Patent Publication Number: US-2021179163-A1

Title: Telescopic shaft and steering system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Japanese Patent Application No. 2019-225632 filed on Dec. 13, 2019, incorporated herein by reference in its entirety. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a telescopic shaft and a steering system including a telescopic shaft. 
     2. Description of Related Art 
     Steering systems mounted on vehicles include a shaft member configured to transmit a rotational torque for steering steered wheels. The shaft member includes a telescopic shaft extensible and contractible in an axial direction. 
     For example, Japanese Unexamined Patent Application Publication No. 2009-191936 (JP 2009-191936 A) discloses a telescopic shaft including a male shaft and a female shaft. The male shaft has a non-circular outer peripheral shape. The female shaft has a non-circular inner peripheral shape, and is externally fitted to the outer periphery of the male shaft so as to be relatively movable in an axial direction and to transmit a rotational torque. In the telescopic shaft, a plurality of partial sleeves is arranged in a clearance between the non-circular inner periphery and the non-circular outer periphery. Each partial sleeve is attached to the male shaft. A groove-shaped grease reservoir is provided on the outer periphery of the partial sleeve. Thus, grease can be supplied between the outer periphery of the partial sleeve and the inner periphery of the female shaft. 
     SUMMARY 
     As in the telescopic shaft described above, it is effective, for improvement in slidability, that the telescopic shaft including the male shaft (internal shaft) and the female shaft (external shaft) has the grease reservoir in a sliding member such as the partial sleeve arranged between the outer periphery of the internal shaft and the inner periphery of the external shaft. In the related-art telescopic shaft having the grease reservoir, however, so-called grease depletion may occur because the grease leaks outward in the axial direction from the sliding member. Thus, a friction force increases between the sliding member and its sliding counterpart (for example, the inner peripheral surface of the external shaft). The increase in the friction force may cause troubles such as a backlash between the internal shaft and the external shaft, or damage to the sliding member. 
     The present disclosure provides a telescopic shaft improved in reliability, and a steering system including the telescopic shaft. 
     A first aspect of the present disclosure relates to a telescopic shaft. The telescopic shaft includes an internal shaft, an external shaft, and a sliding member. The internal shaft has an external toothing on an outer peripheral surface. The external toothing includes a plurality of teeth arrayed in a circumferential direction. The external shaft has an internal toothing on an inner peripheral surface. The internal toothing includes a plurality of teeth disposed at positions where the internal toothing meshes with the external toothing. The sliding member is disposed between the external toothing and the internal toothing, and is fixed to one of the external toothing and the internal toothing. The sliding member is disposed so as to be slidable in an axial direction relative to the other one of the external toothing and the internal toothing. The sliding member includes a wall at an end of the sliding member between the sliding member and the other one of the external toothing and the internal toothing. The wall protrudes toward the other one of the external toothing and the internal toothing. The end is an axial end of the sliding member in a clearance extending in the axial direction. 
     According to the structure described above, it is possible to provide the telescopic shaft improved in reliability. 
     A second aspect of the present disclosure relates to a steering system. The steering system includes a telescopic shaft configured to transmit a rotational torque to be used for turning a steered wheel. The telescopic shaft includes an internal shaft, an external shaft, and a sliding member. The internal shaft has an external toothing on an outer peripheral surface. The external toothing includes a plurality of teeth arrayed in a circumferential direction. The external shaft has an internal toothing on an inner peripheral surface. The internal toothing includes a plurality of teeth disposed at positions where the internal toothing meshes with the external toothing. The sliding member is disposed between the external toothing and the internal toothing, and is fixed to one of the external toothing and the internal toothing. The sliding member is disposed so as to be slidable in an axial direction relative to the other one of the external toothing and the internal toothing. The sliding member includes a wall at an end of the sliding member between the sliding member and the other one of the external toothing and the internal toothing. The wall protrudes toward the other one of the external toothing and the internal toothing. The end is an axial end of the sliding member in a clearance extending in the axial direction. 
     According to the structure described above, it is possible to provide the steering system including the telescopic shaft improved in reliability. 
