Abstract:
A continuously variable transmission, which comprises a drive race for receiving power input, a driven race for transmitting power output and a set of rollers between the races. Each roller in the set is capable of rotating to transfer motion from the drive race to the driven race, is capable of tilting about a first tilt axis to vary a ratio of the continuously variable transmission, and is capable of tilting about a second tilt axis different from the first tilt axis. The transmission further comprises a ratio change actuator acting on each roller of the set for tilting the roller about its second tilt axis, a tilt of the roller about its second tilt axis inducing a reaction force on the roller tending to tilt the roller about its first tilt axis and thus provoke a ratio change. The transmission also comprises a synchronization member coupling the rollers, when a tilting movement is imparted to the rollers about their respective first tilt axes, the synchronization member constraining the rollers to tilt in unison about the respective first tilt axes to produce a coordinated ratio change.

Description:
TECHNICAL FIELD 
       [0001]    The present invention generally relates to a toric-drive continuously variable transmission and, more particularly, to a drive roller control for use therewith. 
       BACKGROUND ART 
       [0002]    Transmissions are used in motorized vehicles to transmit the engine power to the propelling system of the vehicles (i.e., wheels, propellers, etc.). Various types of transmissions adapt to the different engines and motors in order to propel the vehicle. An internal combustion engine, for instance, does not behave like an electric motor. An electric motor evolves between a full stop to high running speeds at high efficiencies. Therefore, a transmission may couple the electric motor directly to the propelling system. An internal combustion engine, on the other hand, will not run below a minimal revolutions per minute (RPM) and is also limited with respect to the maximal RPM it may attain. Therefore, the transmission used with such engines requires a clutching mechanism in order to allow the internal combustion engine to run while the vehicle is idle. Furthermore, the transmission must allow ratio changes between the engine output and the propelling system input, as high torque is typically required initially to propel the idle vehicle forward, to the detriment of the vehicle speed. Thereafter, lower torque is supplied for higher speed. 
         [0003]    There are generally two main types of transmissions for internal combustion engine vehicles in the automotive industry: the discontinuous ratio transmission and the continuously variable transmission (CVT). The difference between the two types of transmission is comparable to the relation, in mathematics, between integers and real numbers. There are five integers comprised between 1 and 5 inclusively, whereas there is an infinity of real numbers between the same interval. The translation from an integer to the next integer implies a jump, a discontinuity. A discontinuous ratio transmission has such jumps. For instance, a five-speed vehicle has five different ratios, the ratio being the rotational speed at the inlet divided by the rotational speed at the outlet of the transmission. On the other hand, CVT&#39;s have an infinite ratio of speeds between inlet and outlet of the transmission, extending between a minimal ratio and a maximal ratio. 
         [0004]    Discontinuous ratio transmissions are found on most cars, as they are highly efficient (in the vicinity of 95%). and highly reliable as there are no efficiency losses due to slip or overheating, and these transmissions are closed from water and dust damage. On the other hand, the discontinuity between the speed ratios and the necessity for clutching to switch speeds are major inconveniences. There is a loss in engine power, although small, when switching from one ratio to another. These transmissions also are more complex and require synchronization between the ratio changes. Furthermore, in difficult conditions, driver ability comes into account. 
         [0005]    One type of CVT, the toric-drive transmission, involves a drive disk and a driven disk adjacent to one another, and shaped so as to form together a torus-shaped cavity. Rollers are positioned in the torus-shaped cavity so as to transmit motion from the drive disk to the driven disk. The input-to-output ratio changes as a function of the orientation of the rollers with respect to the disks, but is continuous. With CVT&#39;S, the change of speed and ratios is effected without discontinuity. The CVT&#39;s are also very flexible in allowing to optimize the use of the engine to which they are connected. However, CVT&#39;s are typically less energy-efficient than discontinuous ratio transmissions. For instance, in some type of toric-drive transmissions, actuation is required to displace rollers. between the drive disk and the driven disk to change orientation, and hence vary the input-to-output ratio. More specifically, a translation of the rollers is caused to initiate a change in orientation to change the input-to-output ratio, whereby a non-negligible amount of actuation is used to cause the translation. 
       SUMMARY OF INVENTION 
       [0006]    Therefore, it is a feature of the present invention to provide a novel continuously variable transmission. 
