Patent Publication Number: US-2020277930-A1

Title: A hydraulic device and a pivot joint

Description:
The present invention relates to a hydraulic device comprising a housing, a shaft which is mounted in the housing and rotatable with respect to the housing about a first axis of rotation, wherein the shaft has a flange extending transversely to the first axis of rotation, and a partly spherical portion including a centre point remote from the flange, a plurality of pistons including respective piston centrelines which have components extending parallel to the first axis of rotation, which pistons are fixed to the flange at equiangular distance about the first axis of rotation, a plurality of cylindrical sleeves, wherein each sleeve has a sleeve bottom comprising a sleeve opening including a centreline, which sleeves cooperate with the pistons to form respective compression chambers of variable volume, a barrel plate which is mounted on the partly spherical portion of the shaft and which has barrel plate ports including respective centrelines, wherein the barrel plate has a first side which supports the sleeve bottoms and an opposite second side which is supported by a face plate that is mounted in the housing and angled with respect to the first axis such that the sleeves are rotatable about a second axis of rotation which intersects the first axis of rotation at the centre point by an acute swash angle such that during rotating the shaft the volumes of the compression chambers change, wherein the barrel plate is coupled to the shaft in rotational direction thereof by means of a pin-groove coupling where a groove is located in one of said partly spherical portion and the barrel plate and a pin is located in the other one of the partly spherical portion and the barrel plate, wherein the groove has a longitudinal direction extending in the same direction as one of the first and second axis of rotation and the pin is received by said groove, hence creating a pivot axis of the barrel plate which extends perpendicular to the second axis and runs through the centre point, wherein under operating conditions each compression chamber alternatingly communicates via the corresponding sleeve opening and barrel plate port with ports in the face plate. 
     A hydraulic device as described above is known from WO 2006/083163. The known device has a shaft including a partly spherical portion on which a drum plate can swivel. The drum plate has a keyway which interacts with a key in the shaft at the partly spherical portion thereof. Consequently the drum plate rotates together with the shaft about the second axis of rotation. The resulting pivot axis of the drum plate rotates together with the shaft about the second axis of rotation. The key reciprocatingly travels in longitudinal direction of the keyway. The shaft drives the sleeves in rotational direction about the first axis of rotation via the pistons. Because of the key and keyway coupling the rotational speed of the drum plate varies when the shaft rotates at a constant speed or the rotational speed of the shaft varies when the drum plate rotates at a constant speed. Consequently, the relative positions of the sleeves and the drum plate vary in rotational direction about the second axis of rotation under operating conditions. This also means that the positions of the centrelines of the respective sleeve openings and the corresponding centrelines of the respective drum plate ports vary, resulting in varying pressure fields between the drum plate and the sleeves causing a tipping torque on the drum plate. 
     It is desired to minimize tipping torque since it might lead to increased friction between the drum plate and the face plate and under severe conditions this may even cause accelerated wear of the drum plate and the face plate. Besides, minimizing tipping torque means that the maximum allowable speed of the hydraulic device can be higher than in conventional hydraulic devices. 
     An object of the invention is to provide a hydraulic device which minimizes tipping torque. 
     This object is accomplished with the device according to the invention, which is characterized in that it comprises a plurality of said pin-groove couplings creating a plurality of pivot axes at equiangular distance about the second axis of rotation, wherein the widths of the grooves allow the pins to move within the respective corresponding grooves in rotational direction about the second axis of rotation under operating conditions, wherein the relative position of the shaft and the barrel plate in rotational direction about the second axis of rotation is adapted such that under operating conditions each centreline of the respective barrel plate ports fluctuates in rotational direction about the second axis of rotation with respect to the centreline of the corresponding sleeve opening within a range in which the centreline of the sleeve opening lies. 
