Abstract:
A hydrostatic assembly employing a 1 st  hydraulic piston drive unit is described in which the output is increased without using additional servo assemblies by incorporating a 2 nd  hydraulic piston drive unit, coupling the drive shafts of both drive units together, and employing a common means to simultaneously adjust displacement of both the 1 st  and 2 nd  hydraulic piston drive units. In such embodiments, coupling the drive shafts together such that the 1 st  hydraulic piston drive unit is rotationally offset with respect to the 2 nd  hydraulic piston drive allows for a reduction in the amplitude of pressure pulsations associated with the hydrostatic assembly output, thereby smoothing out operation and improving durability.

Description:
INTRODUCTION 
       [0001]    The present invention relates generally to a hydrostatic assembly. 
         [0002]    Hydrostatic modules or assemblies are hydraulic devices used in hydrostatic and power splitting transmissions to effect ratio changes between the transmission input and output. Such assemblies typically comprise two hydraulic piston drive units and may be of a bent axis or an axial piston drive design. The two drive units are in fluid communication with each other. One of the hydraulic piston drive units typically functions as a pump and the other typically functions as a motor. Depending on the transmission design, the role of the pump and motor may be permanently or alternately assigned depending on the transmission mode. The speed and torque ratios between the input and output shafts of the module are determined by the displacement ratio between the two hydraulic piston drive units. By making at least one of the drive units a variable displacement type, the speed and torque ratio of the module may be varied. 
         [0003]    The amount of power and torque to be transferred through the module will determine the size of the components. Generally, greater torque requires larger displacement drive units. With larger displacement drive units the allowable or permitted operating speed may be reduced as the mass of the rotating components is increased due to the increased size of the drive units. In a transmission where the drive units are permanently assigned as each of a pump and motor, a large motor and a small pump are typically used. However, if different size drive units are used, different rotating components for the pump and motor may be required. 
         [0004]    U.S. 2010/0212309 describes a dual hydrostatic assembly with a common shaft driving the two pumps where the two pumps are arranged opposite one another and the input shafts rotate about the same axis. Similarly, the two motors have a common shaft where the two motors are arranged opposite one another and the out shafts rotate about the same axis. Each of the pumps and motors are arranged in separate rotatable yokes. 
         [0005]    DE1064311 discloses a hydraulic module with two bent axis piston drive units, one functioning as pump and the other functioning as motor, where the pump and motor cylinder blocks rotate within a common yoke. However, the cylinder blocks are set at different angles and the angle between respective cylinder blocks and shafts of each bent axis piston drive unit is altered using the common yoke assembly. Despite advances in the art, there remains a continuing need for durable hydrostatic assemblies that can produce greater displacements and transfer greater power and torque, while remaining compact in size, smooth in operation, and simple in design. The present invention addresses this need and provides other advantages as discussed in more detail below. 
       SUMMARY OF THE INVENTION 
       [0006]    The output associated with a 1 st  hydraulic piston drive unit in a hydrostatic assembly can be increased without using additional servo assemblies by incorporating a 2 nd  hydraulic piston drive unit, coupling the drive shafts of both drive units together, and employing a common means to simultaneously adjust displacement of both the 1 st  and 2 nd  hydraulic piston drive units. Alternatively, this method may be used to reduce the piston size needed to provide a given output from a 1 st  hydraulic piston drive in a hydrostatic assembly. 
         [0007]    As a further advantage, this approach may be used to reduce the amplitude of pressure pulsations associated with the output from the hydrostatic assembly. In such embodiments, coupling the drive shafts of the 1 st  and 2 nd  hydraulic piston drive units such that the 1 st  hydraulic piston drive unit is rotationally offset with respect to the 2 nd  hydraulic piston drive allows for a reduction in the amplitude of the pressure pulsations. 