         [0007]    It is a further feature of the present invention to provide a continuously variable transmission having an increased energy efficiency. 
         [0008]    It is a still further feature of the present invention to provide a continuously variable transmission in which the amount of force required to actuate a change in input-to-output ratio is reduced. 
         [0009]    It is a still further feature of the present invention to provide a method for controlling a power input/output ratio in a continuously variable transmission. 
         [0010]    According to the features of the present invention, from a broad aspect, there is provided a toric-drive transmission comprising: a drive disk for receiving a power input; a driven disk for transmitting a power output; a roller device having a roller displaceably mounted between the drive disk and the driven disk, the roller having three rotational degrees of freedom, a first one of the rotational degrees of freedom for transmitting motion from the drive disk to the driven disk so as to convert the power input to the power output, a second one of the rotational degrees of freedom for varying a ratio of the power output to the power input as a function of an orientation of the roller along the second one of the rotational degrees of freedom, and a third one of the rotational degrees of freedom for initiating rotation of the roller about the second one of the rotational degrees of freedom; and a controller system operatively connected to the roller device for changing the orientation of the roller in the second one of the rotational degrees of freedom by actuating a displacement of the roller in the third one of the rotational degrees of freedom. 
         [0011]    Further in accordance with the features of the present invention, there is provided a method for controlling a power input/output ratio of a toric-drive transmission of the type having a pair of disks forming a torus-shaped cavity with a roller in the torus-shaped cavity, the roller having a first rotational degree of freedom associated with a transmission of motion between the disks, a second rotational degree of freedom associated with the power input/output ratio, and a third rotational degree of freedom associated with a path of the roller on the disks, the method comprising the steps of: displacing the roller from a first orientation to a predetermined second orientation in the third rotational degree of freedom so as to change the path of the roller on the disks, in which the roller will tend to return to the first orientation; and guiding the roller into a change of orientation in the second rotational degree of freedom when the roller returns to the first orientation; whereby the power input/output ratio is changed as a function of the predetermined second orientation in the third rotational degree of freedom. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0012]    A preferred embodiment of the present invention will now be described with reference to the accompanying drawings in which: 
           [0013]      FIG. 1  is an exploded view of a toric-drive transmission in accordance with a preferred embodiment of the present invention; 
           [0014]      FIG. 2  is a perspective view of a drive disk and a driven disk in accordance with the preferred embodiment of the present invention; 
           [0015]      FIG. 3  is a perspective view of a drive roller in accordance with the preferred embodiment of the present invention; 
           [0016]      FIG. 4  is a perspective view of a structure elbow in accordance with the preferred embodiment of the present invention; 
           [0017]      FIGS. 5A to 5C  are schematic sectioned views of the toric-drive transmission; and 
           [0018]      FIG. 6  is a graph illustrating the speed vs. the RPM of a CVT in comparison with a discontinuous-drive transmission. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0019]    Referring now to the drawings and more particularly to  FIG. 1 , a toric-drive transmission in accordance with the present invention is generally shown at  10 . A protective casing, along with the necessary seals and joints, has been removed from the figures in order to clarify the views of the transmission  10 . The toric-drive transmission  10  of the present invention is protected from dust and water, as it is enclosed in the casing (not shown). The various elements of the transmission  10  are shown exploded. A drive axis is generally shown at D. 
         [0020]    The transmission  10  comprises a drive disk  12 . As seen in  FIGS. 1 ,  2  and  5 A to  5 C, the drive disk  12  has a groove  14  which is a portion of a torus. The drive disk  12  has on an opposed side a flange  16  ( FIG. 2 ) extending axially with three connection slots  18  therein. A driven disk  22  is a mirror image of the drive disk  12 . The driven disk  22 , therefore, also has a groove  24  which is a portion of a torus, and on an opposed side a flange  26  ( FIG. 1 ) having three connection slots  28 . The drive disk  12  and the driven disk  22  are positioned in the transmission  10  such as to have the grooves  14  and  24  facing each other, and this is well depicted in  FIGS. 2 and 5A  to  5 C. The drive disk  12  and the driven disk  22  together define a torus-shaped cavity (as best seen sectioned in  FIGS. 5A to 5C ), having a circular section. A circle (not shown) is formed by the centers of all circular sections, and will be referred to hereinafter as the toric circle. 
