KR930000132Y1 - Die cushion for applying pressure to a press machine via gas pressurized push rods - Google Patents

Abstract

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Description

Die cushioning device

1 is a partially broken front view schematically showing a press working machine having a die cushioning device according to an embodiment of the present invention.

2 is a cross-sectional view of the die cushion of the apparatus.

3 is an enlarged cross-sectional view of a portion of the Daikushon.

4 is a cross-sectional view of the die cushion at the time of gas encapsulation.

5 is a partially broken front view showing the entire die cushion device.

Figure 6 is a front view showing a die cushion device according to another embodiment of the present invention.

The present invention relates to a gas-sealed die cushion apparatus used in a press working machine or the like.

As the press working machine, for example, various kinds of working machines for use in drawing processing are provided. Press machines of this kind are generally equipped with dies and punches and are used to form blanks. However, during the drawing process, the blank machining portion is pressed into the hole of the die by the punch, so that the peripheral edge of the blank is deformed in the circumferential direction and wrinkles are generated. Therefore, the press working machine is equipped with the pressure pad for a blank holder, and processes a blank, pressing the peripheral part of a blank with a die and a pressure pad.

If the blank holder force, i.e., the load on the pressure pad is too weak, the processed product will be wrinkled. On the contrary, if the load is too strong, the blank will be broken by the punch during processing. Therefore, in order to obtain a complete product, it is necessary to impart an appropriate blank holder force to the blank, and in order to generate such blank holder force, a conventional press working machine is equipped with a die cushioning device.

As a die cushion device, a die cushion device for generating a blank holder force by using a spring pressure such as a coil spring is known. However, in particular, when using this type of die cushioning device in a press working machine having a large working stroke such as deep drawing, a large size spring is required, and thus the entire apparatus is enlarged. In addition, the spring constant of the device becomes large, and the blank holder force in press work greatly changes. Therefore, there is a high possibility of causing breakage or cutting of the blank.

The drawback of such a spring pressure die cushion apparatus is solved, and the die cushion apparatus using gas pressure is provided. The apparatus includes a cylinder in which a high pressure gas of at least several tens of atmosphere is sealed, a piston movably housed in the cylinder, and a rod connected to a pressure pad through a portion of the cylinder in the piston. The gas pressure in the cylinder acts on the piston in the direction of extruding the rod. A seal is installed at a portion where the cylinder and the rod slide reciprocally or at a portion where the cylinder and the piston slide reciprocally.

In such a gas pressure die cushion apparatus, the blank holder force at the initial stage of press working can be set to a size corresponding to the gas encapsulation pressure inside the cylinder, and the spring constant of the apparatus can be set low by increasing the internal volume of the cylinder. Done.

Therefore, compared with the apparatus using a spring etc., there exists an advantage that the change of the blank holder force at the time of press work can be made small.

However, since the high-pressure gas in the cylinder in such a die cushion device is directly sealed by a sealing device provided in a portion in which the rod and the cylinder reciprocate with each other, or in a portion in which the cylinder and the piston reciprocate with each other, this sealing portion Little leakage of gas from the inevitable is unavoidable. Therefore, there is a drawback that the blank holder force (set load) is lowered in a relatively short period of time, so that the desired blank holder force cannot be supplied stably. Also, when pressing a single pressure pad with a plurality of die cushions, Since the degree of gas leak (different) is different, there is a drawback that it is not possible to give a uniform surface pressure to the pressure pad.

The present invention has been completed by reducing the above-described drawbacks, and an object thereof is to provide a die cushion device capable of stably generating a desired set load.

