Patent Publication Number: US-8973545-B2

Title: Valve-timing control apparatus for internal combustion engine

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a valve-timing control apparatus for an internal combustion engine, in which opening and closing timings of intake valve and/or exhaust valve of the internal combustion engine are controlled. 
     Recently, a valve-timing control apparatus is proposed in which opening and closing timings of intake or exhaust valve are controlled by transmitting rotative force of an electric motor through a speed-reduction mechanism to a cam shaft and thereby varying a relative rotational phase of the cam shaft to a sprocket to which rotative force is transmitted from a crankshaft. 
     Japanese Patent Application Publication No. 2011-256798 discloses a previously-proposed valve-timing control apparatus. In this technique, an output shaft of the electric motor is formed in a tubular shape, and a bearing member such as a ball bearing is accommodated inside the tubular output shaft. Accordingly, an axial length of the entire valve-timing control apparatus can be shortened to attain a downsizing thereof. The bearing member is lubricated by supplying lubricating oil into the tubular output shaft. 
     Moreover, in the above previously-proposed valve-timing control apparatus, a brush is provided to a cover member located on a front end side of the electric motor whereas a slip ring is provided to the electric motor. By means of a contact between these brush and slip ring, electric power is supplied to the electric motor. A plug member is provided inside a tip portion of the tubular output shaft in order to prevent lubricating oil retained in the tubular output shaft from flowing out and adhering to the brush and the slip ring. 
     SUMMARY OF THE INVENTION 
     However, in the above previously-proposed valve-timing control apparatus, the plug member includes a core member which is made of a metal material in the form of “U” in cross section. A rubber material integrally coats (i.e., integrally molded to) an entire surface of the core member of the plug member. Hence, once the plug member has been fitted and fixed into the tubular output shaft by press fitting, it is difficult to detach the plug member. Therefore, for example, there is a problem that a maintenance for the inside of the tubular output shaft of the electric motor is not easily performed. 
     It is therefore an object of the present invention to provide a valve-timing control apparatus for an internal combustion engine, devised to easily detach the plug member even after the plug member has been fixed into the output shaft of the electric motor. 
     According to one aspect of the present invention, there is provided a valve-timing control apparatus for an internal combustion engine, comprising: a drive rotating member configured to receive a rotational force from a crankshaft; a driven rotating member fixed to a cam shaft and configured to rotate relative to the drive rotating member; an electric motor configured to rotate the driven rotating member relative to the drive rotating member by means of rotary drive of the electric motor; a housing connected integrally with the drive rotating member, wherein structural components of the electric motor are accommodated in the housing; a cover member fixed to a main body of the internal combustion engine and located to face a front end portion of the housing; a slip ring configured to supply electric power to the electric motor and provided to one of the front end portion of the housing and a facing portion of the cover member which faces the front end portion of the housing; a brush provided to another of the front end portion of the housing and the facing portion of the cover member, and configured to supply electric power to the electric motor by an electrical contact with the slip ring; a tubular motor output shaft provided inside the housing to be rotatable relative to the housing, and configured to be rotated by electric-power supply to the electric motor, wherein lubricating oil is supplied into the tubular motor output shaft; a bearing member provided between an outer circumferential surface of a part of the driven rotating member and an inner circumferential surface of the tubular motor output shaft; a plug member fixed to an inner circumferential surface of a tip portion of the tubular motor output shaft, the cover member facing the tip portion of the tubular motor output shaft, wherein the plug member is configured to inhibit lubricating oil supplied into the tubular motor output shaft from leaking to an external; and a seal member provided between the cover member and the housing and configured to inhibit lubricating oil from entering a gap between the slip ring and the brush, wherein the plug member includes a core member formed in a bottomed tubular shape having a through-hole in a bottom portion of the core member, and an elastic body coating at least the through-hole and an outer circumferential surface of the core member, the elastic body closing the through-hole. 
     According to another aspect of the present invention, there is provided a valve-timing control apparatus for an internal combustion engine, comprising: a drive rotating member configured to receive a rotational force from a crankshaft; a driven rotating member fixed to a cam shaft and configured to rotate relative to the drive rotating member; an electric motor configured to rotate the driven rotating member relative to the drive rotating member by means of rotary drive of the electric motor; a housing connected integrally with the drive rotating member, wherein structural components of the electric motor are accommodated in the housing; a cover member fixed to a main body of the internal combustion engine and located to face a front end portion of the housing; a slip ring configured to supply electric power to the electric motor and provided to one of the front end portion of the housing and a facing portion of the cover member which faces the front end portion of the housing; a brush provided to another of the front end portion of the housing and the facing portion of the cover member, and configured to supply electric power to the electric motor by an electrical contact with the slip ring; a tubular motor output shaft provided inside the housing to be rotatable relative to the housing, and configured to be rotated by electric-power supply to the electric motor, wherein lubricating oil is supplied into the tubular motor output shaft; a bearing member provided between an outer circumferential surface of a part of the driven rotating member and an inner circumferential surface of the tubular motor output shaft; a plug member fixed to an inner circumferential surface of a tip portion of the tubular motor output shaft, the cover member facing the tip portion of the tubular motor output shaft, wherein the plug member is configured to inhibit lubricating oil supplied into the tubular motor output shaft from leaking to an external; and a seal member provided between the cover member and the housing and configured to inhibit lubricating oil from entering a gap between the slip ring and the brush, wherein the plug member includes a core member formed in a bottomed cylindrical shape having a through-hole in a bottom portion of the core member, and a sealing structure configured to maintain a sealed state of the through-hole under a state where the lubricating oil supplied into the tubular motor output shaft takes a maximum pressure level thereof, and to release the sealed state of the through-hole when an axial force greater than the maximum pressure level of the lubricating oil is applied to the through-hole. 
     According to still another aspect of the present invention, there is provided a valve-timing control apparatus for an internal combustion engine, comprising: a drive rotating member configured to receive a rotational force from a crankshaft; a driven rotating member fixed to a cam shaft and configured to rotate relative to the drive rotating member; an electric motor configured to rotate the driven rotating member relative to the drive rotating member by means of rotary drive of the electric motor; a housing connected integrally with the drive rotating member, wherein structural components of the electric motor are accommodated in the housing; a cover member fixed to a main body of the internal combustion engine and located to face a front end portion of the housing; a slip ring configured to supply electric power to the electric motor and provided to one of the front end portion of the housing and a facing portion of the cover member which faces the front end portion of the housing; a brush provided to another of the front end portion of the housing and the facing portion of the cover member, and configured to supply electric power to the electric motor by an electrical contact with the slip ring; a tubular motor output shaft provided inside the housing to be rotatable relative to the housing, and configured to be rotated by electric-power supply to the electric motor, wherein lubricating oil is supplied into the tubular motor output shaft; a bearing member provided between an outer circumferential surface of a part of the driven rotating member and an inner circumferential surface of the tubular motor output shaft; a plug member fixed to an inner circumferential surface of a tip portion of the tubular motor output shaft, the cover member facing the tip portion of the tubular motor output shaft, wherein the plug member is configured to inhibit lubricating oil supplied into the tubular motor output shaft from leaking to an external; and a seal member provided between the cover member and the housing and configured to inhibit lubricating oil from entering a gap between the slip ring and the brush, wherein the plug member is formed in a bottomed cylindrical shape, and a bottom portion of the plug member has a rigidity lower than a rigidity of the other portion of the plug member. 
     The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a longitudinal sectional view of a valve-timing control apparatus in a first embodiment according to the present invention. 
         FIG. 2  is a front view of a plug member in the first embodiment. 
         FIG. 3  is an exploded oblique perspective view showing structural elements in the first embodiment. 
         FIG. 4  is a sectional view of  FIG. 1 , taken along a line A-A. 
         FIG. 5  is a sectional view of  FIG. 1 , taken along a line B-B. 
         FIG. 6  is a sectional view of  FIG. 1 , taken along a line C-C. 
         FIG. 7  is a longitudinal sectional view of a valve-timing control apparatus in a second embodiment according to the present invention. 
         FIG. 8  is a front view of a plug member in the second embodiment. 
         FIG. 9  is a front view of a plug member in another example of the second embodiment. 