         [0021]    Referring now to  FIG. 3 , a drive roller is shown at  30 . The drive roller  30  is disk-shaped and has a peripheral surface  32 . The peripheral surface  32  is slightly convex. The drive roller  30  is held in a drive roller support  34 . The drive roller support  34  consists of a pair of parallel plates  36  spaced from one another to receive the driver roller  30  therebetween. A shaft (not shown) serves as an axle for the drive roller  30  and is held at opposed ends by the parallel plates  36 . Bearings (not shown) ensure the rolling of the drive roller  30  about the axle held by the parallel plates  36 . 
         [0022]    Opposed ends of the parallel plates  36  each comprise a head  38 , which has a spherical contact surface  40 , from which guide pins  42  extend from the middle thereof. One of the two parallel plates  36  has a control arm  44  with a follower sphere  46  at an end thereof. As seen in  FIG. 1 , the follower sphere  46  is adapted for being received in a socket  48 . Each of the sockets  48  is tube-shaped. It is observed in  FIG. 1  that the transmission  10  has three drive rollers  30 , each mounted to a drive roller support  34 . It is pointed out that the transmission  10  of the present invention may be provided with various configurations ranging from one drive roller to a plurality of drive rollers. 
         [0023]    Referring now to  FIGS. 5A to 5C , two drive rollers  30  are shown in various positions between the drive disk  12  and the driven disk  22  in order to illustrate the operation of the toric-drive transmission  10 . The drive rollers  30  are driven by the drive disk  12  and, therefore, transmit the rotative motion to the driven disk  22 . The drive rollers  30  rotate about an X-axis in the transmittal of the rotative motion from the drive disk  12  to the driven disk  22 . A Y-axis intersects the X-axis at the geometric center of each of the drive rollers  30 , and passes through points of contact between the drive rollers  30 , the drive disk  12  and the driven disk  22 . A Z-axis is normal to a plane in which lies the X-axis and Y-axis, and intersects the X-axis and the Y-axis at the origin. It is pointed out that the Z-axis is tangential to the toric circle (see above) when the input-to-output ratio of the toric-drive transmission  10  is constant. 
         [0024]    The drive rollers  30  may also rotate about the Z-axis in order to change the input-to-output ratio of the transmission  10 . The drive roller  30  transmits the rotating motion from the drive disk  12  to the driven disk  22  by being in contact, through its peripheral surface  32 , with thin films of oil on the surfaces of the grooves  14  and  24 . Nonetheless, for simplicity purposes, the points of transfer of motion between the drive rollers  30  and the drive disk  12  and the driven disk  22  will be referred to as points of contact hereinafter. This will be described in further detail hereinafter. The drive rollers  30  have a third rotational degree of freedom, as they may rotate according to the Y-axis. The Y-axis extends between the point of contact of the drive disk  12  with the drive roller  30  and the point of contact of the drive roller  30  with the driven disk  22 . The rotation about the Y-axis will initiate the Z-axis rotation, which will modify the input-to-output ratio. This will be explained in further detail hereinafter. 
         [0025]    The drive rollers  30  are positioned between the drive disk  12  and the driven disk  22  such that their geometric centers (i.e., at the origin of the XYZ coordinate systems described above) are on the toric circle. The geometric center of each of the drive rollers  30  generally remains on the toric circle throughout operation of the toric-drive transmission  10 . 
         [0026]    According to  FIG. 5A , the transmission  10  is in speed reduction. In speed reduction, the input-to-output ratio is above 1 as the drive disk  12  (input) rotates faster than the driven disk  22  (output). As the distance R 1  from the point of contact between the drive disk  12  and the drive rollers  30  to the center of the drive disk  12  is smaller than the distance R 2  from the point of contact between the driven disk  22  and the drive roller  30  to the center of the driven disk  22 , the drive disk  12  will rotate faster than the driven disk  22 . 
