Patent Publication Number: US-2023151875-A1

Title: Drive system architecture for improved motor efficiency

Description:
INTRODUCTION 
     The present disclosure generally relates to motor driven systems and more specifically, to drive systems with gear architectures providing desired and optimized performance by leveraging high speed motor input with minimized driveline origin loss inducing loads. 
     Motor driven systems of apparatus such as vehicles and other equipment and machinery, provide a motive force/torque for a variety of purposes. In applications such as a driveline of an electrified vehicle, power for the motor is at a premium and is preferably conserved. When employing relatively high speed motors, any added loads on the motor shaft tend to significantly increase power consumption leading to reduced operational range of the vehicle. In other various applications, added loads from the driven system may lead to a need to oversize the motor and/or to employ heavier bearings. Any added weight in battery powered vehicle applications may also lead to reduced range and so is preferably avoided. 
     In a number of applications, a motor may be coupled to the driven load through a gearing arrangement that increases or reduces rotational speed and torque. The gearing arrangement may take a variety of forms and generally, the moving parts include gears (simple or planetary), shafts and bearings. Any moving mechanical system has inefficiencies that arise from sources such as friction and other generated forces. Bearings and lubricants are often employed to reduce friction, increasing efficiency and performance while reducing wear. As the desire to further reduce inefficiencies increases, such as in battery powered vehicle applications, additional improvements would be beneficial. 
     Accordingly, it is desirable to provide motor driven systems for a variety of applications that result in appropriate performance characteristics such as torque/force requirements, and that provide desired levels of efficiency at minimized cost. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background. 
     SUMMARY 
     Systems are provided for delivering power through a drive system with desirable performance characteristics such as operating a motor at high efficiency. In a number of embodiments, a drive system includes a motor and a gear system coupled with the motor by a shaft. At least one input gear is disposed on the shaft. One transfer shaft includes a transfer gear meshing with the input gear(s). Another transfer shaft includes an additional transfer gear meshing with the input gear(s). The gear system is configured to cancel axial forces at the shaft to avoid loads on the motor. 
     In additional embodiments, two input gears are opposite handed helix gears configured to cancel the axial forces at the shaft. The gear system is also configured to cancel at least one of radial and tangential forces 
     In additional embodiments, an output shaft carries a pair of output gears that are helix gears with opposite handed helix angles. 
     In additional embodiments, an additional pair of transfer gears are disposed on the transfer shafts and mesh with the output gears. The additional transfer gears are configured to cancel radial and tangential forces of the transfer gears and the output gears. 
     In additional embodiments, one of the input gears and one of the output gears have common handed helix angles, and the other of the input gears and the other of the output gears have different common handed helix angles. 
     In additional embodiments, one of the input gears and one of the output gears have helix angles defining a ratio of tangents approximately equal to a ratio of pitch diameters of two of the transfer gears on opposite transfer shafts. 
     In additional embodiments, bearings disposed on the transfer shaft(s), are configured to allow axial motion of the transfer shaft(s). 
     In additional embodiments, four pairs of meshing gears are included in the gear system. The output shaft carries a pair of output gears. The four pairs of meshing gears include one input gear meshing with a first of the transfer gears, the other input gear meshing with a second of the transfer gears, a third of the transfer gears meshing with one of the output gears and a fourth of the transfer gears meshing with the other output gear. 
     In additional embodiments, two transfer shafts are coaxial. One transfer shaft is a hollow shaft with a portion of the other transfer shaft extending through the hollow shaft. 
     In additional embodiments, an output shaft carries a pair of output gears. At least one output gear is a helix gear with a first helix angle of a first magnitude. At least one input gear is a helix gears with a second helix angle of a second magnitude that differs from the first magnitude enabling self-correction of force generation in the drive system. 
     In a number of additional embodiments, a drive system includes a motor and an input shaft driven by the motor that rotates about an input axis. A gear system is coupled with the motor by the input shaft, and includes first and second input gears disposed on the input shaft. A first transfer shaft includes a first transfer gear meshing with the first input gear, and a second transfer shaft includes a second transfer gear meshing with the second input gear. The first transfer gear and the first input gear include structures configured to cancel at least one of axial, radial and tangential forces of the second transfer gear and the second input gear at the input shaft. The first transfer shaft rotates about a first transfer axis and the second transfer shaft rotates about a second transfer axis. The input axis, the first transfer axis, and the second transfer axis all lie approximately in a common plane. 