    
    
     
       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 signs denote like elements, and wherein: 
         FIG. 1  is a diagram illustrating an overview of the structure of a steering system according to an embodiment; 
         FIG. 2  is a perspective view of an appearance of a telescopic shaft according to the embodiment; 
         FIG. 3  is an exploded perspective view of the telescopic shaft according to the embodiment; 
         FIG. 4  is a perspective view of an appearance of an external shaft according to the embodiment; 
         FIG. 5  is a perspective view of an appearance of a sliding member according to the embodiment; 
         FIG. 6  is a first sectional view of the telescopic shaft according to the embodiment; 
         FIG. 7  is a second sectional view of the telescopic shaft according to the embodiment; 
         FIG. 8  is a third sectional view of the telescopic shaft according to the embodiment; 
         FIG. 9  is a local sectional view of a telescopic shaft according to a modified example of the embodiment; and 
         FIG. 10  is a diagram illustrating a part of the axial end of a sliding member according to the modified example of the embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     An embodiment and its modified example are described below in detail with reference to the drawings. The embodiment and the modified example are comprehensive or specific examples. Numerical values, shapes, materials, constituent elements, arrangements and connection forms of the constituent elements, steps, the order of the steps, and the like described in the embodiment and the modified example are examples, but are not intended to limit the present disclosure. 
     The drawings are schematic drawings in which objects are emphasized, omitted, or adjusted in terms of their proportions as appropriate to demonstrate the present disclosure. Therefore, shapes, positional relationships, and proportions may differ from actual shapes, positional relationships, and proportions. In the following embodiment and the claims, expressions of relative directions or postures such as “parallel” and “orthogonal” may be used, but include directions or postures deviating from the strict directions or postures. For example, an expression “two directions are parallel” means not only that the directions are completely parallel, but also that the directions are substantially parallel, that is, include a difference of about several percent. 
     1. Overview of Structure of Steering System 
     First, an overview of the structure of a steering system  100  according to the embodiment is described with reference to  FIG. 1 .  FIG. 1  is a diagram illustrating the overview of the structure of the steering system according to the embodiment. As illustrated in  FIG. 1 , the steering system  100  according to this embodiment turns steered wheels  210  based on a rotational torque generated by a driver who operates an operation member  200  such as a steering wheel. 
     The steering system  100  includes a steering column shaft  190 , a telescopic shaft  10 , a belt transmission device  101 , a shaft  110 , a ball nut  120 , and a housing  140 . The operation member  200  is coupled to one end of the steering column shaft  190 . The belt transmission device  101  includes an assist motor  150 , a driving pulley  153 , a driven pulley  130 , and a belt  160 . 
     The steering column shaft  190  mechanically transmits a rotational torque generated in response to a driver&#39;s operation for the operation member  200  to the shaft  110  via the telescopic shaft  10 . The shaft  110  is a rod-shaped member having grooves  111  on a part of its outer peripheral surface in an axial direction. The shaft  110  is connected to the steered wheels  210  via tie rods  211  to function as a steering operation shaft configured to turn the steered wheels  210 . The shaft  110  is provided with a rack  112 , and a rack and pinion mechanism is formed such that the rack  112  meshes with a pinion gear of a pinion shaft  191 . The steering system  100  is a so-called electric power steering system in which the rotational torque transmitted via the steering column shaft  190  is converted into a linear motion of the shaft  110  and an assist torque generated by the assist motor  150  is converted into a linear motion of the shaft  110 . In  FIG. 1 , the assist torque generated by the assist motor  150  is transmitted to the shaft  110  via the driving pulley  153 , the belt  160 , the driven pulley  130 , and the ball nut  120 , but the method for transmitting the assist torque to the shaft  110  is not particularly limited. For example, the assist torque generated by the assist motor  150  may be transmitted to the shaft  110  via the pinion gear. The steering system  100  need not be the electric power steering system, but may have a structure for turning the steered wheels  210  only by the rotational torque transmitted via the steering column shaft  190 . 
