EP0273340A1

EP0273340A1 - Pneumatic device for presses

Info

Publication number: EP0273340A1
Application number: EP87118917A
Authority: EP
Inventors: Umberto Gaianigo
Original assignee: Gti Di Gaianigo Umberto & C Snc
Current assignee: Gti Di Gaianigo Umberto & C Snc
Priority date: 1987-01-02
Filing date: 1987-12-21
Publication date: 1988-07-06
Also published as: IT8785501A0; IT1210548B

Abstract

According to the invention, the pneumatic device for presses includes two cylinders (1,2; 31, 32) with different diameters, communicating with each other, within which slide two pistons equippped with shafts (7,9; 33, 34) integrally connected with one another.

The piston (1; 31) transmits a negligible power and it only serves the purpose of moving the upper part of the die until it brings it in the vicinity of its shutting point, while the other piston (2; 32) transmits the maximum power of the press in correspondence with the end stretch of the piston stroke. The device presents accident-prevention characteristics, since the entire stroke previous to the shutting of the die is performed by the cylinder transmitting a reduced power.

A variation of execution of the device foresees that the lowering cylinder (31) and the power cylinder (32) be arranged side by side and with their axes parallel with each other.

Description

The invention concerns the realization of a pneumatic device including a pneumatic power cylinder with special accident-prevention characteristics and functioning with limited air consumption, particularly suited to be installed in presses for the lifting and lowering of the upper part of the die.
At the present technical stage there are on the market pneumatic presses suited to prevent the occurrence of possible accidents to the operators. Said presses usually have two pneumatic cylinders with co-ordinated action. The first one, which is not very powerful and is called "service" cylinder, lowers and lifts - through the inlet of air under pressure - the upper part of the die with a power which is barely sufficient to move the mass connected with it, so that the working punch approaches the working area without actually transmitting any useful power. The shaft of said service cylinder is equipped with a suitable ledge, which, during the downward stroke of the piston itself, goes to touch a micro-switch being integral with the press and controlling a main cylinder. Thus the air enters into said second cylinder, which is the power cylinder of the press and it transmits to the piston connected to the punch the strength necessary to perform the working process with the die.
The just described double-cylinder press is actually equipped with an accident preventing safety device, in that the second cylinder, which is the power-giving one, only begins to function almost at the end of the stroke of the first service cylinder, i.e. shortly before the shutting of the die. In actuality, if the press operator were accidentally to place his hand or a finger under the die, this would happen before the power cylinder of the press begins to perform and the operator's limb would only receive a slight blow from the service cylinder, i.e. a blow of only a few kilograms. Moreover, the hand itself would prevent the service cylindr from completing its stroke and , therefore, the power cylinder would be prevented from functioning.
One of the main limitations of the presses thus conceived consists in the fact that the accident preventing performance is not absolutely guaranteed. In fact the operator can disconnect the service cylinder and keep the control of the power cylinder constantly closed, all this for instance in order to speed up the working cycle.
Another drawback consists in the fact that in these types of presses the air consumption is increased by the presence of the service cylinder.
Not the least drawback is represented by the construction complication due to the presence of a double cylinder and of a more complex pneumatic circuit which implies higher costs both for purchase and maintenance.
The goal of the present invention is that of overcoming the just mentioned drawbacks.
Specifically the goal is that of obtaining a pneumatic device including a pneumatic power cylinder suited to be applied to presses, where the pneumatic cylinder can be stopped without any damage by exerting a minimum amount of pressure, should it meet with an obstacle during its stroke. In the substance, the proposed goal is that, in case of the accidental presence of the operator's hands under the die, the operator may receive only a minimum damage, i.e. a blow of negligible power and of very little effect, such that it will not damage the involved limb. Yet another purpose to be reached is that of realizing a device with a pneumatic power cylinder requiring a low air consumption in comparison with cylinders of equal power of the known construction types.
