Abstract:
A hydraulic swivel drive for a grab that has two gripping arms or shells movable like tongs, including two swivel shafts parallel to each other and rotatably mounted in a drive housing, whose shaft ends are connectable with the gripper arms or shells, wherein the swivel shafts each have two oppositely threaded screw engagement portions, with which two pistons movable in opposite directions by means of hydraulic pressure for driving the swivel shafts are in screw engagement. Two differently large and differently contoured cylinder pressure chambers can be assigned to each piston in order to optimally adapt the cross-sectional area of the pistons for each of the reciprocating positioning movements and to appropriately select the pressure conditions. To be able to run in the differently contoured and dimensioned cylinder pressure chambers, each of the pistons is provided with different piston portions which are adapted to the respective cylinder pressure chamber.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of German Utility Model Application No. 20 2015 006 973.8 filed 6 Oct. 2015, the entire contents and substance of which is hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of Invention 
         [0003]    The present invention relates to a hydraulic swivel drive for a grab which has two gripping arms or shells movable like tongs, comprising two swivel shafts parallel to each other and rotatably mounted in a drive housing, whose shaft ends are connectable with the gripper arms or shells, wherein the swivel shafts each have two oppositely threaded screw engagement portions, with which two pistons movable in opposite directions by means of hydraulic pressure for driving the swivel shafts are in screw engagement. 
         [0004]    2. Background and Related Art 
         [0005]    A swivel drive for a clamshell grab according to the prior art is known for example from DE 201 07 206 U1, in which the swivel shafts each are part of a hydraulic motor which is enclosed by the shell carrier formed as closed housing. On the two swivel shafts, which swivel the gripper arms or gripper shells open and closed, pipe sections each are seated, which are longitudinally shiftably, but non-rotatably guided and are in screw engagement with the respective swivel shaft, so that a longitudinal displacement of the pipe sections leads to a rotation of the swivel shafts. The longitudinal displacement of the pipe sections is effected by means of a common center piece which connects the pipe sections and is formed as piston which is accommodated in the interior of the housing that forms the cylinder for the piston and can be charged with hydraulic pressure through corresponding pressure chambers. 
         [0006]    As in this known swivel drive the piston/cylinder volume is increasing continuously with increasing distance of the swivel shafts and hardly manageable forces thereby act on the housing, DE 203 19 227 U1 proposes to not form the entire shaft drive piece as piston, but to provide plunger pistons in the shaft drive piece, which are movable relative to the shaft drive piece by hydraulic pressure in corresponding plunger piston chambers and thereby can shift the shaft drive piece to and fro in the desired way. 
         [0007]    In hydraulic swivel drives of this type high axial forces occur at the bearings of the swivel shafts, which result from the fact that the linear movement of the shaft drive piece is converted into the desired swivel shaft rotation by a screw engagement. To be able to produce the necessary high swivel shaft moments, the shaft drive piece must be driven axially with correspondingly high forces, which leads to correspondingly high axial reaction forces in the bearings of the swivel shafts. To absorb these forces, the swivel shafts usually are supported by means of axial plain bearings. This provides for the typical compact construction of the swivel drive and the shell carrier, whose outside dimensions cannot by extended arbitrarily by the gripper shells to be connected. Such axial plain bearing of the swivel shafts, however, was found to be disadvantageous for the efficiency of the drive. 
         [0008]    To reduce the bearing resistance of the swivel shaft bearing, DE 20 2004 013 158 U1 proposes hydrostatic axial bearings for the swivel shafts, which are fed by the hydraulic pressure by means of which the shaft drive piece is shifted. However, this solution in turn requires a corresponding constructional effort, and in addition special constructive measures must be taken, in order to supply the pressure pockets of the hydrostatic bearings with pressure to a sufficient extent. 