According to the present invention for achieving the above object, the die cushion device is a cylinder having a closed end and the other end having a through hole, a pressurizing rod fitted in the cylinder through the through hole and movable along the axial direction of the cylinder, A plurality of rings which are accommodated coaxially in the cylinder and fixed to a substantially cylindrical metal bellows body and the outer surface of the bellows body freely stretched along the axial direction of the cylinder and freely reciprocating on the inner circumferential surface of the cylinder. It has a bellows-shaped guide, and seals the cylinder with the bellows for accommodating the pressurizing rod and separating the cylinder with the liquid chamber communicating with the through hole and the gas chamber connected to the closed end of the cylinder. A sealing means, a working liquid before filling in the liquid chamber, and a sealing means in the gas chamber. Control and the load pressure through the bellows and a working liquid having a working gas pressure in a direction to protrude from the cylinder.

According to the die cushion apparatus configured as described above, since the gas pressure enclosed in the gas chamber in the cylinder acts on the working liquid in the liquid chamber through the metal bellows, the pressing rod is loaded in the direction in which the cylinder is extruded. This load is used as the blank holder force of the blank at the time of press working. At the time of press working, if the pressure rod is operated in the direction in which the pressure is pushed in the cylinder, the gas chamber is further compressed as the load press amount into the cylinder increases, causing the bellows to bend in the axial direction. Therefore, gas repulsion force in a gas chamber becomes large. At the time of opening of the mold, when the pressing rod moves in the direction protruding in the cylinder, the volume of the gas chamber is enlarged as the pressing rod moves, causing the bellows to be bent in the zero direction.

Since the gas pressure in the gas chamber also acts on the liquid chamber, the gas pressure acts on the sliding reciprocating portion of the cylinder through the cylinder and the rod through the liquid. Since the sealing means provided in the sliding reciprocating portion only needs to seal the working liquid in the liquid chamber, the sealing means can be reliably sealed as compared with the case of directly sealing the high pressure gas. Since the gas chamber and the liquid chamber are completely separated by the metal bellows, the high pressure gas in the gas chamber does not leak into the liquid in the liquid chamber through the bellows.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

1 shows a press working machine having a die cushioning device according to an embodiment of the present invention. This processing machine is provided with the die 2 in which the hole 6 is formed, and the punch 4 provided in the upper part of the die 2. As shown in FIG. The punch 4 is fixed to the slider 5 and is raised and lowered integrally with the slider 5 by a drive mechanism (not shown). In addition, the slider 5 is assembled with a die cushion device having a plurality of die cushions 20, for example. An annular pressure pad 3 is provided around the punch 4, and the pressure rods 22 of the die cushions 20 described later are connected to the pressure pad 3.

At the time of processing, as the blank holder, the pressure pad 3 is lowered together with the slider 5 to be brought into contact with the periphery of the blank 1 placed on the die 2 so as to be given by the die cushion 20. At the same time the blank 1 is pressed into the die 2 by pressure, the punch 4 pushes the blank 1 into the hole 6 of the die 2. After the lowering is completed, the molded article is extruded from the hole 6 by the knockout member 7 subsequent to the rise of the slider 5.

Subsequently, in describing the die cushion device in detail, since the die cushion 20 of 2 has the same configuration, the following description will be given on behalf of the die cushion 20 on one side.

As shown in FIG. 2, the die cushion 20 includes a hollow cylinder 21 and a pressing rod 22. The upper end and the lower end of the cylinder 21 are each closed by a single wall, and a through hole 10 is formed around the same axis as the axis of the cylinder 21 on the upper wall. And the press rod 22 is fitted in the cylinder so that sliding hole reciprocation with the through-hole 10 is possible.

The inner hole of the cylinder 21 is formed in a stage and has a small diameter portion 21a, a large diameter portion 21b and a shoulder portion 49 positioned therebetween.

In the large diameter portion 21b, an inner cylinder 23 (hole) having the same inner diameter as the small diameter portion 21a is provided. The upper end of the inner cylinder 23 is fixed to the shoulder portion 43, so that the inner cylinder 23 extends coaxially with the small diameter portion 21a. In addition, an end member 24 having a distribution port 25 is provided at the lower end of the inner cylinder 23.