         [0008]    Hydrostatic assemblies of the invention comprise a housing, a pivot axis, 1 st  and 2 nd  hydraulic piston drive units, and common means for simultaneously adjusting displacement of the 1 st  and 2 nd  hydraulic piston drive units. Each of the 1 st  and 2 nd  hydraulic piston drive units comprises a cylinder block with ports, pistons within the cylinders in the cylinder block, and a drive shaft mounted to the housing, in which the drive shafts of 1 st  and 2 nd  hydraulic piston drive units are coupled together (e.g. using a belt and pulleys, using engaged gears, etc. It should also be noted that the coupling may be accomplished outside of the immediate assembly shown and could encompass each shaft connected to a different drive wheel of a vehicle with contact with the ground serving as the final link.) The common means for adjusting is mounted on the pivot axis and is capable of rotation. The hydrostatic assembly further comprises a 3 rd  hydraulic piston drive unit and 2 nd  means for adjusting displacement of the 3 rd  hydraulic piston drive unit. The 3 rd  hydraulic piston drive unit also comprises a cylinder block with ports, pistons within the cylinders in the cylinder block, and a drive shaft mounted to the housing. The 2 nd  means for adjusting is also mounted on the pivot axis and is capable of rotation independently of the common means. Finally, the hydrostatic assembly also comprises at least one fluid passage fluidly connecting the ports of the cylinder blocks in the 1 st  and 2 nd  hydraulic drive units to the ports of the cylinder block in the 3 rd  hydraulic drive unit. 
         [0009]    In one embodiment appropriate for use with bent axis piston drive units, the pivot axis is a yoke pivot axis and the housing comprises the yoke pivot axis. Further, the common means for adjusting displacement is a common yoke comprising at least one fluid passage therein. And the cylinder blocks and ports of the 1 st  and 2 nd  hydraulic drive units are mounted to the common yoke. Further still, the 2 nd  means for adjusting displacement is a 2 nd  yoke mounted on the yoke pivot axis adjacent the common yoke, and the 2 nd  yoke comprises at least one fluid passage therein. And the cylinder block and ports of the 3 rd  hydraulic drive unit are mounted to the 2 nd  yoke. In this embodiment, the at least one fluid passage connects the ports of the cylinder blocks in the 1 st  and 2 nd  hydraulic drive units to a hydraulic rotary joint located between the common yoke and the 2 nd  yoke. And further, the at least one fluid passage fluidly connects the hydraulic rotary joint to the ports of the cylinder block in the 3 rd  hydraulic drive unit. 
         [0010]    In this embodiment of a hydrostatic assembly, the 1 st , 2 nd , and 3 rd  hydraulic piston drive units can all be bent axis piston drive units. And optionally, all of the 1 st , 2 nd , and 3 rd  bent axis piston drive units can essentially be the same type of unit. Advantageously then, only one drive unit type may need to be sourced to manufacture the improved hydrostatic assembly. 
         [0011]    In a second embodiment appropriate for use with axial piston drive units, the pivot axis is a swashplate pivot axis, and the common means for adjusting displacement is a common swashplate. Here, the cylinder blocks and ports of the 1 st  and 2 nd  hydraulic drive units are mounted to the housing. Further, the 2 nd  means for adjusting displacement is a 2 nd  swashplate mounted on the swashplate pivot axis adjacent the common swashplate. And the cylinder block and ports of the 3 rd  hydraulic drive unit are mounted to the housing. In this embodiment, the housing comprises the least one fluid passage connecting the ports of the cylinder blocks in the 1 st  and 2 nd  hydraulic drive units to the ports of the cylinder block in the 3 rd  hydraulic drive unit. 
         [0012]    In this second embodiment of a hydrostatic assembly, the 1 st , 2 nd , and 3 rd  hydraulic piston drive units can all be axial piston drive units. And as before, optionally all of the 1 st , 2 nd , and 3 rd  axial piston drive units can essentially be the same type of unit. 
         [0013]    More complex embodiments can also be considered. For example, the hydrostatic assembly can comprise a 4 th  fourth hydraulic piston drive unit in which the drive shafts of the 3 rd  and 4 th  hydraulic piston drive units are coupled together. (In a like manner to the other drive units, the 4 th  hydraulic piston drive unit also would comprise a cylinder block with ports, pistons within the cylinders in the cylinder block, and a drive shaft mounted to the housing). Also for example, hydrostatic assemblies comprising both bent axis and axial piston drive units which have been appropriately configured together may also be contemplated in principle. 
         [0014]    In certain preferred embodiments of hydrostatic assemblies, the drive shafts of the 1 st  and 2 nd  hydraulic piston drive units can be essentially parallel. And further, the cylinder blocks of the 1 st  and 2 nd  hydraulic piston drive units can be coupled together at the same angle with respect to their drive shafts, thereby functioning as a drive unit which has been doubled in size. In a like manner, the drive shaft of the 3 rd  hydraulic piston drive unit can also be essentially parallel to the drive shafts of the 1 st  and 2 nd  hydraulic piston drive units. Such embodiments are suitable for applications in which the 1 st  and 2 nd  hydraulic piston drive units act as a motor and the 3 rd  hydraulic piston drive unit acts as a pump. 