     An increasing number of pivot axes approaches a homokinetic joint and leads to a reduction of the amplitude of the fluctuation of the barrel plate and the sleeves with respect to each other. Since the relative position of the shaft and the barrel plate is adapted, as well, the hydraulic device according to the invention minimizes tipping torque of the barrel plate, which allows a relatively high maximum speed of the hydraulic device. 
     The combination of an increasing number of pivot axes and adapting the relative position of the shaft and the barrel plate as described above is not obvious when considering the modifications of an existing hydraulic device including a single pin-groove coupling by applying additional pin-groove couplings, which will be explained below. 
     In case of a single pin-groove coupling the pin and a groove edge contact each other at a contact location. When the shaft is driven at a constant speed the contact location travels reciprocatingly with respect to the shaft about the centre point of the partly spherical portion in longitudinal and lateral direction of the groove and in radial direction of the second axis of rotation. Consequently, the barrel plate position fluctuates with respect to the shaft in rotational direction about the second axis of rotation, whereas the constant rotational speed of the shaft causes a constant speed of the sleeves about the second axis of rotation. This results in varying relative rotational positions of the barrel plate ports and the sleeve openings. In this case the single pin-groove coupling has a single contact location which is maintained during an entire revolution of the shaft. 
     In case of a plurality of pin-groove couplings that create a plurality of pivot axes the respective pin-groove couplings alternatingly form contact locations during an entire revolution of the shaft. Since the pivot axes are located at angular distance about the second axis of rotation the relative positions of the pin and groove of the respective pin-groove couplings are different. If each of the pin-groove couplings maintained a contact location during an entire revolution there would be a kinematic conflict because of the fluctuations of each contact location with respect to the shaft in rotational direction about the second axis of rotation as described above. The kinematic conflict is avoided in the device according to the invention, since the widths of the grooves allow the pins to move within the respective corresponding grooves in rotational direction about the second axis of rotation under operating conditions. Because of the freedom of movement of each pin in the corresponding groove, each groove must be wider than the diameter of the cooperating pin. 
     During rotation of the shaft at a constant speed there is still a fluctuation of the actual contact location with respect to the shaft at one of the pin-groove couplings, but after a certain angle of rotation a next pin-groove coupling corresponding to a different pivot axis will form a new contact location whereas the contact at the former pin-groove coupling will be lost. Although there are still fluctuations within each contact period of the successive contact locations, each of the fluctuations has a smaller amplitude in rotational direction about the second axis of rotation than in case of a single pivot axis. In other words, the positions of one of the pin and groove of each pin-groove coupling that corresponds to a pivot axis will fluctuate in rotational direction about the second axis of rotation by a certain amplitude with respect to the shaft, but the amplitude of each of the fluctuating contact locations is smaller. Similarly, the amplitude of fluctuation of the barrel plate with respect to the shaft about the second axis of rotation is smaller than the amplitude of the relative movement of the individual pins and cooperating grooves. 
     The alternating contact locations introduce a phase shift with respect to a device including a single pivot axis, since each time when one of the pin and groove of a pin-groove coupling tends to delay the barrel plate with respect to the shaft, a next pin-groove coupling takes over the driving action of the shaft. As a consequence, each of the contact locations will fluctuate with respect to the shaft in rotational direction about the second axis of rotation about a centre which is remote from the centre of fluctuation in case of a single pivot axis. 
     The mentioned phase shift is solved in the device according to the invention by introducing an offset with respect to a device including a single pivot axis, which offset is between the barrel plate and the shaft in rotational direction about the second axis of rotation. Therefore, the relative position of the shaft and the barrel plate in rotational direction about the second axis of rotation is adapted such that under operating conditions the centrelines of the barrel plate ports fluctuate in rotational direction about the second axis of rotation with respect to the centrelines of the corresponding sleeve openings within a range in which the centrelines of the sleeve openings lie. 
     It is noted that a device which is provided with only two pin-groove couplings that are located with respect to each other at an angular distance of 180° about the second axis of rotation is not covered by the claims since the two pin-groove couplings form a single pivot axis similar to a device including only a single pin-groove coupling. 
