Abstract:
A hydrostatic variator having a common yoke design and two or more bent axis piston drive units is disclosed in which the rotating group of a drive unit is arranged to be movable relative to the yoke to alter the angle between its drive shaft axis and rotating group axis independent of the other drive unit. This can be accomplished using movable sector plates coupled to the rotating groups that are arcuately movable within the yoke about the axis perpendicular to the respective drive shaft axes of the drive units. Such variators enjoy the response and packaging advantages of a common yoke design while still allowing dynamic adjustment of system size. In turn, varying system size allows for efficiency to be improved at conditions other than at the maximum design torque.

Description:
INTRODUCTION 
       [0001]    The present invention relates generally to a hydrostatic variator. 
         [0002]    Although many promising alternative power sources are being developed for motor vehicles, the predominant power source today is the internal combustion engine. Current IC-driven designs have evolved substantially over the last century but there is still room for improvement. The use of continuously variable and infinitely variable transmissions (CVTs and IVTs) allows the engine to operate at its peak performance point improving performance as well as efficiency while reducing the impact on the environment. 
         [0003]    In a split path IVT, power from the engine is split into two branches and recombined at the output. One branch is purely mechanical and may consist of as little as a shaft and a pair of gears. The other branch consists of a continuously variable device able to change the speed and torque ratios of the transmission. Power will flow exclusively through the mechanical branch, through the variator branch or a blend of the two. Since the transmission will be the most efficient when most or all of the power flows thru the mechanical branch, the transmission will be designed such that this condition occurs at the most frequently used operating point in the duty cycle. 
         [0004]    Several different devices such as toroidal drives to electric motor/generator sets have been used to create a continuously variable gearset or “variator” for the IVT. Hydrostatic variators are currently the best suited of these to handle the power demands of large on and off road trucks and construction equipment. In addition, mobile hydraulics technology is very mature offering proven reliability as well a wide range of readily available components. 
         [0005]    In a hydrostatic variator, one or both of the pump and motor are of a variable displacement design. The pump and motor are in fluid communication such that fluid from the pump drives the motor. By changing the displacement ratio between the pump and motor the speed and torque ratios between input and output shafts can be varied. Typically, an electronic controller regulates the pump and motor displacements depending on operator demands and drive train conditions. The sum of the pump and motor displacements is known as the system size. 
         [0006]      FIG. 1  illustrates a prior art hydrostatic variator described in US 2010/0212309. In  FIG. 1 , variator  2  comprises a first bent axis unit or axial piston unit and a second bent axis unit or axial piston unit  4 . It is noted that the first bent axis unit or axial piston unit is not visible in the figure. Output/input shafts  6  of the respective bent axis units are positioned within housing  8  to retain the outputs shafts in a predetermined relative position. Each of the first bent axis unit and the second bent axis unit  4  includes a rotating group of pistons  10  coupled to the respective output/input shafts  6  and are movable within cylinders located in respective cylinder blocks  12 . The cylinder blocks  12  are rotatable within respective yokes  14 ,  16  that also provide fluid channels between respective cylinder blocks  12  of the first bent axis unit and the second bent axis unit  4 . Each of the yokes  14 ,  16  are independently rotatable about axis  18  such that displacement of fluid from the first bent axis unit to the second bent axis unit or vice versa can be varied. The displacement of each bent axis unit depends on the rotational angle of its yoke relative to the housing. The position of each yoke is control independently by its own servo assembly ( 20  or  22 ). 
         [0007]    Most split path IVT power train control algorithms are set up for relatively simplistic power management schemes: maintain the engine either at peak power or at its point of least fuel consumption. This approach does create significant gains over stepped ratio transmissions but more refined control is required to realize the full potential of an IVT. 
         [0008]    The variator of a split path IVT must be designed to handle as much as or more than half of the input power to the transmission. However, depending on the loading cycle of the vehicle, full power may not be required all the time. A hydraulic drive system which is “too large” for the amount of power being transferred will not run as efficiently as a smaller system. A method commonly employed to improve this efficiency under partially loaded conditions is to reduce the pump and motor displacement proportionately (i.e. reducing system size) to increase system pressure while maintaining the desired speed ratio. 
