Abstract:
A hydrostatic machine, has first and second axial-piston units arranged on a common axis and having pistons which cooperate with an adjustable swash plates, wherein the first axial-piston unit has a first one of the swash plates associated with it while the second axial-piston unit has a second one of the swash plates associated with it, and a coupling mechanism which couples the first and the second swash plates mechanically and adjustably.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The invention described and claimed hereinbelow is also described in German Patent Application DE 10 2010 021 708.5 filed on May 27, 2010. This German Patent Application, whose subject matter is incorporated here by reference, provides the basis for a claim of priority of invention under 35 U.S.C. 119(a)-(d). 
       BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to a hydrostatic machine having two axial-piston units arranged on a common axis. A machine of this kind is known from DE 10 2007 022 022 A1. 
         [0003]    In this known machine, the two axial-piston units co-operate with oppositely positioned surfaces of one and the same swash plate. The axial forces which are exerted on the swash plate by the cylinders of the axial-piston units largely compensate for one another in this way. This reduces the requirements which the mounting of the swash plate has to meet with regard to load-bearing capacity and enables the machine to be produced in a compact and light-weight form. 
       SUMMARY OF THE INVENTION 
       [0004]    It is an object of the present invention to continue the development of this known machine in order to widen its field of application, e.g. as a hydraulic pump or hydraulic motor. 
         [0005]    The object is achieved by virtue of the fact that, in a hydrostatic machine having a first second axial-piston unit which are arranged on a common axis and which have pistons which co-operate with an adjustable swash plate, the first axial-piston unit has a first swash plate associated with it and the second axial-piston unit has a second swash plate associated with it, the first and the second swash plate being adjustable in unison parallel to one another by a coupling mechanism. 
         [0006]    The replacement of the conventional single swash plate by two coupled swash plates enables the practical properties of the hydrostatic machine to be improved in two respects. On the one hand, the division into two enables the two swash plates to be so arranged as to be pivotable on two different axes; in particular, the axes of pivot may be so positioned that the distance between the swash plates, as measured in the direction defined by the common axis of the axial-piston units, is all the greater the less inclined is the position of the swash plates. Such an increase in the distance results in the mean volume of the cylinders of the axial-piston units, and hence too the deleterious volume, becoming all the smaller as the inclined position becomes less pronounced. Hence low-loss operation is possible particularly at a not very inclined position. 
         [0007]    To allow a change of this kind in the distance between the swash plates to be achieved in practice, it is possible, when an imaginary plane extending along the common axis of the axial-piston units divides at least the first swash plate into a half adjacent the first axial-piston unit and a half remote from the first axial-piston unit, for the axis of pivot of the first swash plate to be usefully situated on the same side of this plane as that half of the first swash plate which is adjacent the first axial-piston unit. 
         [0008]    It is also useful for the axis of pivot of the first swash plate to extend across a surface of the first swash plate which faces the first axial-piston unit. 
         [0009]    If the longitudinal axis of a cylinder of the axial-piston unit which is situated at its top dead centre also intersects the axis of pivot of the first swash plate, the position of the dead centre of the cylinder is not dependent on the pivoted position of the swash plate. The deleterious volume of the cylinder can thus be made independent of its pivoted position; in particular it may be made equal to virtually zero for any pivoted position. 
         [0010]    The two axial-piston units should be connected together in such a way as to be solid in rotation with one another. This ensures that pressures exerted by the two axial-piston units on their associated swash plates in the axial direction are equal and opposite and are able to compensate for one another at a common mounting for the two swash plates. 
         [0011]    It may also help to relieve the load on such a mounting if the coupling mechanism comprises a support which extends between the first and the second swash plate and which is arranged to pass on pressure exerted by one of the axial-piston units on its associated swash plate to the other swash plate, 
         [0012]    A simple and effective coupling mechanism may comprise a guiding track solid with one of the swash plates and a stop which is solid with whichever is the other swash plate in the given case and which is able to move along the guiding track. A carrier of the guiding track may, at the same time, act as the above-mentioned support. 
         [0013]    Alternatively, gearing may equally well be provided as a coupling mechanism. 
         [0014]    If the hydrostatic machine is intended to operate primarily as a pump, then the two swash plates are preferably pivotable between positive and negative angles of pivot, about a zero position which is perpendicular to the axis of rotation of the machine. 
         [0015]    In the case of a hydrostatic machine which is designed primarily for operation as a motor, it is enough for the swash plates to be pivotable on only one side of the zero position, e.g. each from the zero position to a maximum positive angle of pivot. 
         [0016]    In a refinement of the invention which is a particular preference, one of the swash plates has a greater freedom of pivoting movement than the other swash plate, and the freedom of pivoting movement only of the one swash plate includes a zero position. 
         [0017]    This affords the possibility not only of both axial-piston units being operated with indrawn volumes which are each the same but also of one being operated with an indrawn volume of zero and the other with a small, but not vanishingly so, indrawn volume. This makes the technically usable spread over which the hydrostatic machine can be operated as a motor twice as wide as that of the axial-piston units by which it is formed if each is considered on its own. 
         [0018]    A partial decoupling of this kind of the swash plates, with a transition to the zero position, can easily be accomplished in the case of the above-mentioned coupling mechanism having a guiding track and a stop able to move along the guiding track by giving the guiding track an arcuate portion which is centred on the axis of pivot of the other swash plate. 
         [0019]    Other features and advantages of the invention can be seen from the following description of embodiments, which is given by reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  is an axial schematic section through a first embodiment of hydrostatic machine according to the invention. 
           [0021]      FIG. 2  is a view from the side of a second embodiment of hydrostatic machine according to the invention, showing the swash plates set to their maximum inclination. 
           [0022]      FIG. 3  is a view from the side similar to  FIG. 2  which shows the swash plates coupled at a minimum angle of pivot. 
           [0023]      FIG. 4  is a view similar to  FIG. 2  showing the swash plates decoupled. 
           [0024]      FIG. 5  is a view from the side of a third embodiment of hydraulic machine according to the invention. 
           [0025]      FIG. 6  is a view from the side similar to  FIG. 5  which shows the swash plates coupled at a minimum angle of pivot. 
           [0026]      FIG. 7  is a view from the side which shows the swash plates in a decoupled state. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0027]    The hydrostatic machine which is shown in a schematic section in  FIG. 1  has a cylindrical housing whose end-walls are indicated as  1  and  2  in the drawing. A cylindrical outer shell which connects the end-walls  1 ,  2  is not shown in the drawing. A shaft, which is also not shown in the drawing for the sake of clarity, extends through central bores  3  in the end-walls  1 ,  2  and is connected to the axial-piston units  5 ,  6  to be solid in rotation therewith. In the following description, the housing will be assumed to be fixed in position and the axial-piston units  5 ,  6  to be rotatable but it goes without saying that the reverse would also be possible. The axial-piston units  5 ,  6  in turn each have a central bore  4  through which the shaft extends, and respective cylinder bores  7 ,  8  which are arranged in a circle around the central bores  4  and of which one can be seen in section in each case and in which respective pistons  9  are guided to be movable axially. Jointed, by a ball-joint  10  in each case, to each of the free ends of the pistons  9  which project from the cylinder bores  7 ,  8  is a sliding shoe  11  which presses against a swash plate  13  or  14 , which swash plate  13  or  14  is aligned at an inclination to the longitudinal axis  12  of the machine. 
         [0028]    The swash plates  13 ,  14  are able to pivot on axes of pivot  15 ,  16  which are orientated perpendicularly to the plane of section of  FIG. 1 , and their pivoted position is controlled with the help of rods  17  which engage with respective opposite edges of the swash plates  13 ,  14  and which in turn are connected to pistons  20  which are displaceable in cylinders  18 ,  19  in the end-walls  1 ,  2 .  FIG. 1  shows the swash plates  13 ,  14  in a maximum inclined position where they are deflected at an angle α max , of for example 21° each, from a position perpendicular to the longitudinal axis  12  which will also be referred to as the zero position in what follows. 
         [0029]    In the embodiment shown in  FIG. 1 , the cylinders  18 ,  19  in the end-wall  1  can each be connected via respective shut-off valves  21 ,  22  to a source or sink for hydraulic fluid, whereas the cylinders  18 ,  19  in the end-wall  2  situated opposite are able to communicate with one another via a line  23 . If for example hydraulic fluid is admitted into the cylinder  18  in the end-wall  1  via the shut-off valve  21 , the piston  20  in this cylinder thrusts the swash plate  14  to a less inclined position. At the same time, the piston  20  in the cylinder  18  in the end-wall  2  is slid back and hydraulic fluid is transferred via the line  23  into the cylinder  19  in the end-wall  2 , and the piston  20  in this cylinder drives a pivoting movement of the swash plate  13 , and the two swash plates  13 ,  14  thus maintain their parallel orientation. 
         [0030]    In the case of a hydrostatic machine for operation as a pump, the freedom of pivoting movement of the two mash plates  13 ,  14  may extend over an angular interval [−α max , α max ] on the two sides of the zero position. 
         [0031]    In the case of a hydrostatic machine for operation as a motor, an angular interval of pivoting movement through which the two swash plates  13 ,  14  pivot in parallel with one another extends between α max  and a small positive bottom limiting value α min  of for example 7°. If the angle of pivot of the two swash plates  13 ,  14  reaches this bottom limiting value α min , a shut-off valve  24  in the line  23  is closed, and a shut-off valve  25  is opened to connect the line  23  to a tank holding hydraulic fluid. The closing of the shut-off valve  24  immobilises the pistons in the two cylinders  19  and the swash plate  13  is unable to pivot past the angle α min  to smaller angles of pivot, whereas the swash plate  14 , if the infeed of hydraulic fluid through the shut-off valve  21  continues, can reach the zero position in which it is orientated at right angles to the axis  12  and in which the volume of the cylinder bores  8  does not vary in the course of a revolution of the axial-piston units  5 ,  6 . 
         [0032]    The reason for the differing adjustability of the two swash plates  13 ,  14  is the self-locking effect which occurs when an axial-piston unit is operating as a motor if the angle of pivot of its swash plate goes below a minimum: there is a sharp rise in the frictional forces as the angle of pivot goes down and the minimum angle of pivot is the angle at which they reach 100%. Although operation would be possible just above this minimum angle of pivot, it would not be very economical due to the high losses. Because one swash plate can be pivoted to the zero position in the machine according to the invention while the other remains at the angle of pivot α min  at which losses are still low, operation with appreciably lower losses is possible than if both axial-piston units were to operate at an angle of pivot of α min /2. 
         [0033]      FIG. 2  is a view from the side of a second embodiment of hydraulic machine according to the invention. Once again, the outer shell connecting the end-walls  1 ,  2  has been omitted to enable the inside of the machine to be shown. The plurality of cylinder bores  7 ,  8  of each axial-piston unit can be seen in this case. Of the pistons  9  with which these bores  7 ,  8  are fitted, only one has been shown in each of the axial-piston units  5 ,  6 . 
         [0034]    The swash plate  13  carries two supports  26  in bar or ridge form whose tips rest against contact blocks  27  on the swash plate  14  in such a way as to be able to move by sliding. The supports  26  and contact blocks  27  transmit the forces which are exerted by the pistons  9  of one of the axial-piston units  5 ,  6  on the swash plate  13  or  14  situated opposite the said pistons to whichever is the other swash plate  14  or  13  in the given case, and the pressures from the axial-piston units thus compensate for one another and do not have to be absorbed by the mountings of the axes of pivot  15 ,  16  on the housing or by the rods  17 . 
         [0035]    The tips of the supports  26  are able to move by sliding across the contact blocks  27  and remain in contact with them if the swash plates  13  are pivoted from the most pronounced inclined position possible α max  which is shown in  FIG. 2 , or in other words the position corresponding to the largest possible indrawn volume for the axial-piston units  5 ,  6 , to the position which is shown in  FIG. 3 . In the position shown in  FIG. 3 , the inclined position of the swash plates  13 ,  14  is only α min , and the tip of the upper support  26  is situated directly against the boundary of an arcuate flank  28  of the upper contact block  27 , which arcuate flank  28  is centred on the axis of pivot  15 . 
         [0036]    If the swash plate  13  continues to be pivoted towards the zero position from the position shown in  FIG. 3 , the tip of the support  26  travels over the flank  28 . Because the latter is arcuate and has the axis of pivot  15  as its centre, the swash plate  14  ceases to be entrained if the swash plate  13  continues to pivot towards the zero position.  FIG. 4  shows the final result of such an adjustment: the swash plate  13  has reached the zero position whereas the swash plate  14  has remained in the orientation shown in  FIG. 3 , at the angle of pivot α min . 
         [0037]    As can easily be seen, in the two embodiments considered above the distance between the surfaces of the two swash plates  13 ,  14  with which the sliding shoes  11  are in contact is equal to the distance between the axes of pivot  15 ,  16  and is independent of the orientation of the swash plates, because the axes of pivot  15 ,  16  extend across the surfaces of the swash plates  13 ,  14  which are facing the pistons  9  and intersect the longitudinal axis  12  as they do so. Hence, the total volume of fluid in the cylinder bores  7 ,  8  of the two axial-piston units  5 ,  6  is not dependent on the orientation of the swash plates  13 ,  14  and losses attributable to the compressibility, though small, of the hydraulic fluid circulating in the cylinder bores  7 ,  8  are not dependent on the orientation of the swash plates  13 ,  14 . In contrast to this, the distance as measured along the longitudinal axis between the surfaces of the single swash plate in DE 10 2007 022 022 A1 is smallest when the swash plate is in its neutral position. Losses attributable to the compressibility of the hydraulic fluid are therefore particularly high close to the neutral position in the conventional hydrostatic machine. The swash plates  13 ,  14  according to the invention, which are separate from one another but coupled, enable this disadvantage to be avoided. 
         [0038]      FIG. 5  is a view, similar to  FIG. 2 , of a third embodiment of hydrostatic machine according to the invention. The housing of the machine and the axial-piston units  5 ,  6  are the same as in the embodiments considered above and will not be described again. A plane  29  which is perpendicular to the plane of the drawing and which extends along the longitudinal axis  12  divides each of the two swash plates  13 ,  14  into two halves, namely respective halves  13 ′ and  14 ′ adjacent the associated axial-piston units  5  and  6  respectively, and respective halves  13 ″ and  14 ″ remote from the axial-piston units  5  and  6  respectively. The axes of pivot  15 ,  16  are offset from the said plane  29  in parallel positions and are respectively situated on the same sides of the plane  29  as those halves  13 ′ and  14 ′ of the swash plates which are adjacent the axial-piston units. The axes of pivot  15 ,  16  intersect the longitudinal axes of respective cylinder bores  7  and  8  whose piston  9  (not shown in the case of the axial-piston unit  6 ) is at its top dead centre. At this top dead centre, the volume of the cylinder concerned is virtually zero, and does not change as a function of the orientation of the swash plate  13  or  14 . Consequently, losses of efficiency caused by the compressibility of the hydraulic fluid are negligible in this embodiment. 
         [0039]    Arranged on the edges of the swash plate  13  are two plates  30 , situated in opposite positions from one another, of which one conceals the other in each of the views shown in  FIGS. 5 to 7 . The plates  30  each have a slotted hole  31 . Engaging in the slotted holes  31  are cylindrical projections  32  which project from each of the sides of the swash plate  14 .  FIG. 5  shows the swash plates  13 ,  14  in a stop position at their maximum inclination, once again in a position inclined at α max , in which the projection  32  is at one end of the slotted hole  31 . Starting from this end, the shape of the slotted hole  31  is initially dictated by the requirement that the two swash plates  13 ,  14  are always to pivot in parallel with one another. 
         [0040]      FIG. 6  shows the swash plates  13 ,  14  at the smallest possible inclined position α min  which the swash plate  14  is able to reach. As can be seen, the projection  32  has not yet reached the opposite end of the slotted hole  31  in this position. The portion  33  of the slotted hole  31 , which portion  33  extends from the position shown in  FIG. 6  for the projection  32  to the opposite end of the slotted hole  32 , is an arc of a circle whose centre coincides with the axis of pivot  15 . Consequently, the swash plate  14  no longer follows a pivoting movement by the swash plate  13  if the latter moves towards the zero position beyond the position shown in  FIG. 6 . This produces the configuration shown in  FIG. 7  where the swash plate  13  is in the zero position orthogonal to the longitudinal axis  12  whereas the swash plate  14  is still in the inclined position α min . 
         [0041]    It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of constructions differing from the types described above. 
         [0042]    While the invention has been illustrated and described as embodied in a hydrostatic machine, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. 
         [0043]    Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention.