Hydraulic press with automatic oil compensation

Abstract: Hydraulic press comprising a first chamber (2/10) for the compression of the hydraulic fluid, a second working chamber, respective pistons (2/11,2/15), a sealed conduit (2/4) connecting the inner volumes of said two chambers with wach other, a thrust element (2/12) adapted to press upon the piston of said first chamber, aperture (2/16) that enables the volume containing the hydraulic fluid to communicate with the outside ambient, shutting means (2/18) adapted to shut said aperture and linked with motion transmission means (2/13) that are displaceable into such distinct positions as to cause said shutting means to move into respective positions in which they fully shut and clear said aperture. The same aperture is provided both on a portion extending from said sealed conduit connecting the inner volumes of said two chambers with each other, and in the head of the piston of the hydraulic fluid compression chamber.

Description: The present invention refers to a hydraulic press for forming, mainly by deep-drawing and/or bending, sheet-metal plates, sheets or blanks. Particular sheet-metal forming operations are known to exist which, owing to reasons of process automation, setting-up requirements or other production-related grounds, must be carried out with the working tool moving in a horizontal displacement direction or, anyway, with an inclination and position that are selectively variable with respect to the hydraulic pressure generating means. Such presses are implemented by combining hydraulically the hydraulic pressure generating means, which are usually constituted by a piston, which is driven so as to hermetically slide within the sealed space of a cylinder containing a hydraulic medium, and the actual working means, which comprise the forming or final tool. In this kind of machines, usually presses, the separation of the power cylinder and piston from the actuating cylinder and piston (ram) is most clearly exemplified in the patent EP 0 251 796 to LINVAL. However, the cited document does not disclose any solution to the problem of the thermal expansion of the hydraulic fluid and the consequences thereof. As a matter of fact, as it on the other hand emerges from the European patents EP 0 718 055 and EP 0 589 128, anyone skilled in the art is well aware of the existing need for sheet-metal working machines to comply with two basically conflicting requirements, ie. the requirement concerning the strength and the ability of producing high pressures in a controlled manner and the requirement concerning the high precision in the movement of the metalworking tool. An apparatus for positioning and clamping sheet-metal plates or blanks that must be formed or anyway processed by means of a press to high precision standards, is known from the disclosure in EP 0 589 128 to MORITA AND COMPANY. An apparatus for sheet-metal processing applications comprising a plurality of hydraulic balancing cylinders and corresponding pistons having their respective hydraulic chambers in communication with each other, in view of a more precise and synchronous operation of said pistons in precision forming operations, is further known from EP 0 718 055 to TOYOTA. In particular, the problem connected with such a need is dealt with, for small dies, in the Japanese patent specification 60-40621, application no. 58-147686, priority August 11th, 1993, to NISSAN. The above cited robustness, repeatability and precision requirements are usually complied with by having resort to compromise solutions, such as for instance the ones described in the above cited patents. The problem however is left of the imprecision that is introduced in the actual forming process by the thermal expansion of the hydraulic oil, which may reach a temperature of up to 70°C in the case of mineral oil, or even up to 120°C in the case of synthetic oils, in continuous-duty operations. Such a temperature rise does of course not fail to bring about a volumetric expansion of the hydraulic oil normally used and involved in the process, and since such an oil has a coefficient of volumetric expansion of approx. 0.07% /°C, it ensues that the working temperatures that may be reached in the process will determine the expansion thereof to such an extent as to definitely jeopardize the processing precision, as anyone skilled in the art is well aware of, so that no further explanation shall be given here in this connection. It should furthermore be noticed that the rise in the temperature of the oil depends, further to the actual working load, by a number of other variable and basically uncontrollable elements such as the initial transient state of the apparatus until the latter reaches steady-state conditions, the ambient temperature and the heat dissipation. All such factors therefore contribute to an aggravation of both the variability and the imprecision of a prolonged working process carried out with the use of a hydraulic press, as this is particularly the case in sheet-metal forming operations. From PCT WO 94/25260 to DANLY, a control module associated to a press is also known, in which a power cylinder is provided with an auxiliary compartment adapted to be connected with the working and pressure volume being defined by the displacement of a piston inside the same cylinder at least in the top