All the above-mentioned goals and others which will be better pointed out hereafter, are achieved by a pneumatic device suited to be employed on presses, characterized by the fact that it consists of two pneumatic cylinders having different diameters, communicating with each other and equipped with two different pistons rigidly connected with each other, further characterized by the fact that the descent of the shaft connected with the upper part of the die, prior to the shutting of the die, is obtained by the reduced thrust of the piston of the pneumatic cylinder having the smaller diameter, while the last part of the descent of the shaft corresponding with the shutting of the die, occurs thanks to the thrust of the piston belonging to the cylinder having the larger diameter, said thrust being equal to the nominal power of the press.
Advantageously, according to a preferred form of execution of the invention, the two pneumatic cylinders are coaxial with each other and they have their two respective pistons connected by a rigid shaft coaxial with the shaft of the main piston protruding from the cylinder and connected to the upper part of the working die.
The upper cylinder has a significantly smaller diameter than the lower cylinder, so that, the air pressure being equal, the power of the upper cylinder is negligible in comparison with the power of the lower pneumatic cylinder. Moreover, the diameter of the lower cylinder is not constant, but it rather presents, in its lower terminal part, a stretch having a diameter slightly reduced in comparison with its own diameter. This because the goal is that of making the piston of the lower cylinder work only in correspondence with the reduced diameter, as mentioned above. At first the air under pressure enters into the upper cylinder having a smaller diameter and it exerts on the piston a considerably reduced thrust of negligible power, such as to cause the shaft of the cylinder to descend until the piston of the lower cylinder having a larger diameter reaches the terminal part of said cylinder.
In the bottom part of the lower cylinder, the piston of said cylinder gets into contact with the walls of the chamber of the cylinder itself, since the diameter of said lower part is equal to the diameter of the piston. When the air discharged by a valve activated by the piston itself is let into the smaller chamber which is created by the descent of the lower piston, said piston is subject to the thrust exerted by the air in the remaining part of the lower cylinder, said thrust being exerted only on the upper part of the piston. Thus , in the last stretch of the piston stroke, corresponding with the shutting of the die, the thrust equal to the maximum power of the press is obtained. When, because of work requirements on special dies the piston stroke is very long, in order to avoid excessive overall dimensions, another form of execution of the invention foresees that the two pneumatic cylinders, the one causing the lowering of the shaft and the power one, are no longer coaxial but separate and placed side by side and parallel to each other. In this case also the cylinders are communicating with each other, so that the pressure created inside one of them is equal to the pressure inside the other, moreover, the shafts protruding from the cylinders are connected with each other, so that the two shafts are subject to the same influences.
One of the advantages obtained with the device of the invention is that the thrust exerted on the upper part of the die prior to the shutting of the die is controlled by the cylinder with the negligible power which is barely sufficient to overcome the frictions and to move the masses connected with the device. Thus, if, because of accidental causes, the operator's hands were under the die, they would be hit by a power of only a few kilograms which would not entail traumatic effects.
Yet another advantage obtained is that the air consumed to develop the maximum power of the cylinder is relative to the volume corresponding to the active surface of the lower piston and only to the last part of the stroke, or indeed to the stretch corresponding with the shutting of the die. It is easy to understand that this volume is considerably reduced in comparison with the total volume of the power cylinder, said volume in the present stage of technical development being lost at each stroke of the press.Other constructive and functional details will be better understood from the description of three preferred forms of execution of the invention, which are given by way of illustration only, but are not meant to limit the scope of the invention, such as it is illustrated in the enclosed tables of drawing, where:

- Fig. 1 is a cross-section view of the pneumatic device with coaxial cylinders and with the pistons in their intermediate stroke position;
- Fig. 2 is cross-section view of the pneumatic device of Fig. 1 with the pistons in their end-of-the-stroke position;
- Fig. 3 shows a cross-section of a variation of the solution concerning the air-discharge system;
- Fig. 5 represents the same variation of the solution of Fig. 4 with the piston at the end of its stroke.

With reference to Fig. 1, the pneumatic device according to the invention, indicated with 10 as a whole, consists of a hollow cylinder 1 connected with another cylinder 2 by means of a junction cap 3, said cylinder 2 being coaxial with the first one and having a larger diameter. The cylindrical bodies 1 and 2 are constantly communicating with one another thanks to the manifold hole 4 obtained on cap 3 and also thanks to the fact that there is a backlash between shaft 7 of piston 5 and bushing 8 on which shaft 7 slides.
Inside the cylindrical bodies 1 and 2 respectively are inserted two pistons 5 and 6 having suitable diameters, and being connected with each other by means of shaft 7 presenting a splined profile.
Said shaft 7 is coupled so it can slide with a splined bushing 8 contained within the junction cap 3, so as to avoid the rotation of the shaft itself and, therefore, the possible rotation of the punch.
The just mentioned pistons also divide the cylindrical bodies 1 and 2 into a series of chambers with variable volume in relation to the stroke of the pistons themselves. Chamber 12 is located in cylinder 1 above piston 5 and it communicates through duct 13 with a header 14 which sends in air under pressure.
Chamber 15 is located in cylinder 1 between piston 5 and the junction cap 3 and it communicates with chamber 16, located in cylinder 2 between piston 6 and cap 3 itself, through duct 4.
Chamber 16 communicates also, through duct 17, with header 18 which sends air under pressure into cylinder 2. When air is let in through header 18, it flows through duct 17 and expands in the chambers 15 and 16, communicating with each other, and in chamber 19 further communicating with chamber 16, since the diameter of piston 6 is smaller than the maximum internal diameter of the cylindrical body 2, as can be observed in Fig. 1.
In effect, the cylindrical body 2 presents internally two different diameters, i.e. a larger diameter in correspondence with chamber 16 and a smaller one in correspondence with chamber 19, where piston 6 can perform its action, its diameter being compatible.
Since the shafts 7 and 9, which are coaxial with each other, have bases of different areas, the area of shaft 7 being larger than that of shaft 9, it happens that the same pressure being present in the chambers 15, 16 and 19, and acting perpendicularly on piston 6, it stresses more the lower surface 20 of said piston, due to the larger pressure area. Thus, the resultant of the forces exerted on piston 6 has the effect of pushing upwards the piston itself and, as a consequence, piston 5 which is connected with it through shaft 7, so that shaft 9, protruding from the pneumatic cylinder 10, withdraws causing the punch connected with it to move upwards.
In order to obtain the descent of the pistons 5 and 6, and, therefore, of shaft 9 and of the punch, air under pressure is sent in through header 14. The air, entering through duct 13, expands in the air-tight chamber 12 and pushes piston 5 and, therefore, piston 6 contemporaneously, downwards.
The thrust transmitted to shaft 9, connected with the punch equals, therefore, the difference between the above-mentioned thrust exerted on piston 5 and the thrust, in the contrary direction, exerted on piston 6 by the pressure present in the chambers 15, 16 and 19.
Since the area of piston 5 on which the thrust is exerted is small, the thrust moving downwards shaft 9 and, therefore, the punch is also small. On this subject it has already been seen that an adequate dimensioning of cylinder 1 can limit the downward thrust within 3-5 Kgs. This limited thrust is maintained until the downward stroke brings piston 6 to shut chamber 19. In fact, while coming down, the lower surface 20 of piston 6 comes into contact with the head of valve 21 which lowers itself and lets out the air being present in chamber 19 through channel 22 and header 23.
This occurs since washer 24 of piston 6 rests against the inside vertical wall 25 of the cylindrical body 2 and it causes an air-tightness, thereby preventing the air contained in the chambers 15 and 16 from exiting. Therefore, when piston 6 enters into said chamber 19 and washer 24 causes the air-tightness, the upper surface 27 of piston 6 will be stressed by the uniform pressure existing in the chambers 12, 15 and 16 and thus the nominal power of the cylinder will be produced, since the thrust in the contrary direction exerted on the lower surface 20 of piston 6 remains null, the discharge of the air having taken place through valve 21.
In this way it is obtained that cylinder 10 as a whole does not exert its nominal power until the inside piston of cylinder 2 arrives in chamber 19 and until said piston causes the air contained in said chamber 19 to be let out. It can be well understood then, that up to the just mentioned moment, shaft 9 of cylinder 10 connected with the striking mass of the press is stressed only by the force due the air pressure on piston 5, decreased by the light thrust exerted on surface 20 of piston 6 because of the area difference. This fact allows the operator to work safely and to limit to a simple swelling or bruise an accident due to the presence of his hand under the die during the press draw.
In order to cause the unit made of the pistons 5 and 6 and of the shafts 7 and 9, all these elements being integral with each other, to go back up, it suffices to remove the pressure from chamber 12 by discharging the air in header 14, and to convey pressure into chamber 19 by letting the air into duct 22, through the seat of valve 21 which is still open.
While piston 6 is exiting from chamber 19, valve 21 is gradually recalled by spring 26 and it shuts again the passage to the air. Since washer 24 no longer effects the air tightness, the pressure existing in chamber 19 enters also into the chambers 15 and 16 and the force necessary to lift the pistons will again be given by the difference between the thrusts exerted on the lower part 20 and on the upper part 27 of piston 6.
It can then be observed that for a complete working cycle the consumed air corresponds to the sum of the air volumes being present in chamber 19 and in chamber 12, said volumes being much reduced in comparison with the volume of chamber 16, which in the case of the present invention is never used up, or in comparison with the traditional cylinders of comparable power, which had to use up a quantity of air corresponding almost to the sum of the volumes of chamber 16 and of chamber 19, further increased by the corresponding volume of the service cylinder in the presses provided with the accident-prevention device.
An execution variation of the just described device according to the invention, especially suited when the shutting stroke of the die is sufficiently long, is illustrated in Fig. 3. In fact, in order to avoid exceeding heights, when the length of the pneumatic cylinders exceed certain dimensions, for instance 500-600 millimeters each, in Fig. 3 the device according to the invention is made so that cylinder 31, which controls the descent of the upper part of the die, and cylinder 32, which is the power cylinder, are placed parallel side by side.
The shafts 33 of cylinder 31 and 34 of cylinder 32 are connected with each other by means of shaft 35 which is integral with two bushings 56 and 57 keyed on the shafts 33 and 34 respectively. The pneumatic connections and the mechanical structure inside the two cylinders are practically unchanged in relation to device 10 of the Figs. 1 and 2, exception being made for the discharge valve which, in the case of Fig. 3, is controlled by a selenoid valve.
In fact, in the case under examination, in order to reduce as much as possible the overall dimensions, valve 36, discharging the air in chamber 37 having a smaller diameter than chamber 38, both belonging to the power cylinder 32, is controlled by a selenoid valve 39, which is activated when piston 40 of cylinder 32 lowers itself until it touches the proximity sensor 41. In fact, when the proximity sensor 41 closes the circuit of the selenoid valve 39, valve 36 lets out through duct 42 all the air contained in chamber 37. Said valve continues to let out the air until piston 40 completes its descending stroke, after that the control circuit of the device - which is not represented in the drawings, since it is of a well-known type and rather obvious to the expert in the business, controls the inlet of air under pressure into chamber 37 through the same duct 42, which was previously functioning as an air outlet duct, and at the same time it controls the discharge of the air contained in chamber 43 of cylinder 31 through duct 44.
Since chamber 38 of cylinder 32 and chamber 45 of cylinder 31 communicate with each other through duct 46, it happens that the thrust difference occurring on the surfaces 47 and 48 of piston 40 - said difference being due to the difference in the areas -and which would keep piston 40 in its lower position, is compensated by a further upward thrust being exerted on surface 50 of piston 49 of cylinder 31, said thrust being efficient since chamber 43 is in the air-outlet process. It is thus obtained that shaft 34 and, therefore, the upper part of the die go back to the top position and are ready to begin a new working cycle.
Obviously, the beginning a a new working cycle for device 30 foresees, as it did for device 10, the inlet of air pressure into chamber 43 through duct 44, so that shaft 33 of cylinder 31 drags downwards shaft 34 of cylinder 32 directly connected with the upper part of the die. This descent occurs thanks to the reduced power of cylinder 31 until piston 48 of cylinder 32 comes to shut chamber 37. When that happens, the descent of shaft 34 and, therefore, of the upper part of the die occurs with the transmission of the nominal power of the press, as already described.
A variation of the same inventive idea concerning a different method of air discharge is represented in the Figs. 4 and 5. In order to make the description simpler, said variation will be referred to the Figs. 1 and 2, but it is equally valid if it is applied to the variation illustrated in Fig. 3.
In this execution variation, as can be observed in Fig. 4, shaft 9 for the connection of piston 6 presents an area 60 with an indented diameter.
Shaft 9 slides within seat 61 obtained in the bottom flange 62, where the tightness between seat 61 and shaft 9 is guaranteed by the presence of the washers on lip 63. On the same flange 62 a horizontal duct 64 is also obtained. It communicates with seat 61 in the area comprised between the two washers 63 and it allows said area to be in communication with the outside environment.
When piston 6, during its downward displacement allows duct 64 to come into contact with the indented diameter area 60, a ring-shaped air-space 65 is created, through which duct 64 lets the outside environment come into contact with chamber 19. This allows the air contained in chamber 19 to exit. The air outlet will continue until piston 6 reaches its bottom dead center, which, as can be observed in Fig. 5, corresponds with the complete air discharge from chamber 19. In order to make piston 6 return to its upper position, it will suffice to let air in through duct 64 by performing the same operations previously described.
One of the most obvious advantages given by the application of said variation consists in a better reliability of the entire device. In fact, the indentation 60 on shaft 9 allows the discharge from chamber 19, thereby doing away with valve 21, which contributes to reduce the overall reliability of the device, since it is a dynamic component.
Yet another advantage consists in the possibility of realizing bottom flanges 65 having a reduced thickness, since it is no longer dependent on the stroke length of valve 21.
As has already been mentioned, the execution variation described, which is necessary for the material and functional replacement of valve 21 represented in the Figs. 1 and 2, can just as well be usefully applied for the material and functional replacement of valve 36 in Fig. 3.
Thus, with the described invention, a pneumatic device with considerable accident-prevention characteristics has been obtained, which guarantees the rapid descent of the punch for almost the entire stroke, previous to the shutting of the die, with a power which is almost null.
The goal, which is as important, of substantially reducing the air consumption by discharging outside only a reduced volume of air, in order to considerably decrease production costs has also been achieved.

