Abstract:
A variator ( 2 ) having a pair of semi-toroidally recessed races ( 11, 12 ) mounted to a housing ( 6 ) for rotation about a common variator axis. A roller ( 14 ) is mounted between the races to transfer drive between them at a variable drive ratio. A sun gear ( 25 ) is mounted between the races, as is a ring (gear  26 ). The ring gear is radially outboard of the sun gear. Mounted between the sun and ring and meshing with both is a carrier gear ( 33 ) upon which the roller is mounted in a manner which enables it to both spin about its own axis, when turned by the races, and also to tilt relative to the races to vary the drive ratio. A control pinion ( 27 ) is also arranged between the sun and ring gears meshing with both. The control pinion is able to rotate about its own axis freely, but the pinion axis is fixed with respect to the housing.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is the U.S. national phase of PCT application no. PCT/GB2009/051723 filed 16 Dec. 2009 which claims priority to Japanese application JP 2008-319493 filed 16 Dec. 2008. 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to rolling-traction variators of the type in which drive is transmitted from one race to another by at least one roller whose orientation is variable in accordance with changes in variator drive ratio. More specifically it concerns such a variator in which the roller is mounted upon a carrier gear which is controlled through sun and ring gears. 
     (2) Description of Related Art 
     The word “variator” is used herein to refer to a device which transmits rotary drive from a rotary input to a rotary output at a continuously variable variator drive ratio (the ratio of the input&#39;s speed to the output&#39;s speed). Variators are particularly, but not exclusively, applicable in motor vehicle transmission applications. One known form of rolling-traction type variator uses at least two co-axially mounted races having opposed faces which are shaped so that the races together form an approximately toroidal space. At least one roller is positioned in the space between the races and runs upon their shaped faces to transmit drive from one race to the other. Changes in the inclination of the roller are associated with changes in the relative speeds of the races, and hence in the variator drive ratio. 
     Some mechanism is needed to control roller inclination, and the prior art contains numerous examples. Typically such mechanisms do not act by directly applying a torque to the roller&#39;s mountings to tilt the roller. Instead, the roller is mounted in such a manner that displacing it causes it to steer itself, due to the forces exerted on it by the races, to a new inclination. The steering effect arises because the roller seeks a position in which its own axis coincides with the common axis of the variator races, since in any other condition the motion of the roller is non-parallel to that of the races in the area where they engage with each other. The control mechanism serves to regulate the roller&#39;s displacement. 
     Examples of such mechanisms are found in the applicants&#39; prior published patent cases including PCT/GB03/00259 (WO 03/062670) and JP2006-292079A. In many examples, the displacement needed to cause the roller to steer itself is along the circumferential direction (about the common axis of the variator races) and, by allowing the rollers to tilt about an axis which is inclined to the radial plane, a relationship is established between roller displacement and roller inclination. An actuator is provided for urging the roller along the circumferential direction and so influencing its displacement, and correspondingly influencing the variator ratio. 
     The principles are illustrated in  FIGS. 1 to 3 , which are highly simplified representations of a variator viewed along the direction of the races&#39; common axis ( FIGS. 1   b ,  2   b  and  3   b ) and along a direction radial to said axis ( FIGS. 1   a ,  2   a ,  3   a ). The variator&#39;s input and output races  111 ,  112  are represented in the radial views by straight lines, although in a real variator they are, as noted above, shaped to form a substantially toroidal cavity, and in the axial views they are seen to be circular. Roller  113  (which is one of a set, although the others are omitted from the drawings for the sake of simplicity) is arranged between the input and output races  111 ,  112 , which are urged toward one another to provide traction between the roller and the races. The roller is mounted in a carriage  116  in a manner which permits the roller to rotate freely about its own axis P. The carriage  116  is connected to a piston of a hydraulic actuator  115 . Power can in principle flow in either direction through the variator—from input to output or vice versa. Consider the case in which power flows from input to output. In this case the input race  111  turns the roller  113  (its direction of rotation is indicated as w 1  in the drawings) and the roller  113  drives the output race  112  (whose direction of rotation is shown as w 3 ). A traction force F 11  is applied to the roller  113  by the input race  111 , which is driving the roller  113 . A traction force F 12  is applied to the roller by the output disc, which is driven by the roller  113 . The sum of the traction forces F 11 +F 12  is reacted through the hydraulic actuator  115  and must be balanced by the actuator&#39;s force. 