         [0009]    For the solution of the above-mentioned bearing problems and to manage the axial forces, DE 20 2006 013 101 U1 proposes the use of two pistons moving in opposite directions, which each are in screw engagement with both swivel shafts and thereby utilize oppositely threaded screw engagement portions of the swivel shaft, so that the swivel shafts are rotated into the one or other direction by the pistons moving apart from each other and inversely by the pistons moving towards each other. By such pistons movable in opposite directions the axial bearing pressure can be minimized, as the driving forces and reaction forces of the pistons largely compensate each other. However, in the known swivel drive according to DE 20 2006 013 101 U1 relatively high transversal forces act on the housing in a direction transversely to the longitudinal shaft axes, which require a correspondingly massive and therefore heavy formation of the housing. Due to the large-surface pistons, the housing shell which defines the cylinder pressure chambers also has a correspondingly large surface area, which under the influence of hydraulic pressure leads to large forces on the housing. In addition to the transversal forces high axial forces also act on the lateral housing covers through which the swivel shafts extend. Regardless of the compensation of the bearing forces on the shaft bearings, the bearing caps still are strongly loaded by the hydraulic pressure even with pistons moving in opposite directions, so that these bearing caps and their attachment to the housing base body also must be formed correspondingly massive. 
         [0010]    Therefore, it is the object underlying the present invention to create an improved swivel drive of the type mentioned above, which avoids the disadvantages of the prior art and develops the latter in an advantageous way. Preferably, a lighter construction of the drive housing should be achieved, without therefore having to make concessions as regards the driving torque of the swivel shafts and the gripping force of the grab. 
       BRIEF SUMMARY OF THE INVENTION 
       [0011]    Briefly described, in a preferred form, the present invention comprises a hydraulic swivel drive for a grab which has two gripping arms or shells movable like tongs, comprising two swivel shafts rotatably mounted in a drive housing and parallel to each other, whose shaft ends are connectable with the gripping arms, wherein the swivel shafts each have two oppositely threaded screw engagement portions, with which two pistons movable in opposite directions for driving the swivel shafts are in screw engagement, characterized in that the pistons each have two piston running surfaces on the outer shell surface, which are shaped differently in cross-section and are arranged to run in differently large cylinder pressure chambers shaped differently in cross-section, which are formed by the drive housing. 
         [0012]    The present invention further comprises a grab, in particular clamshell grab, with a hydraulic swivel drive which is formed according to the above outlined hydraulic swivel drive. 
         [0013]    It hence is proposed to assign two differently large and differently contoured cylinder pressure chambers to each piston, in order to optimally adapt the cross-sectional area of the pistons for each of the reciprocating positioning movements and to appropriately select the pressure conditions. To be able to run in the differently contoured and dimensioned cylinder pressure chambers, each of the pistons is provided with different piston portions which are adapted to the respective cylinder pressure chamber. According to the invention the pistons each have two piston running surfaces on the outer shell surface, which are shaped differently in cross-section and are arranged to run in cylinder pressure chambers formed by the drive housing, which are differently large and are shaped differently in cross-section. Due to the stepped, sectionally varying contouring of the pistons, differently high positioning forces and also different positioning speeds can be achieved in opposite positioning directions, without therefore requiring different pressure levels and different flow rates of the hydraulic pressurization, which of course can be provided nevertheless. In particular, one piston portion and the associated cylinder pressure chamber can optimally be adapted to the opening movement and the other piston portion and the associated cylinder pressure chamber can be adapted to the closing and gripping movement. Despite the specification to arrange each piston on both swivel shafts and the involved spatial constraints, the pressurization resulting for the housing can be limited and controlled by the sectionally different contouring of the pistons, without therefore having to make functionally relevant concessions as regards the driving torque of the swivel shafts and the gripping force of the grab. 