dead center of said piston; in such a position of the piston, the hydraulic liquid contained in said working volume can therefore transfer its possible excess volume, brought about by the thermal expansion, towards said compartment. Such a solution, although seemingly simple in itself, depends on the sealing effectiveness of the gaskets and is therefore strongly affected and impaired in its durability by such gaskets unavoidably wearing out and gradually loosing their sealing effectiveness. In order to eliminate all of the afore cited kinds of drawbacks, ie. overtemperature and resulting thermal expansion, imprecision and/or inconstant precision of the forming operations, high pressures, sealing effectiveness of the gaskets, and the like, it is a generally known practice, so as this is shown in Figure 1, to provide between the cylinder receiving the pressure of the die and the actuating cylinder (ram) a hermetically sealed hydro-pneumatic accumulator containing a pre-determined volume of hydraulic liquid and a pre-determined volume of gas, said liquid and said gas being completely separated from each other by a moving piston inside said accumulator. When the volume of the oil increases, it causes the inner pressure to rise as well, so that said pressure then tends to discharge itself onto the above cited piston which is thereby caused to displace in the direction in which it compresses the gas and, as a result, leaves a greater volume available for the liquid. Such a solution, however, has a clear drawback in that the compression of the gas occurs actually each time that the liquid is put under pressure and, therefore, specially at each actuation of the die. Such an occurrence therefore leads practically to a worsening precision in the transmission of the motion to the actuating cylinder (ram), as well as to a considerably reduced capability in transmitting high working pressures. Another generally known practice, so as this is shown in Figure 2, calls for the use of an oil reservoir associated to a stop valve, or electromagnetic stop valve, installed between the two above cited cylinders. Such a valve is opened for a very short period of time between the moment in which the piston (pump) relating to the die terminates its return stroke, and the moment in which the same piston starts its subsequent delivery stroke; during such a period of time, any possible increase in the volume tending to occur in the hydraulic fluid is practically counteracted and neutralized by a sufficient amount of fluid passing automatically from the hydraulic circuit to the oil reservoir, which thing is effective in restoring correct pressure conditions in the fluid and, as a result, maintaining a correct volume thereof. Such a solution, although fully effective in solving the afore described problem, is however quite complicated and expensive in its implementation; it further requires constant maintenance owing to the electromagnetic valve and the related control circuits. From GB 542,871 and from DE 3002850 a type of hydraulic mechanism, of the kind wherein an actuating member is required to be responsive through hydraulic agency to the movement of a directly operable controlling member, is known. Said mechanism is provided with the combination in series of a leakage passage which is interposed between a liquid reservoir and a part the hydraulic system which is subject to a pressure when the control mechanism is in use and which has a leakage capacity which is negligible or non-existent during the period of time required for the performance of a controlling operation, but which nevertheless is sufficient to permit of the requisite passage of liquid between the said liquid reservoir and part of the hydraulic system during the comparatively long periods of time when the control mechanism is at rest and a separate valve means which permits the passage of liquid to and from the liquid reservoir by way of the leakage passage during periods of rest but which cuts off the means of communication between the leakage passage and the liquid reservoir when a controlling operation is initiated. However said kind of mechanism is able to work just because the periods between operations are long enough to allow the correct and complete leakage of the hydraulic fluid from the hydraulic system to the liquid reservoir; as a matter of fact said type of mechanism cannot correctly work in continuous-duty operations, as in the present advanced press operations in the automotive industry. In this case the slowness of the leakage of oil towards the reservoir 8 cannot enssure the constancy of the oil volume into the cylinder 7 between one operation and the following one, if said operations are close enough. It therefore is a main purpose of the present invention to provide a technical solution. for application to the pressure generating means (pump) of a hydraulic apparatus, which is actually capable of eliminating the afore cited drawbacks and ensuring full independence from the variations in the volume of the hydraulic fluid, whichever might the cause of such variations also be. It is a further purpose of the present invention to ensure full automaticity of the action aimed at compensating for the variations in the characteristics of the hydraulic fluid, whichever may the cause of such variations also be, such as for instance variations