         [0027]    A rotation of the drive rollers  30  about their respective Z-axes leads to other ratios, such as that shown in  FIGS. 5B and 5C .  FIG. 5B  illustrates a direct drive between the drive disk  12  and the driven disk  22 . In the direct drive, the drive disk  12  and the driven disk  22  rotate at the same speed, giving a ratio of 1 between input (drive disk  12 ) and output (driven disk  22 ). This is achieved by distance R 1  being equal to distance R 2 .  FIG. 5C  illustrates an overdrive between the drive disk  12  and the driven disk  22 , and the ratio is therefore below 1. In this case, distance R 1  is greater than distance R 2 , to have the driven disk  22  rotate faster than the drive disk  12 . As mentioned above, a vehicle having the toric-drive transmission  10  of the present invention initially has the drive rollers  30  in a speed-reduction position. The input-to-output ratio at that point is above 1, which means that the output speed is below the input speed. Therefore, the torque of the engine is used to provide torque to the wheels. As the vehicle increases speed, the input-to-output ratio is gradually decreased in order to lessen the torque transmitted to the wheels while increasing the speed of the wheels. 
         [0028]    Returning to  FIG. 1 , the toric-drive transmission  10  is shown with the three drive rollers  30 , each having a drive roller support  34 . For simplicity purposes, each drive roller  30 /drive roller support  34  assembly will be referred to hereinafter as assembly  35 . Assemblies  35  are each-supported between structure elbows  50 . The toric-drive transmission  10  has three structure elbows  50  in order to support all three assemblies  35 . As best seen in  FIG. 4 , each structure elbow  50  has an arcuate flange  52 , by which it is secured to the casing (not shown) of the transmission  10 . The structure elbows  50  are structural, and are thus immovable in the transmission  10 . The structure elbows  50  each have an arcuate body  54  from which the arcuate flange  52  projects outwardly. On opposed ends, the arcuate body  54  has spherical contact surfaces  56 . The spherical contact surfaces  56  of the structure elbows  50  are adapted for receiving in a coplanar and sliding relationship the spherical contact surfaces  40  of the driver roller supports  34 . Therefore, the assemblies  35  may move with respect to the structure elbows  50 , which, as mentioned above, are idle in the transmission  10 . The spherical contact surfaces  56  each have a channel  58  therein in order to receive the guide pins  42  of the spherical contact surfaces  40 . The drive rollers  30  of the assemblies  35  may thus pivot about the Z-axis direction ( FIGS. 1 and 3 ). Furthermore, translation of the assemblies  35  in the channels  58  (via guide pins  42  of the drive roller supports  34 ) causes rotation of the drive rollers  35  about their respective Y-axes ( FIGS. 1 and 3 ). 
         [0029]    As the drive rollers  30  must remain with their geometric center on the toric circle, the spherical contact surfaces  40  and  56  are shaped so as to have their centers coincident with the geometric center of their respective drive roller  30 . Furthermore, it is contemplated to provide only one of the spherical contact surfaces  56  of each one of the structure elbows  50  with a channel  58 , with the roller supports  34  each provided with a single guide pin  42 . This configuration would lessen the risk of the roller supports  34  getting jammed between the structure elbows  50  in a Y-axis rotation, as will be described hereinafter. 
         [0030]    Y-axis and Z-axis rotations of the drive rollers  30  will cause changes to the input-to-output ratio of the transmission  10 . The control of the Y-axis and Z-axis rotations is achieved by three sleeves: an internal sleeve  60 , a drive-mode external sleeve  66 , and a reverse-mode external sleeve  72 . 
         [0031]    The internal sleeve  60  is tube-shaped with three equidistantly spaced holes  62  therein. The holes  62  are adapted to immovably hold the sockets  48 . As mentioned above, the sockets  48  host the follower spheres  46  of the drive roller supports  34 . The internal sleeve  60  further comprises three generally rectangular openings  64 . The openings  64  are equidistantly spaced on the periphery of the internal sleeve  60 . When the transmission  10  is assembled, the arcuate flanges  52  of the structure elbows  50  extend through the openings  64  of the internal sleeve  60 . The internal sleeve  60  has two degrees of freedom with respect to the casing of the transmission  10 . First, the internal sleeve  60  may translate with respect to the drive axis D. The openings  64  are provided in a rectangular shape for this reason, i.e., so as to allow the internal sleeve  60  to translate with the structure elbows remaining idle in supporting the assemblies  35 . Second, the internal sleeve  60  can rotate about the drive axis D. Therefore, the openings  64  are longer than the arcuate flanges  52 , and the rotational displacement of the internal sleeve  60  is limited by the abutting of the sides of the opening  64  with the arcuate flange  52  (e.g., 4° of play in rotation). The rotation of the internal sleeve  60  with respect to the drive axis D will rotate the drive rollers  30  about their Y-axes, i.e., as the internal sleeve  60  rotates, the sockets  48 , which are secured in the holes  62 , will guide the drive rollers  30  in rotating about their respective Y-axes, as the follower spheres  46  follow the movement of the sockets  48 . This rotation is possible, as mentioned above, by the channels  58  in the structure elbows  50  guiding the guide pins  42 . As the channels  58  are oriented in the same direction as the rotation of the internal sleeve  60 , the assemblies  35  may be driven into rotating about the Y-axis by a rotation of the internal sleeve  60 , thereby making the drive rollers  30  rotate in the Y-axis. 
