Abstract:
An electro-hydraulic pressing device, which is particularly suitable for producing pipe joints by means of press fittings, has a pressure piston, which acts on pressing jaws. The pressure piston divides a pressure piston chamber into a pressure chamber and a rear chamber. A hydraulic pump for feeding hydraulic fluid from a reservoir is connected to the pressure chamber. A balance piston is arranged in the piston in addition to an overpressure valve. Up until a switch-off pressure is reached, i.e. throughout the pressing process, the balance piston closes a connection opening and a return flow channel.

Description:
BACKGROUND 
       [0001]    1. Field of the Disclosure 
         [0002]    The disclosure relates to an electro-hydraulic pressing device, particularly for producing tube connections by means of press fittings, and for producing crimp connections in electrical engineering. 
         [0003]    2. Discussion of the Background Art 
         [0004]    Such a pressing device, as described e.g. in DE 20 2004 000 215, comprises e.g. two pliers-like press jaws. These are actuated by an electro-hydraulic drive. For this purpose, a pressure piston is provided in a piston chamber, the pressure chamber of the pressure piston being connected to the hydraulic pump. In this arrangement, conveyance of fluid into the pressure chamber will cause the pressure piston to move. This will result in the closing of the press jaws, wherein the pressure piston, optionally via a roller head, will actuate the press jaws. Further, alternatively to pliers-like press means, also so-called press loops are known. These comprise a plurality of mutually articulated press jaws which will be laid e.g. around a press fitting for pressing the same. The opening of the press loop will be connected to a pressing device so that, by contracting the press loop, a press connection will be realized. The pressing device used herein is designed substantially corresponding to the pressing device described in DE 20 2004 000 215. Further, axial pressing devices are known, in which the pressure build-up is also generated electro-hydraulically for displacement of a pressure piston. 
         [0005]    For pressure build-up in the pressure chamber, hydraulic fluid is conveyed into the pressure chamber by the hydraulic pump. Thereby, the pressure piston is moved and the pressing is performed. If the pressing has been performed in a reliable manner, a switch-off pressure in the pressure chamber at the end of the pressing. Once the switch-off pressure has been reached, the pressing process will be automatically terminated. This is achieved by provision of an overpressure valve in the pressure piston. Said valve can be e.g. a needle valve as described in DE 20 2004 000 215 which, when the defined switch-off pressure has been reached, will open a connection channel provided in the pressure piston. When the switch-off pressure has been reached, the piston of the overpressure valve will be advanced into the pressure piston against the force of a bias spring, thus causing fluid to flow from the pressure chamber through the connection channel into a rear chamber of the piston chamber opposite to the pressure chamber. In this process, fluid will laterally past the piston of the overpressure valve. 
         [0006]    Further, in the flow channel connecting the hydraulic pump to the pressure chamber, a control valve is provided. When the switch-off pressure has been reached, the resultant opening of the overpressure valve will cause a change of the pressure in the pressure chamber and thus also in the connection channel between hydraulic pump and pressure chamber. This will result in a switching of the control valve, resulting in the opening of a return flow channel. The return flow channel connects the pressure chamber to the fluid reservoir. The control valve as described in DE 20 2004 000 215 is of such a design that, during the pressing process, fluid will be pumped by the hydraulic pump through a narrow channel provided in the valve piston of the control valve. When the switch-off pressure has been reached and the pressure in the pressure chamber is thus sinking, the spring-biased valve piston of the control valve will be displaced and thus will clear the return flow channel. The process of pressing the pressure piston back into the starting position is performed by a return spring arranged in the pressure chamber. Said spring, when performing its return movement, will press the fluid in the pressure chamber—through the return flow channel cleared by the control valve—into the reservoir. A disadvantage of the pressing device described in DE 20 2004 000 215 resides in that, for performing the pressing process, the hydraulic fluid has to be pumped through the opening in the control valve that is small in cross section. This adversely affects the efficiency. Further, openings having a small cross section are susceptible to contamination. 