The rod 22 is fitted so as to be movable in the axial direction with respect to the cylinder 21. The piston 30 is formed in the inner end of the rod 22, that is, the lower end. The piston 30 slides reciprocally on the inner circumferential surface of the small diameter portion 21a and on the inner circumferential surface of the inner cylinder portion 23 through a sliding bearing 31 fixed to the outer circumference thereof.

An inner circumferential portion of the through hole 10 through which the rod 22 penetrates is provided with a sealing means 35 for sealing the sliding bearing 34 and the sliding reciprocating portion with respect to the rod 22. The sealing means 35 includes a high pressure seal 36 and a low pressure seal 37 and a dust seal provided on a lower pressure side than the high pressure seal 36, that is, on the atmospheric side. seal: 38) The upper plate of the small diameter portion 21a is provided with a ring-shaped lip stopper 39 made of an elastic material such as urethane elastomer or the like. The stopper 39 defines the stroke end in this direction when the rod 22 moves in the protruding direction A. FIG.

The metal bellows 45 is accommodated in the large diameter part 21b of the cylinder 21. This bellows 45 is provided with the cylindrical bellows body 46 which can be stretched and contracted in the axial direction of the cylinder 21, and the bellows cap 47 which closes the end of this bellows body 46. As shown in FIG. The other end 48 of the bellows body 46 is fixed to the shoulder 49 of the cylinder 21 in such a way that a reliable sealing is possible, whereby the bellows body 46 surrounds the inner cylinder 23 and It is provided coaxially with the cylinder 21. The bellows main body 46 is made of a stainless steel sheet having a thickness of about 0.1 to 0.3 mm. However, metals other than stainless steel may be used, and may be formed in the thickness of the said plate | board thickness.

An annular valve body 50 is provided on the inner surface side of the bellows cap 47. The valve body 50 is made of a rubbery elastic material such as urethane elastomer or silicone resin, and is opposed to the annular valve seat 51 formed around the above-described flow port 25.

These valve bodies 50 and the valve seat 51 are self-sealing means for confining oil between the inner cylinder 23 and the bellows main body 46 when the bellows 45 is contracted by a predetermined amount as described later. Self-locking seal: 52).

Ring-shaped bellows guide members 55 and 56 are fixed to the outer circumference of the bellows main body 46. These guide members 55 and 56 slide between the bellows main body 46 and the cylinder 21 in order to secure a predetermined clearance between the bellows main body 46 and the cylinder 21. It is used to reduce the resistance of the reciprocating motion. In addition, each guide member is formed with a plurality of air passages extending in the axial direction of the cylinder 21.

The inner space of the cylinder 21 is separated into the liquid chamber 60 and the gas chamber 80 by the bellows 45. The liquid chamber 60 is defined by the side where the sealing means 35 is provided, that is, the inner surface of the bellows 45 and the inner surface of the small diameter portion 21a. The gas chamber 80 is defined by the outer surface of the bellows 45 and the inner surface of the large diameter portion 21b. The liquid chamber 60 is filled with oil as a working liquid. An oil injection hole 63 is formed in the cylinder 21, one end of which is connected to the liquid chamber 60, and the other end thereof is blocked by the ball 61 and the screw 62.

The liquid chamber 60 is the first liquid chamber 60a formed by the inner space of the inner cylinder 23 and the small diameter portion 21a, and the second liquid chamber 60b defined by the outer surface of the inner cylinder and the inner surface of the bellows 45. It consists of). In addition, the first liquid chamber 60a is separated by the piston 30 into a first region 60c located on the through hole 10 side and a second region 60d located on the inner cylinder 23 side. The second region 60d is connected to the second liquid chamber 60b through a distribution port 25 formed in the end member 24 of the inner cylinder.