     In additional embodiments, the first and second input gears comprise opposite handed helix gears with helix angles of a common magnitude and are configured to cancel the axial forces at the input shaft. 
     In additional embodiments, an output shaft is disposed on an output shaft axis. A first output gear is disposed on the output shaft, and a second output gear is disposed on the output shaft. The output gears comprise helix gears with opposite handed helix angles, and the output shaft axis lies outside the common plane. 
     In additional embodiments, a third transfer gear is disposed on the first transfer shaft and meshes with the first output gear. A fourth transfer gear is disposed on the second transfer shaft and meshes with the second output gear. The first output gear and the second output gear have a common pitch diameter. 
     In additional embodiments, the first and second input gears comprise a first double helix arrangement on the input shaft, and the first and second output gears comprise a second double helix arrangement on the output shaft. The first input gear and the first output gear have first common handed helix angles. The second input gear and the second output gear have second common handed helix angles. 
     In additional embodiments, a first bearing is disposed on the first transfer shaft and a second bearing is disposed on the first transfer shaft. A third bearing supports the second transfer shaft, and a fourth bearing supports the second transfer shaft. The first and second bearings are configured to allow axial motion of the first transfer shaft, and the third and fourth bearings are configured to allow axial motion of the second transfer shaft. 
     In additional embodiments, four pairs of meshing gears are included in the gear system, and an output shaft carries a first output gear and a second output gear. The four pairs of meshing gears include the first input gear meshing with the first transfer gear, the second input gear meshing with the second transfer gear, a third transfer gear meshing with the first output gear and a fourth transfer gear meshing with the second output gear. A first power flow path is defined from the input shaft, through the first input gear to the first transfer gear, through the first transfer shaft, and through the third transfer gear to the first output gear and to the output shaft. A second power flow path is defined from the input shaft, through the second input gear to the second transfer gear, through the second transfer shaft, and through the fourth transfer gear to the output shaft. 
     In additional embodiments, an output shaft in included in the gear system. The first and second transfer shafts are disposed at equal offset angles relative to the output shaft. 
     In additional embodiments, first and second output gears are disposed on an output shaft. The first and second input gears have a first common pitch diameter. The first and second output gears have a second common pitch diameter. The first and second output gears comprise output helix gears with first helix angles of a first magnitude. The first and second input gears comprise input helix gears with second helix angles of a second magnitude. The first magnitude differs from the second magnitude enabling self-correction of force generation in the drive system. 
     In a number of other embodiments, a drive system includes a motor driving an input shaft. A gear system drives an output shaft and is coupled with the motor by the input shaft. The gear system includes first and second input gears disposed on the input shaft. A first transfer shaft includes a first transfer gear meshing with the first input gear, and a second transfer shaft includes a second transfer gear meshing with the second input gear. The first transfer gear and the first input gear are configured to cancel radial and tangential forces of the second transfer gear and the second input gear at the input shaft. The gear system includes at least one output gear on the output shaft, the at least one output gear coupled with at least one of the first and second transfer shafts through a third transfer gear. The first and second input gears comprise a first double helix gear arrangement on the input shaft. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The exemplary embodiments will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and wherein: 
         FIG.  1    is a schematic illustration of a vehicle with a drive system including a motor driven gear system, in accordance with various embodiments; 
         FIG.  2    is a schematic illustration of helix gears for the system of  FIG.  1   , in accordance with various embodiments; 
         FIG.  3    is a schematic diagram of part of the drive system of  FIG.  1    with double helical first and second stages and two transfer shafts on two sides of the input shaft, in accordance with various embodiments; 
         FIG.  4    is an output end view schematic diagram of the motor and gear system of  FIG.  3   , in accordance with various embodiments; 
         FIG.  5    is a force diagram output end view for part of the drive system of  FIG.  1   , in accordance with various embodiments; 
         FIG.  6    is a schematic diagram for part of the drive system of  FIG.  1    with alternate gear locations, in accordance with various embodiments; 
         FIG.  7    is a schematic diagram for part of the drive system of  FIG.  1    with an alternate bearing arrangement, in accordance with various embodiments; 
         FIG.  8    is a schematic diagram for part of the drive system of  FIG.  1    with a bearing between the input gears, in accordance with various embodiments; 
         FIG.  9    is a schematic diagram for part of the drive system of  FIG.  1    with four transfer shafts, in accordance with various embodiments; 
         FIG.  10    is a schematic diagram for part of the drive system of  FIG.  1    with transfer shafts on one side, in accordance with various embodiments; 
         FIG.  11    is a schematic diagram for part of the drive system of  FIG.  1    with single helical output, in accordance with various embodiments; and 
         FIG.  12    is a schematic diagram for part of the drive system of  FIG.  1    with alternate gear locations in a single helical output, in accordance with various embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely exemplary in nature and is not intended to limit the application and uses. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding introduction, brief summary or the following detailed description. 
     For the systems disclosed herein, motor input is delivered to a load through a gear system with balanced axial, radial and/or tangential forces. Balancing the axial, radial and/or tangential forces reduces loads, such as those introduced by the gearing onto the motor, leading to optimal performance, reduced weight, and improved efficiency. In a number of embodiments, a pair of transfer shafts include transfer gears that mesh with input gears on an input shaft and gears that mesh with output gears on the output shaft. The transfer shafts may be physically disposed to balance radial forces on the input and/or output shafts, when desired. In embodiments, the gearing arrangement enables the transfer shafts to each have gears with opposite helix angles and therefore to impose axial thrust reactions on the input and the output shafts in opposite directions cancelling the axial forces on the motor and on the downstream driveline. Reference to cancel forces herein means avoiding, eliminating, negating and/or nullifying forces, fully, or at least partially. Axial forces may be canceled on select transfer shafts through the use of axially floating shafts via appropriate bearings to self-correct for variations. Two independent load paths from input to output may be provided through individual and/or double gear meshes. Specific helix angles and paired arrangements of the gears on the shafts optimize alignment and other aspects of operation of the gearing. Radial and/or tangential forces may be avoided or canceled, at least partially, so as to not cause loads on the motor shaft. Losses may be further reduced by locating the transfer shafts at or near opposite sides of the motor axis. Minimizing the axial, radial and/or tangential forces on the motor provides a number of benefits such as lower loads leading to reduced power consumption. In addition, lighter weight components such as bearing may be used to support the various shafts under lower loads. Friction losses may be minimized by balancing the loads and providing smoother quieter operation. 
     Referring to  FIG.  1   , an example application involves a vehicle  20  with a drive system  21  generally including a power supply  22 , a motor  24 , a gear system  26 , and a differential  28 , driving a pair of wheels  30 . The power supply  22  is coupled with the motor  24  by a powerline  32 . The motor  24  is coupled with the gear system  26  by a shaft  34  as an input shaft, and the gear system  26  is coupled with the differential  28  by a shaft  36  as an output shaft. The differential  28  is coupled with the wheels  30  by half-shafts  38 . Accordingly, the motor  24  drives the wheels  30  through the drive system  21  including the gear system  26 . Although the current embodiment is disclosed in the context of a vehicle  20 , other applications will benefit from the balancing/reduction of forces and the mechanisms disclosed herein. Accordingly, the current disclosure is not limited to any specific application, but may be applied wherever reduced loads on a motor or otherwise on an upstream and/or downstream driveline is desirable. 
     In the current embodiment, the vehicle  20  may be any type of vehicle. The motor  24  may be operated by any means and in the current embodiment is an electric motor and accordingly, the power supply  22  may be an electrical power supply including a battery bank. As such, operation of the drive system  21  to propel the vehicle  20  may be limited by the storage capacity of power supply  22  leading to a limited electric operation range of the vehicle  20 . Any reduction in power consumption is therefore beneficial in extending the range of the vehicle  20 . The motor  24  may be configured to run at a variety of speeds including relatively high speeds which may compound any loads or losses introduced by any characteristics of, or inefficiencies in, the drive system  21 . In a number of embodiments, the motor may spin at 10-25 times the number of revolutions per minute of the shaft  36  leaving the gear system  26 , and so any effects introduced into the motor may be amplified by the speed. For example, the motor may operate up to 30,000 revolutions per minute and the output shaft may turn at a respective 1200 revolutions per minute. In other embodiments, any gearing ratio appropriate for the application may be used. 
     The gear system  26  may be any of a variety of configurations of gears and shafts. Mechanical excitation may occur during operation including from the mesh of the gears in the gear system  26  as a source. The excitation may lead to the transmission of forces and motions through the shafts and bearings and to the gear housing  40 , which may in turn radiate noise. Accordingly, in the current embodiment the gear system  26  may employ helical gears for benefits including noise avoidance. Helical gears may run more smoothly and quietly than other types of gears such as spur gears with less noise and vibration being generated. 
     Example helical gears  41 ,  42  are illustrated in  FIG.  2   , to which reference is directed. The line of contact  43  of the helical gears  41 ,  42  is diagonal across the tooth trace  44 . The helical gear teeth  45  are cut at angles  48 ,  49  to the rotational axes  46 ,  47  of the respective gear  41 ,  42  and follow a spiral path. The angle  48 ,  49  at which the gear teeth  45  are cut is referred to as the helix angle and may be either a right-hand helix (R) as in gear  41  or a left-hand helix (L) as in gear  42 . When employing helix gears  41 ,  42  on parallel axes  46 ,  47 , for the gears  41 ,  42  to mesh together, gear  41  has a right-hand helix and the meshing gear  42  has a left-hand helix. On both of the meshing gears  41 ,  42  the helix angles will be of the same magnitude. Helical gears, such as gears  41 ,  42  have a sliding contact of the meshing teeth  45 . However, higher friction may accompany this sliding action leading to the generation of loads such as forces  50  that may result in drag on the system and a side thrust (axial force) may arise from the helix angles. Radial and tangential forces may also be generated as described below. The various forces, when not addressed by the approaches described herein, may transfer through the system to other components, such as the motor  24  creating undesirable loads and inefficiencies. 
     As shown in  FIGS.  3  and  4   , an example of the drive system  21  of the vehicle  20  includes the gear system  26  with the shaft  34  providing input from the motor  24 , and with the shaft  36  delivering output to the differential  28 . In this embodiment, the shafts  34  and  36  are disposed in a parallel relationship with one another, and disposed offset with the shaft  36  and located further back than the shaft  34  as viewed in the illustration of  FIG.  3    and visible in the illustration of  FIG.  4   . The shaft  34  is supported by bearings  51 - 53  on an axis  57  and the shaft  36  is supported at least by bearings  54 - 55  on an axis  58 . The bearings  54 ,  55  directly support a hub  56 , which may be formed as part of, or connected with, the shaft  36 . 
     The gear system  26  includes a pair of transfer shafts  61 ,  62 . The transfer shaft  61  is supported by bearings  63 ,  64  and rotates about an axis  65 , and the transfer shaft  62  is supported by bearings  66 ,  67  and rotates about an axis  68 . The bearings  63 - 64  and  66 - 67  are of a configuration that allows the shafts  61 ,  62  to move, at least slightly, along their respective axis  65 ,  68 . This axial movement enables the shafts  61 ,  62  to seek positions, such as in response to the force  50  and/or as a result of variations in cutting of the teeth  45 , to assist in relieving the axial forces/thrust without transferring those to the motor  24  or to the differential  28 . For example, the bearings  63 - 64  and  66 - 67  may be of the cylindrical or needle roller type with a sleeve/cup  69  and rollers  70 . The gears  71 - 78  may be rigidly fixed to their respective shafts  34 ,  36 ,  61 ,  62  and the bearings  63 - 64  and  66 - 67  relieve the axial forces/thrust. 
     To transfer rotation, movement, and power from the shaft  34  to the shaft  36 , a split power path is provided through the transfer shafts  61 ,  62  and through gears  71 - 78 . In the current embodiment, all of the gears  71 - 78  are helical gears with meshing gears of opposite handed configuration so each meshing pair includes a right handed version (R) and a left handed version (L). In other embodiments, other gear types may be used. A first power flow path is provided from the shaft  34 , through the gears  71  and  75 , through the transfer shaft  62 , through the gears  76  and  77 , and to the shaft  36  at the hub  56 . A second power flow path is provided from the shaft  34 , through the gears  72  and  73 , through the transfer shaft  61 , and through the gears  74  and  78  to the shaft  36  at the hub  56 . 