     The housing  140  houses the shaft  110 . In this embodiment, the housing  140  is fixed to a vehicle body by using a bracket formed integrally with the housing  140 , retains the housed shaft  110 , and guides movement of the shaft  110 . For example, the housing  140  is made of an aluminum alloy. Bellows-shaped tubular boots  142  made of rubber or the like are attached to both ends of the housing  140 . The boots  142  seal both open ends of the housing  140  in its longitudinal direction, thereby preventing water or dirt from entering the housing  140 . 
     For example, the telescopic shaft  10  of this embodiment is a shaft member called “intermediate shaft”, which is extensible and contractible, and mechanically connects the steering column shaft  190  to the pinion shaft  191 . The telescopic shaft  10  includes a tubular external shaft  20  and a solid internal shaft  30 . In this embodiment, the external shaft  20  and the internal shaft  30  are made of a metal such as iron. The external shaft  20  and the internal shaft  30  may be made of a material having predetermined rigidity and durability. A non-metallic material such as a resin may be employed as a part or entirety of the material. 
     The internal shaft  30  is attached to the external shaft  20  in a state in which the end of the internal shaft  30  is inserted into the external shaft  20 . More specifically, the external shaft  20  and the internal shaft  30  are fitted together in a state in which one of the external shaft  20  and the internal shaft  30  is movable relative to the other in an axial direction of the telescopic shaft  10  and is non-rotatable about an axis of the other. That is, the telescopic shaft  10  has a structure in which the external shaft  20  and the internal shaft  30  slide relative to each other in the axial direction and engage with each other in a circumferential direction. The telescopic shaft  10  is connected to the steering column shaft  190  via a universal joint  29 , and to the pinion shaft  191  via a universal joint  39 . Thus, the steering column shaft  190  and the pinion shaft  191  having different axial directions can mechanically be connected together, and the rotational torque received from the steering column shaft  190  can be transmitted to the pinion shaft  191 . The steering column shaft  190  and the telescopic shaft  10  may be referred to collectively as “steering shaft”. In this case, the telescopic shaft  10  is a part of the steering shaft. The telescopic shaft  10  according to this embodiment is described below in more detail. 
     2. Structure of Telescopic Shaft 
       FIG. 2  is a perspective view of an appearance of the telescopic shaft  10  according to the embodiment.  FIG. 2  illustrates a state in which the universal joints  29  and  39  are attached to both axial ends of the telescopic shaft  10 . In  FIG. 2  and other subsequent figures, a long dashed short dashed line and a dot with a reference symbol A indicate a rotational axis A (imaginary axis) of the telescopic shaft  10 . Unless otherwise noted, the “axial direction” hereinafter refers to a direction parallel to the rotational axis A, and coincides with an X-axis direction in this embodiment.  FIG. 3  is an exploded perspective view of the telescopic shaft  10  according to the embodiment.  FIG. 4  is a perspective view of an appearance of the external shaft  20  according to the embodiment.  FIG. 5  is a perspective view of an appearance of a sliding member  50  according to the embodiment.  FIG. 6  is a first sectional view of the telescopic shaft  10  according to the embodiment.  FIG. 6  is a sectional view of the telescopic shaft  10  in a YZ plane passing through a line VI-VI in  FIG. 2 . 
     As illustrated in  FIG. 2 , the telescopic shaft  10  includes the internal shaft  30  and the external shaft  20 . As illustrated in  FIG. 3  and  FIG. 6 , an external toothing  31  including a plurality of teeth  32  arrayed in the circumferential direction is formed on an outer peripheral surface  30   a  of the internal shaft  30 . In this embodiment, the external toothing  31  includes four teeth  32  arrayed at regular intervals in the circumferential direction. As illustrated in  FIG. 4  and  FIG. 6 , an internal toothing  21  is formed on an inner peripheral surface  20   a  of the external shaft  20 . The internal toothing  21  includes a plurality of (four in this embodiment) teeth  22  arranged at positions where the internal toothing  21  meshes with the external toothing  31  of the internal shaft  30 . The teeth  32  of the external toothing  31  extend in the axial direction on the internal shaft  30 . The teeth  22  of the internal toothing  21  extend in the axial direction on the external shaft  20 . 