         FIG. 10  is a longitudinal sectional view of a valve-timing control apparatus in a third embodiment according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, embodiments of valve-timing control (VTC) apparatus for an internal combustion engine according to the present invention will be explained referring to the drawings. 
     First Embodiment 
     As shown in  FIGS. 1 to 3 , a valve-timing control apparatus includes a timing sprocket  1 , a cam shaft  2 , a cover member  3  and a phase change mechanism  4 . The timing sprocket  1  (functioning as a drive rotating member) is rotated and driven by a crankshaft of the internal combustion engine. The cam shaft  2  is rotatably supported on a cylinder head  40  through a bearing  42 , and is rotated by a rotational force transmitted from the timing sprocket  1 . The cover member  3  is provided on a front side (in an axially frontward direction) of the timing sprocket  1 , and is fixedly attached to a chain cover  49 . The phase change mechanism  4  is provided between the timing sprocket  1  and the cam shaft  2 , and is configured to change a relative rotational phase between the timing sprocket  1  and the cam shaft  2  in accordance with an operating state of the engine. 
     Whole of the timing sprocket  1  is integrally formed of an iron-based metal in an annular shape. The timing sprocket  1  includes a sprocket main body  1   a , a gear portion  1   b  and an internal-teeth constituting portion (internal-gear portion)  19 . An inner circumferential surface of the sprocket main body  1   a  is formed in a stepped shape to have two relatively large and small diameters as shown in  FIG. 1 . The gear portion  1   b  is formed integrally with an outer circumference of the sprocket main body  1   a , and receives rotational force through a wound timing chain (not shown) from the crankshaft. The internal-teeth constituting portion  19  is formed integrally with a front end portion of the sprocket main body  1   a.    
     A large-diameter ball bearing  43  which is a bearing having a relatively large diameter is interposed between the sprocket main body  1   a  and an after-mentioned follower member  9  provided on a front end portion of the cam shaft  2 . The timing sprocket  1  is rotatably supported by the cam shaft  2  through the large-diameter ball bearing  43  such that a relative rotation between the cam shaft  2  and the timing sprocket  1  is possible. 
     The large-diameter ball bearing  43  includes an outer race  43   a , an inner race  43   b , and a ball(s)  43   c  interposed between the outer race  43   a  and the inner race  43   b . The outer race  43   a  of the large-diameter ball bearing  43  is fixed to an inner circumferential portion (i.e., inner circumferential surface) of the sprocket main body  1   a  whereas the inner race  43   b  of the large-diameter ball bearing  43  is fixed to an outer circumferential portion (i.e., outer circumferential surface) of the follower member  9 . 
     The inner circumferential portion of the sprocket main body  1   a  is formed with an outer-race fixing portion  60  which is in an annular-groove shape as obtained by cutting out a part of the inner circumferential portion of the sprocket main body  1   a . The outer-race fixing portion  60  is formed to be open toward the cam shaft  2 . 
     The outer-race fixing portion  60  is formed in a stepped shape to have two relatively large and small diameters. The outer race  43   a  of the large-diameter ball bearing  43  is fitted into the outer-race fixing portion  60  by press fitting in an axial direction of the timing sprocket  1 . Thereby, one axial end of the outer race  43   a  is placed at a predetermined position, that is, a positioning of the outer race  43   a  is performed. 
     The internal-teeth constituting portion  19  is formed integrally with an outer circumferential side of the front end portion of the sprocket main body  1   a . The internal-teeth constituting portion  19  is formed in a cylindrical shape (circular-tube shape) extending in a direction toward an electric motor  12  of the phase change mechanism  4 . An inner circumference of the internal-teeth constituting portion  19  is formed with internal teeth (internal gear)  19   a  which function as a wave-shaped meshing portion. 
     Moreover, a female-thread constituting portion  6  formed integrally with an after-mentioned housing  5  is placed to face a front end portion of the internal-teeth constituting portion  19 . The female-thread constituting portion  6  is formed in an annular shape. 
     Moreover, an annular retaining plate  61  is disposed on a (axially) rear end portion of the sprocket main body  1   a , on the side opposite to the internal-teeth constituting portion  19 . This retaining plate  61  is integrally formed of metallic sheet material. As shown in  FIG. 1 . An outer diameter of the retaining plate  61  is approximately equal to an outer diameter of the sprocket main body  1   a . An inner diameter of the retaining plate  61  is approximately equal to a diameter of a radially center portion of the large-diameter ball bearing  43 . 
     Therefore, an inner circumferential portion  61   a  of the retaining plate  61  faces and covers an axially outer end surface  43   e  of the outer race  43   a  through a predetermined clearance. Moreover, a stopper convex portion  61   b  which protrudes in a radially-inner direction of the annular retaining plate  61 , i.e. protrudes toward a central axis of the annular retaining plate  61  is provided at a predetermined location of an inner circumferential edge (i.e., radially-inner edge) of the inner circumferential portion  61   a . This stopper convex portion  61   b  is formed integrally with the inner circumferential portion  61   a.    
     As shown in  FIGS. 1 and 5 , the stopper convex portion  61   b  is formed in a substantially fan shape. A tip edge  61   c  of the stopper convex portion  61   b  is formed in a circular-arc shape in cross section, along a circular-arc-shaped inner circumferential surface of an after-mentioned stopper groove  2   b . Moreover, an outer circumferential portion of the retaining plate  61  is formed with six bolt insertion holes  61   d  each of which passes through the retaining plate  61 . The six bolt insertion holes  61   d  are formed at circumferentially equally-spaced intervals in the outer circumferential portion of the retaining plate  61 . A bolt  7  is inserted through each of the six bolt insertion holes  61   d.    
     An annular spacer  62  is interposed between an axially inner surface of the retaining plate  61  and the outer end surface  43   e  of the outer race  43   a  of the large-diameter ball bearing  43 . Thereby, the inner surface of the retaining plate  61  faces the outer end surface  43   e  through the annular spacer  62 . By this spacer  62 , the inner surface of the retaining plate  61  applies a slight pressing force to the outer end surface  43   e  of the outer race  43   a  when the retaining plate  61  is jointly fastened to the timing sprocket  1  and the housing  5  by the bolts  7 . However, a thickness of the spacer  62  is set at a certain degree at which a minute clearance between the outer end surface  43   e  of the outer race  43   a  and the retaining plate  61  is produced within a permissible range for an axial movement of the outer race  43   a.    
     An outer circumferential portion of the sprocket main body  1   a  (the internal-teeth constituting portion  19 ) is formed with six bolt insertion holes  1   c  each of which axially passes through the timing sprocket  1   a . The six bolt insertion holes  1   c  are formed substantially at circumferentially equally-spaced intervals in the outer circumferential portion of the sprocket main body  1   a . Moreover, the female-thread constituting portion  6  is formed with six female threaded holes  6   a  at its portions respectively corresponding to the six bolt insertion holes  1   c  and the six bolt insertion holes  61   d . By the six bolts  7  inserted into the six bolt insertion holes  61   d , the six bolt insertion holes  1   c  and the six female threaded holes  6   a ; the timing sprocket  1   a , the retaining plate  61  and the housing  5  are jointly fastened to one another from the axial direction. 
     It is noted that the sprocket main body  1   a  and the internal-teeth constituting portion  19  function as a casing for an after-mentioned speed-reduction mechanism  8 . 
     The timing sprocket  1   a , the internal-teeth constituting portion  19 , the retaining plate  61  and the female-thread constituting portion  6  have outer diameters substantially equal to one another. 
     As shown in  FIG. 1 , the chain cover  49  is fixed to a front end portion of a cylinder block and the cylinder head  40  which constitute a main body of the engine. The chain cover  49  is disposed along an upper-lower direction to cover a chain (not shown) wound around the timing sprocket  1   a . The chain cover  49  is formed with an opening portion  49   a  at a location corresponding to the phase change mechanism  4 , and includes an annular wall  49   b . The annular wall  49   b  constituting the opening portion  49   a  is formed with four boss portions  49   c . The four boss portions  49   c  are formed integrally with the annular wall  49   b  and are located at circumferential four spots of the annular wall  49   b . A female threaded hole  49   d  is formed in the annular wall  49   b  and each boss portion  49   c  to pass through the annular wall  49   b  and reach an interior of the each boss portion  49   c . That is, four female threaded holes  49   d  corresponding to the four boss portions  49   c  are formed. 