     In a preferred embodiment the centreline of each of the barrel plate ports fluctuates about a centreline which substantially coincides with the centreline of the corresponding sleeve opening, since this provides a symmetrical fluctuation and minimizes tipping torque. 
     In a specific embodiment each of the grooves has parallel groove edges. In this case the distance between the groove edges must be wider than the diameter of the cooperating pin in order to allow the pin and groove to move with respect to each other under operating conditions as described above. This embodiment is preferably used for applications in which a hydraulic device is operated in one rotational direction, since rotating the shaft in opposite direction would introduce a phase shift as described above. 
     In an alternative embodiment each of the grooves has a midpoint from which the groove width increases in opposite directions in longitudinal direction of the groove. This embodiment provides the opportunity to allow the respective contact locations between the pins and the cooperating grooves of the respective pin-groove couplings to follow a path along the respective groove edges which is substantially the same or close to the path which is followed by one of the pin and groove during the corresponding contact period. This provides the opportunity to adapt the groove edge profile such that the above-mentioned phase shift in case of a plurality of pivot axes is compensated. Besides, this embodiment provides the opportunity to create mirrored profiles of the groove edges such that the same effect is achieved in opposite rotational directions of the shaft, hence minimizing any phase shift when switching the rotational direction. The relative positions of the shaft and the barrel plate in rotational direction about the second axis of rotation may be adapted such that when one of the pins is located in the midpoint of the cooperating groove the centreline of each barrel plate port substantially coincides with the centreline of the corresponding sleeve opening. 
     At least at the midpoint each of the groove edges may be angled with respect to the longitudinal centreline of the groove by a half of the swash angle. If the swash angle is variable each of the groove edges may be angled by a half of the maximum swash angle. Increasing the swash angle above this maximum would lead to a kinematic conflict as described above. Decreasing the swash angle introduces a slight phase shift between the shaft and the barrel plate. 
     Preferably, the pins accurately fit in the respective grooves at their midpoints so as to minimize any play in rotational direction of the shaft. This minimizes any phase shift upon reversing the rotational direction of the shaft. The widths of the grooves at their midpoints and the dimensions of the respective pins can be matched to each other within allowable tolerance requirements. 
     The groove edges of each groove may be substantially straight from the midpoint to at least a location where the pins and groove edges loose contact under operating conditions. A straight groove edge is advantageous in terms of manufacturing effort. 
     In a preferred embodiment the width of the groove outside a repetitive contact area between the pin and the groove edge is increased, at least by a slope which is larger than any slope within the contact area, since this allows the pins which are temporarily out of contact with the groove edge to move into the widened portion of the groove. This means that the width of a central portion of each groove where the cooperating pin temporarily contacts the groove edge may be relatively narrow which minimizes any phase shift if the shaft is rotated in opposite directions, even in case of parallel groove edges. 
     In a practical embodiment the pins of the respective pin-groove couplings are fixed in the shaft and the grooves are located in the barrel plate. The pins may be cylindrical including a circular cross-section. 
     The invention is also related to a pivot joint comprising a support, a first shaft which is mounted to the support and rotatable with respect to the support about a first axis of rotation, wherein the first shaft has a partly spherical portion including a centre point, a second shaft which is mounted to the support, on the one hand, and to the partly spherical portion of the first shaft, on the other hand, such that the second shaft is rotatable with respect to the support about a second axis of rotation which intersects the first axis of rotation at the centre point by an acute angle, wherein the second shaft is coupled to the first shaft in rotational direction thereof by means of a plurality of pin-groove couplings, wherein each pin-groove coupling comprises a groove which is located in one of the partly spherical portion and the second shaft and a pin which is located in the other one of the partly spherical portion and the second shaft, wherein the groove has a longitudinal direction extending in the same direction as one of the first and second axis of rotation and the pin is received by the groove, wherein the pin-groove couplings are arranged such that a plurality of pivot axes about which the first and second shafts pivot with respect to each other are created at equiangular distance about one of the first and second axes of rotation, wherein each pivot axis extends perpendicular to the one of the first and second axes of rotation and runs through the centre point, wherein each of the grooves has a midpoint from which the groove width increases in opposite directions in longitudinal direction of the groove, wherein preferably the pins are cylindrical and have a circular cross-section. 
