Abstract:
The invention relates to a hydromechanical drive system for at least one press ram of a cutting and/or forming press, at least one drive cylinder being connected to a following kinematic chain. This kinematic chain is designed in such a way that a complete ram stroke, i.e. a downward motion and an upward motion, is effected during a single piston stroke in only one direction. In addition, the proposed drive system has mechanical controlled synchronization.

Description:
FIELD OF THE INVENTION 
     The invention relates to a drive for presses, in particular cutting and/or forming presses, having pressurized drive cylinders which drive a press ram in interaction with articulated elements. If two or more pressure points are used for initiating displacement or force at the ram, the articulated elements are synchronized in a controlled manner. A plurality of rams may also be driven by the use further of [sic] coupling articulations. 
     BACKGROUND OF THE INVENTION 
     Continuous-path-control and adaptive-control presses differ, inter alia, according to the type of press drive. Mechanically driven presses work with continuous-path control, i.e. not until the kinematic drive means, e.g. an eccentric, approaches bottom dead center does the desired forming or rated force greatly increase. In contrast, in the adaptive-control, hydraulically driven press, the maximum forming force is available in every ram position. Set against this advantage of the hydraulically driven press are a number of considerable disadvantages, such as, for example, low number of strokes and high drive power to be installed with poor overall efficiency. 
     A detailed account of the advantages and disadvantages of a hydraulic press compared with a mechanical press may be gathered from the article “Eine Stückkosten-Analyse erleichtert die Wahl” [A unit cost analysis facilitates choice], published in Industrieanzeiger 16/97, pages 30 and 31. 
     A disadvantage of the mechanically driven press is the complicated and expensive drive, consisting of, for example, motor, clutch/brake combination, flywheel with drive, gear chain with gears, eccentric wheels, connecting rod, shafts and bearings. The attempt to combine the advantages of a hydraulically driven press with those of a mechanical press can be seen from EP 0 616 882. The object of the latter was to achieve an essentially constant forming speed during the entire forming operation. The contents of this publication are essentially the hydraulic activation of the working cylinder and the open-loop and closed-loop control blocks required for this; the actual drive kinematics are not a subject matter of the patent rights. The kinematics shown in diagrammatic sketches correspond to the known toggle-lever or toggle-joint drive known from die-casting or injection-molding machines. This toggle-joint drive, in particular when approaching the extended position of the articulations, requires very large displacements of the drive cylinder with correspondingly large oil quantities. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to propose a drive system for a press, which drive system, with a low drive power, achieves a higher number of strokes than a hydraulically driven press and is more cost effective than a mechanical press due to a reduction in the drive elements without dispensing with the controlled synchronization, present in mechanical presses, of the force- or displacement-initiating components. 
     The accompanying drawings, which are incorporated herein and constitute part of this specification, illustrate presently preferred embodiments of the invention and, together with the general description given above and the detailed description given below, serve to explain features of the invention. 
     The invention is based on the idea of designing the drive system in such a way that the complete ram stroke, i.e. downward and upward stroke, can be performed with a single cylinder stroke in one direction without reversing connection. In this case, any desired ram stroke from the minimum to the maximum displacement is to be capable of being performed with this drive concept. Minimum strokes are realized for pure cutting operations, whereas maximum strokes, for example, are required when using transfer systems with corresponding degrees of freedom. In particular in the case of a ram having a large area, a plurality of drive points are necessary in order to avoid tilting of the ram. In this case, mechanical controlled synchronization is provided between the drive levers. 
     The motion characteristic of the proposed lever systems is characterized by a continually increasing transmission ratio in the direction of bottom dead center and thus by a correspondingly increasing press force. 
     In contrast to the conventional hydraulic press, a drive cylinder of substantially smaller diameter may therefore be selected; the number of strokes is markedly increased despite the lower drive power to be installed. The use of a so-called synchronous cylinder is especially advantageous for producing identical stroke or force ratios with the simplest closed-loop control. Since in each case the same diametral and thus area ratios are present in this cylinder in both stroke directions, there is also a very favorable control response. 
     Due to the smaller drive cylinder, the oil quantity required for operation is also reduced, as a result of which the undesirable compressibility of the pressure medium decreases. However, on account of the lever mechanisms connected in between, this compressibility is only of secondary importance, which has a favorable effect in particular for the reduction of the cutting impact occurring during cutting operations. 
     The motion and speed profile known from the mechanical presses also has an advantageous effect. In the cutting and forming region, the force increases on the one hand, but the speed is reduced. This speed, which continually decreases toward bottom dead center, is necessary in particular during the material flow during the forming operation in order to obtain good workpieces. 
     This advantageous kinematic sequence of motion is achieved by a considerably small number of components compared with a mechanical press. The press becomes more cost-effective and thus has corresponding efficiency with optimum sequences of motion from the forming point of view. 
     As already mentioned, any desired size of stroke can be performed with the proposed drive within the limits of the predetermined geometry. To this end, the drive cylinder may be provided with a displacement measuring system, which can be extended in combination with a displacement measuring system at the ram to form a position control loop. At the same time, it is also ensured in this application that the cylinder in each case travels only in one direction for a complete ram stroke, i.e. downward and upward movements. The ram stroke required in each case is then realized by the size of the associated stroke of the drive cylinder. Thus different ram strokes, e.g. for a drawing operation with subsequent sizing impact, are also possible in a forming process. 
