Abstract:
A continuously variable transmission ( 2 ) has a first helical gear ( 204 A) for its rotation output. The fixed speed ratio transmission ( 3 ) has a second helical gear ( 203 B) for its rotation output. The third helical gear ( 207 ) rotates according to the difference of the rotation speeds of the first helical gear ( 204 A) and second helical gear ( 203 B). These helical gears rotate on a same rotation shaft ( 204 ). One of the three helical gears positioned between the other two helical gears applies a bending moment on the rotation shaft ( 204 ) due to a radial force given by a gear meshed therewith. The direction of the tooth trace of the other two helical gears is set so that at least one of them produces a bending moment in an opposite direction to the first bending moment.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to the setting of the tooth trace of helical gears in an infinitely variable transmission.  
         BACKGROUND OF THE INVENTION  
         [0002]    Tokkai 2000-213623 published by the Japanese Patent Office in 2000 discloses an infinitely variable transmission (IVT) for a vehicle which combines a fixed speed ratio transmission, a continuously variable transmission (CVT) and a planetary gear set.  
           [0003]    In this IVT, the output of the CVT is transmitted to a sun gear which is fixed to a central shaft of the planetary gear set through a chain. The output of the fixed speed ratio transmission is transmitted to the planet carrier of the planetary gear set. The ring gear of the planetary gear set is fixed to the output shaft of the IVT which is supported on an outer periphery of the central shaft. The planetary gear set varies the speed and direction of rotation of the ring gear in response to the difference between the rotation speed of the planet carrier and the sun gear and transmits torque to the drive shaft of the vehicle through an IVT output gear which is fixed on the output shaft.  
           [0004]    A low-noise helical gear is used on the IVT output gear and in the fixed speed ratio transmission. The tooth trace of these helical gears is set to eliminate the bending moment applied on the central shaft by the tension of the chain.  
         SUMMARY OF THE INVENTION  
         [0005]    Tokkai Hei 11-63139 published by the Japanese Patent Office in 1999 discloses an IVT which transmits the output of the CVT to the sun gear of the planetary gear set by combining two helical gears without the use of a chain. The two helical gears transmit rotations in an opposite direction to the direction of rotation of the chain. As a result, the direction of rotation of the sun gear is opposite to the direction of rotation of the sun gear in the IVT disclosed in Tokkai 2000-213623. In this IVT, the direction of rotation of the sun gear and the planet carrier are arranged to reverse the direction of the output rotations of the fixed speed ratio transmission using a counter gear.  
           [0006]    Since this IVT transmits the CVT output to the sun gear using a helical gear, the application of a bending moment on the central shaft differs from the IVT disclosed in Tokkai 2000-213623.  
           [0007]    It is therefore an object of this invention to reduce the bending moment applied to the output shaft in an IVT in which the CVT output is transmitted to the planetary gear via helical gears.  
           [0008]    In order to achieve the above object, this invention provides a special arrangement of an infinitely variable transmission comprising an input shaft, a continuously variable transmission which outputs a rotation of the input shaft to a first helical gear at an arbitrary speed ratio, a fixed speed ratio transmission which outputs a rotation of the input shaft to a second helical gear at a fixed speed ratio, and a third helical gear which changes a rotation direction and a rotation speed according to a difference between a rotation speed of the first helical gear and a rotation speed of the second helical gear.  
           [0009]    In this infinitely variable transmission, the first helical gear, the second helical gear and the third helical gear are supported on a common rotation shaft such that one of the first helical gear, the second helical gear and the third helical gear is disposed between the other two gears. One of the other two gears has a tooth trace which generates a thrust force exerting a bending moment on the rotation shaft in a direction opposite to a direction of a bending moment exerted by a radial force acting on the gear which is disposed between the other two gears on the rotation shaft.  
           [0010]    The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a schematic diagram of an infinitely variable transmission (IVT) according to this invention.  
         [0012]    [0012]FIG. 2 is a schematic diagram of the IVT showing the setting of the tooth traces of helical gears according to this invention.  
         [0013]    [0013]FIG. 3 is a schematic sectional view of helical gears.  
         [0014]    FIGS.  4 A- 4 C are schematic diagrams of the essential parts of the IVT describing the bending moment applied to the IVT output shaft and the thrust force applied to the helical gears.  
         [0015]    [0015]FIG. 5 is similar to FIG. 3, but showing a second embodiment of this invention.  
