Patent Publication Number: US-2020284289-A1

Title: Method for manufacturing spline telescopic shaft and spline telescopic shaft

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Japanese Patent Application No. 2019-042509 filed on Mar. 8, 2019, incorporated herein by reference in its entirety. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a method for manufacturing a spline telescopic shaft, and to a spline telescopic shaft. 
     2. Description of Related Art 
     Japanese Unexamined Patent Application Publication No. 2014-238173 (JP 2014-238173 A) discloses a spline telescopic shaft mounted on a vehicle steering system. The spline telescopic shaft is provided by spline-fitting an inner shaft and a tubular outer shaft so that the inner shaft and the outer shaft are slidable along an axial direction and a torque is transmissible therebetween. The outer peripheral surface of the inner shaft is coated with a resin by fluidized-bed coating to provide a resin layer. The resin layer is provided with a spline to be fitted to a spline formed on the inner peripheral surface of the outer shaft. The resin layer may be provided with recesses formed by laser beam machining to use the recesses as lubricant reservoirs. 
     SUMMARY 
     Various metals may be used as materials for forming the inner shaft. Depending on the metals, the strength may decrease due to influence of heat generated during the fluidized bed coating. In view of the circumstances, the resin layer may be formed by injection molding. When the resin layer is formed by injection molding, the resin layer may detach from the inner shaft due to low adhesion to the inner shaft. Even if the recesses serving as lubricant reservoirs are formed in this resin layer by laser beam machining, the resin layer including the recesses may peel off due to the low adhesion. Similar problems may arise in a case where the resin layer is provided on the outer shaft. 
     The present disclosure provides a method for manufacturing a spline telescopic shaft, and a spline telescopic shaft in which lubricant reservoirs are easily formed while maintaining retention of a resin layer on an inner shaft or an outer shaft. 
     A method for manufacturing a spline telescopic shaft according to a first aspect of the present disclosure is a method for manufacturing a spline telescopic shaft including an inner shaft provided with an external spline on an outer peripheral surface of the inner shaft, and an outer shaft provided with an internal spline on an inner peripheral surface of the outer shaft and configured such that the external spline of the inner shaft slides in an axial direction relative to the internal spline. The method includes a toothless portion forming step, a resin layer forming step, and a cooling step. The toothless portion forming step is a step of forming a toothless portion in one of an external tooth provided on an outer peripheral surface of a shaft body included in the inner shaft to extend in the axial direction and an internal tooth provided on an inner peripheral surface of a tubular body included in the outer shaft to extend in the axial direction. The resin layer forming step is a step of forming a resin layer, which covers one of the external tooth and the internal tooth and fills the toothless portion, by arranging one of the shaft body including the toothless portion and the tubular body including the toothless portion in a mold including a cavity and injecting a resin into the cavity. The cavity is included in a flat receiving surface that faces tooth flanks of one of the external tooth of the shaft body including the toothless portion and the internal tooth of the tubular body including the toothless portion. The cooling step is a step of cooling one of the shaft body including the resin layer and the tubular body including the resin layer to form a lubricant reservoir such that a portion of the resin layer corresponding to the toothless portion is recessed as compared to other portions. 
     A spline telescopic shaft according to a second aspect of the present disclosure includes an inner shaft provided with an external spline on an outer peripheral surface of the inner shaft, and an outer shaft provided with an internal spline on an inner peripheral surface of the outer shaft and configured such that the external spline of the inner shaft slides relative to the internal spline. The inner shaft includes a shaft body including an external tooth serving as at least a part of the external spline on the outer peripheral surface. The outer shaft includes a tubular body including an internal tooth serving as at least a part of the internal spline on the inner peripheral surface. One of the shaft body and the tubular body includes a resin layer, which covers one of the external tooth and the internal tooth. A toothless portion filled with the resin layer is provided in one of the external tooth covered with the resin layer and the internal tooth covered with the resin layer. A portion of the resin layer corresponding to the toothless portion is a lubricant reservoir recessed as compared to other portions. 