As shown in FIG. 2 and FIG. 3, the damping force generation mechanism 70 is provided in the press rod 22. As shown in FIG. This mechanism 70 includes a valve chamber 68, a first orifice 71, a second orifice 72, and a third orifice 73. Each of the first orifices 71 and 71 penetrates the piston 30 in the axial direction to connect the first region 60c and the second region 60d. Each of the second orifices 72 and 72 extends in the radial direction of the rod 22 to connect the first region 60c and the valve chamber 68. The third orifice 73 extends in the axial direction of the rod 22 to connect the second region 60d and the valve chamber 68.

The third orifice 73 is opened and closed by a one-way valve 75 of a poppet valve type provided in the valve chamber 68. The one-way valve 75 is elastically supported in the direction of closing the third orifice 73 by the compression spring 76 accommodated in the valve chamber 68. The one-way valve 75 opens the valve while the spring 76 resists the elastic bearing force when the pressure in the second region 60d becomes higher than the pressure in the first region 60c. have.

In addition, the damping force generating mechanism 70 may not be provided in the rod 22 as in the above embodiment, but may be provided in, for example, the distribution port 25 formed in the end member 24 of the inner cylinder 23. In some cases, the rod 22 may be provided where the liquid (oil) in the liquid chamber 60 flows when the rod 22 is relatively moved. The damping force generating mechanism 70 may be configured to adjust the die cushion damping force when the rod 23 moves in the protruding direction A to two or more stages or to continuously adjust steplessly. In addition, the damping force generating mechanism 70 is not limited to a poppet valve-type valve, for example, a combination of several plate valves is used, and the damping force when the rod 22 moves in the protruding direction A is applied. It may be comprised so that it may become larger than the damping force at the time of moving to the pulling direction B. FIG. In a situation where the damping force generating mechanism 70 is not required, structures other than the orifice 71 of the mechanism 70 may be omitted.

The gas chamber 80 is filled with an inert gas such as nitrogen, for example. The sealing pressure of the gas is determined depending on the blank holder force required by the die cushion 20, but is sealed at a high pressure of, for example, several 10 kg / cm 2 or more. This gas pressure acts in the direction of shrinking the bellows 45, that is, the direction in which the rod 22 is extruded from the cylinder 21. The gas supply port 82 provided on the end wall of the cylinder 21 is blocked by the cap 83. In addition, when the pressure in the gas chamber 80 rises to a predetermined value at a proper position of the die cushion 20, a safety valve (not shown) is provided for opening the valve to release the gas.

As shown in FIG. 4, they are connected to each other by the gas communication pipe 95 of the gas chamber 80 of the pair of die cushion 20 which consists of the above-mentioned structure. The end 95a side of the communication tube 95 is blocked.

However, other cylinders, accumulators or other gas supply ports may be connected to the gas passage 95 as necessary.

The process of supplying gas to the gas chamber 80 is as follows.

When supplying gas, the liquid chamber 60 is filled with oil in advance, and the hole 63 is opened. Subsequently, as shown in FIG. 5, the pressurized gas supply source 90 is connected to the gas supply port 82. A valve 91 is provided between the supply source 90 and the supply port 82, and opens the valve 91 to supply gas into the gas chamber 80. As the gas amount in the gas chamber 80 increases, the bellows main body 46 contracts in the axial direction. Therefore, a part of the oil in the liquid chamber 60 is discharged to the outside through the hole 63.

The gas is divided into two stages and enclosed in the gas chamber 80. In detail, first, gas is supplied to the gas chamber 80 at low pressure. When the bellows main body 46 is reduced by a predetermined stroke, the valve body 50, which is the magnetic sealing means 52, comes into close contact with the valve seat 51 to close the flow hole 25. Therefore, the bellows body 46 is no longer bent in the axial direction. Subsequently, the hole 63 is closed with the ball 61 and the screw 62 so that the second liquid chamber 60b is sealed while the oil is accommodated.

Thereafter, the high pressure gas is supplied to the gas chamber 80.