that may be brought about by a replacement of the existing oil or a restoration of the oil level or amount through the addition of fluid having different thermal expansion properties, when the operation is basicaliy of continuous type. All such aims of the present invention shall furthermore be capable of being reached through the use of simple techniques and materials that are readily available on the market. The present invention describes a type of apparatus having the characteristics as substantially described with particular reference to the appended claims. The characteristics and the advantages of the invention will anyhow be more readily understood from the description that is given below by way of non-limiting example with reference to the accompanying drawings, in which: Figures 1 and 2 are schematical views of two different configurations of an apparatus according to prior-art technique; Figure 3 is a schematical view of the main component parts of an apparatus according to the prior art and not provided with any means for the correction of variations in the volumes of the hydraulic fluid; Figure 4 is a schematical view of the architecture and the arrangement of the component parts of a module according to a further variant of the present invention, in a first operational state thereof; Figures 5 and 6 are schematical views of the arrangement of the component parts of the above cited module, in a second and third operational state thereof, respectively; Figure 7 is a magnified view of a detail of the module of Figure 4; the pairs of Figures 8-9, 10-11 and 12-13 are views of corresponding constructive and operational variants of the solution illustrated in the preceding Figures 6 and 7. According to prior-art technique, as best represented in Figure 3, the apparatus comprises two working chambers 1 and 2, wherein in the first chamber 1 the hydraulic fluid contained therein is put under pressure by the respective piston 3. which may be driven in any of the plurality of manners known in the art. The so generated pressure is transmitted to the fluid contained in the chamber 2 through the conduit 4 which connects the inner volumes of said two chambers 1 and 2 so as to enable them to communicate with each other. As a result, the so transferred pressure actuates the working piston 5 associated to the chamber 2 and, therefore, the tool (not shown) that is solidly applied on to said piston 5. It clearly appears that, given the hermetically sealed construction of the chamber, any variation in the volume of the hydraulic fluid, however it may also be produced, will bring about a corresponding variation in the "container" that may be defined here as the "aggregate of the volumes occupied by the fluid which is put under pressure", and in particular in the position and/or shape of the walls of said chambers, and therefore only in the position of the pistons, since it is assumed that the geometry of the chambers remains fixed and unaltered, except for the position of the pistons. It also clearly appears that an expansion in the volume of the fluid, if the initial position of the piston of the first working chamber 1 is kept constant, will automatically have repercussions on the initial position, and therefore also on all other subsequent positions, of the working piston 5, with easily imaginable consequences in terms of a poorer precision of the deep-drawing and/or bending operations. In order to eliminate such a drawback, the present invention therefore proposes the herein described solution, which does not propose a compensation of the total volume being occupied by the fluid, but rather the use of means capable of ensuring the constancy of the volume of the "container", so as the latter has been defined above, and in particular of the two chambers 1 and 2 and the conduit 4, regardless of the overall volume of the fluid which may on the contrary vary within even relatively wide limits. This solution is based essentially on the fact that, for the volume of the "container" to be kept constant, the therein contained fluid in excess of a pre-determined volume is expelled or, better, "purged" out through at least an aperture that is provided in a wall of the same container, said aperture leading into an expansion (or even feeding) reservoir adapted to receive the excess amount of fluid flowing out through said aperture or, conversely, to feed in an amount of fluid as appropriate to restore the ideal conditions of total filling of said "container" when the fluid contracts. The pre-determined amount of fluid is established in accordance with the geometry of the volumes involved, the general architecture of the apparatus, the initial working position, and all other factors that intervene in the process and are generally well-known and normally considered by anyone skilled in the art when identifying and setting the operational conditions of the apparatus for achieving the desired results. With reference to the above cited Figures, and in particular to Figure 4 which is a simplified schematical view of the basic features of the above solution according to present invention, these features can be noticed to include a compression chamber 2/10, the related piston 2/11, a thrust element 2/12 adapted to act, after a short stroke, on means 2/13 provided for transmitting such thrust and, after that