     As described above, the telescopic shaft  10  according to this embodiment has a structure in which the external toothing  31  provided on the internal shaft  30  and including the teeth  32  projecting radially outward and the internal toothing  21  provided on the external shaft  20  and including the teeth  22  projecting radially inward mesh (engage) with each other in the circumferential direction. Thus, a rotational torque applied to one of the internal shaft  30  and the external shaft  20  is transmitted to the other. Since the external toothing  31  and the internal toothing  21  extend in the axial direction, the telescopic shaft  10  can change its total axial length while keeping the function of transmitting the rotational torque from one axial end to the other axial end. 
     In this embodiment, when the telescopic shaft  10  extends or contracts, the external shaft  20  and the internal shaft  30  do not directly slide, but slide via the sliding member  50  arranged between the external shaft  20  and the internal shaft  30  in a radial direction. 
     Specifically, as illustrated in  FIG. 3 , the telescopic shaft  10  includes the sliding member  50  arranged between the external toothing  31  and the internal toothing  21  and fixed to one of the external toothing  31  and the internal toothing  21 . In this embodiment, the sliding member  50  is fixed to the external toothing  31  of the internal shaft  30 . Although the fixing method is not particularly limited, the sliding member  50  is fixed to the external toothing  31  of the internal shaft  30  by an adhesive, welding, screw fastening, crimping, arrangement of a stopper, or any combination of those methods. 
     The sliding member  50  is made of a material such as resin having relatively small sliding friction. This material improves axial slippage of one of the internal shaft  30  and the external shaft  20  relative to the other. At a part where the internal shaft  30 , the external shaft  20 , and the sliding member  50  are arrayed in the radial direction, a clearance is present between the members, thereby improving the slippage as well. 
     Specifically, as illustrated in  FIG. 6 , the sliding member  50  fixed to the external toothing  31  of the internal shaft  30  is arranged substantially in close contact with the outer surface of the external toothing  31  (teeth  32 ). A plurality of clearances  59  is present between the sliding member  50  and the internal toothing  21  (teeth  22 ) of the external shaft  20 . More specifically, clearances  59  extending in the axial direction are present between tooth tips  22   a  of the teeth  22  of the internal toothing  21  and receding threads  52  of the sliding member  50 . The receding threads  52  face the tooth tips  22   a , and recede radially inward. Further, clearances  59  extending in the axial direction are present between bottom lands  23  of the internal toothing  21  and projecting threads  51  of the sliding member  50 . Each bottom land  23  is formed between two adjacent teeth  22  of the internal toothing  21 . The projecting threads  51  face the bottom lands  23 , and project radially outward. The axial length of the clearance  59  is larger than the circumferential length (width) of the clearance  59 . That is, the clearance  59  of this embodiment is a space elongated in the axial direction (depth direction in  FIG. 6 ) (see  FIG. 7  and  FIG. 8 ). 
     As described above, the clearances  59  are present between the sliding member  50  and the internal toothing  21  of the external shaft  20  in the radial direction. The clearances  59  prevent an excessive increase in friction between the internal toothing  21  and the sliding member  50  fixed to the internal shaft  30 . As a result, the sliding member  50  meshes with the internal toothing  21  so as to be slidable in the axial direction. As illustrated in  FIG. 6 , the sliding member  50  and the internal toothing  21  of the external shaft  20  have a relationship in which the sliding member  50  and the internal toothing  21  abut against each other in the circumferential direction. Therefore, the rotational torque of one of the internal shaft  30  and the external shaft  20  is securely transmitted to the other. 
     To further improve the slippage between the sliding member  50  and the internal toothing  21 , a lubricant (for example, grease) is applied between the sliding member  50  and the internal toothing  21  of the external shaft  20 . The grease spreads not only over the clearances  59 , but also between the directly abutting circumferential side faces of each projecting thread  51  of the sliding member  50  and each tooth  22  of the internal toothing  21 . Thus, the sliding member  50  and the internal toothing  21  slip more easily. As a result, the telescopic shaft  10  extends and contracts in the axial direction more smoothly. 
