Patent Publication Number: US-RE39647-E

Title: Disk for toroidal type continuously variable transmission

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a disk for use in a toroidal type continuously variable transmission which can be used in vehicles, various kinds of industrial machines, and the like. 
     2. Description of the Related Art 
     A toroidal type continuously variable transmission comprises, for example, as shown in  FIG. 15 , input and output disks a and b which are disposed concentrically with each other, and a power roller c which is interposed between the respective traction surfaces f and i of the input and output disks a and b 1 . 
     In the input disk a, between the small diameter end portion d and large diameter portion e thereof, there is formed a traction surface f the cross section of which provides a concave-arc shape and, in the output disk b 1 , similarly, between the small diameter end portion g and large diameter portion h thereof, there is formed a traction surface i the cross section of which provides a concave-arc shape. On the side of the input disk a that is distant from the power roller c, there is concentrically disposed a loading cam through a plurality of engaging rollers (both of which are not shown), so that, due to the oil pressure that is supplied between the loading cam and input disk a, a driving force proportional to a torque can be applied toward the input disk a. 
     The power roller c is a device which can be frictionally engaged with the respective traction surfaces f and i of the input and output disks a and b 1  to thereby transmit power; and, the power roller c is supported by a trunnion j in such a manner that it can be inclined in the diameter direction of the input and output disks a and b 1 . And, if the trunnion j is operated by a drive mechanism (not shown) to thereby change the contact positions of the power roller c in the diameter direction thereof with respect to the input and output disks a and b 1 , then a rotation speed ratio between the input and output disks a and b 1 , that is, a speed change ratio can be varied continuously. 
     By the way, the toroidal type continuously variable transmission is required to transmit a higher torque and, for this reason, the input and output disks, a, b 1  and power roller c receive very large repeated bending stress and repeated shearing stress when compared with ordinary mechanical parts (such as ordinary gears and bearings); and, in the input and output disks a and b 1 , especially, as shown by fine hatchings in  FIG. 17 , the traction surfaces f, i, small diameter end portions d, g, and inside diameter surfaces at the small diameter end portion side d (g) receive large repeated bending stress and repeated shearing stress. Therefore, when manufacturing the input and output disks a and b 1 , it is necessary to use such highly durable material that can resist such repeated bending stress and repeated shearing stress. 
     Conventionally, to manufacture the input and output disks a and b 1 , for example, as shown in  FIG. 12 , cylindrical-shaped material (carburized steel or the like) having a length equal to the axial length of the input and output disks a and b 1  is shaved or cut to thereby produce such a final shape as shown in FIG.  16 . 
     However, in the conventional method for manufacturing the input and output disks a and b 1 , the yield of the material is poor and it takes long time to cut or shave the material, with the result that the production costs of the input and output disks a and b 1  are soaring. 
     Also, because a metal flow (the flow of structure) k is arranged along the axial direction of the disk, in the traction surfaces f and i with which the power roller c is frictionally engaged with a large pressure, the metal flow k comes to an end and, actually, does not extend along the traction surfaces f and i. As a result of this, not only the material is easy to peel off in the portions of the traction surfaces f and i with which the power roller c is frictionally engaged, but also an impact crack or a fatigue crack is easy to occur in the input and output disks a and b starting at the broken portions of the metal flow k, thereby providing an obstacle to the long lives of the input and output disks a and b 1 . 
     SUMMARY OF THE INVENTION 
     The present invention aims at eliminating the drawbacks found in the disks used in the conventional toroidal type continuously variable transmission. Accordingly, it is an object of the invention to provide a disk for use in a toroidal type continuously variable transmission which not only can reduce the production cost thereof but also can realize a long life. 
     By the way, in the center of the cylindrical-shaped material and in the neighboring portion of such center (that is, in  FIGS. 12 and 16 , 0.3 D portions: where, D designates the diameter of the cylindrical-shaped material), nonmetallic inclusions, which have a great influence on the fatigue breaking strength of the disk, are high in density (see  FIG. 13 ) and, therefore, it is desired that the nonmetallic inclusions are not present in the heavy bending stress areas of the disk (for example, the inside diameter surface of the small diameter end portions and the like) and in the areas of the traction surfaces that receive the heaviest shearing stress. 
     Referring now to the nonmetallic inclusions, it is known that the strength of material with respect to repeated bending stress is greatly influenced by the size of the defect thereof at which the breaking of the material can start. For example, in a book titled “Effects of small defects and nonmetallic inclusions” (Written by Yukitaka Murakami, published by Yokendo Ltd.), there is stated as follows: that is, 
     The fatigue limit of material when repeated bending is applied to the material can be expressed by the following equation; 
               σ   w     =       K   ⁡     (     Hv   +   120     )           (       (   area   )       1   /   2       )       1   /   6                 (   1   )             
         where K: 1.43 (when a defect is present on the surface of the material);   K: 1.41 (when a defect is present in such a manner as to be in contact of the material surface)   K: 1.56 (when a defect is present in the interior of the material)   Σ w : fatigue limit   Hv: hardness of material (relating to the strength of the matrix of the material), and,   (area) 1/2 : a square root of a projection area obtained when a defect or a crack is projected in the greatest main stress direction (an amount representing the dimension of a defect or a crack).       