         [0016]    [0016]FIG. 6 is a schematic diagram of the essential parts of the IVT describing the bending moment applied to the IVT output shaft and the thrust force applied to the helical gears according to the second embodiment of this invention.  
         [0017]    [0017]FIG. 7 is similar to FIG. 3, but showing a third embodiment of this invention.  
         [0018]    [0018]FIG. 8 is similar to FIG. 6, but showing a third embodiment of this invention.  
         [0019]    [0019]FIG. 9 is similar to FIG. 3, but showing a fourth embodiment of this invention.  
         [0020]    [0020]FIG. 10 is similar to FIG. 6, but showing the fourth embodiment of this invention.  
         [0021]    [0021]FIG. 11 is a schematic diagram of an infinitely variable transmission (IVT) according to a fifth embodiment of this invention.  
         [0022]    [0022]FIG. 12 is similar to FIG. 3, but showing the fifth embodiment of this invention.  
         [0023]    [0023]FIG. 13 is similar to FIG. 6, but showing the fifth embodiment of this invention.  
         [0024]    [0024]FIG. 14 is similar to FIG. 3, but showing a sixth embodiment of this invention.  
         [0025]    [0025]FIG. 15 is similar to FIG. 6, but showing the sixth embodiment of this invention.  
         [0026]    [0026]FIG. 16 is similar to FIG. 3, but showing a seventh embodiment of this invention.  
         [0027]    [0027]FIG. 17 is similar to FIG. 6, but showing the seventh embodiment of this invention.  
         [0028]    [0028]FIG. 18 is similar to FIG. 3, but showing an eighth embodiment of this invention.  
         [0029]    [0029]FIG. 19 is similar to FIG. 6, but showing the eighth embodiment of this invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]    Referring to FIG. 1 of the drawings, an infinitely variable transmission (hereinafter referred to as an IVT) for a vehicle comprises an IVT input shaft  1 A and an IVT output shaft  206  disposed in parallel, a fixed speed ratio transmission  203 , a toroidal continuously variable transmission (hereinafter referred to as a (VT)  2  and planetary gear set  205 , a power recirculation clutch  209 , a direct clutch  210  and an IVT output gear  207 .  
         [0031]    The IVT input shaft  1 A is engaged with the engine. The IVT output shaft  206  is a hollow shaft which is supported to rotate freely about and coaxially with respect to a central shaft  204  of the planetary gear set. Both ends of the central shaft  204  are respectively supported on the IVT housing (not shown) through bearings  251 ,  252 .  
         [0032]    The fixed speed ratio transmission  203  comprises an input gear  203 A fixed to the IVT input shaft IA, an output gear  203 B supported to rotate freely on an outer periphery of the central shaft  204 , and a counter gear  203 D which meshes with the output gear  203 B and the input gear  203 A and which is supported on a counter shaft  203 E. The gears  203 A,  203 E,  203 B comprise helical gears.  
         [0033]    The CVT  2  is provided with a CVT input shaft  1 B which is disposed coaxially with respect to the IVT input shaft  1 A, and a pair of toroidal units  2 A,  2 B arranged on the CVT input shaft  1 B.  
         [0034]    The toroidal units  2 A,  2 B comprise an input disk  21  which is fixed to the CVT input shaft lB, an output disk  222  which is supported to rotate freely on the CVT input shaft  1 B and a pair of power rollers  20  which are gripped between the disks. The output disks  222  of the toroidal units  2 A and  2 B are integrated with, a CVT output gear  230  being fixed to the outer periphery of the output disk  222 .  
         [0035]    The input disk  21  of the toroidal unit  2 A is biased in an axial direction by a loading cam device  23  which generated a thrust force in response to the rotation of the IVT input shaft  1 A. As a result, the input disks  21  rotate together with the IVT input shaft  1 A, and the input disk  21  of the toroidal unit  2 A and the input disk  21  of the toroidal unit  2 B tend to move towards each other and grip the power rollers  20  as a result of the thrust force. The CVT  2  performs arbitrary variation of the speed ratio of the input disk  21  and the output disk  22  by varying the gyration angle of the power rollers  20  using hydraulic pressure. U.S. Pat. No. 5,935,039 discloses CVT speed ratio control using hydraulic pressure in the above manner.  
         [0036]    The CVT output gear  230  meshes with a gear  204 A which is fixed on the central shaft  204 . The gears  230  and  204 A comprise helical gears.  
