Patent Publication Number: US-2022220948-A1

Title: Axial Piston Machine with High Drive Rotational Speed

Description:
This application claims priority under 35 U.S.C. § 119 to patent application no. DE 10 2021 200 205.6, filed on Jan. 12, 2021 in Germany, the disclosure of which is incorporated herein by reference in its entirety. 
     The disclosure relates to an axial piston machine. 
     BACKGROUND 
     DE 10 2015 208 925 A1 has disclosed a swashplate-type axial piston machine. The axial piston machine is optimized for a high drive rotational speed by virtue of the mouth openings of the cylinders being arranged very close to the axis of rotation. It is consequently the case that low centrifugal forces act there even when the cylinder drum is rotating at very high speed. The suction limit is hereby shifted toward higher rotational speeds, wherein the risk of cavitation at the suction side is reduced. The suction limit is the rotational speed at which the axial piston machine still just aspirates as intended. If the rotational speed is increased further, so-called suction separation arises, that is to say the axial piston machine no longer aspirates at all, or the aspirated volume flow is considerably lower than the product of rotational speed and swept volume. 
     SUMMARY 
     An advantage of the disclosure consists in that the respective axial piston machine can be operated at an even higher rotational speed without cavitation occurring at the suction side. Here, the axial piston machine is of exceptionally simple construction and is consequently very inexpensive. The advantage of the ability to operate at higher rotational speeds comes at the cost of substantially only a slight increase in the structural space requirement. 
     It is proposed that the first rotary bearing is arranged with a spacing to the distributor plate in the direction of the axis of rotation, wherein the distributor plate is held on the support surface transversely with respect to the axis of rotation, wherein the first and the second section of the at least one working channel run parallel to the axis of rotation, wherein the first and second section are arranged entirely within, or are intersected by, the reference cylinder. Which of the two latter alternatives is used is dependent significantly on the selected construction of the first rotary bearing. The sliding surface is preferably of spherical design. It is preferably convexly curved with respect to the distributor plate. The first and/or the second rotary bearing are preferably designed as tapered-roller bearings. 
     Provision may be made for the first section of the at least one working channel to extend through the distributor plate with a constant cross-sectional shape, wherein the cross-sectional shape has at least one section that is configured as a slot which is curved about the axis of rotation. It is preferable for the cross-sectional shapes of the first sections of both working channels to each be defined by a single curved slot. Each slot preferably has a constant width over its entire length in the circumferential direction. The ends of the slot that are situated opposite one another with respect to the circumferential direction preferably comprise a straight section adjoined by two rounded corners. The prior art has disclosed cross-sectional shapes of the first section that are made up of multiple separate apertures. Such an embodiment is preferably specifically not used, so as to keep the cavitation tendency low. 
     Provision may be made for the second section to be directly adjoined by a third section of the at least one working channel, wherein the third section is arranged in the housing, wherein the cross-sectional central point of the third section runs along a curve with an unchanging curvature direction, wherein the corresponding curvature is selected such that the respective working channel runs, over its entire length, with a spacing to the first rotary bearing, wherein the working channel has the smallest spacing to the first rotary bearing in the third section. With the third section, the working channel is led past the first rotary bearing, wherein the cavitation tendency at the suction port is substantially not impaired. The cross-sectional central point is preferably to be understood to mean a center of area of the cross-sectional area of the working channel. 
     Provision may be made for a curvature direction of an inner delimiting surface of the third section to reverse along its course from the respective working port toward the distributor plate. This gives rise to a particularly low cavitation tendency at the suction port. For further details, reference is made to the corresponding statements relating to  FIG. 1 . 
     Provision may be made for exactly two working channels to be provided, the working ports of which point away from one another. The two working channels that result from this can be of particularly streamlined design, wherein they have identical flow characteristics. It is likewise conceivable for two working ports to be provided which point away from the cylinder drum in the direction of the axis of rotation. The latter embodiment is selected if the structural space in the superordinate machine necessitates this. The former embodiment however makes higher rotational speeds possible. 
     Provision may be made for the two working ports to each have a center of area, wherein the two centers of area define a straight reference line, wherein the straight reference line intersects the first rotary bearing. This arrangement of the working ports results in particularly streamlined working channels, wherein, in particular, the cavitation tendency at the suction port is low. 
