Abstract:
A toroidal type continuously variable transmission has an input shaft to which a rotation torque is input, an input side disk and an output side disk that are concentrically and rotatably supported by the input shaft in a state that an inner peripheral surface of the input side disk opposes to an inner peripheral surface of the output side disk, a power roller held between the input side disk and the output side disk and a hydraulic loading device applying predetermined pressing force so that the power roller is pressed between the input and output side disks. Fine grooves are formed on at least a radially inner part of the inner peripheral surface of the input side disk and at least radially outer part of the inner peripheral surface of the output side disk, respectively.

Description:
The present invention claims foreign priority to Japanese patent application Nos. P.2004-311392 and P.2004-311397, both of which filed on Oct. 26, 2004, the contents of which are incorporated herein by references. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a toroidal type continuously variable transmission which can be used to a transmission for an automobile and various industrial machines. 
   2. Description of the Background Art 
   For example, a double-cavity-type toroidal type continuously variable transmission used as a transmission for an automobile is structured as shown in  FIGS. 3 and 4 . As shown in  FIG. 3 , inside a casing  50 , there is supported an input shaft (center shaft)  1  in such a manner that it can be rotated. On the outer periphery of the input shaft  1 , there are mounted two input side disks  2 ,  2  and two output side disks  3 ,  3 . Also, on the outer periphery of the middle portion of the input shaft  1 , there is rotatably supported an output gear  4 . The output gear  4  includes in the central portion thereof cylindrical-shaped flange portions  4   a ,  4   a  which are respectively connected to the output side disks  3 ,  3  through spline engagement. 
   The input shaft  1  can be driven and rotated by a drive shaft  22  through a loading-cam-type pressing device  12  interposed between a cam plate  7  and the input side disk  2  that is positioned on the left side in  FIG. 3 . Also, the output gear  4  is supported within the casing  50  through a partition wall  13  constructed by connecting two members to each other, whereby the output gear  4  can be rotated about the center axis O of the input shaft  1  but it is prevented from shifting in the direction of the center axis O. 
   The output side disks  3 ,  3  are respectively supported by needle bearings  5 ,  5  respectively interposed between the input shaft  1  and the respective disks in such a manner that they can be rotated about the center axis O of the input shaft  1 . Also, the input side disk  2  shown on the left side in  FIG. 3  is supported on the input shaft  1  through a ball spline  6  and the input side disk  2  shown on the right side in  FIG. 3  is spline connected to the input shaft  1 , while these two input side disks  2  can be rotated together with the input shaft  1 . Further, between the inner surfaces (concave-shaped surfaces)  2   a ,  2   a  of the input side disks  2 ,  2  and the inner surfaces (concave-shaped surfaces)  3   a ,  3   a  of the output disks  3 ,  3 , there are held power rollers  11  (see  FIG. 4 ) in such a manner that they can be rotated. 
   On the inner peripheral surface  2   c  of the input side disk  2  positioned on the right side in  FIG. 3 , there is provided a stepped portion  2   b  against which a stepped portion  1   b  provided on the outer peripheral surface  1   a  of the input shaft  1  is butted, while the back surface (the right surface in  FIG. 3 ) of the input side disk  2  is butted against a loading nut  9 . This substantially prevents the input side disk  2  from shifting from the input shaft  1  in the center axis O direction. Also, between the cam plate  7  and the collar portion  1   b  of the input shaft  1 , there is interposed a counter sunk spring  8  which applies a pressing force to contact portions where the concave-shaped surfaces  2   a ,  2   a ,  3   a ,  3   a  of the respective disks  2 ,  2 ,  3 ,  3  and the peripheral surfaces  11   a ,  11   a  of their corresponding power rollers  11 ,  11  are contacted with each other. 
   Now,  FIG. 4  is a section view taken along the line A-A shown in  FIG. 3 . As shown in  FIG. 4 , inside the casing  50 , there are provided a pair of trunnions  15 ,  15  which can be swung about a pair of pivot shafts  14 ,  14  torsionally positioned with respect to the input shaft  1 . Here, in  FIG. 4 , the illustration of the input shaft  1  is omitted. The respective trunnions  15 ,  15  include, in the two end portions thereof in the longitudinal direction (in  FIG. 4 , in the vertical direction) of their respective support plates  16 , a pair of bent wall portions  20 ,  20  which are formed so as be bent in the direction of the inner surface side of the support plates  16 . Thanks to the formation of these bent wall portions  20 ,  20 , in the respective trunnions  15 ,  15 , there are formed concave-shaped pocket portions P which are used to store their respective power rollers  11  therein. Also, on the outer surfaces of the respective bent wall portions  20 ,  20 , there are provided the pivot shafts  14 ,  14  such that they are concentric with each other. 