         [0009]    In current variator systems using independently adjustable pumps and motors, a microcontroller controls the speed and torque ratios via hydroelectric servos. Problems arise when multiple servos are required to respond quickly and in a synchronized manner. Stick/slip conditions can occur which vary with system pressure; creating a control algorithm to address all situations can be complex and difficult. In the current state of the art, servo mechanisms have limitations with regards to response to transient forces. All of this acids a degree of unpredictability to the system. 
         [0010]    One approach to create a more predictable system is to reduce the number of interfaces where stick/slip can occur. This can be done by using a common yoke for the pump and motor rather than a separate one for each, for instance such as the arrangement shown in DE1064311B. The pump and motor are hydraulically close coupled which leads to a smaller physical envelope, and dynamic rotary seals between pump and motor are not required. As well, only one servo assembly is required to move both pump and motor. The problem with this approach is the angle between the pump and motor yokes is fixed, and hence the system size is fixed. When applied in a split path transmission, potential efficiency gains by altering system size are not possible. 
         [0011]    Therefore, there is a need for a hydrostatic variator which can combine the response and packaging advantages of a common yoke design while still allowing dynamic adjustment of system size. The present invention addresses these needs and provides other related benefits as described below. 
       SUMMARY OF THE INVENTION 
       [0012]    A hydrostatic variator providing dynamic adjustment of system size in a common yoke design comprises a housing, first and second bent axis piston drive units which each comprise a rotating group, a yoke common to both drive units, and a yoke servo assembly to position the yoke relative to the housing. The respective rotating groups in each piston drive unit are arranged to rotate within a yoke. And the yoke includes one or more fluid channels for fluid communication between the first and second bent axis piston drive units. The yoke is rotatable relative to the housing about a yoke axis perpendicular to both a drive shaft of the first bent axis piston drive unit and to a drive shaft of the second bent axis piston drive unit to simultaneously alter an angle between the drive shaft axis and the rotating group axis of each of the first and second bent axis piston drive units. For purposes of dynamic adjustment, the rotating group of the first bent axis piston drive unit is arranged to be movable relative to the yoke in order to alter the angle between the drive shaft axis and the rotating group axis of the first bent axis piston drive unit independent of the second bent axis piston drive unit. Since the system size can be varied, efficiency can be improved for conditions other than the maximum design torque of the variator. 
         [0013]    In one embodiment, dynamic adjustment can be achieved by employing a first movable sector plate that is coupled to the rotating group of the first bent axis piston drive unit and that is arcuately movable within the yoke about a first sector plate axis perpendicular to the drive shaft axis of the first bent axis piston drive unit. (The first sector plate axis and the yoke axis may be the same or different axes.) A first servo assembly can be employed to position the first movable sector plate relative to the yoke. Further, the first movable sector plate can comprise an elongate opening in order to allow fluid to flow between the rotating group of the first bent axis piston drive unit and the fluid channel of the yoke independent of the arcuate position of the first movable sector plate. 
         [0014]    In another embodiment, the rotating group of the second bent axis piston drive unit can be arranged, or also be arranged, to be movable relative to the yoke to alter the angle between the drive shaft axis and the rotating group axis of the second bent axis piston drive unit independent of the first bent axis piston drive unit. With adjustment capability for both rotating groups, the possible range of adjustment of system size of the variator can be increased. 
         [0015]    As with the first bent axis piston drive unit, a second movable sector plate can thus be coupled to the rotating group of the second bent axis piston drive unit that is arcuately movable within the yoke about a second sector plate axis perpendicular to the drive shaft axis of the second bent axis piston drive unit. (As before, the second sector plate axis and the yoke axis may be the same or different axes.) In a preferred embodiment, both first and second movable sector plates are employed. A second servo assembly can be employed to position the second movable sector plate relative to the yoke. And as before, the second movable sector plate can comprise an elongate opening in order to allow fluid to flow between the rotating group of the second bent axis piston drive unit and the fluid channel of the yoke independent of the arcuate position of the second movable sector plate. 