     Provision may be made for the two working channels to be of mirror-symmetrical form with respect to one another, wherein the corresponding plane of symmetry encompasses the axis of rotation. The axial piston machine can thus be used in 4-quadrant operation, wherein similar delivery characteristics are realized in each of the four correspondingly possible operating states. 
     Provision may be made for the cylinder drum to have multiple cylinders which are of identical form to one another and which are arranged in uniformly distributed fashion about the axis of rotation, wherein each cylinder has a circular cylindrical section with a first cross-sectional area, wherein each cylinder has, in the region of the sliding surface, a mouth opening with a second cross-sectional area, wherein the second cross-sectional area is smaller than the first cross-sectional area, wherein the hydrostatic force that results from this during operation forces the cylinder drum against the sliding surface, wherein the cylinder drum is otherwise forced against the sliding surface exclusively by a single spring, wherein the spring lies against the end side of the cylinder drum. Accordingly, no spring is arranged radially between the cylinder drum and the drive shaft. The mouth openings and the first and the second sections can consequently be relocated inward to a very great extent. This results in a relatively large difference between the first and second cross-sectional area, which results in an intense hydrostatic contact pressure of the cylinder drum against the distributor plate. The second cross-sectional area is preferably between 40% and 70% of the first cross-sectional area. The claimed spring, which can be configured to be only relatively weak, is therefore sufficient for pressing the cylinder drum against the distributor plate. The spring is preferably supported on a separate pressure-exerting part with a spherical surface, wherein the spherical surface is in turn supported on a retraction plate, which in turn is supported on the slide shoes of the pistons. 
     Provision may be made for the reference cylinder to intersect the circular cylindrical section of the cylinder, wherein the mouth openings are arranged entirely within, or are intersected by, the reference cylinder. The circular cylindrical sections of the cylinders are preferably arranged radially further to the outside than the respective mouth openings. 
     Provision may be made for the mouth openings to each be defined by a mouth channel with a constant cross-sectional shape, wherein the mouth channels are arranged so as to be inclined with respect to the axis of rotation such that they open out in each case in a corner region of the associated circular cylindrical section, in which the circular cylindrical section transitions into a base of the cylinder. The mouth channels accordingly have a small inclination relative to the axis of rotation. The change in direction of the fluid flow in the region of the mouth openings is consequently small, whereby the cavitation tendency is reduced. 
     Provision may be made for the second rotary bearing to comprise an inner ring, an outer ring and multiple rolling bodies, wherein all of the parts are carbonitrided. The first and the second rotary bearing are arranged far apart from one another in relation to a conventional axial piston machine. At the same time, the diameter of the drive shaft is relatively thin, because the first sections are arranged very far to the inside. This results in a relatively high degree of bending of the drive shaft. This has the effect, in particular, that the first rotary bearing, which is preferably designed as a tapered-roller bearing, is subjected to high load. By means of the proposed carbonitriding, the second rotary bearing nevertheless achieves the desired service life. 
     Provision may be made for the housing to comprise a first and a second housing part, wherein the first housing part is of pot-shaped form, wherein the first housing part defines an opening, wherein the opening is completely covered by the second housing part, wherein the at least one working channel is, outside the distributor plate, delimited entirely by the second housing part, wherein the first rotary bearing is accommodated in the second housing part. The second rotary bearing is preferably accommodated in the first housing part. The first and the second housing part may be sealed off against one another by means of a sealing ring or by means of a flat seal. 
     Provision may be made for the cylinder drum to have a rotational drive connection to the drive shaft by means of a spline toothing, wherein a circular cylindrical inner circumferential surface of the distributor plate is arranged approximately in alignment with a root circle diameter of the spline toothing of the drive shaft. The diameter of the inner circumferential surface is preferably between 95% and 110% of the root circle diameter of the spline toothing of the drive shaft. 
     It is self-evident that the features mentioned above and the features yet to be discussed below may be used not only in the respectively specified combination but also in other combinations or individually without departing from the scope of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure will be discussed in more detail below on the basis of the appended drawings, in which: 
         FIG. 1  shows a longitudinal section through an axial piston machine according to the disclosure; and 
         FIG. 2  shows an enlarged detail of  FIG. 1  in the region of the distributor plate. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a longitudinal section through an axial piston machine  10  according to the disclosure. The axial piston machine  10  comprises a housing  20  which is made up of a first and a separate second housing part  21 ;  22 . The first housing part  21  is of pot-shaped form, such that it has an opening, which points to the right in  FIG. 1 . The opening is completely covered by the second housing part  22 . The first and the second housing part  21 ;  22  lie against one another at a planar sealing surface, wherein a seal  24  is arranged there, which seal closes off the housing  20  in fluid-tight fashion. The seal  24  may be designed as an O-ring or as a flat seal. 