         [0015]    In a simple arrangement, the drive shafts of the 1 st  and 2 nd  hydraulic piston drive units are coupled to drive at the same speed. However, the drive shafts can advantageously be coupled such that the 1 st  hydraulic piston drive unit is rotationally offset with respect to the 2 nd  hydraulic piston drive unit. In this way, the pressure pulsations associated with the 1 st  hydraulic piston drive unit are staggered with respect to those of the 2 nd  drive unit. And as a consequence, the magnitude of the pressure pulsations is less than it would be if the drive units were synchronized and the output fluid pressure profile is smoothed out, thereby improving durability. In one convenient arrangement, the drive shafts can be coupled such that the  1 st hydraulic piston drive unit is rotationally offset at half the angle between two rotationally adjacent cylinders. 
         [0016]    Typically, the 1 st  and 2 nd  hydraulic piston drive units each comprise a plurality of ports and pistons. In a practical exemplary embodiment, the 1 st  and 2 nd  hydraulic piston drive units can for instance each comprise nine pistons. And the two drive units can be rotationally offset such that the 1 st  hydraulic piston drive unit is rotationally offset 20 degrees between two rotationally adjacent cylinders in the 2 nd  drive unit. 
         [0017]    As mentioned, the invention provides for improvements in output from hydrostatic assemblies without the need for additional servo assemblies. A controllable hydrostatic assembly of the invention thus comprises the aforementioned hydrostatic assembly, a single servo assembly to control the angle of the displacement adjusting common means on the pivot axis, and a single servo assembly to control the angle of the 2 nd  displacement adjusting means on the pivot axis. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    For a better understanding of the invention and to show how the same may be carried into effect reference is now made by way of example to the accompanying drawings in which: 
           [0019]      FIG. 1  schematically illustrates a hydrostatic assembly according to a first embodiment of the invention; 
           [0020]      FIG. 2  schematically illustrates a section in the plane of and through the bucket portions of the common and second yokes of the hydrostatic assembly of  FIG. 1 , when both the common and second yokes are in alignment; 
           [0021]      FIG. 3  schematically illustrates a section in the plane of the arm portions of the common and second yokes and through a fluidic passage of the hydrostatic assembly of  FIG. 1 , when both the common and second yokes are in alignment; 
           [0022]      FIG. 4  schematically illustrates a section through the common yoke illustrated in  FIG. 1 ; and 
           [0023]      FIG. 5  schematically illustrates a hydrostatic assembly according to a second embodiment of the invention. 
           [0024]      FIG. 6  schematically illustrates a hydrostatic assembly according to a third embodiment of the invention. 
           [0025]      FIG. 7  schematically illustrates a section in a plane parallel to the drive shafts of the axial piston drive units and through a fluidic passage of the hydrostatic assembly of  FIG. 6 . 
           [0026]      FIG. 8  schematically illustrates a section through a fourth embodiment of the invention comprising two bent axis piston drive units and one axial piston drive unit. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]      FIG. 1  schematically illustrates a hydrostatic assembly or hydraulic module  1  according to a first embodiment of the invention. The hydrostatic assembly  1  comprises a common housing  2  that supports respective drive shafts  12 ,  22 ,  32  of each of a first bent axis piston drive unit  10 , second bent axis piston drive unit  20 , and third bent axis piston drive unit  30 . Housing  2  includes a bearing or bearings (not shown) mounted within housing  2  to allow each of the drive shafts  12 ,  22 ,  32  to rotate. That is to say that each of drive shafts  12 ,  22 ,  32  are rotatably arranged or mounted within the housing  2 . Each of the drive shafts  12 ,  22 ,  32  of the bent axis piston drive units include splines to allow the shafts to be coupled to other input or output mechanisms. 
