Abstract:
The invention relates to an axial piston engine having a swash plate, or a bent axis with a valve segment, which can be adjusted by means of a servo piston, and having an adjustment unit for electrically proportionally adjusting the supply volume. The adjustment unit has a regulating piston for controlling the oil pressure which moves the servo piston, electrically actuable proportional magnets which act on the regulating piston, and a feedback device for feeding the instantaneous swash plate position or bent axis valve segment position back to the regulating piston. The feedback device has a tappet which is mechanically coupled to the swash plate or to the bent axis valve segment, in each case one leaf spring being arranged in the longitudinal direction at both sides of said tappet, said leaf springs enclosing a feedback lever which is mounted in the regulating piston.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The invention relates to an axial piston engine having an adjustment unit for electrically proportionally adjusting the supply volume according to the features of claim  1 , and to a type series of axial piston engines having different supply volumes according to claim  13 .  
         [0002]     Axial piston engines such as hydraulic pumps and motors, of both open-circuit and closed-circuit type, and both of swash plate design and bent axis design, are often operated with electrically proportional adjustment. The input signal into said adjustment unit is an electric current. Its output signal is a hydraulic pressure. The oil pressure which is output acts on servo pistons of the axial piston engine, said servo pistons being displaced along their movement axes as a result. Said movement is transmitted to a swash plate or to a bent axis valve segment, which permit variable adjustment of the axial piston engine through variation of their angular position.  
         [0003]     To regulate the axial piston engine to the value of the supply volume set by the adjustment device, relevant systems have a regulating piston which is moved by means of at least one proportional magnet, but is generally displaced along its movement axis by means of two proportional magnets which are arranged opposite one another at the end faces of said regulating piston, and said regulating piston as a result connecting or blocking ducts such that oil at a certain pressure is available for displacing the servo piston.  
         [0004]     The mechanical regulator which is conventionally provided is composed of a lever and spring system, which ensures that, when the set value is reached, the regulating piston is returned to its initial position by means of a spring force which is directed counter to the setting force of the 35 proportional magnets. For this purpose, for example, the setting movement of the servo piston which is connected to the swash plate is fed back to the regulating valve, or the instantaneous position of the swash plate or of the bent axis valve segment is fed back directly to the electrically operated adjustment unit via a feedback pin which is fastened to said swash plate or bent axis valve segment. Said position feedback closes the regulating circuit, and ensures that the supply volume of the axial piston engine is proportional to the electric current to the adjustment unit.  
         [0005]     An axial piston engine having a device for adjusting the supply volume is known from DE 24 56 381 A1, said device comprising a regulating piston (“slide valve”) which can be displaced in two directions along a common axis by means of electromagnets. Here, the control edges of the slide valve alternately connect the lines leading to the piston of a servo motor to discharge and pressure lines, resulting in the piston of the servo motor being displaced in one or the other direction and in the supply volume of the pump being correspondingly adjusted. The piston of the servo motor is connected to the slide valve via a return spring which acts as a tension and compression spring. The force of the return spring, which increases during adjustment, finally returns the slide valve and the electromagnets into their initial positions.  
         [0006]     The document DE 102 20 889 B3 describes an adjustment device for hydrostatic piston engines. The tappet of a proportional magnet opens pressure lines for adjusting a servo piston (“setting piston”) which is in turn connected to the regulating piston via a setting lever which is formed in the manner of a pair of tongs. When the setting piston is adjusted, the arms, which are formed in the manner of a pair of tongs, of the setting lever are spread apart counter to the spring force of a tension spring until force equilibrium is reached, a feedback force being generated which acts counter to the control force of the proportional magnet. When force equilibrium has been achieved, the regulating piston is situated in a deflected position such that the servo pressures are in equilibrium with the external forces acting on the servo piston.  
         [0007]     All the known systems require several levers and springs. Production tolerances and friction losses between the individual parts have an adverse effect on the sensitive force equilibrium. In addition, an individually dimensioned adjustment unit must be provided for each installation size of a type series.  