         [0032]    On the other hand, a translation of the internal sleeve  60  along the drive axis D will have the drive rollers  30  rotate in their respective Z-axes. The internal sleeve  60  will rotate the drive rollers  30  through the action of the sockets  48  on the follower spheres  46 . More specifically, the drive roller supports  34  will each pivot with respect to the structure elbows  50 , as the guide pins  42  will pivot in the channels  58 . It is pointed out that the coplanar engagement of the spherical contact surfaces  56  of the structure elbows  50  and the spherical contact surfaces  40  of the drive roller supports  34  enables this rotation of the assemblies  35  with respect to the structure elbows  50 . 
         [0033]    The drive-mode external sleeve  66  is tube-shaped and has an internal diameter slightly larger than the external diameter of the internal sleeve  60 , so as to be in sliding contact therewith. The drive-mode external sleeve  66  has three obround openings  68  which are equidistantly spaced and are each of the same dimensions. The drive-mode external sleeve  66  further comprises helical channels  70  between adjacent obround openings  68 . The obround openings  68  are adapted for receiving therethrough the arcuate flanges  52  of the structure elbows  50 . As the obround openings  68  are only slightly larger than the arcuate flanges  52 , the drive-mode external sleeve  66  is limited to movement along one rotational degree of freedom about the drive axis D. The helical openings  70  are adapted for receiving therein a portion of the sockets  48 . With the orientation of the helical openings  70 , and the fact that the drive-mode external sleeve  66  is kept from moving in translation on the drive axis D, a rotation of the drive-mode external sleeve  66  will cause the internal sleeve  60  to translate with respect to the drive axis D as the sockets  48  will move upward or downward in the helical openings  70 . As mentioned earlier, a translation of the internal sleeve  60  on the drive axis D will have the drive rollers  30  rotating in the Z-axis. 
         [0034]    The reverse-mode external sleeve  72  has an internal diameter slightly greater than the external diameter of the drive-mode external sleeve  66 , so as to be in sliding contact therewith. The reverse-mode external sleeve  72  also has obround openings  74  which are equidistantly spaced on the periphery of the reverse-mode external sleeve  72 . Helical openings  76  are positioned between adjacent ends of obround openings  74 . However, when compared with the drive-mode external sleeve  66 , the helical openings are oriented in an opposite direction. The helical openings  76  are adapted for receiving therein a portion of the sockets  48 , whereas the obround openings  74  receive the arcuate flanges  52  therethrough such that the reverse-mode external sleeve  72  is restricted in rotating about the drive axis D, i.e., has one rotational degree of freedom. Once more, a rotation of the reverse-mode external sleeve  72  will be transformed into a translation motion of the internal sleeve  60  (through the sockets  48  engaging displacements in the helical openings  76 ), and thus into a rotation of the drive rollers  30  in the Z-axis. 
         [0035]    Still referring to  FIG. 1 , the toric-drive transmission  10  is provided with various gears and shafts to receive the driving input from the engine and output the transmitted motion to the wheels. It is pointed out that the toric-drive transmission given as an example in  FIG. 1  has both the input and output on the same side. The toric-drive transmission  10  may also be provided with the input on one side and the output on opposite sides of the transmission  10 . The same-side input/output mechanisms, which will be described briefly hereinafter, are only provided for describing an embodiment of the present invention, and are by no means limitative. The input from the engine (not shown) is received by gear  100 . The gear  100  has teeth  102  at a front end thereof for meshing with a gear at the output of the engine, and has claws  104  projecting outwardly from a flanged portion thereof. The claws  104  mate with the connection slots  18  ( FIG. 2 ) in the drive disk  12  so as to rotate therewith. A bearing  106  is sandwiched between the gear  100  and the drive disk  12  and will support both the gear  100  and the drive disk  12  on a driveshaft  108 . Spacers  110  are received in the gear  100  and allow the latter to rotate freely about the driveshaft  108 . 