     In this embodiment, the sliding member  50  has a plurality of circumferential grooves  58  that functions as portions that store grease (so-called grease reservoirs) to increase the amount of grease applied between the sliding member  50  and the internal toothing  21  (see  FIG. 5 ). The grease is stored in the circumferential grooves  58  by applying the grease to the outer peripheral surface of the sliding member  50  fixed to the internal shaft  30 . In this state, the end of the internal shaft  30  where the sliding member  50  is fixed is inserted into the internal toothing  21  of the external shaft  20 . Thus, the telescopic shaft  10  is assembled. When the telescopic shaft  10  extends or contracts, the sliding member  50  moves in the axial direction relative to the internal toothing  21 , thereby supplying a part of the grease stored in the circumferential grooves  58  to a boundary between the sliding member  50  and the internal toothing  21 . 
     As illustrated in  FIG. 5 , each circumferential groove  58  extends from the receding thread  52  to the projecting thread  51  on the sliding member  50 . That is, the circumferential groove  58  is arranged so as to connect the clearance  59  between the tooth tip  22   a  and the receding thread  52  to the clearance  59  between the bottom land  23  and the projecting thread  51  in  FIG. 6 . Therefore, the grease is supplied to the two clearances  59  and the part between the two clearances  59  where the sliding member  50  and the internal toothing  21  abut against each other in the circumferential direction. Thus, the grease is distributed over a wide range between the sliding member  50  and the internal toothing  21  of the external shaft  20 . 
     Since the clearance  59  extends from one axial end to the other axial end of the sliding member  50 , the grease may leak from the axial ends of the sliding member  50  in the clearance  59 . When the grease leakage amount increases, the grease retained between the sliding member  50  and a sliding counterpart (in this embodiment, the internal toothing  21  of the external shaft  20 ) decreases. Therefore, the sliding friction between the sliding member  50  and the internal toothing  21  increases. As a result, the smoothness of the extending and contracting motion of the telescopic shaft  10  is lost. Further, damage may be caused by, for example, wear of the sliding member  50 . 
     In this embodiment, the sliding member  50  has walls  55  at positions of its axial ends in the clearances  59  as illustrated in  FIG. 5 ,  FIG. 7 , and  FIG. 8 .  FIG. 7  is a second sectional view of the telescopic shaft  10  according to the embodiment.  FIG. 8  is a third sectional view of the telescopic shaft  10  according to the embodiment.  FIG. 7  illustrates a part of a cross section VII-VII in  FIG. 6 .  FIG. 8  illustrates a part of a cross section VIII-VIII in  FIG. 6 . 
     As illustrated in  FIG. 5 ,  FIG. 7 , and  FIG. 8 , the sliding member  50  has the walls  55  provided upright in the radial direction at its axial ends in the clearances  59  to suppress axially outward leakage of the grease from the clearances  59 . As illustrated in  FIG. 5  and  FIG. 7 , the wall  55  located at the end of the sliding member  50  in the clearance  59  between the bottom land  23  and the projecting thread  51  is referred to as “wall  55   a ”. As illustrated in  FIG. 5  and  FIG. 8 , the wall  55  located at the end of the sliding member  50  in the clearance  59  between the tooth tip  22   a  and the receding thread  52  is referred to as “wall  55   b”.    