     Gears  71  and  72  are disposed on, and rotate with, the shaft  34  as an input shaft from the motor  24 . Gears  73  and  74  are disposed on, and rotate with, the transfer shaft  61 . Gears  75  and  76  are disposed on, and rotate with, the transfer shaft  62 . Gears  77  and  78  are coupled and rotate with the shaft  36  as an output shaft. Gears  71  and  75  mesh with each other, are opposite handed relative to one another, and have helix angles of equal magnitude. Gears  72  and  73  mesh with each other, are opposite handed relative to one another, and have helix angles of equal magnitude. Gears  76  and  77  mesh with each other, are opposite handed relative to one another, and have helix angles of equal magnitude. Gears  74  and  78  mesh with each other, are opposite handed relative to one another, and have helix angles of equal magnitude. Gears  71  and  72  have a common pitch diameter. Gears  77  and  78  have a common pitch diameter. The gear system  26  may provide a reduction ratio between the shaft  34  to the shaft  36  of approximately 10:1 to 20:1. The rotational speed of the transfer shafts  61  and  62  may be approximately one-third that of the input shaft  34 . 
     The gearing arrangement of  FIGS.  3  and  4    provides balancing of the forces generated during operation of the gear system  26 . For example, the gears  71  and  72  on the shaft  34  of the motor  24  have opposite handedness and helix angles of equal magnitude to result in a balancing of the generated axial forces. The two transfer shafts  61  and  62  are disposed on opposite sides of the shaft  34  provide radial balancing. As shown in  FIG.  4   , the lines of the axes  57 ,  65  and  68  are disposed in a common plane  79  (viewed on edge), with the axes  65  directly opposite the axis  68  across from the axis  57  for the balance. In other embodiments, the axes  65  and  68  may not be directly across the axis  57  from one another and may lie outside the plane  79 , such as by a small angle deviating from the plane  79  or other angle as appropriate for the application. The transfer shafts  61  and  62  are disposed on axis  65  and  68  respectively, which lie at offset angles  87  and  89  relative to the axis  58  of the shaft  36 . The offset angles  87  and  89  are optimized for balancing of forces on the drive system  21  and for packaging considerations. For example, the offset angels  87  and  89  have equal magnitudes and each has a sufficient magnitude for balancing optimization. Also, for example, the offset angles  87  and  89  are maintained at a relatively small magnitude for packaging considerations, while accommodating the gear ratios required. 
     Referring additionally to  FIG.  5   , force balancing is schematically shown. It will be appreciated that axial/thrust forces (not shown), will be directed into or out of the view and are balanced as described above. To assist in avoiding or canceling those axial forces, the gears  71 ,  72  are opposite handed. A radial force  80  is depicted at the axis  68  resulting from meshing gears  71  and  75 . A tangential force  81  is depicted at the gear  75  resulting from meshing gears  71  and  75 . A radial force  82  is depicted at the axis  65  resulting from meshing gears  72  and  73 . A tangential force  83  is depicted at the gear  73 , resulting from meshing gears  72  and  73 . Forces on the shaft  36  and its gearing are not shown in this illustration. Because the gears  71  and  72  have the same number of teeth at common helix angles and common pitch diameters and the gears  73 ,  75  have the same number of teeth at common helix angles, the radial forces  80 ,  82  balance and cancel each other and the tangential forces  81 ,  83  balance and cancel each other, resulting on a net zero or near-zero force on the shaft  34  and at the axis  57  of the motor  24 . A similar result may be accomplished at the axis  58  at the output to the differential  28 . 