         [0036]    The driveshaft  108  is elongated and has at a first end thereof threads  112  and splines  114 . The other end of the driveshaft  108  is equipped with a gear portion  116 . When the toric-drive transmission  10  is assembled, with the rollers  30  in contact with both the drive disk  12  and the driven disk  22 , the driveshaft  108  extends beyond the driven disk  22  such that the spline  114  and the threads  112  emerge outwardly therefrom. A transmission ring  118 , having a through bore comprising slots corresponding to splines  114 , is secured to the splines  114  of the driveshaft  108 . The transmission ring  118  has claws  120 , which mate with the connection slots  28  of the driven disk  22 . Therefore, the transmission ring  118  rotates with the driven disk  22 . Furthermore, as the transmission ring  118  is secured to the spline  114  of the driveshaft  108 , the driveshaft  108  rotates with the driven disk  22 . A nut  122  which is tapped is received on the threads  112  of the driveshaft  108  and ensures that the transmission ring  118  stays on the driveshaft  108  by pushing a washer  121  thereagainst. Furthermore, keys  124  ensure that the transmission ring  118  and the washer  121  rotate together, and thus that the nut  122  does not become loosened. 
         [0037]    A Belleville spring  126  is sandwiched between the driven disk  22  and the transmission ring  118 . The Belleville spring  126  consists of a ring of resilient material, whereby it may be squeezed so as to allow the driven disk  22  to slightly translate on the drive axis D when engaged with the transmission ring  118 . Although the spacing between the drive disk  12  and the driven disk  22  is in theory constant, slight variations of the spacing therebetween are anticipated due to thermal expansion or contraction for instance. Therefore, the Belleville spring  126  provides the contact force in order for the driven disk  22  to be in contact with the drive rollers  30  at all times. The toric-drive transmission  10  does not require an overly large casing, as the contact force between the drive rollers  30  and the disks  12  and  22  is in the axial direction and is thus sustained by the driveshaft one way. 
         [0038]    Now that the toric-drive transmission  10  has been described in detail, the steps for changing ratios will be described. For clarity purposes, all components rotating about the drive axis D will be referred to as turning in direction A, or in direction B, which is opposite direction A.  FIGS. 1 and 5A  to  5 C have been added with vectors A and B for illustrating the rotation direction. According to the driving mode of the vehicle, the drive disk  12  will be rotating in either direction A or B. If the vehicle is moving forward, the drive disk  12  will, for instance, be rotating in direction A upon receiving the engine output. When the drive disk  12  is rotating in direction A, the driven disk  22  will be rotating in direction B, as a result of the transmitted rotation by the rollers  30 . As mentioned above, in transmitting the rotation from drive disk  12  to driven disk  22 , the rollers  30  rotate about their respective X-axes. 
         [0039]    Initially, the drive rollers  30  are in a speed reduction position within the transmission  10 , as illustrated in  FIG. 5A . In this position, the drive disk  12  rotates faster than the driven disk  22 . In speed reduction, more torque is provided to the wheels, and this position is preferably used when the vehicle is idle or needs high torque. As the drive rollers  30  rotate about their Z-axes toward a direct drive (as shown in  FIG. 5B ), the input-to-output ratio gradually decreases and, in doing so, the driven disk  22  increases speed with respect to the drive disk  12 . 
         [0040]    When the vehicle is in the drive mode, the drive-mode external sleeve  66  will be active in allowing to change speed ratios between the drive disk  12  and the driven disk  22  while the reverse-mode external sleeve  72  is inactive. To increase the rotating speed of the driven disk  22 , and thus reduce the input-to-output ratio, the drive-mode external sleeve  66  is rotated in the A direction, thereby entraining the sockets  48  (and ultimately the drive rollers  30 ) with the respective helical openings  70  pressing against the sockets  48 . The internal sleeve  66  is entrained in this rotation by the action of the sockets  48  against the holes  62 . As a result, and as mentioned above, the drive rollers  30  will rotate about their respective Y-axes. More specifically, the drive-mode external sleeve  66  will guide the follower spheres  46  into rotating the assemblies  35 , and the engagement of the guide pins  42  in the channels  58  allows this pivoting. In other words, the assemblies  35  will slide against the spherical contact surfaces  56  while being guided by their respective pins  42  following the paths defined by the channels  58 , and thus having the rollers  30  rotate with respect to their Y-axes. This will result in the rollers  30 , which were defining a circular path on the drive disk  12  and the driven disk  22  (i.e., R 1  and R 2  remaining constant), changing to a spiral path on the drive disk  12  and the driven disk  22  (i.e., with, in this case, R 1  continuously decreasing while R 2  continuously increases). In taking a spiral path, the rollers  30  will further pivot with respect to their Y-axes. 
         [0041]    When the desired actuation input on the external sleeve  66  is reached (e.g., a desired amount of tilt about the Y-axis has been given), the drive-mode external sleeve  66  is fixed with respect to the toric-drive transmission  10 . Once in the spiral path, the rollers  30  will have a tendency to move back to their initial Y-axis orientation, i.e., as they were prior to being displaced by the internal sleeve  60 /external sleeve  66  rotation. They will thus exert pressure on the internal sleeve  60  and the drive-mode external sleeve  66  in order to return to that Y-axis orientation. As the sockets  48  are immovably secured to the internal sleeve  60 , this force exerted by the drive rollers  30 , to move out of their spiral pattern, will be exerted on the drive-mode external sleeve  66 , which, as mentioned above, is now fixed and immovable with respect to the toric-drive transmission. Since the sockets  48  are in the helical openings  70 , a return of the drive rollers  30  to their initial Y-axis orientations will displace the sockets  48  in the direction of arrow  1  in the helical opening  70 . As the drive-mode external sleeve  66  is immovable, this will result in the internal sleeve  60  moving toward the driven disk  22  in the D-axis direction, i.e., to the left in  FIG. 1 . This will cause the drive rollers  30  to rotate in their respective Z-axes as a result thereof and, to return to a circular path and a constant input/output ratio, but having caused a decrease in the input-to-output ratio toward a direct-drive position, as illustrated in  FIG. 5B , or an overdrive position, as shown in  FIG. 5C , in having pivoted along the Y-axis. 
         [0042]    To increase the input-to-output -ratio when the drive disk  12  is in the drive mode, i.e., rotates in A, the rotation of the drive-mode external sleeve together with the internal sleeve  60  would be opposite, and thus in the B direction. This will cause the sockets  48  to move in the direction opposite arrow  1  in the helical openings  70 , as shown in  FIG. 1 . 
         [0043]    Throughout the changes of ratios in the drive mode of the toric-drive transmission  10 , the reverse-mode external sleeve  72  is unrestricted from rotating about the D-axis, to comply with the motion of the sockets  48  in the helical openings  70  of the drive-mode external sleeve  66 . If the toric-drive transmission  10  were in reverse mode, the drive-mode external sleeve  66  would be unrestricted from rotating about the D-axis, while the reverse-mode external sleeve  72  would be controlled as described above for the drive-mode external sleeve  66 . In the case of the reverse mode, the drive disk  12  will be rotating in direction B, and the driven disk  22  will thus be rotating in direction A. Accordingly, with the same logic as for the drive mode explained above, the initiating of a decrease in the input-to-output ratio will be achieved by a rotation of the reverse-mode external sleeve  72  with the internal sleeve  60  in the B direction, whereas an A-direction rotation would cause an increase in the input-to-output ratio. A rotation of the external sleeve  72 /internal sleeve  60  causes a Y-axis rotation of the rollers  30 , resulting in a spiral path of the rollers  30  with respect to the disks  12  and  22 . When the desired actuation input of the internal sleeve  60  is reached and the external sleeve  72  is locked, the rollers  32  are subjected to an opposite Y-axis rotation, hence causing a Z-axis rotation and circular motion of the rollers  30 , to a desired input/output ratio. 