from said element 2/12 to said piston 2/11, the fluid expansion/working chamber 2/14, the conduit 2/4 connecting said two chambers and the actuation piston (ram) 2/15, to which the final tool is applied. Such a final tool is not shown in the Figures due to both reasons of greater simplicity and substantial irrelevance to the purposes of the present invention. Connected to said conduit 2/4 there is provided a portion 2/16, which might be defined as an equalization oil passage portion, and which is therefore included in said "container", has a completely sealed construction, but for the provision of an aperture 2/17, which is best to be seen in Figure 7, where the thrust element 2/12 has been considerably raised with respect to such an aperture. Said aperture 2/17 is capable of being closed with any appropriate shutting means 2/18 known in the art. Motion transmission means 2/13 are provided between said thrust element 2/12 and said shutting means 2/18 and are so configured as to be capable of detecting whether said thrust element is sufficiently, ie. by an appropriate value, raised from the piston 2/11 or is in its lowest position from which it starts to press upon said piston. In the first case shown in Figure 4, the thrust element 2/12 is so arranged as to be able to first act, while moving downwards, on the motion transmission means 2/13 and, only after it has fully actuated such means, it then actuates also said piston 2/11 in correspondence of the contact zones 2/11a and 2/11b thereof. The means 2/13 are adapted to transfer the movement imparted thereto by said thrust element to said shutting means 2/18 up to the extent of causing the latter to fully and tightly shut the aperture 2/17. In the preferred embodiment of the present invention illustrated in the accompanying Figures, the thrust element 2/12 and the motion transmission means 2/13 are two organs that move orthogonally, and their movement is transferred by means of two mating surfaces 2/12a and 2/13a, that are inclined with respect to the angle formed by the movement of said organs, so that the "cam effect" originated by their rubbing against each other brings about the desired displacement of the motion transmission means 2/13, which are represented as a slide in this case, in the desired direction, ie. rightwards in the Figures which reference is being made to. Said slide 2/13 is adapted to move within a respective guide 2/20 which is open on a lower portion thereof and from which there is protruding a projection 2/21 that is provided with an inclined lower side, too. Such a projection, the aperture 2/17 and the shutting means 2/18, the latter being constituted by a simple sphere, are sized and arranged so that, according to the position of the thust element and, as a consequence, the slide 2/13, said projection 2/21 presses said sphere 2/18 downwards for the latter to clear the aperture 2/17 or enables said sphere 2/18 to rise until it eventually shuts said aperture. The so arranged machine needs following initial settings and adjustments: an initial position is first of all defined for the piston 2/11, to which a coinciding initial position is then found for the actuation piston 2/15. This enables an inner volume to be defined for the chambers 2/10 and 2/14, the conduit 2/4 and the portion 2/16 (in other words, the "container'), which has then to be kept not only constant under any operating condition whatsoever, but also, and in particular, independent of any volumetric variation of the hydraulic fluid contained therein. In order to do so, it is at this point necessary for a position of the shutting means 2/18 to be adjusted, along with the position of the therewith associated organs, ie. the slide 2/13 and the thrust element 2/12, in such a manner that the same shutting means are then able to hermetically shut the aperture 2/17. At the same time, however, it is also necessary for said associated organs to be articulated in such an initial position in such a manner that, each time that the thrust element 2/12 rises starting from said initial position, it causes said sphere 2/18 to move downwards, owing to the pushing effect exerted by said projection 2/21, so as to clear the aperture 2/17. The operating principle of the present invention will be now fully apparent: upon completing the initial settings and adjustments according to the above described principle, the operation therefore starts from the thrust element 2/12 being raised beyond the initial position (Figures 4 and 7), ie. a condition in which the aperture 2/17 is not shut. At this point, when the said thrust element starts to move downwards, the hydraulic fluid, the amount of which is such as to be able to completely fill up the portion 2/16 under any condition, starts to flow out of the latter through the aperture 2/17. As the thrust element 2/12 keeps moving downwards the same element comes to reach the previously defined initial position (Figure 5), in which the slide 2/13 enables the shutting means 2/18 to shut the aperture 2/17. At this point, therefore, the afore mentioned "container" is sealed hermetically and, as a result, can be put under pressure and the piston 2/11 can start its useful working stroke (Figure 6) since, due to the only aperture 2/17 being shut, the force transmitted by the latter, and received by the element 2/12, can be fully transmitted