     To further optimize performance, including to minimize losses and loads on the motor  24 , the gears  77  and  78  at the shaft  36  have opposite handed helix angles of equal magnitude and have a common pitch diameter. In addition, the gears  71  and  77  have common handed helix angles and the gears  72  and  78  have common handed helix angles. In addition, the gear  71  and the gear  77  have helix angle magnitudes with a ratio of tangents equal, or approximately equal to, a ratio of pitch diameters of the gears  75  and  76 . Further, the offset angles  87  and  89  between the axis  58  and the transfer shafts  61 ,  62  for optimized for packaging and force reduction purposes. The helix angles of the gears  77 ,  78  have magnitudes that differ, by a number of degrees, from the helix angles of the gears  71 ,  72  providing an additional degree of freedom to self-correct for force generation in the drive system  21  and to avoid restriction. As a result, loads on the motor  24  and on the bearings  51 - 55  are minimized resulting in optimized performance with maximum efficiency, and enabling the use of smaller lighter weight components, for maximized vehicle range. Force generation in the drive system may occur, such as due to variations in manufacturing tolerances and/or inexact meshing or rotation. Examples include index error, wobble, eccentricity error, or other irregularities. For example, index error may arise due to the angular relationship of gear teeth between decks or planes. Wobble may occur under operating conditions of a shaft where a combination of support stiffness and shaft stiffness may cause movement from the shaft&#39;s center axis. Eccentricity may occur under operating conditions of a gear and its shaft where the center axis of the shaft in not concentric with the reference center axis of the gear. 
     An alternate gear location arrangement is depicted in  FIG.  6   , with the shafts  34 ,  36  extending parallel to, and partially alongside each other. A double helix gear arrangement is provided on both the shafts  34 ,  36 . In this example, the gears  71  and  72  are located axially between the gears  77  and  78 . A zero, or near zero net thrust is provided through a combination of the shafts  34  and  36 . The transfer shafts  61  and  62  are disposed on opposite sides of the shaft  34  and may be shorter than in the embodiment of  FIG.  3   , providing a more compact package, which may enable further weight reduction. Two power/load paths are provided through the gear system  26 . The first power flow path is from the shaft  34 , through the gears  71  and  75 , through the transfer shaft  62 , through the gears  76  and  78 , to the shaft  36 . The second power flow path is from the shaft  34 , through the gears  72  and  73 , through the transfer shaft  61 , and through the gears  74  and  77  to the shaft  36 . Balancing of forces in the axial, radial and tangential directions is accomplished similar to the embodiment of  FIG.  3   . 
     An alternate bearing arrangement is shown in the drive system  21  of  FIG.  7   . A double helix gear arrangement is provided on both the shafts  34 ,  36 , similar to that of the embodiment of  FIG.  3   . The input shaft from the motor  24  is supported by bearings  52  and  53 , with the bearing  51  of  FIG.  3    is omitted. This bearing arrangement is enabled by balancing and reducing forces and results in the ability to use a shorter shaft  34 . As a result, additional weight reduction is achieved. 
     As illustrated in  FIG.  8   , in a gearing arrangement similar to that of  FIG.  3   , the bearing  52  is located on the input shaft between the input gears  71  and  72 . This enables omitting the bearing  51  and shortening the input shaft  34 , saving cost and weight. The bearings  52  and  53  are configured to allow the input shaft  34  to move axially to cancel axial forces at the motor  24 . This arrangement of a floating input shaft  34  may be desirable in other embodiments to cancel forces, such as that of  FIG.  11    where a single output gear  77  is used. 
     An alternative gear arrangement for the gear system  26  is illustrated in  FIG.  9    with a total of eight meshing gear pairs. In this and following illustrations, the bearings and some other elements are omitted for simplicity. The shafts  34  and  36  are each in double meshing relationship with each of the transfer shaft axes  65  and  68 . The axis  65  has coaxial shafts  100  and  102 , with the shaft  102  being hollow so that the shaft  100  extends through the hollow interior of the shaft  102 . Similarly, the axis  68  has coaxial shafts  104  and  106 , with the shaft  106  being hollow so that the shaft  104  extends through the hollow interior of the shaft  106 . Similar to the embodiment of  FIG.  3   , the shaft  34  includes two helical gears  71  and  72  and the shaft  36  includes two helical gears  77  and  78 . The transfer shafts  100  and  102  carry four gears  108 ,  73 ,  110  and  74 . The transfer shafts  104  and  106  carry four gears  75 ,  112 ,  76  and  114 . The gears  108  and  74  are disposed on the shaft  100 . The gears  73  and  110  are disposed on the shaft  102 . The gears  75  and  114  are disposed on the shaft  104 . The gears  112  and  76  are disposed on the shaft  106 . The gears  73  and  108  are opposite handed, the gears  75 ,  112  are opposite handed, the gears  74  and  110  are opposite handed and the gears  76  and  114  are opposite handed. The result is four separate power flow paths through the gear system  26 . A first power path is from the shaft  34  through the gears  71  and  75 , through the transfer shaft  104 , through the gears  114  and  78  and to the shaft  36  at the hub  56 . A second power path is from the shaft  34  through the gears  71  and  108 , through the transfer shaft  100 , and through the gears  74  and  78  to the shaft  36  at the hub  56 . A third power path is from the shaft  34 , through the gears  72  and  73 , through the transfer shaft  102 , and through the gears  110  and  77  to the shaft  36  at the hub  56 . A fourth power path is from the shaft  34 , through the gears  72  and  112 , through the transfer shaft  106 , and through the gears  76  and  77  to the shaft  36  at the hub  56 . Balancing of forces in the axial, radial and tangential directions is accomplished similar to the embodiment of  FIG.  3   . As a result of this gear arrangement, a higher level of balancing may be achieved by the additional offsetting gear arrangements. In addition, a higher torque carrying capacity may be provided and/or lighter weight gears may be used. 