         [0014]    In an advantageous development of the invention the pistons can be arranged to face each other with their respectively larger piston running surfaces, so that the cross-sectional area of the outer pressure chambers, which are arranged towards the lateral end walls, in particular housing covers, of the drive housing, is smaller than the cross-sectional area of the at least one inner pressure chamber located between the pistons. The compressive forces on the end walls thereby become smaller due to the smaller surface area. In the larger pressure chamber located between the pistons such axial forces do not play a relevant role. In the case of a common pressure chamber between the pistons there is no dividing wall which would be acted upon axially by the hydraulic pressure. Even if two separate pressure chambers are provided between the pistons and separated from each other by a dividing wall, the axial pressures on the dividing wall compensate each other, as the dividing wall then is exposed to corresponding pressures from opposite sides. 
         [0015]    Advantageously, the orientation of the screw engagement portions at the pistons and swivel shafts can cooperate with the different pressure chambers such that the smaller pressure chambers, i.e. smaller in terms of the effective cross-sectional area, effect swiveling open of the gripper arms or shells coupled to the swivel shafts, while inversely the swiveling together of the gripper arms is effected by the larger cylinder pressure chamber cross-section. At actually the same pressure level, higher gripping forces thereby can be achieved, while inversely smaller driving torques are sufficient for swiveling open. In addition, a natural adaptation so to speak of the positioning speeds to the grab functions can be achieved without special control means for the flow rates: While on the one hand a finer, more easily controllable positioning movement takes effect on gripping, the grab is swiveled open more quickly with the same flow rate of the hydraulic fluid, as the pressure chambers with smaller cross-section are filled more quickly. 
         [0016]    The different contouring of the various piston portions can be adapted to the different sizes of the cross-sectional areas, in particular such that in the cylinder pressure chamber of larger cross-sectional area the piston and cylinder pressure chamber cross-section is flattened less than in the pressure chambers of smaller surface area. This is based on the consideration that in the pressure chambers of larger cross-sectional area the circumference of the piston running surface and hence the circumference of the shell surface of the cylinder pressure chamber is longer than in the pressure chambers of smaller cross-sectional area, so that the housing wall in the region of the cylinder pressure chamber of larger cross-sectional area has a larger surface area and therefore is subjected to larger radial compressive forces than in the region of the cylinder pressure chamber of smaller cross-sectional area. Correspondingly, the higher radial compressive forces on the housing in the region of the cylinder pressure chamber of larger cross-sectional area can be absorbed better by the housing wall than this would be the case in a cross-section flattened more strongly, as flattened cross-sections under high internal pressures tend to be pressed open, i.e. the short main axis is elongated, while the long main axis is shortened. 
         [0017]    In so far, the material of the housing wall can optimally be utilized due to the different cross-sectional contouring in conjunction with the different cross-sectional areas, and the radial pressurization can optimally be absorbed with a thin-walled, light construction, although the deviation from the flattened piston contour as such, which actually is specified by the existence of the two swivel shafts spaced from each other, does not seem to make sense, as sacrifices would have to be made in terms of a small overall size and hence a higher weight. However, since the radial compressive forces have little spreading effect, when the housing cross-section is flattened less, a weight advantage can be achieved due to a thinner wall thickness. 
         [0018]    Advantageously, the piston portion flattened less can have an at least approximately circular cross-sectional contouring. The associated cylinder pressure chamber, which preferably is the cylinder pressure chamber of larger cross-sectional area, then correspondingly has the shape of a circular cylinder. 
         [0019]    The piston portion flattened more strongly can be formed oval or elliptical or similarly rounded, wherein the cylinder pressure chamber in which this oval or elliptical or otherwise rounded piston portion extends has a correspondingly oval or elliptical or otherwise rounded cylinder cross-sectional contour. 
         [0020]    The circular-cylindrical formation of the larger cylinder pressure chamber can optimally absorb the correspondingly higher radial forces on the cylinder wall, whereby small wall thicknesses are employed here and a corresponding weight saving can be achieved, when this is seen in comparison to a correspondingly flattened cylinder chamber. 