     A controlled drive cylinder, by imposing a corresponding speed profile, permits any desired speed behavior of the ram. In addition to the forming process, this also offers considerable advantages during the use of automation equipment by optimizing the degrees of freedom. 
     A further embodiment according to the invention is shown by way of example by the drive of a plurality of rams through the use of additional coupling articulations. This arrangement is desirable, for example, in transfer presses which require a superimposed cutting ram for cutting die plates. 
     Further details and advantages of the invention follow from the description below of exemplary embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a hydromechanical drive system with a drive cylinder, showing the start and the end of a maximum ram stroke; 
     FIG. 2 shows the same as FIG. 1 but with the ram position at bottom dead center; 
     FIG. 3 shows the same as FIG. 1 but with a smaller ram stroke; 
     FIG. 4 shows a hydromechanical drive system with two drive cylinders and ram position at bottom dead center; 
     FIG. 5 shows a hydromechanical drive system for 2 rams; and 
     FIG. 6 shows an embodiment variant of FIG.  5   
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows the top part of a press  1  having a column section  2  and ram  3 . Drive cylinder  4 , consisting of cylinder housing  5 , piston  6  and piston rod  7 ,  8 , is attached perpendicularly to column  2 , Articulated lever  9  is fastened with one end to the piston rod  7  and with the other end to toothed segment  10 . Toothed segment  10  can pivot in an arc of a circle about pivot  11 . Coupling lever  12  is likewise fastenend with one end to the toothed segment  10 , while the other end is connected to double lever  13 . The pivot of double lever  13  is identified by  14 . Furthermore, double lever  13  is connected to thrust lever  15 , between which there is a connection to pressure point  16 . This pressure point  16  known from mechanical presses may serve as overload protection to safeguard the press  1  and the tool and may also contain a device for adaptation to various tool heights. 
     The construction of the lever system shown in the left-hand press half is completely identical to the lever system on the right-hand side of the press. The representation in FIG. 1 shows two different positions of the articulated system  17 . This representation has been selected in order to show that a complete ram stroke, i.e. downward and upward, is achieved with a cylinder stroke in only one direction, here downward. The left-hand side of the drawing shows the piston  6  of the drive cylinder  4  in the top position and the ram in the top dead center position. The right-hand half of the drawing shows the situation after the piston  6  has performed the downward stroke. The piston  6  is now in the bottom position, but the ram is again in the top dead center position. The change in position of the articulated system  17  can be seen from FIG.  1 . The initial position for the next complete ram stroke is now with the piston  6  at the bottom. 
     As in the following examples too, the toothed segments  10  serve to synchronize the sequences of motion when using a plurality of ram pressure points. 
     In FIG. 2, the ram  3  is shown at bottom dead center. Piston  6  is then located exactly in the center position of the cylinder housing  5 . The desired effect of high forces at a low forming or cutting speed is achieved by the design of the articulated system  17 . The effect of the transmission ratio accordingly consists in the fact that the ram speed is reduced when approaching the bottom dead center and the force increases in return. The extreme case is shown in FIG. 2, in which the speed is zero at a theoretically infinite magnitude of force. This is symbolized by the double lever  13  and thrust lever  15  located in an extended position relative to one another. The upward stroke of ram  3  is now initiated by further downward travel of piston  6 . 
     FIG. 3 shows a further possible sequence of motion. If a small ram stroke is required as the maximum stroke, e.g. for cutting or embossing work, this may be realized without problem by a corresponding smaller stroke of the drive cylinder. For this purpose, drive cylinder  4  is equipped with a displacement measuring system (not shown in any more detail). Due to the smaller drive stroke, the symmetrical pivoting angle W of the double lever  13  about the pivot  14  is also correspondingly smaller, which has a direct effect on the size of the ram stroke. This operation described permits an infinite variation of the ram stroke. Different ram strokes following one after the other during corresponding forming operations are likewise possible. 
     The importance of the synchronization function by means of the toothed segments  10  can be seen in particular when using two drive cylinders  4 , as shown in FIG.  4 . The fitting of two separate drive cylinders  4  may be necessary for constructional and functional reasons. The basic function of the articulated system  17  does not differ from the description of the preceding exemplary embodiments. The corresponding levers have been provided with the same item numbers as in the other exemplary embodiments. From the extended position of double lever  13  and thrust lever  15  and the position of piston  6 , it can be seen that the ram  3  is located in the bottom dead center position. 
     FIG. 5 shows the drive of a plurality of rams. Apart from the ram  3  for forming processes, the ram  18  is superimposed as a further ram, e.g. for cutting operations. This is therefore a press concept having 2 working rams  3 ,  18 . It is important, then, that the drive system described can be extended with minimum outlay for driving further rams. In principle, the drive system need only be extended by a double lever  13 , thrust lever  15  and coupling lever  19 . Any different drive force which may be required can be provided by appropriately varied pressurizing of the drive cylinders  4 . FIG. 6 shows a space-saving variant of FIG.  5 . In addition to a simple design of press column  2 , a single pivoted lever  20 , instead of a double lever, may be used for the drive of the ram  18 . 
     The invention is not restricted to the exemplary embodiment shown and described. It also comprises all developments by the person skilled in the art within the scope of the present invention.