hydraulically to the actuation slide 2/15. It is now fully apparent that any possible increase in the volume of the hydraulic fluid within said "container" is entirely reversed by the fact that the excess amount of fluid is caused to escape by the downward displacement of the piston 2/11, so that it flows out through the aperture 2/17 before said piston actually reaches its initial position, so that, in any case, when said piston reaches said initial position the excess amount of fluid has been transferred into the expansion reservoir, the aperture 2/17 is tightly shut and, as a result, the "container" is automatically put under pressure with the inner volume thereof kept at a constant, pre-determined value. It can therefore be clearly appreciated that the initial position of the piston 2/11 is made fully independent of the variations in the overall volume of the hydraulic fluid used, and is defined solely by an appropriate combination of the initial geometries alone. As far as the return stroke is concerned, the above described sequence is repeated exactly in the reverse order, in a manner that is easily imaginable and, therefore, will not be described anew for reasons of greater simplicity. The problem arising from the deformability of the conduit connecting the compression chamber 2/10 and the fluid expansion/working chamber 2/14 with each other can be radically overcome by making as a single rigid, preferably one-piece body the walls which, generally indicated at 2/31 in Figure 5, define said chambers 2/10 and 2/14, and therefore especially the conduit 2/4 which, as a result, is created automatically by simply hollowing out an appropriate portion of said rigid body 2/31. It will be appreciated that a number of further variants and improvements are of course possible. For instance, the shutting means 2/18, represented by a sphere, are contained in an enlargement 2/23 of the top end of the portion 2/16 extending from said conduit 2/4, and said enlargement comprises walls 2/24 oriented towards said aperture 2/17; furthermore, an elastic separation means 2/19, preferably a cylindrical spiral spring, is engaged between said shutting means 2/18 and said walls, so that said shutting means is in all cases forced to shut the aperture 2/17 when so enabled by the position of the slide 2/13. Second elastic means 2/26 are further provided which are adapted to act on said motion transmission means 2/13 in such a manner as to force it to take the position to which a state of the aperture 2/17 corresponds in which the same aperture is not shut by said shutting means 2/18 appropriately pushed inwards by said projection 2/21, when said shutting means 2/18 are not moved away from said position by said thrust element 2/12. With reference to Figures 8 and 9 it may be noticed that the shutting means can be advantageously constituted by a wedge 2/28 capable of sliding within the enlargement 2/23 of the top end of said portion 2/16; when the motion transmission means are pushed backwards, it releases the movement of said wedge which, by making also use of the side walls of said enlargement 2/23 as a guide, perfectly fits into the aperture 2/17, thereby shutting it tightly. With reference to Figures 10 and 11 it may be observed that the shutting means are constituted by the same motion transmission means 2/13 that are formed by an actuator capable of sliding, also as a small piston, within the respective guide 2/20. Said small piston is configured with two prominent and opposite portions 2/38 and 2/48, which are arranged and sized in such a manner that, in a first operational position shown in Figure 10, said portions plug the passage between the portion of conduit 2/16 and either the outside ambient or an appropriate sealed expansion chamber 2/200. The advantage of using a sealed chamber derives from the fact that, in this way, contaminants or foreign matters are prevented from mixing with the working hydraulic fluid and thereby altering the properties thereof. For the tightness of said expansion chamber to be more effectively ensured, it is a preferred solution to provide for said chamber to be brought at a low overpressure above the atmospheric pressure using a suitable neutral gas. Such a configuration is of course made to correspond to the position of the slide displaced by the lowering motion of said thrust element 2/12, whereas, with reference to Figure 11, when said thrust element is raised and separated from the slide 2/13, the latter elastically moves into such a position as to clear the respective passages toward the expansion chamber 2/200 and the portion of conduit 2/16 from the respective plugging prominent portions 2/38 and 2/48. A further embodying variant is illustrated in Figures 12 and 13, wherein the clearness and the simplicity of the afore given illustrations and explanations are such as to exempt from giving further explanations. In this connection, it may be sufficient to point out how, in this case, the motion transmission means 2/13 are formed by a rocker lever hinged on an axis 2/35 and having the tappet end 2/36 adapted to be actuated by the inclined surface 2/12a of said thrust element 2/12, whereas the opposite end 2/37 is adapted to actuate said plugging means 2/18, which may in turn be configured exactly as the sphere that is substantially represented in Figures 4 to 7.