     As described above, the telescopic shaft  10  according to this embodiment is extensible and contractible in the axial direction, and includes the internal shaft  30 , the external shaft  20 , and the sliding member  50 . The internal shaft  30  includes the external toothing  31  formed on the outer peripheral surface  30   a . The external toothing  31  includes the teeth  32  arrayed in the circumferential direction. The external shaft  20  includes the internal toothing  21  formed on the inner peripheral surface  20   a . The internal toothing  21  includes the teeth  22  arranged at the positions where the internal toothing  21  meshes with the external toothing  31 . The sliding member  50  is arranged between the external toothing  31  and the internal toothing  21 , fixed to the external toothing  31 , that is, one of the external toothing  31  and the internal toothing  21 , and slidable in the axial direction relative to the internal toothing  21 , that is, the other one of the external toothing  31  and the internal toothing  21 . The sliding member  50  has the walls  55  at its axial ends in the clearances  59  extending in the axial direction between the sliding member  50  and the internal toothing  21 . The walls  55  protrude toward the internal toothing  21 . 
     According to this structure, the telescopic shaft  10  can extend and contract such that the external shaft  20  and the internal shaft  30  slide via the sliding member  50 , and the rotational torque applied to one of the external shaft  20  and the internal shaft  30  can be transmitted to the other such that the internal toothing  21  and the external toothing  31  mesh with each other via the sliding member  50 . Since the clearances  59  are present between the sliding member  50  and the internal toothing  21 , the lubricant such as grease can be applied to the clearances  59 . The walls  55  of the sliding member  50  suppress the leakage of the grease from the clearances  59 . Therefore, it is possible to attain an effect of improvement in the slidability by the grease, and to suppress troubles with the extending and contracting motion of the telescopic shaft  10  due to grease depletion, or damage that may be caused by, for example, wear of the sliding member  50 . Thus, the telescopic shaft  10  according to this embodiment can be improved in reliability. 
     In this embodiment, as illustrated in  FIG. 6 , the clearance  59  is present between the tooth tip  22   a  of the internal toothing  21  and the receding thread  52  of the sliding member  50  that faces the tooth tip  22   a . As illustrated in  FIG. 5  and  FIG. 8 , the walls  55   b  are provided at the axial ends of the receding thread  52 . 
     In the telescopic shaft  10  according to this embodiment, the clearance  59  where the walls  55   b  are arranged to suppress the grease leakage is present between the tooth tip  22   a  and the receding thread  52  that face each other in the radial direction and are substantially unrelated to the transmission of the rotational torque. Thus, the telescopic shaft  10  can keep or improve the smoothness of the extending and contracting motion without impairing the function of transmitting the rotational torque. 
     In this embodiment, as illustrated in  FIG. 6 , the clearance  59  is present between the bottom land  23  of the internal toothing  21  and the projecting thread  51  of the sliding member  50  that faces the bottom land  23 . As illustrated in  FIG. 5  and  FIG. 7 , the walls  55   a  are provided at the axial ends of the projecting thread  51 . 
     In the telescopic shaft  10  according to this embodiment, the clearance  59  where the walls  55   a  are arranged to suppress the grease leakage is present between the bottom land  23  and the projecting thread  51  that face each other in the radial direction and are substantially unrelated to the transmission of the rotational torque. Thus, the telescopic shaft  10  can keep or improve the smoothness of the extending and contracting motion without impairing the function of transmitting the rotational torque. 
     In this embodiment, as illustrated in  FIG. 7  and  FIG. 8 , the sliding member  50  has the walls  55  at both axial ends of the sliding member  50  in the clearances  59 . The clearances  59  are present continuously between the walls  55  at both ends. Specifically, the sliding member  50  has the walls  55   a  at both axial ends of the projecting thread  51 , and the walls  55   b  at both axial ends of the receding thread  52 . 
     In the sliding member  50  according to this embodiment, the walls  55  are arranged at both axial ends of the sliding member  50  in the clearance  59  that stores the grease. Therefore, the grease leakage is suppressed more securely. Thus, the smoothness of the extending and contracting motion of the telescopic shaft  10  is kept for a long time. The end of the sliding member  50  where the wall  55  is arranged falls within a predetermined range in the axial direction, including an axial edge of the sliding member  50  (for example, a range of several millimeters from the edge, or a range from the edge to a point corresponding to 1/10 of the axial length of the sliding member  50 ). The wall  55  need not be provided upright from the axial edge of the sliding member  50 , but may be provided upright to prevent the grease leakage at a position on an inner side of the axial edge of the sliding member  50  (position closer to the opposite edge). 