     According to the present disclosure, it is possible to provide the method for manufacturing a spline telescopic shaft, and the spline telescopic shaft in which the lubricant reservoirs are easily formed while maintaining the retention of the resin layer on the inner shaft or the outer shaft. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, advantages, and technical and industrial significance of exemplary embodiments of the disclosure will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein: 
         FIG. 1  is a schematic structural diagram of a vehicle steering system including an intermediate shaft to which a spline telescopic shaft according to an embodiment is applied; 
         FIG. 2  is a partially cutaway side view of the intermediate shaft according to the embodiment; 
         FIG. 3  is a sectional view illustrating the sectional profile of a part of the intermediate shaft according to the embodiment; 
         FIG. 4  is a perspective view illustrating an inner shaft according to the embodiment; 
         FIG. 5  is a plan of the inner shaft according to the embodiment that is viewed in an axial direction; 
         FIG. 6  is a side view illustrating the inner shaft according to the embodiment; 
         FIG. 7  is a flowchart illustrating a flow of a method for manufacturing the intermediate shaft according to the embodiment; 
         FIG. 8  is a sectional view illustrating the condition of a shaft body in a resin layer forming step according to the embodiment; 
         FIG. 9  is a sectional view illustrating a cutting plane including a line IX-IX in  FIG. 8 , illustrating the condition of the shaft body in the resin layer forming step according to the embodiment; 
         FIG. 10  is a perspective view illustrating a toothless portion of a shaft body according to Modified Example 1; 
         FIG. 11  is a sectional view illustrating the toothless portion and its surrounding structure according to Modified Example 1; 
         FIG. 12  is a perspective view schematically illustrating an inner shaft according to Modified Example 2; 
         FIG. 13  is a sectional view illustrating the sectional profile of a part of an intermediate shaft according to Modified Example 3; and 
         FIG. 14  is a sectional view illustrating a cutting plane including a line XIV-XIV in  FIG. 13 . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     An embodiment is described below in detail with reference to the drawings. The embodiment described below demonstrates a comprehensive or specific example. Numerical values, shapes, materials, constituent elements, arrangement positions and connection forms of the constituent elements, and the like described in the following embodiment are examples, and are not intended to limit the present disclosure. Among the constituent elements of the following embodiment, constituent elements that are not described in independent claims that demonstrate the broadest concept are described as optional constituent elements. 
     The drawings are schematic drawings in which elements are emphasized, omitted, or adjusted in their proportions as appropriate to demonstrate the present disclosure. Shapes, positional relationships, or proportions may differ from actual shapes, positional relationships, or proportions. 
       FIG. 1  is a schematic structural diagram of a vehicle steering system including an intermediate shaft to which a spline telescopic shaft according to the embodiment is applied. As illustrated in  FIG. 1 , a vehicle steering system  1  includes a steering shaft  3 , an intermediate shaft  5 , a pinion shaft  7 , and a rack shaft  8 . The steering shaft  3  is coupled to a steering member  2  such as a steering wheel. The intermediate shaft  5  serves as the spline telescopic shaft coupled to the steering shaft  3  through a universal joint  4 . The pinion shaft  7  is coupled to the intermediate shaft  5  through a universal joint  6 . The rack shaft  8  serves as a steering operation shaft including a rack  8   a  that meshes with a pinion  7   a  provided near the end of the pinion shaft  7 . 
     A steering operation mechanism A 1  is constructed of a rack and pinion mechanism including the pinion shaft  7  and the rack shaft  8 . The rack shaft  8  is supported by a housing (not illustrated) so as to be movable in an axial direction along a lateral direction of a vehicle (direction along the drawing sheet). Although illustration is omitted, the ends of the rack shaft  8  are coupled to corresponding steered wheels through corresponding tie rods and corresponding knuckle arms. 
     The steering shaft  3  includes an upper shaft  13  and a lower shaft  14  fitted together by spline coupling so as to be rotatable together and slidable relative to each other in an axial direction. One of the upper shaft  13  and the lower shaft  14  is an inner shaft, and the other is a tubular outer shaft. The upper shaft  13  and the lower shaft  14  are supported on a vehicle body through a steering column  20 . 
       FIG. 2  is a partially cutaway side view of the intermediate shaft  5  according to the embodiment.  FIG. 3  is a sectional view illustrating the sectional profile of a part of the intermediate shaft  5  according to the embodiment. Specifically,  FIG. 3  is a sectional view illustrating a cutting plane including a line III-III in  FIG. 2 . 