         [0021]    Depending on how much the cross-sectional area of the flattened cylinder chamber deviates from the cross-sectional area of the cylinder chamber flattened less, in particular how much it is reduced, the wall thicknesses of the shell-surface-side cylinder pressure chamber walls can be chosen differently, as then the decrease of the radial forces occurring due to the reduction of the cross-sectional area becomes noticeable more or less. In a development of the invention the drive housing in a portion between the pistons, which defines the pressure chamber of larger cross-sectional area, can have a smaller wall thickness than in a housing portion which defines the outer, lateral pressure chambers on the side of the shell surface. Alternatively, however, equal wall thicknesses also can be provided in the shell surface portions, wherein in the case of stronger differences in the cross-sectional areas of the pressure chambers it can also be taken into consideration to form the wall thickness of the housing shell in the region of the lateral, smaller pressure chambers thinner than in the central housing portion which defines the larger pressure chamber on the side of the shell surface. 
         [0022]    Depending on the intended use, the swivel shafts can perform swivel movements in the same or in opposite directions, wherein in use as grab drive in particular oppositely directed swivel shaft drive movements can be provided. In a manner known per se, the two swivel shafts each can be two oppositely oriented screw engagement portions, which each are in screw engagement with one of the two shaft drive pieces, so that a rotation of the swivel shafts in the one direction is achieved by moving the two shaft drive pieces towards each other, and a rotation of the swivel shafts in the opposite direction is achieved by moving the two shaft drive pieces apart from each other. Rotating in the one direction or rotating in the other direction advantageously does not mean that both shafts rotate in the same direction, so that the two swivel shafts each rotate in opposite directions, in particular to in one case achieve swiveling apart of the grab shells and in the other case achieve swiveling together of the grab shells. 
         [0023]    In a development of the invention the pistons preferably can have sealing portions arranged towards their sides facing each other, which run on the swivel shafts. The swivel shafts advantageously can each have a toothing-free portion between their oppositely threaded screw engagement portions, on which the aforementioned sealing portions of the pistons are running. In particular, the toothing-free portions can have a smooth, cylindrical surface and can have a diameter at least as large as the screw engagement portions, so that the sealing portions of the pistons can slide on the toothing-free portions with a precise fit. In this way, not only a stabilization of the pistons with respect to tilting moments, but also a sealing of the pistons at the swivel shafts can be achieved. 
         [0024]    In addition to the sealing of the pistons with respect to the swivel shafts, the pistons advantageously also are sealed towards the outside, i.e. the inner shell surfaces of the cylinder pressure chambers. In a development of the invention each of the differently contoured piston running surfaces of each piston has a sealing portion with which the two piston running surfaces of each piston are sealed on the one hand towards the shell surface of the larger cylinder pressure chamber and on the other hand towards the shell surface of the smaller cylinder pressure chamber. 
         [0025]    The pitch of the screw engagement portions can be chosen differently in principle and be adapted to the swivel angle of the swivel shafts to be achieved on the one hand and to the torques to be achieved at the swivel shafts on the other hand. In a development of the invention, a favorable compromise for the proposed arrangement of two oppositely threaded shaft drive pieces comprises in that the oppositely threaded screw engagement portions of the swivel shafts each have a pitch angle in the range from 30° to 60°. Preferably, the pitch angle is between 40° and 50°, wherein according to an advantageous embodiment it can be about 45°. As mentioned already, different pitch angles however are possible in principle, depending on which swivel angles must be achieved by the swivel shafts and which torques must be achieved thereby. 