     A second rotary bearing  32  is accommodated on the base of the first housing part  21 , wherein a first rotary bearing  31  is accommodated in the second housing part  22 . The first and the second rotary bearing  31 ;  32  are designed in the present case as tapered-roller bearings, which are installed in an X arrangement. The rotary bearings support a drive shaft  30 , rotatably about an axis of rotation  11 , on the housing  20 . The drive shaft  30  is surrounded by a separate cylinder drum  40 , wherein the drive shaft  30  and the cylinder drum  40  have a rotational drive connection by means of a spline toothing  34 . Here, the spline toothing  34  on the cylinder drum  40  is shorter than the spline toothing  34  on the drive shaft  30 . No spring is arranged between the cylinder drum  40  and the drive shaft  30 , in order that the mouth openings (number  43  in  FIG. 2 ) can be arranged as far to the inside as possible so as to allow high drive rotational speeds. The spring  44  is instead arranged at an end side on the cylinder drum  42 , wherein the spring is supported on a separate pressure-exerting part  84 . The pressure-exerting part  84  likewise engages into the spline toothing of the drive shaft  30  so as to rotate conjointly with the latter. The pressure-exerting part has a spherical surface, against which a separate retraction plate  83  is supported in the direction of the axis of rotation  11 . The retraction plate  83  is therefore pivotable relative to the drive shaft  30 , wherein the retraction plate follows the pivoting movement of the pivot cradle  80 . 
     In the present case, the drive shaft  30  projects with a drive journal  35  out of the housing  20  at the first housing part  21 . Drive journals or similar drive means may however also be provided at both sides of the housing  20  or only at the opposite side of the housing  20 . 
     Multiple, for example seven or nine, cylinders  41  are arranged in the cylinder drum  40  so as to be distributed uniformly about the axis of rotation  11 . The cylinders  41  have a circular cylindrical section  42 , which in the present case is formed by a separate slide bushing that is fixedly installed in the cylinder drum  40 . The circular cylindrical section  42  may however also be formed directly by the cylinder drum  40 . In each case one associated piston  81  is received in linearly movable fashion in the circular cylindrical section  42 , so as to form a cylinder chamber with variable volume. Each cylinder chamber has a mouth opening (number  43  in  FIG. 2 ) via which the cylinder chamber has a fluid-exchanging connection to in each case one of the two working channels  60  in a manner dependent on the rotational position of the cylinder drum  40 . In the context of the disclosure, the mouth openings (number  43  in  FIG. 2 ) should be arranged as close as possible to the axis of rotation  11  in order that only relatively low centrifugal forces act on the pressure fluid there. The cylinder drum  40  can consequently rotate at a high rotational speed without cavitation occurring at the suction side. The pressure fluid is preferably a liquid, and most preferably hydraulic oil. 
     That end of each piston  81  which projects out of the cylinder drum  40  is connected by means of a ball joint to a separate slide shoe  82 , which is supported on a planar control surface of the pivot cradle  80 . In the unpressurized state in particular, the slide shoes  82  are pushed against the pivot cradle  80  by the spring  40  via the retraction plate  83 . The corresponding opposing force pushes the cylinder drum  40  against the distributor plate  50 , and this in turn against the second housing part  22 . In the context of the disclosure, this force is relatively low, in particular in relation to axial piston machines that have a further spring between the cylinder drum  40  and the drive shaft  30 . The pivot cradle  80  is pivotable about a pivot axis that is arranged perpendicular to the axis of rotation  11 . In the present case, the pivot axis intersects the axis of rotation  11 , wherein the pivot axis may also be arranged so as to be offset somewhat with respect to the axis of rotation  11 . The pivot cradle  80  can be adjusted for example by means of a pivot cylinder (not illustrated) in order to adjust the displacement volume of the axial piston machine  10 . 