     Therefore, for a mechanical part which is used under severe conditions such as the toroidal type continuously variable transmission (that is, under such severe conditions, the mechanical part receives not only great repeated bending stress but also great repeated shearing stress), it is desirable to use material in which a defect providing a starting point of the breaking of the material has been controlled. 
     Generally, it is known that the main defect cause of steel requiring high strength is an oxide-related nonmetallic inclusions. As a method for controlling such oxide-related nonmetallic inclusions, there are known the JIS method (JIS-G-0555), the ASTM method (ASTM-E45) and the like. Also, for bearing material requiring especially high cleanliness, there are known a method which, for example, as disclosed in Japanese Patent Publication No. Hei. 3-294435, melts material again using an electron beam melting method to float large-sized oxide-related nonmetallic inclusions of the material to thereby control the cleanliness of the material, and an extreme statistical method disclosed in the above-cited book “Effects of small defects and nonmetallic inclusions” (Written by Yukitaka Murakami, published by Yokendo Ltd.) (that is, a method in which the greatest diameter of oxide-related nonmetallic inclusions per unit area S 0  is investigated from several test pieces and, after then, the thus investigated result is processed statistically, thereby predicting the greatest diameter of oxide-related nonmetallic inclusions in an area S required). 
     For a ball-and-roller bearing, a gear and the like, using the above-mentioned cleanliness controlling methods, the steel cleanliness is controlled so that they are able to perform their expected functions. However, in the disks and power roller which constitute the toroidal type continuously variable transmission, the absolute values of the stress applied thereto (in particular, the contact surface pressure thereof is of the order of 4.0 GPa and the bending stress thereof is 90 kgf/mm 2 ) are large when compared with the ball-and-roller bearing, a gear and the like to which normal repeated stress is applied. Besides, not only the repeated bending stress and repeated shearing stress are applied simultaneously to the disks and power roller but also the volume thereof receiving such stress is large. For these reasons, in the toroidal type continuously variable transmission, it is difficult to obtain sufficient strength using the above-mentioned nonmetallic inclusions control method. That is, there is desired new means which can cope with the influence of the nonmetallic inclusions. 
     In attaining the above object, according to the invention, there is provided a disk for use in a toroidal type continuously variable transmission which comprises input and output disks each including a traction surface of a concave-arc-shaped cross section interposed between a small diameter end portion and a large diameter end portion and disposed concentrically with each other with their respective traction surfaces opposed to each other, and a power roller frictionally engageable with the respective traction surfaces of the input and output disks to thereby transmit power. The disk is used as one of the input disk and the output disk, and has metal flows. Under the finished state of the disk after all necessary finishing operations are executed, the metal flows of the disk which has the following positional relationship with respect to the surface of the disk is defined as “metal flows along the disk surface”, that is, an angle θ formed between the metal flow on the surface side and the tangent of the surface is smaller than or equal to 30 degrees, preferably, smaller than or equal to 20 degrees. The disk is structured such that the metal flows along the disk surface exist along a part of all surfaces including the traction surface. 
     Here, when the angle θ of a metal flow with respect to the tangent of the surface of the disk, especially, the traction surface exceeds 30 degrees, then the metal flow becomes equivalent to an end flow (that is, a metal flow which does not extend along the disk surface), which not only causes the material to peel off but also incurs the bending fatigue thereof or the like to thereby cause the disk to break (that is, the crack life of the disk is shortened). 
     Also, although it is desirable that the lower limit value of the angle θ may be infinitely approximate to 0, (θ=0), as can be seen from the relationship between a state of the material after forged (shown by a two-dot chained line) and a state of the material after completion of working (shown by a solid line) in  FIGS. 5 and 9 , for example, in the case of the traction surface  4  and inside diameter surface  2 , the margin between them varies, with the result that the angles of metal flows  6  intersecting with the traction surface  4  and inside diameter  2  after worked vary from the angles thereof after forged. For example, if a special attention is paid to one metal flow  6  after forged, then it is clear that it does not intersect with above-mentioned respective surfaces at a constant angle but the intersecting angles of the present metal flow  6  is changed between a state of the material after forged and a state of the material after completion of working. And, the intersecting angle θ of the metal flow with respect to the respective surfaces used in the present application, that is, the angle θ of the metal flow having an influence on the peel-off and bending fatigue of the material is defined that it does not mean the state of the material after forged but means the state of the material after worked, namely, the practically usable state of the material. 
     Therefore, it may be sufficient that the metal flow  6  after forged shows a state which, when the working is completed, is believed to be able to provide the range of the angle θ defined in the present application by means of a given margin. However, although a product having the angle θ equal to 0 or infinitely approximate to 0 is the most desirable from the viewpoint of the performance of the disk, it is also desirable to reduce the margin as much as possible. Also, in order to avoid the variations in the margin, or in order to remove the margin at right angles or at other angles with respect to the surfaces, that is, due to the working need, if the angle θ is obtained extremely severely, then poor working or similar inconveniences can occur, that is, the yield of the products can be degraded, with the result that the manufacturing cost of the disk is caused to rise. 
     Since the present invention has an object to provide a disk which not only can improve the performance thereof but also can reduce the manufacturing cost thereof and a method for manufacturing the same disk, there is employed θ=2-30 degrees, preferably, θ=5-20 degrees. In particular, the lower limit of the angle θ is defined mainly from the viewpoint of the improvement in both of the performance and yield, whereas the lower limit thereof is defined, as described above, mainly from the viewpoint of the improvements in the peel-off and bending fatigue. 
     The foregoing description is the definition of the metal flow existing along the disk surface. 
     (i) Also, the “metal flow along the disk surface”, preferably, may exist not only on the above-mentioned traction surface but also on the portion of the inside diameter surface of the disk that extends axially from the small diameter end face of the disk at least in the range of ⅓ A where the length of the disk in the axial direction thereof is expressed as A. 
     In this manner, the “metal flow along the disk surface” is formed in such a manner as to exist also on the inside diameter surface of the disk axially from the small diameter end face of the disk at least in the range of ⅓ A. The reason for this is as follows: that is, since a peripheral groove for a snap ring as shown in FIG.  17  and the like are formed in the inside diameter surface, the inside diameter surface is a portion which can be affected severely by the bending stress or the like; and, therefore, up to the area of ⅓ A of the disk, it is necessary to dispose the metal flow along the disk surface in the range of θ angle according to the invention. 
     (ii) In this case, if the “metal flow existing along the disk surface” is formed in such a manner as to exist also in the end face of the disk on the small diameter end portion side thereof, then the bending fatigue and the concentration of the stress applied to the peripheral groove can be relieved, thereby being able to extend the life of the disk still further. 
     (iii) Further, referring to  FIG. 14 , preferably, the “metal flow existing along the disk surface” may exist along the traction surface in the peripheral direction thereof in the range of an angle α of 45 degrees or more, preferably, 48 degrees or more, where the angle α is an angle which is formed by the traction surface with respect to a horizontal line (a line extending in parallel to the axis of the disk) passing through the center of radius O. 
     In this case, the area that receives the severest bending stress and the like in the traction surface (see the fine hatching portions shown in  FIG. 17 ) can be covered by the “metal flow existing along the disk surface”, which makes it possible to prevent the breakage of the disk caused by the bending stress and the like. 
     (iv) Further, the toroidal type continuously variable transmission disk according to the invention is manufactured by forging (which will be described later) using a mold. In this case, referring to  FIGS. 10 and 15 , where the small radius of a contact ellipse between the traction surface and power roller is expressed as b when the power roller is set horizontal (that is, parallel to the axis of the disk), that is, when a speed change ratio is 1:1, preferably, nonmetallic inclusions having a high density may exist in an area which is distant by 1.5 b or longer in the depth direction from the traction surface. Hereupon, “high density” is defined by the number of nonmetallic inclusions of 10 μm or larger as shown in FIG.  13 . “High density area” is such an area of 0.3 D portion in FIG.  13 . 
     The reason for the above is as follows: that is, the area that receives the severest shearing stress in the traction surface is an area which exists within 1.5 b in the depth direction from the traction surface and, therefore, if no nonmetallic inclusions are present in this area, then the life of the disk cannot be influenced at all (see FIG.  11 ). 
     (v) Also, as described above, since the portion of the inside diameter surface that extends axially in the range of ⅓ A from the end face on the small diameter end portion side is a portion which can be affected severely by the bending stress or the like due to the formation of the peripheral groove for the snap ring and the like, preferably, no nonmetallic inclusions may be present in ⅓ A range portion of the present inside diameter surface. 
     By the way,  FIGS. 18A  to  18 D and  19 A to  19 D respectively show methods respectively for manufacturing a disk for a toroidal type continuously variable transmission, which are disclosed in Japanese Patent Publication No. Hei. 9-126289. These disk manufacturing methods can be used to manufacture only the disk that includes a metal flow of θ=0 degree among the disks according to the present invention, but they are not able to manufacture the remaining disks including metal flows of other angles than θ=0 degree. In brief, in the above-disclosed manufacturing methods, there are found some problems to be solved. 