         [0016]    In certain exemplary practical embodiments, the yoke can be rotatable to simultaneously alter the angle between the drive shaft axis and the rotating group axis of each of the first and second bent axis piston drive units over a range from at least 0 to about 40 degrees. And the angle between the drive shaft axis and the rotating group axis of the first bent axis piston drive unit can be altered over the range from at least 0 to about 20 degrees independent of the second bent axis piston drive unit. 
         [0017]    In the first and second bent axis piston drive units, the axes of the drive shafts can be parallel, while the axes of the rotating groups may not be parallel. 
         [0018]    Depending on the intended application, either one of the first and second bent axis piston drive units can serve as a pump and the other of the first and second bent axis piston drive units can serve as a motor. 
         [0019]    In an associated method of the invention, the angle between the drive shaft axis and the rotating group axis of a first bent axis piston drive unit is altered independent of a second bent axis piston drive unit in a hydrostatic variator having a common yoke design. The method comprises arranging the rotating group of the first bent axis piston drive unit to be movable relative to the yoke to alter the angle between the drive shaft axis and the rotating group axis of the first bent axis piston drive unit independent of the second bent axis piston drive unit. In one embodiment, a first movable sector plate is incorporated that is arcuately movable within the yoke about the axis perpendicular to the drive shaft axis of the first bent axis piston drive unit, and the first movable sector plate is coupled to the rotating group of the first bent axis piston drive unit. In another embodiment, a second movable sector plate is instead or is additionally incorporated and coupled in a like manner in the second bent axis piston drive unit. In other embodiments, other means may be employed to arrange the rotating group of the first and/or second bent axis piston drive units so as to be movable relative to the yoke, and thereby obtain dynamic adjustment of angle between the first and second bent axis piston drive units. For instance, miniature yoke configurations may be considered within the main yoke of the variator. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    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: 
           [0021]      FIG. 1  illustrates a prior art hydrostatic variator in US 2010/0212309; 
           [0022]      FIG. 2  illustrates a perspective view of a hydrostatic variator according to an embodiment of the invention; 
           [0023]      FIG. 3  illustrates a side view of the hydrostatic variator of  FIG. 2 ; 
           [0024]      FIG. 4  illustrates a front view of the hydrostatic variator of  FIG. 2 ; 
           [0025]      FIG. 5  illustrates a perspective view of the hydrostatic variator shown in  FIG. 2  with a section of the yoke removed to reveal fluid connection channels formed within the yoke; 
           [0026]      FIG. 6  illustrates a perspective view of the hydrostatic variator shown in  FIG. 2  with a section of the yoke removed to reveal the two bent axis piston drive units; 
           [0027]      FIG. 7  illustrates a cross section A-A through the hydrostatic variator shown in  FIG. 4 ; 
           [0028]      FIG. 8  illustrates a cross section B-B through the hydrostatic variator illustrated in  FIG. 4 ; and 
           [0029]      FIG. 9  illustrates the first and second bent axis piston drive units and sector plates of the hydrostatic variator of  FIG. 2  with the housing and the yoke removed. 
       
    
    
     DETAILED DESCRIPTION 
       [0030]    As is commonly used in the art, herein “system size” refers to the sum of the pump and motor displacements in a hydrostatic variator. 
         [0031]      FIG. 2  illustrates a perspective view of a hydrostatic variator or hydraulic module  50  according to an embodiment of the invention. Hydrostatic variator  50  comprises a housing  56  that supports first bent axis piston drive unit  52 , shown on the left hand side of the figure and second bent axis piston drive unit  54  shown on the right hand side of the figure. Housing  56  supports a distal end of each of the bent axis piston drive units  52 ,  54  and allows each of the bent axis piston drive units  52 ,  54  to rotate relative to the housing  56 . Housing  56  is manufactured such that output shafts (not shown in the figure) of each of the bent axis piston drive units  52 ,  54  are parallel to one another and are generally perpendicular to the housing. Each of the bent axis piston drive units  52 ,  54  includes at least two pistons  74  (in this example, nine pistons) that are coupled to a respective output shaft (not shown in the figure) and are movable in and out of a cylinder block  72 . A distal end of the piston includes a spherical portion to allow the angle between the piston and the shaft to be altered while the shaft rotates. (In other embodiments, the output shafts need not he parallel.) 