Claims

1) A pneumatic device, particularly suited for the operation of presses and for the lifting and lowering of the upper part of the die, characterized in that it includes two cylinders (1, 2; 31, 32) having different diameters, communicating with each other and being provided with two pistons with shafts (7,9; 33, 34) integrally connected with each other, the one (2, 32) being suited to transmit the maximum power of the press in correspondence with the end part of the stroke of the piston sliding on an area (19, 37) with a smaller diameter of said cylinder and the other (1, 31) having a negligible power and being suited only to move the upper part of the die, further characterized in that the descent of the shaft (9, 34) connected with the upper part of the die is obtained, prior to the shutting of the die itself, through the reduced thrust of the piston belonging to the cylinder with the smaller diameter (1, 31), while the last stretch of the shaft descent, corresponding with the shutting of the die, occurs under the thrust of the power piston belonging to the cylinder with the larger diameter (2, 32).

2) A pneumatic device according to claim 1, characterized in that the two pneumatic cylinders (1, 2) are coaxial with each other, communicate with each other through a duct (4) and in that the pistons (5, 6) of said cylinders are integrally connected with each other through the single shaft (9) protruding from the device and connected with the upper part of the die.

3) A pneumatic device according to claim 2, characterized by the fact that the shaft (7) connecting the two pistons has a splined profile and is connected, so it can slide, with a bushing (8), equally splined, preventing the cylinder shaft (9) from rotating around its own axis.

4) A pneumatic device according to one of the preceeding claims, characterized in that the power cylinder presents at its bottom an area (19, 37) having a smaller diameter than the inside diameter of the cylinder, such that the piston (6; 40) entering into contact with the walls of the just mentioned area transmits to the shaft (9; 34) protruding from the cylinder itself all the nominal power, the air compressed between the piston (6; 40) and the bottom of the cylinder (2; 32) being discharged through an oulet valve (21; 36) activated by the piston itself.

5) A pneumatic device according to one of the preceeding claims, characterized in that the air discharge from the chamber (19) is obtained thanks to an indentation (60) on shaft (9) communicating with the horizontal duct (64) made on the bottom flange (62) of the cylinder (2).

6) A pneumatic device according to claim 4), characterized in that the valve (21) placed at the bottom of the power cylinder (2) is made to let the air out by the direct contact of the piston (6) of said cylinder acting on the shaft of said valve by overcoming the force of a spring (26).

7) A pneumatic device according to claim 4, characterized in that the valve (36) placed at the bottom of the power cylinder is activated by a selenoid valve (39) controlled by a proximity sensor (41) or by similar means, which excites said selenoid valve when the power cylinder piston (6; 40) slides on the smaller-diameter area (19; 37) of said power cylinder.

8) A pneumatic device according to claim 1, characterized in that the power cylinder (32) and the cylinder (31) lowering and lifting the upper part of the die are placed side by side and parallel, that they communicate with one another through an air duct (46) and with shafts (33, 34) integrally connected with each other.