Since the gas continues to be supplied to the gas chamber 80 even after the valve body 50 is in close contact with the valve seat 51, the gas chamber 80 and the pressure increase gradually. Since the oil is substantially incompressible, the inner surface of the bellows body 46 is uniformly fronted by the oil filled in the gap between the first liquid chamber 60b, that is, the outer circumferential surface of the inner cylinder 23 and the inner circumferential surface of the bellows body 46. Supported. Therefore, even if gas is continuously supplied at a high pressure, the bellows main body 45 is not excessively bent in the radial direction thereof. Therefore, the bellows main body 46 maintains a cylindrical shape and maintains a predetermined gap between the bellows main body 46 and the outer peripheral surface of the inner cylinder 23. When the pressure in the gas chamber 80 reaches a predetermined value, the supply of gas is stopped, and the supply port 82 is closed with a stopper 83.

As described above, the die cushion 20 in which the gas of the predetermined pressure is sealed in the gas chamber 80 is attached to the slider 5 of the press working machine to hold the pressure pad 3 in a pressable position. When the punch 4 is lowered together with the slider 5 at the time of press working, the pressure pad 3 abuts against the periphery of the blank 1, and the rod 22 moves in a manner of being pressed into the cylinder 21. . At this time, the insertion amount of the rod 22 with respect to the cylinder 21 increases. Therefore, as the rod 22 moves, the gas chamber 80 is compressed, and while the bellows 45 is extended, the pressure in the gas chamber 80 increases.

As the rod 22 moves on the inlet shaft, the pressure in the second region 60d becomes higher than the pressure in the first region 60c. Therefore, as shown in FIG. 3, the one-way valve 75 is opened, and a part of the oil in the second area 60d passes through the second and third orifices 72 and 73 to the first area 60c. Inflow. At the same time, a part of the oil in the second region 60d flows into the first region 60c through the first orifice 71. Most of the oil in the second region 60d flows into the second liquid chamber 60b through the communication port 25.

Therefore, since the bellows main body 46 is extended and the gas in the gas chamber 80 is compressed, the pressure of the gas rises. The pressure of the gas is then applied to the pressing rod 22 as the blank holder force through the bellows 45 and the oil.

On the other hand, when the rod 22 moves in the inlet direction B, the one-way valve 75 is opened, so that the first region 60c and the second region (1) by the first to third orifices 71 and 72. A plurality of flow paths connecting 60d) are formed. Therefore, the oil in the second region 60d can be relatively easily introduced into the first region 60c, and a relatively small damping force is applied to the rod 22. Accordingly, the rod 22 is quickly drawn into the cylinder 21, so that a suitable blank holder force can be exerted very quickly.

When the press is finished and the slider 5 is raised, the rod 22 is pressed by the gas pressure in the gas chamber 80 and moves in the direction A which protrudes from the cylinder 21. At this time, since the pressure of one area | region 60c becomes higher than the pressure of the 2nd area | region 60d, the one way valve 75 is closed. Therefore, oil flows from the first region 60c to the second region 60d through only the first orifice 71, and a large damping force acts on the rod 23. Therefore, since the movement speed of the rod 22 in the projecting direction A is pressed so as not to be fast, the pressure pad 3 does not rise rapidly. Therefore, since the rising speed of the pressure pad 3 can be set slower than the rising speed of the punch 4, the problem that the product rises with the punch 4 can be prevented, and the product can be reliably released from the punch. . In addition, when the rod 22 moves in the protruding direction A, the oil in the second liquid chamber 60b flows into the second region 60d as much as the rod 22 moves, so that the bellows 45 moves in the gas chamber 80. Return to the original bending state by the gas pressure in the cylinder.

The gas in the gas chamber 80 is completely separated from the oil in the liquid chamber 60 by the metal bellows 45. The metal bellows 45 can exhibit extremely good gas barrier properties even if its plate thickness is thin, so that the gas in the gas chamber 80 does not mix with the oil in the liquid chamber 60.