   In the central portion of each support plate portion  16 , there is formed a circular hole  21 ; and, in the circular hole  21 , there is supported the base end portion (first shaft portion)  23   a  of a shift shaft  23 . And, if the trunnions  15 ,  15  are respectively swung about their respective pivot shafts  14 ,  14 , the inclination angles of the shift shafts  23  supported on the central portions of the respective trunnions  15 ,  15  can be adjusted. Also, on the peripheries of the leading end portions (second shaft portions)  23   b  of the shift shafts  23  projected out from the inner surfaces of the respective trunnions  15 ,  15 , there are respectively supported the power rollers  11 ,  11  in such a manner that they can be rotated; and, the power rollers  11 ,  11  are respectively held between the input side disks  2 ,  2  and output side disks  3 ,  3 . Here, the base end portions  23   a  and leading end portions  23   b  of the respective shift shafts  23 ,  23  are eccentric to each other. 
   Also, the pivot shafts  14 ,  14  of the respective trunnions  15 ,  15  are supported in such a manner that they can be swung with respect to a pair of yokes  23 A,  23 B and can be shifted in the axial direction thereof (in the front and back direction in  FIG. 3 , in the vertical direction in  FIG. 4 ); and, the movements of the trunnions  15 ,  15  in the horizontal direction thereof are restricted by their respective yokes  23 A,  23 B. The yokes  23 A,  23 B are respectively formed into a rectangular shape by press working or forging metal such as steel. Each of the yokes  23 A,  23 B includes in the four corners thereof four circular support holes  18 , and the four pivot shafts  14  disposed on the two end portions of the trunnions  15  are respectively supported on the four holes  18  in such a manner that they can be swung. In the central portions of the yokes  23 A,  23 B in the width direction thereof (in the right and left direction in  FIG. 3 ), there are formed circular-shaped securing holes  19 . The inner peripheral surfaces of the securing holes  19  are formed as spherically concave-shaped surfaces, while spherical surface posts  64 ,  68  are respectively fitted into the spherically concave-shaped surfaces of the securing holes  19 . That is, the upper yoke  23 A is swingably supported by the spherical surface post  64  which is supported on the casing  50  through a fixing member  52 , while the lower yoke  23 B is swingably supported by the spherical surface post  68  and the upper cylinder body  61  of a cylinder  31  supporting the spherical surface post  68 . 
   Here, the shift shafts  23 ,  23  disposed on the respective trunnions  15 ,  15  are set in positions which are present 180 deg. opposite each other with respect to the input shaft  1 . The direction, in which the leading end portions  23   b  of the respective shift shafts  23 ,  23  are eccentric to the base end portions  23   a , is the same direction (in the reversed vertical direction in  FIG. 4 ) with respect to the direction of rotation of the input side and output side disks  2 ,  2 ,  3 ,  3 . Also, the eccentric direction is a direction which is substantially perpendicular to the direction of disposition of the input shaft  1 . Thus, the respective power rollers  11 ,  11  are supported in such a manner that they can be shifted to some extent in the longitudinal direction of the input shaft  1 . As a result of this, even when the respective power rollers  11 ,  11  tend to shift in the axial direction of the input shaft  1  due to the elastic deformation or the like of the respective composing members caused by a thrust load produced by the pressing device  12 , unreasonable forces are not applied to the respective composing members but the shifting motion of the power roller  11  can be absorbed. 
   Also, between the outer surfaces of the respective power rollers  11  and the inner surfaces of the respective support plate portions  16  of the trunnions  15 , in order starting from the outer surfaces of the power rollers  11 , there are interposed thrust ball bearings  24  consisting of thrust ball-and-roller bearings and thrust needle bearings  25 . Of these elements, the thrust ball bearings  24  not only support thrust-direction loads applied to the respective power rollers  11  but also allow the power rollers  11  to rotate. Each of the thrust ball bearings  24  consists of two or more balls  26 ,  26 , an annular-shaped retainer  27  for retaining the respective balls  26 ,  26  in such a manner that the balls  26 ,  26  are free to roll, and an annular-shaped outer race  28 . Also, the inner race tracks of the thrust ball bearings  24  are formed on the outer surfaces (large end faces) of their respective power rollers  11 , while the outer race tracks of the ball bearings  24  are formed on the inner surfaces of their respective outer races  28 . 