         [0037]    The power recirculation clutch  209 , the planetary gear set  205  and the direct clutch  210  are disposed between the output gear  203 B of the fixed speed ratio transmission  203  and the gear  204 A which are on the central shaft  204 .  
         [0038]    The planetary gear set  205  comprises a sun gear  205 A which is fixed to the central shaft  204 , a ring gear  205 C which is disposed coaxially to the outer side of the sun gear  205 A, planet gears  205 D which are pinions arranged between the sun gear  205 A and ring gear  205 C, and a planet carrier  205 B which supports the planet gears  205 D so as to be free to rotate and free to turn around the sun gear  205 A. The sun gear  205 A, the planet gears  205 D and the ring gear  205 C comprise helical gears. The sun gear  205 A and the planet gears  205 D are external contact gears. The ring gear  205 C is an internal contact gear.  
         [0039]    The ring gear  205 C is fixed to the IVT output shaft  206  and the IVT output shaft  206  is engaged with the gear  204 A through the direct clutch  210 .  
         [0040]    The planet carrier  205 B is engaged with the output gear  203 B of the fixed speed ratio transmission  203  through the power recirculation clutch  209 .  
         [0041]    When in an engaged state, the direct clutch  210  connects the gear  204 A and the IVT output shaft  206  to rotate together. When in a disengaged state, the direct clutch  210  allows relative rotation of the gear  204 A and the IVT output shaft  206 . When in an engaged state, the power recirculation clutch  209  connects the gear  203 B of the fixed speed ratio transmission  203  and the planet carrier  205 B to rotate together. When in a disengaged state, the power recirculation clutch  209  allows relative rotation of the output gear  203 B and the planet carrier  205 B. The direct clutch  210  and the power recirculation clutch  209  are respectively operated using hydraulic pressure. The above mentioned U.S. Pat. No. 5,935,039 discloses the operation of these clutches using hydraulic pressure.  
         [0042]    The IVT output gear  207  is fixed to the IVT output shaft  206 . The rotation of the IVT output gear  207  is transmitted to a final gear  12  of a differential  208  through a gear  213  and a gear  214  fixed to a counter shaft  215 . A pair of drive shafts  211  connected to drive wheels of the vehicle are rotated by the differential  208 . The IVT output gear  207  and the gear  213  comprise helical gears.  
         [0043]    In this IVT, the transmission of rotational force to the drive wheels from the engine is performed through the five shafts comprising the IVT input shaft  1 A, the IVT output shaft  206 , the counter shaft  203 E, the counter shaft  215  and the drive shaft  211 . Although these shafts are shown in a coplanar orientation in FIG. 1 in order to simplify description, the actual orientation as shown in FIG. 3 is not coplanar.  
         [0044]    This IVT selectively switches between power recirculation mode in which the power recirculation clutch  209  is engaged and the direct clutch  210  is disengaged and direct mode in which the power recirculation clutch  209  is disengaged and the direct clutch  210  is engaged.  
         [0045]    In power recirculation mode, the output rotation of the fixed speed ratio transmission  203  is transmitted to the planet carrier  205 B of the planetary gear set  205  from the output gear  203 B. The output rotation of the CVT  2  is transmitted to the sun gear  205 A of the planetary gear set  205  through the central shaft  204 . The planetary gear set  205  rotates the ring gear  205 C in response to the relative rotation of the planet carrier  205 B and the sun gear  205 A.  
         [0046]    The rotation of the ring gear  205 C is output to the drive shafts  211  through the IVT output gear  207  and the gear  213 , the gear  214 , the final gear  212  and the differential  208 . The direction of rotation of the ring gear  205 C depends on the relative rotation of the planet carrier  205 B and the sun gear  205 A, i.e., the differential between the output rotation speed of the fixed speed ratio transmission  203  and the output rotation speed of the CVT  2 . The output rotation speed of the fixed speed ratio transmission  203  is normally fixed with respect to the engine rotation speed.  
         [0047]    However, the CVT output rotation speed varies continuously in response to the gyration angle of the power rollers  20 . Thus the IVT transmits the engine output rotation to the drive shafts  211  at an arbitrary speed ratio from forward to reverse vehicle operation by varying the gyration angle of the CVT  2 .  
         [0048]    In direct mode, the output rotation of the CVT  2  is output directly to the IVT output gear  207  through the direct clutch  210  in the engaged state. In contrast, the output gear  230 B of the fixed speed ratio transmission  203  is cut off from the planet carrier  205 B and rotates freely because the power recirculation clutch  209  is disengaged. Thus the output rotation of the CVT  2  is transmitted directly to the drive shaft  211  during direct mode.  