     As shown in  FIGS. 1 and 3 , the cover member  3  is made of aluminum alloy material and is integrally formed in a cup shape. The cover member  3  includes a cover main body  3   a  and a mounting flange  3   b . The cover main body  3   a  bulges out in the cup shape (protrudes in an expanded state) frontward in the axial direction. The mounting flange  3   b  is in an annular shape (ring shape) and is formed integrally with an outer circumferential edge of an opening-side portion of the cover main body  3   a . The cover main body  3   a  is provided to face and cover a front end portion of the housing  5 . An outer circumferential portion of the cover main body  3   a  is formed with a cylindrical wall  3   c  extending in the axial direction. The cylindrical wall  3   c  is formed integrally with the cover main body  3   a  and includes a retaining hole  3   d  therein. An inner circumferential surface of the retaining hole  3   d  functions as a guide surface for an after-mentioned brush retaining member  28 . 
     The mounting flange  3   b  includes four boss portions  3   e . The four boss portions  3   e  are formed substantially at circumferentially equally-spaced intervals (approximately at every 90-degree location) on the mounting flange  3   b . Each boss portion  3   e  is formed with a bolt insertion hole  3   g . The bolt insertion hole  3   g  passes through the boss portion  3   e . Each bolt  54  is inserted through the bolt insertion hole  3   g  and is screwed in the female threaded hole  49   d  formed in the chain cover  49 . By these bolts  54 , the cover member  3  is fixed to the chain cover  49 . 
     As shown in  FIGS. 1 and 3 , an oil seal  50  which is a seal member having a large diameter is interposed between an outer circumferential surface of the housing  5  and an inner circumferential surface of a stepped portion (multilevel portion) of outer circumferential side of the cover main body  3   a . The large-diameter oil seal  50  is formed in a substantially U-shape in cross section, as shown in  FIG. 1 . A core metal is buried inside a base material formed of synthetic rubber. An annular base portion of outer circumferential side of the large-diameter oil seal  50  is fixedly fitted in a stepped annular portion (annular groove)  3   h  formed in the inner circumferential surface of the cover member  3 . 
     The housing  5  includes a housing main body (tubular portion)  5   a  and a sealing plate  11 . The housing main body  5   a  is formed in a tubular shape having its bottom by press molding. The housing main body  5   a  is formed of iron-based metal. The sealing plate  11  is formed of non-magnetic synthetic resin, and seals a front-end opening of the housing main body  5   a.    
     The housing main body  5   a  includes a bottom portion  5   b  at a rear end portion of the housing main body  5   a . The bottom portion  5   b  is formed in a circular-disk shape. Moreover, the bottom portion  5   b  is formed with a shaft-portion insertion hole  5   c  having a large diameter, at a substantially center of the bottom portion  5   b . An after-mentioned eccentric shaft portion  39  is inserted through the shaft-portion insertion hole  5   c . A hole edge of the shaft-portion insertion hole  5   c  is formed integrally with an extending portion (exiting portion)  5   d  which protrudes from the bottom portion  5   b  in the axial direction of the cam shaft  2  in a cylindrical-tube shape. Moreover, an outer circumferential portion of a front-end surface of the bottom portion  5   b  is formed integrally with the female-thread constituting portion  6 . 
     The cam shaft  2  includes two drive cams per one cylinder of the engine. Each drive cam is provided on an outer circumference of the cam shaft  2 , and functions to open an intake valve (not shown). The front end portion of the cam shaft  2  is formed integrally with a flange portion  2   a.    
     As shown in  FIG. 1 , an outer diameter of the flange portion  2   a  is designed to be slightly larger than an outer diameter of an after-mentioned fixing end portion  9   a  of the follower member  9 . An outer circumferential portion of a front end surface of the flange portion  2   a  is in contact with an axially outer end surface of the inner race  43   b  of the large-diameter ball bearing  43 , after an assembly of respective structural components. Moreover, the front end surface of the flange portion  2   a  is fixedly connected with the follower member  9  from the axial direction by a cam bolt  10  under a state where the front end surface of the flange portion  2   a  is in contact with the follower member  9  in the axial direction. 
     As shown in  FIG. 5 , an outer circumference of the flange portion  2   a  is formed with a stopper concave groove  2   b  into which the stopper convex portion  61   b  of the retaining plate  61  is inserted and engaged. The stopper concave groove  2   b  is formed along a circumferential direction of the flange portion  2   a . (A bottom surface of) The stopper concave groove  2   b  is formed in a circular-arc shape in cross section when taken by a plane perpendicular to the axial direction of the cam shaft  2 . The stopper concave groove  2   b  is formed in an outer circumferential surface of the flange portion  2   a  within a predetermined range given in a circumferential direction of the cam shaft  2 . The cam shaft  2  rotates within this circumferential range relative to the sprocket main body is so that one of both end edges of the stopper convex portion  61   b  becomes in contact with the corresponding one of circumferentially-opposed edges  2   c  and  2   d  of the stopper concave groove  2   b . Thereby, a relative rotational position of the cam shaft  2  to the timing sprocket  1  is restricted between a maximum advanced side and a maximum retarded side. 
     The stopper convex portion  61   b  is disposed axially away toward the cam shaft  2  from a point at which the outer race  43   a  of the large-diameter ball bearing  43  is pressed by the spacer  62  for fixing the outer race  43   a  in the axial direction. Accordingly, the stopper convex portion  61   b  is not in contact with the fixing end portion  9   a  of the follower member  9 . Therefore, an interference between the stopper convex portion  61   b  and the fixing end portion  9   a  can be sufficiently suppressed. 
     The stopper convex portion  61   b  and the stopper concave groove  2   b  constitute a stopper mechanism. 
     As shown in  FIG. 1 , the cam bolt  10  includes a head portion  10   a  and a shaft portion  10   b . A washer portion  10   c  formed in an annular shape is provided on an end surface of the head portion  10   a  which is located on the side of the shaft portion  10   b . An outer circumference of the shaft portion  10   b  includes a male thread portion  10   d  which is screwed into a female threaded portion of the cam shaft  2 . The female threaded portion of the cam shaft  2  is formed from the end portion of the cam shaft  2  toward an inside of the cam shaft  2  in the axial direction. 
     The follower member  9  which functions as a driven rotating member is integrally formed of an iron-based metal. As shown in  FIG. 1 , the follower member  9  includes the fixing end portion  9   a , a cylindrical portion (circular tube portion)  9   b  and a cylindrical retainer  41 . The fixing end portion  9   a  is in a circular-plate shape and is formed in a rear end side of the follower member  9 . The cylindrical portion  9   b  protrudes in the axial direction from a front end of an inner circumferential portion of the fixing end portion  9   a . The retainer  41  is formed integrally with an outer circumferential portion of the fixing end portion  9   a , and retains or guides a plurality of rollers  48 . 
     A rear end surface of the fixing end portion  9   a  is in contact with the front end surface of the flange portion  2   a  of the cam shaft  2 . The fixing end portion  9   a  is pressed and fixed to the flange portion  2   a  in the axial direction by an axial force of the cam bolt  10 . 
     As shown in  FIG. 1 , the cylindrical portion  9   b  is formed with an insertion hole  9   d  passing through a center of the cylindrical portion  9   b  in the axial direction. The shaft portion  10   b  of the cam bolt  10  is passed through the insertion hole  9   d . Moreover, a needle bearing  38  functions as a bearing member is provided on an outer circumferential side of the cylindrical portion  9   b.    
     As shown in  FIGS. 1 ,  3  and  4 , the retainer  41  is formed in a cylindrical shape (circular-tube shape) having its bottom and protruding from the bottom in the extending direction of the cylindrical portion  9   b . The retainer  41  is bent in a substantially L-shape in cross section from a front end of the outer circumferential portion of the fixing end portion  9   a . A tubular tip portion  41   a  of the retainer  41  extends and exits through a space portion  44  toward the bottom portion  5   b  of the housing  5 . The space portion  44  is an annular concave portion formed between the female-thread constituting portion  6  and the extending portion  5   d . Moreover, a plurality of roller-retaining holes  41   b  are formed in the tubular tip portion  41   a  substantially at circumferentially equally-spaced intervals. Each of the plurality of roller-retaining holes  41   b  is formed in a substantially rectangular shape in cross section, and functions as a roller retaining portion which retains the roller  48  to allow a rolling movement of the roller  48 . The total number of the roller-retaining holes  41   b  (or the total number of the rollers  48 ) is smaller by one than the total number of the internal teeth  19   a  of the internal-teeth constituting portion  19 . 