     In this embodiment, the surface of the sliding member  50  that faces the internal toothing  21  across the clearances  59  has the circumferential grooves  58  extending in the circumferential direction and receding away from the internal toothing  21 . The walls  55  protrude toward the internal toothing  21  from the surface of the sliding member  50  that faces the external toothing  31 . 
     According to this structure, the grease is supplied via the circumferential grooves  58  from the clearance  59  that stores the grease to the space between two members abutting against each other in the circumferential direction (between the circumferential side face of the projecting thread  51  and the circumferential side face of the tooth  22 ). Thus, the slidability between the internal toothing  21  and the sliding member  50  is further improved. 
     As described above, the steering system  100  according to this embodiment is configured to steer the vehicle, and includes the telescopic shaft  10  configured to transmit a rotational torque to be used for turning the steered wheels  210 . 
     That is, the steering system  100  includes the telescopic shaft  10  according to this embodiment as a member having an important role of transmitting the rotational torque for turning the steered wheels  210 . Thus, the reliability of the steering system  100  can be improved. 
     Although the steering system  100  according to the embodiment is described above, the steering system  100  may have a telescopic shaft having a structure different from the structure illustrated in  FIG. 1  to  FIG. 8  as the telescopic shaft configured to transmit the rotational torque for turning the steered wheels  210 . A modified example of the telescopic shaft is described below mainly about a difference from the embodiment. 
     Modified Example 
       FIG. 9  is a local sectional view of a telescopic shaft  10   a  according to the modified example of the embodiment. The position of a cross section in  FIG. 9  coincides with the position of the cross section in  FIG. 6 .  FIG. 10  is a diagram illustrating a part of the axial end of a sliding member  90  according to the modified example of the embodiment. 
     The telescopic shaft  10   a  according to this modified example may be a substitute for the telescopic shaft  10  in the steering system  100  according to the embodiment (see  FIG. 1 ). Similarly to the telescopic shaft  10  according to the embodiment, the telescopic shaft  10   a  includes the internal shaft  30  and the external shaft  20 . The sliding member  90  is arranged between the external toothing  31  of the internal shaft  30  and the internal toothing  21  of the external shaft  20 . The overall shape of the sliding member  90  is in common with the shape of the sliding member  50  according to the embodiment (see  FIG. 5 ), but the positions of walls  95  differ from the positions of the walls of the sliding member  50 . 
     Specifically, the sliding member  90  of this modified example is fixed to the internal toothing  21  of the external shaft  20  instead of the external toothing  31  of the internal shaft  30 . Although the fixing method is not particularly limited, the sliding member  90  is fixed to the internal toothing  21  by an adhesive, welding, screw fastening, crimping, arrangement of a stopper, or any combination of those methods. 
     Since the sliding member  90  is fixed to the internal toothing  21 , the sliding member  90  is arranged substantially in close contact with the outer surface of the internal toothing  21  (teeth  22 ). The clearances  59  are present between the sliding member  90  and the external toothing  31  (teeth  32 ) of the internal shaft  30 . More specifically, the clearances  59  are present between tooth tips  32   a  of the teeth  32  of the external toothing  31  and receding threads  92  of the sliding member  90 . The receding threads  92  face the tooth tips  32   a , and recede radially outward. Further, the clearances  59  are present between bottom lands  33  of the external toothing  31  and projecting threads  91  of the sliding member  90 . Each bottom land  33  is formed between two adjacent teeth  32  of the external toothing  31 . The projecting threads  91  face the bottom lands  33 , and project radially inward. 
     As illustrated in  FIG. 10 , the sliding member  90  according to this modified example has the walls  95  for the clearances  59 . The walls  95  are provided at the axial ends of the sliding member  90  in the clearances  59 . Specifically, walls  95   b  are arranged at the axial ends of the receding threads  92  of the sliding member  90 , and walls  95   a  are arranged at the axial ends of the projecting threads  91  of the sliding member  90 . The walls  95   a  and  95   b  are provided upright toward the radially inner side on the sliding member  90 . The walls  95   a  and  95   b  suppress grease leakage from the clearances  59  present between the sliding member  90  and the external toothing  31 . Thus, the telescopic shaft  10   a  can keep or improve the smoothness of the extending and contracting motion without impairing the function of transmitting the rotational torque. 