     In particular, referring at first to the conventional disk manufacturing method shown in  FIGS. 18A  to  18 D, cylindrical-shaped material (carburized steel or the like) n with its metal flows m extending on the outer peripheral surface of the material along the axial direction thereof is concentrically held by and between an upper mold o and a lower mold p, and the material n is then molded by a given amount (see FIG.  18 B). Here, the upper mold o includes a small diameter end portion molding surface s for molding the small diameter end portion r of a disk q and a traction surface molding surface u for molding the traction surface t having a concave-arc-shaped cross section of the disk q, whereas the lower mold p includes a large diameter end portion molding surface w for molding the large diameter end portion v of the disk q. And, the upper mold o and lower mold p are moved further closely to each other to thereby pressure forge the material n in the axial direction thereof several times, so that not only the small diameter end portion r and large diameter end portion v are respectively molded in the upper and lower end portions of the material n respectively, but also the traction surface t is molded between the small diameter end portion r and large diameter end portion v. 
     Next, as shown in  FIG. 18C , the upper mold o and lower mold p are moved most closely to each other to thereby pressure forge the material n into the final shape of the disk q and, after then, not only the thus forged and molded final shape is ground or finished but also the inside diameter surface portion thereof is cut to thereby produce the inside diameter surface x of the disk q 1 , thereby completing the final product of the disk q 1  that is shown in FIG.  18 D. 
     However, in the present conventional manufacturing method, since the cylindrical-shaped material n is forged up to the final shape of the disk q 1  using one kind of upper and lower molds o and p, the contact time between the upper and lower molds o, p and the material n is long, which makes it easy for the upper and lower molds o and p to be influenced by heat which is generated in the molding or forging operation. As a result of this, there is raised an inconvenience that the surface hardness of the upper and lower molds o and p can be lowered and the lives of the upper and lower molds o and p can be thereby shortened. 
     Also, in the final stage of the above molding operation, because the space between the upper and lower molds o and p is filled with the material n in a tightly closed state, the thicknesses of the corner portions of the upper and lower molds o and p can be easily reduced or burrs are easy to occur in the present corner portions. Besides, if trying to improve the shape of disk q 1  by force, then an excessive molding load must be applied to the upper and lower molds o and p, with the result that the upper and lower molds o and p can be broken. 
     Further, in the step to be executed after the material is mold-forged, the forged material is finished by grinding. Therefore, the grinding margin must be minimized in order to shorten the working time necessary for the grinding operation as much as possible. As a result of this, it is necessary to reduce the degree of abrasion in the upper and lower molds o and p during the forging operation, so that the lives of the upper and lower molds o and p can be inconveniently shortened. 
     In addition, since the upper and lower molds o and p are not structured such that they hold the cylindrical-shaped material n within their interior portions, the cylindrical-shaped material n is easy to shift from the centers of the upper and lower molds o and p, which results in the worsened working precision. 
     Thus, in order to eliminate the above-mentioned inconveniences found in the conventional disk manufacturing method, the present inventors have developed a conventionally unknown new disk manufacturing method as follows: that is, this disk manufacturing method is suitable for manufacturing a disk according to the invention and, especially, in addition to the disk according to the invention, the present method is ideal for manufacturing a disk which includes the before-mentioned respective means (i) and (iii). 
     In particular, the present disk manufacturing method comprises: a first step in which cylindrical-shaped material with its metal flows existing on the outer peripheral surface thereof and extending along the axial direction of the material is swaged using a first forging mold; a second step in which the swaged material is molded using a second forging mold to thereby form a portion of the inside diameter surface of the disk in the central portion of the upper end face of the material, and the shape of the molding surface of the second forging mold is transferred to the present inside diameter surface portion; and a third step in which the material obtained in the second step is molded using a third forging mold to thereby form a small diameter end portion, a traction surface and a large diameter end portion, the inside diameter surface portion molded in the second step is pushed further to such an extent as allows a residual wall to be left between the back surface of the large diameter end portion and the present inside diameter surface portion, and further a burr is formed on the outside diameter surface of the present large diameter end portion, wherein, after the burr and residual wall of the mold forged product obtained through the respective steps are removed, the mold forged product is ground to thereby form a disk having its final shape, and the thus ground disk is heat treated and is finished. 
     In the present disk manufacturing method, since the mold forging operation is carried out in three steps using three kinds of molds, the contact time between the mold and material can be shortened, which makes it possible to reduce the heat influence on the molds during the molding operation. As a result of this, the mold surface hardness can be maintained in a good level to thereby be able to improve the lives of the molds used. 
     Also, if the amount of swaging of the cylindrical-shaped material is increased in the first step, then it is possible to reduce the molding amounts not only in the second step but also in the third step in which a high molding load is required in order to obtain a shape approximate to the final shape of the product. As a result of this, the working burden of the forging mold in the second and third steps can be relieved so that the lives of the molds can be extended. 
     Further, the increased swaging amount of the cylindrical-shaped material in the first step can reduce not only the degree of push-in of the inside diameter surface portion of the forging mold in the second step but also the degree of further push-in of the forging mold inside diameter portion molded in the second step. This makes is possible to enhance greatly the life of the portion of the material of the forging mold that can be most influenced by heat. 
     Still further, since the mold forging operation is carried out in three steps using three kinds of molds, the flow of the material during the forging operation can be set freely. This means that a shape matched to the final shape of the product can be set in the previous steps (that is, in the first and second steps), thereby being able to provide a well-balanced mold forged product. 
     Yet further, because the disk is obtained by grinding the mold forged product, even if the forged product before ground is a rough forged product (for example, a hot-forged product), the present disk manufacturing method is sufficiently able to deal with such forged product; and, because there is little need to pay attention to the abrasion of the molds (that is, because, even if the molds wear to some degree, they can still be used), the costs of the molds can be reduced in the long run. 
     In addition, due to the fact that, in the third step, the burr is produced on the outside diameter surface of the large diameter end portion at the time when the molding operation is completed, tightly closed forging is avoided to thereby be able to prevent an unnecessary molding load from increasing. This makes it possible to improve the lives of the molds. 
     By the way, if the height H 1  of the central portion of the material after it is swaged in the first step is set in the range of 80-120% of the height  2  of the material when the forged product is completed in the third step, then the enhanced lives of the molds in the second and third steps can be achieved effectively. 
     Also, in the above-mentioned first to third steps, by disposing positioning means which is capable of centering or positioning the respective forging molds with respect to the material, the material can be positioned accurately and positively in the molding center of the forging molds in each step. Thanks to this, the material can be always molded at the correct position to thereby be able to provide a high-precision mold forged product. That is, there can be provided a forged product in which a metal flow existing along the surface of the disk according to the present invention can be obtained by a post-machining operation. 
     Next, description will be given below of the conventional disk manufacturing method which is shown in  FIGS. 19A  to  19 D. This is a method in which part of the inside diameter surface x of a disk q 2  is molded when the material is forged by molds. That is, at first, as shown in  FIG. 19B , the upper end portion of the cylindrical-shaped material n is drawn to thereby set the diameter of the present upper end portion smaller than the diameter of the small diameter end portion r and, next, the cylindrical-shaped material n is concentrically held by and between an upper mold y and a lower mold z and is then molded by a given amount (see FIG.  19 C). The upper mold y includes a small diameter end portion molding surface s for molding the small diameter end portion r of the disk q 2 , a traction surface molding surface u for molding a traction surface t having a concave-arc shaped cross section, and a projection a 1  which is provided in the central portion of the small diameter end portion molding surface s for molding a portion of the inside diameter surface x of the disk q 2  from the small diameter end portion r side. On the other hand, the lower mold z includes a large diameter end portion molding surface w for molding the large diameter end portion v, and a projection a 2  which is provided in the central portion of the large diameter end portion molding surface w for molding a portion of the inside diameter surface x of the disk q 2  from the large diameter end portion v side. 
     Next, the upper and lower molds y and z are moved to approach each other further and the material n is forged several times in the axial direction thereof, whereby not only the small diameter end portion r and large diameter end portion v are molded respectively in the upper and lower end portions of the material n but also the traction surface t is molded between the small diameter end portion r and large diameter end portion v; and, then an operation to mold the inside diameter surface x by the projections a 1  and a 2  is started. 