     In a particular embodiment at least at the midpoint opposite groove edges are mirrored with respect to a longitudinal centreline of the groove and angled with respect to each other, preferably by the acute angle, and preferably by a predefined maximum acute angle if the acute angle is variable. In this embodiment the groove has a waisted shape as seen in plan view, wherein the narrowest width is located at the midpoint. The opposite groove edges extending from the narrowest width of the groove may be straight. 
     The pins preferably fit accurately in the respective grooves at their midpoints. 
     The width of the groove outside a repetitive contact area between the pin and the groove edge may be increased, at least by a slope which is larger than any slope within the contact area. The length of the repetitive contact area can be defined by 2*R*sin(n/k)*sin(acute angle), wherein k equals the number of pivot axes. 
    
    
     
       The invention will hereafter be elucidated with reference to very schematic drawings showing embodiments of the invention by way of example. 
         FIG. 1  is a cross-sectional view of an embodiment of a hydraulic device according to the invention. 
         FIG. 2  is a perspective view of a barrel plate of the embodiment as shown in  FIG. 1 . 
         FIG. 3  is a cross-sectional view along the line III-III in  FIG. 1 , illustrating only one sleeve that is placed at one cooperating barrel plate port and without showing the pistons. 
         FIG. 4  is an illustrative enlarged view of a pin-groove coupling of the embodiment as shown in  FIG. 1 . 
         FIG. 5  is an illustrative view of a path that is followed by a pin of a pin-groove coupling of  FIG. 4  on a large scale. 
         FIG. 6  is a similar view as  FIG. 4  of another embodiment. 
         FIG. 7  is a similar view as  FIG. 4  of still another embodiment. 
         FIGS. 8 and 9  are perspective views of pins of alternative pin-groove couplings. 
         FIG. 10  is a cross-sectional view of an embodiment of a pivot joint according to the invention. 
         FIG. 11  is an enlarged sectional view of a part of the second shaft of the embodiment of  FIG. 10 , showing one of the grooves. 
     
    
    
       FIG. 1  shows internal parts of a hydraulic device  1 , such as a pump or hydromotor, which are fitted into a housing  2  in a known manner. The hydraulic device  1  is provided with a shaft  3  which is supported by bearings at both sides of the housing  2  and it is rotatable about a first axis of rotation  4 . The housing  2  is provided on the one side with an opening with a shaft seal  5  in a known manner, as a result of which the end of the shaft  3 , which is provided with a toothed shaft end  6 , protrudes from the housing  2 . A motor can be coupled to the toothed shaft end  6  if the hydraulic device  1  is a pump, and a driven tool can be coupled thereto if the hydraulic device  1  is a motor. 
     The hydraulic device  1  comprises face plates  7  which are mounted inside the housing  2  at a distance from each other. In the embodiment as shown the face plates  7  have fixed positions with respect to the housing  2 . The shaft  3  extends through central through-holes in the face plates  7 . 
     The shaft  3  is provided with a flange  8  which extends perpendicularly to the first axis of rotation  4 . A plurality of pistons  9  are fixed at both sides of the flange  8  at equiangular distance about the first axis of rotation  4 , in this case fourteen pistons  9  on either side.  FIG. 1  shows that each piston  9  is fixed to the flange  8  by means of a piston pin  10  which is pressed into the flange  8 . The pistons  9  have centrelines which extend parallel to the first axis of rotation  4 . The planes of the face plates  7  are angled with respect to each other and with respect to the plane of the flange  8 . 
