compact form, and it can eliminate the large-size buffer tank, the high-pressure large-capacity compressor, the quick-acting large-size exhaust valve, etc. of the conventional type equipment. With low operating cost, the equipment can attain the proper die cushion capability accurately and quickly. Moreover, the differential pressure is controlled according to the capability diagram memorized by the capability memorizing means. This makes it possible to change and adjust the die cushion capability during the press fabrication to suit the product to be fabricated, and this contributes to the production of a wide variety of the high quality products at higher efficiency. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a general schematical view of the first embodiment of this invention. Fig. 2 is a drawing to explain the operation in comparison with the operation of the conventional equipment. Fig. 3 is a general schematical view to show the second embodiment. Fig. 4 is a general schematical view to show the third embodiment of the invention. Fig. 5 is a general schematical view to show the fourth embodiment. Fig. 6 is a drawing to explain the operation of the third and the fourth embodiments in comparison with the operation of a die cushion equipment of conventional type. Fig. 7 is a general schematical view of a die cushion equipment of conventional type. In the figures, 1 refers to a cylinder, 1U an upper chamber, 1L a lower chamber, 2 a piston, 10 a check valve, 11 a pipe, 20 a first shut-off valve, 21 a pipe, 30 a first air passage, 31 a pipe, 40 a first shut-off valve, 41 a main unit, 50 a second air passage, 51 a second shut-off valve, 60 and 70 a pressure regulating valve and a control unit respectively to form the differential pressure setting means, 71 a means to generate the capability signal, 72 a capability memorizing means, 73 a selection means, 74 a standard capability setter, 75 and 76 a controller and a pressure setter respectively to form the control means, and 77 and 78 a controller and a pressure setter respectively to form the initial pressure setting means. DESCRIPTION OF THE PREFERRED EMBODIMENTS In the following, description will be given in connection with the drawings. (Embodiment 1) Fig. 1 is a general schematical view of an embodiment of this invention, and Fig. 2 is a diagram to explain the operation of the equipment. The same number or symbol is used to the same component as that of the Fig. 7, which is given to illustrate a die cushion equipment of the conventional type, in order to simplify or omit the explanations. As shown in Fig. 1, the die cushion equipment of the first embodiment of this invention consists of a cylinder unit (cylinder 1 and piston 2), a check valve 10, a shut-off valve 20, etc. To the cylinder unit, the compressed air is supplied from the air source 15 comprising a small compressor, a pressure regulating valve 16 and a check valve 17. In the initial condition where the piston 2 is at the upper limit UL, the initial pressure PlS is established in the lower chamber 1L, which is formed below the piston 2 of the cylinder 1. This initial pressure PlS is set by the pressure regulating valve 16. The movable stroke (ST) of the piston 2 is between the upper limit UL and the lower limit LL in the cylinder 1. 1U represents the upper chamber formed above the piston 2. Therefore, the die cushion capability of this equipment consisting of a closed cylinder unit is determined by the differential pressure between the lower chamber pressure Pl and the upper chamber pressure Pu if the sectional area A2 of the piston rod 3 is neglected. The check valve 10 is a means to communicate the lower chamber 1L with the upper chamber 1U, and it is opened when the differential pressure Δ (= Pl - Pu) between the lower chamber pressure Pl and the upper chamber pressure Pu, changing as the piston 2 goes down, reaches the preset differential pressure value ΔS (e.g. 3 kgf/cm²). The check valve is installed in the middle of the pipe 11, connecting two chambers 1L and 1U. The check valve 10 of this embodiment is constructed in such manner that the die cushion capability corresponding to the proper blank holding pressure can be changed and adjusted. Specifically, when the setting signal is received from the differential pressure setter 12, the check valve 10 is opened when the differential value reaches the preset value ΔS (e.g. 2.8 kgf/cm², 3.0 kgf/cm², 3.2 kgf/cm², .....). On the other hand, the shut-off valve 20 is installed in the middle of the pipe 21, connecting the upper chamber 1U and the lower chamber 1L, and it is a means to communicate the upper chamber 1U with the lower chamber 1L at appropriate time. In other words, it prevents the excessive increase of the upper chamber pressure Pu when the piston 2 goes up. It is closed when the piston 2 goes down, and it is opened at appropriate time when the piston goes up. The excessive increase of the pressur as mentioned in the present invention is defined as the