     A separate distributor plate  50  is arranged between the cylinder drum  40  and the second housing part  22 . In the case of conventional axial piston machines, the distributor plate is held transversely with respect to the axis of rotation  11  by the outer ring of the first rotary bearing  31 . In the context of the disclosure, the first rotary bearing  31  is arranged with a spacing to the distributor plate  50  in the direction of the axis of rotation  11 , such that the two working channels  60  can in this region be brought very close to the axis of rotation  11 . 
     The second housing part  22  has a substantially planar support surface  23  through which the two working channels  60  extend, wherein the support surface is oriented perpendicular to the axis of rotation  11 . The support surface  23  is provided with a retaining projection (number  24  in  FIG. 2 ) of circular cylindrical form about the axis of rotation  11 , which retaining projection holds the distributor plate  50  on the housing transversely with respect to the axis of rotation  11 . Furthermore, the distributor plate  50  is secured against rotation relative to the housing  20 , for example by means of a cylindrical pin (not illustrated). The distributor plate  50  is thus positionally fixed relative to the housing  20 . 
     A relative movement between the cylinder drum  40  and the distributor plate  50  occurs at the sliding surface  51  of the distributor plate  50 . The sliding surface  51  is rotationally symmetrical with respect to the axis of rotation  11  so as to allow a rotation of the cylinder drum  40 . The sliding surface is furthermore concavely curved so as to hold the cylinder drum  40  transversely with respect to the axis of rotation  11 . A planar sliding surface is likewise conceivable. In the present case, the rotational support of the cylinder drum  40  in the axial and radial directions is realized exclusively by way of the sliding surface  51 . The first and the second rotary bearing  31 ;  32  alone support the drive shaft  30 . The spline toothing  34  is designed such that substantially only a torque about the axis of rotation  11  can be transmitted. 
     As already discussed, the mouth openings (number  43  in  FIG. 2 ) of the cylinders  41  should be brought as close as possible to the axis of rotation  11  in order that the cylinder drum  40  can be operated with a high rotational speed. This basic principle is known from DE 10 2015 208 925 A1. In the context of the disclosure, it is the intention for the working channels  60  to be improved such that the rotational speed of the cylinder drum  40  can be increased yet further without the suction separation discussed above occurring at the suction side, and with no cavitation occurring in the region of the mouth openings (number  43  in  FIG. 2 ). 
     The inventors have recognized that, for this purpose, it is advantageous if the fluid flow is diverted as little as possible in the region of the mouth openings (number  43  in  FIG. 2 ). The mouth channels  45  that define the mouth openings (number  43  in  FIG. 2 ) are therefore arranged with a shallow inclination with respect to the axis of rotation  11 . The mouth channels therefore open out in a corner region of the respective cylinder  41 , in which the circular cylindrical section  42  transitions into a base of the cylinder  41 . In the case of a conventional axial piston machine, the mouth channel opens out entirely at the base of the cylinder. 
     The two working channels  60  are mirror-symmetrical with respect to a plane of symmetry that encompasses the axis of rotation  11 . The working channels have in each case one first, one second and one third section (numbers  61 ;  62 ;  63  in  FIG. 2 ). The first section (number  61  in  FIG. 2 ) runs entirely in the distributor plate  50 . In the present case, the distributor plate has a single aperture for each working channel  50 , which aperture runs with a constant cross-sectional shape parallel to the axis of rotation  11 . The aperture is of kidney-shaped form. It can also be stated that the aperture is designed in the form of a slot that runs with a circular curvature about the axis of rotation  11 . The two apertures are of identical design, because it is the intention for the present axial piston machine to be capable of 4-quadrant operation. That is to say, it is the intention for the direction of rotation of the drive shaft  30  to be reversible, wherein it is furthermore the intention for both working ports  64  to be operable selectively as a suction port or as a pressure port. 
     The second section (number  62  in  FIG. 2 ) directly adjoins the first section (number  61  in  FIG. 2 ), wherein the second section runs in the housing  20 , specifically in the second housing part  22 . The second section (number  62  in  FIG. 2 ) likewise runs parallel to the axis of rotation  11 . The second section has a constant cross-sectional shape which forms an aligned continuation of the cross-sectional shape of the first section (number  61  in  FIG. 2 ). At the transition between the first and the second section (numbers  61 ;  62  in  FIG. 2 ), there is accordingly no step and no bend that could influence the fluid flow. The first and the second section (numbers  61 ;  62  in  FIG. 2 ), considered together, are of such a length that a substantially turbulence-free flow running parallel to the axis of rotation  11  can form, specifically in particular at the suction side. This flow is subjected to only a minimal diversion at the mouth opening (number  43  in  FIG. 2 ). This disruption of the fluid flow is small in relation to the disruption caused by the rotating cylinder drum  40 . Accordingly, despite the centrifugal forces acting in the mouth channel  45 , the occurrence of cavitation is substantially avoided, even if the cylinder drum  40  is rotating at very high speed. 