     Each of the pistons  9  cooperates with a cylindrical sleeve  11  to form a compression chamber  12  of variable volume. The hydraulic device  1  as shown in  FIG. 1  has 28 compression chambers  12 . Each cylindrical sleeve  11  comprises a sleeve bottom  13  including a sleeve opening  14 . Each piston  9  is sealed directly to the inner wall of the sleeve  11  through a ball-shaped piston head. 
     The sleeve bottoms  13  are supported by two barrel plates  15  which are fitted around the shaft  3  at respective partly spherical portions  16  thereof. The spherical portions  16  have respective centre points CP which are located remote from the flange  8 . The barrel plates  15  are coupled to the shaft  3  by means of pin-groove couplings. Each coupling comprises a pin  17  which is fixed in the shaft  3  and a groove  18  in the barrel plates  15  in which the pin  17  is received. Each pin  17  is cylindrical and has a circular cross-section. Each of the grooves  18  has a longitudinal direction which extends perpendicular to the plane of the corresponding face plate  7 . In this embodiment each of the grooves  18  has parallel groove edges. Each pin  17  and cooperating groove  18  together with the corresponding spherical portion  16  form a pivot joint. The pin-groove couplings serve to rotate the barrel plates  15  together with the shaft  2  under operating conditions. 
     The barrel plates  15  rotate about respective second axes of rotation  19  which are angled with respect to the first axis of rotation  4  by respective swash angles. The second axes of rotation  19  intersect the first axis of rotation  4  at the respective centre points CP. This means that the cylindrical sleeves  11  also rotate about the respective second axes of rotation  19 . As a consequence, upon rotating the shaft  2  the volumes of the compression chambers  12  change. During rotation of the barrel plates  15  each cylindrical sleeve  11  makes a combined translating and swivelling motion around the cooperating piston  9 . Therefore, the outer side of each piston head is ball-shaped. The ball-shape creates a sealing line between the piston  9  and the cylindrical sleeve  11  which extends perpendicularly to the centreline of the cooperating cylindrical sleeve  11 . The pistons  9  are conical and their outer diameters outside the respective piston heads decrease towards the flange  8  in order to allow the relative motion of the cooperating cylindrical sleeves  11  about the pistons  9 . 
     The sides of the respective barrel plates  15  which are directed away from the flange  8  are supported by the face plates  7 . Due to the inclined orientation of the face plates  7  with respect to the flange  8  the barrel plates  15  pivot about the partly spherical portions  16  during rotation with the shaft  3 . 
     The barrel plates  15  are pressed against the respective face plates  7  by means of springs  20  which are mounted in holes in the shaft  3 . The compression chambers  12  communicate via the sleeve openings  14  in the respective sleeve bottoms  13  with cooperating barrel plate ports  21  in the barrel plates  15 . Each of the barrel plate ports  21  has a centreline  22  extending parallel to the second axis of rotation  19  and running through a centre of the barrel plate port  21  which centre lies in rotational direction about the second axis of rotation  19 . Each of the sleeve openings  14  has a centreline  23 . It is desired to keep the centreline  22  of the barrel plate port  21  and the centreline  23  of the corresponding sleeve opening  14  as close as possible to each other under operating conditions in order to keep the pressure fields between the barrel plate  15  and the sleeves  11  aligned as much as possible. 
     The barrel plate ports  21  alternatingly communicate via face plate ports in the face plates  7  with a high-pressure port and a low-pressure port in the housing  2 . 
       FIG. 2  shows the barrel plate  15  as a separate part and  FIG. 3  shows a part of the barrel plate  15  on which one sleeve  11  is placed.  FIG. 3  shows three pin-groove couplings. The pins  17  are located at equiangular distance about the first axis of rotation  4  and the grooves  18  are located at equiangular distance about the second axis of rotation  19 . This configuration results in three pivot axes  24  which are located at an equiangular distance from each other about the second axis  19  and intersect each other at the centre point CP. It is also possible to create the three pivot axes  24  by six pin-groove couplings, wherein each coupling has a mirrored coupling located at an angular distance of 180° about the second axis of rotation  19 . 