increase of the upper chamber pressure Pu to the value higher than the lower chamber pressure Pl or as the increase of the upper chamber pressure Pu to the value, at which the piston 2 is no more capable to go up if the sectional area A2 of the piston rod 3 is taken into account. Actually, its opening and closing are controlled by the control means 22. As shown in Fig. 2, the control means 22 of this embodiment is constructed in such manner that the opening signal is issued to the shut-off valve 20 when the piston 2 goes up from the lower limit LL with the lower chamber pressure Pl reduced and the upper chamber pressure Pu increased and the pressures in two chambers Pl and Pu become equal to each other (with the lower chamber pressure Pl slightly higher), and that the closing signal is issued immediately before the piston 2 reaches the upper limit UL. The time to issue the opening signal can be determined by the ascending speed of the piston 2, and the signal may be issued at any point of the stroke (ST). It is also desirable to set the timing for the closing signal in such manner that the better upper limit damping effect can be obtained until the piston 2 can reach the upper limit UL. Also, such timing can be defined by the present position of the piston 2 (piston rod 3) detected by a position detector (not shown). However, the timing may be defined by detecting the differential pressure between the lower chamber pressure Pl and the upper chamber pressure Pu or by detecting the axial angle of the crank. The shut-off valve 20 and the control means 22 are not limited to the structures as described above. A pilot check valve may be provided to directly connect the upper chamber 1U with the lower chamber 1L, and the pipe 21 and the like may be omitted. 19 represents an exhaust valve to move the cushion downward. Next, description will be given on the operation of the first embodiment. Suppose that the sectional area of the piston 2 is A1 when the piston 2 is at the upper limit UL. Then, the die cushion capability F is determined by the following equation: F = Pl·A1 - Pu·(A1 - A2) As shown in Fig. 2, the upper chamber pressure Pl is supposed to be equal to the lower chamber pressure Pu at the upper limit UL in this embodiment, the upward movement, i.e. the differential pressure Δ is based on the effective area difference due to the presence of the sectional area A2 of the piston rod 3. In the structure of the conventional equipment as shown in Fig. 7, the upper chamber 1U is opened to the atmospheric air. Accordingly, the die cushion capability gradually increases as shown by one-dot chain line of Fig. 2 and reaches Fpe at the lower limit LL. Thus, the predetermined die cushion capability Fp cannot be maintained at constant level. The blank holding pressure becomes too high by 20 - 25% at the lower limit LL. If the differential pressure ΔS is set on the differential pressure setter 12 in advance to communicate the lower chamber 1L with the upper chamber 1U in this embodiment, the predetermined die cushion capability can be quickly obtained because lower chamber pressure Pl is increased and the differential pressure Δ from the upper chamber pressure Pu is increased when the piston moves downward. When the differential pressure Δ becomes equal to the preset differential pressure value ΔS, the check valve 10 is opened, connecting two chambers 1L and 1U with each other. As the result, the lower chamber pressure Pl decreases and the upper chamber pressure Pu increases. Then, the differential pressure 4 becomes smaller than lthe preset differential pressure ΔS, and the check valve 10 is closed again. Thus, the check valve 10 repeatedly performs the opening and closing operation according to the differential pressure 4 until the piston 2 reaches the lower limit LL. Therefore, the die cushion capability F1 is controlled within the band with constant width even though there is slight change of differential pressure (as shown by the solid line zigzag in Fig. 2) according to the opening and closing operation of the check valve 10. The width of the band B is defined as the allowable range of the proper blank holding pressure. The width of the band B can be selected by setting the bore diameter, flow resistance, etc. of the check valve 10 and the pipe 11 against the volume of the cylinder 1 to appropriate values. Also, the die cushion capability can be varied according to the preset differential pressure value ΔS of the differential pressure setter 12. When it is set to the capability F2 as shown in Fig. 2, the preset differential pressure value ΔS should be set to the value lower than that of the capability F1. On the other hand, the upward movement of the piston 2 from the lower limit LL depends upon the differential pressure Δ between the pressures Pl and Pu of two chambers and matches the upward movement of the slide in the initial stage. Then, it is performed smoothly in non-loaded status. As the lower chamber pressure Pl decreases and the upper chamber