     As illustrated in  FIG. 1  to  FIG. 3 , the intermediate shaft  5  serving as the spline telescopic shaft is formed by spline-fitting an inner shaft  35  and a tubular outer shaft  36  so that the inner shaft  35  and the outer shaft  36  are slidable along an axial direction X 1  and a torque is transmissible therebetween. One of the inner shaft  35  and the outer shaft  36  is an upper shaft, and the other is a lower shaft. In this embodiment, the outer shaft  36  is coupled to the universal joint  4  as the upper shaft, and the inner shaft  35  is coupled to the universal joint  6  as the lower shaft. 
     In this embodiment, description is given of a case where the spline telescopic shaft is applied to the intermediate shaft  5 . The spline telescopic shaft of the present disclosure may be applied to the steering shaft  3 , and the steering shaft  3  may exert a telescopic adjustment function and a shock absorbing function. In this embodiment, description is given of a case where the vehicle steering system  1  is a manual steering system. The spline telescopic shaft of the present disclosure may be applied to an electric or hydraulic power steering system. 
     An outer peripheral surface  35   a  of the inner shaft  35  is provided with an external spline  37 . An inner peripheral surface  36   a  of the outer shaft  36  is provided with an internal spline  38 . The external spline  37  and the internal spline  38  are slidable while being fitted together. The inner shaft  35  and the outer shaft  36  move relative to each other, such that the entire intermediate shaft  5  extends or contracts. 
     Next, the inner shaft  35  is described in detail. 
       FIG. 4  is a perspective view illustrating the inner shaft  35  according to the embodiment.  FIG. 5  is a plan view of the inner shaft  35  according to the embodiment that is viewed in the axial direction.  FIG. 6  is a side view illustrating the inner shaft  35  according to the embodiment. In  FIG. 4  to  FIG. 6 , a resin layer  50  provided on the inner shaft  35  is partially omitted. In actuality, the resin layer  50  continuously covers the outer peripheral surface of a shaft body  40  so as to cover the entire circumference of the shaft body  40 . 
     As illustrated in  FIG. 4  to  FIG. 6 , the inner shaft  35  includes the shaft body  40  and the resin layer  50 . The shaft body  40  is an elongated member extending along the axial direction X 1 . The shaft body  40  is made of a metal that is relatively light in terms of specific gravity. Specifically, the shaft body  40  is integrally formed by using aluminum or an aluminum alloy. A plurality of external teeth  41  are provided on the outer peripheral surface of one end of the shaft body  40 . The external teeth  41  are radially provided about an axis center of the shaft body  40 . The number of external teeth  41  to be provided in a circumferential direction may be two or more, but may be four or more from the viewpoint of achieving stable torque transmission. 
     The external teeth  41  extend along the axial direction X 1 . Thus, a plurality of tooth spaces  43  each provided between the external teeth  41  in the circumferential direction also extend along the axial direction X 1 . 
     A plurality of toothless portions  44 ,  45 , and  46  are provided in tip surfaces  42  of the external teeth  41 . The toothless portion  44  is arranged closest to the distal end of the shaft body  40 . The toothless portion  46  is arranged closest to the other end of the shaft body  40 . The toothless portion  45  is arranged between the toothless portion  44  and the toothless portion  46 . The lengths of the toothless portion  44  and the toothless portion  46  in the axial direction X 1  are equal. The length of the toothless portion  45  in the axial direction X 1  is larger than the length of the toothless portion  44  in the axial direction X 1 . The toothless portions  44 ,  45 , and  46  are formed by cutting the external teeth  41  in a range from the tip surfaces  42  of the external teeth  41  to the bottom lands of the tooth spaces  43 . That is, the bottom surfaces of the toothless portions  44 ,  45 , and  46  are flush with the bottom lands of the tooth spaces  43 . 
     The toothless portions  44  of the external teeth  41  are arrayed in line over the entire circumference of the shaft body  40 . The toothless portions  45  of the external teeth  41  are arrayed in line over the entire circumference of the shaft body  40  at positions different from those of the toothless portions  44  in the axial direction X 1 . The toothless portions  46  of the external teeth  41  are arrayed in line over the entire circumference of the shaft body  40  at positions different from those of the toothless portions  44  and the toothless portions  45  in the axial direction X 1 . 