       FIG. 3  shows a condition in which the centrelines  22 ,  23  of the barrel plate port  21  and the sleeve opening  14  coincide. However, under operating conditions the rotational speed of the barrel plate  15  and the shaft  3  vary with respect to each other during each revolution of the shaft  3 . The extent of variation depends on the selected swash angle. Since the pins  17  rotate about the first axis of rotation  4  and the grooves  18  rotate about the second axis of rotation  19  they have a relative motion with respect to each other during each revolution of the shaft  3 . When rotating the shaft  3  at a constant speed each pin  17  moves in three directions with respect to its corresponding groove  18 : in longitudinal, depth and lateral direction of the groove  18 , whereas each of the three pins  17  contacts a groove edge of its corresponding groove  18  during a part of each revolution. During such a contact period the barrel plate  15  does not rotate at a constant speed when the shaft  3  rotates at a constant speed. As a consequence, the centrelines  22 ,  23  of each barrel plate port  21  and the corresponding sleeve opening  14  fluctuate with respect to each other in rotational direction about the second axis of rotation  19 . 
     According to the invention the relative position of the shaft  3  and the barrel plate  15  in rotational direction about the second axis of rotation  19  is adapted such that under operating conditions the centreline  22  of each barrel plate port  21  fluctuates in rotational direction about the second axis of rotation  19  within a range in which the centreline  23  of the corresponding sleeve opening  14  lies. In order to achieve this condition the widths of the grooves must be such that they allow the respective pins  17  to move within the corresponding grooves  18  in rotational direction about the second axis of rotation  19  under operating conditions. 
     The relative movements of the three pins  17  and the corresponding grooves  18  will be illustrated by means of  FIGS. 4 and 5 .  FIG. 4  shows three pins  17  in three respective grooves  18  which have an angular distance of 120° with respect to each other, but which are superimposed in  FIG. 4  for explanatory reasons. For that reason, it seems that the three pins  17  are located in a single groove  18 , but it should be noted that a single pin  17  is received by a single groove  18 . The width of each groove  18  is shown in vertical direction and the longitudinal direction of each groove  18  is shown in horizontal direction in  FIG. 4 . A lemniscate line describes a path which is followed by the centrelines of each of the pins  17  in their respective grooves  18  when the shaft  3  rotate at a constant speed.  FIG. 4  shows the actual positions of the pins  17  in their respective grooves  18 . The respective pins  17  follow the same lemniscate path during each revolution, but at mutual phase shifts. Because of the relative movement of the pins within the grooves  18  in lateral direction thereof, the width of each groove  18  must be larger than the diameter of the pin  17 . 
       FIG. 4  shows that one of the pins  17  contacts a groove edge at a contact location CL when the shaft  3  including the pins  17  is driven in one direction which is indicated by the arrow in  FIG. 4 . During rotation of the shaft  3  the contact location CL will shift along the groove edge and at a certain moment the contact between that pin  17  and the groove edge will be lost and another pin  17  in another corresponding groove  18  will take it over and contact its corresponding groove edge. During a period of contact between one pin  17  and its corresponding groove edge the contact location CL will not only travel along the groove edge, but also rotate with respect to the shaft  3  about the second axis of rotation  19  by an angle which varies with respect to the angle about which the shaft  3  rotates about the first axis of rotation  4  within the same period. As a result the centreline  22  of each of the barrel plate ports  21  also varies with respect to the centreline  23  of each of the corresponding sleeve openings  14 . Consequently, pressure fields of the sleeves  11  and the barrel ports  21  are temporarily not in line causing barrel tipping torque load. 