pressure Pu increases, the differential pressure Δ rapidly decreases. Then, the shut-off valve 20 is opened by the signal from the control means 22, and the two chambers 1U and 1L are connected with each other. Accordingly, the pressures Pl and Pu of two chambers become equal to each other, while the piston 2 moves further upward by the pushing force generated by the effective area difference due to the presence of the sectional area A2 of the piston rod 3. This piston may be left to go up to the upper limit UL, whereas the shut-off valve 20 is closed again in this embodiment immediately before the upper limit UL. Therefore, the upper chamber pressure Pu becomes slightly higher and rapidly reduces the pushing force of the piston 2. Thus, the better damping effect can be obtained at the upper limit UL. It is also effective that the shut-off valve 20 is opened and closed again momentarily when the piston 2 closely approaches the upper limit UL. In the first embodiment, the lower chamber 1L and the upper chamber 1U are communicated with each other, and the communication is opned by the check valve 10 when the differential pressure between the pressures Pl and Pu of two chambers reaches the preset differential pressure ΔS. Accordingly, the differential pressure between the upper chamber pressure Pu and the lower pressure Pl can be kept at constant level, and the die cushion capability can be maintained at constant level during the press fabrication. Because this embodiment is based on a closed construction where the upper chamber 1U and the lower chamber 1L are connected with each other through the check valve 10, the pipe 11, the shut-off valve 20 and the pipe 21 and where the air in the lower chamber 1L is released into the upper chamber 1U when the piston 2 goes down. Hence, there is no need to install the extra-large buffer tank 6 as in the conventional equipment, and this provides much conveniences and advantages in terms of operation economy, space requirements and air consumption. Because the die cushion capability can be adjusted by the cracking pressure (differential pressure) of the check valve 10, there is no need to install the high-pressure large-capacity compressor or the large-size exhaust valve as in the conventional equipment. This contributes to the compact design of the equipment and quick adjustment operation. Therefore, the production efficiency of the press can be raised with shorter waiting time. More economic operation can be achieved because there is no need to release large quantity of compressed air into the atmosphere. Further, the upper chamber pressure Pu can be left unconsumed near the upper limit UL of the piston 2, i.e. residual pressure may be provided by setting the proper timing to close the shut-off valve 20. This is helpful to extensively decrease the impact force at the upper limit UL. Because the cracking pressure of the check valve 11 can be easily set and changed by the differential pressure setter 12, the necessary blank holding pressure can be quickly and accurately obtained. Moreover, the inconveniences such as the folds or damage of the flange can be avoided because the blank holding pressure remains constant regardless of the downward movement of the piston 2. This contributes to the stable production of high-quality products. Further, because the shut-off valve 20 is so constructed that its opening and closing are controlled properly by the control means 22, the return speed of the piston 2 to the upper limit UL, the upper limit damping effect, etc. can be set to the values as desired. (Embodiment 2) In contrast to the embodiment 1, in which the cracking pressure can be set and changed by the differential pressure setter 12 equipped with the check valve 10, the embodiment 2 is constructed in such manner that the check valves 10 (10A, 10B and 10C) are installed in parallel as shown in Fig. 3, and that one check valve can be selected by the selection means 14 through changeover of the changeover switches 13 (13A, 13B and 13C) consisting of the corresponding electromagnetic valves. The other details are the same as the embodiment 1, and the explanation is omitted. Therefore, it is possible by this embodiment to obtain the same operation and effect as the embodiment 1, to set the die cushion capability more accurately because the cracking pressure of the check valve 10 is constant, and to perform the setting changeover more quickly because the changeover is carried out by simply opening or closing the changeover valve. The embodiments 1 and 2 above consist of a complete closed type cylinder unit (cylinder 1 and piston 2), whereas an opening valve 25 may be provided to open the upper chamber 1U to atmospheric air at proper time and temporarily as shown in Fig. 3 by two-dot chain line. For example, if it is opened by the control means 22 only for the moment when the piston 2 goes down from the upper limit UL, it is possible to perform the startup of the die cushion capability [F, (F2)] more quickly. Thus, it is possible to obtain and to promote the better effect and operation of the present invention. Of course, in the special-purpose press with no need to change and adjust the die cushion capability, it is not necessary to change the check valve 10 to the cracking pressure variable type, and this invention is also applicable to this case. (Embodiment 3) Compared with the embodiments 1 and 2, which have constant die cushion capability during press fabrication, the die cushion capability is changeable and adjustable during press fabrication in the embodiment 3. As shown in Fig. 4, this die cushion equipment comprises a cylinder unit consisting of a cylinder 1, a piston 2, etc., an initial pressure setting means, a differential pressure setting means, a capability memorizing means 72, a selection means 73 to control the first air passage 30 and a second air passage 50, and a control means to control the opening and the closing of the second air passage 50. The cylinder unit consists of a cylinder 1, a piston 2, a piston rod 3, etc., and it has basically the same structure as the equipment of Fig. 7. However, because one of the objects of the present invention is to eliminate the buffer tank (6) of the conventional equipment, the connecting pipe (7) is not installed in the lower chamber, and the upper chamber 1U is designed in closed type without opening (5). The initial pressure setting means is to set the pressure in the lower chamber 1L under the initial condition (with the piston 2 at upper limit UL), and it comprises a pressure regulating valve 36 of pressure variable type and a check valve 37 installed on the pipe 38 connecting an air source 15 consisting of a small compressor with the lower chamber 1L and a controller 77 to regulate the pressure regulating valve 36 according to the setting value of the pressure setter 78. The first air passage 30 is to connect the lower chamber 1L with the upper chamber 1U of the cylinder 1 when the piston 2 goes down, and it consists of a pipe 31 connecting the two chambers 1L and 1U and of a first shut-off valve 40 installed in the middle of this pipe 31 and serving as a check valve. The cracking pressure as the check valve function is controlled by the preset differential pressure value inputted from the differential pressure setting means (60 and 70). It opens when the differential pressure between the lower chamber pressure Pl and the upper chamber pressure Pu exceeds the preset differential pressure value. Here, the first shut-off valve 40 consists of a hollow cylindrical main unit 41 and a cylindrical valve disc 46 slidably inserted in the main unit 41, and it is closed at all times. At the front end of the main unit 41, an inlet 42I and an outlet 420 are provided to connect with the pipe 31, and an inlet 43I is furnished on the rear end to apply the upper chamber pressure Pu through the pipe 31. On the other hand, a spring is mounted in the hollow portion 47H at the rear end of the valve disc 46, and a valve unit 47V is provided on the front end to close the inlet 42I. Consequently, when the lower chamber pressure Pl is increased, the valve disc 46 is moved rightward in Fig. 4 against the resilient force of the spring 49, and the lower chamber 1L and the upper chamber 1U of the cylinder 1 are connected with each other through the inlet 42I and the outlet 42O. Also, when the air is released from the lower chamber 1L, the lower chamber pressure Pl is reduced, and the valve 40 is closed again by the resilient force of the spring 49. In this case, the cracking pressure serving as a check valve to the first shut-off valve 40 is determined primarily by the resilient force of the spring 49. In the meantime, the first shut-off valve 40 of this invention is characterized in that the cracking pressure is variable. For this reason, compressed air is supplied from the inlet 43C furnished in the middle of the main unit 41. The compressed air serving as a control signal is to increase the cracking pressure in addition to the resilient force of the spring 49. In other words, minimum cracking pressure is determined by the spring 49, and the higher cracking pressure is determined by the pressure of the air supplied to the inlet 43C. The outlet 42O and the inlet 43I are communicated with each other to keep the balance. Also, 43O is opened to the atmospheric air. Here, the differential pressure setting means is a means to set the cracking pressure of the first shut-off valve 40 and is to set the pressure of the air supplied to the inlet 43C of the first shut-off valve 40. Specifically, it consists of a pressure regulating valve 60 serving as an electropneumatic converter installed on the pipe 61, which connects the inlet 43C of the valve 40 with the air source 15, and a control unit 70. The control unit 70 and the like are accommodated in the control panel 80 together with the other controllers (75) and the like. The control unit 70 in this embodiment 3 uses the lower chamber pressure Pl from the pressure