         [0028]    Two yokes are employed in the embodiment of  FIG. 1 , namely common yoke  14  and second yoke  16 , and both are rotatably mounted on pivot axis  40 . Second yoke  16  supports a cylinder block  28  of third bent axis piston drive unit  30 . Cylinder block  28  of third bent axis piston drive unit  22  is rotatably mounted within second yoke  16 . Second yoke  16  is generally u-shaped and includes two extending arms that are rotatably mounted to the housing  2 . The axis of rotation of second yoke  16  is that of pivot axis  40  and crosses (e.g., is perpendicular to) the axis of the drive shaft  32  of third bent axis piston drive unit  30 . Rotation of second yoke  16  with respect to housing  2  is provided by a servo assembly mechanism  44 . In this example, servo assembly mechanism  44  is provided by two antagonistic hydraulic actuators. In other words, two hydraulic actuators are used to rotate second yoke  16 , a first actuator to rotate second yoke  16  in a clockwise direction and a second actuator to rotate second yoke  16  in a counter clockwise direction. 
         [0029]    Common yoke  14  supports cylinder block  24  of first bent axis piston drive unit  10  and cylinder block  26  of second bent axis piston drive unit  20 . The respective cylinder blocks  24 ,  26  of first and second bent axis piston drive units  10 ,  20  are rotatably mounted within common yoke  14 . Common yoke  14  is generally u-shaped and includes two extending arms that are rotatably mounted to housing  2 . The axis of rotation of common yoke  14  is that of pivot axis  40  and crosses (e.g., is perpendicular to) the axes of drive shafts  12 ,  22  of first and second bent axis piston drive units  10 ,  20 . Rotation of common yoke  14  with respect to housing  2  is provided by servo assembly mechanism  42 . In this example, servo assembly mechanism  42  is provided by two antagonistic hydraulic actuators. That is to say that two hydraulic actuators are used to rotate common yoke  14 , a first actuator to rotate common yoke  14  in a clockwise direction and a second actuator to rotate common yoke  14  in a counter clockwise direction. 
         [0030]    Cylinder block  28  of third bent axis piston drive unit  30  is fluidically coupled to second yoke  16 . That is two say that fluid may pass between cylinder block  28  of third bent axis piston drive unit  30  and second yoke  16 . The respective cylinder blocks  24 ,  26  of first and second bent axis piston drive units  10 ,  20  are fluidically coupled to one another and to common yoke  14 . That is two say that fluid may pass between respective cylinder blocks  24 ,  26  of first and second bent axis piston drive units  10 ,  20  and common yoke  14 . Further, common and second yokes  14 ,  16  are in fluid communication with one another such that fluid may pass between the first, second and third bent axis piston drive units. The connection between common and second yokes  14 ,  16  is made via a hydraulic rotary joint  5  such as is described in detail in U.S. 2010/0212309. 
         [0031]    According to embodiments of the invention, control of servo assemblies  42 ,  44  may be by any means such as mechanical, hydraulic, electronic or combination thereof. According to the first embodiment each servo assembly  42 ,  44  is controlled by an electronically actuated control valve, a sub-system to supply pressurized control fluid to the control valve and a microprocessor to control the opening and close of the control valve. 
         [0032]    Each of common and second yokes  14 ,  16  are manufactured in at least two parts. A first support portion that includes a recess or bucket for supporting the cylinder blocks of the respective bent axis piston drive units and includes fluidic channels for coupling with the cylinder blocks of the respective bent axis piston drive units. Each yoke also includes second arm portions that extend from the first support portion and also include fluidic channels that are coupled to the fluidic channels of the first support portion and allow fluid to enter and exit the yoke. In this example, fluid enters and exits each yoke at the pivot point between common and second yokes  14 ,  16 . 
         [0033]    Drive shafts  12 ,  22  of first and second bent axis piston drive units  10 ,  20  are mechanically coupled together. In this example, the mechanical coupling is achieved using two engaged gears  13 ,  23 . Also in this example, drive shafts  12  and  22  rotate opposite each other but the addition of an idler gear(s) between them would allow for rotation in the same direction. Rotation in the same direction though would require different fluid routing in common yoke  14  between the cylinder blocks  26  and  28 . In other examples, first and second bent axis piston drive units  10 ,  20  may be coupled using other mechanical couplings, for example each drive shaft  12 ,  22  may include sprockets and the sprockets are coupled using a chain. Furthermore here, drive shafts  12 ,  22  of first and second bent axis piston drive units  10 ,  20  are parallel to each other. Common and second yokes  14 ,  16  illustrated in the figure are independently rotatable. 