     Next, as shown in  FIG. 19D , the upper and lower molds y and z are moved further to approach each other most closely and the material n is mold-forged into the final shape of the disk q 2 . In this forging operation, the inside diameter surface x is held in a state where the residual wall a 3  thereof is still left. After then, the residual wall a 3  of the inside diameter surface x is removed by cutting or by grinding to thereby complete the inside diameter surface x and, at the same time, the material n is further ground, which completes the final product of the disk q 2 . 
     In the above-mentioned conventional disk manufacturing method, there occurs a phenomenon in which the end points of the metal flows m appearing in the upper and lower ends of the cylindrical-shaped material n in  FIG. 19B  are pulled into the interior of the cylindrical-shaped material n in  FIG. 19C ; and, as a result of this, the metal flows exist along the surface shape of the disk q 2  (θ=0 degree) ranging from the traction surface t to the small diameter end portion r and the portion of the inside diameter surface x that is located on the small diameter end portion r side thereof. 
     However, in the step shown in  FIG. 19C , it is very difficult to mold the cylindrical-shaped material n in such a manner that the end points of the metal flows m are pulled into the interior of the cylindrical-shaped material n and, therefore, in most cases, the end points of the metal flows m are left somewhere on the upper and lower end faces of the material n. As a result of this, it is difficult to allow the metal flows having the angle of θ=0 degree to exist positively along the surface of the disk q 2 . 
     Also, there is a high possibility that high-density nonmetallic inclusions existing in the neighboring portion of the central portion of the cylindrical-shaped material n can be left in the portion to which the bending stress is applied severely, that is, a portion ranging from the small diameter end face of the disk q 2  axially to the portion of the inside diameter surface that extends in the ⅓ A range thereof, which has an ill effect on the life of the disk. 
     Thus, in order to eliminate the inconveniences found in the above-mentioned conventional disk manufacturing method, the present inventors have developed a conventionally unknown new disk manufacturing method as follows: that is, this disk manufacturing method is suitable for manufacturing a disk according to the invention and, especially, in addition to the disk according to the invention, the present method is ideal for manufacturing a disk which includes all of the before-mentioned means (i) to (v). 
     In particular, the present disk manufacturing method comprises: a first step in which a cylindrical-shaped material with metal flows existing on the outer peripheral surface thereof and extending along the axial direction thereof is swaged using a first forging mold and, at the same time, the upper end portion of the present cylindrical-shaped material is drawn; and, a second step in which the material obtained in the first step is molded using a second forging mold to thereby form a traction surface and a large diameter end portion and, at the same time, a portion of an inside diameter surface is formed in the central portion of the present material in such a manner that a residual wall is left between the back surface of the large diameter end portion and the present inside diameter surface portion. And, the present disk manufacturing method is characterized in that, when forming a portion of the inside diameter portion in the central portion of the material in the second step, the upper end portion of the present material is restricted by a portion of the second forging mold to thereby prevent the present upper end portion from increasing in diameter during the molding operation and, at the same time, high-density nonmetallic inclusions existing in the central portion of the present material are pushed into the lower end side of the material to thereby expand the lower end side of the material outwardly in the diameter direction thereof; and, further, after the residual wall of the mold forged product obtained through the respective steps is removed, the material is ground to thereby mold the same into the final shape of the disk and the thus molded disk is heat treated and finished. 
     In the present disk manufacturing method, it is possible to obtain a disk (a finished product) simply and positively in which the “metal flows existing along the disk surface” each having the angle of θ=2-30 degrees exist respectively in the end face of the disk on the small diameter end portion side thereof, the traction surface of the disk, the outer peripheral surface of the large diameter end portion of the disk, and the back surface of the large diameter end portion thereof. 
     Also, partly because use of the mold forging operation makes it possible to reduce the diameter of the cylindrical-shaped material, partly because, in the second step, the upper end portion of the present material is restricted by a portion of the second forging mold to thereby prevent the present upper end portion from increasing in diameter during the molding operation, and partly because the high-density nonmetallic inclusions existing in the central portion of the present material are pushed into the lower end side of the material to thereby expand the lower end side of the material outwardly in the diameter direction thereof, there can be obtained simply and positively a disk in which the high-density nonmetallic inclusions do not exist in the following areas: that is, the area of the traction surface portion that receives the severest shearing stress in the traction surface portion, that is, the area extending in the depth direction from the traction surface by a distance less than 1.5 b; and, the area of the disk ranging axially from the end face of the small-diameter-end-portion side inside diameter surface portion receiving a severe bending stress due to formation of the peripheral groove for the snap ring up to the ⅓ A (A is the axial length of the disk) range portion. 
     By the way, preferably, in the second step, a burr may be produced on the outside diameter surface of the large diameter end portion at the time when the molding is completed. That is, the thus produced burr can avoid a tightly closed forging operation to thereby prevent an unnecessary molding load from increasing, which in turn makes it possible to enhance the lives of the molds. 
     Also, in the above-mentioned first and second steps, by disposing positioning means which is capable of centering or positioning the respective forging molds with respect to the material, the material can be positioned accurately and positively in the molding center of the forging molds in each step. Thanks to this, the material can be always molded at the correct position to thereby be able to provide a high-precision mold forged product. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is an is an explanatory view of a toroidal type continuously variable transmission disk which is a first embodiment according to the invention; 
         FIG. 2  is an explanatory view of a first step employed in a disk manufacturing method, in particular, the left half section shows a state of a disk material before molded, while the right half section shows a state of a disk material after molded; 
         FIG. 3  is an explanatory view of a second step employed in the disk manufacturing method, in particular, the left half section shows a state of a disk material before molded, while the right half section shows a state of a disk material after molded; 
         FIG. 4  is an explanatory view of a third step employed in the disk manufacturing method, in particular, the left half section shows a state of a disk material before molded, while the right half section shows a state of a disk material after molded; 
         FIG. 5  is an explanatory view of an example of a final step employed in the disk manufacturing method; 
         FIG. 6  is an explanatory view of a toroidal type continuously variable transmission disk which is a second embodiment according to the invention; 
         FIG. 7  is an explanatory view of a first step employed in a disk manufacturing method, in particular, the left half section shows a state of a disk material before molded, while the right half section shows a state of a disk material after molded; 
         FIG. 8  is an explanatory view of a second step employed in the disk manufacturing method, in particular, the left half section shows a state of a disk material before molded, while the right half section shows a state of a disk material after molded; 
         FIG. 9  is an explanatory view of an example of a final step employed in the disk manufacturing method, 
         FIG. 10  is an explanatory view of the existing portion of the high-density nonmetallic inclusions in a disk; 
         FIG. 11  is a graphical representation to explain the relationship between the depth from the surface of a traction surface and the distribution of shearing stress; 
         FIG. 12  is an explanatory view of the existing portion of the high-density nonmetallic inclusions in a cylindrical-shaped material before molded; 
         FIG. 13  is a graphical representation to explain the relationship between the diameter of the cylindrical-shaped material before molded and the number of nonmetallic inclusions; 
         FIG. 14  is a view to explain the meanings of α and θ; 
         FIG. 15  is a section view to explain a toroidal type continuously variable transmission; 
         FIG. 16  is an explanatory view of a conventional disk; 
         FIG. 17  is an explanatory view of a portion of a disk which receives large repeated bending stress and repeated shearing stress; 
       FIG.  18 A- FIG. 18D  are explanatory views of a conventional disk manufacturing method; and 
       FIG.  19 A- FIG. 19E  are explanatory views of another conventional disk manufacturing method. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now, description will be given below of the preferred embodiments of a toroidal type continuously variable transmission according to the invention with reference to the accompanying drawings In particular,  FIG. 1  is an explanatory view of the input and output disks of a toroidal type continuously variable transmission which are a first embodiment according to the invention;  FIG. 2  is an explanatory view of a first step employed in a disk manufacturing method;  FIG. 3  is an explanatory view of a second step employed in the disk manufacturing method;  FIG. 4  is an explanatory view of a third step employed in the disk manufacturing method;  FIG. 5  is an explanatory view of an example of a final step employed in the disk manufacturing method;  FIG. 6  is an explanatory view of the input and output disks of a toroidal type continuously variable transmission which are a second embodiment according to the invention;  FIG. 7  is an explanatory view of a first step employed in a disk manufacturing method;  FIG. 8  is an explanatory view of a second step employed in the disk manufacturing method; and,  FIG. 9  is an explanatory view of an example of a final step employed in the disk manufacturing method. 