detector 32, the upper chamber pressure Pu from the pressure detector 53 and the crank axial angle from the angle detector 85 as input data. It issues the electric signal to control the pressure regulating valve 60 so that the differential pressure obtained by the comparative computation of the lower chamber pressure Pl and the upper chamber pressure Pu becomes equal to the differential pressure corresponding to the capability signal. In other words, it is to control the cracking pressure of the first shut-off valve 40 by a closed loop. This capability signal is outputted from the capability signal generating means 71. The capability signal generating means 71 is to determine the die cushion capability to obtain the necessary blank holding pressure during press fabrication in relation to the stroke of the piston 2, and a capability memorizing means 72 to memorize two or more capability diagrams, i.e. the die cushion capability - piston stroke curve is included in it. The capability diagram is as shown by the curves (2) - (5) in Fig. 6, and the capability is changed during the change of the piston stroke, i.e. during press fabrication. For wider application, the curve (1) is also memorized, in which the capability is at constant level as in the cases of the embodiments 1 and 2. The selection of the curve from the capability diagram is carried out by the selection changeover means 73. Further, the capability signal generating means 71 in this embodiment 3 is designed in such manner that the capability signal for the quick rising up to the standard capability F as set by the standard capability setter 74 until the piston stroke ST1 as shown in Fig. 6 is reached. Of course, the diagram specifying from the upper limit UL to the lower limit LL and from the lower limit LL to the upper limit UL may be memorized by the capability signal generating means 71 and the capability signal generating means 71 may issue the capability signal corresponding to the diagram selected by the selection means 73 to the control unit 70. In this case, the standard capability setter 74 can be omitted. (See the embodiment 4.) The piston stroke is to be determined by the axial angle of the crank inputted from the angle detector 85. Because the capability memorizing means 72 consists of reloadable ROM, the capability diagram can be changed, added or deleted as appropriate. Next, the second air passage 50 is to connect the upper chamber 1U with the lower chamber 1L at appropriate time during the upward movement of the piston 2, and it is composed of a pipe 52 to connect two chambers 1U and 1L and of a second shut-off valve 51 consisting of an electromagnetic valve. The control means to control the opening and the closing of the second shut-off valve 51 consists of a pressure setter 76 and a controller 75. The controller 75 issues the signal to excite the solenoid when the upper chamber pressure Pu detected by the pressure detector 53 is equal to the setting value of the setter 76. When solenoid is excited, the second shut-off valve 51 is opened. Also, the controller 75 turns off the signal when the piston 2 closely approaches the upper limit UL and closes the second shut-off valve 51 again. The differential pressure setter may be provided instead of the pressure setter 76, and the lower chamber pressure Pl may be inputted to the controller 75 so that the opening and the closing of the second shut-off valve 51 can be controlled according to the differential pressure between the pressures Pl and Pu of two chambers. 19 represents an exhaust valve to move the cushion downward, and 37 a check valve. In the embodiment 3 with the structure as described above, the operation is performed as follows: First, the initial pressure in the lower chamber 1L is set by the pressure setter 78. Next, the standard capacity F to be determined up to the piston stroke ST1 is set by the standard capability setter 74, and the curve, in which the capability after the stroke ST1 matches the forms of the products, is selected by the selection means 73. (e.g. the curve (5) of Fig. 6) Also, the upper chamber pressure Pu to close the second shut-off valve 51 is set by the pressure setter 76. In Fig. 6, the curves (2) - (5) are more simplified than the curve (1) (indicating the average capability only), and the process during the upward movement of the piston is not shown because it is easily imaginable from the curve (1). In the meantime, 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 structure as shown in Fig. 7, the predetermined capability Fa is determined as soon as the piston rod 3 moves down. Then, the die cushion capability gradually increases up to Fae at the lower limit LL as shown by the one-dot chain line of Fig. 6. Because the volume of the buffer tank 6 is 5 - 8 times more than that of the cylinder, it is not possible to maintain the die cushion capability Fa at constant value, At the lower limit LL, the blank holding pressure becomes excessive by 20 - 25%. When the piston 2 moves