         [0034]      FIG. 2  schematically illustrates a section in the plane of and through the bucket portions of common yoke  14  and second yoke  16  of hydrostatic assembly  1  when both the common and second yokes  14 ,  16  have been rotated to be in alignment with each other on pivot axis  40 . The sectioned areas of common and second yokes  14 ,  16  are shown with diagonal lines in  FIG. 2 . Like features of  FIGS. 1 and 2  are labeled using the same reference numerals. 
         [0035]    Cylinder blocks  24 ,  26 ,  28  of bent axis piston drive units  10 ,  20 ,  30  rotate about their respective drive shafts  12 ,  22 ,  32 . Each cylinder block comprises a plurality of ports and pistons within the cylinders of the blocks. Certain ports  25 ,  27 , and  29  of cylinder blocks  24 ,  26 , and  28  respectively are visible in the view of  FIG. 2 . Common yoke  14  includes first fluidic passage  43  that couples together certain ports from each of first and second bent axis piston drive units (not visible in this view). Common yoke  14  also includes a second fluidic passage  45  that couples together other ports from each of first and second bent axis piston drive units (again not visible in this view). Pistons  7  (visible in  FIG. 1  but not in  FIG. 2 ) of each of first and second bent axis piston drive units  10 ,  20  are attached to pivot axis  40  and in operation either draw hydraulic fluid in or push hydraulic fluid out of their respective cylinder blocks depending on their position in the rotation cycle. The amount of fluid drawn in or pushed out depends on the bend angle the cylinder block makes with respect to its drive shaft axis. First fluidic passage  43  is in fluidic communication with certain ports  25 ,  27  of first and second bent axis piston drive units  10 ,  20  via respective arcuate shapes formed in channel  43 . These arcuate shapes of first fluidic passage  43  provide a fluidic connection between several of the pistons in each of first and second bent axis piston drive units  10 ,  20 . All of the pistons connected by first fluidic passage  43  are drawing fluid in or all of the pistons connected by first fluidic passage  43  are pushing fluid out. In a like manner, second fluidic passage  45  is in fluidic communication with first and second bent axis piston drive units  10 ,  20  via respective arcuate shapes formed in passage  45 . Each of these arcuate shapes in second fluidic passage  45  provides a fluidic connection between several other pistons in each of first and second bent axis piston drive units  10 ,  20 . First fluidic passage  43  and second fluidic passage  45  provide for allow hydraulic fluid to be drawn in and to be pushed out respectively, or vice versa, depending on the bend angle direction that the cylinder blocks make with respect to pivot axis  40 . 
         [0036]    In the embodiment of  FIG. 2 , first and second bent axis piston drive units  10 ,  20  are directly coupled using a pair of engaged gears  13 ,  23  such that when first and second bent axis piston drive units  10 ,  20  rotate, each gear  13 ,  23  rotates in an opposite direction. That is to say that when first bent axis piston drive unit  10  rotates in a clockwise direction, second bent axis piston drive unit  20  rotates in a counterclockwise direction. 
         [0037]    As depicted in  FIG. 2 , second yoke  16  is aligned with common yoke  14 . Second yoke  16  includes third fluidic passage  47  and fourth fluidic passage  49  that connect to certain ports  29  of third bent axis piston drive unit (not visible in this view). Third fluidic passage  47  and fourth fluidic passage  49  are also coupled to first fluidic passage  43  and second fluidic passage  45  via hydraulic rotary joint  5 , thereby allowing for appropriate passages of hydraulic fluid between common yoke  14  and second yoke  16 . 
         [0038]      FIG. 3  schematically illustrates a section in the plane of the arm portions of the common and second yokes and taken through second fluidic passage  45  of hydrostatic assembly  1 , again when both the common and second yokes are in alignment as in  FIG. 2 . Like features of  FIGS. 1, 2 and 3  are labeled using the same reference numerals. 
         [0039]    Each of first and second fluidic connections  43 ,  45  of common yoke  14  extend from bucket portion  14   a  to arm portion  14   b  illustrated on the left hand side of common yoke  14  in  FIG. 3 . Each of first and second fluid connections  43 ,  45  extend from bucket portion  14   a  through to left hand arm portion  14   b  of common yoke  14 . Left hand arm portion  14   b  of common yoke  14  includes spigot  62  that is engaged with opening  64  of second yoke  16  that allows common yoke  14  to rotate with respect to second yoke  16 . Right hand arm portion  14   c  of common yoke  14  includes spigot  60  that is rotatable within opening  66  of housing  2  that allows common yoke  14  to rotate with respect to housing  2 . 