         [0049]    Power recirculation mode is used to obtain a large IVT speed ratio for reverse or low-speed forward vehicle operation. Direct mode is used to obtain a small IVT speed ratio for high-speed forward vehicle operation.  
         [0050]    Referring to FIGS. 2, 3 and  4 A- 4 C, the setting of the direction of the face advance of the tooth trace of the IVT output gear  207 , the gear  204 A and the output gear  203 B of the fixed speed ratio transmission  203  will be described. (Note: It is convenient to the readers that the figure number reference is indicated at the top of the paragraph.)  
         [0051]    In power recirculation mode, the torque applied to the output gear  203 B, the gear  204 A and the IVT output gear  207  is larger than the torque applied to the same gears during direct mode. As a result, a thrust force which is applied to the engagement sections  203 G,  204 G, and  207 G of these three helical gears with the other gears is greater than the thrust force which is applied to the same engagement sections during direct mode. Consequently a larger bending moment is applied to the central shaft  204  during power recirculation mode than during direct mode.  
         [0052]    The time required for the IVT to transmit torque for forward vehicle operation is conspicuously greater than the time required to transmit torque for reverse vehicle operation. Therefore the direction of the tooth trace of respective helical gears is set with the object of reducing the bending moment applied to the central shaft  204  during forward vehicle operation in power recirculation mode.  
         [0053]    Referring now to FIG. 3, the IVT input shaft  1 A and the CVT input shaft  1 B rotate in a clockwise direction when the IVT is viewed from the left-hand side of FIG. 2.  
         [0054]    Thus in the fixed speed ratio transmission  203 , the input gear  203 A fixed to the IVT input shaft  1 A rotates in a clockwise direction, the counter gear  203 D rotates in a counter-clockwise direction and the output gear  203 B and the planet carrier  205 B respectively rotate in a clockwise direction.  
         [0055]    With respect to the CVT  2 , the input disks  21  fixed to the CVT input shaft  1 B rotate in a clockwise direction. The output disks  22  normally rotate in the opposite direction to the input disk  21 . Thus the output disks  22  and the CVT output gear  230  rotate in a counter-clockwise direction. The gear  204 A which meshes with the CVT output gear  230  and the sun gear  205 A which engages with the gear  204 A rotate in a clockwise direction.  
         [0056]    The sun gear  205 A and the planet carrier  205 B both rotate in a clockwise direction in the planetary gear set  205 . The rotational direction of the ring gear  205 C varies in response to the speed ratio of the rotation of the sun gear  205 A and the planet carrier  205 B. When the vehicle is travelling forward, the ring gear  205 C rotates in the same clockwise manner as the sun gear  205 A and the planet carrier  205 B. Consequently the IVT output gear  207  which is joined to the ring gear  205 C via the IVT output shaft  206  rotates in a clockwise direction and the gear  213  rotates in a counter-clockwise direction.  
         [0057]    The torque transmission of force between the gears will be described hereafter. During forward vehicle operation in power recirculation mode, torque is transmitted to the planet carrier  205 B from the output gear  203 B of the fixed speed ratio transmission  203 . The carrier  205 B rotates the ring gear  205 C and the sun gear  205 A.  
         [0058]    Torque which is transmitted to the sun gear  205 A is recirculated through the CVT  2  to the input gear  203 A of the fixed speed ratio transmission  203 . During reverse vehicle operation in power recirculation mode, the direction of torque recirculation is opposite to that described above.  
         [0059]    When torque is transmitted between the helical gears, a force acting in a central direction and a force in a direction which is tangential to a base circle are applied to the engaging tooth faces. The composite force thereof applies a radial force to the central axes of the helical gears. In contrast, the sloping tooth faces of the helical gears apply a thrust force on the helical gears. The thrust force applies a bending moment on the central axes of the helical gears.  
         [0060]    Both ends of the central shaft  204  are supported on bearings  251 ,  252 . The output gear  203 B of the fixed speed ratio transmission  203  is disposed in proximity to the bearing  251 . The gear  204 A is disposed in proximity to the bearing  252 . Thus the radial force which acts on these gears apply little bending moment to the central shaft  204 . The radial force which exerts a large bending moment on the central shaft  204  is a radial force Ro acting on the IVT output gear  207 .  