down from the upper limti UL in the embodiment 3, te lower chamber pressure Pl increases, and the differential pressure from the upper chamber pressure Pu increases, Thus, the standard capability F can be generated quickly. The higher the initial pressure in the lower chamber 1L is, the quicker the startup is. When the piston 2 tends to go down after the standard capability F is established, the control unit 70 including the differential pressure setting means controls the pressure regulating valve 60 in such manner that the differential pressure obtained from the input given by the pressure detectors 32 and 53 becomes equal to the differential pressure value corresponding to the capability signal issued from the capability signal generating means 71. This determines the cracking pressure, and the first shut-off valve 40 is opened by this cracking pressure. Then, the lower chamber 1L is connected with the upper chamber 1U. The lower chamber pressure Pl decreases and the first shut-off valve 40 closes again. Therefore, the differential pressure changes, repeating the slight fluctuation within the allowable range of the necessary blank holding pressure. The standard capability F can be substantially maintained up to the stroke ST1. If the curve (1) of Fig. 6 is selected by the selection means 73, the capability signal generating means 71 reads the curve (1) from the diagrams memorized in the capability memorizing means 72 and issues this to the control unit 70. In this case, the first shut-off valve 40 is controlled by the differential pressure setting mans (60 and 70) in such manner that the capability F can be maintained at the constant level until the piston 2 reaches the lower limit LL. If the curve (5) is selected, for example, the control unit 70 controls the pressure regulating valve 60 to change the die cushion according to the curve (5) based on the input from the capability signal generating means 71, the input from both pressure detectors 32 and 53, and the input from the angle detector 33. Thus, the opening and the closing of the first shut-off valve 40 are controlled. The die cushion capability is decreased stepwise after the piston stroke passed ST1 during press fabrication, and it is maintained at constant level until the piston reaches the lower limit LL. In case of the curve (4), it gradually decreases, whereas in case of the curve (2), it gradually increases. On the other hand, the upward movement of the piston 2 from the lower limit LL matches initially the upward movement of the slide according to the differential pressure between the pressures Pl and Pu in two chambers, and the piston moves smoothly upward thereafter in non-loaded status. Because the lower chamber pressure Pl decreases and the upper chamber pressure Pu increases, the differential pressure rapidly decreases. When the upper chamber pressure Pu exceeds the preset value determined by the pressure setter 76, the second shut-off valve 51 is opened by the signal of the controller 75, and two chambers 1U and 1L are connected with each other. Accordingly, the pressures Pl and Pu in two chambers become equal to each other, whereas the piston 2 moves further upward by the pushing force generated by the effective area difference due to the presence of the sectional area A2 of the piston rod 3. The piston may be left to reach the upper limit UL, while, in this embodiment, the second shut-off valve 51 is closed again immediately before the upper limit UL. Because the upper chamber pressure Pu slightly increases and reduces the pushing foece of the piston 2, the damping effect at the upper limit UL can be extensively increased. It is also effective to open and close the shut-off valve 51 again momentarily when the piston 2 closely approaches the upper limit UL. In the embodiment 3, the first and the second air passages 30 and 50 are provided to connect te upper chamber 1U with the lower chamber 1L of the cylinder 1. When the piston goes down, the opening and the closing of the first air passage 30 (the first shut-off valve 40) are adjusted to control the differential pressure between the lower chamber pressure Pl and the upper chamber pressure Pu. When the piston goes up, the opening and the closing of the second air passage 50 (the second shut-off valve 51) are adjusted to hinder the excessive increase of the upper chamber pressure Pu. Consequently, there is no need to install extra-large buffer tank (6), and it provides many advantages in terms of operation economy, space requirements and air consumption. Also, the first shut-off valve 40 having the function of a check valve is included in the first air passage 30, and the setting of the cracking pressure is changed by the differential pressure setting means (60 and 70) in the first shut-off valve 40. Moreover, the differential pressure setting means (60 and 70) controls the first shut-off valve 40 according to the capability signal issued from the capability signal generating means 71 specified by the selection means 