     The resin layer  50  is laminated on the external teeth  41  and the tooth spaces  43  at a substantially uniform thickness. The tip end faces of the resin layer  50  corresponding to the external teeth  41  are referred to as tip surfaces  59 . The resin layer  50  fills the toothless portions  44 ,  45 , and  46  provided in the external teeth  41 . With the resin layer  50 , the outer profile of the entire external spline  37  is substantially uniform along the axial direction X 1 . That is, each tip surface  59  of the resin layer  50  has a uniform profile along the axial direction X 1 . Portions of the resin layer  50  that fill the toothless portions  44 ,  45 , and  46  are referred to as filling portions  51 . The thickness of the filling portion  51  is larger than those of portions of the resin layer  50  that are laminated on the external teeth  41  and the tooth spaces  43 . Since the filling portions  51  fill the toothless portions  44 ,  45 , and  46 , the filling portions  51  are caught on the external teeth  41  in the toothless portions  44 ,  45 , and  46  even if the resin layer  50  may move in the axial direction X 1  relative to the shaft body  40 . Thus, the movement of the resin layer  50  in the axial direction X 1  is restricted, and the resin layer  50  is unlikely to detach from the shaft body  40 . 
     As illustrated in  FIG. 6 , the outer surfaces of the filling portions  51  are provided with lubricant reservoirs  52  recessed as compared to other portions. Specifically, the lubricant reservoirs  52  are provided at positions corresponding to those of the toothless portions  44 ,  45 , and  46 . That is, the lubricant reservoirs  52  are arrayed in a plurality of lines corresponding to the toothless portions  44 ,  45 , and  46  over the entire circumference of the shaft body  40 . Thus, the lubricant reservoirs  52  are successively provided in the tooth flanks of the external spline  37 . For example, a lubricant is stored in the lubricant reservoir  52 . The stored lubricant can increase the slidability of the internal spline  38  relative to the external spline  37 . The lubricant includes a liquid lubricant such as lubricating oil, a semi-solid lubricant such as grease, and a solid lubricant composed of a soft metal such as molybdenum disulfide. 
     As illustrated in  FIG. 3 , the tooth flanks of the external spline  37  mesh with the tooth flanks of the internal spline  38 . The slidability of the internal spline  38  relative to the external spline  37  can be increased if the lubricant enters the meshing portions. As described above, the lubricant reservoirs  52  are successively provided in the tooth flanks of the external spline  37 . Therefore, the lubricant can enter the meshing portions. 
     Next, description is given of a method for manufacturing the intermediate shaft  5  serving as the spline telescopic shaft.  FIG. 7  is a flowchart illustrating a flow of the method for manufacturing the intermediate shaft  5  according to the embodiment. 
     In a tooth forming step S 1 , the shaft body  40  is formed by forming the external teeth  41  on a metal round bar as illustrated in  FIG. 5 . Specifically, the external teeth  41  are formed on the outer peripheral surface of the round bar by drawing the round bar. 
     In a toothless portion forming step S 2 , the toothless portions  44 ,  45 , and  46  are formed by, for example, cutting the tip surfaces  42  of the external teeth  41  of the shaft body  40  formed in the tooth forming step. 
     In a resin layer forming step S 3 , the resin layer  50  is formed by injection molding of the shaft body  40  using a resin.  FIG. 8  and  FIG. 9  are sectional views illustrating the condition of the shaft body  40  in the resin layer forming step S 3  according to the embodiment.  FIG. 9  is a sectional view illustrating a cutting plane including a line IX-IX in  FIG. 8 . As illustrated in  FIG. 8  and  FIG. 9 , in the resin layer forming step S 3 , one end of the shaft body  40  is housed in a cavity  91  of a mold  90 . The cavity  91  of the mold  90  has a surface profile conforming to the outer profile of the resin layer  50 . In the cavity  91 , each receiving surface  92  for the tip surface  42  of the shaft body  40  has a flat profile. 