         [0061]    The thrust force which exerts a large bending moment of the central shaft  204  is a thrust force Fc applied to the gear  204 A by the CVT output gear  230  and a thrust force Fg applied to the output gear  203 B by the counter gear  203 D of the fixed speed ratio transmission  3 . In this IVT, the tooth traces of the helical gears  203 B and  204 A are determined according to the positional relationship of the rotation shafts  1 B,  203 E and  204  so that the bending moment created by the radial force Ro on the central shaft  204  is reduced by the thrust force Fc and the thrust force Fg.  
         [0062]    When the vehicle is running forward in power recirculation mode, the output gear  203 B of the fixed speed ratio transmission  203  is a driven gear which is driven by the input gear  203 A through the counter gear  203 D. The IVT output gear  207  is a drive gear which drives the final gear  212  through the gears  213  and  214 . The gear  204 A is also a drive gear since power is recirculated to the CVT  2  through the gear  204 A from the sun gear  205 A.  
         [0063]    Referring now to FIG. 3, the angle subtended by a line connecting the central shaft  204  with the IVT input shaft  1 A and the direction of application of the radial force Ro is designated as θ1. The angle subtended by a line connecting the central shaft  204  with the counter shaft  203 E and the direction of application of the radial force Ro is designated as θ2. In this configuration, the IVT input shaft  1 A, the counter shaft  203 E and the counter shaft  215  forming the center of the gear  213  are disposed so that both the angles θ1 and θ2 are less than ninety degrees.  
         [0064]    Referring to FIG. 4A, the tooth traces of both the output gear  203 B and the gear  204 A of the fixed speed ratio transmission  203  are respectively set as a right-handed thread. In contrast, the tooth trace of the counter gear  203 D is set as a left-handed thread and the tooth trace of the input gear  203 A of the fixed speed ratio transmission  203  is set as a right-handed thread. The tooth trace of the CVT output gear  230  which meshes with the gear  204 A is set as a left-handed thread.  
         [0065]    Referring again to FIG. 3, during forward vehicle operation in power recirculation mode, the gear  204 A which is joined to the sun gear  205 A rotates in a clockwise direction and drives the CVT output gear  230  in a counter-clockwise direction. A thrust force Fc oriented towards the right side of  
         [0066]    [0066]FIG. 4A is applied to the meshing sections of the gear  204 A and the CVT output gear  230  as a result of the reaction force acting when the gear  204 A drives the CVT output gear  230 .  
         [0067]    The output gear  203 B of the fixed speed ratio transmission  203  which is joined to the planet carrier  205 B via the power recirculation clutch  209  rotates in a clockwise direction. A thrust force Fg oriented towards the left side of  
         [0068]    [0068]FIG. 4A is applied to the meshing sections of the counter gear  203 D and the output gear  203 B due to the driving force applied to the output gear  203 B from the counter gear  203 D.  
         [0069]    The total bending moment acting on the central shaft  204  is suppressed to a small value due to the fact that the bending moments M 1  and M 2  which are applied to the central shaft  204  by the thrust forces Fc and Fg act in the opposite direction to the bending moment applied to the central shaft  204  by the radial force Ro.  
         [0070]    [0070]FIG. 4B shows the application of forces when the tooth trace of the gear  204 A is set as left-handed and the tooth trace of the gear  203 B is set as right-handed. FIG. 4C shows the application of forces when the tooth trace of the gear  204  is set as right-handed and the tooth trace of the gear  203 B is set as left-handed. The tooth trace of the input gear  203 A and the counter gear  203 D of the fixed speed ratio transmission  203  is set to correspond to the tooth trace of the gear  203 B. The tooth trace of the CVT output gear  230  is set to correspond to the gear  204 A.  
         [0071]    The settings of the tooth traces shown in both FIG. 4B and FIG. 4C adapt the direction of the bending moment applied on the central shaft  204  so that one of the three forces Ro, Fg, Fc is oriented in an opposite direction to the bending moment applied to the central shaft  204  by the other two forces. As a result, it is possible to suppress the total amount of the bending moment applied to the central shaft  204  to a small level. Consequently, it is possible to reduce the load on the bearings  251  and  252 .  
         [0072]    The dimension and point of application of the forces Ro, Fc, and Fg determine which setting of the tooth trace shown in FIGS.  4 A- 4 C minimizes the bending moment acting on the central shaft  204 . Thus it is preferred that a setting which minimizes the bending moment is selected from the three settings for the tooth trace as shown in FIGS.  4 A- 4 C according to the configuration of the IVT.  