   Each thrust needle bearing  25  is held between the inner surface of the support plate portion of its corresponding trunnion  15  and the outer surface of the corresponding outer race  28 . The thus-arranged thrust needle bearings  25  not only support thrust loads applied to the respective outer races  28  from their corresponding power rollers  11  but also allow the power rollers  11  and outer races  28  to swing about the base end portions  23   a  of the respective shift shafts  23 . 
   On the one-end portions (lower end portions in  FIG. 4 ) of the respective trunnions  15 ,  15 , there are respectively disposed drive rods (trunnion shafts)  29 ,  29 , while drive pistons (oil pressure pistons)  33 ,  33  are fixedly secured to the outer peripheral surfaces of the middle portions of the respective drive rods  29 ,  29 . And, the drive pistons  33 ,  33  are respectively fitted in an liquid tight manner into the drive cylinder  31  consisting of the upper cylinder body  61  and lower cylinder body  62 . The drive pistons  33 ,  33  and drive cylinder  31  constitute a drive unit  32  which can shift the respective trunnions  15 ,  15  in the axial directions of the pivot shafts  14 ,  14  of these trunnions  15 ,  15 . 
   In the case of the thus-structured toroidal type continuously variable transmission, the rotation of the input shaft  1  is transmitted through the pressing device  12  to the respective input side disks  2 ,  2 . And, the rotational movements of these input side disks  2 ,  2  are then transmitted through the pair of power rollers  11 ,  11  to the respective output side disks  3 ,  3 ; and, further, the rotational movements of the output side disks  3 ,  3  are taken out from the output gear  4 . 
   To change a rotation speed ratio between the input shaft  1  and output gear  4 , the pair of drive pistons  33 ,  33  may be shifted in the mutually opposing directions. When the respective drive pistons  33 ,  33  are shifted in this manner, the pair of trunnions  15 ,  15  are shifted in the mutually opposing directions. For example, the power roller  11  shown on the left side in  FIG. 4  is shifted downward in  FIG. 4 , whereas the power roller  11  shown on the right side in  FIG. 4  is shifted upward in  FIG. 4 . This changes the directions of tangential-direction forces applied onto the contact portions between the peripheral surfaces  11   a ,  11   a  of the respective power rollers  11 ,  11  and the inner surfaces  2   a ,  2   a ,  3   a ,  3   a  of the input side disks  2 ,  2  and output side disks  3 ,  3 . With the change in the directions of the forces, the respective trunnions  15 ,  15  are swung in the mutually opposing directions about the pivot shafts  14 ,  14  which are pivotally supported on the yokes  23 A,  23 B. 
   This changes the contact positions between the peripheral surfaces  11   a ,  11   a  of the respective power rollers  11 ,  11  and the corresponding inner surfaces  2   a ,  3   a , which then changes the rotation speed ratio between the input shaft  1  and output gear  4 . Also, when the torque to be transmitted between the input shaft  1  and output gear  4  varies to thereby change the quantities of elastic deformation of the respective composing members, the respective power rollers  11 ,  11  and the outer races  28 ,  28  belonging to these power rollers  11 ,  11  are rotated slightly about the base end portions  23   a ,  23   a  of the respective shift shafts  23 ,  23 . Such rotational movements are executed smoothly because the thrust needle bearings  25 ,  25  are present between the outer surfaces of these outer races  28 ,  28  and the inner surfaces of the support plate portions  16  constituting the respective trunnions  15 ,  15 . This can reduce a force which is used to change the inclination angles of the respective shift shafts  23 ,  23  in such a manner as described above. 
   Now, in the thus-structured toroidal type continuously variable transmission, power is transmitted through the shearing force of oil (traction oil) existing between the input and output side disks  2 ,  3  and their corresponding power rollers  11  (between traction surfaces (rolling surfaces)) (see e.g. Japanese Patent Unexamined Publications No. JP-A-2003-343675 and JP-A-2003-278869). Because the coefficient of traction oil is fixed, to transmit high torque, a large load (pressing force) must be applied to the contact points between the input side and output side disks  2 ,  3  and power rollers  11 . 