         [0026]    These and other objects, features and advantages of the present invention will become more apparent upon reading the following specification in conjunction with the accompanying drawing figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    Various features and advantages of the present invention may be more readily understood with reference to the following detailed description taken in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and in which: 
           [0028]      FIG. 1  shows a perspective, schematic representation of a hydraulic swivel drive with two swivel shafts and a pair of shaft drive pieces drivable in opposite directions for driving the swivel shafts in a partial section, wherein in the portion of the representation cut free the two shaft drive pieces and one of the swivel shafts can be seen, 
           [0029]      FIG. 2  shows a partial perspective representation of the two pistons and the two swivel shafts, on which the pistons are seated, which shows the differently shaped piston running surfaces, 
           [0030]      FIG. 3  shows a frontal side view of the swivel drive, which shows the two protruding portions of the swivel shafts, 
           [0031]      FIG. 4  shows a longitudinal section through the two swivel shafts of the swivel drive along line C-C in  FIG. 3 , wherein the two pistons are shown in their position moved together, which can correspond to a position of the grab swiveled open, 
           [0032]      FIG. 5  shows a longitudinal section through the two swivel shafts of the swivel drive similar to  FIG. 4 , wherein the two pistons are shown in their position moved apart, which can correspond to a position of the grab swiveled together, 
           [0033]      FIG. 6  shows a frontal view of the swivel drive similar to  FIG. 3 , wherein further sectional planes are depicted, which then are shown in  FIGS. 7 and 8 , 
           [0034]      FIG. 7  shows a longitudinal section through one of the swivel shafts along line A-A in  FIG. 6 , which corresponds to a longitudinal section which as compared to the longitudinal sections of  FIGS. 4 and 5  is made in the vertical plane, and 
           [0035]      FIG. 8  shows a longitudinal section through the swivel drive between its two swivel shafts along line B-B in  FIG. 6 , according to which one of the two swivel shafts is not shown in section. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0036]    To facilitate an understanding of the principles and features of the various embodiments of the invention, various illustrative embodiments are explained below. Although exemplary embodiments of the invention are explained in detail, it is to be understood that other embodiments are contemplated. Accordingly, it is not intended that the invention is limited in its scope to the details of construction and arrangement of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or carried out in various ways. Also, in describing the exemplary embodiments, specific terminology will be resorted to for the sake of clarity. 
         [0037]    It must also be noted that, as used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural references unless the context clearly dictates otherwise. For example, reference to a component is intended also to include composition of a plurality of components. References to a composition containing “a” constituent is intended to include other constituents in addition to the one named. 
         [0038]    Also, in describing the exemplary embodiments, terminology will be resorted to for the sake of clarity. It is intended that each term contemplates its broadest meaning as understood by those skilled in the art and includes all technical equivalents which operate in a similar manner to accomplish a similar purpose. 
         [0039]    Ranges may be expressed herein as from “about” or “approximately” or “substantially” one particular value and/or to “about” or “approximately” or “substantially” another particular value. When such a range is expressed, other exemplary embodiments include from the one particular value and/or to the other particular value. 
         [0040]    Similarly, as used herein, “substantially free” of something, or “substantially pure”, and like characterizations, can include both being “at least substantially free” of something, or “at least substantially pure”, and being “completely free” of something, or “completely pure”. 
         [0041]    By “comprising” or “containing” or “including” is meant that at least the named compound, element, particle, or method step is present in the composition or article or method, but does not exclude the presence of other compounds, materials, particles, method steps, even if the other such compounds, material, particles, method steps have the same function as what is named. 
         [0042]    It is also to be understood that the mention of one or more method steps does not preclude the presence of additional method steps or intervening method steps between those steps expressly identified. Similarly, it is also to be understood that the mention of one or more components in a composition does not preclude the presence of additional components than those expressly identified. 
         [0043]    The materials described as making up the various elements of the invention are intended to be illustrative and not restrictive. Many suitable materials that would perform the same or a similar function as the materials described herein are intended to be embraced within the scope of the invention. Such other materials not described herein can include, but are not limited to, for example, materials that are developed after the time of the development of the invention. 
         [0044]    The swivel drive  3  shown in  FIG. 1  comprises a drive housing  1  which is formed as shell carrier and in a usual manner can be coupled to the arm of an excavator or another lifting gear. On its upper side, the drive housing  1  can include a ring-shaped pivot bearing  2  and a non-illustrated rotary drive associated to the same, in order to be able to rotate the grab about an upright axis. 