         [0073]    The preferred direction for the tooth traces of the gear  204 A and the gear  203 B differs corresponding to the setting of the angles θ1 and θ2. Preferred settings for the tooth traces according to the setting of the angles θ1 and θ2 will be described as second to fourth embodiments.  
         [0074]    Referring to FIGS. 5 and 6, the second embodiment of this invention will be described.  
         [0075]    In this embodiment, as shown in FIG. 5, the IVT input shaft  1 A, the counter shaft  203 E and the counter shaft  215  are disposed so that the angle θ1 is smaller than ninety degrees and the angle θ2 is greater than ninety degrees. As shown in FIG. 6, the tooth trace of the gear  203 B is set as left-handed and the tooth trace of the gear  204 A is set as right-handed. The tooth traces of the input gear  203 A and the counter gear  203 D of the fixed speed ratio transmission  203  are set corresponding to the tooth trace of the gear  203 B. The tooth trace of the CVT output gear  230  is set corresponding to the gear  204 A.  
         [0076]    In this embodiment, in the same manner as the case of FIG. 4A in the first embodiment, the bending moments M 1  and M 2  produced by the action of the thrust forces Fg and Fc are oriented in an opposite direction as shown in FIG. 6 to the bending moment applied by the radial force Ro on the central shaft  204 . This results in the same effect as the arrangement shown in FIG. 4A.  
         [0077]    When the tooth trace of one of the gear  203 B and the gear  204 A is set in the opposite direction, the same effect as the case of FIG. 4B or FIG. 4C in the first embodiment is obtained.  
         [0078]    Referring to FIGS. 7 and 8, the third embodiment of this invention will be described.  
         [0079]    In this embodiment, as shown in FIG. 7, the IVT input shaft  1 A, the counter shaft  203 E and the counter shaft  215  are disposed so that the angle θ1 is greater than ninety degrees and the angle θ2 is smaller than ninety degrees. As shown in FIG. 8, the tooth trace of the gear  203 B is set as right-handed and the tooth trace of the gear  204 A is set as left-handed. The tooth traces of the input gear  203 A and the counter gear  203 D of the fixed speed ratio transmission  203  are set corresponding to the tooth trace of the gear  203 B. The tooth trace of the CVT output gear  230  is set corresponding to the gear  204 A.  
         [0080]    In this embodiment, in the same manner as the case of FIG. 4A in the first embodiment, the bending moments M 1  and M 2  produced by the action of the thrust forces Fg and Fc are oriented in an opposite direction as shown in FIG. 8 to the bending moment applied by the radial force Ro on the central shaft  204 . This results in the same effect as the arrangement shown in FIG. 4A.  
         [0081]    When the tooth trace of one of the gear  203 B and the gear  204 A is set in the opposite direction, the same effect as the case of FIG. 4B or FIG. 4C in the first embodiment is obtained.  
         [0082]    Referring now to FIGS. 9 and 10, the fourth embodiment of this invention will be described.  
         [0083]    In this embodiment, as shown in FIG. 9, the IVT input shaft  1 A, the counter shaft  203 E and the counter shaft  215  are disposed so that the angle θ1 and the angle θ2 are greater than 90 degrees. As shown in FIG. 10, the tooth trace of the gear  203 B is left-handed and the tooth trace of the gear  204 A is left-handed. The tooth trace of the input gear  203 A and the counter gear  203 D of the fixed speed ratio transmission  203  is set corresponding to the tooth trace of the gear  203 B. The tooth trace of the CVT output gear  230  is set corresponding to the gear  204 A.  
         [0084]    In this embodiment, in the same manner as the case of FIG. 4A in the first embodiment, the bending moments M 1  and M 2  produced by the action of the thrust forces Fg and Fc are oriented in an opposite direction as shown in FIG. 10 to the bending moment applied by the radial force Ro on the central shaft  204 . This results in the same effect as the arrangement shown in FIG. 4A.  
         [0085]    When the tooth trace of one of the gear  203 B and the gear  204 A is set in the opposite direction, the same effect as the case of FIG. 4B or FIG. 4C in the first embodiment is obtained.  
         [0086]    Referring now to FIGS.  11 - 13 , a fifth embodiment of this invention will be described.  
         [0087]    This embodiment describes the application of this invention to an IVT as disclosed in the aforementioned Tokkai Hei 11-63139.  