73 and the capability memorizing means 72. Thus, the die cushion capability can be changed during the press fabrication, and a wide variety of the products can be produced efficiently. Also, the material cost can be reduced, and there is no restriction on the forms of the material. The start-up and the stopping of the operation can be quickly adjusted. The cylinder units (1, 2) are designed in the closed type, and the die cushion capability is determined by connecting or separating two chambers 1U and 1L through the control of the first air passage 30 and the second air passage 50. This contributes to the elimination of the high-pressure large-capacity compressor or the quick-acting large-size exhaust valve as seen in the conventional equipment, and a compact and economical equipment can be obtained. To adjust the die cushion capability, only the setting of the cracking pressure of the first shut-off valve 40 should be changed. This means quick and accurate operation as well as the shorter waiting time and higher production efficiency of the press. Moreover, the quantity of the air released for the adjustment of die cushion capability is very low in principle, and it is very advantageous for economical operation. Further, the upper chamber pressure Pu can be left unused when the piston approaches the upper limit UL, i.e. the differential pressure can be reduced by choosing the closing time of the second shut-off valve 51 appropriately. This is helpful to extensively decrease the impact force at the upper limit UL. (Embodiment 4) The embodiment 4 is-shown in Fig. 5. The embodiment 4 has more simplified components and facilities than the embodiment 3. Specifically, when the forms and the characteristics of the cylinder unit (1, 2), the first shut-off valve 40 and the second shut-off valve 51 are determined, the lower chamber pressure Pl, the upper chamber pressure Pu and the differential pressure between the pressures Pl and Pu of two chambers during the upward and the downward movement of the piston 2 as well as the relation between the upward and the downward movement of the slide and the necessary blank holding pressure can be made clear if the type of the press and the products to be fabricated are specified. Therefore, the differential pressure is controlled by the first shut-off valve 40, whereas the pressures in two chambers and the differential pressure are not directly detected and these are substituted by the axial angle of the crank. Therefore, the pressure detectors 32 and 53 in the embodiment 3 are not included. Also, the standard capability setter 74 is not provided, and the die cushion capability for all strokes of the piston 2 from the upper limit UL to the lower limit LL is memorized by the capability memorizing means 72. Thus, the control unit 70 controls the pressure regulating valve 60 by specifying the capability signal, inputted from the capability signal generating means 71, by the crank axial angle from the angle detector 85, and the cracking pressure of the first shut-off valve 40 is set. That is, the embodiment 3 forms a closed loop, while the embodiment 4 forms an open loop with one curve read from the capability memorizing means 72 as the preset value. Further, the control means to control the opening and the closing of the second shut-off valve 51 consists of the controller 75, which is a program sequencer. It control the opening and the closing of the second shut-off valve 51 in relation to the crank axial angle according to the predetermined procedure during the upward movement of the piston 2. However, the opening and the closing are controlled in the timing similar to the embodiment 3. The opening and closing procedure as well as the timing can be changed. It is also possible in the case of the embodiment 4 to obtain the same effect as the embodiment 3 by using the crank axial angle as input, (i.e. the effects such as the elimination of large size buffer tank, high-pressure large-capacity compressor, quick-acting large-size exhaust valve, and the adjustment and alteration of the die cushion capability during press fabrication). Moreover, compared with the embodiment 3, this embodiment can provide more simplified structure and lower cost by omitting two pressure detectors (32 and 53) and by simplifying the control unit 70. Because each operation is determined not as the differential pressure between the pressures Pl and Pu in two chambers but is indirectly determined by the crank axial angle, the automatic adjustment of the die cushion capability can be freely, easily and simply performed during press fabrication if the capability diagram to be memorized by the capability memorizing means 72 is clearly defined. In the embodiments 3 and 4, the control unit 70, the capability signal generating means 71, the capability memorizing means 72, and the controllers 75 and 77 are furnished separately, whereas these means may be organically and integrally incorporated by the computer consisting of CPU, RAM, ROM, etc.