     An inner-race fixing portion  63  is formed in a cut-out manner between the outer circumferential portion of the fixing end portion  9   a  and a bottom-side connecting portion of the retainer  41 . The inner-race fixing portion  63  fixes or fastens the inner race  43   b  of the large-diameter ball bearing  43 . 
     The inner-race fixing portion  63  is formed by cutting the follower member  9  in a stepped manner (multilevel manner) such that the inner-race fixing portion  63  faces the outer-race fixing portion  60  in the radial direction. The inner-race fixing portion  63  includes an outer circumferential surface  63   a  and a second fixing stepped surface (multilevel-linking surface)  63   b . The outer circumferential surface  63   a  is in an annular shape (tubular shape) extending in the axial direction of the cam shaft  2 . The second fixing stepped surface  63   b  is formed integrally with the outer circumferential surface  63   a  on a side opposite to an opening of the outer circumferential surface  63   a , and extends in the radial direction. The inner race  43   b  of the large-diameter ball bearing  43  is fitted into the outer circumferential surface  63   a  in the axial direction by means of press fitting. Thereby, an inner end surface  43   f  of the press-fitted inner race  43   b  becomes in contact with the second fixing stepped surface  63   b , so that an axial positioning of the inner race  43   b  is done. 
     The phase change mechanism  4  includes the electric motor  12  and the speed-reduction mechanism  8 . The electric motor  12  is disposed on a front end side of the cam shaft  2 , substantially coaxially to the cam shaft  2 . The speed-reduction mechanism  8  functions to reduce a rotational speed of the electric motor  12  and to transmit the reduced rotational speed to the cam shaft  2 . 
     As shown in  FIGS. 1 and 3 , the electric motor  12  is a brush DC motor. The electric motor  12  is constituted by the housing  5 , a motor output shaft  13 , a pair of permanent magnets  14  and  15 , and a stator  16 . The housing  5  is a yoke which rotates integrally with the timing sprocket  1 . The motor output shaft  13  is arranged inside the housing  5  to be rotatable relative to the housing  5 . The pair of permanent magnets  14  and  15  are fixed to an inner circumferential surface of the housing  5 . Each of the pair of permanent magnets  14  and  15  is formed in a half-round arc shape. The stator  16  is fixed to the sealing plate  11 . 
     The motor output shaft  13  is formed in a stepped tubular shape (in a cylindrical shape having multileveled surface), and functions as an armature. The motor output shaft  13  includes a large-diameter portion  13   a , a small-diameter portion  13   b , and a stepped portion (multilevel-linking portion)  13   c . The stepped portion  13   c  is formed at a substantially axially center portion of the motor output shaft  13 , and is a boundary between the large-diameter portion  13   a  and the small-diameter portion  13   b . The large-diameter portion  13   a  is located on the side of the cam shaft  2  whereas the small-diameter portion  13   b  is located on the side of the brush retaining member  28 . An iron-core rotor  17  is fixed to an outer circumference of the large-diameter portion  13   a . The eccentric shaft portion  39  is fitted and fixed into the large-diameter portion  13   a  in the axial direction by means of press fitting, so that an axial positioning of the eccentric shaft portion  39  is done by an inner surface of the stepped portion  13   c.    
     On the other hand, an annular member (tubular member)  20  is fitted over and fixed to an outer circumference of the small-diameter portion  13   b  by press fitting. A commutator  21  is fitted over and fixed to an outer circumferential surface of the annular member  20  by means of press fitting in the axial direction. Hence, an outer surface of the stepped portion  13   c  performs an axial positioning of the annular member  20  and the commutator  21 . An outer diameter of the annular member  20  is substantially equal to an outer diameter of the large-diameter portion  13   a . An axial length of the annular member  20  is slightly shorter than an axial length of the small-diameter portion  13   b.    
     The axial positioning (i.e., location setting) for both of the eccentric shaft portion  39  and the commutator  21  is performed by the inner and outer surfaces of the stepped portion  13   c . Accordingly, an assembling work is easy while an accuracy of the positioning is improved. 
     A front edge of the small-diameter portion  13   b  faces an inner surface  3   f  of the cover main body  3   a  of the cover member  3 . A space S 1  having a predetermined width is formed between the front edge of the small-diameter portion  13   b  and the inner surface  3   f  of the cover main body  3   a.    
     Lubricating oil is supplied to an inside space of the motor output shaft  13  and the eccentric shaft portion  39  in order to lubricate the bearings  37  and  38 . A plug member (plug)  55  is fixedly fitted into an inner circumferential surface of the small-diameter portion  13   b  by press fitting. The plug member  55  inhibits the lubricating oil from leaking to the external. 
     As shown in  FIGS. 1 and 2 , the plug member  55  is formed in a substantially U-shape in cross section. The plug member  55  includes a core member  56  and an elastic body  57 . The core member  56  is made of metal. The elastic body  57  coats (is molded to) an entire surface of the core member  56 , i.e. coats an entire exterior of the core member  56 . 
     The core member  56  includes a disk-like main body  56   a , and an outer circumferential portion  56   b  formed integrally with an outer circumferential edge of the main body  56   a . The core member  56  is formed in a flange shape by bending the outer circumferential portion  56   b  toward the ball bearing  37  in a manner of L-shape in cross section. Whole of the core member  56  is substantially in the form of “[” (square bracket) or “U” in cross section. Moreover, the disk-like main body  56   a  is formed with a circular through-hole  56   c  having a relatively large diameter. The circular through-hole  56   c  passes through a substantially center portion of the disk-like main body  56   a . That is, whole of the core member  56  is formed in a bottomed tubular shape (bottomed cylindrical shape) having the circular through-hole  56   c  in the bottom of the core member  56 . 
     On the other hand, the elastic body  57  is made of a flexible or pliant material such as a synthetic rubber. The elastic body  57  is integrally attached and fixed to whole of inner and outer circumferential surfaces of the main body  56   a  and also whole of inner and outer circumferential surfaces of the outer circumferential portion  56   b , by means of vulcanization adhesion. A circular wall portion  57   a  of the elastic body  57  which is located at a center of the elastic body  57  closes (fills) the circular through-hole  56   c  of the disk-like main body  56   a . An outer diameter of an outer circumferential portion  57   b  of the elastic body  57  is formed to be slightly larger than an inner diameter of the small-diameter portion  13   b  of the motor output shaft  13 . Thereby, a margin of the plug member  55  which causes the press-fitting against the inner circumferential surface of the small-diameter portion  13   b  is secured. Hence, the plug member  55  is elastically in contact with the inner circumferential surface of the small-diameter portion  13   b  so that the plug member  55  liquid-tightly seals between the axial inside and outside of the motor output shaft  13 . 
     The iron-core rotor  17  is formed of magnetic material having a plurality of magnetic poles. An outer circumferential side of the iron-core rotor  17  constitutes bobbins each having a slot. (A coil wire of) An electromagnetic coil  18  is wound on the bobbin. 
     The commutator  21  is made of electrical conductive material and is formed in an annular shape. The commutator  21  is divided into segments. The number of the segments is equal to the number of poles of the iron-core rotor  17 . Each of the segments of the commutator  21  is electrically connected to a terminal of the coil wire (not shown) of the electromagnetic coil  18 . That is, a tip of the terminal of the coil wire is sandwiched by a turn-back portion of the commutator  21  which is formed on an inner circumferential side of the electromagnetic coil  18 , so that the commutator  21  is electrically connected to the electromagnetic coils  18 . 