         [0088]    Firstly referring to FIG. 11, the position of the fixed speed ratio transmission  203  and the CVT  2  are reversed in this IVT when compared to the IVT as shown in FIG. 1. The CVT  2  is disposed in proximity to the engine.  
         [0089]    The gear  204 A is disposed between the IVT output gear  207  and the output gear  203 B of the fixed speed ratio transmission  203 . As a result, the central shaft  204  is arranged on an outer side of the IVT output shaft  206  and supported by the IVT output shaft  206 . In this embodiment, the IVT output shaft  206  is supported by the bearings  251  and  252  rather than the central shaft  204 . During forward vehicle operation in power recirculation mode, the gear  203 B is a driven gear and the IVT output gear  207  is a drive gear in the same manner as in the case of the first-fourth embodiments..  
         [0090]    In this embodiment, direction of the tooth trace of the gears  203 B and  207  and the positional relationship of the respective shafts are set so that the bending moment applied to the IVT output shaft  206  by the radial force Rc acting on the gear  204 A is reduced.  
         [0091]    Referring now to FIG. 12, the angle subtended by a line connecting the IVT output shaft  206  and the counter shaft  203 E and the direction of application of the radial force Rc is designated as θ3. The angle subtended by a line connecting the IVT output shaft  206  with the counter shaft  215  and the direction of application of the radial force Rc is designated as θ4.  
         [0092]    In this IVT, the IVT input shaft  1 A, the counter shaft  203 E and the counter shaft  215  are disposed so that both the angles θ3 and θ4 are less than ninety degrees. As shown in FIG. 13, the tooth traces of both the gear  203 B and the gear  204 A are set as a left-hand thread. The tooth traces of the counter gear  203 D and the input gear  203 A of the fixed speed ratio transmission  203  are set corresponding to the tooth trace of the gear  203 B. The tooth trace of the gear  213  is set corresponding to the gear  207 .  
         [0093]    During forward vehicle operation in power recirculation mode, the thrust force Fg to the right side of FIG. 13 is applied to a gear  203 B and the thrust force Fo to the left side of the figure is applied to the gear  207 . The bending moment M 2  applied by the thrust force Fg on the IVT output shaft  206  and the bending moment M 3  applied by the thrust force Fo on the IVT output shaft  206  both act in an opposite direction to the bending moment applied by the radial force Rc acting on the IVT output shaft  206 . Thus it is possible to suppress the total bending moment applied on the IVT output shaft  206  to a small level.  
         [0094]    The above embodiment shows that this invention can be effectively applied to an IVT which supports the IVT output shaft  206  with the bearings  251  and  252 .  
         [0095]    Referring to FIGS. 14 and 15, a sixth embodiment of this invention will be described.  
         [0096]    In this embodiment, as shown in FIG. 14, the IVT input shaft  1 A, the counter shaft  203 E and the counter shaft  215  are disposed so that the angle θ3 is smaller than ninety degrees and the angle θ4 is greater than ninety degrees.  
         [0097]    As shown in FIG. 15, the tooth trace of the gear  203 B is set as left-handed and the tooth trace of the gear  207  is set as right-handed. The tooth traces of the counter gear  203 D and the input gear  203 A of the fixed speed ratio transmission  203  are set corresponding to the tooth trace of the gear  203 B. The tooth trace of the gear  213  is set corresponding to the gear  207 .  
         [0098]    During forward vehicle operation in power recirculation mode, the thrust force Fg to the right side of FIG. 15 is applied to a gear  203 B and the thrust force Fo to the right side of the figure is applied to the gear  207 . The bending moment M 2  applied by the thrust force Fg on the IVT output shaft  206  and the bending moment M 3  applied by the thrust force Fo on the IVT output shaft  206  both act in an opposite direction to the bending moment applied by the radial force Rc acting on the IVT output shaft  206 . Thus this embodiment also makes it possible to suppress the total bending moment applied on the IVT output shaft  206  to a small level in the same manner as the fifth embodiment.  
         [0099]    In this embodiment, it is possible to set the tooth trace of one of the gears  203 B and the gear  207  in an opposite direction. As a result, the direction of the bending moment resulting on the IVT output shaft  206  from one of the three forces Ro, Fg, Fc is oriented in an opposite direction to the direction of the bending moment applied to the IVT output shaft  206  by the other two forces. As a result, the above settings also make it possible to suppress the total amount of the bending moment applied to the central shaft  204  to a small level. Consequently, it is desirable to select settings which minimize the bending moment according to the configuration of the IVT.  