         [0032]    Hydrostatic variator  50  further comprises yoke  58 , which may be referred to as a common yoke, that supports a proximal end of each of the bent axis piston drive units  52 ,  54  and allows each of bent axis piston drive units  52 ,  54  to rotate relative to yoke  58 . Yoke  58  is manufactured such that it includes two portions  58   a,    58   b  that are joined together to form a single or common yoke and that are angularly offset about axis  60  with respect to one another. In this example, yoke portions  58   a,    58   b  are offset by an angle of 20 degrees, but other offset angles are envisaged. Therefore, an angle between an output (or input) shaft (not shown in the figure) and a cylinder block  70  of first bent axis piston drive unit  52  will be different than an angle between an output (or input) shaft (not shown in the figure) and cylinder block  72  of second bent axis piston drive unit  54 . 
         [0033]    Yoke  58  is rotatable with respect to housing  56  about axis  60 . By rotating yoke  58  in this manner, the angle between an output (or input) shaft and cylinder block  70 ,  72  of each of first and second bent axis piston drive units  52 ,  54  can be varied, as is described in more detail below. The angle of the yoke may be adjusted in various ways, such as by use of electro-mechanical jack screws, rotary stepper motors, etc. Here, a yoke servo assembly  170  consisting of a pair of setting pistons  66  (visible in  FIGS. 2 ) and  80  (not visible in  FIG. 2 ) is coupled at a distal end to yoke  58  and a proximal end can be moved in and out of cylinder  68  (visible in  FIGS. 2 ) and  84  (not visible in  FIG. 2 ) positioned on the housing to rotate yoke  58  with respect to housing  56 . Yoke servo assembly  66  is generally cylindrical and is driven using a suitable hydraulic fluid, e.g., mineral oil. It will be appreciated that the piston may be positioned such that the distal end is coupled to housing  56  and the proximal end is movable within a cylinder positioned on yoke  58 . The housing also includes relief valves  76   a,    76   b  that are used for introducing hydraulic fluid into or removing hydraulic fluid from yoke  58 . (Valves  76   a,    76   b  may also be used for relieving pressure from the high pressure port to the low pressure port. Valves  76   a,    76   b  are used in combination with a channel within yoke  58  that is opened or closed using the valves. A boost port  102   c  to provide make-up oil for leakage past the dynamic seals may also be incorporated in yoke  58 .) 
         [0034]    Yoke  58  also includes two servo assemblies or setting mechanisms  62 ,  64 . Each servo assembly  62 ,  64  includes a piston and a cylinder, as described below. First servo assembly  62  is movable, as indicated by the arrows on the figure, to move cylinder block  70  arcuately within yoke  58 , and relative to yoke  58  and independently of cylinder block  72  of second bent axis piston drive unit  54 . A second servo assembly  64  is movable, as indicated by the arrows on the figure, to move cylinder block  72  arcuately within yoke  58 , and relative to yoke  58  and independently of cylinder block  70  of first bent axis piston drive unit  52 . Arcuately is used to describe the movement of cylinder blocks  70 ,  72  because cylinder blocks  70 ,  72  are moved in an arc at a fixed distance from the rotational axis of yoke  58 . 