         [0007]    An electro-hydraulic pressing device is also known from DE 198 25 160. In this device, a needle valve is arranged in the return flow channel. When the switch-off pressure has been reached, this needle valve will be opened so that the hydraulic fluid will be pressed back from the pressure chamber into the reservoir with the aid of the return spring. The needle valve arranged in the return flow channel comprises a channel having a small diameter, which during the pressing process is closed by the valve needle. Since also this device is provided with a channel of small cross section through which the fluid has to be pressed, the efficiency of the pressing device according to DE 198 25 160 in the process of pressing back the pressure piston is low. Further, there is again the disadvantage that the narrow-sectioned channel provided in the needle valve is easily contaminated. 
         [0008]    It is the object of the disclosure to provide an electro-hydraulic pressing device which is suited particularly for producing tube connections by means of press fittings, wherein this device has a high efficiency both when opening and when closing the press jaws. 
       SUMMARY 
       [0009]    The electro-hydraulic pressing device according to the disclosure comprises a pressure piston acting on press jaws. Herein, the pressure piston can act on the press jaws directly or by means of intermediate elements, such as e.g. a roller head. Further, using a pressing device connected to a press loop, it is possible, via intermediate elements, to act on the press jaws, i.e. to close the press loop, in an indirect manner. Such electro-hydraulic pressing devices are suited particularly for producing tube connections by means of press fittings but also for crimping cable shoes and the like. 
         [0010]    The pressure piston is arranged for displacement in a piston chamber, said piston chamber being divided, by the pressure piston, into a pressure chamber and a rear chamber. With the aid of a hydraulic pump, hydraulic fluid can be conveyed from a reservoir into the pressure chamber. This will result in a movement of the pressure piston, wherein the pressure in the pressure chamber will increase along with the progress of the pressing. In the rear chamber arranged opposite to the pressure chamber, a return spring is provided. This spring serves for urging the pressure piston back into its starting position after completion of the pressing. In the pressure piston, an overpressure valve is arranged which particularly is designed as a needle valve. When a working pressure is reached which is obtained in the pressure chamber when the pressing has been completed, the overpressure valve will open a connection channel arranged between the rear chamber and a piston interior. The channel herein can be guided laterally past a valve piston of the overpressure valve, as also described in DE 20 2004 000 215. 
         [0011]    According to the disclosure, a balance piston is arranged in the piston interior. The balance piston is operative to open a connection opening to allow for the pressure piston to be pushed back into the starting position after pressing. In correspondence therewith, the connection opening is closed by the balance piston when, during the pressing process, the piston is being moved out of its starting position. The connection opening is arranged between the pressure chamber and the piston interior. Further, the balance piston closes a return flow channel until the switch-off pressure is reached. When the switch-off pressure has been reached, this will result in a movement of the balance piston whereby, on the one hand, the connection opening between the pressure chamber and the piston interior and, further, also the return flow channel will be opened. 
         [0012]    Thus, the balance piston is arranged within the pressure piston and during the pressing process will be moved together with the pressure piston while, during the pressing process, no relative movement will occur between the balance piston and the pressure piston. This relative movement will occur only when the switch-off pressure has been reached. After the switch-off pressure has been reached and the resultant displacement of the balance piston, the pressure piston will be pushed back into its starting position by the return spring. 
         [0013]    In the process, the fluid that is present in the pressure chamber will flow through the connection opening into the return flow channel. Since the connection channel is preferably arranged in the pressure piston, a return flow of the fluid will occur through the pressure piston. Since, during the pressing process, the fluid will flow directly into the pressure chamber and since no valve is arranged in the flow path, the pressing process performed by the movement of the pressure piston during the pressing process can be achieved with high efficiency because the connection opening by which the balance piston can be closed does not need to be designed as a needle valve. Thereby, also contamination problems are avoided or at least reduced. When the switch-off pressure has been reached, the connection opening, preferably having a large cross section, will be opened by the balance piston, thus causing the working piston to automatically travel back into the rear end position and respectively starting position. Due to the provision of a large cross section, this can take place very quickly. 
         [0014]    On the rear side of the balance piston, a pressure spring could be provided which during the pressing process will be urging the balance piston in the direction of the connection opening for closing the same. In a particularly preferred embodiment of the disclosure, this spring can be omitted or at least be designed as a weak, merely supporting spring. In this arrangement, the piston interior is connected to the pressure chamber via a supply channel. During the pressing process, fluid will flow through this supply channel into the piston interior onto the rear side of the balance piston and will push the latter into the connection opening and respectively into a valve seat surrounding the connection opening or against an edge of the connection opening. Since the supply channel is open during the entire pressing process, the pressure on the rear side of the balance piston will rise corresponding to the pressure increase in the pressure chamber. As a result, the balance piston will be pressed ever more tightly against the connection opening. This way, in spite of the increase of the pressure in the pressure chamber, a displacement of the balance piston caused by said pressure increase and a resultant opening of the connection opening are avoided. 
         [0015]    A supportive measure for holding the balance piston by means of a spring arranged on the rear side of the balance piston can be entirely omitted particularly if the rear side of the balance piston comprises a pressure surface whose pressure-relevant cross section is larger than the opening cross section of the connection opening. This design of the ratios between the surface areas has the consequence that the force acting on the rear side of the balance piston is larger than the force acting on a front side of the balance piston closing the connection opening. 
         [0016]    According to a further preferred embodiment of the pressing device of the disclosure, it is provided that, at the start of a pressing process, i.e. before the pressure piston will begin to move, a front side of the pressure piston is in abutment on a sealing seat of the piston chamber. Said front side is that front side of the pressure piston in which the connection opening is arranged. Said sealing seat surrounds the connection opening. Since, in this embodiment, the feeding of the hydraulic fluid to the pressure chamber takes place externally of said sealing seat, initially no force or merely a slight force will act on the balance piston surface closing the connection opening. Instead, initially, the pressure on the rear side of the balance piston will be increased, thus pressing the balance piston into the connection opening. As soon as the pressure piston starts moving and pressure is thus being built up at the front so side of the balance piston, the force built up at the rear side of the balance piston is already higher so that an unintended displacement of the balance piston with resultant unclosing of the connection opening is avoided. 
         [0017]    Further, it is preferred that the pressure piston comprises a head element arranged in the pressure chamber. Preferably, the balance piston is arranged entirely or at least partially within the head element. The head element is designed to the effect that, at least partially, it is not in abutment on the inner side of the pressure chamber. As a result, particularly at the beginning of the pressing process, the hydraulic fluid flowing into the pressure chamber can flow past the head element and will first flow via the supply channel into the chamber provided behind the balance piston, so as to exert a corresponding force onto the rear side of the balance piston. Particularly, the head element is designed cylindrically corresponding to the pressure chamber and has a smaller diameter than the pressure chamber so that an annular gap is formed between the inner side of the pressure chamber and the head element. 
         [0018]    Preferably, the return flow channel is arranged in such a manner that, in the opened state of the balance piston, it will connect the pressure chamber to the rear chamber. For this reason, the return flow channel is preferably provided in the pressure piston. In this manner, particularly, a compact design can be realized. Further, it is advantageous if the fluid that is flowing back into the reservoir when the piston is being pressed back, is conducted into the rear chamber since, preferably, the rear chamber is anyway connected to the reservoir. This is required because, during the pressing process, fluid which is present in the rear chamber will be pressed out of the rear chamber by the piston. This is performed by a return duct or channel which particularly is arranged in the housing of the pressing tool and which connects the rear chamber to the reservoir. 
         [0019]    The connection channel which during the pressing process is closed by the overpressure valve is preferably arranged in a partition wall between the piston interior and a valve chamber. Thus, the overpressure valve is preferably arranged internally of the pressure piston. The valve chamber herein is that chamber in which the piston of the overpressure valve is arranged. When the connection channel is opened by displacement of the piston of the overpressure valve, the piston will be displaced into the valve chamber. Thereby, at least a small quantity of fluid will flow through the connection channel into the valve chamber and will preferably proceed from the valve chamber into the rear chamber. Herein, it is preferred that the fluid will laterally flow past the piston of the overpressure valve. For this purpose, the piston can comprise corresponding grooves or channels extending in the longitudinal direction or can be arranged with a correspondingly large tolerance or gap in this space. It is to be considered in this regard that, according to the disclosure, it is sufficient if merely a small quantity of fluid is flowing through the connection channel when the switch-off pressure is reached because, thereby, only the pressure in the interior has to be reduced so that the balance piston will be moved and the connection opening will be opened. As soon as the connection opening is open, a return flow of medium will occur from the pressure chamber via the return flow channel and preferably through the rear chamber into the reservoir. According to preferred embodiment, the overpressure valve is a needle valve so that a connection channel with small diameter can be provided. This is not of disadvantage herein because merely small quantities of fluid need flow through the connection channel and, therefore, neither the efficiency will be degraded nor a danger of clogging will arise. 
         [0020]    Further, in the pressing tool of the disclosure, the provision of the balance piston makes it possible to switch off the hydraulic pump and respectively the electric motor driving the hydraulic pump also during the pressing process. In this situation, the working piston will remain at that the respective position wherein then, by renewed switch-on of the hydraulic pump, the pressing process can be continued. Thus, a so-called “inching operation” is possible. According to the disclosure, this is possible because the balance piston comprises a pressure surface whose pressure-effective cross-sectional area is larger than the opening cross section of the connection opening. Due to this differential surface, the balance piston will remain in the closed position also during an interruption of the pressing process. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    The disclosure will be explained in greater detail hereunder by way of a preferred embodiment with reference to the accompanying drawings. In the drawings: 
           [0022]      FIG. 1  shows a schematic plan view of a pressing tool, 
           [0023]      FIG. 2  shows a schematic longitudinal sectional view of a pressing tool according to a preferred embodiment, 
           [0024]      FIG. 3  shows a longitudinal sectional partial view of a pressure piston at the start of a pressing process, and 
           [0025]      FIG. 4  shows a longitudinal sectional partial view of a pressure piston at the end of a pressing process. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0026]    In  FIG. 1 , the head area of a pliers-like pressing tool is schematically illustrated. Said head area comprises two press jaws  10  which are connected to each other by a connection element  12  and are pivotable about pins  14 . In accordance with the respective design of the openings  16  on the two press jaws  14 , a pressing of press fittings or other component parts can be performed. One can also use the recesses  16  or slightly differently designed recesses in order to connect these to the free end of the press loop and to then employ the pressing tool for contracting a press loop. 
         [0027]    The closing of the press jaws  10  is carried out with the aid of a pressure piston  18  which preferably is driven electrohydraulically. Pressure piston  18  is connected to a roller head  20 . By displacement of pressure piston  18  and roller head  20  in the direction indicated by arrow  22 , the rollers of roller head  20  will be moved into abutment on the flanks  24  of the press jaws  10  so that the—as viewed in FIG.  1 —left-hand ends of the press jaws  10  will be pressed together. Thereby, the press jaws will be pivoted about the pins  14  and the press jaws will thus be closed. 
         [0028]    The press jaws  10  can be inserted into a housing  26  of the pressing tool and be fixed within the housing by a holding pin  28 . Thus, the press jaws  10  are exchangeable so that, by use of the same pressing tool, different pressing jaws can be operated. 
         [0029]    In  FIG. 2 , the pressing tool is shown in longitudinal sectional view, without the press jaws  10  inserted into housing  26 . The piston  18  is arranged in a piston chamber  30 , wherein this pressure piston  18  divides the piston chamber  30  into a pressure chamber  32  and a rear chamber  34 . Pressure chamber  32  is connected via a channel  36  to a hydraulic pump  38 . Said hydraulic pump  38  is operative to convey hydraulic fluid from a reservoir  40  through channel  36  into pressure chamber  32 . The hydraulic pump  38  is driven with the aid of a rechargeable battery  42  and an intermediate transmission  44 . 
         [0030]    Hereunder, the environment of piston  18  arranged in piston chamber  30  will be explained in greater detail with reference to  FIGS. 3 and 4 . 
         [0031]    For subdivision of piston chamber  30  into said pressure chamber  32  and said rear chamber  34 , the piston comprises a plate-shaped projection  46  which is sealed against an inner side  50  of piston chamber  30 , particularly by additional use of an annular sealing element  48 . Via channel  36 , fluid is supplied into pressure chamber  32  so that the pressure in the chamber will be increased and, for performing a pressing process, piston  18  will be moved, from the starting position shown in  FIG. 3 , toward the right. After completion of the pressing process, pressure piston  18  will be moved back by a return spring  52  arranged in rear chamber  34 . 
         [0032]    In the illustrated embodiment, an overpressure valve  54  designed as a needle valve is arranged internally of piston  18 . A valve piston  56  of overpressure valve  54  comprises a needle  58  which is operative to close a connection channel  60  as long as the switch-off pressure has not yet been reached. The closing of a connection channel  60  is effected by a pressure spring  62  whose bias can be set with the aid of a screw element  64 . 