     A momentary imbalance between the traction forces F 11 +F 12  and the force of the actuator  115  causes the roller  113  to move. Suppose for example that, starting from a condition in which the variator is in equilibrium, the force applied by the actuator  115  is reduced. The traction force F 11 +F 12  will then momentarily dominate, and the roller will move toward the actuator  115 , as seen in  FIG. 2 . Referring to  FIG. 2   b , if the velocity vector V′r of the roller  113  at the region  117  where it engages the output race  112  were to remain unchanged, it would then be non-parallel to the velocity vector V′d of the surface of the output race  112  in the same region. The result is a traction force vector F 14  acting on the roller  113  tending to cause it to tilt. A similar action between the input race  111  and the roller  113  produces a traction force on the roller at its region of contact (not shown) with the input race, and the two forces on the roller  113  form a couple, producing a steering effect upon the roller  113 . That is, the roller  113  is caused to tilt about a steering axis  119 . Note that this steering axis  119  forms a castor angle B to the radial plane  118 . As a result of this castor angle, the tilting motion of the roller  113  is able to restore parallelism of the vectors V′r and V′d. The roller thus tilts (as in  FIG. 3   b ) until it reaches a position in which the said vectors are parallel, and the aforesaid couple is thus reduced to zero. As the roller moves back and forth, its tilt (and correspondingly the variator&#39;s drive ratio) varies accordingly. 
     The total torque that must be reacted from the rollers to the variator&#39;s housing is often referred to as the “reaction torque”, and is equal to the sum of the torques upon the variator&#39;s input race  111  and output race  112 . Note that this torque can only be reacted through the hydraulic actuators  115 . Hence by regulating the hydraulic fluid pressures in the actuators  115 , the reaction torque is directly regulated. The rollers automatically move to positions which cause the variator to generate a reaction torque corresponding to the said fluid pressures. It is thus the reaction torque that is directly regulated, and not the variator&#39;s actual drive ratio. This mode of control is thus sometimes referred to as “torque control”. 
     A different arrangement for controlling the variator rollers is described in Torotrak (Development) Limited&#39;s published International patent applications WO2007/065900 and WO2005/121602, both of which disclose variators in which each of a set of rollers is carried upon a respective carrier gear which meshes with a radially inner sun gear and a radially outer ring gear in the manner of a planetary gear in an epicyclic gear train. In this type of arrangement, rotation of the sun gear relative to the ring gear causes the carrier gear to turn, and it is this turning of the carrier gear that causes the steering effect upon the roller needed to control the roller&#39;s tilt. 
     In this type of arrangement it is necessary to control the motion of the sun and ring gears in order to control the variator, and the aforementioned International patent applications contain various mechanisms for this purpose. Among these are arrangements in which one or a set of control pinions is provided which, like the carrier gears, mesh with the sun and ring. In particular, WO2007/065900 discloses an arrangement in which a control pinion (referred to therein as a planet) is coupled to a hydraulic actuator in a manner which enables the actuator to move the pinion back and forth, but prevents the pinion from rotating. In this way the actuator controls the positions of both sun and ring gears. 
     All the arrangements in this prior art document permit the control pinion(s) and the carrier gears to move back and forth about the common axis of the variator races, with the forces on the control pinions being reacted through, and hence controlled by, some form of actuator. In this way the reaction torque can be regulated. 
     SUMMARY OF THE INVENTION 
     When torque from a rotary power source (such as, for example, an engine) is transmitted from the input disc to the roller, a traction force (F 11 ) acts, and when torque transmission is effected from the roller to the output disc, a traction force (F 12 ) acts, so, in order to support the roller it is necessary to apply a reaction force (F 13 ) that balances these traction forces (F 11 +F 12 ). Consequently it is necessary to apply a reaction force to the power roller through the sun gear, ring gear and hydraulic servos. 
     An object of the present invention is to provide a variator of the aforementioned type, having sun and ring gears and a carrier gear on which the roller is mounted, in which the means used to drive the sun and ring are not required to react the variator reaction torque. 
     In accordance with a first aspect of the present invention, there is a variator comprising a housing, a pair of semi-toroidally recessed races each mounted to the housing for rotation about a variator axis, and a roller mounted between the races to transfer drive between them at a variable drive ratio, the variator further comprising:
         a sun gear between the races;   a ring gear between the races, the ring gear being radially outward of the sun gear;   a carrier gear which is disposed between the sun and ring gears and meshes with both, and upon which the roller is mounted in a manner which enables the roller to spin about its own axis and to tilt relative to the races to vary the drive ratio; and   a control pinion which is disposed between the sun and ring gears and meshes with both, the variator being characterised in that   the control pinion is mounted to the housing in a manner which permits it to rotate freely about a pinion axis but prevents the pinion axis from moving with respect to the housing.       

     According to a second aspect of the present invention there is a variator ( 2 ) comprising:
         a casing ( 6 ),   a power roller ( 14 ) gripped between two discs ( 11 ,  12 ), namely an input disc ( 11 ) and an output disc ( 12 );   a sun gear ( 25 A,  25 B) provided between the two discs ( 11 ,  12 ), on the circumferentially inward side;   a ring gear ( 26 A,  26 B) provided between the two discs, on the circumferentially outward side; and   a planet gear ( 33 ) having a rotation and inclination support part ( 31 ) which supports the power roller ( 14 ) in a freely rotatable fashion and with freedom to incline relative to the two discs ( 11 ,  12 ), and having a support shaft ( 32 ) which supports the rotation and inclination support part ( 31 ) and which at one end meshes with the sun gear ( 25 A,  25 B) and at the other end meshes with the ring gear ( 26 A,  26 B)   characterised in that:   the variator ( 2 ) comprises:   rotational drive means ( 29 ) which drives in rotation at least one or other of the sun gear ( 25 A,  25 B) and ring gear ( 26 A,  26 B);   a control pinion ( 27 ) which is positioned between the sun gear ( 25 A,  25 B) and the ring gear ( 26 A,  26 B), and which at one end meshes with the sun gear ( 25 A,  25 B) and at the other end meshes with the ring gear ( 26 A,  26 B); and   a carrier ( 28 ) which supports the control pinion ( 27 ) in a freely rotatable fashion, and in that   in altering the gear ratio in the toroidal type of continuously variable device ( 2 ), when the angle of inclination of the support shaft ( 32 ) of the planet gear ( 33 ) is altered by driving the sun gear ( 25 A,  25 B) and the ring gear ( 26 A,  26 B) in rotation in respectively opposite directions via the control pinion ( 27 ) by means of the rotational drive means ( 29 ), the angle of the power roller ( 14 ) relative to the rotational direction of the two discs ( 11 ,  12 ) is altered via the rotation and inclination support part ( 31 ), and, because of the resulting difference in the rotational direction of the angle-altered power roller relative to the two discs ( 11 ,  12 ) at the contact regions ( 17 ), its attitude is automatically altered in such a way that the power roller is inclined in a direction whereby the contact radii of the contact regions ( 17 ) are altered and at the same time returns to a direction tangential to the rotational direction of the two discs ( 11 ,  12 );   and also in that   the carrier ( 28 ) is secured to the casing ( 6 ).       

     Preferably the rotational drive means consists of a motor ( 29 ). 
     Preferably the rotation and inclination support part ( 31 ) has:
         a central support part ( 35 ) which is fixedly supported on the support shaft ( 32 ) and which is formed in the shape of a cylinder centred on a first axis (I) given a caster angle (γ) with respect to an axis (H) orthogonal to the support shaft;   a roller rotational support part ( 37 ) which is supported with freedom of rotation with respect to a cylindrical arcuate surface ( 36 ) of the central support part ( 35 ), and which is formed in the shape of a cylinder centred on a second axis (J) orthogonal to the first axis (I); and   the power roller ( 14 ) is supported with freedom of rotation, centred on the second axis (J), with respect to an arcuate surface ( 38 ) of the cylindrical shape of the roller rotational support part ( 37 );   the rotation and inclination support part ( 31 ) supports the roller rotational support part ( 37 ) and the power roller ( 14 ) by means of cylindrical parallel surfaces of the central support part ( 35 ), and when the support shaft ( 32 ) is inclined by rotational control of the planet gear ( 33 ), makes the power roller ( 14 ) incline relative to the rotational direction of the two discs ( 11 ,  12 ), and, when the power roller is made to incline in a direction which changes the contact radius of the contact regions ( 17 ) due to the difference in the rotational direction at the contact regions ( 17 ), the power roller is made to incline about the first axis (I), by means of the cylindrical arcuate surface ( 36 ) of the central support part ( 35 ), and at the same time is made to return to a direction tangential to the rotational direction of the two discs ( 11 ,  12 ) in accordance with the caster angle.       

     In the variator according to the invention, movement of the power rollers in the circumferential direction relative to the two discs can be eliminated, so the carriers that support the control pinions in a freely rotatable fashion can be fixed to the casing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Specific embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: — 
         FIGS. 1   a  and  1   b  represent a balanced condition of the forces of a conventional type of toroidal continuously variable device, (a) being a view in the radial direction and (b) being a view in the axial direction. 
         FIGS. 2   a  and  2   b  represent a conventional type of toroidal continuously variable device in the condition in which the power rollers are displaced, (a) being a view in the radial direction and (b) being a view in the axial direction. 
         FIGS. 3   a  and  3   b  represent a conventional type of toroidal continuously variable device in the condition in which the power rollers are inclined, (a) being a view in the radial direction and (b) being a view in the axial direction. 
         FIG. 4  is a section in an axial plane through a variator according to the present invention. 
         FIG. 5  is a section in a radial plane through a variator according to the present invention. 
         FIG. 6  is a perspective view of a roller unit. 
         FIGS. 7   a - 7   c  illustrate parts of the variator, (a) representing the roller positions at a variator ratio of −1, (b) being a view of the condition in which the roller units are rotated, and (c) being a view of a condition in which the rollers are inclined. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The variator  2  seen in  FIGS. 4 and 5  is of double cavity, full toroidal type. It may be used in a motor vehicle transmission, and in this context it may be combined with a known type of planetary gear (not depicted herein) to provide both forward and reverse drive, and in suitable cases to provide two or more “regimes”—i.e. two or more different transmission ratio ranges. 
     The variator  2  has a pair of input races  11  mounted upon an input shaft  3  to rotate along with it. The input shaft  3  forms the variator&#39;s rotary input in this embodiment and may for example be coupled through suitable gearing to a source of rotary drive such as an internal combustion engine. An output race  12  is connected at its outer periphery to an output member  16  which forms the variator&#39;s output and may for example be coupled to gearing leading to the wheels of a motor vehicle. Roller units  5  (see  FIG. 6 ), to be described in detail later, each comprise a roller  14  that is arranged between one of the input races  11  and the output race  12 . Each of the input races  11  has a semi-toroidally recessed surface upon which the associated rollers  14  run. The output race  12  has two semi-toroidally recessed faces so that the races together form two generally toroidal cavities containing the rollers  14 . For the sake of simplicity  FIG. 4  shows only a single roller  14  in each cavity, but in practice a plurality of rollers  14  (for example three power rollers in each cavity) are arranged at regular intervals about the cavities&#39; circumferences. In order to provide traction between the rollers  14  and the races  11 ,  12  they must be biased into engagement with one another. In a manner known in the art, this is achieved by means of a hydraulic actuator  8  which acts upon one of the input races  11 , urging it along the input shaft  3  toward the other races. The other of the input races  11  is prevented from moving along the shaft  3 , so that the force of the actuator  8  is transmitted through the rollers  14  and the output race  12  to the shaft  3 . 
     A roller control device  4  controls the inclination of the rollers  14 . Thus continuous (stepless) variation of the variator drive ratio can be achieved by alteration of the radii at which the rollers  14  engage with the input discs  11  and the output disc  12 . The output disc  12  rotates in the opposite direction to the input discs  11 , so the variator drive ratio is negative. 
     The roller control device  4  is formed as follows. Sun gears  25 A,  25 B are arranged circumferentially inwards of the respective rollers  14 , being supported in a freely rotatable fashion on the input shaft  3  and linked by a sleeve member  25   a  passing through the output race  12 , so that the sun gears  25 A,  25 B rotate together. Ring gears  26 A,  26 B are arranged circumferentially outwards of the respective rollers  14  and supported in a freely rotatable fashion with respect to the input shaft  3 . 
     A pinion mounting part  28  is fixed with respect to the transmission casing  6  and supports in a freely rotatable fashion control pinions  27  that are arranged between the sun gears  25 A,  25 B and the ring gears  26 A,  26 B, to mesh with both. In addition, the roller control device  4  includes the roller units  5  referred to above and a motor unit  29  (see  FIG. 5 ) constituting a rotary drive means that drives the ring gear  26 A in rotation. 
     The carrier  28  comprises a front main carrier plate  28   a , a front subcarrier plate  28   b , a rear subcarrier plate  28   c , and a rear main carrier plate  28   d . The front main carrier plate  28   a  is formed on its circumferentially inward side with a sleeve  28   g . The sleeve  28   g  is supported in a freely rotatable fashion on the input shaft  3  by means of a bearing  43 . The front main carrier plate  28   a  is formed on its circumferentially outward side with a connecting part  28   s . The connecting part  28   s  is fixed to the inside face of the transmission casing  6 . In addition, the front main carrier plate  28   a  is integrally formed with pinion shafts  28   e . Control pinions  27 , to be described, are supported thereon in a freely rotatable fashion by means of bearings  41 . 
     The front subcarrier plate  28   b  is formed with holes  28   h  into which are fitted projections  28   f  that are formed on the pinion shafts  28   e , and is thereby fixed with respect to the main carrier plate  28   a  by means of the holes  28   h . Also, the front subcarrier plate  28   b  is formed with a sleeve  28   i  on the circumferentially inward side thereof and is arranged so as to be on the circumferentially outward side of the sleeve member  25   a.    
     The rear main carrier plate  28   d  is formed with a sleeve  28   m  on the circumferentially inward side thereof. The sleeve  28   m  is supported in a freely rotatable fashion on the input shaft  3  by means of a bearing  44 . Also, pinion shafts  28   j  are integrally formed on the rear main carrier plate  28   d  and the control pinions  27  are supported in a freely rotatable fashion thereon by means of bearings  42 . 
     The rear subcarrier plate  28   c  is formed with holes  28   l  into which are fitted projections  28   k  that are formed on the pinion shaft  28   j  and is thereby fixed with respect to the rear main carrier plate  28   d  by means of the holes  28   l . The rear subcarrier plate  28   c  is formed on its circumferentially inward side with a sleeve  28   n  arranged so as to be on the circumferentially outward side of the sleeve member  25   a . The front subcarrier plate  28   b  and the rear subcarrier plate  28   c  are linked on the circumferentially inward side of the output disc  12  and in this way the carrier  28 , which is constituted by the front main carrier plate  20   a , the front subcarrier plate  28   b , the rear subcarrier plate  28   c  and the rear main carrier plate  28   d , is fixed with respect to the transmission casing  6 . Also, at the portion where the front subcarrier plate  28   b  and the rear subcarrier plate  28   c  are linked, the output disc  12  is supported in a freely rotatable fashion by means of bearings  45 . 
     The control pinions  27  on the left side of  FIG. 4  are arranged between adjacent power rollers  14  in the cavity  13  and mesh with the sun gear  25 A and the ring gear  26 A. They are supported in a freely rotatable fashion by means of the bearings  41 , on the pinion shafts  28   e  that are integrally formed with the carrier  28 . Similarly, the control pinions  27  on the right side of  FIG. 4  are arranged between adjacent rollers  14  in the cavity  13  and mesh with the sun gear  25 B and the ring gear  26 B. They are supported in a freely rotatable fashion by means of the bearings  42 ,  42  on the pinion shaft  28   j  that is integrally formed with the carrier  28 . 
     In the present embodiment six roller units  5  as described above are arranged in the two cavities (three per cavity). All of the roller units  5  are constructed in the same way. As shown in  FIG. 6 , the roller units  5  have a shape in which the range of movement of the aforesaid power roller  14  is cut out from an annular gear, and comprise: a planet gear  33  whose circumferentially inward side (the lower side in  FIG. 4 ) meshes with the sun gear  25 A ( 25 B) and whose circumferentially outward side (the upper side in  FIG. 4 ) meshes with the ring gear  26 A ( 26 B); and a rotation and inclination support part  31  that is fixed and supported by the central portion of this planet gear  33  and supports the power rollers  14  in freely rotatable fashion in the ω 1  direction and with freedom to incline in the ω 2  direction. 
     The rotation and inclination support part  31  comprises a central support part  35  and roller rotation support part  37 . This central support part  35  is integrally formed with a support shaft  32  and is formed in cylindrical shape centred on an axis (the first axis) I that is inclined by a caster angle γ in a plane orthogonal to the support shaft  32  from an axis H parallel with the two discs  11 ,  12  and orthogonal to this support shaft  32 . The roller rotational support part  37  is formed in a cylindrical shape centred on an axis (the second axis) J constituting the axis of rotation of the power roller  14  and orthogonal to the above-mentioned axis I, and is supported in a freely rotatable fashion with respect to the cylindrically shaped arcuate surface  36  of this central support part  35 . Also, the roller rotational support part  35  supports in a freely rotatable fashion a roller  14  that rotates about the axis J, by means of a cylindrical arcuate surface  38  thereof. 
     As shown in  FIG. 5 , the motor unit  29  is arranged below the variator  2  within the transmission casing  6  and comprises a drive section  29   a  constituted by a stepping motor, and a rack  29   b . In the drive section  29   a  there is provided a rotor (not shown) that is controlled in accordance with the operating conditions of the vehicle, based on a signal from an electronic control device, not shown. The rack  29   b  is connected with the rotor through a screw mechanism (not shown) that converts rotary motion into linear motion. The rack  29   b  is a rack-shaped member that is formed with a plurality of teeth  29   c  on its upper surface. The teeth  29   c  mesh with external teeth  26   a  formed on the outer circumference of the ring gear  26 A. 
     Next, the operation of the variator  2  will be described with reference to  FIG. 7 . 
     When the variator  2  is mounted in a vehicle, rotation of the input shaft  3  that is connected with the output shaft of the engine is transmitted to the input discs  11  of the variator  2 . The power rollers  14  are rotated by the rotation of the input discs  11 , and the output disc  12  is rotated in the direction ω 3  as shown in  FIG. 7(   a ) by this rotation. When this happens, the traction force F 1  that is generated when torque is transmitted from the input discs  11  to the output disc  12  acts on the power rollers  14  in the cavities  13  and this traction force F 1  is in balance with the reaction forces F 2 ′, F 3 ′ received by the ring gear  26 A and sun gear  25 A. 
       FIG. 7(   b ) shows what happens when, for example, the ring gear  26 A is driven in rotation by the motor unit  29  in the ω 4  direction. The planet gear  33  of the roller units  5  is rotated in the same direction as the ring gear  26 A and the control pinion  27  is rotated about the pinion shaft  28   e  of the carrier  28 , causing the sun gear  25 A that is meshed therewith to be rotated in the ω 5  direction, which is the opposite direction to that of the ring gear  26 A. Consequently, the power roller units  5  are rotated in the ω 4  direction from the ring gear  26 A and rotated in the ω 5  direction from the sun gear  25 A, so that the power roller units rotate (by themselves) to their original position and the angles of the support shafts  32  of the roller units  5  are altered as shown in  FIG. 7(   b ). At this point, the rollers  14  are inclined towards the rotational direction ω 3  of the output disc  12  together with the roller rotational support part  37 , by means of the cylindrically shaped parallel faces that are provided on the central support part  35  of the rotation and inclination support part  31 . 
     When this happens, at the contact regions  17  of the output disc  12  and the power rollers  14 , the velocity vector Vr of the power roller  14  is facing more towards the circumferentially inward side than the tangential direction of the output disc  12 , while the velocity vector Vd of the output disc  12  is in the tangential direction of this output disc  12 . Consequently, the velocity vector Vd of the output disc  12  and the velocity vector Vd of the power roller  14  become non-parallel. Furthermore, at the contact regions  17 , a traction force F 4  in the same direction as the difference of the velocity vector Vd and the velocity vector Vr is generated, which traction force F 4  acts on the power rollers  14 . 
     A similar action takes place between the power rollers  14  and the input discs  11 , but a traction force that is in the opposite direction to the traction force F 4  acts on the power rollers  14 . By the action of this traction force that is generated between the rollers  14  and the input discs  11  and the traction force F 4 , as shown in  FIG. 4(   c ), the axis of rotation J of the rollers  14  (see  FIG. 6)  is tilted about the axis I along the arcuate surface  36  provided on the central support part  35 , and the rollers  14  are brought into positions, by the action of the caster angle γ, in which the velocity vector Vr of the power roller  14  and the velocity vector Vd of the output disc  12  become parallel. In other words, the transmission ratio (contact radii) of the output disc  12  with respect to the input discs  11  are automatically changed. 
     Next, the balance of forces in the variator  2  will be described. Even when the variator  2  is at a fixed transmission ratio in which no transmission ratio control is being performed, a traction force F 1  is generated on the rollers  14  whilst torque transmission is being performed between the input discs  11  and the output disc  12 . As shown in  FIG. 5 , the traction force F 1  can be divided into a force F 2  that acts at the circumferentially outward region of the variator  2  on the planet gear  33  and a force F 3  that acts on the circumferentially inward region of the variator  2  on the planet gear  33 . 
     The force F 2  is transmitted to the ring gear  26 A as a force F 5  tending to rotate the ring gear  26 A in the clockwise direction in  FIG. 5 , through the meshing of the planet gear  33  with the ring gear  26 A. The force F 5  that is transmitted to the ring gear  26 A is transmitted to the control pinion  27  as a force F 7  tending to rotate the control pinion  27  in the clockwise direction in  FIG. 5  through the meshing of ring gear  26 A and the control pinion  27 . 
     Furthermore, the force F 3  is transmitted to the sun gear  25 A as a force F 6  tending to rotate the sun gear  25 A in the clockwise direction in  FIG. 5 , through the meshing of the planet gear  33  and the sun gear  25 A. The force F 6  that is transmitted to the sun gear  25 A is transmitted to the control pinion  27  as a force F 8  tending to rotate the control pinion  27  in the anticlockwise direction in  FIG. 5 , by meshing of the sun gear  25 A with the control pinion  27 . 
     Forces F 7  and F 8  tend to rotate the control pinion  27  in mutually opposite directions, i.e. they constitute a force F 9  that acts on the pinion shaft  28   e , tending to move the control pinion in the clockwise direction in  FIG. 5  (the direction of rotation of the input discs  11 ). The force F 9  acts on the transmission casing  6  (see  FIG. 4 ) through the carrier  28 . 
     Force F 9  is the sum of F 7  and F 8 , which are of the same magnitude as the forces F 2 , F 3  that were resolved from the traction force F 1  as described above, so it is of the same magnitude as the traction force F 1 . Furthermore, the fraction forces F 1  and F 9  can be placed in a balanced condition (in which they do not rotate the planet gear  33  and control pinion  27 ) since the centre of rotation of the planet gear  33  (rollers  14 ) on which they respectively act, and the pinion shafts  28   e  are at substantially the same radius as the input discs  11 . In other words, all of the traction forces F 1  that are generated on the rollers  14  act on the transmission casing  6 , so all of the reaction force in respect of these transmission forces F 1  can be borne by the transmission casing  6 . 
     It should be noted that, although the reactions to these fraction forces F 1  to F 9  are omitted from  FIG. 5 , these reactions are the reaction forces F 1 ′ to F 9 ′ and can be represented by arrows in the opposite direction to the respective arrows that are shown. More specifically, the reaction forces F 2 ′, F 3 ′ are shown in order to describe the forces acting on the roller units  5 , as illustrated in  FIG. 7(   a ). 
     When the variator  2  performs transmission ratio control, rotational drive of the ring gear  26 A is effected by the motor unit  29  as described above, while torque is transmitted between the input discs  11  and the output disc  12 . Although the ring gear  26 A is driven in rotation by the motor unit  29  so that the planet gear  33  and control pinion  27  are rotated, there is no change in the respective meshing positions of the planet gear  33  and control pinion  27  and of the ring gear  26 A and sun gear  2 A. Therefore, as mentioned above, the force relationship is unchanged in that the reactions to the traction forces F 1  generated on the power rollers  14  are borne by the transmission casing  6 . Consequently, the drive force provided by the motor unit  29  can drive the ring gear  26 A without needing to bear these reaction forces. Mechanisms such as a hydraulic servo or hydraulic circuitry in motor unit  29 , are therefore not required to output a drive force that is larger than the traction force F 1 . 
     It should be noted that, in the variator  2  according to the present embodiment, control pinions  27  are arranged between the rollers  14  so that no movement of the power rollers  14  in the direction of rotation of the two discs  11  and the disc  12  takes place when transmission ratio control of the roller units  5  that support the rollers  14  is performed. There is therefore no risk of fouling of the power rollers  14  and control pinions  27 . 
     In the variator  2  described above, the carrier  28 , immovably mounted with respect to the transmission casing  6 , supports control pinions  27  meshing with the sun gears  25 A,  25 B and ring gears  26 A,  26 B in a freely rotatable fashion. Consequently, even when a traction force F 1  acts on the rollers  14 , causing reaction to be transmitted through the planet gear  33  to the sun gears  25 A,  25 B and ring gears  26 A,  26 B, the reaction of the traction force F 1  transmitted to the sun gears  25 A,  25 B and ring gears  26 A,  26 B is referred to the transmission casing  6  through the control pinions  27  and carrier  28 . In this way, the reaction to the traction force F 1  can be borne by the transmission casing  6  and it becomes unnecessary for the motor unit  29  to react the traction force F 1  produced by the rollers  14 . In this way, mechanisms (hydraulic servo and/or hydraulic circuitry etc) for outputting larger drive force than the traction force F 1  can be made unnecessary, making it possible to make the variator  2  more compact. 
     The traction force F 1  produced by the power rollers  14  is reacted by the transmission casing  6 , even when the transmission force F 1  varies in accordance with unpredictable operating conditions. It therefore becomes unnecessary to perform output control of the reaction force produced by the motor unit  29  so as to track the traction force F 1 . Thus, a control mechanism for performing complicated control such as, for example, feedback control is made unnecessary and simplification and cost reduction can thus be achieved. 
     Also, since the rotary drive means is constituted by a motor, a simple construction can be adopted. Also, in particular if a stepping motor, i.e. a motor that is capable of controlling its own position in response to an electrical instruction, is employed for the motor unit  29 , a device for performing feedback by detecting the position of, for example, the sun gears  25 A,  25 B or ring gears  26 A,  26 B becomes unnecessary, so the variator  2  can be simplified and costs can be reduced. 
     Also, the rotation and inclination support part  31  can incline the rollers  14  with respect to the direction of rotation of the two discs  11 ,  12  by means of the cylindrically shaped parallel faces of the central support part  35 , when the support shaft  32  is inclined by rotational control of the planet gear  33 , and can subsequently, incline the power rollers  14  and return the power rollers  14  in the tangential direction of the direction of rotation of the two discs  11 ,  12 , in accordance with the caster angle γ, by means of the cylindrically shaped arcuate surfaces  36  of the central support part  35 , when these power rollers  14  are inclined in a direction with altered contact radii of the contact regions  17  by the difference in direction of rotation at the contact regions  17 . It is therefore possible to alter the contact radii of the power rollers  14  with respect to the two discs  11 ,  12  and return the power rollers  14  automatically in the tangential direction of the direction of rotation of the two discs  11 ,  12 , without moving the centre of rotation of the power rollers  14 , simply through rotational control of the planet gear  33 . In this way, movement of the power rollers  14  in the circumferential direction relative to the two discs  11 ,  12  can be eliminated, so the carriers  28  that support the control pinions  27  in a freely rotatable fashion can be fixed to the casing  6 . 
     In the example described above the ring gear was driven in rotation by the rotational drive means. The present invention could also be applied in a construction in which the sun gear, or sun gear and ring gear are driven in rotation by the rotational drive means. 
     The rotational drive means need not necessarily be a motor unit  2  having a stepping motor and screw mechanism. For example, a hydraulic servo could be employed.