         [0044]    An advantage of the present invention resides in the fact that no great forces need to be applied to the drive rollers in order to initiate input-to-output ratio changes. As mentioned above, the initiation of the ratio change is achieved by rotating the internal sleeve  60 , which entrains the drive-mode external sleeve  66  or the reverse-mode external sleeve  72 . Therefore, there is no need for a hydraulic control in order to initiate the ratio changing. The toric-drive transmission  10  of the present invention is thus advantageous when used with vehicles having low-power engines, as the power of the engine is not used for running a hydraulic system, and thus has its energy efficiency increased. As the control of the ratio is initiated in a direction perpendicular to the traction force, the control may be a low-power control, such as an electronic or electromechanical control, actuating displacements of the respective sleeves  60 ,  66  and  72 . 
         [0045]    It is contemplated to provide alternative controller systems to the sleeves  60 ,  66  and  72 . With the level of quality of linear actuators, a fully electronic controller system could be provided to actuate the displacement of the drive rollers  30  as described above. The above described controller system with the sleeves  60 ,  66  and  72  is advantageous in that it ensures a synchronization of the displacement of the drive rollers  30 , while remaining relatively simple. Moreover, in case of failure of the electrical system of the vehicle, the input-to-output ratio of the toric-drive transmission  10  remains constant with the use of the above described controller system. 
         [0046]    As mentioned above, the power transmittal between the drive rollers  30  and the drive disk  12 /driven disk  22  will be made through a film of oil which is on the surface of the grooves  14  and  24 . Due to the forces evolving in such a power transmission, the oil ensuring the contact between the drive rollers  30  and the disks  12  and  22  will be in a semi-solid state under high pressure, the phase being referred to as elastohydrodynamic phase. A traction oil is thus needed, as the instantaneous viscosity and the shear modulus will be increased many times their normal condition. The oil film allows to increase the longevity of the toric-drive transmission, as metal-to-metal contact would damage the pieces at an incredibly fast rate. Santotrac™ oil from Monsanto is an example of a traction oil adapted for being used with the toric-drive transmission  10  of the present invention. 
         [0047]    The toric-drive transmission  10  is provided with an adequate lubrication system, which will ensure that the grooves  14  and  24  of the drive disk  12  and the driven disk  22 , respectively, have the required oil film thereon. Deflectors may be installed in the toric-drive transmission  10  to direct oil toward the contact surfaces between the drive rollers  30  and the disks  12  and  22 . It is pointed out that the spin is equal but in opposite directions on opposed sides of the contact surface between the drive rollers  30  and the disks  12  and  22 . The effect of the spin is thus cancelled. 
         [0048]    As mentioned above, the peripheral surface  32  of the drive roller  30  is slightly convex, in order to minimize the contact surface between the peripheral surface and the oil film on the disk  12  or  22 . The contact surface is typically oval shaped and has radii of 1 and 2 mm. This allows reduction to the spin resulting from the power transmittal between the disks  12  and  22  and the rollers  30 . Spin is a phenomenon which occurs due to the fact that the rotating speed is generally the same at all points of the peripheral surface  32  of the drive roller  30 , whereas the rotational speed changes on the disks  12  and  22  according to the radial position. The rotational speed transmitted to the peripheral surface  32  by the drive disk  12  increases on the contact surface therebetween, as the outermost point of the contact surface with respect to the center of the drive disk  12  has a greater rotational speed than the closest point of the contact surface with respect to the center of the drive disk  12 . Accordingly, spin occurs and energy losses therewith. Therefore, by minimizing the contact surface with the peripheral surface  32  of the drive rollers  30 , both the spin and the energy losses are minimized. 
         [0049]    Referring now to  FIG. 6 , a graph is shown illustrating the speed vs. the RPM in a comparison of a CVT, such like the toric-drive transmission  10  of the present invention, and a discontinuous-ratio transmission. Curve  100  shows the various speed ratios of a discontinuous-ratio transmission, whereas curve  101  shows the constantly changing ratios of the toric-drive transmission  10 . The areas under the curves show the acceleration potential of the two types of transmissions. As the area under the curve is greater for the toric-drive transmission  10 , the toric-drive transmission  10  may thus uniformly accelerate while keeping the motor at its highest power. Furthermore, although the graph represents vertical lines between the change of gears of the discontinuous-ratio transmission, there is a slight loss of vehicle velocity when a ratio change occurs with the discontinuous-drive transmission. This loss of velocity may be troublesome in harsh conditions, for instance, when the vehicle is sunk into snow or mud.