         [0035]      FIG. 3  illustrates a side view of the hydrostatic variator of  FIG. 2 . The reference numerals used in  FIG. 2  are also used in  FIG. 3  to identify the same features. Setting piston  80  and cylinder  84  are illustrated in the figure. In operation, yoke setting pistons  66 ,  80  are operated antagonistically. That is to say that in the figure, yoke setting piston  66  is used to rotate yoke  58  in a clockwise direction and yoke setting piston  80  is used to rotate yoke  58  in a counter-clockwise direction. Output shaft  82  of first bent axis piston drive unit  52  is illustrated in the figure. Shaft  82  includes a number of splines  86  for coupling the shaft to another device or assembly. 
         [0036]      FIG. 4  illustrates a front view of the hydrostatic variator of  FIG. 2 . The reference numerals used in  FIG. 2  are also used in  FIG. 4  to identify the same features.  FIG. 4  also indicates the locations of cross sections A-A and B-B which appear in subsequent Figures. 
         [0037]      FIG. 5  illustrates a perspective view of the hydrostatic variator of  FIG. 2  with a section of yoke  58  removed to reveal the fluid connection channels formed within yoke  58 . The cut surface of yoke  58  is illustrated in the figure by the diagonal hatching. The reference numerals used in  FIG. 2  are also used in  FIG. 5  to identify the same features. 
         [0038]    In  FIG. 5 , a first and a second piston  90 ,  92  of the fast and second servo assemblies  62 ,  64  are illustrated. Pistons  90 ,  92  are movable in the directions illustrated by the arrows in the figure. First and second pistons  90 ,  92  are movable within a respective cylinder (not shown in the figure) and are coupled to respective first and second bent axis piston drive units  52 ,  54  such that when first or second piston  90 ,  92  is moved as indicated by the arrows in the figure, the cylinder block of the respective bent axis unit moves with the respective piston  90 ,  92 . Each piston  90 ,  92  includes a lip seal or o-ring seal  94 ,  96  at each end which is used to seal pistons  90 ,  92  within their cylinders (not shown). Each piston  90 ,  92  is effectively two antagonistic pistons, such that an upper piston portion (according to the orientation of the figure) is operated to move the position in a downward direction (according to the orientation of the figure) and a lower piston portion (according to the orientation of the figure) is operated to move the position in a upward direction (according to the orientation of the figure). Pistons  90 ,  92  are driven using a suitable hydraulic fluid, e.g., mineral oil. 
         [0039]    First fluid channel  98  and second fluid channel  100  are illustrated in the figure. First and second fluid channels  98 ,  100  fluidically couple the first and second bent axis piston drive units  52 ,  54 . That is to say that the first and second fluid channels  98 ,  100  provide fluid communication between the first and second bent axis piston drive units  52 ,  54 . Through holes  106   a,    106   b,    106   c,    106   d  are provided in yoke  58  at each end of each fluid channels  98 ,  100  going between fluid channels  98 ,  100  and bent axis piston drive units  52 ,  54 . During operation, for example, first fluid channel  98  carries fluid from first bent axis piston drive unit  52  to second bent axis piston drive unit  54  and second fluid channel  100  carries fluid from second bent axis piston drive unit  54  to first bent axis piston drive unit  52 . However, this will depend on the rotational direction of each of bent axis piston drive units  52 ,  54 . 
         [0040]      FIG. 5  and the aforementioned description relate to a closed loop system. An open loop system familiar to those skilled in the art may also be employed. With an open loop configuration, one of fluid channels  98  or  100  would be eliminated and replaced with a direct opening to the case which contains the whole assembly and would be flooded with hydraulic fluid. 
         [0041]      FIG. 6  illustrates a perspective view of the hydrostatic variator of  FIG. 2  with a section of yoke  58  removed to reveal the bent axis piston drive units  52 ,  54 . The cut surface of yoke  58  is illustrated in the figure by the diagonal hatching. The reference numerals used in  FIG. 2  are also used in  FIG. 6  to identify the same features. 