     The receiving surface  92  faces the tip surface  42  at a predetermined distance. The receiving surface  92  is a surface for molding the tip surface  59  of the resin layer  50 . The receiving surface  92  has a uniform profile along the axial direction X 1 . For example, if the tip surface  59  of the resin layer  50  has a linear profile when viewed in the axial direction, the receiving surface  92  also has a linear profile when viewed in the axial direction. The profile is uniform along the axial direction X 1 . If the tip surface  59  of the resin layer  50  has a curved profile when viewed in the axial direction, the receiving surface  92  also has a curved profile when viewed in the axial direction. The profile is uniform along the axial direction X 1 . That is, the flat profile of the receiving surface  92  means that the entire receiving surface  92  has a uniform profile along the axial direction X 1 . 
     In the resin layer forming step S 3 , when one end of the shaft body  40  is arranged in the cavity  91  of the mold  90 , the resin is injected into the cavity  91  for injection molding. Thus, the resin layer  50  that covers the external teeth  41  and the tooth spaces  43  is formed. At this time, the filling portions  51  are formed by filling the toothless portions  44 ,  45 , and  46  with the resin. 
     In a cooling step S 4  illustrated in  FIG. 7 , the shaft body  40  including the resin layer  50  is cooled. The shaft body  40  may be cooled in still air or by using a cooler. The resin layer  50  is cured by the cooling. The resin layer  50  shrinks during the cooling. The thickness of the filling portion  51  of the resin layer  50  is larger than those of portions of the resin layer  50  that are laminated on the external teeth  41 . That is, the shrinkage of the filling portion  51  is larger than those of the portions laminated on the external teeth  41 . Therefore, the outer surface of the filling portion  51  is recessed as compared to the surrounding portions. The recess serves as the lubricant reservoir  52  (see  FIG. 6 ). When the resin layer  50  is cured, the shaft body  40  is released from the mold  90 . The shaft body  40  including the resin layer  50  serves as the inner shaft  35  including the external spline  37 . 
     In an assembling step S 5 , the inner shaft  35  is attached to the outer shaft  36  including the internal spline  38  provided on the inner peripheral surface. Specifically, lubricating oil is applied to at least one of the outer surface of the external spline  37  and the inner surface of the internal spline  38 . Then, the inner shaft  35  is attached to the outer shaft  36  by inserting and fitting the external spline  37  of the inner shaft  35  into the internal spline  38  of the outer shaft  36 . After the attachment, the lubricating oil is located between the external spline  37  and the internal spline  38 . In particular, the lubricating oil is stored in the lubricant reservoirs  52  of the external spline  37 . Therefore, the relative slidability between the external spline  37  and the internal spline  38  can be maintained stably over a long period. Thus, the intermediate shaft  5  is completed. 
     As described above, a part of the resin layer  50  formed by injection molding fills the toothless portions  44 ,  45 , and  46  provided in the external teeth  41  of the shaft body  40 . Since the filling portions  51  of the resin layer  50  fill the toothless portions  44 ,  45 , and  46 , the filling portions  51  are caught on the external teeth  41  in the toothless portions  44 ,  45 , and  46  even if the resin layer  50  may move in the axial direction X 1  relative to the shaft body  40 . Thus, the movement of the resin layer  50  in the axial direction X 1  is restricted. Accordingly, retention on the shaft body  40  can be maintained even in the case of the resin layer  50  formed by injection molding. 
     From the viewpoint of manufacture, portions of the resin layer  50  corresponding to the toothless portions  44 ,  45 , and  46  (outer surfaces of the filling portions  51 ) are recessed as compared to other portions when cured. The recesses serve as the lubricant reservoirs  52 . That is, the lubricant reservoirs  52  can definitely be formed during the curing without cutting or melting the resin layer  50 . Therefore, the lubricant reservoirs  52  can be formed without performing additional work (machining). 
     The lubricant reservoirs may be formed such that protrusions for forming the lubricant reservoirs be provided in the cavity of the mold. In this case, the protrusions are obstacles that make it difficult to pull out the shaft body from the mold in the axial direction after the resin layer is cured. Therefore, it is necessary to prepare the mold so that the mold is splittable into many parts. This case is not preferable because the mold is complicated. This embodiment is advantageous in that the lubricant reservoirs  52  can be formed without providing the protrusions for the lubricant reservoirs in the cavity of the mold. 