     As illustrated in  FIG.  10   , an embodiment includes two transfer shafts  100  and  102  on the single axis  65 , with both located on one side of the shaft  34  of the motor  24 . The gears  71  and  72  are opposite handed from one another, as are the gears  77  and  78 . As in the embodiment of  FIG.  9   , the axial forces at the gear set  72 ,  73  cancel and balance the forces at the gear set  71 ,  108 . Similarly, the axial forces at the gear set  77 ,  110  cancel and balance the forces at the gear set  74 ,  78 . In the embodiment of  FIG.  10   , providing transfer shafts on one side enables weight reductions and packaging space reductions over the embodiment of  FIG.  9    and balances the axial forces to avoid loading the shaft  34  of the motor  24  to avoid associated losses. In addition, even with all of the gears rigidly mounted to their respective shafts, the bearings allow the shafts to move axially to adjust for varying loads as described above. 
     An embodiment as illustrated in  FIG.  11    eliminates the gear  78  as compared to the embodiment of  FIG.  3   . The embodiment is a double helix input and single helix output with two transfer shafts  61 ,  62 . The gears  74  and  76  both engage and mesh with the single gear  77  on the shaft  36 . Eliminating the gear  78  reduces weight while balancing of axial, radial and tangential forces is provided at the shaft  34  by the gears  71 ,  72 ,  73  and  75 . Accordingly, efficient operation of the motor is accomplished in a lighter lower cost approach. 
     Another double helix input and single helix output with two transfer shafts embodiment is illustrated in  FIG.  12   . The shaft  34  to the motor  24  includes the two opposite handed helix gears  71  and  72 . The shaft  36  to the differential  28  includes one helix gear  77 , which is common handed with the gear  72 . One power path through the gear system  26  is from the shaft  34  through the gear  71 , the gear  75 , the transfer shaft  62 , the gear  76  and the gear  77  to the shaft  36 . A second power path through the gear system  26  is from the shaft  34  through the gear  72 , the gear  73 , the transfer shaft  61 , the gear  74  and the gear  77  to the shaft  36 . Accordingly, the gear  77  is in both power paths. Balancing of axial, radial, and tangential forces is provided at the shaft  34  by the gears  71 ,  72 ,  73  and  75 . 
     Accordingly, motor driven systems are provided that address axial, radial and tangential force balancing to reduce loads, including on the motor. Pairs of transfer shafts with gears engage double gears on the input (motor) shaft and/or output (differential) shaft. The transfer shafts may be arranged on opposite sides, or on a common side, of the input and/or output shafts to reduce net radial loading of the input and/or output shafts, especially the input shaft. Opposing input helix angles and opposing output helix angles are provided to eliminate/optimize total thrust on the transfer shafts. Transfer shafts may be mounted to allow axial movement, such as with cylindrical roller bearings, for example, to allow each shaft to seek the optimum axial location to accommodate input and output gearing with little or axial movement, such as between an electric traction motor and a differential drive to wheels through half-axles. In applications, the handedness of the helix angles of the gears in an embodiment may be modified. For example, in the embodiment of  FIG.  11   , the right and left helixes of the gears  71  and  72  may be swapped, and the helix hands of the remaining gears may be adjusted accordingly. 
     While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the disclosure in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the exemplary embodiment or exemplary embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope of the disclosure as set forth in the appended claims and the legal equivalents thereof.