     The permanent magnets  14  and  15  are formed in a cylindrical shape (circular-tube shape), as a whole. The permanent magnets  14  and  15  have a plurality of magnetic poles along a circumferential direction thereof. An axial location of the permanent magnets  14  and  15  is deviated (offset) in the frontward direction from an axial location of the iron-core rotor  17 . That is, with respect to the axial direction, a center of the permanent magnet  14  or  15  is located at a frontward site beyond a center of the iron-core rotor  17  by a predetermined distance, as shown in  FIG. 1 . In other words, the stator  16  is closer to the center of the permanent magnet  14  or  15  than to the center of the iron-core rotor  17  by the predetermined distance, with respect to the axial direction. 
     Thereby, a front end portion of the permanent magnet  14 ,  15  overlaps with the commutator  21  and also an after-mentioned first brush  25   a ,  25   b  of the stator  16  and so on, in the radial direction. 
     As shown in  FIG. 6 , the stator  16  mainly includes a resin plate  22 , a pair of resin holders  23   a  and  23   b , a pair of first brushes  25   a  and  25   b  each functioning as a switching brush (commutator), inner and outer slip rings  26   a  and  26   b , and pigtail harnesses  27   a  and  27   b . The resin plate  22  is formed in a circular plate shape, and is formed integrally with an inner circumferential portion of the sealing plate  11 . The pair of resin holders  23   a  and  23   b  are provided on an inside portion (cam-shaft-side portion) of the resin plate  22 . The pair of first brushes  25   a  and  25   b  are received or accommodated respectively in the pair of resin holders  23   a  and  23   b  such that the first brushes  25   a  and  25   b  are able to slide in contact with the resin holders  23   a  and  23   b  in the radial direction. Thereby, a tip surface of each of the first brushes  25   a  and  25   b  is elastically in contact with an outer circumferential surface of the commutator  21  in the radial direction by a spring force of coil spring  24   a ,  24   b . Each of the inner and outer slip rings  26   a  and  26   b  is formed in an annular shape. The inner and outer slip rings  26   a  and  26   b  are buried in and fixed to front end surfaces of the resin holders  23   a  and  23   b  under a state where outer end surfaces (front end surfaces) of the slip rings  26   a  and  26   b  are exposed to the space S 1 . As shown in  FIG. 1 , the inner and outer slip rings  26   a  and  26   b  are disposed at an identical axial location and are disposed at radially inner and outer locations in a manner of radially-double layout. The pigtail harness  27   a  electrically connects the first brush  25   a  with the slip ring  26   b  whereas the pigtail harness  27   b  electrically connects the first brush  25   b  with the slip ring  26   a . It is noted that the slip rings  26   a  and  26   b  constitute a part of a power-feeding mechanism according to the present invention. Moreover, the first brushes  25   a  and  25   b , the commutator  21 , the pigtail harnesses  27   a  and  27   b  and the like constitute an energization switching section (switching means) according to the present invention. 
     A positioning of the sealing plate  11  is given by a concave stepped portion formed in an inner circumference of the front end portion of the housing  5 . The sealing plate  11  is fixed into the concave stepped portion of the housing  5  by caulking. A shaft insertion hole  11   a  is formed in the sealing plate  11  to pass through a center portion of the sealing plate  11  in the axial direction. One end portion of the motor output shaft  13  and so on are passing through the shaft insertion hole  11   a.    
     The brush retaining member  28  is fixed to the cover main body  3   a . The brush retaining member  28  is integrally molded by synthetic resin material, and constitutes the power-feeding mechanism. As shown in  FIG. 1 , the brush retaining member  28  is substantially formed in an L-shape as viewed laterally, i.e., in cross section taken by a plane parallel to the axial direction and parallel to an extending direction of an after-mentioned terminal strip  31 . The brush retaining member  28  mainly includes a brush retaining portion  28   a , a connector portion  28   b , a pair of bracket portions  28   c  and  28   c , and a pair of terminal strips  31  and  31 . The brush retaining portion  28   a  is substantially in a cylindrical shape, and is inserted in the retaining hole  3   d . The connector portion  28   b  is located on an upper end portion of the brush retaining portion  28   a . The pair of bracket portions  28   c  and  28   c  are formed integrally with the brush retaining portion  28   a , and protrude from both sides of the brush retaining portion  28   a  in both directions perpendicular to the axial direction and perpendicular to the extending direction of the terminal strip  31 . Through the pair of bracket portions  28   c  and  28   c , the brush retaining member  28  is fixed to the cover main body  3   a . A major part of the pair of terminal strips  31  and  31  is buried in the connector portion  28   b.    
     The pair of terminal strips  31  and  31  extend in the upper-lower direction, and extend parallel to each other. The pair of terminal strips  31  and  31  are formed in a crank shape. One end (lower end)  31   a  of each of the terminal strips  31  and  31  is exposed at a bottom portion of the brush retaining portion  28   a  whereas another end (upper end)  31   b  of each of the terminal strips  31  and  31  is introduced in a female fitting groove  28   d  of the connector portion  28   b  and protrudes from a bottom of the female fitting groove  28   d , as shown in  FIG. 1 . Moreover, the another ends  31   b  and  31   b  of the terminal strips  31  and  31  are electrically connected through a male connector (not shown) to a battery power source. 
     The brush retaining portion  28   a  is provided to extend in a substantially horizontal direction (i.e., in the axial direction). The brush retaining portion  28   a  is formed with through-holes each having a cylindrical-column shape, at upper and lower portions of an inside of the brush retaining portion  28   a . Sliding members  29   a  and  29   b  each having a sleeve shape are provided respectively in the upper and lower through-holes of the brush retaining portion  28   a , and are respectively fixed to the upper and lower through-holes of the brush retaining portion  28   a . Second brushes  30   a  and  30   b  are received and retained respectively in the sliding members  29   a  and  29   b  to allow the second brushes  30   a  and  30   b  to slide in contact with the sliding members  29   a  and  29   b  in the axial direction. A tip surface of each of the second brushes  30   a  and  30   b  is in contact with the slip ring  26   a ,  26   b  in the axial direction. 
     Each of the second brushes  30   a  and  30   b  is formed in a substantially rectangular-parallelepiped shape. Each of second coil springs  32   a  and  32   b  is elastically disposed between the second brush  30   a ,  30   b  and the one end  31   a  of the terminal strip  31  which is exposed to a bottom portion of the through-hole of the brush retaining portion  28   a . The second brushes  30   a  and  30   b  are biased respectively toward the slip rings  26   b  and  26   a  by spring forces of the second coil springs  32   a  and  32   b . The large-diameter oil seal  50  prevents lubricating oil from entering a gap between the slip ring  26   a ,  26   b  and the second brush  30   a ,  30   b.    
     A pigtail harness  33   a  having a flexibility is disposed between a front end portion (a hole-bottom-side end portion) of the second brush  30   a  and one of the one ends  31   a  and  31   a  of the terminal strips  31  and  31 , and is attached to the front end portion of the second brush  30   a  and the one of the one ends  31   a  and  31   a  by welding. In the same manner, a pigtail harness  33   b  having a flexibility is disposed between a front end portion of the second brush  30   b  and another of the one ends  31   a  and  31   a  of the terminal strips  31  and  31 , and is attached to the front end portion of the second brush  30   b  and the another of the one ends  31   a  and  31   a  by welding. Thereby, the second brushes  30   a  and  30   b  are electrically connected to the terminal strips  31  and  31 . A length of each of the pigtail harnesses  33   a  and  33   b  is designed to restrict a maximum sliding position of the second brush  30   a ,  30   b  such that the second brush  30   a ,  30   b  is prevented from dropping out from the sliding member  29   a ,  29   b  when the second brush  30   a ,  30   b  has moved and slid in an axially-outward direction at the maximum by the second coil spring  32   a ,  32   b.    
     Moreover, an annular (ring-shaped) seal member  34  is fitted into and held by an annular fitting groove which is formed on an outer circumference of a base portion side of the brush retaining portion  28   a . The annular seal member  34  becomes elastically in contact with a tip surface of the cylindrical wall  3   c  to seal an inside of the brush retaining portion  28   a  when the brush retaining portion  28   a  is inserted into the retaining hole  3   d.    
     The male connector (not shown) is inserted into the female fitting groove  28   d  which is located at an upper end portion of the connector portion  28   b . The another ends  31   b  and  31   b  which are exposed to the female fitting groove  28   d  of the connector portion  28   b  are electrically connected through the male connector to a control unit (not shown). 