         [0042]    First bent axis piston drive unit  52  is illustrated on the left hand side of variator  50 . First bent axis unit  52  includes a first sector plate  106  that is movable in an are within the yoke  58  about axis  60 . (Although drawn this way here, the first sector plate axis that the first sector plate rotates about does not have to be the same as yoke axis  60  that the yoke rotates about. It could be offset and parallel to  60 ). First sector plate  106  is coupled to cylinder block  70  of first bent axis piston drive unit  52  such that cylinder block  70  is able to rotate with respect to sector plate  106 . First sector plate  106  includes a coupling or socket  112  to allow a cooperating coupling or ball of piston  90  to be coupled together (piston  90  is not shown in the figure). Here, a ball and socket  112  arrangement is used so that the movement of piston  90  can be linear and the movement of sector plate  106  can be arcuate. First sector plate  106  includes first and second through holes  118 ,  120 , such that fluid from cylinder block  70  of first bent axis piston drive unit  52  can pass through sector plate  106  to fluid channels  98 ,  100  (channel  100  is not shown in the figure) in yoke  58 . Through holes  118 ,  120  are elongated so that as sector plate  106  is moved arcuately at least a portion of the openings  118 ,  120  are aligned with through holes  106   c,    106   d  in yoke  58  (through hole  106   d  is not shown in this figure). In operation, when piston  90  is moved upward or downward, first sector plate  106  is moved arcuately clockwise or counter clockwise. 
         [0043]    Second bent axis piston drive unit  54  is illustrated on the right hand side of variator  50 . Second bent axis piston drive unit  54  includes second sector plate  108  that is movable in an arc within Yoke  58  about axis  60 . (Again, although drawn this way here, the second sector plate axis that the second sector plate rotates about does not have to be the same as yoke axis  60  that the yoke rotates about. It could be offset and parallel to  60 ). Second sector plate  108  is coupled to cylinder block  72  of second bent axis piston drive unit  54  such that cylinder block  72  is able to rotate with respect to second sector plate  108 . Second sector plate  108  includes a coupling or socket  116  to allow a cooperating coupling or ball  110  of piston  92  to be coupled together. A ball  110  and socket  116  arrangement is used so that the movement of piston  92  can be linear and the movement of second sector plate  108  can be arcuate. Second sector plate  108  also includes first and second through holes  112 ,  114 , such that fluid from cylinder block  72  of second bent axis piston drive unit  54  can pass through second sector plate  108  to fluid channels  98 ,  100  (channel  100  not shown in the figure) in yoke  58 . Through holes  112 ,  114  are elongate so that as second sector plate  108  is moved arcuately at least a portion of the openings  112 ,  114  are aligned with through holes  106   a,    106   b  in yoke  58  (through holes  106   a,    106   b  are not shown in this figure). In operation, when piston  92  is moved upward or downward, second sector plate  108  is moved arcuately clockwise or counter clockwise. 
         [0044]      FIG. 7  illustrates a cross section A-A through hydrostatic variator  50  shown in  FIG. 2 . The location of cross section A-A appears in  FIG. 4 . In particular, the cross section illustrates a section through first bent axis piston chive unit  52 . The cut surface of the various elements is illustrated in the figure by the diagonal hatching. The reference numerals used in  FIGS. 1 to 6  are also used in  FIG. 7  to identify the same features. 
         [0045]    Shaft  82  is positioned in housing  56  using one or more bearings  132  to allow the shaft to freely rotate. Piston assembly  62  in the figure is shown in cross section such piston  90  and associated seal  94  can be viewed with cylindrical chamber or cylinder  138 . Piston  90  includes ball  136  that is coupled to socket  112  of first sector plate  106 , such that when piston  90  is moved in a linear manner, first sector plate  106  is moved in an arcuate manner, as illustrated in the figure. The socket or opening  112  in first sector plate  106  is elongate to allow movement of the ball  136  of the piston in relation to first sector plate  106 , as illustrated by the arrows in the figure, as the piston is moved. 
         [0046]    First sector plate  106  of first bent axis piston drive unit  52  includes a spindle  134  that is located in the centre of cylinder block  70  to allow cylinder block  70  to rotate. First sector plate  106  is located in an elongate recess in yoke  58  which provides a track in which first sector plate  106  can be moved in an arcuate manner. 