     At first, description will be given below of a disk (a finished product) which is a first embodiment according to the invention with reference to FIG.  1 . This disk  1  includes a traction surface  4  having a concave-arc-shaped cross section interposed between a small diameter end portion  2  and a large diameter end portion  3 ; and, in the central portion of the end face of the disk ion the small diameter end portion  2  side thereof, there is formed a through hole which extends therethrough up to the back surface of the large diameter end portion  3 , while the inner peripheral surface of the through hole provides an inside diameter surface  5 . Here, with reference to FIGS. land  14 , among the metal flows  6  that exist in the disk  1 , a metal flow  6 , which has such a positional relationship with respect to the surface of the disk  1  that the metal flow  6  on the traction surface  4  side has an angle θ of 2-30 degrees, preferably, 5-20 degrees with respect to a tangent P of the traction surface  4 , is defined as a “metal flow  6  along the disk surface”. 
     In the disk  1 , a “metal flow  6  along the disk surface” having the angle θ=2-30 degrees exists in the traction surface  4 ; a “metal flow  6  along the disk surface” having the angle θ=2-30 degrees exists in the area of the inside diameter surface  5  ranging from the end face of the inside diameter surface  5  on the small diameter end portion  2  side thereof to the range of ⅓ A, where the length of the disk  1  in the axial direction thereof is expressed as A; and, a “metal flow  6  along the disk surface” having the angle θ=2-30 degrees exists in the outside diameter surface of the large diameter end portion  3  and in a portion of the back surface of the present large diameter end portion  3 . 
     Also, a “metal flow  6  along the disk surface” having the angle θ=2-30 degrees exists in the traction surface  4  in such a manner that, with reference to  FIG. 14 , it extends along the peripheral direction of the traction surface  4  in the range of an angle α of 45 degrees with respect to a horizontal line passing through the center of curvature O of the traction surface  4  (that is, a line extending in parallel to the axis of the disk  1 ). By the way, a metal flow  6  having the angle θ of greater than 30 degrees with respect to the tangent P of the traction surface  4  becomes equivalent to an end flow (a metal flow which does not exist along the disk surface), so that such metal flow not only causes the material to peel off but also gives rise to the bending fatigue to the disk and thus the rupture of the disk (the lowered crack life). 
     In the thus structured disk  1 , since the “metal flow  6  along the disk surface” having the angle θ=2-30 degrees exists in the traction surface with which the power roller can be frictionally engaged with a large pressure, especially when the disk  1  is used under a low load condition, not only the material in the engaged portion of the traction surface  4  where it is frictionally engaged with the power roller can be prevented from peeling off, but also an impact crack or a fatigue crack is hard to occur in the disk  1  so that the long life of the disk  1  can be realized. 
     Also, because the “metal flow  6  along the disk surface” having the angle θ=2-30 degrees exists in the area of the inside diameter surface  5  ranging from the end face of the inside diameter surface  5  on the small diameter end portion  2  side thereof to the range of ⅓ A, within the portion of the inside diameter surface  5  that is relatively weak against the bending stress and the like due to the formation of the peripheral groove for the snap ring, the metal flow can be prevented from providing an end flow, which makes it possible to realize the further longer life of the disk  1 . 
     Further, since the “metal flow  6  along the disk surface” having the angle θ=2-30 degrees exists in the traction surface  4  in the range of an angle α of 45° C., the area of the traction surface  4  that receives the severest bending stress and the like in the traction surface  4  (see the fine hatching portion in  FIG. 17 ) is covered by the “metal flow  6  along the disk surface”, thereby being able to prevent effectively the breakage of the disk  1  caused by the bending stress and the like. 
     Next, description will be given below of a method for manufacturing the disk  1  with reference to  FIGS. 2  to  5 . 
     (First Step) 
     In  FIG. 2 , there is shown a first step (a swaging step) employed in the present disk manufacturing method. In particular, the left half section of  FIG. 2  shows a state before the material is swaged, whereas the right half section of  FIG. 2  shows a state after the material is swaged. In the first step, a cylindrical material (carburized steel or the like) W 1  with metal flows  6  existing on the outer peripheral surface of the material and extending along the axial direction of the material is interposed between an upper mold  11  and a lower mold  12 ; and, next, the upper mold  11  is moved in the axial direction of the cylindrical-shaped material W 1  to swage the present cylindrical-shaped material W 1 , thereby molding the same into a material W 2  having such a shape as shown in the right half section of FIG.  2 . Here, in the first step, a swaging ratio is set larger than an ordinary swaging ratio; and, in this case, if the height H 1  of the central portion of the material W 2  after swaged is set in the range of 80-120% of the height H 2  of a material W 4  which is shown in FIG.  4  and is obtained when the forging operation is completed, then the lives of forging molds used in second and third steps (which will be discussed later) can be enhanced effectively. 
     In the plane portion  12 a of the lower mold  12 , there is formed a recessed portion  13  into which the lower end portion of the cylindrical-shaped material W 1  can be fitted; that is, if the lower end portion of the cylindrical-shaped material W 1  is fitted into the recessed portion  13 , then the cylindrical-shaped material W 1  can be aligned with the lower mold  12  with accuracy. Also, in the central portion of the upper mold  11 , there is provided a circular projecting portion  14  having a diameter smaller than the diameter of the cylindrical-shaped material W 1 ; and, the circular projecting portion  14 , in the swaging operation, molds the central portion of the cylindrical-shaped material W 1  to thereby spread the diameter of the material. Further, between the outer peripheral side plane portion  11 a outside of the circular projecting portion  14  of the upper mold  11  and the circular projecting portion  14 , there is formed a curve molding surface  15  which is curved outwardly in the diameter direction of the upper mold from the circular projecting portion  14  in such a manner as to expand gradually and upwardly and also which continues with the plane portion  11 a of the upper mold. The present curve molding surface  15  is capable of transferring the curved shape to the swaged material W 2 . 
     (Second Step: Intermediate Step) 
     Now,  FIG. 3  shows a second step employed in the present disk manufacturing method; and, the left half section of  FIG. 3  shows a state of the material before molded, whereas the right half section thereof shows a state of the material after molded. The function of the second step is to give the optimum shape to the material in order that the volume of the material can be distributed properly, that is, in order to be able to prevent the reduced thickness and burrs from occurring in a third step. If the shape of the material formed in the second step is not proper, then, when the material is molded in the third step, the burrs or the reduced thickness can be generated on the inside diameter corners (on the upper end side) of the material, or burrs on the outside diameter surface of the large diameter end portion of the material (which will be discussed later) can be reduced in the thickness thereof. Also, another function of the second step is to give the material such a shape which allows alignment to be achieved positively between the material W 3  and mold in the third step. 
     Referring firstly to upper and lower molds  21  and  22  employed in the second step, in the central portion of the plane portion  21 a of the upper mold  21 , there is projectingly disposed a middle mold  25  which has a substantially conical-shaped projection. On the other hand, the lower mold  22  includes, in the portion thereof that is located near the outer periphery thereof, an inclined portion  23  which is inclined obliquely upwardly, that is, outwardly in the diameter direction of the material W 2  swaged in the first step; and, the material W 2  can be aligned with the lower mold  22  where the lower end side outer peripheral edge of the material W 2  is in contact with the inclined portion  23  of the lower mold  22 . At the then time, the lower end face of the material W 2  is set in such a manner as to float slightly from the upper end face of a projecting portion  24  provided on the central plane portion  22 a of the lower mold  22 . 
     And, in this state, if the upper mold  21  and middle mold  25  are moved down integrally, then not only the middle mold  25  invades into the upper end face central portion of the material W 2  to thereby form a recessed portion  5 a which is a portion of the inside diameter surface  5 , but also the plane portion  21 a of the upper mold  21  presses against the upper end face of the material W 2  to thereby apply a molding pressure; and, due to this molding pressure, as shown in the right half section of  FIG. 3 , the shapes of the plane portion  22 a, inclined portion  23  and projecting portion  24  of the lower mold  22  are respectively transferred to the lower end portion of the material W 2 , so that the material W 2  is molded into the shape of the material W 3 . 
     (Third Step) 
     Now,  FIG. 4  shows a third step employed in the present disk manufacturing method; and, in particular, the left half section of  FIG. 4  shows a state of the material before molded, whereas the right half section thereof shows a state of the blank after molded. In the third step, an upper mold  31  includes a small diameter end portion molding surface  33  for molding the small diameter end portion  2  of the disk  1 , a traction surface molding surface  34  for molding the traction surface  4  having a concave-arc-shaped cross section, and a middle mold  35  which is disposed in the central portion of the small diameter end portion molding surface  33  and is used to mold a portion of the inside diameter surface  5  from the small diameter end portion  2  side; and, a lower mold  32  includes a large diameter end portion molding surface  36  for molding the large diameter end portion  3 , and a projecting portion  37  which is disposed in the central portion of the large diameter end portion molding surface  36  concentrically with the middle mold  35 . Onto the projecting portion  37 , there can be fitted the recessed portion  26  of the material W 3  to which the shape of the projecting portion  24  of the lower mold  22  was transferred in the second step. That is, if the recessed portion  26  is fitted with the projecting portion  37 , then the material W 3  can be positioned in the central portion of the lower mold  32  accurately and positively. Also, on the outer peripheral portion of the lower mold  32 , there is disposed an outer mold  38 ; and, the outer mold  38  and lower mold  32  cooperate together in forming a molding space for forming a recess-shaped large diameter end portion. 