     The walls  95   a  and  95   b  may be provided at both axial ends of the sliding member  90 . Thus, the grease leakage suppression effect is improved. Circumferential grooves extending in the circumferential direction may be formed on the surface of the sliding member  90  that faces the external toothing  31  across the clearances  59 . Therefore, the grease is supplied via the circumferential grooves from the clearance  59  that stores the grease to the space between two members abutting against each other in the circumferential direction (between the circumferential side face of the receding thread  92  and the circumferential side face of the tooth  32 ). Thus, the slidability between the internal toothing  21  and the sliding member  90  is further improved. 
     Other Embodiments 
     The steering system according to the present disclosure has been described above based on the embodiment and its modified example. However, the present disclosure is not limited to the embodiment and the modified example. Without departing from the spirit of the present disclosure, the scope of the present disclosure encompasses various modifications to the embodiment or the modified example that are conceivable to persons having ordinary skill in the art, or modes obtained by combining a plurality of the constituent elements described above. 
     For example, the number of teeth of each of the external toothing  31  and the internal toothing  21  may be a number other than four. The external toothing  31  and the internal toothing  21  may have pluralities of teeth that engage with each other in the circumferential direction. Each of the external toothing  31  and the internal toothing  21  preferably has two or more teeth evenly arrayed in the circumferential direction. 
     The sliding member  50  may have at least one of the wall  55   a  provided at the axial end of the projecting thread  51  and the wall  55   b  provided at the axial end of the receding thread  52 . Further, the wall  55   a  may be provided at only one of the two axial ends of the projecting thread  51 , and the wall  55   b  may be provided at only one of the two axial ends of the receding thread  52 . That is, when the clearances are present between the sliding member  50  and the sliding counterpart, the amount of grease leaking from the space between the sliding member  50  and the sliding counterpart can be reduced as long as the wall is provided at, at least one axial end of the sliding member  50  in the clearance. 
     Although the telescopic shaft  10  according to the embodiment is arranged between the steering column shaft  190  and the pinion shaft  191 , the telescopic shaft  10  may be employed as a part or entirety of the steering column shaft  190 . The steering column shaft  190  may have a function of extending and contracting in its axial direction to, for example, adjust the position of the operation member  200 . By employing the telescopic shaft  10  as a part or entirety of the steering column shaft  190 , the rotational torque generated by the driver who operates the operation member  200  can be transmitted to, for example, the pinion shaft  191  on the downstream side. Further, smoothness of movement of the operation member  200  can be kept or improved. 
     The telescopic shaft  10  may be mounted on a steer-by-wire system in which the operation member  200  is not mechanically connected to the steered wheels. For example, the operation member  200  of the steer-by-wire system may require a function of moving in the axial direction in order to retract the operation member  200  into a predetermined retraction area in front of a driver&#39;s seat. By employing the telescopic shaft  10  as a shaft member to which the operation member  200  is coupled at one end, the steering operation can appropriately be performed based on the rotational torque generated through an operation for the operation member  200 . Further, the smoothness of the movement of the operation member  200  can be kept or improved. 
     The shape and number of the circumferential grooves  58  need not be the shape and number illustrated in  FIG. 5  or the like. For example, the circumferential groove  58  may be provided in one or more loops in the circumferential direction around the outer peripheral surface of the sliding member  50 . 
     The circumferential groove  58  may be omitted from the sliding member  50 . Also in this case, the sliding member  50  can attain the grease leakage suppression effect by providing the walls  55 . 
     Various supplementary remarks described above about the sliding member  50  according to the embodiment may be applied to the sliding member  90  according to the modified example. 
     The steering system according to the present disclosure is useful as a steering system to be provided in a vehicle such as an automobile.