       SUMMARY OF THE INVENTION  
       [0008]     The invention is therefore based on the object of producing an axial piston engine having a simplified, universally applicable adjustment system.  
         [0009]     According to the invention, said object is achieved by means of an axial piston engine having a swash plate, or a bent axis with a valve segment, which can be adjusted by means of a servo piston, and having an adjustment unit for electrically proportionally adjusting the supply volume.  
         [0010]     The adjustment device comprises a regulating piston for controlling the oil pressure which moves the servo piston, electrically actuable proportional magnets which act on the regulating piston, and a feedback device for feeding the instantaneous swash plate position or bent axis valve segment position back to the regulating piston, the feedback device having a tappet is which is mechanically coupled to the swash plate or to the bent axis valve segment. In each case one leaf spring is arranged along each side of the tappet, said leaf springs enclosing a feedback lever which is mounted in the regulating piston. This results in a comparatively simple construction, with the “pivot angle” regulating variable being fed back directly to the regulating piston.  
         [0011]     The leaf springs preferably have a width and/or stiffness which vary, in particular continuously, in the longitudinal direction of the tappet. This both provides a sufficient material thickness in the fastening region of the spring, and simultaneously permits fine-tuning of the stiffness of the leaf spring as a result.  
         [0012]     For the purpose of optimizing the force transmission, one end of the feedback lever is mounted in a ball guide in the regulating piston, and the feedback lever head, which is situated opposite said end, is clamped between the leaf springs. The latter extend along the tappet and beyond its end, and between them, at their ends, form a space for holding the feedback lever, the feedback lever head preferably being spherical or cylindrical.  
         [0013]     In a refinement of the invention, the leaf springs are fastened to the tappet by means of in each case at least one setting screw in such a way that in each case one preload, with which the leaf springs act on the feedback lever, can be set by means of the setting screws, said preload defining a dead band. The width and/or stiffness of the leaf springs preferably vary in the longitudinal direction of the tappet in the direction of the feedback lever in such a way that a desired dead band can be obtained, that is to say that a correspondingly increased force level in order to move the regulating piston out of its neutral position can be provided. This is significant inter a/ia when gravity influences have an effect (for example where assembled vertically). Without corresponding preload, the system would pivot out, even if no external signal were applied. Another possibility for generating a dead band is to provide an idle stroke of the regulating piston in which the servos are connected only to the tank.  
         [0014]     It is particularly advantageous if the dead band of each leaf spring can be set individually. This can be set particularly effectively if the tappet has screw-on faces on the tappet, said screw-on faces forming a contact surface for fastening the leaf springs by means of fastening screws, and a gap, by means of which the preload of the leaf springs can be set using the setting screws.  
         [0015]     The engagement point of the feedback lever in the regulating piston is preferably situated outwith the central longitudinal axis of the latter and on the longitudinal axis of actuating elements which are actuated by the proportional magnets ( 12 ,  12 ′). This avoids undesired tilting moments.  
         [0016]     In a refinement of the invention, the regulating piston has a longitudinal bore. Any leakage oil can be easily discharged through said longitudinal bore.  
         [0017]     The invention also relates to a type series of axial piston engines having 30 different supply volumes, with all the models of said type series being provided with the same adjustment unit.  
         [0018]     Further features and advantages of the invention can be gathered from the following description of an exemplary embodiment.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     In the figures:  
         [0020]      FIG. 1 : shows an exemplary embodiment of the feedback device of an axial piston engine according to the invention,  
         [0021]      FIG. 2 : shows the adjustment device in section along the regulating piston,  
         [0022]      FIG. 3 : is a detail illustration from  FIG. 2 ,  
         [0023]      FIG. 4 : shows a view of the feedback device with the mounting of the feedback lever,  
         [0024]      FIGS. 5   a,    5   b:  are illustrations of the fastening of the leaf springs. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]      FIG. 1  shows an exemplary embodiment of the feedback device I of an axial piston engine according to the invention in a perspective view. The regulating piston  2  is acted on at the end sides by two proportional magnets (not illustrated), and can be displaced in its longitudinal direction in this way. Control edges are formed, in the known way, at the periphery is of said regulating piston  2 , said control edges forming throttle points with grooves formed in a bore which holds the regulating piston, said throttle points being variable as a function of the axial position of the regulating piston. In this way, hydraulic ducts which supply oil at a certain pressure to the servo system (not illustrated here) for displacing the servo piston are connected or blocked. A swash plate is mechanically coupled to the servo piston, the angular position of said swash plate being determined by the position of the servo piston.  