     And, in this state, if the upper mold  31  and middle mold  35  are moved down integrally, then not only the shapes of the small diameter end portion molding surface  33 , traction surface molding surface  34  and large diameter end portion molding surface  36  are respectively transferred to the material W 3 , but also the middle mold  35  invades into the recessed portion  5 a of the material W 3  to thereby mold a recessed portion  5 b, that is, a portion of the inside diameter  5  with a residual wall  39  left between the lower end side recessed portion and the recessed portion  5 b. Thanks to this, as shown in the right half section of  FIG. 4 , the material W 3  is molded into a material W 4  having a shape approximate to the final shape of the disk  1 . By the way, in the third step, at the time when the molding is completed, there is formed a clearance C between the upper mold  31  and outer mold  38  to thereby allow a burr S to be produced on the outside diameter surface of the large diameter end portion  3 . That is, the production of the burr S can avoid a tightly closed forging operation to thereby prevent an unnecessary molding load from increasing, which makes it possible to improve the lives of the molds used. 
     In the mold forged product W 4  obtained in the above manner, in a post-step, from a state shown by a two-dot chained line in  FIG. 5 , the burr S is trimmed and removed by a press and the residual wall  39  of the inside diameter surface  5  is removed by a press; and, after then, the whole surface of the mold forged product W 4  is ground so that it is molded into the disk  1  having the final shape shown by a solid line in FIG.  5 . And, after the disk  1  is molded in this manner, the disk  1  is carburized or carbonitrided, that is, the disk  1  is heat treated and further the heat treated disk  1  is ground in such a manner as to have a required precision, before the disk  1  is incorporated into a toroidal type continuously variable transmission. 
     In the present disk manufacturing method, since the mold forging operation is carried out in three steps using three kinds of molds, the contact time between the molds and material is shortened to thereby be able to reduce the heat influence on the molds in the molding operation. As a result of this, the mold surface hardness can be maintained in a good level, which in turn makes it possible to improve the lives of the molds. 
     Also, because the swaging amount of the cylindrical-shaped material W 1  is set larger than the ordinary swaging amount in the first step, not only in the second step but also in the third step which requires a high molding load in order to obtain a state close to the shape of the product, the molding amount can be reduced, with the result that the working burdens of not only the forging molds  21  and  22  in the second step but also the forging molds  31  and  32  in the third step can be reduced, thereby being able to extend the lives of these forging molds. 
     Further, since the swaging amount of the cylindrical-shaped material W 1  is set larger than the ordinary swaging amount in the first step, the push-in degree of the middle mold  25  in the second step as well as the push-in degree of the middle mold  35  in the third step can be reduced, which makes it possible to enhance greatly the tool lives of the middle molds  25  and  35  which can be most susceptible to the heat influence of the material. 
     Still further, due to the fact that the material is positioned at the molding centers of the molds accurately and positively in the respective steps including the first to third steps, the material can be always molded at a correct position to thereby obtain a mold forged product of high precision. 
     Yet further, since the mold forging operation is carried out in three steps using three kinds of molds, the material flow in the forging operation can be set freely. As a result of this, if the shape that corresponds with the final shape of the disk is set in the pre-steps (first and second steps), then a well-balanced mold forged product can be obtained. 
     In addition, because the disk  1  is obtained by grinding the mold forged product W 4 , even if the forged product before it is ground is a rough forged product (for example, a hot-forged product and the like), the present disk manufacturing method is surely able to deal with such forged product. Also, since it is little necessary to pay attention to the abrasion of the molds (that is, since the molds can be used even if they are abraded to some degree), the costs of the molds can be reduced in the long run. 
     Next, description will be given below of a disk for a toroidal type continuously variable transmission, which is a second embodiment according to the invention. 
     As shown in  FIG. 6 , the present disk (finished product)  51  includes a small diameter end portion  2 , a large diameter end portion  3 , and a traction surface  4  which is interposed between the small diameter end portion  2  and large diameter end portion  3  and has a concave-arc shaped cross section; and, in the central portion of the end face of the disk on the small diameter end portion  2  side thereof, there is formed a through hole which extends through the disk  51  up to the back surface of the large diameter end portion  3 , while the inner peripheral surface of the through hole is used as an inside diameter surface  5  of the disk. Here, with reference to  FIGS. 6 and 14 , among the metal flows  6  that exist in the disk  51 , a metal flow  6 , which has such a positional relationship with respect to the surface of the disk  51  that the metal flow  6  on the traction surface  4  side has an angle θ of 2-30 degrees, preferably, 5-20 degrees with respect to a tangent P of the traction surface  4 , is defined as a “metal flow  6  along the disk surface”. In the disk  51 , “metal flow  6  along the disk surface” each having the angle θ=2-30 degrees exist continuously in the end face of the disk  51  on the small diameter end portion  2  side thereof, in the traction surface  4 , in the outside diameter surface of the large diameter end portion  3 , and in the back surface of the large diameter end portion  3 ; and, a “metal flow  6  along the disk surface” having the angle θ=2-30 degrees exists in the area of the inside diameter surface  5  ranging from the end face of the inside diameter surface  5  on the small diameter end portion  2  side thereof to the range of ⅓ A, where the length of the disk  51  in the axial direction thereof is expressed as A. 
     By the way, a metal flow  6  having the angle θ of greater than 30 degrees with respect to the tangent P of the traction surface  4  becomes equivalent to an end flow (a metal flow which does not exist along the disk surface), so that such metal flow not only causes the material to peel off but also gives rise to the bending fatigue of the disk and thus the rupture of the disk (the lowered crack life). 
     Also, in the present disk  51 , with further reference to  FIGS. 10 and 15 , when the power roller is set horizontal (that is, parallel to the axis of the disk), that is, where the small radius of the contact ellipse between the traction surface and power roller is expressed as b when a speed change ratio is 1:1, a nonmetallic inclusions  52  of high density exists in an area which is distant by 1.5 b or longer in the depth direction from the traction surface. Further, no nonmetallic inclusions are present in an area which exists within the range of ⅓ A (A is the length of the disk  51  in the axial direction thereof) from the end face of the inside diameter surface  5  on the small diameter end portion  2  side thereof. 
     In the thus structured disk  51 , since the “metal flows  6  along the disk surface” each having the angle θ=2-30 degrees exist continuously in the traction surface  4  with which the power roller can be frictionally engaged with a large pressure, especially when the disk  51  is used under a high load condition, not only the material in the engaged portion of the traction surface  4  where it is frictionally engaged with the power roller can be prevented from peeling off, but also an impact crack or a fatigue crack is hard to occur in the disk  51  so that the long life of the disk  51  can be realized. 
     Also, because the “metal flow  6  along the disk surface” having the angle θ=2-30 degrees exists in the area of the inside diameter surface  5  ranging from the end face of the inside diameter surface  5  on the small diameter end portion  2  side thereof to the range of ⅓ A, within the portion of the inside diameter surface  5  that is relatively weak against the bending stress and the like due to the formation of the peripheral groove for the snap ring, the metal flow can be prevented from providing an end flow, and, at the same time, since the “metal flow  6  along the disk surface” having the angle θ=2-30 degrees exists in the end face of the disk  51  on the small diameter end portion  2  side thereof as well, the bending fatigue as well as the concentration of the stress on the peripheral groove can be relieved, which makes it possible to extend the life of the disk  51  further. 
     Further, the nonmetallic inclusions  52  of high density are absent not only in the area of the traction surface  4  that extends within the range of 1.5 b in the depth direction from the traction surface and receives the severest shearing stress in the traction surface, but also in the portion of the inside diameter surface  5  that extends axially within the range of ⅓ A from the small diameter end portion side end face of the inside diameter surface that can be affected severely by the bending stress or the like due to the formation of the peripheral groove for the snap ring and the like. This can avoid the ill effects of the nonmetallic inclusions on the life of the disk  51 . 
     Next, description will be given below of a method for manufacturing the disk  51  with reference to  FIGS. 7  to  9 . 
     (First step: Swaging step) 
     Now,  FIG. 7  shows a first step (a swaging step) employed in the present disk manufacturing method; and, in particular, the left half section of  FIG. 7  shows a state of a material before swaged, whereas the right half section of  FIG. 7  shows a state thereof after swaged. In the first step, a cylindrical material (carburized steel or the like) W 11  with metal flows  6  existing on the outer peripheral surface of the material and extending along the axial direction of the material is interposed between an upper mold  51  and a lower mold  152 ; and, next, the upper mold  51  is moved in the axial direction of the cylindrical-shaped material W 11  to swage the present cylindrical-shaped material W 11 , thereby molding the same into a material W 12  having such a shape as shown in the right half section of FIG.  7 . 
     In the plane portion  152 a of the lower mold  152 , there is formed a recessed portion  53  into which the lower end portion of the cylindrical-shaped material W 11  can be fitted; that is, if the lower end portion of the cylindrical-shaped material W 11  is fitted into the recessed portion  53 , then the cylindrical-shaped material W 11  can be aligned with the lower mold  152  accurately and positively. 
     On the other hand, in the central portion of the upper mold  51 , there is formed a tapered recessed portion  51 a which decreases in diameter in the upward direction thereof in such a manner as to be concentric with the recessed portion  53  of the lower mode  152 , while the bottom surface of the present tapered recessed portion  51 a is formed as a flat surface  54 . And, the peripheral edge (a boundary portion between the outer peripheral surface and upper end face of the cylindrical-shaped material W 11 ) of the upper end face of the cylindrical-shaped material W 11  is in contact with the slanting portion  55  of the tapered recessed portion  51 a. Thanks to this, when the upper mold  51  is moved down, the upper mold  51  restricts the upper end portion of the cylindrical-shaped material W 11  to thereby be able to not only align the cylindrical-shaped material W 11  with the upper mold  51  accurately and positively but also transfer the shape of the slanting portion  55 . 
     (Second step) 
     Now,  FIG. 8  shows a second step employed in the present disk manufacturing method; and, in particular, the left half section of  FIG. 8  shows a state of the material before molded, whereas the right half section of  FIG. 8  shows a state thereof after molded. In the second step, the material W 12  swaged in the first step is interposed between a lower mold  62  and an upper mold  61  mounted on an outer mold  65  and the upper mold  61  is moved in the axial direction of the material W 12  to thereby mold the material W 12  into a material W 13  which has such a shape approximate to the final shape of the disk  51  as shown in the right half section of FIG.  8 . 
     The lower mold  62  includes a large diameter end portion molding surface  63  for molding the large diameter end portion  3  of the disk  51  and, in the central portion of the large diameter end portion molding surface  63 , there is a recessed portion  64  into which a projecting portion  56  (to which the shape of the recessed portion  53  of the lower mold  52  was transferred in the first step) can be fitted. That is, if the projecting portion  56  of the material W 12  is fitted into the recessed portion  64  of the lower mold  62 , then the material W 12  is prevented from playing with respect to the lower mold  62  so that the alignment of the material W 12  with the lower mold  62  can be achieved accurately and positively. 
     The outer mold  65  includes a small diameter end portion molding surface  66  for molding the small diameter end portion  2  of the disk  51  and a traction surface molding surface  67  for molding the traction surface  4 ; and, in the central portion of the small diameter end portion molding surface  66 , there is projectingly provided the upper mold  61  having a cylindrical shape. 
     In the lower end face of the upper mold  61 , there is formed a shallow tapered recessed portion  68  which reduces gradually in diameter as it goes upwardly. The bottom surface of the tapered recessed portion  68  is formed as a flat surface  69  and the diameter of the flat surface  69  is set larger than the area diameter of nonmetallic inclusions  52  of high density which exist in the central portion of the upper end face of the material W 12  swaged in the first step. Also, since the outer peripheral surface of the upper end portion of the material W 12  is in contact with the slanting portion  70  of the tapered recessed portion  68 , the alignment of the material W 12  with the upper mold  61  can be achieved accurately and positively. Therefore, the inside diameter D 2  of the slanting portion  70  of the tapered recessed portion  68 , in particular, the inside diameter D 2  of the contact position of the slanting portion  70  with the material W 12  is larger than the diameter D 1  of the upper end face of the material W 12 . 
     And, in this state, if the outer mold  65  and upper mold  61  are moved down integrally, then not only the shapes of the small diameter end portion molding surface  66 , traction surface molding surface  67  and large diameter end portion molding surface  63  are respectively transferred to the material W 12 , but also the upper mold  61  invades into the central portion of the material W 12  to thereby mold a recessed portion  5 d, that is, a portion of the inside diameter  5  with a residual wall  71  left between the recessed portion  64  and the present recessed portion  5 d. Thanks to this, as shown in the right half section of  FIG. 8 , the material W 12  is molded into a material W 13  having a shape approximate to the final shape of the disk  51 . By the way, in the second step, at the time when the molding is completed, there is formed a clearance C between the lower mold  62  and outer mold  65  to thereby allow a burr S to be produced on the outside diameter surface of the large diameter end portion  3 . That is, the production of the burr S can avoid a tightly closed forging operation to thereby prevent an unnecessary molding load from increasing, which makes it possible to improve the lives of the molds used. 
     Also, when the upper mold  61  invades into the central portion of the material W 12 , not only the tapered recessed portion  68  of the upper mold  61  restricts the upper end portion of the material W 12  to thereby prevent the present upper end portion from increasing in diameter during molding, but also the nonmetallic inclusions  52  of high density existing in the central portion of the material W 12  are pushed into the lower end side of the material W 12  to thereby expand the present lower end side outwardly in the diameter direction of the material W 12 . 
     In the mold forged product W 13  obtained in the above manner, in a post-step, from a state shown by a two-dot chained line in  FIG. 9 , the burr S is trimmed and removed by a press and the residual wall  71  of the inside diameter surface  5  is removed by a press; and, after then, the whole surface of the mold forged product W 13  is ground so that it is molded into the disk  51  having the final shape shown by a solid line in FIG.  9 . And, after the disk  51  is molded in this manner, the disk  51  is carburized or carbonitrided, that is, the disk  51  is heat treated and further the heat treated disk  51  is ground in such a manner as to have a required precision, before the disk  51  is incorporated into a toroidal type continuously variable transmission. 
     As can be seen clearly from the above description, in the present disk manufacturing method, it is possible to obtain the disk  51  (finished product) simply and positively in which “metal flows  6  along the disk surface” each having the angle θ=2-30 degrees exist in the end face of the disk  51  on the small diameter end portion  2  side thereof, in the traction surface  4 , in the outside diameter surface of the large diameter end portion  3 , and in the back surface of the large diameter end portion  3 . 
     Also, not only since use of the mold forging method can reduce the diameter of the cylindrical-shaped material W 11  in the first step, but also since, in the second step, the tapered recessed portion  68  of the upper mold  61  restricts the upper end portion of the material W 12  to thereby prevent the present upper end portion from increasing in diameter during molding and also the nonmetallic inclusions  52  of high density existing in the central portion of the material W 12  are pushed into the lower end side of the material W 12  to thereby expand the present lower end side outwardly in the diameter direction of the material W 12 , there can be obtained the disk  51  simply and positively in which the nonmetallic inclusions  52  of high density are absent not only in the area of the traction surface  4  that extends within the range of 1.5 b in the depth direction from the traction surface and receives the severest shearing stress in the traction surface, but also in the portion of the inside diameter surface  5  that extends axially within the range of ⅓ A from the small diameter end portion side end face of the inside diameter surface that can be affected severely by the bending stress or the like due to the formation of the peripheral groove for the snap ring and the like. 
     Now, Table 1 shows the results of a disk durability test in which, when it is assumed that a load is 5 tons and a load position is the groove bottom of the traction surface, the respective disks are tested in the durability thereof by changing the angle α (see FIG.  14 ). Here, the angle α is an angle formed between the traction surface  4  and a horizontal line passing through the center of curvature O (that is, a line extending in parallel to the axis of a disk). 
     In Table 1, disks No. 1 to No. 6 are respectively embodiments according to the invention. In particular, in each of the disks No. 1 to No. 4, the “metal flow  6  along the disk surface” having the angle θ=2-30 degrees exists in the traction surface thereof. Also, among these disks, in the No. 1 and No. 2 disks, the angle is set such that α&lt;45 degrees and, in the No. 3 and No. 4 disks, the angle is set such that α≧45 degrees. And, as the No. 5 and No. 6 disks, there were used disks in which the “metal flows  6  along the disk surface” having the angle θ=2-30 degrees exist continuously in the traction surface thereof. On the other hand, as the No. 7 and No. 8 disks, there were used conventional disks which were manufactured by cutting. Except for the above-mentioned conditions, the same test conditions (such as size, material, load condition and the like) were set for all the disks in the present disk durability test. 
     By the way, the angle was adjusted to θ=2-30 degrees by previously checking a metal flow after forged and by adjusting the margin. In particular, a metal flow having the angle θ smaller than or equal to the angle α is assumed as a metal flow along the disk surface which satisfies the θ range according to the invention; and, a metal flow having the angle θ larger than the angle α is assumed as a metal flow which has an angle out of the θ range according to the invention. That is, the disks were observed for breakage under the above conditions. The forged disks were manufactured by using the above-mentioned two manufacturing methods properly. 
                                     TABLE 1               No.       α[deg.]   Test Result   Judgment                  1   Embodiment   27   Broken in 123 hrs.   Δ       2   Embodiment   37   Broken in 194 hrs.   Δ       3   Embodiment   48   Vibrated in 289 hrs.   ∘                   Crack in traction surface       4   Embodiment   50   Vibrated in 272 hrs.   ∘                   Crack in traction surface       5   Embodiment   —   Nothing wrong in 350 hrs.   ⊚       6   Embodiment   —   Nothing wrong in 350 hrs.   ⊚       7   Conventional   —   Broken in 97 hrs.   X           Example       8   Conventional   —   Broken in 63 hrs.   X           Example                    
Load: approx. 5 ton
 
Load position: Groove bottom
 
     As can be seen clearly from Table 1, the disks (No. 1 to No. 6) according to the invention are greatly enhanced in the durability of the traction surface thereof when compared with the conventional disks (No. 7 and No. 8). 
     Also, among the disks according to the invention, the disks having α≧45 degrees (No. 3 and No. 4) are enhanced in durability when compared with the disks having α&lt;45 degrees; and, in the disks (No. 5 and No. 6) in which the “metal flows  6  along the disk surface” having the angle θ=2-30 degrees exist continuously in the traction surface thereof, even after 350 hours have passed, there was found nothing wrong on the traction surface thereof, that is, it can well be said that they have the greatest durability. 
     By the way, although the present durability test was finished in 350 hrs., in the disks having θ≈0 degree and θ≈2 degrees, it is believed that nothing wrong can be found even after the passage of 350 hrs. and thus it can be expected that these disks are almost equivalent to the disks No. 5 and No. 6 in performance. 
     Now, Table 2 shows the results of a second disk durability test in which the depth of a disk ranging from the traction surface thereof to the area of the high-density nonmetallic inclusions (0.3 D portion: see  FIGS. 10 and 12 ) is caused to vary. By the way, in Table 2, b designates the small radius of the contact ellipse between the traction surface and power roller when a speed change ratio is 1:1 (see FIG.  15 ). 
     In Table 2, disks No. 1 to No. 8 are all embodiments according to the invention and, for each of the disks, there was used a disk in which the “metal flow  6  along the disk surface” having the angle θ=2-30 degrees exists in the traction surface thereof. The present durability test was conducted for all disks under the same conditions, except that the depth of a disk ranging from the traction surface thereof to the area of the high-density nonmetallic inclusions varies in the respective disks. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                 Depth of 
                   
                   
                   
               
               
                   
                 0.3 D 
                   
                   
                   
               
               
                   
                 portion 
                   
                   
                   
               
               
                   
                 from 
                   
                   
                   
               
               
                 No. 
                 surface 
                   
                 Test Result 
                 Judgment 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 1 
                 0 
                   
                 Traction surface peels 
                 Δ 
               
               
                   
                   
                   
                 off in 170 hrs. 
               
               
                 2 
                 0.25 
                 b 
                 Traction surface peels 
                 Δ 
               
               
                   
                   
                   
                 off in 173 hrs. 
               
               
                 3 
                 0.60 
                 b 
                 Traction surface peels 
                 ∘ 
               
               
                   
                   
                   
                 off in 212 hrs. 
               
               
                 4 
                 0.90 
                 b 
                 Traction surface peels 
                 ∘ 
               
               
                   
                   
                   
                 off in 208 hrs. 
               
               
                 5 
                 1.25 
                 b 
                 Traction surface peels 
                 ∘ 
               
               
                   
                   
                   
                 off in 239 hrs. 
               
               
                 6 
                 1.6 
                 b 
                 Nothing wrong in 250 hrs. 
                 ⊚ 
               
               
                 7 
                 2.55 
                 b 
                 Nothing wrong in 250 hrs. 
                 ⊚ 
               
               
                 8 
                 4.5 
                 b 
                 Nothing wrong in 250 hrs. 
                 ⊚ 
               
               
                   
               
               
                 b: Small radius of contact ellipse between traction surface and power roller when speed change ratio is 1:1  
               
            
           
         
       
     
     As can be clearly understood from Table 2, as the depth of the 0.3 D portion from the traction surface to the high-density nonmetallic inclusions area increases, the durability of the traction surface is enhanced and, especially, the depth is larger than or equal to 1.5 b, nothing wrong is found even after 250 hrs. have passed, which shows that the traction surface having the depth larger than or equal to 1.5 b is most excellent in durability. 
     Now, Table 3 shows the results of a third durability test conducted on disks which are different from each other in the existing area of the high-density nonmetallic inclusions of the disk inside diameter surface, in particular, on the durability of the inside diameter surfaces of the disks on their respective small diameter end portion sides. Here, A expresses the axial length of the disk (that is, the length of the disk in the axial direction thereof), and B expresses the axial length of the inside diameter surface from the end face of the inside diameter surface on the small diameter end portion side thereof. 
     In Table 3, No. 3 to No. 8 test pieces are disks which were manufactured according to the embodiments of the invention. In the present durability test, as the embodiments of the invention, there were used disks in each of which the “metal flow  6  along the disk surface” having the angle θ=2-30 degrees exists in the inside diameter surface thereof in the range from the small diameter end portion side end face of the inside diameter surface to the axial depth of (B/A)×100% where the axial length of the disk is expressed as A. Also, as No. 1 and No. 2 disks, there were used conventional disks which were manufactured by cutting. That is, in the present durability test, all disks were tested under the same conditions, except that the disks differ from each other in the existing area of the high-density nonmetallic inclusions of the disk inside diameter surface. By the way, in the disks No. 3 to No. 8, the relationship between the nonmetallic inclusions and their respective existing area (B/A)×100% were adjusted by adjusting the margins of the respective disks when they were manufactured. In this case as well, as the test disks, there were properly used forged disks which were manufactured according to the above-mentioned two manufacturing methods. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                   
                   
                   
                 (B/A) × 
                   
                   
               
               
                   
                   
                 Working 
                 100 
                   
                 Judg- 
               
               
                 No. 
                   
                 Method 
                 [%] 
                 Test Result 
                 ment 
               
               
                   
               
             
            
               
                 1 
                 Conventional 
                 Cutting 
                 — 
                 Inside diameter portion 
                 X 
               
               
                   
                 Example 
                   
                   
                 on small diameter side 
               
               
                   
                   
                   
                   
                 was broken in 68 hrs. 
               
               
                 2 
                 Conventional 
                 Cutting 
                 — 
                 Inside diameter portion 
                 X 
               
               
                   
                 Example 
                   
                   
                 on small diameter side 
               
               
                   
                   
                   
                   
                 was broken in 59 hrs. 
               
               
                 3 
                 Embodiment 
                 Forging 
                 15 
                 Inside diameter portion 
                 Δ 
               
               
                   
                   
                   
                   
                 on small diameter side 
               
               
                   
                   
                   
                   
                 was broken 171 hrs. 
               
               
                 4 
                 Embodiment 
                 Forging 
                 22 
                 Inside diameter portion 
                 ∘ 
               
               
                   
                   
                   
                   
                 on small diameter side 
               
               
                   
                   
                   
                   
                 was broken 211 hrs. 
               
               
                 5 
                 Embodiment 
                 Forging 
                 34 
                 No problem after 
                 ⊚ 
               
               
                   
                   
                   
                   
                 passage of 250 hrs. 
               
               
                 6 
                 Embodiment 
                 Forging 
                 41 
                 No problem after 
                 ⊚ 
               
               
                   
                   
                   
                   
                 passage on 250 hrs. 
               
               
                 7 
                 Conventional 
                 Forging 
                 53 
                 No problem after 
                 ⊚ 
               
               
                   
                 Example 
                   
                   
                 passage of 250 hrs. 
               
               
                 8 
                 Conventional 
                 Forging 
                 51 
                 No problem after 
                 ⊚ 
               
               
                   
                 Example 
                   
                   
                 passage of 250 hrs. 
               
               
                   
               
            
           
         
       
     
     As can be seen obviously from Table 3, the disks (No. 3 to No. 8) according to the invention are greatly improved in the durability of the inside diameter surface on the small diameter end portion side thereof when compared with the conventional disks (No. 1 and No. 2). 
     Also, out of the disks according to the invention, in the disks in which the high-density nonmetallic inclusions exist in the inside diameter surface over the ((B/A)×100%) area of more than 33%, nothing wrong was found in the small diameter end portion side inside diameter surface thereof even after the passage of 250 hrs., which shows that these disks are most excellent in durability. 
     As can be clearly understood from the foregoing description according to the invention, there can be provided a toroidal type continuously variable transmission disk which not only permits the reduction of the production cost thereof but also can extend the life thereof. 
     Although the invention has been described in its preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form can be changed in the details of construction and in the combination and arrangement of parts without departing from the spirit and the scope of the invention as hereinafter claimed.