     As shown in  FIG. 3 , each of the bracket portions  28   c  and  28   c  is formed in a substantially triangular shape and is formed with a bolt insertion hole  28   e . Theses bolt insertion holes  28   e  and  28   e  located at both sides of the brush retaining portion  28   a  axially pass through the bracket portions  28   c  and  28   c . A pair of bolts are respectively inserted through the bolt insertion holes  28   e  and  28   e , and are screwed into a pair of female threaded holes (not shown) formed in the cover main body  3   a . Thereby, the brush retaining member  28  is fixed to the cover main body  3   a  through the bracket portions  28   c  and  28   c.    
     The motor output shaft  13  and the eccentric shaft portion  39  are rotatably supported by the small-diameter ball bearing  37  and the needle bearing  38 . The small-diameter ball bearing  37  is a bearing member provided on an outer circumferential surface of a head-portion-side portion of the shaft portion  10   b  of the cam bolt  10 . The needle bearing  38  is provided on an outer circumferential surface of the cylindrical portion  9   b  of the follower member  9 , and is located axially adjacent to the small-diameter ball bearing  37 . 
     The needle bearing  38  includes a cylindrical retainer  38   a  and a plurality of needle rollers  38   b . The retainer  38   a  is formed in a cylindrical shape (circular-tube shape), and is fitted in an inner circumferential surface of the eccentric shaft portion  39  by press fitting. Each needle roller  38   b  is a rolling element supported rotatably inside the retainer  38   a . The needle rollers  38   b  roll on the outer circumferential surface of the cylindrical portion  9   b  of the follower member  9 . 
     An inner race of the small-diameter ball bearing  37  is fixed between a front end edge of the cylindrical portion  9   b  of the follower member  9  and a washer  10   c  of the cam bolt  10  in a sandwiched state. On the other hand, an outer race of the small-diameter ball bearing  37  is fixedly fitted in a stepped diameter-enlarged portion of the inner circumferential surface of the eccentric shaft portion  39  by press fitting. The outer race of the small-diameter ball bearing  37  is axially positioned by contacting a step edge (barrier) formed in the stepped diameter-enlarged portion of the inner circumferential surface of the eccentric shaft portion  39 . 
     A small-diameter oil seal  46  is provided between the outer circumferential surface of the motor output shaft  13  (eccentric shaft portion  39 ) and an inner circumferential surface of the extending portion  5   d  of the housing  5 . The oil seal  46  prevents lubricating oil from leaking from an inside of the speed-reduction mechanism  8  into the electric motor  12 . The oil seal  46  separates the electric motor  12  from the speed-reduction mechanism  8  by a searing function of the oil seal  46 . 
     The control unit detects a current operating state of the engine on the basis of information signals derived from various kinds of sensors and the like, such as a crank angle sensor, an air flow meter, a water temperature sensor and an accelerator opening sensor (not shown). Thereby, the control unit controls the engine. Moreover, the control unit performs a rotational control for the motor output shaft  13  by supplying electric power to the electromagnetic coils  18 . Thereby, the control unit controls a relative rotational phase of the cam shaft  2  to the timing sprocket  1 , through the speed-reduction mechanism  8 . 
     As shown in  FIGS. 1 and 3 , the speed-reduction mechanism  8  is mainly constituted by the eccentric shaft portion  39 , a medium-diameter ball bearing  47 , the rollers  48 , the retainer  41 , and the follower member  9  formed integrally with the retainer  41 . The eccentric shaft portion  39  conducts an eccentric rotational motion. The medium-diameter ball bearing  47  is provided on an outer circumference of the eccentric shaft portion  39 . The rollers  48  are provided on an outer circumference of the medium-diameter ball bearing  47 . The retainer  41  retains (guides) the rollers  48  along a rolling direction of the rollers  48 , and permits a radial movement of each roller  48 . 
     The eccentric shaft portion  39  is formed in a stepped cylindrical shape (stepped circular-tube shape) having a multilevel diameter. A small-diameter portion  39   a  of the eccentric shaft portion  39  which is located in a front end side of the eccentric shaft portion  39  is fixedly fitted in an inner circumferential surface of the large-diameter portion  13   a  of the motor output shaft  13  by press fitting. As shown in  FIG. 4 , an outer circumferential surface of a large-diameter portion  39   b  of the eccentric shaft portion  39  which is located in a rear end side of the eccentric shaft portion  39 , i.e. a cam surface of the eccentric shaft portion  39  has a center (axis) Y which is eccentric (deviated) slightly from a shaft center X of the motor output shaft  13  in the radial direction. 
     Substantially whole of the medium-diameter ball bearing  47  overlaps with the needle bearing  38  in the radial direction, i.e., the medium-diameter ball bearing  47  is located approximately within an axial existence range of the needle bearing  38 . The medium-diameter ball bearing  47  includes an inner race  47   a , an outer race  47   b , and a ball(s)  47   c  interposed between both the races  47   a  and  47   b . The inner race  47   a  is fixed to the outer circumferential surface of the eccentric shaft portion  39  by press fitting. The outer race  47   b  is not fixed in the axial direction, and thereby is in an axially freely-movable state. That is, one of axial end surfaces of the outer race  47   b  which is closer to the electric motor  12  is not in contact with any member whereas another of the axial end surfaces of the outer race  47   b  faces an inside surface of the retainer  41  to have a first clearance (minute clearance) C between the another of the axial end surfaces of the outer race  47   b  and the inside surface of the retainer  41 . Moreover, an outer circumferential surface of the outer race  47   b  is in contact with an outer circumferential surface of each of the rollers  48  so as to allow the rolling motion of each roller  48 . An annular second clearance C 1  is formed on the outer circumferential surface of the outer race  47   b . By virtue of the second clearance C 1 , whole of the medium-diameter ball bearing  47  can move in the radial direction in response to an eccentric rotation (of the outer circumferential surface of the large-diameter portion  39   b ) of the eccentric shaft portion  39 , i.e., can perform an eccentric movement. 
     Each of the rollers  48  is formed of iron-based metal. With the eccentric movement of the medium-diameter ball bearing  47 , the respective rollers  48  move in the radial direction and are fitted in the internal teeth  19   a  of the internal-teeth constituting portion  19 . Also, with the eccentric movement of the medium-diameter ball bearing  47 , the rollers  48  are forced to do a swinging motion in the radial direction while being guided in the circumferential direction by both side edges of the roller-retaining holes  41   b  of the retainer  41 . That is, the rollers  48  are moved closer to the internal teeth  19   a  and are moved away from the internal teeth  19   a , repeatedly, by the eccentric movement of the medium-diameter ball bearing  47 . 
     Lubricating oil is supplied into the speed-reduction mechanism  8  by a lubricating-oil supplying means (supplying section). This lubricating-oil supplying means includes an oil supply passage, an oil supply hole  51 , an oil hole  52  having a small hole diameter, and three oil discharge holes (not shown) each having a large hole diameter. The oil supply passage is formed inside the bearing of the cylinder head. Lubricating oil is supplied from a main oil gallery (not shown) to the oil supply passage. The oil supply hole  51  is formed inside the cam shaft  2  to extend in the axial direction as shown in  FIG. 1 . The oil supply hole  51  communicates though a groove(s) with the oil supply passage. The oil hole  52  is formed inside the follower member  9  to pass through the follower member  9  in the axial direction. One end of the oil hole  52  is open to the oil supply hole  51 , and another end of the oil hole  52  is open to a region near the needle bearing  38  and the medium-diameter ball bearing  47 . The three oil discharge holes are formed inside the follower member  9  to pass through the follower member  9  in the same manner. 
     By the lubricating-oil supplying means, lubricating oil is supplied to the space portion  44  and held in the space portion  44 . Thereby, the lubricating oil lubricates the medium-diameter ball bearing  47  and the rollers  48 . Moreover, the lubricating oil flows to the inside of the eccentric shaft portion  39  and the inside of the motor output shaft  13  so that moving elements such as the needle bearing  38  and the small-diameter ball bearing  37  are lubricated. It is noted that the small-diameter oil seal  46  inhibits the lubricating oil held in the space portion  44  from leaking to the inside of the housing  5 . 