Claims: Pneumatic die cushion equipment, comprising a lower chamber (7L) and an upper chamber (7U) separated by a movable piston (2), and means for selectively placing the said two chambers in communication, characterised in that the means comprises: - (a) a check valve (10), which opens when the differential pressure between the lower chamber pressure and the upper chamber pressure, changing with the downward movement of the piston, exceeds a preset differential pressure value, and (b) at least one shut-off valve (20) which opens to hinder excessive increase of the upper chamber pressure when the piston goes up. A pneumatic die cushion equipment as claimed in Claim 1, characterised in that a differential pressure setter (12), adjustable to a predetermined value and sensitive to the said differential pressure, is incorporated to control opening or closing of the said check valve (10). A pneumatic die cushion equipment as claimed in Claims 1 or 2, characterised in that said shut-off valve (20) is opened by an opening signal of control means (22), and closed by a closing signal of such control means. A pneumatic die cushion equipment as claimed in Claim 3, characterised in that said control means (22) is adapted to issue an opening or closing signal when the piston or a supporting rod therefor, arrives at a predetermined position. A pneumatic die cushion equipment as claimed in Claims 3 or 4, characterised in that said shut-off valve (20) and its control means (22) are jointly incorporated into a pilot check valve which directly places the upper chamber and the lower chamber of said cylinder in communication. A pneumatic die cushion equipment as claimed in any one preceding claim, characterised in that it comprises a plurality of associated check valves (a) each capable of being preset so that the plurality of valves is sensitive to two or more differential pressures. Pneumatic die cushion equipment, as claimed in any one of the preceding claims, wherein the said two chambers are selectively placeable into communication by a first air passage (30) equipped with a first shut-off valve (40) and by a second air passage (50) equipped with a second shut-off valve (51), said first shut-off valve being constructed in such manner that it opens when the differential between the lower chamber pressure and the upper chamber pressure exceeds a preset differential pressure as inputted from a differential pressure setting means, said equipment further comprising (a) a control means to control the opening and the closing of said second shut-off valve at appropriate times as the piston goes up, and (b) a cushioning-capability memorizing means to memorize two or more cushioning-capability diagrams, and (c) a selection means (73) capable of selecting one or other of the cushioning-capability diagrams memorized by the cushioning-capability memorizing means; whereby the preset differential pressure value issued from the differential pressure setting means may be changed according to the cushioning-capability diagram selected by the selection means to change and adjust the die cushion capability during use. A pneumatic die cushion equipment as claimed in Claim 7, characterised in that said first shut-off valve comprises a hollow main unit (41) having an outlet connecting to the lower chamber and an outlet connected to the upper chamber, and a spring-biassed valve disc (46) slidably engaged within this main unit; said inlet being closed by said valve disc, (a) as biassed by the spring, to determine a minimum opening pressure, and (b) as under air pressure to determine a further opening pressure. A pneumatic die cushion equipment as claimed in Claims 7 or 8, characterised in that said differential pressure setting means comprises a pressure-regulating valve (60) provided between an air source and the first shut-off valve (40), and a control unit (70) to issue an electric signal for control of the pressure regulating valve (60). A pneumatic die cushion equipment as claimed in Claim 9, characterised in that said control unit issues the electric signal for control such that the differential between the lower chamber pressure and the upper chamber pressure is equalized to the differential pressure corresponding to the die cushion capability inputted by a capability signal generating means (71). A pneumatic die cushion equipment as claimed in Claim 10, characterised in that said capability signal generating means (71) comprises said cushioning-capability memorizing means (72) and issues the capability signal according to the capability diagram as selected by said selection means. A pneumatic die cushion equipment as claimed in Claims 11 or 12, characterised in that said capability signal generating means (71) comprises the function to generate a capability signal for rapid startup up to standard capability as set by the standard capability setter (74), until said piston reaches the specific stroke. A pneumatic die cushion equipment as claimed in Claims 9, 10, 11, or 12, characterised in that said control unit (70) comprises the capability memorizing means (72) to memorize the die cushion capability defined for all strokes of said piston and issues an electric signal for control corresponding to the stroke for the crank angle detected by the angle detector (85), of the output signals from the capability signal generating means (71), to the pressure regulating valve (60). A pneumatic die cushion equipment as claimed in Claims 8, 9, 10, 11, 12 or 13, characterised in that said control means opens said second shut-off valve (51) when the value set by the pressure setter (76) becomes equal to the upper chamber pressure detected by the pressure detector. A pneumatic die cushion equipment as claimed in Claims 8, 9, 10, 11 or 12, characterised in that said control means comprises a second shut-off valve (51) connecting said upper chamber with said lower chamber, and a program sequencer.

Inventors: IMANISHI SHOZO, IMANISHI, SHOZO
Applicants: AIDA ENG LTD, AIDA ENGINEERING LTD.
Application year: 1989
Main classification: B30B15
IPC codes: F16F15, F16F9, B21D24