     The third section  63  of the working channel  60  directly adjoins the second section (number  62  in  FIG. 2 ), wherein the third section runs in the housing  20 . The third section opens out at the outer side of the housing  20  at a circular working port  64 . The third section  63  firstly has the task of changing the circular shape of the working port into the kidney shape of the respective mouth opening (number  43  in  FIG. 2 ) without turbulence being generated in the fluid flow. Furthermore, the working channel  60  must be led past the first rotary bearing  31 . The inherently optimum arrangement of the working ports in terms of flow, in the direction of the axis of rotation  11  and in alignment with the mouth openings (number  43  in  FIG. 2 ), is obstructed by the first rotary bearing  31 . Instead, the working ports  64  are arranged on opposite sides of the housing  20 , in particular of the second housing part  22 . The centers of area of the two working ports  64 , specifically the corresponding circle central points, define a straight reference line  68  that intersects the first rotary bearing  31 . The resulting curvature of the third section  63  yields particularly favorable flow conditions. Firstly, the cross-sectional central point  65  of the working channel  64  runs along a path with a uniform and smooth curvature. There are preferably no steps in the profile of the radius of curvature. Furthermore, there is a resulting characteristic profile of the inner delimiting surface  67  of the third section  63 . Proceeding from the working port  64 , the inner delimiting surface is initially concavely curved, and is convexly curved in the further profile towards the mouth opening. Specifically this profile contributes significantly to the formation of a substantially turbulence-free fluid flow, running parallel to the axis of rotation, in the first and in the second section (numbers  61 ;  62  in  FIG. 2 ). It would basically be conceivable for the working ports  64  to be relocated to the right in  FIG. 1 , wherein the curvature reversal discussed above would be omitted. Tests carried out by the applicant have however shown that an axial piston machine of such design allows only lower rotational speeds than the axial piston machine  10  shown in  FIG. 1 . 
       FIG. 2  shows an enlarged detail of  FIG. 1  in the region of the distributor plate  50 . Here, it is firstly possible to see the retaining projection  24  on the support surface  23 , by means of which retaining projection the distributor plate  50  is held transversely with respect to the axis of rotation. It is also possible to see the profile of the first and of the second section  61 ;  62  of the working channel  60  parallel to the axis of rotation. Also shown in  FIG. 2  is the smallest spacing  67  between the working channel  60  and the first rotary bearing  31 , the smallest spacing being arranged in the third section  63 . 
     In  FIG. 2 , it is also possible to see the root circle diameter  36  of the spline toothing  34  of the drive shaft  30 . The root circle diameter is situated approximately in alignment with the inner circumferential surface  52  of the distributor plate  50 . With this arrangement, mouth openings  43  can be brought very close to the axis of rotation, without the drive shaft being excessively weakened. 
     REFERENCE DESIGNATIONS 
     
         
           10  Axial piston machine 
           11  Axis of rotation 
           20  Housing 
           21  First housing part 
           22  Second housing part 
           23  Support surface 
           24  Retaining projection 
           24  Seal 
           32  Drive shaft 
           31  First rotary bearing 
           32  Second rotary bearing 
           33  Outer circumferential surface of the first rotary bearing 
           34  Spline toothing 
           35  Drive journal 
           36  Root circle diameter of the spline toothing 
           40  Cylinder drum 
           41  Cylinder 
           42  Circular cylindrical section 
           43  Mouth opening 
           44  Spring 
           45  Mouth channel 
           50  Distributor plate 
           51  Sliding surface 
           52  Inner circumferential surface 
           60  Working channel 
           61  First section of the working channel 
           62  Second section of the working channel 
           63  Third section of the working channel 
           64  Working port 
           65  Cross-sectional central point of the working channel 
           66  Smallest spacing between the working channel and the first rotary bearing 
           67  Inner delimiting surface of the third section 
           68  Straight reference line 
           80  Pivot cradle 
           81  Piston 
           82  Slide shoe 
           83  Retraction plate 
           84  Pressure-exerting part