     Next, operations in this embodiment according to the present invention will now be explained. At first, when the crankshaft of the engine is drivingly rotated, the timing sprocket  1  is rotated through the timing chain  42 . This rotative force is transmitted through the internal-teeth constituting portion  19  and the female-thread constituting portion  6  to the housing  5 . Thereby, the electric motor  12  rotates in synchronization. On the other hand, the rotative force of the internal-teeth constituting portion  19  is transmitted through the rollers  48 , the retainer  41  and the follower member  9  to the cam shaft  2 . Thereby, the cam of the cam shaft  2  opens and closes the intake valve. 
     Under a predetermined engine-operating state after the start of the engine, the control unit supplies electric power to the electromagnetic coils  17  of the electric motor  12  through the terminal strips  31  and  31 , the pigtail harnesses  33   a  and  33   b , the second brushes  30   a  and  30   b  and the slip rings  26   b  and  26   a  and the like. Thereby, the rotation of the motor output shaft  13  is driven. This rotative force of the motor output shaft  13  is transmitted through the speed-reduction mechanism  8  to the cam shaft  2  so that a reduced rotation is transmitted to the cam shaft  2 . 
     That is, (the outer circumferential surface of) the eccentric shaft portion  39  eccentrically rotates in accordance with the rotation of the motor output shaft  13 . Thereby, each roller  48  rides over (is disengaged from) one internal tooth  19   a  of the internal-teeth constituting portion  19  and moves to the other adjacent internal tooth  19   a  with its rolling motion while being radially guided by the roller-retaining holes  41   b  of the retainer  41 , every one rotation of the motor output shaft  13 . By repeating this motion sequentially, each roller  48  rolls in the circumferential direction under a contact state. By this contact rolling motion of each roller  48 , the rotative force is transmitted to the follower member  9  while the rotational speed of the motor output shaft  13  is reduced. A speed reduction rate which is obtained at this time can be set at any value by adjusting the number of rollers  48  and the like. 
     Accordingly, the cam shaft  2  rotates in the forward or reverse direction relative to the timing sprocket  1  so that the relative rotational phase between the cam shaft  2  and the timing sprocket  1  is changed. Thereby, opening and closing timings of the intake valve are controllably changed to its advance or retard side. 
     As shown in  FIG. 5 , a maximum positional restriction (angular position limitation) for the forward/reverse relative rotation of cam shaft  2  to the timing sprocket  1  is performed when one of respective lateral surfaces (circumferentially-opposed surfaces) of the stopper convex portion  61   d  becomes in contact with the corresponding one of the circumferentially-opposed surfaces  2   c  and  2   d  of the stopper concave groove  2   b.    
     Specifically, when the follower member  9  rotates (at a higher speed) in the same rotational direction as that of the timing sprocket  1  with the eccentric rotational motion of the eccentric shaft portion  39 , one lateral surface of the stopper convex portion  61   d  becomes in contact with the surface  2   c  of the stopper concave groove  2   b  so that a further relative rotation of the follower member  9  in the same direction is prohibited. Thereby, the relative rotational phase of the cam shaft  2  to the timing sprocket  1  is changed to the advance side at maximum. 
     On the other hand, when the follower member  9  rotates in a relatively opposite rotational direction to that of the timing sprocket  1  (i.e., at a lower speed than the timing sprocket  1 ), another lateral surface of the stopper convex portion  61   d  becomes in contact with the surface  2   d  of the stopper concave groove  2   b  so that a further rotation of the follower member  9  in the relatively-opposite direction is prohibited. Thereby, the relative rotational phase of the cam shaft  2  to the timing sprocket  1  is changed to the retard side at maximum. 
     As a result, the opening and closing timings of the intake valve can be changed to the advance side or the retard side up to its maximum. Therefore, a fuel economy and an output performance of the engine are improved. 
     In this embodiment, the plug member  55  is fitted into and fixed to the inner circumferential surface of the small-diameter portion  13   b  of the motor output shaft  13  by press fitting. By means of liquid-tight sealing of the plug member  55 , lubricating oil supplied from the small-diameter oil hole  52  of the lubricating-oil supplying means to the inside of the eccentric shaft portion  39  in order to lubricate the respective bearings  38  and  37  and the like is prohibited from leaking from a front end side of the motor output shaft  13  toward the external. 
     The plug member  55  is constructed by coating the entire surface (entire appearance) of the core member  56  with the elastic body  57 . Hence, a sealing performance is enhanced by the elastic force of the elastic body  57 . Since the outer circumferential portion  57   b  of the elastic body  57  applies a large press-contact force to the inner circumferential surface of the small-diameter portion  13   b , an easy movement of the plug member  55  by oil pressure can be suppressed. 
     Moreover, in a case that the plug member  55  is desired to be detached from the inside of the small-diameter portion  13   b  of the motor output shaft  13  for the purpose of maintenance of the small-diameter ball bearing  37  or the like after the plug member  55  was fixed to the inner circumferential surface of the small-diameter portion  13   b  by press fitting, the plug member  55  can be easily detached from the inside of the motor output shaft  13  in the following manner. That is, for example, a jig or tool (not shown) having a tip portion formed in a hook shape is used to push and break the wall portion  57   a  which is a center portion of the elastic body  57 , from the outside of the plug member  55  (i.e., from the outside of the small-diameter portion  13   b ). Then, a portion of the core member  56  located near a hole edge of the through-hole  56   c  is made to be hooked or caught on the hook-shaped tip portion of the jig at the inside of the small-diameter portion  13   b . Then, by pulling (drawing) the hooked core member  56  toward the outside of the small-diameter portion  13   b , the plug member  55  is easily detached from the motor output shaft  13 . Therefore, a follow-up maintenance is easy. 
     Second Embodiment 
       FIG. 7  is a view showing a second embodiment according to the present invention. In the second embodiment, a structure of the core member  56  of the plug member  55  is changed in some degree. The main body  56   a  of the core member  56  in the second embodiment is formed with four circular through-holes  56   c  each having a relatively small diameter, also as shown in  FIG. 8 . The respective through-holes  56   c  are formed at circumferentially equally-spaced intervals in the main body  56   a . Specifically, the four through-holes  56   c  are located substantially at 90-degree intervals in the circumferential direction of the main body  56   a . An inner diameter of each of the four through-holes  56   c  is set at a size that enables to insert the hook-shaped tip portion of the jig through the through-hole  56   c.    
     The elastic body  57  is integrally formed to coat or enclose the entire surface of the core member  56  by means of vulcanization adhesion, in the similar manner as in the first embodiment. At this time, four wall portions  57   a  of the elastic body  57  respectively close (fill) the four through-holes  56   c . That is, each of the four wall portions  57   a  is in a circular shape having a small diameter which is substantially equal to the diameter of the through-hole  56   c.    
     Since the other structures are similar to those of the first embodiment, the same operations and advantageous effects as the first embodiment are obtained. In particular, at the time of maintenance, the plug member  55  can be easily detached from the inside of the motor output shaft  13  by breaking one of the four wall portions  57   a  by use of the hook-shaped tip portion of the jig, by hooking an inside portion of the main body  56   a  located near the hole edge of the through-hole  56   c , and then by pulling out the main body  56   a.    
     In the second embodiment, the four through-holes  56   c  are provided. Accordingly, a target for the breaking by the tip portion of the jig can be selected from the four wall portions  57   a  positioned at different locations. Hence, a disinstallation (detaching operation) of the plug member  55  is made easier. 
     Moreover, in the second embodiment, the plurality of through-holes  56   c  are dotted (scattered) in the main body  56   a  of the core member  56 . Accordingly, a central-portion side of the main body  56   a  has a high rigidity, so that the press-contact force that is applied by the elastic body  57  against the inner circumferential surface of the small-diameter portion  13   b  can be set at a large level. 
       FIG. 9  is a view showing a modified example in the second embodiment. In this example, each of the four through-holes  56   c  of the core member  56  is formed in a different shape. That is, the shape of each of the four through-holes  56   c  is changed from the circular shape to a square shape, as viewed from the axial direction. Also, each of the four wall portions  57   a  corresponding to the four through-holes  56   c  is formed in a square shape. 