         [0045]    The housing  1  comprises two end walls  4  and  5  parallel to each other, which are connected with each other by a possibly multipart housing shell  6 , so that the end walls  4  and  5  together with the housing shells  6  define an interior space  7 . 
         [0046]    In the interior of the housing  1  the two swivel shafts  8  and  9  are arranged parallel to each other and spaced from each other. They are rotatably, but axially firmly mounted in the end walls  4  and  5  by means of a bearing  10  and protrude out of the housing with their peg-shaped shaft ends  11 . To the peg-shaped shaft ends  11  grab shells, which are not shown in the drawing, can be coupled non-rotatably in a manner known per se, so that the grab shells can be swiveled open and closed by rotating the swivel shafts  8  and  9 . 
         [0047]    To rotatorily drive the swivel shafts  8  and  9 , two pistons  12  and  13  drivable in opposite directions are provided, which each enclose the two swivel shafts  8  and  9  and are in screw engagement with the respective swivel shaft  8  and  9 . As shown in  FIGS. 1 and 2 , each piston  12  and  13  can shiftably be seated in the interior of the drive housing  1  parallel to the swivel shafts  8  and  9 . The housing  1  forms the cylinder for the shaft drive pieces  12  and  13 , on both of whose outer sides a pressure chamber  15  and  16  is formed and between which a common pressure chamber  17  or possibly two separate pressure chambers is/are formed. By introducing hydraulic fluid into the pressure chambers  15  and  16 , the shaft drive pieces  12  and  13  can be moved towards each other and by introduction into the pressure chamber  17  can be moved apart from each other and hence the swivel shafts  8  and  9  can be rotated correspondingly. 
         [0048]    As shown in  FIG. 3  or  FIG. 8 , each swivel shaft  8  and  9  has two oppositely threaded screw engagement portions  19  and  20 , whose screw toothings have opposite pitches. In  FIG. 8 , the screw engagement portion  19  shown there on the left is formed as right-hand thread and the screw engagement portion  20  shown there on the right is formed as left-hand thread. Although this need not necessarily be provided, the two screw engagement portions  19  and  20  advantageously have the same amount of pitch angle, which advantageously can lie in the range from 30° to 60°, for example amount to 45°. 
         [0049]    Between the two screw engagement portions  19  and  20 , each swivel shaft  8  and  9  has a toothing-free portion  21 , which has a smooth cylindrical surface and advantageously can have a slightly larger diameter than the external diameter of the screw engagement portions  19  and  20 . As shown in  FIG. 4 or 7 , the toothing-free portion  21  can be offset from the screw engagement portions  19  and  20  by an undercut  22 . Although not depicted especially, the toothing-free portion  21  can be divided into two sub-portions, each of which is associated to one of the screw engagement portions  19  and  20 , respectively. 
         [0050]    As shown in  FIGS. 4 and 5 , the two pistons  12  and  13  each have a sealing portion  23 , which likewise is formed tubular and encloses the respective swivel shaft  8  and  9 , respectively. As is illustrated in  FIGS. 4 and 5 , the sealing portions  23  can run on the toothing-free portion  21  of the respective swivel shaft, in order to achieve a guidance and a sealing of the pistons  12  and  13  with respect to the swivel shafts  8  and  9 . 
         [0051]    As shown in  FIG. 1 , the outer contour of the pistons  12  and  13  is adapted to the inner contour defined by the housing  1 , which forms the cylinders and the pressure chambers  15 ,  16  and  17 , and is sealed by suitable sealing means. 