     The shaft body  40  is made of aluminum or an aluminum alloy. 
     Aluminum or an aluminum alloy is light in weight. By employing aluminum or an aluminum alloy for the shaft body  40 , the weight of the spline telescopic shaft can be reduced. However, the melting point of aluminum or an aluminum alloy is relatively low. If aluminum or an aluminum alloy is employed for the shaft body  40  and the resin layer is laminated by fluidized-bed coating, the strength is likely to decrease due to influence of heat generated during the fluidized-bed coating. With the method for manufacturing the spline telescopic shaft and with the spline telescopic shaft, the decrease in the strength of the shaft body  40  made of aluminum or an aluminum alloy can be suppressed because the resin layer  50  is formed by injection molding. 
     The toothless portions  44 ,  45 , and  46  are provided in all the tip surfaces  42  of the external teeth  41  so as to be arrayed over the entire circumference of the shaft body  40 . The lubricant reservoirs  52  are provided at portions of the resin layer  50  corresponding to all the toothless portions  44 ,  45 , and  46 . 
     Since the lubricant reservoirs  52  are provided at portions corresponding to all the toothless portions  44 ,  45 , and  46  arrayed over the entire circumference of the shaft body  40 , the lubricating oil stored in the lubricant reservoirs  52  can be distributed over the entire circumference of the shaft body  40 . Thus, the relative slidability between the external spline  37  and the internal spline  38  can further be stabilized. 
     The toothless portions  44 ,  45 , and  46  are provided in all the tip surfaces  42  of the external teeth  41  at a plurality of positions spaced away from each other in the axial direction X 1  of the shaft body  40  so as to be arrayed over the entire circumference of the shaft body  40 . 
     Since the lubricant reservoirs  52  are provided at portions corresponding to all the toothless portions  44 ,  45 , and  46  arrayed in a plurality of lines over the entire circumference of the shaft body  40 , the lubricating oil can be retained at a plurality of positions in the axial direction X 1 . Thus, the relative slidability between the external spline  37  and the internal spline  38  can further be stabilized. 
     Modified Example 1 
     In the embodiment described above, description is given of the exemplary case where the toothless portions  44 ,  45 , and  46  are formed by cutting. Any method may be employed as the method for forming the toothless portions. In Modified Example 1, description is given of a case where the toothless portions are formed by pressing. In the following description, the same parts as those of the embodiment described above may be represented by the same reference symbols to omit their description. 
       FIG. 10  is a perspective view illustrating a toothless portion  44   a  of a shaft body  40   a  according to Modified Example 1.  FIG. 11  is a sectional view illustrating the toothless portion  44   a  and its surrounding structure according to Modified Example 1. 
     As illustrated in  FIG. 10  and  FIG. 11 , the toothless portion  44   a  provided in the shaft body  40   a  according to Modified Example 1 is formed by pressing each external tooth  41   a . Therefore, a part of the external tooth  41   a  is plastically deformed. Thus, a bottom  47   a  of the toothless portion  44   a  laterally protrudes from the tooth flanks of the external tooth  41   a . Those portions are referred to as protruding portions  49   a . In Modified Example 1, description is given of an exemplary case where the protruding portions  49   a  protrude from the respective tooth flanks of the external tooth  41   a . The protruding portion  49   a  may protrude from one tooth flank alone. The protrusion amount of the protruding portion  49   a  is smaller than the thickness of a resin layer  50   a . Thus, the entire protruding portion  49   a  is embedded in the resin layer  50   a.    
       FIG. 11  illustrates a state in which the external tooth  41   a  meshes with the internal spline  38 . A long dashed double-short dashed line in  FIG. 11  represents the outer profile of the external tooth  41   a  at a portion that is not pressed. At this portion, the resin layer  50   a  is evenly laminated. The resin layer  50   a  overlaps the protruding portions  49   a  when viewed in the axial direction (portions represented by dashed-line hatching in  FIG. 11 ). Thus, the resin layer  50   a  is caught on the protruding portions  49   a , and the movement of the resin layer  50   a  in the axial direction X 1  is restricted. 