Claims: Hydraulic apparatus comprising: a first chamber (10) for the hydraulic fluid, a second chamber (2/14) for the same hydraulic fluid, a sealed conduit (2/4) connecting the inner volumes of said two chambers with each other, a piston (2/11) slidably arranged within said first chamber, adapted to slide hermetically relative to the inner walls thereof and pressurise the hydraulic fluid contained therein, a thrust element (2/12) adapted to press upon the outer perimeter of said piston (2/11), a container adapted to delimit the surfaces containing the hydraulic fluid that is put under pressure, said container being provided with an aperture (2/17) that enables the inner volume thereof to communicate with the outside ambient, or with a tightly sealed expansion chamber (2/200), and with shutting means (2/18) that are capable of shutting said aperture, said shutting means being linked with motion transmission means (2/13) comprising move-away means (2/26) adapted to automatically keep said shutting means open when said motion transmission means (2/13) are disengaged from said thrust element, said motion transmission means being adapted to determinate the position of said shutting means according to the positions that can be taken by said piston (2/11) until said shutting means eventually tightly shut the aperture (2/17), characterized in that said aperture is provided in an hydraulic fluid passage portion (2/16) included in said container and extending from said sealed conduit (2/4). Hydraulic apparatus according to claim 1, characterized in that it is capable of going into a first operational state in which said thrust element does not act either on said motion transmission means (2/13) or on said piston, a second operational state in which said thrust element acts on said motion transmission means so as to induce said shutting means (2/18) to shut said aperture (2/17), while however not acting on said piston yet, and a third operational state in which said thrust element eventually acts on said piston, thereby inducing it to reduce the inner volume of said first chamber (10), while keeping said shutting means (2/18) in their shut condition. Hydraulic apparatus according to claim 2, characterized in that said shutting means comprise a valve in the shape of a sphere (2/18) or of a wedge (2/28) contained in said enclosed portion (2/16) and adapted to take at least two distinct positions, ie. a first position in which said aperture is not shut, and a second position in which said aperture is shut by said valve. Hydraulic apparatus according to claim 3, characterized in that said valve is contained in an enlargement (2/23) of said enclosed portion (2/16), said enlargement being provided with walls (2/24) oriented towards said aperture, and that said move away means (2/26) consist of at least a first elastic means (2/19) engaged between said walls and said valve. Hydraulic apparatus according to claim 4, characterized in that said elastic separation means (19) are formed by a cylindrical spiral spring. Hydraulic apparatus according to any of the claims 1, 2 or 3, characterized in that a side (2/12a) of said thrust element is adapted to engage a corresponding side (2/13a) of said motion transmission means (2/13) comprising a slide, with respect to which it acts in a cam-like manner, said sides (2/12a, 2/13a) being inclined relative to the direction of movement of said means and said thrust element so as to be able to slide on at least a portion of the respective surfaces. Hydraulic apparatus according to any of the preceding claims, characterized in that there are provided second elastic means (2/26) acting on said motion transmission means (2/13) and adapted to force the latter to automatically move into a position enabling the passage of hydraulic fluid through said aperture during said first operational state in which said thrust element is not acting on said motion transmission means. Hydraulic apparatus according to claim 1 or 2 or 3, characterized in that said motion transmission means (2/13) are themselves forming said shutting means, which are provided with prominent portions (2/38, 2/48) that are arranged and sized so that, in a first operational position of said motion transmission means, said prominent portions plug the passageway between said enclosed portion (2/16) and the expansion chamber (2/200) and, in a second operational position thereof, a passage of hydraulic fluid between said enclosed portion (2/16) and the expansion chamber (2/200) is on the contrary enabled. Hydraulic apparatus according to any of the claims 1, 2 or 3, characterized in that the motion transmission means (2/13) comprise a rocker lever hinged on an intermediate axis (2/35) and having a tappet end (2/36) adapted to be actuated by said inclined side (2/12a) of said thrust element (2/12), whereas the opposite end (2/37) thereof is adapted to actuate said shutting means (2/18), which are preferably configured in the shape of a sphere. Hydraulic apparatus according to any of the preceding claims, characterized in that said expansion chamber (2/200) is kept at a pressure which is slightly above the atmospheric pressure.

Inventors: BORDIGNON SIMONE, BORDIGNON, SIMONE
Applicants: BORDIGNON SIMONE, BORDIGNON, SIMONE
Application year: 1997
Main classification: F15B7
IPC codes: F15B7, B30B1