         [0026]     The illustrated feedback device  1  feeds the angular position of the swash  25  plate back to the regulating piston  2 . For this purpose, a tappet  9  is provided which, at its mounting end  11 , is connected to the swash plate and follows the pivoting movement of the latter. The tappet  9  therefore reflects the actual value of the angular position, that is to say the regulating variable of the regulating system. 30  
         [0027]     Screw-on faces  16  are formed on the tappet  9 , leaf springs  6 ,  6 ′ being attached to said screw-on faces  16  by means of fastening screws  14 ,  14 ′ at both sides in the movement direction of the tappet  9 . The leaf springs  6 ,  6 ′ vary in width; they narrow conically in the longitudinal direction of the tappet  9 , and project beyond the other end  10 , which faces away from the mounting end II, of the tappet  9 , so that they form between them, at their ends, a space for holding the feedback lever  3 . There, the leaf springs enclose a spherical head  4  of the feedback lever  3  which is embodied as a twin-armed lever and is aligned approximately perpendicularly to the tappet  9  and its movement direction. The feedback lever  3  is rotatably mounted on the eccentric bolt  7  and, with its other end which faces away from the feedback lever head  4 , engages in the regulating piston  2 .  
         [0028]     The dimensions of the tappet  9  are selected such that the thickness of the tappet end  10  between the leaf springs  6 ,  6 ′ approximately corresponds to the diameter of the feedback lever head  4 , while adjacent to the screw-on faces  16 , the tappet  9  is narrowed under the leaf springs  6 ,  6 ′ in such a way that in each case one gap  15  is formed between the tappet  9  and the spring  6 . In each case one setting screw  13  is provided in the region of said gap  15 , by means of which setting screws  13  a degree of bending of the leaf springs  6 ,  6 ′ can be set and a preload can be obtained. This is particularly important because it is possible in this way to set a dead band which is desired when reaching the neutral position or going beyond the neutral position to a certain extent.  
         [0029]     When the regulating piston  2  is adjusted by means of the proportional magnets, the feedback lever  3  rotates about the rotational axis set by the eccentric bolts  7  and, with the feedback lever head  4 , pushes one of the springs  6 ,  6 ′ away from the tappet  9 , which therefore generates a force, which counteracts the setting force of the magnet, on the regulating piston. The adjustment of the regulating piston  2  activates the servo system which adjusts the swash plate of the axial piston engine in the direction of the desired pivot angle. As a result of the adjustment of the swash plate, the tappet  9  also moves in such a way that the counterforce exerted by the leaf spring increases further, said counterforce then prevailing over the setting force of the magnet and returning the regulating piston until the latter has approximately arrived back in its neutral position when the nominal value is reached.  
         [0030]     The same reference symbols are also used in the following figures to denote the same structural features.  FIG. 2  shows the adjustment device in section along the regulating piston  2 . The proportional magnets  12 ,  12 ′ act, by means of their tappets  19 ,  19 ′, on the end faces of said regulating piston  2 . A longitudinal bore  18  in the regulating piston  2  serves to discharge leakage oil. When the regulating piston  2  is displaced out of its neutral position, ducts leading to the servo system are opened or closed.  
         [0031]     The feedback lever  3 , which is embodied as a twin-armed lever, engages  5  by means of its spherical or cylindrical feedback lever base  20  in the regulating piston  2 . Here, the engagement point is situated, offset relative to the central longitudinal axis  17  of the regulating piston, on the straight connecting lines between the magnet plungers  19 ,  19 ′. This avoids tilting moments and resulting undesired force effects on the regulating piston.  