     A lubricant reservoir  52   a  recessed as compared to other portions is formed at a portion of the resin layer  50   a  (filling portion  51   a ) corresponding to the toothless portion  44   a . In  FIG. 11 , dot hatching represents a state in which lubricating oil G is stored in the lubricant reservoir  52   a . Although illustration is omitted in  FIG. 11 , lubricating oil is also applied to the surface of the resin layer  50   a  and the surface of the internal spline  38  in actuality. 
     As illustrated in  FIG. 11 , the position of a bottom surface  48   a  of the bottom  47   a  may be substantially the same position as those of tip surfaces  38   a  of the internal spline  38 . Thus, the lubricant reservoir  52   a  to be formed by the presence of the toothless portion  44   a  can be formed to have a size corresponding to those of the tooth flanks where the inner shaft  35  and the outer shaft  36  slide in contact with each other. Accordingly, the lubricating oil G stored in the lubricant reservoir  52   a  can securely be distributed over the tooth flanks of the internal spline  38 . 
     In the toothless portion forming step S 2  for forming the toothless portion  44   a , a shaft body  40   a  including external teeth  41   a  with no toothless portions  44   a  is prepared first. The toothless portions  44   a  are formed by, for example, pressing tip surfaces  42   a  of the external teeth  41   a  of the shaft body  40   a . At this time, the tip surface  42   a  of each external tooth  41   a  is plastically deformed. Thus, a part of the bottom  47   a  of the toothless portion  44   a  laterally protrudes from the tooth flanks of the external tooth  41   a  as the protruding portions  49   a.    
     The part of the bottom  47   a  of the toothless portion  44   a  is caught on the resin layer  50   a  as the protruding portions  49   a . Thus, the movement of the resin layer  50   a  in the axial direction X 1  can be restricted. Accordingly, retention on the shaft body  40   a  can be maintained more securely even in the case of the resin layer  50   a  formed by injection molding. 
     Modified Example 2 
     In the embodiment described above, description is given of the exemplary case where the toothless portions  44 ,  45 , and  46  are provided in all the tip surfaces  42  of the external teeth  41  so as to be arrayed over the entire circumference of the shaft body. In Modified Example 2, description is given of a case where the toothless portions are provided in at least two tip surfaces of the external teeth. 
       FIG. 12  is a perspective view schematically illustrating an inner shaft  35   b  according to Modified Example 2. In  FIG. 12 , toothless portions  44   b  and lubricant reservoirs  52   b  are schematically represented by long dashed double-short dashed lines so that their arrangement positions coincide with each other. As illustrated in  FIG. 12 , the toothless portions  44   b  and the lubricant reservoirs  52   b  are provided in a plurality of lines along a circumferential direction of a shaft body  40   b . Specifically, the toothless portions  44   b  are provided in at least two tip surfaces  42   b  of external teeth  41   b  at a plurality of positions spaced away from each other in the axial direction X 1  of the shaft body  40   b  so as to be arrayed in the circumferential direction. The lubricant reservoirs  52   b  are provided at portions of a resin layer  50   b  corresponding to all the toothless portions  44   b.    
     In  FIG. 12 , each array of the toothless portions  44   b  and the lubricant reservoirs  52   b  is provided in an area corresponding to a semiperimeter of the shaft body  40   b . If a first array is an odd-numbered array and a second array is an even-numbered array from the distal end of the shaft body  40   b  in the axial direction X 1 , the second array is located so as not to overlap the first array in the axial direction X 1 . 
     As described above, the toothless portions  44   b  are provided in at least two tip surfaces  42   b  of the external teeth  41   b  at a plurality of positions spaced away from each other in the axial direction X 1  of the shaft body  40   b  so as to be arrayed in the circumferential direction. With this structure, the number of processes for forming the toothless portions can be reduced as compared to the case where the toothless portions are formed over the entire circumference. If the first arrays and the second arrays are alternately provided as described above, lubricating oil stored in the lubricant reservoirs  52   b  can be distributed over the entire circumference of the shaft body  40   b.    