         [0052]    As shown in the Figs., the lateral external pressure chambers  15  and  16  on the one hand and the internal pressure chamber  17  between the pistons  12  and  13  on the other hand have various cross-sectional contours and also various sizes of cross-sectional areas. To allow each piston  12  and  13  to run in such differently shaped and differently large pressure chambers  15  and  17  or  16  and  17 , each piston  12  and  13  has two differently shaped piston running surfaces  24  and  25  which are provided on axially different piston portions and are formed by corresponding external shell surface portions of the piston  12  or  13 . As shown in  FIG. 2 , each of the pistons  12  and  13  has a more strongly flattened piston running surface  24  and a less flattened piston running surface  25 , wherein the arrangement of the two piston running surfaces  24  and  25  advantageously can be made such that—as seen in longitudinal direction of the swivel shafts—the more strongly flattened piston running surface  24  is arranged completely within the contour of the less flattened piston running surface  25 . In particular, the longer main axis of the more strongly flattened piston running surface  24  also can be even shorter than the corresponding main axis and/or the diameter of the less flattened piston running surface  25 , cf.  FIG. 2 . 
         [0053]    In an advantageous development of the invention the less flattened piston running surface  25  can at least approximately have a circular contour, while—independent of a circular or not exactly circular contouring of the less flattened piston portion  25 —the more strongly flattened piston running surface advantageously can have an oval or elliptical or similarly rounded circumferential contour, cf.  FIG. 2 . 
         [0054]    In an advantageous development of the invention the longer main axis of the cross-section of the flattened piston running surface  25  extends parallel to a connecting line through the two swivel shafts  8  and  9 , while the shorter main axis of the cross-section of the more flattened piston running surface  24  extends vertically to a plane connecting the two swivel shafts  8  and  9 . When the less flattened piston running surface  25  is not exactly circularly contoured on its circumference, but also slightly flattened, the longer and shorter main axes of the cross-sectional contour of the less flattened piston running surface  25  can be aligned correspondingly. 
         [0055]    As shown in  FIG. 2 , each of the pistons  12  and  13  can have a beveled contour in the region between the piston running surfaces  24  and  25 , so that the respective piston  12  or  13  does not form a rectangular step between the two piston running surfaces  24  and  25 , cf.  FIG. 2 . 
         [0056]    The shell surfaces of the associated cylinder pressure chambers  15  and  16  as well as  17  formed by housing portions of the drive housing  1  are adapted to the piston running surfaces  24  and  25 . As shown in particular in  FIG. 1 , the shell-surface-side walls of the drive housing  1  in the region of the lateral, outer pressure chambers  15  and  16  are contoured oval or elliptical or similarly rounded, so that the pressure chambers  15  and  16  have an oval, elliptical or similarly rounded cylindrical shape. In contrast thereto, the central portion of the housing shell  6  which defines the inner pressure chamber  17  in the region between the pistons  12  and  13  is flattened less strongly and in particular contoured at least approximately circular—as seen in cross-section vertically to the shaft longitudinal axes,—so that the piston running surfaces  25  of the two pistons  12  and  13  run in an at least approximately circular-cylindrical cylinder pressure chamber  17 . 
         [0057]    As shown in  FIGS. 1, 7 and 8 , a beveled shell surface transition is provided at the housing shell  6  between the circular-cylindrical inner pressure chamber  17  and the lateral, outer pressure chambers  15  and  16 , cf. reference numeral  26  in  FIGS. 1, 7 and 8 , in order to provide for a bevel between the piston running surfaces  24  and  25 . For example, between the piston running surfaces  24  and  25  and/or between the inner pressure chamber  17  and the outer pressure chambers  15  and  16  a bevel can be provided at an angle of about 45° to a plane which is vertical to the swivel shaft longitudinal axes. Other bevel angles and/or rounded bevel transitions can, however, also be provided. 
         [0058]    Numerous characteristics and advantages have been set forth in the foregoing description, together with details of structure and function. While the invention has been disclosed in several forms, it will be apparent to those skilled in the art that many modifications, additions, and deletions, especially in matters of shape, size, and arrangement of parts, can be made therein without departing from the spirit and scope of the invention and its equivalents as set forth in the following claims. Therefore, other modifications or embodiments as may be suggested by the teachings herein are particularly reserved as they fall within the breadth and scope of the claims here appended.