Since there is no sliding reciprocating portion for passing to the atmosphere side in the gas chamber 80, even if the reciprocating movement of the rod 22 is repeated at high speed, the gas chamber 80 is provided through the sliding reciprocating portion of the rod 22. The gas inside does not leak directly to the atmosphere. Since the sealing means 35 which seals the sliding reciprocating part of the rod 22 is provided in the liquid chamber 60 side, this sealing means 35 only needs to be able to seal oil. Oil having a higher viscosity than gas can be sealed relatively easily and reliably.

If, for some reason, oil leaks from the cylinder 21 through the through hole 10, the volume of the gas chamber 80 is relatively increased while the volume (oil content) of the liquid chamber 60 decreases.

However, since the gas chambers 80 of the two die cushions 20 are connected to each other by the gas communication tube 95, oil leaks from the die cushion 20 on one side or the oil leakage amount of each die cushion is different. Even if there is a pressure between the gas chambers 80 is equal. Therefore, the pressure pad 3 can always be pressed with an equal surface pressure.

In addition, the die cushion 20 having the above-described configuration seals the second liquid chamber 60b when the deflection of the bellows 45 exceeds a predetermined stroke to seal the oil on the inner surface side of the bellows body 46. Means 52 are provided. Therefore, even if the oil in the liquid chamber 60 leaks due to any cause and the pressure of the high pressure gas acts in the direction to bend the bellows 45, the position where the valve body 50 is in close contact with the valve seat 51 Since the bellows 45 is bent until the oil is confined in the second liquid chamber 60b, the bellows body 46 can be supported evenly on the inner surface side by this oil. Therefore, the bellows main body 46 can be prevented from being excessively bent in the radial direction, which is effective for protecting the bellows 45.

6 shows a die cushion apparatus according to a second embodiment of the present invention. In this embodiment, the liquid transfer means, for example the hydraulic circuit 101, is connected to the liquid chamber 60 of the pair of die cushions 20 via the liquid passage 100. The hydraulic circuit 101 includes a first pressure regulating valve 102, a second pressure regulating valve 103, an accumulator 104, a check valve 105, an oil feed pump 106 and a reservoir 107. Etc.).

The hydraulic pressure in the conduit from the pump 106 to the first pressure regulating valve 102 is kept constant by the action of the second pressure regulating valve 103.

The gas communication pipe 95 is provided with a valve 108 and a pressure gauge 109. In addition, another cylinder or accumulator, a gas supply source, or the like may be connected to the communication tube 95.

According to the second embodiment, when the oil in the liquid chamber 60 leaks through the through hole 10 of the cylinder 21 and the pressure in the cylinder 21 is lowered, the pressure regulating valve 102 is operated to maintain a constant pressure. The oil is sent from the hydraulic circuit 101 to the liquid chamber 60 until it reaches this point. When the pressure in the cylinder 21 reaches the set value, the pressure regulating valve 102 is closed. Therefore, since the fall of the spring constant and setting load (blank holder force) of the die cushion 20 by oil leakage is prevented, a constant spring constant is always ensured. In addition, even if the amount of oil leakage is different between the two die cushions, the pressure in the liquid chamber 60 can be made uniform immediately. Therefore, a uniform surface pressure can always be applied to the blank through the pressure pad.

The pump 105 may be driven by an electric motor, but a small amount of manual pump may be sufficient because the amount of oil leaking from the sealing means 35 is extremely small.

In addition, since the other structure is the same as that of 1st Embodiment mentioned above, the same code | symbol is attached | subjected to the same part, and the detailed description is abbreviate | omitted.

The present invention is not limited to the above-described embodiment and can be variously modified within the scope of the present invention. For example, the number of die cushions may not be limited to two but may be three or more. In addition, the die cushion apparatus of the present invention can be applied to various kinds of presses, such as a single action press and a double action press.