   A method for applying the above load includes a case using the before-described pressing device  12  of a loading cam type which mechanically generates a load in proportion to input torque, and a case using a pressing device of a hydraulic pressure type. In the case where only the pressing device  12  of a loading cam type, there is generated a thrust force (the pressing force of the input side disk) in proportion only to the input torque; and, therefore, depending on the transmission gear ratios, there is a fear that an excessive pressing force can act on the contact portions between the disks and rollers to thereby reduce the transmission efficiency and durability of the composing members. On the other hand, when the pressing device of hydraulic pressure type is used, the optimum pressing force can be applied in compliance with the transmission gear ratios, oil temperatures, numbers of revolutions and the like, which makes it possible to enhance the transmission efficiency and durability of the transmission when compared with the pressing device of a loading cam type. 
   Now, in order to widen the transmission range and disuse a starting device, there is conventionally known a transmission which is a combination of a toroidal type continuously variable transmission and a planetary gear mechanism and is capable of switching speed modes over to each other (that is, includes two or more speed modes). 
   In the above-mentioned transmission including a planetary gear mechanism, there occurs torque reversal at the mode switching areas, which makes it easy to cause torque shift (a phenomenon in which the power roller  11  is out of its center axis and thus slips sideways to thereby shift the set transmission gear ratio). This torque shift is caused by a difference in transformation between the composing members due to variations in the pressing force or by the rickety motion and worsened rigidity of the trunnions and power rollers due to variations in the traction force. When such torque shift is produced, in the mode switching time, a shock is given to the transmission and thus an automobile carrying the transmission on board, which reduces the riding feeling of the automobile. Also, when sudden torque shift occurs, there is a possibility that inconveniences such as a gross slip can be produced. 
   However, in the low speed time, due to the contact radius and transmission torque between the input side disk and power roller, there is required a high pressing force (in a toroidal type continuously variable transmission, the contact radius between the disk and power roller varies in accordance with transmission gear ratios and thus the necessary pressing force varies). Therefore, the surface pressure of the contact portion between the disk and power roller (in the case of a half toroidal type continuously variable transmission, the surface pressure of the power roller  1011  as well) becomes high, which has ill effects on the durability of the composing parts. Also, when the durability of the composing parts is raised, they become large in size. 
   SUMMARY OF THE INVENTION 
   The present invention is made in the light of the above-mentioned conventional circumstances. That is, it is one of objects of the invention to provide a toroidal type continuously variable transmission which not only can apply the optimum pressing force in accordance with transmission gear ratios but also can obtain a desired transmission gear ratio with a lower pressing force than the conventional transmission to thereby reduce the occurrence of torque shift. 
   In order to achieve the above mentioned object, according to a first aspect of the present invention, there is provided a toroidal type continuously variable transmission comprising: 
   an input shaft to which a rotation torque is input; 
   an input side disk and an output side disk that are concentrically and rotatably supported by the input shaft in a state that an inner peripheral surface of the input side disk opposes to an inner peripheral surface of the output side disk; 
   a power roller held between the input side disk and the output side disk; and 
   a hydraulic loading device applying predetermined pressing force so that the power roller is pressed between the input side disk and the output side disk, 
   wherein fine grooves are formed on at least a radially inner part of the inner peripheral surface of the input side disk and at least radially outer part of the inner peripheral surface of the output side disk, respectively. 
   According to a second aspect of the present invention, there is provided a toroidal type continuously variable transmission apparatus comprising: 
   a toroidal type continuously variable transmission comprising:
         an input shaft to which a rotation torque is input;   an input side disk and an output side disk that are concentrically and rotatably supported by the input shaft in a state that an inner peripheral surface of the input side disk opposes to an inner peripheral surface of the output side disk;   a power roller held between the input side disk and the output side disk; and   a hydraulic loading device applying predetermined pressing force so that the power roller is pressed between the input side disk and the output side disk; and       

   a planetary gear transmission unit adopted to switch a pluralities of speed mode, 
   wherein fine grooves are formed on around speed mode changing areas of respective the inner peripheral surfaces of the input side disk and the output side disk, and 
   wherein the speed mode switching areas contact with the power roller when the planetary gear transmission unit switches the speed mode. 
   According to a third aspect of the present invention, as set forth in the second aspect of the present invention, it is preferable that the fine grooves are formed on a radial outer part of the inner peripheral surface of the input side disk and on a radial inner part of the inner peripheral surface of the output side disk, respectively. 
   According to a fourth aspect of the present invention, as set forth in the second aspect of the present invention, it is preferable that the planetary gear transmission unit is connected with the input side disk and the output side disk. 
   According to fifth and seventh aspects of the present invention, it is preferable that depth of the fine groove is 1 μm or more and 10 μm or less. 
   According to sixth and eighth aspects of the present invention, it is preferable that pitch of the fine grooves is 100 μm or more and 300 μm or less. 
   In a toroidal type continuously variable transmission according to the invention, due to use of a pressing device of a hydraulic type, the optimum pressing force can be applied in accordance with transmission gear ratios; and, at the same time, because there are formed fine grooves in the inner peripheral surface areas of the input and output side disks where the disks are contacted with the power rollers at least at the switching areas of the speed modes or in the neighborhood of such peripheral surfaces areas, the design traction coefficient can be increased, thereby being able to reduce the pressing force. Thanks to this, by reducing the pressing force in the periphery of the switching areas of the speed modes, the torque shift can be reduced, or it is possible to secure an allowance for the gross slip at the switching areas of the speed modes. 
   By the way, the above-mentioned fine groove may preferably have a depth of about 1-10 μm and a pitch of about 100-300 μm. Also, if the roughness of the fine groove is large, there is a possibility that the rolling life thereof can be shortened. Therefore, preferably, the fine groove may not be formed on the entire traction surface but they may be formed only at the switching areas of the speed modes. This arrangement is advantageous in working as well. That is, in the case of the fine grooves, it is necessary to round the head portions thereof or the corner portions thereof, which requires an extra finishing step for execution of such rounding operations. However, when the fine grooves are formed only at the speed mode switching areas to thereby narrow the finishing range, the working cost of the fine grooves can be reduced. 
   In a toroidal type continuously variable transmission according to the invention, due to use of a pressing device of a hydraulic pressure type, the optimum pressing force can be applied in accordance with transmission gear ratios; and, at the same time, because there are formed fine grooves at least in the radially inner part of the inner peripheral surface of the above-mentioned input side disk and in the radially outer part of the inner peripheral surface of the above-mentioned output side disk, that is, in the contact areas between the input and output side disks and their respective power rollers which correspond to the low speed (speed reducing) time, the design traction coefficient can be increased. Thanks to this, not only the pressing force can be reduced but also the durability of the respective parts (for example, disks, power rollers, trunnions, shafts and the like) can be enhanced, whereby the size of the transmission can be reduced. 
   By the way, each of the above-mentioned fine grooves preferably has a depth of about 1-10 μm and a pitch of 100-300 μm. Also, when the roughness of the fine groove is large, there is a possibility that the rolling life can be shortened. Therefore, preferably, the fine grooves may not be formed in the entire areas of the traction surface but may be formed only on the low speed side which is low in the frequency of use and provides a high load. This arrangement is advantageous also in working. That is, in forming the fine grooves, the head portions or corner portions thereof must be rounded, which requires a separate finishing step of rounding these portions. If the fine grooves are formed only on the low speed side to thereby narrow the finishing range, the working cost of the fine grooves can be reduced. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a section view of the main portions of a toroidal type continuously variable transmission according to an embodiment of the invention; 
       FIG. 2A  is an enlarged section view of the main portions of the toroidal type continuously variable transmission shown in  FIG. 1 ; 
       FIG. 2B  is an enlarged section view of the fine groove according to the embodiment of the invention; 
       FIG. 3  is a section view of a concrete example of a conventionally known half toroidal type continuously variable transmission; 
       FIG. 4  is a section view taken along the line A-A shown in  FIG. 3 ; 
       FIG. 5  is a section view of the main portions of a half toroidal type continuously variable transmission according to an embodiment of the invention; 
       FIG. 6A  is an enlarged section view of the main portions of the half toroidal type continuously variable transmission shown in  FIG. 5 ; 
       FIG. 6B  is an enlarged section view of the fine groove according to another embodiment of the invention; and 
       FIG. 7  is a section view of the main portions of a full toroidal type continuously variable transmission according to a modification of the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Now, description will be given below of an embodiment according to the invention with reference to the accompanying drawings. Here, the characteristic aspect of the invention consists in a disk surface structure for reducing a pressing force applied by a pressing device and the other remaining structures and operations thereof are similar to the before-mentioned conventional structures and operations. Therefore, in the following description of this specification, only the characteristic portions of the invention will be described and other remaining portions will be explained only briefly with the same reference characters as shown in  FIGS. 3 and 4  given. 
   Now,  FIG. 1  shows a structure in which a toroidal type continuously variable transmission is incorporated into a continuously variable transmission apparatus of a geared neutral type. The continuously variable transmission is constructed by combining a toroidal type continuously variable transmission unit  147  substantially similar in structure to the before-mentioned structure shown in  FIG. 3  with first to third planetary gear type transmission units  148 ,  149 ,  150 , and includes an input shaft  1  and output shaft  151 . Also, between the input shaft  1  and output shaft  151 , there is interposed a transmission shaft  152  which is concentric with these shafts  1 ,  151  and can be rotated with respect to these shafts  1 ,  151 . Here, a pressing device  12 A is of a hydraulic pressure type, while input side and output side disks  2 ,  3  are supported with respect to a hollow shaft  153  through which the input shaft  1  penetrates. Also, the input shaft  1  is arranged such that it receives a rotational force from a drive shaft  22  through the pressing device  12 A. As known well, according to this structure, by taking out power from the respective planetary gear type transmission units, transmission can be achieved between two or more speed modes (for example, between a low speed mode and a high speed mode). 
   As shown in  FIG. 2 , the pressing device  12 A includes a first cylinder portion  141  to be connected to the input end portion  1   a  of the input shaft  1 , a second cylinder portion  159  formed integral with the input side disk  2 , a first circular-shaped body  161 , and a second circular-shaped body  160 . 
   The first cylinder portion  141  is situated outside the outer periphery of the second cylinder portion  159  and is arranged such that it is opposite the back surface  2   d  of the input side disk  2 . Also, the second cylinder portion  159  is formed in a cylindrical shape and extends from the outer peripheral edge of the input side disk  2  toward the first cylinder portion  141 . 
   The second circular-shaped body  160  is arranged such that its inner peripheral surface is fitted with the outer peripheral surface of the input shaft  1  and its outer peripheral surface is fitted with the inner peripheral surface of the second cylinder portion  159 , while the second circular-shaped body  160  is disposed so as to be opposite the back surface  2   d  of the input side disk  2 . Also, the first circular-shaped body  161  is arranged such that its inner peripheral surface is fitted with the outer peripheral surface of the input shaft  1  and its outer peripheral surface is fitted with the inner peripheral surface of the first cylinder portion  141 , while the first circular-shaped body  161  is interposed between the second circular-shaped body  160  and first cylinder portion  141 . 
   A space existing between the first cylinder portion  141  and first circular-shaped body  161  forms a first hydraulic pressure chamber (oil chamber)  170 . This first hydraulic pressure chamber  170  is kept fluid tightly by two or more seal members  171 . Also, a space between the second cylinder portion  159  and second circular-shaped body  160  forms a second hydraulic pressure chamber (oil chamber)  167 . This second hydraulic pressure chamber  167  is kept fluid tightly by two or more seal members  168 . And, a space  175  existing between the second circular-shaped body  160  and first circular-shaped body  161  provides an air chamber. The air chamber  175  is kept fluid tightly by two or more seal members  168 ,  171 . Also, the second cylinder portion  159  includes, between the first circular-shaped body  161  and itself, a clearance s which also serves as a communication groove to allow the air chamber  175  to communicate with the outside; and, the second cylinder portion  159  can be contacted with the first circular-shaped body  161  through the clearance s. By the way, for supply of hydraulic to the respective hydraulic pressure chambers  167 ,  170 , there are formed liquid passages respectively in the drive shaft  22  and input shaft  1 . 
   Also, in the radially outer part (near-to-outer-periphery portion) of the inner peripheral surface  2   a  of the input side disk  2  and in the radially inner part (near-to-center portion) of the inner peripheral surface  3   a  of the output side disk  3 , that is, in the inner peripheral surface areas of the input and output side disks  2 ,  3  where the disks are contacted with their respective power rollers  11  at the before-mentioned speed mode switching areas or in the neighborhood of such areas, there are formed fine grooves  200 . Each of the fine grooves  200  is set such that it has a depth D of about 1-10 μm and a pitch P 1  of about 100-300 μm (see  FIG. 2B ). By the way, the head portion  202  or corner portions  204  of each fine groove  200  are rounded in a separate finishing step. Also, the fine grooves  200  may also be formed in other disk inner peripheral areas than the areas shown in  FIG. 2  (for example, in the entire areas of the disk inner peripheral surface). 
   As described above, in a toroidal type continuously variable transmission according to the present embodiment, due to use of the pressing device  12 A of a hydraulic pressure type, the optimum pressing force can be applied in accordance with transmission gear ratios. Also, in the present embodiment, because there formed the fine grooves  200  in the inner peripheral surface areas of the input and output side disks  2 ,  3  where the disks are contacted with their respective power rollers  11  at the before-mentioned speed mode switching areas or in the neighborhood of such areas, the design traction coefficient can be increased, which makes it possible to reduce the pressing force. Thanks to this, by reducing the pressing force in the periphery of the speed mode switching areas, the torque shift can be reduced, or it is possible to secure an allowance for the gross slip at the speed mode switching areas. 
   Here, when the roughness of the fine groove  200  is large, there is a possibility that the rolling life can be shortened. Therefore, the fine grooves  200  are not formed in the entire areas of the traction surface but, preferably, as in the present embodiment, there may be formed the fine grooves  200  only at the speed mode switching areas. This is advantageous in working as well. That is, in the case of the fine groove  200 , as described before, the head portion  202  thereof or the corner portions  204  thereof must be rounded, which requires a separate finishing step to be executed. In this case, if the fine grooves  200  are formed only at the speed mode switching areas to thereby narrow the finishing range, the working costs of the fine grooves can be reduced. 
     FIGS. 5 and 6  respectively show a half toroidal type continuously variable transmission according to another embodiment of the invention. As shown in  FIG. 5 , on the back surface  1002   d  of an input side disk  1002  located on the input side of an input shaft  1001 , there is provided a pressing device  1012 A of an hydraulic pressure type for pressing against the input side disk  1002  toward the axial direction. This pressing device  1012 A includes a first cylinder portion  1141  to be connected to the input end portion  1001   a  of the input shaft  1001 , a second cylinder portion  1159  formed integral with the input side disk  1002 , a first circular-shaped body  1161 , and a second circular-shaped body  1160 . 
   The first cylinder portion  1141  is engaged with the outer periphery of the second cylinder portion  1159  and is arranged such that it is opposite the back surface  1002   d  of the input side disk  1002 . Also, the second cylinder portion  1159  is formed in a cylindrical shape and extends from the outer peripheral edge of the input side disk  1002  toward the first cylinder portion  1141 . 
   The second circular-shaped body  1160  is formed such that its inner peripheral surface is fitted with the outer peripheral surface of the input shaft  1001  and its outer peripheral surface is fitted with the inner peripheral surface of the second cylinder portion  1159 ; and, the second circular-shaped body  1160  is arranged to be opposite the back surface  1002   d  of the input side disk  1002 . Also, the first circular-shaped body  1161  is formed such that its inner peripheral surface is fitted with the outer peripheral surface of the input shaft  1001  and its outer peripheral surface is fitted with the inner peripheral surface of the first cylinder portion  1141 ; and, the first circular-shaped body  1161  is interposed between the second circular-shaped body  1160  and first cylinder portion  1141 . 
   A space existing between the first cylinder portion  1141  and first circular-shaped body  1161  forms a first hydraulic pressure chamber (oil chamber)  1170 . This first hydraulic pressure chamber  1170  is kept fluid tightly by two or more seal members  1171 . Also, a space between the second cylinder portion  1159  and second circular-shaped body  1160  forms a second hydraulic pressure chamber (oil chamber)  1167 . This second hydraulic pressure chamber  1167  is kept fluid tightly by two or more seal members  1168 . Further, a space  1175  situated between the second circular-shaped body  1160  and first circular-shaped body  1161  provides an air chamber. The air chamber  1175  is kept fluid tightly by the two or more seal members  1168 ,  1171 . And, the second cylinder portion  1159  includes, between the first circular-shaped body  1161  and itself, a clearance s to serve also as a communication groove which allows the air chamber  1175  to communicate with the outside; and, the second cylinder portion  1159  can be contacted with the first circular-shaped body  1161  through this clearance s. For supply of working fluid to the respective hydraulic pressure chambers  1167 ,  1170 , there are formed liquid passages in the input shaft  1022  located on the engine side. 
   Also, as shown clearly in  FIG. 6 , there are formed fine grooves  1200  respectively in the radially inner part of the inner peripheral surface  1002   a  of the input side disk  1002  and in the radially outer part of the inner peripheral surface  1003   a  of the output side disk  1003 . Each of the fine grooves  1200  is formed such that its depth D is of the order of 1-10 μm and its pitch P 1  is of the order of 100-300 μm (see  FIG. 6B ). By the way, the head portion  1202  or corner portions  1204  of the fine groove  1200  are rounded in a separate finishing step. Further, the fine grooves  1200  may also be formed in the other disk inner peripheral surface areas than the areas shown in  FIG. 6  (for example, in the entire area of the disk inner peripheral surface). 
   According to the above-mentioned structure, to change the rotation speed between the input shaft  1001  and output shaft (not shown), specifically, to reduce the rotation speed between the input shaft  1001  and output shaft, the respective trunnions  1015  may be swung about their corresponding pivot shafts  1014  to thereby incline the respective shift shafts  1023  so that the peripheral surfaces  1011   a  of the respective power rollers  1011  can be contacted with the radially inner parts (the near-to-center portions) of the inner peripheral surfaces  1002   a  of the input side disks  1002  and also with the radially outer parts (the near-to-outer-periphery portions) of the inner peripheral surfaces  1003   a  of the output side disks  1003 . On the other hand, to increase the rotation speed, the respective trunnions  1015  may be swung to thereby incline the respective shift shafts  1023  so that the peripheral surfaces  1011   a  of the respective power rollers  1011  can be contacted with the radially outer parts (the near-to-outer-periphery portions) of the inner peripheral surfaces  1002   a  of the input side disk  1002  and also with the radially inner parts (the near-to-center portions) of the inner peripheral surfaces  1003   a  of the output side disks  1003 . In other words, the present embodiment provides a structure in which the fine grooves  1200  are formed in the contact areas between the input and output side disks  1002 ,  1003  and their respective power rollers  1011  which substantially correspond to the low speed (reduction) time. 
   As has been described above, in a toroidal type continuously variable transmission according to the present embodiment, due to use of the pressing device  1012 A of a hydraulic pressure type, the optimum pressing force can be applied in accordance with transmission gear ratios. Also, in the present embodiment, the fine grooves  1200  are formed at least in the radially inner parts of the inner peripheral surfaces  1002   a  of the input side disk  1002  and in the radially outer parts of the inner peripheral surfaces  1003   a  of the output side disks  1003 , that is, in the contact areas between the input and output side disks  1002 ,  1003  and their respective power rollers  1011  which substantially correspond to the low speed (reduction) time. Thanks to this, the design traction coefficient can be increased. This not only can reduce the pressing force but also can enhance the durability of the respective parts (for example, disks  1002 ,  1003 , power rollers  1011 , trunnions  1015 , and shaft  1001 ), thereby being able to reduce the size of the transmission. 
   By the way, when the roughness of the fine groove  1200  is large, there is a possibility that the rolling life can be shortened. Therefore, the fine grooves  1200  may not be formed in the whole area of the traction surface but, preferably, as in the present embodiment, the fine grooves  1200  may be formed only on the low speed side which is small in the frequency of use and provides a high load. This arrangement is advantageous also in working. In other words, when forming the fine grooves  1200 , as described above, the head portions  1202  thereof or the corner portions  1204  thereof must be rounded, which requires a separate finishing step for rounding them. However, if the fine grooves  1200  are formed only on the low speed side to thereby narrow the finishing range, the working cost can be reduced. 
   The present invention can apply not only to a half toroidal type continuously variable transmission of various types such as a single cavity type or a double cavity type but also to a full toroidal type continuously variable transmission having no trunnions. 
   The present invention is not limited to the above-described embodiment but, of course, it can also be enforced in variously modified manners without departing from the subject matter thereof. For example, in the above-described embodiment, the invention is applied to a half toroidal type continuously variable transmission. However, the invention can also be applied to such a full toroidal type continuously variable transmission having no trunnion as shown in  FIG. 7 . That is, in the full toroidal type continuously variable transmission shown in  FIG. 7 , the fine grooves  1200  may be formed respectively in the radially inner parts of the inner peripheral surfaces  1002   a  of the input side disk  1002  and in the radially outer parts of the inner peripheral surfaces  1003   a  of the output side disks  1003 . 
   While there has been described in connection with the preferred embodiments of the present invention, it will be obvious to those skilled in the art that various changes and modification may be made therein without departing from the present invention, and it is aimed, therefore, to cover in the appended claim all such changes and modifications as fall within the true spirit and scope of the present invention.