         [0047]      FIG. 8  illustrates a cross section B-B through hydrostatic variator  50  shown in  FIG. 2 . The location of cross section B-B appears in  FIG. 4 . In particular the cross section illustrates a section through second bent axis piston drive unit  54 . The cut surface of the various elements is illustrated in the figure by the diagonal hatching. The reference numerals used in  FIGS. 1 to 6  are also used in  FIG. 8  to identify the same features. 
         [0048]    The components, configuration and operation of the elements of second bent axis piston drive unit  54  shown in  FIG. 8  are similar to those shown in the cross section through first bent axis piston drive unit  52  shown in  FIG. 7 . Shown is shaft  148  positioned in housing  56  using one or more bearings  152  and includes a plurality of splines  164 . The axis of rotation  160  of shaft  148  is illustrated in the figure. Piston assembly  64  is shown in cross section such piston  92  and associated seals  96  can be viewed with a cylindrical chamber or cylinder  158 . Piston  92  includes a ball  110  that is coupled to the socket  116  of second sector plate  108 . Second sector plate  108  includes a spindle  154  and one or more seals (not shown in the figure). Cylinder block  72  here includes nine cylinders  162  and nine respective pistons  74 . 
         [0049]      FIG. 9  illustrates only the first and second bent axis piston drive units  52 ,  54  with housing  56  and yoke  58  removed. The reference numerals used in  FIGS. 1 to 8  are also used in  FIG. 8  to identify the same features. 
         [0050]    The operation of variator  50  is now described using the elements described in association with  FIGS. 2 to 9 . 
         [0051]    In operation, first bent axis piston drive unit  52  can be operated as a pump and second bent axis piston drive unit  54  can be operated as a motor. It will be appreciated that either bent axis piston drive units can be operated as a pump or a motor. 
         [0052]    While shaft  148  of the pump is rotated, the angle between shafts  82 ,  148  and spindles  134 ,  154  can be adjusted simultaneously by rotating yoke  58  about yoke axis  60  using yoke servo assembly  170 . Also, the angle between shafts  82 ,  148  and spindles  134 ,  154  can be adjusted independently using piston assemblies  90 ,  92  and cylinders  138 ,  158  of the individual first and second servo assemblies  62 ,  64 . As previously described, the movement of the individual first and second servo assemblies moves sector plates  106 ,  108  in an arcuate manner within and relative to yoke  58 . Therefore, by rotating yoke  58 , the speed ratio between the input shaft  82  (the shaft  82  of first bent axis piston drive unit  52 ) and the output shaft  148  (the shaft  148  of the second bent axis piston drive unit  54 ) is altered and by moving the first and/or second sector plates  106 ,  108 , the size of the system is altered. By the system size, the efficiency can be maximized for conditions other than the maximum design torque. When running at smaller system sizes, it is possible to absorb small shocks to the system by “growing” the system size in response to the torque spike felt at the output. This could reduce and/or eliminate severe pressure spikes. 
         [0053]    Another possible embodiment of the invention involves only a single movable sector plate, i.e. the angle of only one of the rotating groups is variable relative to the yoke, while the other rotating group has no movable sector plate and its angle with respect to the yoke is fixed. This of course reduces the possible differential angle range that may be achieved if two sector plates are employed. 
         [0054]    With the present invention, advantageously the range of angle adjustment is similar to that of variators with independent yokes. In this regard, present bent axis piston drive units have a practical limit of about 40° to 45° because of limitations of the piston and ball socket geometry employed. The additional adjustment provided by the sector plates is expected to be slightly less than half of the full yoke displacement. Thus for instance, if the maximum total displacement angle between the pump and the motor was 40°, the sector plates might provide an additional adjustment range of about 20° each. (Note: in prior art variators that use only sector plates to adjust angle, it is difficult to have large angle ranges because of the limitations of the porting slots between the sector plate and the housing. At some point these slots choke off the ports because of the extreme angle and efficiency will be lost.) 
         [0055]    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.