Claims (5)

A cylinder 21 having a closed end and the other end having a through hole 10, a press rod 22 fitted into the cylinder through the through hole 10 and movable along the axial direction of the cylinder, and in the cylinder. A plurality of cylindrical cylindrical metal bellows body 46, which is coaxially accommodated and freely stretched along the axial direction of the cylinder, is fixed to the outer surface of the bellows body and freely slides on the inner circumferential surface of the cylinder. It has ring-shaped bellows guides 55 and 56, and separates the inside of the cylinder into the liquid chamber 60 that communicates with the through hole and the gas chamber 80 connected to the closed end of the cylinder. Bellows 45, sealing means 35 for sealing between the pressing rod 22 and the through-hole 10, the working liquid filled in the liquid chamber 60 and the gas chamber 80 is sealed Belos (4 5) and a die cushioning apparatus of a press working machine comprising a working gas which presses the pressing rod 22 in a direction A protruding from the cylinder through a working liquid. 2. The cylinder 21 has a small diameter portion 21a positioned on the through hole 10 side, a large diameter portion 21b positioned on a closed end side of the cylinder, and a shoulder portion located between the small diameter portion and the large diameter portion. An inner hole having a 49 is provided, and the bellows body 46 is disposed in the large diameter portion 21b, and the other end 48 and the bellows body 46 fixed to the shoulder portion 49 are provided. It has a bellows cap 47 for closing one end, the liquid chamber 60 is defined by the inner space of the bellows and the small diameter portion, the gas chamber 80 is the outer surface of the bellows and the inner surface of the large diameter portion 21b. Daikushon apparatus of press processing machine prescribed by the law. The damping force of the working liquid applied to the pressing rod when the pressing rod 22 moves in the direction A protruding from the cylinder 21 is the direction in which the pressing rod pulls into the cylinder. And a damping force generating mechanism (70) for adjusting the damping force so as to be larger than the damping force of the working liquid acting on the pressure rod when moving to the die. The cylinder 21 has the inner cylinder 23 installed coaxially in the bellows main body 46, wherein the inner cylinder has an upper end and the bellows fixed to the shoulder 49 of the cylinder. It is provided with an end member 24 facing the bellows cap 47 of 45 and having a communication port 25 at the other end thereof, and the liquid chamber 60 is connected to the pressure rod by the inner cylinder 23. 22 is defined between the first liquid chamber 60a and the second liquid chamber 60b connected to the first liquid chamber through the communication port 25 and defined between the outer surface of the inner cylinder and the inner surface of the bellows. When the bellows main body 46 is contracted beyond a predetermined stroke, the device closes the communication port 25 of the inner cylinder 23 to seal the second liquid chamber 60b so that the bellows main body 46 exceeds the predetermined stroke. Of the press-working machine having self-sealing means 52 for preventing the contraction The cushion device. A plurality of die cushion 20, each die cushion 20 is a cylinder 21 having a closed end and the other end having a through hole 10, and inserted into the cylinder through the through hole 10, the cylinder shaft A pressing rod 22 movable along the direction, and fixed to the bellows body 46 made of a substantially cylindrical metal and freely stretched along the axial direction of the cylinder while being accommodated coaxially within the cylinder. And a plurality of annular bellows guides (55, 56) allowing the reciprocating movement of the inner surface of the cylinder to be free. The liquid chamber (60) which accommodates the pressure rod and communicates with the through hole is provided in the cylinder. And a bellows 45 for separating the gas chamber 80 connected to the closed end of the cylinder, a sealing means 35 for sealing between the pressing rod 22 and the through hole 10, and the liquid chamber 60. With working fluid filled in, The working gas is enclosed in the gas chamber 80 and pressurizes the pressing rod 22 in the direction A protruding from the cylinder through the bellows 45 and the working liquid. A die cushioning apparatus of a press working machine having a gas communication pipe (95) for connecting the gas chambers (80) to each other to maintain a uniform gas pressure in the gas chambers.