     Modified Example 3 
     In the embodiment described above, description is given of the exemplary case where the resin layer  50  is provided on the shaft body  40  of the inner shaft  35 . The resin layer may be provided on the outer shaft. In Modified Example 3, description is given of a case where the resin layer is provided on a tubular body of the outer shaft. 
       FIG. 13  is a sectional view illustrating the sectional profile of a part of an intermediate shaft  5 C according to Modified Example 3. Specifically,  FIG. 13  corresponds to  FIG. 3 . As illustrated in  FIG. 13 , an inner shaft  35   c  according to Modified Example 3 does not include the resin layer, and a resin layer  50   c  is provided on an outer shaft  36   c . Specifically, the outer shaft  36   c  includes a tubular body  361  and the resin layer  50   c . The tubular body  361  is a cylindrical body. A plurality of internal teeth  362  extending in the axial direction X 1  are formed on the inner peripheral surface of the tubular body  361 . The internal teeth  362  are radially provided about an axis center of the tubular body  361 . 
       FIG. 14  is a sectional view illustrating a cutting plane including a line XIV-XIV in  FIG. 13 . As illustrated in  FIG. 14 , at least one toothless portion  364  is formed in a tip surface  363  of each of the internal teeth  362 . 
     The resin layer  50   c  is laminated on an inner peripheral surface  369  of the outer shaft  36   c  so as to cover the entire inner peripheral surface  369 . The resin layer  50   c  fills the toothless portions  364  provided in the internal teeth  362 . Portions of the resin layer  50   c  that fill the toothless portions  364  are referred to as filling portions  51   c . The thickness of the filling portion  51   c  is larger than those of portions of the resin layer  50   c  that are laminated on the internal teeth  362 . Since the filling portions  51   c  fill the toothless portions  364 , the filling portions  51   c  are caught on the internal teeth  362  in the toothless portions  364  even if the resin layer  50   c  may move in the axial direction X 1  relative to the tubular body  361 . Thus, the movement of the resin layer  50   c  in the axial direction X 1  is restricted, and the resin layer  50   c  is unlikely to detach from the tubular body  361 . 
     The outer surfaces of the filling portions  51   c  are provided with lubricant reservoirs  52   c  recessed as compared to other portions. Specifically, the lubricant reservoirs  52   c  are provided at positions corresponding to those of the toothless portions  364 . For example, a lubricant is stored in the lubricant reservoir  52   c . The stored lubricant can increase the slidability of the internal spline  38  relative to the external spline  37 . 
     The outer shaft  36   c  according to Modified Example 3 can be formed through steps similar to the steps for manufacturing the inner shaft  35  described above (tooth forming step S 1 , toothless portion forming step S 2 , resin layer forming step S 3 , and cooling step S 4 ). 
     Others 
     Although the method for manufacturing the spline telescopic shaft and the spline telescopic shaft according to the present disclosure are described above based on the embodiment, the present disclosure is not limited to the embodiment. 
     For example, in the embodiment described above, description is given of the exemplary case where the shaft body  40  is made of aluminum. The shaft body  40  may be made of other metals. 
     In the embodiment described above, description is given of the exemplary case where the plurality of toothless portions  44 ,  45 , and  46  are provided in the tip surface  42  of one external tooth  41 . It is only necessary that at least one toothless portion be provided in the tip surface  42  of one external tooth  41 . In the embodiment described above, description is given of the exemplary case where the toothless portions  44 ,  45 , and  46  are provided in all the tip surfaces  42  of the external teeth  41 . It is only necessary that the toothless portions be provided in the tip surface  42  of at least one of the external teeth  41 . In any case, the lubricant reservoir is formed at a portion of the resin layer corresponding to the toothless portion. 
     In the embodiment described above, description is given of the exemplary case where the toothless portions  44 ,  45 , and  46  are formed in the shaft body  40  including the external teeth  41  formed in advance. The external teeth may be formed after recesses serving as the toothless portions are formed in a shaft body including no external teeth. 
     The present disclosure encompasses embodiments attained by various modifications conceivable by persons skilled in the art to the embodiment, and embodiments attained by arbitrarily combining the constituent elements and functions of the embodiment and the modified examples without departing from the spirit of the present disclosure. 
     The present disclosure is applicable to a spline telescopic shaft including an external spline that has a resin layer.