         [0040]    Hydraulic rotary joint  5  making the fluid connection between common and second yokes  14 ,  16  is described in U.S. 2010/0212309. Generally, each of fluid passages  43 ,  45  of common yoke  14  terminate at the surface of spigot  62  with a circumferential channel on the outer surface of spigot  62  that aligns with a corresponding circumferential channel formed on the inner surface of opening  64  of second yoke  16 . Each of the corresponding circumferential channels formed on the inner surface of opening  64  of second yoke  16  are in fluidic communication with third bent axis piston drive unit  30  in a similar manner to that described for first and second bent axis piston drive units  10 ,  20 . 
         [0041]    Each of bent axis piston drive units  10 ,  20 ,  30  is generally the same in operation and arrangement. Looking at first bent axis piston drive unit  10 , for example, there is provided a drive shaft  12  for coupling the bent axis piston drive unit to a rotating source (e.g., an electric motor) or to a mechanism or shaft to be rotated. The bent axis piston drive unit includes cylinder block  24  that includes at least two cylinders and typically an odd number of cylinders. In this example each of the bent axis units includes nine cylinders. Each cylinder includes a piston  7  that is movable linearly within the cylinder. Cylinder block  24  is rotatable about a spigot (e.g. spigot  52  associated with bent axis piston drive unit  20 ) that is provided in common yoke  14 . The distal ends of pistons  7  are movable within the cylinders and the proximal ends of pistons  7  are coupled to drive shaft  12 , typically using a ball and socket arrangement. In operation, drive shaft  12  and cylinder block  24  rotate at the same speed, since drive shaft  12  and cylinder block  24  are coupled via the pistons or other timing method such as a synchronization shaft. Also, as cylinder block  24  rotates, the pistons  7  will displace fluid dependent on the angle between drive shaft  12  and the cylinder block  24 . This angle is set by the angle of common yoke  14  within housing  2 . That is to say that if the cylinder block  24  and drive shaft  12  are in line, the pistons will not displace fluid. Cylinder block  24  may be driven by drive shaft  12  or cylinder block  24  may drive the shaft dependent on whether the bent axis piston drive unit is arranged as a motor or a pump. 
         [0042]      FIG. 4  schematically illustrates a section through the common yoke illustrated in  FIG. 1  such that the cylinder blocks of first and second bent axis piston drive units  10 ,  20  are visible. The sectioned elements of common yoke  14  are illustrated with diagonal lines. Like features of  FIGS. 1, 2, 3 and 4  are labeled using the same reference numerals. 
         [0043]    Gears  13 ,  23  that couple the respective drive shafts  12 ,  22  of each of first and second bent axis piston drive units  10 ,  20  allow first and second bent axis piston drive units  10 ,  20  to be timed relative to each other as is illustrated in  FIG. 4 . That is to say that arrangement of gears  13 ,  23  allows first and second bent axis piston drive units to be rotationally offset with respect to one another. If, for example, first and second bent axis piston drive units  10 ,  20  are timed such that an opening event of a cylinder of first bent axis piston drive unit  10  occurs at the same time as the opening event of a cylinder of second bent axis piston drive unit  20 , the amplitude of the resulting pressure pulsation within common yoke  14  may be quite high. An opening event will be understood to be a piston beginning to draw fluid into a cylinder or a piston starting to push fluid out of a cylinder. The resulting pressure pulsation may be reduced by staggering the opening events of the cylinders of first and second bent axis piston drive units  10 ,  20 . As depicted in  FIG. 4 , the opening events of the cylinders of first bent axis piston drive unit  10  are staggered by an angle α from the opening events of the cylinders of second bent axis piston drive unit  20 . Accordingly, the number of pulsations per cycle is doubled, but the amplitude is halved compared to the scenario in which the opening events of cylinders in first and second bent axis piston drive units  18 ,  20  coincide. By reducing the amplitude of the pressure pulsations, this staggering technique may reduce noise, smooth out operation, and improve fatigue life of the hydraulic assembly. Further, such assemblies can operate at higher shaft speeds because the individual components such as bearings are smaller and can tolerate higher speeds. These benefits can all be very important in commercial applications. The angle α is determined by the number of cylinders in the cylinder block and is less than the angle of rotation between two adjacent cylinders. In this example, there are nine cylinders such that the angle between adjacent cylinders is 40 degrees. And thus, angle α is 20 degrees, which is half way between two rotationally adjacent cylinders. It will be appreciated that angle α may be an integer value between 1 and 39 degrees in this example. 