     The fluctuations can be explained further by means of  FIG. 5 , which shows the lemniscate path of one of the pins  17  on a larger scale. The lemniscate path is depicted with respect to an imaginary radial plane RP which extends radially from the second axis of rotation  19  and rotates about the axis of rotation  19  at a constant speed synchronously with the shaft  3 . The lemniscate path has two portions which are indicated by thick lines in  FIG. 5 , where the pin  17  contacts the groove edge in the contact location CL, for example when driving the barrel plate  15  by the shaft  3 . In other words, each pin  17  contacts its groove edge along two longitudinal contact ranges LCR during each revolution of the shaft  3 , in the present embodiment two periods of 60 degrees about the second axis of rotation  19 . In the period outside these thick lines the pin  17  does not contact the groove edge in the corresponding groove  18 , but one of the other pins  17  will contact its corresponding groove edge in order to drive the barrel plate  15 . As a result, each of the pins  17  fluctuates about its corresponding imaginary radial plane RP, but the contact location CL varies within a rotational contact range RCR during a period in which the pin  17  contacts the groove edge.  FIG. 5  shows that the amplitude of the fluctuation of the contact location CL within the rotational contact range RCR is smaller than the amplitude of a pin  17  with respect to its corresponding imaginary radial plane RP. 
     The fluctuation of the contact location CL within the rotational contact range RCR causes a fluctuation of the relative locations of the centrelines  22 ,  23  of the barrel plate ports  21  and the sleeve openings  14  respectively. Due to the plurality of the pivot axes  24  the amplitude of this fluctuation is relatively small when compared with a prior art device that has a single pivot axis. However, the rotational contact range RCR fluctuates about a second imaginary radial plane RP′, as indicated in  FIG. 5 , which second imaginary radial plane RP′ extends radially from the second axis of rotation  19  and rotates about the axis of rotation  19  at a constant speed synchronously with the shaft  3 . The second imaginary radial plane RP′ has an angular phase shift PS with respect to the imaginary radial plane RP about the second axis of rotation  19 . In order to minimize the phase shift PS the relative position of the shaft  3  and the barrel plate  15  in rotational direction about the second axis of rotation  19  is adapted such that under operating conditions the centreline  22  of each barrel plate port  21  fluctuates in rotational direction about the second axis of rotation  19  with respect to the centreline of the corresponding sleeve opening  14  within a range in which the centreline  23  of the corresponding sleeve opening  14  lies. 
     The compensation of the phase shift only functions if the hydraulic device  1  is operated in one direction. If the device is operated in opposite rotational direction a phase shift would be re-introduced. For that reason it should be preferably applied in a device  1  which is used in only one rotational direction. 
     A still larger number of pivot axes leads to a narrower rotational contact range RCR.  FIG. 6  shows a similar illustration as  FIG. 4  of another embodiment of the hydraulic device  1 , which is provided with seven pivot axes that are formed by seven pin-groove couplings located at equiangular distance about the second axes of rotation  19 . In this case the groove edges of each of the grooves  18  are also parallel.  FIG. 6  shows the lemniscate path which is followed by the centreline of each of the pins  17  in their respect grooves  18  and the actual positions of the pins  17  in their respective grooves  18 . Because of the relative movement of the pins  17  within the grooves  18 , the width of each groove  18  must be larger than the diameter of the corresponding pin  17 . However, as shown in  FIG. 6 , the longitudinal contact range LCR is now located at a central portion of the groove  18  in longitudinal direction thereof. Theoretically the length of the groove  18  may have the same length as the longitudinal contact range LCR. This means that the width of the groove  18  outside the longitudinal contact range LCR may be increased in order to allow the pins  17  which are temporarily out of contact with the groove edge to move into the widened portions of the respective grooves  18 . In other words, the width of the central portion may be relatively narrow which minimizes any phase shift as described above in relation to  FIG. 5 . 