         [0032]     The feedback lever  3  is mounted so as to rotate about the pivot axis  5  and, as described previously, is held on the feedback lever head  4  in the space between the ends of two leaf springs  6 ,  6 ′ which are attached at both sides of the tappet  9 .  
         [0033]      FIG. 3  shows a detailed section from  FIG. 2 , with the regulating piston  2 , the feedback lever  3  and the leaf springs  6 ,  6 ′ being illustrated, said leaf springs  6 ,  6 ′ between them enclosing the feedback lever head  4 . The regulating piston  2  can be displaced along the axis  17  by means of the proportional magnets  12 ,  12 ′. The feedback lever  3  which is rotatable about the pivot axis  5  engages by means of its base in the regulating piston  2 , and is guided there by means of the ball guide  8 . The leaf springs  6 ,  6 ′ bear against the spherical feedback lever head  4  at both sides in the movement direction of the tappet  9  which is connected to the swash plate, it being possible to apply a preload to said leaf springs  6 ,  6 ′ by means of the setting screws  13 ,  13 ′ in such a way that a defined dead band can be provided.  
         [0034]      FIG. 4  shows the feedback device  1  as seen in the direction of the longitudinal extent of the regulating piston  2 . The longitudinal bore  18  of the latter, for discharging leakage oil, can again be seen. The pivot axis of the feedback lever  3  is provided by a cylindrical eccentric bolt  7  which is mounted in the housing and has an eccentric section on its middle part on which the feedback lever  3  is mounted. Here, the eccentricity is to be dimensioned such that rotation of the eccentric bolt  7  permits a displacement of the pivot axis which is sufficient to be able to adjust the zero position. The transverse dimension of the leaf spring  6  narrows towards the feedback lever head  4 . This is one of several possibilities for fine-tuning the active spring force, though this can also be achieved by means of a varying stiffness in the longitudinal direction or, if appropriate, suitable bending of the leaf springs. That end of the leaf spring  6  which projects beyond the end  10  of the tappet encloses between itself and the opposite leaf spring the feedback lever head  4  and is fastened to the tappet  9  by means of the fastening screw  14 . The setting screw  13  again serves to set a preload of the leaf spring  6 .  
         [0035]     The generation of the preload and of a dead band can be gathered in more detail from  FIGS. 5   a  and  5   b.    FIG. 5   a  shows a section A-A from  FIG. 4 .  
         [0036]     The leaf springs  6 ,  6 ′ are fastened to the screw-on faces  16 ,  16 ′ of the tappet  9  by means of screws  14 ,  14 ′. Adjacent to the fastening faces  16 ,  16 ′, the transverse dimension of the tappet  9 , with the exception of its end  10 , is reduced such that in each case one gap  15 ,  15 ′ is formed between the leaf springs  6 ,  6 ′ and the tappet  9 .  
         [0037]     Setting screws  13 ,  13 ′ are provided at a suitable point in the region of the gap  15 ,  15 ′, by means of which setting screws  13 ,  13 ′ the leaf springs  6 ,  6 ′ can be clamped against the transverse dimension, which is reduced here, of the tappet  9 . The leaf springs  6 ,  6 ′ can have a corresponding preload applied to them in this way. If, for example, the unloaded leaf spring  6 ,  6 ′ bears against the end  10  of the tappet and against the feedback lever  4 , it is possible in this way to set a defined dead band which is desired for the regulation of the supply volume of axial piston engines. By suitably adapting the geometric dimensions regarding the position of the screw-on faces  16 ,  16 ′, of the tappet end  10  and of the feedback lever head  4  in conjunction with the selection of the point at which the setting screws  13 ,  13 ′ bridge the gap  15 ,  15 ′, it is possible to fine-tune both the spring constants of the leaf springs and also the force level which determines the dead band.