         [0044]    During operation, third bent axis piston drive unit  30  may be operated as a pump and first and second bent axis piston drive units  10 ,  20  may be operated as a motor driven by the pump. Common and second yokes  14 ,  16  are rotatable with respect to housing  2  to alter the angle between the drive shaft and the cylinder block of each of the bent axis piston drive units. By altering the angle between the drive shaft and the cylinder block of each of the bent axis piston drive units, the relative speed and size of the system is altered. It will be appreciated that in this example, the angle between the drive shaft and the cylinder block of each of first and second bent axis piston drive units  10 ,  20  is altered at the same time and independent of the angle between the drive shaft and the cylinder block of third bent axis piston drive unit  30 . 
         [0045]      FIG. 5  schematically illustrates a hydrostatic assembly according to a second embodiment of the invention. Like features of  FIGS. 1 and 5  are labeled using the same reference numerals. Hydrostatic assembly  80  comprises common housing  82  that supports respective drive shafts  12 ,  22 ,  32 ,  92  of each of first, second, third and fourth bent axis piston drive units  10 ,  20 ,  30 ,  90 . Housing  82  includes a bearing (not shown) mounted within housing  82  to allow each of drive shafts  12 ,  22 ,  32 ,  92  to rotate. That is to say that each of the shafts  12 ,  22 ,  32 ,  92  are rotatably arranged or mounted within housing  82 . Each of drive shafts  12 ,  22 ,  32 ,  92  of the bent axis piston drive units include splines to allow the shafts to be coupled to other input or output mechanisms. 
         [0046]    Second common yoke  84  supports third and fourth bent axis piston drive units  30 ,  90 . Common yoke  14  and associated bent axis piston drive units  10 ,  20  are the same as those described in association with the aforementioned first embodiment. Second common yoke  84  and associated bent axis piston drive units  30 ,  90  are similar in form and operation as common yoke  14  and associated bent axis piston drive units  10 ,  20  described in association with the first embodiment. 
         [0047]    The shafts  32 ,  92  of third and fourth bent axis piston drive units  30 ,  90  are mechanically coupled together. In this example, the mechanical coupling is achieved using a gearset of two engaged gears  96 ,  98 . Furthermore, the shafts  32 ,  92  of third and fourth bent axis piston drive units  30 ,  90  are parallel to each other. 
         [0048]    In accordance with embodiments of the inventions, it is possible to use one size of rotating kit (bent axis piston drive unit, axial piston drive unit) to build multiple sizes of hydrostatic assemblies or modules. Thus, fewer distinct elements may be required to achieve multiple configurations. Furthermore, using two bent axis piston drive units (or axial piston drive units) rather than a single large hydraulic unit may result in a higher speed range of rotation being achieved because generally smaller units have a greater speed range than larger units. This can significantly improve the power density for a given size of hydrostatic assembly. 
         [0049]    A hydrostatic assembly may be constructed using swashplate design units employing axial piston drive units according to a third embodiment of the invention. Here, the cylinder blocks and drive shafts of three or more drive units are supported in a housing. The pistons of the first and second axial piston drive units are supported by a common swashplate. A third axial piston drive unit is supported on a second swashplate. The first and second axial piston drive units are partnered together and function simultaneously as pump or motor. 
         [0050]      FIGS. 6 and 7  schematically illustrate a hydrostatic assembly according to such a third embodiment. In  FIG. 6 , hydrostatic assembly  101  comprises a common housing  102  that supports respective drive shafts  112 ,  122 ,  132  of each of first, second, and third axial piston drive units  110 ,  120 ,  130  respectively. Housing  102  includes a bearing (not called out in the figures) mounted within to allow each of the drive shafts  112 ,  122 ,  132  to rotate. And each of the drive shafts  112 ,  122 ,  132  include splines to allow the shafts to be coupled to other input or output mechanisms. 
         [0051]    Two swashplates are employed in the embodiment of  FIG. 6 , namely common swashplate  114  and second swashplate  116 , and both are rotatably mounted on common pivot axis  140 . Housing  102  supports cylinder block  128  and second swashplate  116  supports the pistons of third axial piston drive unit  130 . Cylinder block  128  of third axial piston drive unit  122  is rotatably mounted to housing  102 . Rotation of second swashplate  116  with respect to housing  102  is provided by servo assembly mechanism  144 . Housing  102  also supports cylinder block  124  of first axial piston drive unit  110  and cylinder block  126  of second axial piston drive unit  120 . Common swashplate  114  supports the pistons of first axial piston drive unit  110  and second axial piston drive unit  120 . The respective cylinder blocks  124 ,  126  of first and second axial piston drive units  110 ,  120  are rotatably mounted to housing  102 . Rotation of common swashplate  114  with respect to housing  102  is provided by another servo assembly mechanism  142 . 
         [0052]    In a like manner to the preceding embodiments, cylinder block  128  of third axial drive unit  130  is fluidically coupled to first and second axial piston drive units  110 ,  120  through a similar arrangement of ports and passages in housing  102 . No hydraulic rotary joint is required however in this swashplate embodiment. Also, control of the servo assemblies may be accomplished in a like manner to the preceding embodiments. 
         [0053]    Drive shafts  112 ,  122  of first and second axial piston drive units  110 ,  120  are mechanically coupled together. In this example, the mechanical coupling is achieved using an additional gear  133  to couple together gears  113 ,  123  such that drive shafts  112  and  122  both rotate in the same direction. 
         [0054]      FIG. 7  schematically illustrates a section in a plane parallel to the drive shafts of the axial piston drive units and through a fluidic passage of the hydrostatic assembly of  FIG. 6 . Like features of  FIGS. 6 and 7  are labeled using the same reference numerals. As will be apparent to those in the art, except for differences relating to the use of swashplates in place of yokes, the elements and functions of the third embodiment are similar to those of the first embodiment. 
         [0055]      FIG. 8  schematically illustrates a section of an exemplary hydrostatic assembly according to yet other embodiments in which the hydraulic piston drive units include at least one bent axis piston drive unit and at least one axial piston drive unit. The section is in a plane parallel to the drive shafts of the hydraulic piston drive units and through a fluidic passage of the hydrostatic assembly. In  FIG. 8 , hydrostatic assembly  201  comprises a common housing  202  that supports respective drive shafts  212 ,  222 ,  232  of each of first, second, and third hydraulic drive units  210 ,  220 ,  230  respectively. Here however, first and second hydraulic drive units  210 ,  220  are bent axis piston drive units and third hydraulic drive unit  230  is an axial piston drive unit. 
         [0056]    Further, the embodiment of  FIG. 8  employs common yoke  214  and second swashplate  216  which are both rotatably mounted on common pivot axis  240 . (Note in related embodiments, the swashplate axis could differ from the yoke axis.) In a similar manner to the preceding embodiments, second swashplate  216  supports the pistons of third axial piston drive unit  230 . And this cylinder block is rotatably mounted to housing  202 . Rotation of second swashplate  216  with respect to housing  202  is provided by a servo assembly mechanism  244 . Common yoke  214  supports the two cylinder blocks of the first and second bent axis piston drive units  210 ,  220 . As in the first embodiment, these two cylinder blocks are rotatably mounted within common yoke  214 . Rotation of common yoke  214  with respect to housing  202  is provided by servo assembly mechanism  242 . 
         [0057]    In a like manner to the preceding embodiments, the cylinder block of third axial drive unit  230  is fluidically coupled to first and second bent axis piston drive units  210 ,  220  through a similar arrangement of ports and passages in housing  202 . A modified hydraulic rotary joint  205  is employed. Control of the servo assemblies may be accomplished in a like manner to the preceding embodiments. 
         [0058]    “Hybrid” embodiments like that illustrated in  FIG. 8  can thus be considered for a hydrostatic assembly where the advantages of bent axis piston drive units might be preferred for a motor function and those of axial piston drive units might be preferred for a pump function, or vice versa. Further, use of an axial piston drive unit and accompanying swashplate allows for the output drive shaft to be a through-shaft extending through the top of the assembly. 
         [0059]    All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification, are incorporated herein by reference in their entirety. 
         [0060]    While the invention is described herein by way of example for several embodiments and illustrative drawings, those skilled in the art will recognize that the invention is not limited to the embodiments or drawings described. It should be understood, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention. The headings used herein are for organizational purposes only and are not meant to be used to limit the scope of the description. As used throughout this application, the word “may” is used in a permissive sense (i.e. meaning “might”) rather than the mandatory sense (i.e., meaning “must”). Similarly, the words “include”, “including”, and “includes” mean including, but not limited to.