         [0033]    In the interior of a head element  66  which is a part of piston  18 , a balance piston  68  is arranged. Said balance piston  68  is displaceable within a piston interior  70 . At the start of the pressing process and until the switch-off pressure is reached, balance piston  68  closes a connection opening  72  and a return flow channel  74  arranged in head element  66  and in piston  18 . 
         [0034]    Head element  66  comprises a sealing seat  76  at a front side of head element  66  arranged in pressure chamber  32 . Said sealing seat  76  cooperates with an annular projection  78  provided on the inner side  50  of pressure chamber  32 , thus forming a sealing surrounding the connection opening  32 . 
         [0035]    Further, a supply channel  80  is provided for the passage of fluid from pressure chamber  32  into the piston interior  70 . The fluid entering the interior  70  via supply channel  80  will effect a pressure build-up in the interior  70 , which will exert a force onto a rear side  82  of balance piston  68  that will cause the balance piston  68  to close the connection opening  32 . 
         [0036]    Further, the connection channel  60  of overpressure valve  54  is connected to interior  70 . In the illustrated embodiment, rear chamber  34  is connected to reservoir  40  via two return ducts  84 . 
         [0037]    When the pressing process is started, hydraulic fluid is pumped through channel  36  into pressure chamber  32 . Here, the fluid will first enter the annular gap between the head element  66  and the inner side  50  of pressure chamber  32 . At first, due to the sealing seat  76 ,  78 , no fluid or only small quantities of fluid will enter the area of connection opening  72 . Instead, via supply channel  80 , fluid will first reach the interior  70 , causing a pressure build-up on the rear side  82  of balance piston  68 . In this manner, the connection opening  72  will be safely closed by balance piston  68 . Now, a movement of piston  18  will take place toward the right in  FIG. 3 . During this movement, the sealing seat  76 ,  78  will be released so that fluid will also flow in the area of the connection opening. However, since a pressure has already been built up in the interior  70 , said fluid flow will not cause a displacement of balance piston  68  to the interior and thus will also not cause the connection opening  72  to be opened. Since, further, the effective pressure surface on the rear side of balance piston  68  is larger than the cross section of the connection opening, the force acting in the direction of connection opening  72  is larger than the counterforce generated by the pressure in pressure chamber  32 , so that the connection opening  72  will be kept closed during the entire closing and pressing process. 
         [0038]    After completion of the pressing process, i.e. as soon as the switch-off pressure in pressure chamber  32  has been reached, there will occur a displacement of valve piston  56  of overpressure valve  54  so that the needle  58  will be pulled out from connection channel  60  and will open the same. As a result, a small quantity of fluid will flow from the piston interior  70  through connection channel  60  into a valve chamber  86  where the piston  56  of overpressure valve  54  is displaceably arranged. From chamber  86 , the fluid can flow laterally past the piston  56  and will enter the rear chamber  34  via transverse bores, not shown. 
         [0039]    Opening the overpressure valve  54  will effect a pressure drop in the piston interior  70 . Thereby, the force acting on the rear side  82  is reduced, so that the now distinctly higher pressure prevailing in pressure chamber  32  will displace the balance piston into the interior of head element  66 . Thereby, the connection opening  72  as well as the return flow channel  74  will be opened. Thus, hydraulic fluid can flow from pressure chamber  32  into rear chamber  34  via the connection opening  72  and the return flow channels  74  which are arranged in head element  66  and respectively piston  18 . 
         [0040]    Due to the resultant decreasing pressure in pressure chamber  32 , the entire piston  18  will be moved by the return spring  52  toward the left in  FIG. 4 . Thereby, the fluid will be conveyed from pressure chamber  32  through connection opening  72  and the return flow channels  74  into rear chamber  34 . 
         [0041]    As soon as the piston  18  has reached the position shown in  FIG. 3 , the pressing device is ready for the next pressing process. In this situation, the position of balance piston  68  may possibly not yet correspond to the position shown in  FIG. 3 . This position will, however, be ensured because, at the start of the next pressing process, hydraulic oil will first flow through supply conduit  80  into the interior  70 . 
         [0042]    During the pressing process, the hydraulic oil in rear chamber  34  will be pressed back again into the reservoir  40  via the return ducts  84 .