         [0100]    Referring to FIGS. 16 and 17, a seventh embodiment of this invention will be described.  
         [0101]    In this embodiment, as shown in FIG. 16, the IVT input shaft  1 A, the counter shaft  203 E and the counter shaft  215  are disposed so that the angle θ3 is greater than ninety degrees and the angle θ4 is smaller than ninety degrees.  
         [0102]    As shown in FIG. 17, the tooth trace of the gear  203 B is set as right-handed and the tooth trace of the gear  207  is set as left-handed. The tooth traces of the counter gear  203 D and the input gear  203 A of the fixed speed ratio transmission  203  are set corresponding to the tooth trace of the gear  203 B. The tooth trace of the gear  213  is set corresponding to the gear  207 .  
         [0103]    As a result, during forward vehicle operation in power recirculation mode, the thrust force Fg to the left side of FIG. 17 is applied to a gear  203 B and the thrust force Fo to the left side of the figure is applied to the gear  207 . The bending moment M 2  applied by the thrust force Fg on the IVT output shaft  206  and the bending moment M 3  applied by the thrust force Fo on the IVT output shaft  206  both act in an opposite direction to the bending moment applied by the radial force Rc acting on the IVT output shaft  206 .  
         [0104]    Thus this embodiment also makes it possible to suppress the total bending moment applied on the IVT output shaft  206  to a small level in the same manner as the fifth and sixth embodiments. It is also possible to set the tooth trace of one of the gears  203 B and the gear  207  in an opposite direction according to the configuration of the IVT.  
         [0105]    Referring to FIGS. 18 and 19, an eighth embodiment of this invention will be described.  
         [0106]    In this embodiment, as shown in FIG. 18, the IVT input shaft  1 A, the counter shaft  203 E and the counter shaft  215  are disposed so that the angles θ3 and θ4 are both greater than ninety degrees.  
         [0107]    As shown in FIG. 19, the tooth traces of both the gear  203 B and the gear  207  are right-handed. The tooth traces of the counter gear  203 D and the input gear  203 A of the fixed speed ratio transmission  3  are set corresponding to the tooth trace of the gear  203 B. The tooth trace of the gear  213  is set corresponding to the gear  207 .  
         [0108]    As a result, during forward vehicle operation in power recirculation mode, the thrust force Fg to the left side of FIG. 19 is applied to the gear  203 B and the thrust force Fo to the right side of the figure is applied to the gear  207 . The bending moment M 2  applied by the thrust force Fg on the IVT output shaft  206  and the bending moment M 3  applied by the thrust force Fo on the IVT output shaft  206  both act in an opposite direction to the bending moment applied by the radial force Rc acting on the IVT output shaft  206 .  
         [0109]    This arrangement makes it possible to suppress the total bending moment applied on the IVT output shaft  206  to a small level in the same manner as the fifth to seventh embodiments. It is also possible to set the tooth trace of one of the gears  203 B and the gear  207  in an opposite direction according to the configuration of the IVT as described above.  
         [0110]    The first to fourth embodiments are applied to an IVT which disposes the IVT output gear  207  between the output gear  203 B of the fixed speed ratio transmission  203  and the gear  204 A. The fifth to eighth embodiments are applied to an IVT which disposes the gear  204 A between the output gear  203 B of the fixed speed ratio transmission  203  and the IVT output gear  207 . This invention can also be applied to an IVT which disposes the output gear  203 B between the gear  204 A and the IVT output gear  207 .  
         [0111]    In each of the above embodiments, the gear  4 A corresponds to the first helical gear in the claims, the output gear  203 B of the fixed speed ratio transmission  3  corresponds to the second helical gear in the claims, and the IVT output gear  207  corresponds to the third helical gear in the claims. In the first to the fourth embodiments, the central shaft  204  corresponds to the rotation shaft in the claims. In the fifth to the eighth embodiments, the IVT output shaft  206  corresponds to the rotation shaft in the claims.  
         [0112]    The contents of Tokugan 2001-62938, with a filing date of Mar. 7, 2001 in Japan, are hereby incorporated by reference.  
         [0113]    Although the invention has been described above by reference to certain embodiments of the invention, the invention is not limited to the embodiments described above. Modifications and variations of the embodiments described above will occur to those skilled in the art, in light of the above teachings.  
         [0114]    The embodiments of this invention in which an exclusive property or privilege is claimed are defined as follows: