Patent Publication Number: US-2022221031-A1

Title: Frictional roller reducer

Description:
TECHNICAL FIELD 
     The present invention relates to a frictional roller reducer that is incorporated, for example, in a drive system of an electric vehicle for transmitting torque to drive wheels after reducing the rotation (increasing the torque) of an electric motor. 
     BACKGROUND ART 
     In an electric vehicle, in order to improve the efficiency of the electric motor that is the drive source and increase the distance that can be traveled per charge, the rotation of the output shaft of a small electric motor is transmitted to the drive wheels after being decelerated by a reducer. As such a reducer, a frictional roller type reducer can be used. 
       FIG. 6  illustrates a frictional roller reducer described in JP 2012-207778A. The frictional roller reducer  100  includes a housing  101 , an input shaft  102 , an output shaft  103 , a sun roller  104 , an annular roller  105 , a plurality of planetary rollers  106 , and a pair of pressing devices  107 . 
     The input shaft  102  and the output shaft  103  are supported inside the housing  101  coaxially with each other and capable of relative rotation. 
     The sun roller  104  is a combination of a pair of sun roller elements  108  having shapes symmetrical with respect to the axial direction. The pair of sun roller elements  108  are supported around the input shaft  102  being coaxial with the input shaft  102 , and are able to rotate relative to the input shaft  102  in a state in which a gap is interposed between the tip-end surfaces that face each other. The pair of sun roller elements  108  have conical surface-shaped inner-diameter side rolling contact surfaces  109  on the outer circumferential surface, the outer diameter dimension of which increases as the distance from each other in the axial direction increases, and have driven side cam surfaces  110  on the base-end surfaces that each face the opposite side in the axial direction (facing the opposite side in the axial direction from the tip-end surface). The driven-side cam surfaces  110  are formed by arranging driven-side cam concave portions  111 , the depth in the axial direction of which changes in the circumferential direction, at a plurality of locations equidistantly spaced in the circumferential direction. 
     The annular roller  105  is arranged around the sun roller  104  so as to be coaxial with the sun roller  104 , and is connected by a connecting portion  112  having an L-shaped cross section to the output shaft  103  so as to be able to transmit torque. The annular roller  105  has a cylindrical shaped outer-diameter side rolling contact surface  113  on the inner circumferential surface. 
     Each of the planetary rollers  106  has a support shaft  114  that is arranged in parallel with the input shaft  102 , and is supported by the housing  101  so as to be able to rotate (spin) around the support shaft  114  and to displace in the radial direction of the input shaft  102 , and so as not to be able to rotate (revolve) around the input shaft  102 . Each of the planetary rollers  106  has a rolling surface  115  having an arc-shaped generating line on the outer circumferential surface. The planetary rollers  106  are arranged at a plurality of locations in the circumferential direction of the annular space between the sun roller  104  and the annular roller  105 , and causes the rolling surfaces  115  to come into rolling contact with the inner-diameter side rolling contact surfaces  109  of the pair of sun roller elements  108  and with the outer-diameter side rolling contact surface  113  of the annular roller  105 . 
     The pair of pressing devices  107  include a loading cam type pressing device that presses the pair of sun roller elements  108  in directions approaching each other, and each of the pair of pressing devices  107  includes a cam disk  116  and a plurality of balls  117 . 
     Each cam disk  116  is externally fitted and fixed to the input shaft  102  so as to be able to integrally rotate with the input shaft  102 , and has a drive-side cam surface  118  on the side surface in the axial direction facing the driven-side cam surface  110  of the sun roller element  108 . The drive-side cam surface  118  is formed by arranging drive-side cam concave portions  119 , the depth in the axial direction of which changes in the circumferential direction, at a plurality of locations equidistantly spaced in the circumferential direction. 
     One ball  117  is held between each driven-side cam concave portion  111  of the driven-side cam surface  110  of the sun roller element  108  and each drive-side cam concave portion  119  of the drive-side cam surface  118  of the cam disk  116 . 
     In the frictional roller reducer  100 , when torque is inputted to the input shaft  102 , each of the balls  117  of the pair of pressing devices  107  rides up on the shallow portion of the depth in the axial direction of the driven-side cam concave portion  111  and the drive-side cam concave portion  119 . As a result, when the dimension in the axial direction of the pair of pressing devices  107  increases and the pair of sun roller elements  108  are pressed in directions approaching each other, the outer-diameter dimension of the portions of the inner-diameter side rolling contact surfaces  109  that come in rolling contact with the rolling surfaces  115  of the planetary rollers  106  becomes larger. As a result, the surface pressure at the traction portions (rolling contact portions) between the inner-diameter side rolling contact surfaces  109  of the sun roller elements  108  and the rolling surfaces  115  of the planetary rollers  106  increases. Furthermore, when the planetary rollers  106  are pushed outward in the radial direction of the input shaft  102  as the surface pressure increases, the surface pressure at the traction portions between the rolling surfaces  115  of the planetary rollers  106  and the outer-diameter side rolling contact surface  113  of the annular roller  105  also increases. As a result, torque that is inputted to the pair of sun roller elements  108  from the input shaft  102  via the pair of pressing devices  107  can be transmitted to the annular roller  105  via the planetary rollers  106  and obtained from the output shaft  103  without causing excessive slipping at each of the traction portions. 
     In the frictional roller reducer  100  as described above, when the torque that is applied to the input shaft  102  increases, the amount that the balls  117  of the pair of pressing devices  107  ride up from the bottom portions of the driven-side cam concave portions  111  and drive-side cam concave portions  119  increases, and the dimension in the axial direction of the pair of pressing devices  107  increases even more. As a result, the surface pressure at the traction portions between the rolling surfaces  115  and the inner-diameter side rolling contact surfaces  109  and the outer-diameter side rolling contact surfaces  113  further increases, and large torque can be transmitted without the occurrence of excessive slipping at the traction portions. Therefore, by appropriately regulating the inclination angle (gradient angle), the dimension in the circumferential direction, and the like of the driven-side cam concave portions  111  and the drive-side cam concave portions  119 , the surface pressure at the traction portions is automatically adjusted to an appropriate value according to the torque to be transmitted between the input shaft  102  and the output shaft  103 , and specifically, to a value obtained by multiplying the minimum required value by an appropriate safety factor. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: JP 2012-207778A 
     Patent Literature 2: JP 2016-223468A 
     Patent Literature 3: JP 2008-196657A 
     SUMMARY OF THE INVENTION 
     Technical Problem 
     However, in the case of the frictional roller reducer  100  described in JP 2012-207778A, loading cam type pressing devices are used as the pressing devices  107  that press the pair of sun roller elements  108  in directions toward each other, and thus there is a possibility that problems such as those described below may occur. In other words, at the traction portions between the rolling surfaces  115  and the inner-diameter side rolling contact surfaces  109  and the outer-diameter side rolling contact surfaces  113 , the limit value (limit traction coefficient μ max ) of the traction coefficient (=tangential force/normal force) at which torque can be transmitted without causing the occurrence of harmful slipping called gross slipping is also affected by parameters other than the torque to be transmitted between the input shaft  102  and the output shaft  103 . 
     For example, the traction coefficient changes according to the temperature (oil temperature) of the traction oil that is supplied to the traction portions. More specifically, in a normal temperature environment (for example, in an environment of 0° C. or higher), the higher the oil temperature, the lower the viscosity of the traction oil, and thus the traction coefficient also decreases. On the other hand, as described in JP 2016-223468A, it is known that in an extremely low temperature environment (for example, in an environment of less than 0° C.), as the oil temperature decreases, the viscosity of the traction oil increases, but the traction coefficient decreases. 
     Moreover, JP 2008-196657A describes that the traction coefficient is influenced by and changes due to a slip rate S (=(U 1 −U 2 )/U 1 ), which represents a delay of the peripheral speed U 2  of the driven side rotating body with respect to the peripheral speed U 1  of the driving side rotating body. 
     Regardless of the influence of parameters other than the torque that is to be transmitted between the input shaft  102  and the output shaft  103 , such as the oil temperature of the traction oil and the peripheral speed at the traction portions, setting a high safety factor for the traction coefficient in order to transmit torque from the sun roller  104  to the annular roller  105  without causing gross slipping to occur at each of the traction portions is effective. However, in a case where the safety factor for the traction coefficient is excessively increased and the surface pressure at each traction portion becomes excessive, the rolling resistance, for example, will increase unnecessarily, the transmission loss will increase, and the transmission efficiency of the frictional roller reducer  100  will decrease. 
     In view of the circumstances described above, it is an object of the present invention to provide a frictional roller reducer having a structure capable of ensuring excellent transmission efficiency while preventing the occurrence of gross slip at the traction portions. 
     Solution to Problem 
     The frictional roller reducer of the present invention includes an input shaft, an inner-diameter side rolling contact surface, an output shaft, a pair of annular roller elements, a plurality of planetary rollers, a carrier, a pressing device, and a controller. 
     The input shaft is supported by a housing, for example, so as to be able to rotate freely. 
     The inner-diameter side rolling contact surface is configured by an outer circumferential surface of the input shaft, or by an outer circumferential surface of a portion that integrally rotates with the input shaft, for example, the outer circumferential surface of a sun roller or the like that is supported by and fixed to the input shaft. 
     The output shaft is coaxial with the input shaft and supported so as to be able to rotate relative to the input shaft. 
     The pair of annular roller elements are prevented from rotating around the input shaft and are arranged with a gap between tip-end surfaces facing each other. 
     The inner circumferential surface of each of the pair of annular roller elements is configured by an outer-diameter side rolling contact surface that faces the inner-diameter side rolling contact surface, and is inclined in a direction in which an inner diameter dimension increases as going toward the tip-end surface side. 
     At least one annular roller element of the pair of annular roller elements is supported so as to be able to displace in an axial direction, and has a driven side cam surface on a base end surface facing toward an opposite side in the axial direction from the tip-end surface. 
     Each of the plurality of planetary rollers has a rotation shaft arranged parallel to the input shaft, and a rolling surface that comes in rolling contact with the inner-diameter side rolling contact surface and the outer-diameter side rolling contact surfaces. 
     The carrier supports the plurality of planetary rollers so at to be able to rotate freely around the rotation shaft and to be able to displace in a radial direction, and is integrally configured with the output shaft or configured by a member that integrally rotates with the output shaft. 
     The pressing device includes a cam disk, a plurality of rolling bodies, and a pressing force adjusting motor. The cam disk is supported around the input shaft so as to be able to rotate relative to the input shaft and so as not to be able to displace in the axial direction, and has a drive side cam surface on a side surface in the axial direction that faces the driven side cam surface. The plurality of rolling bodies is held between the driven side cam surface and the drive side cam surface. 
     The pressing force adjusting motor rotationally drives the cam disk. Based on rotational driving of the cam disk by the pressing force adjusting motor, the pressing device is able to press the pair of annular roller elements in directions approaching each other. 
     The controller, by adjusting the rotational drive of the pressing force adjusting motor, is able to adjust surface pressure at traction portions (rolling contact portions) between the rolling surfaces and the inner-diameter side rolling contact surface and the outer-diameter side rolling contact surface to a target value. 
     The frictional roller reducer of the present invention can include a temperature sensor that measures the temperature of traction oil supplied to the traction portions. In this case, the controller can use the temperature of the traction oil measured by the temperature sensor to calculate the target value. 
     The frictional roller reducer of the present invention can include a rotational speed sensor that measures rotational speed of the output shaft. In this case, the controller can use the rotational speed of the output shaft measured by the rotational speed sensor to calculate the target value. 
     The pressing device can include a reducer having a drive-side gear that is rotationally driven by the pressing force adjusting motor, and a driven-side gear that meshes with the drive-side gear and integrally rotates with the cam disk. In this case, the drive-side gear is a worm, and the driven-side gear is a worm wheel. In other words, the reducer can be configured by a worm reducer. In a case where the reducer is a worm reducer, preferably the reducer has a self-locking function that does not transmit the rotation of the worm wheel to the worm. 
     The controller, by adjusting the rotational drive of the pressing force adjusting motor, is able to make the surface pressure at the rolling contact portions between the rolling surfaces and the inner-diameter side rolling contact surface substantially zero. 
     In this case, the frictional roller reducer of the present invention can include a planetary roller pressing means that elastically presses the planetary rollers outward in the radial direction. The frictional roller reducer of the present invention can additionally or alternatively include a roller element pressing means that elastically presses the pair of annular roller elements in directions away from each other in the axial direction. 
     Effect of Invention 
     The frictional roller reducer of the present invention, by adjusting the amount and direction of rotation of the pressing force adjusting motor, is able to adjust the surface pressure at the traction portions to an arbitrary value, and thus it is possible to ensure good transmission efficiency while preventing the occurrence of gross slipping at the traction portions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view illustrating a frictional roller reducer of a first example of an embodiment of the present invention in a state during normal running. 
         FIG. 2  is a schematic view illustrating the frictional roller reducer of the first example in a state during coasting. 
         FIG. 3A  is a schematic view illustrating a state in which a rolling body is located in an intermediate portion in the circumferential direction of a drive-side cam concave portion and a driven-side cam concave portion;  FIG. 3B  is a schematic view illustrating a state in which a rolling body has moved from the position illustrated in  FIG. 3A  to the side of the drive-side cam concave portion and driven-side cam concave portion having a higher height in the axial direction; and  FIG. 3C  is a schematic view illustrating a state in which a rolling body is located at the bottom portion of the drive-side cam concave portion and the bottom portion of the driven-side cam concave portion. 
         FIG. 4  is a diagram similar to  FIG. 2  illustrating a frictional roller reducer of a second example of an embodiment of the present invention. 
         FIG. 5  is a diagram similar to  FIG. 2  illustrating a frictional roller reducer of a third example of an embodiment of the present invention. 
         FIG. 6  is a cross-sectional view illustrating an example of a conventional structure of a frictional roller reducer. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     FIRST EXAMPLE 
       FIG. 1  to  FIG. 3C  illustrate a first example of an embodiment of the present invention. A frictional roller reducer  1  of this example includes a housing  2 , an input shaft  3 , an output shaft  4 , an inner-diameter side rolling contact surface  5 , a pair of annular roller elements  6   a ,  6   b , a plurality of planetary rollers  7 , a carrier  8 , a pressing device  9 , and a controller  31 . 
     The input shaft  3  is supported by a motor output shaft of an electric motor, which is a drive source of an electric vehicle, so as to be able to rotate coaxially and integrally with the motor output shaft. The input shaft  3  is rotatably supported inside the housing  2  by a bearing device  10 . 
     The output shaft  4  is supported coaxially with the input shaft  3  so as to be able to rotate relative to the input shaft  3 . In this example, the output shaft  4  is a circular tubular hollow shaft, is arranged around the input shaft  3  so as to be coaxial with the input shaft  3 , and is rotatably supported inside the housing  2  by a pair of bearings  11   a ,  11   b . Each of the pair of bearings  11   a ,  11   b  includes, for example, an angular contact ball bearing or a tapered roller bearing in which contact angles in different directions to each other are given to the rolling bodies. 
     The inner-diameter side rolling contact surface  5  is directly formed by the outer circumferential surface of the tip-end portion of the input shaft  3 . Note that the inner-diameter side rolling contact surface  5  may be formed by the outer circumferential surface of a sun roller that is supported by and fixed to the input shaft  3 . In any case, the inner-diameter side rolling contact surface  5  rotates integrally with the input shaft  3  when the input shaft  3  is rotated by being rotationally driven by the electric motor. Moreover, in this example, the inner-diameter side rolling contact surface  5  is formed by a concave curved surface having a single arc-shaped generating line. However, the inner-diameter side rolling contact surface  5  may be formed by a simple cylindrical surface. 
     The pair of annular roller elements  6   a ,  6   b  are supported inside the housing  2  in a state around the input shaft  3  and coaxial with the input shaft  3  and prevented from rotating around the input shaft  3 , with a gap being interposed between the tip-end surfaces facing each other. The pair of annular roller elements  6   a ,  6   b  have conical surface shaped outer-diameter side rolling contact surfaces  12   a ,  12   b , respectively on the inner circumferential surface of the tip-end portions that face the inner-diameter side rolling contact surface  5  and that are inclined in a direction such that the inner diameter dimension becomes larger as going toward the tip-end sides (in directions toward each other). 
     Of the pair of annular roller elements  6   a ,  6   b , one of the annular roller elements  6   a  (right side in  FIGS. 1 and 2 ) is supported by the housing  2  so that relative rotation is not possible and so as to be able to displace in the axial direction. The annular roller element  6   a  has a driven side cam surface  13  on a base-end surface (right side surface in  FIGS. 1 and 2 ) facing toward the opposite side in the axial direction from the tip-end surface. The driven side cam surface  13  is configured by arranging driven-side cam concave portions  15  that have a depth in the axial direction that is deepest at the bottom portion  14  and that becomes shallower going toward one side in the circumferential direction (upper side in  FIGS. 3A to 3C ) at a plurality of locations in the circumferential direction. 
     Of the pair of annular roller elements  6   a ,  6   b , the other annular roller elements  6   b  (left side in  FIGS. 1 and 2 ) is supported by the housing  2  so that relative rotation is not possible and so as not to be able to displace in the axial direction. The annular roller element  6   b  can also be configured by a part of the housing  2 . 
     Each of the plurality of planetary rollers  7  has a rotation axis C arranged in parallel with the input shaft  3  so as to be able to displace in the radial direction of the input shaft  3 , and has a rolling surface  16  on the outer circumferential surface that comes in rolling contact with the inner-diameter side rolling contact surface  5  and the outer-diameter side rolling contact surfaces  12   a ,  12   b . In this example, the rolling surface  16  is configured by a convex curved surface having a single arc-shaped generating line with a radius of curvature smaller than the radius of curvature of the generating line of the inner-diameter side rolling contact surface  5 . Therefore, the rolling surface  16  makes rolling contact with the inner-diameter side rolling contact surface  5  at an intermediate portion in the axial direction, and also makes rolling contact with the outer-diameter side rolling contact surfaces  12   a ,  12   b  at both end portions in the axial direction. 
     In this example, each of the planetary rollers  7  is rotatably supported by a bearing device  18  around a column-shaped support shaft  17  that is arranged coaxially with the rotation axis C. The support shaft  17  is freely supported at both ends in the axial direction by the carrier  8  so as to displace freely in the radial direction of the input shaft  3 . 
     During operation of the frictional roller reducer  1 , traction oil is continuously supplied from a nozzle (not illustrated), to traction portions (rolling contact portions) between the rolling surfaces  16  of the planetary rollers  7  and the inner-diameter side rolling contact surface  5  of the input shaft  3  and the outer-diameter side rolling contact surfaces  12   a ,  12   b  of the pair of annular roller elements  6   a ,  6   b.    
     The carrier  8  supports each of the planetary rollers  7  so as to freely rotate about the rotation axis C and so as to be able to displace in the radial direction of the input shaft  3 . In this example, the carrier  8  is integrally configured with the output shaft  4 . However, the carrier  8  may be configured by a member separate from the output shaft  4 , and may be coupled and fixed to the output shaft  4 . In any case, as the carrier  8  rotates, the output shaft  4  also integrally rotates. 
     The carrier  8  includes a pair of annular portions  19  that protrude toward the outside in the radial direction from the outer circumferential surface of the output shaft  4 , and are arranged at intervals in the axial direction. Each of the pair of annular portions  19  includes concave portions  20  having oval-shaped openings at a plurality of locations in the circumferential direction of the inner side surfaces that face each other. The concave portions  20  are formed on the inner side surfaces of the annular portions  19  so that the major axis is directed in the radial direction centered on the input shaft  3  (center axis O). Moreover, the concave portions  20  have a minor axis dimension that is slightly larger than the outer-diameter dimension of the support shaft  17 . In this example, by arranging (engaging) both end portions in the axial direction of the support shaft  17  of each of the planetary rollers  7  on the inner side of the concave portions  20  so that there is no looseness in the circumferential direction around the input shaft  3 , the planetary rollers  7  are supported so as to be able to displace in the radial direction of the input shaft  3 . 
     Each of the planetary rollers  7  is configured so that by being supported around the support shaft  17  via a bearing device  18 , the planetary roller  7  is able to freely rotate around the rotation axis C. However, by externally fitting and fastening the planetary roller  7  directly around the support shaft  17 , or by integrally forming the support shaft  17  and the planetary roller  7 , and arranging (engaging) both end portions in the axial direction of the support shaft  17  on the inner side of the concave portions  20  so as to be able to rotate freely via bearings as necessary, it is also possible to allow the planetary roller  7  to rotate about the rotation axis C. 
     Moreover, in this example, by arranging (engaging) both end portions of the support shaft  17  on the inner side of the concave portions  20  so as to be able to displace in the lengthwise direction of the concave portions  20 , each of the planetary rollers  7  is able to displace in the radial direction of the input shaft  3 . However, by supporting each of the planetary rollers  7  by the carrier  8  using a pivoting frame having a pair of support plate portions, the planetary roller  7  is able to displace in the radial direction of the input shaft  3 . In this case, both end portions in the axial direction of the support shaft  17  are supported on the inner side surfaces of the pair of support plate portions facing each other, and the pivoting frame is supported by the carrier  8  so as to be able to pivot around a pivot shaft that is eccentric with respect to the support shaft  17 . 
     The pressing device  9  has a function of bringing the pair of annular roller elements  6   a ,  6   b  closer to each other in the axial direction, and includes a cam disk  21 , a plurality of rolling bodies  22 , and a pressing force adjusting motor  23 . 
     The cam disk  21  is supported around the input shaft  3  coaxially with the input shaft  3  so as to be able to rotate relative to the input shaft  3  and the output shaft  4  and so as not to be able to displace in the axial direction, and has a drive-side cam surface  24  on a side surface in the axial direction (left side surface in  FIGS. 1 and 2 ) facing the driven-side cam surface  13  of the annular roller element  6 a. The drive-side cam surface  24  is configured by arranging drive-side cam concave portions  26 , which have a depth in the axial direction that is deepest at the bottom portion  25  and that becomes shallower as going toward the other side in the circumferential direction (lower side in  FIGS. 3A to 3C ), at a plurality of locations in the circumferential direction. 
     In this example, the cam disk  21  is rotatably supported on the inner side of the housing  2  by a bearing  27 , and has a driven-side gear  28  on the outer circumferential surface. In this example, the driven-side gear  28  is configured by a helical gear (worm wheel) having a tooth trace that is inclined with respect to the axial direction. 
     Each of the plurality of rolling bodies  22  is held between the driven-side cam concave portion  15  of the driven-side cam surface  13  and the drive-side cam concave portion  26  of the drive-side cam surface  24 . Each of the rolling bodies  22  is configured by a ball or a column-shaped roller. 
     In this example, the pressing device  9 , by causing the rolling bodies  22  to ride up on the shallow side from the bottom portion  25  of the drive-side cam concave portion  26  and from the bottom portion  14  of the driven-side cam concave portion  15  due to the rotational drive of the cam disk  21 , and causing the annular roller element  6   a  to displace in the axial direction, is able to press the pair of annular roller elements  6   a ,  6   b  in directions toward each other. 
     The pressing force adjusting motor  23  rotationally drives the cam disk  21 . Therefore, in this example, the drive-side gear  29  that meshes with the driven-side gear  28  of the cam disk  21  is supported by and fixed to the motor output shaft of the pressing force adjusting motor  23 . In this example, the drive-side gear  29  is configured by a worm having a screw-shaped tooth trace. In other words, in this example, the cam disk  21  is configured to be rotationally drivable by the pressing force adjusting motor  23  via a worm reducer  30  including the driven-side gear  28  and the drive-side gear  29 . In this example, the lead angle of the drive-side gear  29 , which is a worm, is reduced so that the worm reducer  30  has a self-locking function. 
     Note that the driven-side gear  28  and the drive-side gear  29  can both be spur gears or bevel gears. Alternatively, configuration may be such that a pulley is supported and fixed to the motor output shaft of the pressing force adjusting motor  23 , and by a continuous belt spanning between the pulley and the outer circumferential surface of the cam disk  21 , the cam disk  21  can be rotationally driven by the pressing force adjusting motor  23 . 
     The pressing force adjusting motor  23  is configured by an electric motor capable of positioning control such as a stepping motor, a DC motor, or the like. 
     A controller  31  has a function of adjusting surface pressure at traction portions between the rolling surfaces  16  of the planetary rollers  7  and the inner-diameter side rolling contact surface  5  of the input shaft  3  and the outer-diameter side rolling contact surfaces  12   a ,  12   b  of the pair of annular roller elements  6   a ,  6   b  to target values by adjusting the rotational drive of the pressing force adjusting motor  23 , and specifically, by adjusting the amount of rotation and the direction of rotation. 
     In other words, the controller  31  causes the annular roller element  6   a  to displace in the axial direction by adjusting the amount of rotation and the direction of rotation of the pressing force adjusting motor  23 , adjusting the amount of rotation and direction of rotation of the cam disk  21 , and adjusting the ride-up amount d 1  of the rolling bodies  22  riding up from the bottom portion  25  of the drive-side cam concave portion  26  and the ride-up amount d 2  of the rolling bodies  22  riding up from the bottom portion  14  of the driven-side cam concave portion  15 . When the outer-diameter dimension of the portion of the outer-diameter side rolling contact surfaces  12   a ,  12   b  of the pair of annular roller elements  6   a ,  6   b  that come in rolling contact with the rolling surfaces  16  of the planetary rollers  7  changes due to displacement in the axial direction of the annular roller element  6   a , the planetary rollers  7  displace in the radial direction of the input shaft  3 . As a result, the surface pressure at the traction portions between the rolling surfaces  16  of the planetary rollers  7  and the inner-diameter side rolling contact surface  5  of the input shaft  3  and outer-diameter side rolling contact surfaces  12   a ,  12   b  of the pair of annular roller elements  6   a ,  6   b  is adjusted to a desired value. 
     More specifically, as illustrated from  FIGS. 3A to 3B , the annular roller element  6   a  is caused to displace in a direction toward the annular roller element  6   b  (left direction in  FIG. 1  and  FIG. 2 ) by rotationally driving the cam disk  21  in a specified direction (upward in  FIG. 3A  and  FIG. 3B ) and increasing the ride-up amount d 1  of the rolling bodies  22  riding up from the bottom portion  25  of the drive-side cam concave portion  26  and the ride-up amount d 2  of the rolling bodies  22  riding up from the bottom portion  14  of the driven-side cam concave portion  15 . As a result, when the outer-diameter dimension of the portion of the outer-diameter side rolling contact surfaces  12   a ,  12   b  of the pair of annular roller elements  6   a ,  6   b  that makes rolling contact with the rolling surfaces  16  of the planetary rollers  7  becomes smaller, the planetary rollers  7  displace inward (downward in  FIGS. 1 and 2 ) in the radial direction of the input shaft  3 . As a result, the surface pressure at the traction portions between the rolling surfaces  16  of the planetary rollers  7  and the inner-diameter side rolling contact surface  5  of the input shaft  3  and outer-diameter side rolling contact surfaces  12   a ,  12   b  of the pair of annular roller elements  6   a ,  6   b  increases. 
     On the other hand, as illustrated from  FIGS. 3B  to  FIG. 3A , when the cam disk  21  is rotationally driven in a direction opposite to a specified direction (downward in  FIGS. 3A and 3B ), the planetary rollers  7  displace outward in the radial direction of the input shaft  3  (upward in  FIG. 1  and  FIG. 2 ), the annular roller element  6   a  displaces in a direction away from the annular roller element  6   b  (toward the right in  FIG. 1  and  FIG. 2 ), and the ride-up amount d 1  of riding up from the bottom portion  25  of the drive-side cam concave portion  26  and the ride-up amount d 2  of riding up from the bottom portion  14  of the driven-side cam concave portion  15  of the rolling body  22  decreases. Note that a centrifugal force acts on the planetary rollers  7  due to the rotation (revolution) of the planetary rollers  7  about the input shaft  3 , and based on this centrifugal force, an outward force is applied in the radial direction of the input shaft  3  to the outer-diameter side rolling contact surfaces  12   a ,  12   b  from the rolling surfaces  16 . The outer-diameter side rolling contact surfaces  12   a ,  12   b  are conical surfaces, and thus when the outward force is applied from the rolling surfaces  16  to the outer-diameter side rolling contact surfaces  12   a ,  12   b  in the radial direction of the input shaft  3 , a component force acts on the annular roller element  6   a  in a direction away from the annular roller element  6   b  (toward the right in  FIGS. 1 and 2 ). Therefore, in a case where the cam disk  21  is rotationally driven in a direction opposite to a specified direction, the planetary rollers  7  displace outward in the radial direction of the input shaft  3  due to a centrifugal force accompanying the revolution. At this time, the annular roller element  6   a  displaces in a direction away from the annular roller element  6   b  while reducing the ride-up amount d 1  of riding up from the bottom portion  25  of the drive-side cam concave portion  26  and the ride-up amount d 2  of riding up from the bottom portion  14  of the driven-side cam concave portion  15  of the rolling body  22 . Accordingly, the planetary rollers  7  are allowed to displace outward in the radial direction of the input shaft  3  and the outer-diameter dimension of the portion of the outer-diameter side rolling contact surfaces  12   a ,  12   b  of the pair of annular roller elements  6   a ,  6   b  that comes in rolling contact with the rolling surfaces  16  of the planetary rollers  7  increases. As a result, the surface pressure at the traction portions between the rolling surfaces  16  of the planetary rollers  7  and the inner-diameter side rolling contact surface  5  of the input shaft  3  and outer-diameter side rolling contact surfaces  12   a ,  12   b  of the pair of annular roller elements  6   a ,  6   b  decreases. 
     In this example, the controller  31  is configured so as to set a target value for the surface pressure at each traction portion in consideration of the torque (transmission torque) transmitted between the input shaft  3  and output shaft  4 , as well as other parameters other than the transmission torque such as the traction oil temperature (oil temperature), the rotational speed of the input shaft  3 , output shaft  4  and/or the planetary rollers  7 , and the like. In order for this, the relation between parameters including the transmission torque and the appropriate value of the surface pressure at the traction portions according to the parameters, or in other words, a value obtained by multiplying the minimum required value by the appropriate safety factor, is found in advance by experimentation, simulation, or the like, and stored in memory of the controller  31  as a map, a calculation formula, or the like. 
     When the frictional roller reducer  1  is in operation, parameters such as the transmission torque and oil temperature are measured by various sensors (not illustrated), and the output values of these sensors are inputted to the controller  31 . The controller  31  finds an appropriate value (target value) for the surface pressure at the traction portions according to the output values of various sensors based on a map, a calculation formula, or the like stored in the memory. In order to adjust the surface pressure at the traction portions to the target value found in this way, the controller  31  adjusts the rotation direction and the amount of rotation of the pressing force adjusting motor  23  to adjust the position in the axial direction of the annular roller element  6   a.    
     The output torque of the electric motor can be used as the transmission torque of the input parameters to be inputted to the map or the calculation formula. Alternatively, the transmission torque may be found by measuring the rotational torque of the input shaft  3  or the output shaft  4  with a torque sensor. As the torque sensor, it is possible to use, for example, a magnetostrictive torque sensor arranged around the input shaft  3  or the output shaft  4 , or a pulse phase difference type torque sensor in which encoders are supported at two positions separated from each other in the axial direction of the input shaft  3  or the output shaft  4 , and magnetic detection elements are made to face each of the encoders. 
     The oil temperature is measured by a temperature sensor installed at an arbitrary position in the traction oil circulation path, such as near a nozzle that sprays traction oil toward a traction portion. 
     The rotational speed of the input shaft  3 , the output shaft  4  and/or the planetary rollers  7  can be measured, for example, by a rotational speed sensor in which a magnetic detection element is made to face an encoder that is supported by and fixed to the input shaft  3 , the output shaft  4  and/or the planetary rollers  7 . Note that in a case where the rotational speeds of the input shaft  3 , the output shaft  4 , and the planetary rollers  7  are measured independently, the amount of slipping at each traction portion can be obtained, which is preferable. 
     In this example, the controller  31  has a function of making the surface pressure at the traction portions between the inner-diameter side rolling contact surface  5  of the input shaft  3  and the rolling surfaces  16  of the planetary rollers  7  substantially zero by adjusting the rotational drive of the pressing force adjusting motor  23 . Therefore, for example, it is possible to cause the output shaft  4  to idle with respect to the input shaft  3  during coasting. In other words, during coasting, the output shaft  4  rotates due to rotation of the wheels, and the planetary rollers  7  rotate (revolve) around the input shaft  3 . Therefore, a centrifugal force based on the revolution acts on the planetary rollers  7 . In this state, when the controller  31  rotationally drives the cam disk  21  in a direction opposite to a specified direction by the pressing force adjusting motor  23  via the worm reducer  30 , the planetary rollers  7  displace outward in the radial direction while the annular roller element  6   a  is caused to displace in a direction away from the annular roller element  6   b  due to the action of centrifugal force. As a result, as illustrated in  FIG. 3C , the rolling bodies  22  move to the bottom portion  14  of the driven-side cam concave portion  15  and to the bottom portion  25  of the drive-side cam concave portion  26 , and as illustrated in  FIG. 2 , the rolling surfaces  16  of the planetary rollers  7  are separated from the inner-diameter side rolling contact surface  5  of the input shaft  3 , a gap  34  occurs between the rolling surfaces  16  and the inner-diameter side rolling contact surface  5 , and the output shaft  4  is able to idle with respect to the input shaft  3 . 
     In the frictional roller reducer  1  of this example, the magnitude of the pressing force generated by the pressing device  9  can be adjusted to an arbitrary value by adjusting the rotational drive amount of the pressing force adjusting motor  23 , and therefore it is possible to adjust the surface pressure at the traction portions between the rolling surface  16  and the inner-diameter side rolling contact surface  5  and the outer-diameter side rolling contact surfaces  12   a , 12   b  to an arbitrary value. More specifically, the surface pressure at the traction portions can be adjust to an appropriate value in consideration of the torque that is transmitted between the input shaft  3  and the output shaft  4 , as well as parameters other than the transmission torque such as the traction oil temperature (oil temperature), rotational speed (rpm) of the input shaft  3 , the output shaft  4  and/or the planetary rollers  7 , or the like. Therefore, it is possible to prevent the occurrence of gross slip at each traction portion without excessively increasing the safety factor for the traction coefficient, and it is possible to ensure good transmission efficiency of the frictional roller reducer  1 . 
     Note that in this example, the relation between conditions such as transmission torque, oil temperature, and the like and the target value of the surface pressure at the traction portions according to the conditions is found by performing experimentation and simulation in advance. Therefore, it is considered that not only the parameters directly measured by various sensors, but also the elastic deformation of each member under the parameters, the slippage at the traction portions, the influence of the skew of the planetary rollers  7  and the like can be taken into consideration. However, it is also possible to measure the inclination of the rotation axis C of the planetary rollers  7  with a displacement sensor and use the output value of the displacement sensor to calculate the target value of the surface pressure at the traction portions. In other words, it is possible to measure the inclination of the rotation axis C by abutting the tip-end portion of a measuring element (probe) of a displacement sensor at two positions on the side surfaces in the axial direction of the planetary roller  7 , preferably at two positions on opposite sides in the radial direction. 
     Moreover, in this example, the worm reducer  30  has a self-locking function, and therefore even in a case where the pressing force adjusting motor  23  is stopped after adjusting the surface pressure at the traction portions to the target value, it is possible to maintain the position in the axial direction of the annular roller element  6   a.    
     Furthermore, the controller  31  of the frictional roller reducer  1  of this example has a function of making the surface pressure at the traction portions between the inner-diameter side rolling contact surface  5  of the input shaft  3  and the rolling surfaces  16  of the planetary rollers  7  substantially zero. Therefore, by activating this function when the accelerator is OFF, such as during high-speed cruising or the like, the traveling distance due to coasting can be lengthened. In other words, in a case where the surface pressure at the traction portions between the inner-diameter side rolling contact surface  5  and the rolling surfaces  16  is substantially set to zero during coasting, the output shaft  4  rotates as the wheels rotate, and the planetary rollers  7  rotate (revolve) around the input shaft  3 . Due to the action of centrifugal force based on the revolution of the planetary rollers  7 , the planetary rollers  7  displace outward in the radial direction while the annular roller element  6   a  is displaced in a direction away from the annular roller element  6   b , and the rolling surfaces  16  of the planetary rollers  7  are separated from the inner-diameter side rolling contact surface  5  of the input shaft  3 , and gaps  34  are formed between the rolling surfaces  16  and the inner-diameter side rolling contact surface  5 . As a result, the output shaft  4  is able to idle with respect to the input shaft  3 , and therefore the rotational resistance of the wheels can be kept low during coasting, and it is possible to improve the electric cost performance of the electric vehicle equipped with the frictional roller reducer  1 . 
     SECOND EXAMPLE 
       FIG. 4  illustrates a second example of an embodiment of the present invention. The frictional roller reducer la of this example further includes planetary roller pressing means  32  that elastically press the planetary rollers  7  outward in the radial direction of the input shaft  3 . The planetary roller pressing means  32  of this example is configured by holding elastic members such as compression coil springs in an elastically compressed state between the end portions on the inner side in the radial direction of the input shaft  3  of the concave portions  20  and both end portions in the axial direction of the support shaft  17 . 
     With the frictional roller reducer  1   a  of this example, in a case where the surface pressure at the traction portions between the inner-diameter side rolling contact surface  5  and the rolling surfaces  16  is substantially zero during coasting, the rolling surfaces  16  of the planetary rollers  7  can be reliably separated from the inner-diameter side rolling contact surface  5  of the input shaft  3 . 
     In the frictional roller reducer  1  of the first example, even in a case where the function of making the surface pressure at the traction portions between the inner-diameter side rolling contact surface  5  and the rolling surfaces  16  substantially zero is activated, and the rolling bodies  22  are moved to the bottom portion  14  of the driven-side cam concave portion  15  and the bottom portion  25  of the drive-side cam concave portion  26 , there is a possibility that the output shaft  4  will not be able to idle with respect to the input shaft  3 . In other words, in a case where the rotational speed of the output shaft  4  is slow, the centrifugal force applied to the planetary rollers  7  is small and the component force in the axial direction acting on the annular roller element  6   a  is small, and thus there is a possibility that the annular roller element  6   a  will not be displaced in a direction away from the annular roller element  6   b , and the rolling surfaces  16  may not be separated from the inner-diameter side rolling contact surface  5 . 
     On the other hand, with the frictional roller reducer  1   a  of this example, the planetary roller pressing means  32  elastically presses the planetary rollers  7  outward in the radial direction of the input shaft  3 , and thus in a case where the function of making the surface pressure at the traction portions between the inner-diameter side rolling contact surface  5  and the rolling surfaces  16  substantially zero is activated, the rolling surfaces  16  can be easily separated from the inner-diameter side rolling contact surface  5  regardless of the rotational speed of the output shaft  4 . The configuration and operational effect of other parts are the same as in the first example. 
     THIRD EXAMPLE 
       FIG. 5  illustrates a third example of an embodiment of the present invention. The frictional roller reducer lb of this example further includes a roller element pressing means  33  that elastically presses the annular roller element  6   a  in a direction away from the annular roller element  6   b . The roller element pressing means  33  of this example is configured by holding an elastic member such as a compression coil spring between the tip-end surfaces of the pair of annular roller elements  6   a ,  6   b  in an elastically compressed state. 
     With the frictional roller reducer  1   b  of this example, in a case where the surface pressure at the traction portions between the inner-diameter side rolling contact surface  5  and the rolling surfaces  16  is substantially zero during coasting, the rolling surfaces  16  of the planetary rollers  7  can be easily separated from the inner-diameter side rolling contact surface  5  of the input shaft  3 . The structure of this example can be implemented in combination with the structure of the second example. In other words, the frictional roller reducer of the present invention can simultaneously include both planetary roller pressing means  32  and roller element pressing means  33 . The configuration and operational effect of other parts are the same as in the first example and second example. 
     REFERENCE SIGNS LIST 
       1 ,  1   a ,  1   b  Frictional roller reducer 
       2  Housing 
       3  Input shaft 
       4  Output shaft 
       5  Inner-diameter side rolling contact surface 
       6   a ,  6   b  Annular roller element 
       7  Planetary roller 
       8  Carrier 
       9  Pressing device 
       10  Bearing device 
       11   a ,  11   b  Bearing 
       12   a ,  12   b  Outer-diameter side rolling contact surface 
       13  Driven-side cam surface 
       14  Bottom portion 
       15  Driven-side cam concave portion 
       16  Rolling surface 
       17  Support shaft 
       18  Bearing device 
       19  Annular portion 
       20  Concave portion 
       21  Cam disk 
       22  Rolling body 
       23  Pressing force adjusting motor 
       24  Drive-side cam surface 
       25  Bottom portion 
       26  Drive-side cam concave portion 
       27  Bearing 
       28  Driven-side gear 
       29  Drive-side gear 
       30  Worm reducer 
       31  Controller 
       32  Planetary roller pressing means 
       33  Roller element pressing means 
       34  Gap 
       100  Frictional roller reducer 
       101  Housing 
       102  Input shaft 
       103  Output shaft 
       104  Sun roller 
       105  Annular roller 
       106  Planetary roller 
       107  Pressing device 
       108  Sun roller element 
       109  Inner-diameter side rolling contact surface 
       110  Driven-side cam surface 
       111  Driven-side cam concave portion 
       112  Connecting portion 
       113  Outer-diameter side rolling contact surface 
       114  Support shaft 
       115  Rolling surface 
       116  Cam disk 
       117  Ball 
       118  Drive-side cam surface 
       119  Drive-side cam concave portion