     Third Embodiment 
       FIG. 10  is a view showing a third embodiment according to the present invention. In the third embodiment, on the assumption that the structures of the first embodiment are basically adopted, a protruding portion  58  is integrally formed with the cover main body  3   a  at a substantially central portion of the inner surface of the cover main body  3   a . The protruding portion  58  protruding toward the plug member  55  is formed in a cylindrical-column shape, and is located substantially coaxially to the motor output shaft  13 . That is, an axis of the protruding portion  58  is substantially identical with an axis of the motor output shaft  13 . Moreover, an outer diameter d of the protruding portion  58  is formed at a substantially constant size over whole the protruding portion  58 . The outer diameter d is smaller than the inner diameter of the small-diameter portion  13   b  of the motor output shaft  13 , and is greater than the diameter of the through-hole  56   c  of the core member  56 . The protruding portion  58  includes a tip portion  58   a  having a tip surface  58   b  formed in a flat shape. The tip portion  58   a  is located radially inside the front end portion of the tubular motor output shaft  13 . In other words, the tip portion  58   a  of the protruding portion  58  overlaps with the motor output shaft  13  in the radial direction, as shown in  FIG. 10 . 
     According the third embodiment, even if the plug member  55  has moved in the frontward direction by an oil pressure (hydraulic pressure) of lubricating oil supplied to the inside space of the tubular motor output shaft  13  or the like, the tip surface  58   b  of the protruding portion  58  becomes in contact with a front end surface of the plug member  55  so as to prevent a further frontward movement of the plug member  55 . Therefore, the plug member  55  can be inhibited from dropping out from a front end of the motor output shaft  13 . 
     In particular, the tip portion  58   a  of the protruding portion  58  extends up to a radially-inside location of the front end portion of the small-diameter portion  13   b  of the motor output shaft  13 . Accordingly, the space S 1  between the front edge of the small-diameter portion  13   b  of the motor output shaft  13  and the inner surface  3   f  of the cover main body  3   a  can be set as a relatively large space. Therefore, a contact between the cover member  3  and the motor output shaft  13  can be avoided even if an oscillating motion (vibrations) or the like occurs. 
     Although the invention has been described above with reference to certain embodiments of the invention, the invention is not limited to the embodiments described above. Modifications and variations of the embodiments described above will occur to those skilled in the art in light of the above teachings. 
     For example, the shape and/or size of the through-hole  56   c  of the core member  56  can be changed to any desired shape and/or size. 
     Configurations 
     Some technical configurations obtainable from the above embodiments according to the present invention will now be listed as follows. 
     [a] A valve-timing control apparatus for an internal combustion engine, comprising: a drive rotating member (e.g.,  1  in the drawings) configured to receive a rotational force from a crankshaft; a driven rotating member ( 9 ) fixed to a cam shaft ( 2 ) and configured to rotate relative to the drive rotating member ( 1 ); an electric motor ( 12 ) configured to rotate the driven rotating member ( 9 ) relative to the drive rotating member ( 1 ) by means of rotary drive of the electric motor ( 12 ); a housing ( 5 ) connected integrally with the drive rotating member ( 1 ), wherein structural components of the electric motor ( 12 ) are accommodated in the housing ( 5 ); a cover member ( 3 ) fixed to a main body of the internal combustion engine and located to face a front end portion of the housing ( 5 ); a slip ring ( 26   a ,  26   b ) configured to supply electric power to the electric motor ( 12 ) and provided to one of the front end portion of the housing ( 5 ) and a facing portion of the cover member ( 3 ) which faces the front end portion of the housing ( 5 ); a brush ( 30   a ,  30   b ) provided to another of the front end portion of the housing ( 5 ) and the facing portion of the cover member ( 3 ), and configured to supply electric power to the electric motor ( 12 ) by an electrical contact with the slip ring ( 26   a ,  26   b ); a tubular motor output shaft ( 13 ) provided inside the housing ( 5 ) to be rotatable relative to the housing ( 5 ), and configured to be rotated by electric-power supply to the electric motor ( 12 ), wherein lubricating oil is supplied into the tubular motor output shaft ( 13 ); a bearing member ( 38 ) provided between an outer circumferential surface of a part of the driven rotating member ( 9 ) and an inner circumferential surface of the tubular motor output shaft ( 13 ); a plug member ( 55 ) fixed to an inner circumferential surface of a tip portion of the tubular motor output shaft ( 13 ), the cover member ( 3 ) facing the tip portion of the tubular motor output shaft ( 13 ), wherein the plug member ( 55 ) is configured to inhibit lubricating oil supplied into the tubular motor output shaft ( 13 ) from leaking to an external; and a seal member ( 50 ) provided between the cover member ( 3 ) and the housing ( 5 ) and configured to inhibit lubricating oil from entering a gap between the slip ring ( 26   a ,  26   b ) and the brush ( 30   a ,  30   b ), wherein the plug member ( 55 ) includes a core member ( 56 ) formed in a bottomed tubular shape having a through-hole ( 56   c ) in a bottom portion of the core member ( 56 ), and an elastic body ( 57 ) coating at least the through-hole ( 56   c ) and an outer circumferential surface of the core member ( 56 ), the elastic body ( 57 ) closing the through-hole ( 56   c ). 
     [b] Alternatively, the plug member (e.g.,  55  in the drawings) includes a core member ( 56 ) formed in a bottomed cylindrical shape having a through-hole ( 56   c ) in a bottom portion of the core member ( 56 ); and a sealing structure configured to maintain a sealed state of the through-hole ( 56   c ) under a state where the lubricating oil supplied into the tubular motor output shaft ( 13 ) takes a maximum pressure level thereof, and to release the sealed state of the through-hole ( 56   c ) when an axial force greater than the maximum pressure level of the lubricating oil is applied to the through-hole ( 56   c ). 
     [c] Further alternatively, the plug member (e.g.,  55  in the drawings) is formed in a bottomed cylindrical shape, and a bottom portion ( 57 ) of the plug member ( 55 ) has a rigidity lower than a rigidity of the other portion ( 56 ) of the plug member ( 55 ). 
     [d] The valve-timing control apparatus as described in the item [a], wherein the elastic body (e.g.,  57  in the drawings) integrally coats the through-hole ( 56   c ) and the outer circumferential surface of the core member ( 56 ) to continue from the through-hole ( 56   c ) to the outer circumferential surface of the core member ( 56 ). 
     [e] The valve-timing control apparatus as described in the item [b], wherein the elastic body (e.g.,  57  in the drawings) coats whole of the core member ( 56 ). 
     [f] The valve-timing control apparatus as described in the item [e], wherein the elastic body (e.g.,  57  in the drawings) coats whole of the core member ( 56 ) such that an outer circumferential portion of the plug member ( 55 ) is a thickest part of the plug member ( 55 ). 
     [g] The valve-timing control apparatus as described in the item [a], wherein the elastic body (e.g.,  57  in the drawings) is made of a rubber material. 
     [h] The valve-timing control apparatus as described in the item [a], wherein the through-hole (e.g.,  56   c  in the drawings) is in a circular shape. 
     [i] The valve-timing control apparatus as described in the item [a], wherein the core member (e.g.,  56  in the drawings) is made of a metal material. 
     [j] The valve-timing control apparatus as described in the item [a], wherein the cover member (e.g.,  3  in the drawings) includes a protruding portion ( 58 ) protruding toward the plug member ( 55 ) from a surface of the cover member which faces the plug member, and at least a part of a tip of the protruding portion ( 58 ) faces at least a part of the core member in an axial direction of the tubular motor output shaft ( 13 ). 
     [k] The valve-timing control apparatus as described in the item [j], wherein an outer diameter of a tip portion of the protruding portion (e.g.,  58  in the drawings) is greater than an inner diameter of the through-hole. 
     [l] A detaching method for the plug member in the valve-timing control apparatus as described in the item [a], the detaching method comprising steps of: inserting a jig through the through-hole by breaking the elastic body (e.g.,  57  in the drawings); and detaching the plug member from the inner circumferential surface of the tubular motor output shaft ( 13 ) by pulling the inserted jig. 
     This application is based on prior Japanese Patent Application No. 2012-275226 filed on Dec. 18, 2012. The entire contents of this Japanese Patent Application are hereby incorporated by reference. 
     The scope of the invention is defined with reference to the following claims.