     An alternative solution to minimize any phase shift is provided by an embodiment as shown in  FIG. 7 . Similar to the embodiment as shown in  FIG. 6  there are seven pins  17  which follow respective lemniscate paths, but in this case the opposite groove edges at the contact areas between the pins  17  and the groove edges have a different shape. Each of the grooves  18  has a midpoint MP from which the groove width increases gradually in opposite directions in longitudinal direction of the groove  18 . The groove width at the midpoint MP may be such that the pins  17  exactly fit between the opposite groove edges. The shape of the groove edges are similar or approach the path of the contact locations between the pins  17  and their groove edges. For example, at least at the midpoint MP of each of the grooves  18  each of the groove edges is angled with respect to the longitudinal centreline of the groove  18  by an angle α which equals a half of the swash angle. In this case there is no phase shift upon rotating the shaft  3  in opposite directions such that no compensation is required. If the hydraulic device  1  has a variable swash angle the groove edges should be angled by a half of the maximum swash angle. Turning to a lower swash angle will lead to a phase shift, but this effect decreases with an increasing number of pivot axes  24 . 
     In order to create some degree of flexibility in the pin-groove couplings the pins  17  may be flexible, for example a pin which is provided with a longitudinal slot as shown in  FIG. 8  or a coiled configuration as shown in  FIG. 9 . 
     The pin-groove couplings as described hereinbefore are not limited to hydraulic devices, but are also useful in other applications where rotational speed fluctuations and phase shifts are undesired.  FIG. 10  shows an embodiment of a pivot joint  25  according to the invention. The pivot joint  25  comprises a first shaft  26  and a second shaft  27 . Both shafts  26 ,  27  are supported by bearings  28 . The first shaft  26  is rotatable with respect to its bearing  28  about a first axis of rotation  29  and the second shaft  27  is rotatable with respect to its bearing  28  about a second axis of rotation  30 . The first axis of rotation  29  and the second axis of rotation  30  intersect each other at a centre point CP by an acute angle. The centre point CP is the centre of a partly spherical portion  31  of the first shaft  26 . The second shaft  27  is mounted to the partly spherical portion  31  and coupled to the first shaft  26  in rotational direction thereof by means of a plurality of pin-groove couplings. 
     In the embodiment as shown in  FIG. 10  each pin-groove coupling comprises a groove  32  which is located in an interior portion of the second shaft  27  and a pin  33  which is fixed in the partly spherical portion  31 . The pin  33  is cylindrical and has a circular cross-section. The groove  32  has a longitudinal direction which extends in the same direction as the second axis of rotation  30 . Each of the pins  33  is received by its corresponding groove  32 . Each pin-groove coupling forms a pivot axis about which the second shaft  27  rotates with respect to the first axis  26  upon rotation of the shafts  26 ,  27  about their axes of rotation  29 ,  30 . Each pivot axis extends perpendicular to the second axis of rotation  30  and runs through the centre point CP. The pin-groove couplings are arranged such that a plurality of pivot axes are created at equiangular distance about the second axis of rotation  30 . 
       FIG. 11  shows one of the grooves  32  in more detail. It can be seen that the groove  32  has a midpoint MP from which the groove width increases in opposite directions in longitudinal direction of the groove  32 . The width of the groove  32  at the midpoint MP may be such that the cooperating pin  33  accurately fits in the groove  32 . The opposite groove edges are angled with respect to each other by an angle β which depends on the acute angle between the first and second axes of rotation  29 ,  30 . Preferably the angle β equals the acute angle. If the acute angle is variable the angle β is preferably a predefined maximum acute angle. An advantage of the pivot joint  25  is that the pin groove couplings are relatively simple and low-cost, whereas the shapes of the groove edges minimize a phase shift between the first and second shaft  26 ,  27  under operating conditions. 
     The invention is not limited to the embodiments as shown in the drawings and described hereinbefore, which may be varied in different manners within the scope of the claims and their technical equivalents. For example, it is also conceivable to provide the grooves in the partly spherical portion of the shaft and the pins in the barrel plate. Furthermore, the hydraulic device according to the invention may also be a hydraulic transformer.