Abstract:
A webbing winding device having a pre-tensioner for tensioning a webbing for restraining a passenger in a passenger restraining direction in an emergency deceleration of a vehicle, comprising a winding shaft to which an end of the webbing is fixed and a gear train mechanism comprising a plurality of gears arranged in such a manner that adjacent gears mesh with each other, wherein a driving gear which receives an external driving force when the pre-tensioner is actuated is driven from a starting end position to a terminating end, position which are set in advance, so as to rotate the winding shaft in a direction for retracting the webbing, and when a driving of the driving gear is completed, at least a pair of adjacent gears are separated so as to enable the winding shaft to be rotated in the direction for retracting the webbing by inertial force.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a webbing winding device having a pre-tensioner which tenses a passenger restraining webbing of a seat belt apparatus in a passenger restraining direction during an emergency deceleration of a vehicle cause by a vehicle collision or the like. 
     2. Description of the Related Art 
     Some types of webbing winding device for use in a vehicle seat belt unit have a pre-tensioner for removing play in the webbing by retracting the webbing so that it becomes tense during an emergency deceleration of a vehicle caused by a collision or the like. This type of webbing winding device has been described in, for example, Japanese Patent No. 2500192. The belt retractor which is a webbing winding device described in Japanese Patent No. 2500192 comprises a belt drum around which a belt web (webbing) is wound, a pinion connected to this belt drum via a free wheel connecting apparatus and gear transmitting means for transmitting rotation in one direction only, a rack which meshes with this pinion and a piston driving apparatus in which a piston is connected to this rack. When a vehicle is in a collision, the piston driving apparatus is actuated by gas generated by a gas generator so as to move the rack. The pinion is then rotated in a direction for winding up the belt web and the gear transmitting means increases the angular velocity of the rotation of the pinion and transmits it to the belt drum. 
     Further, even if the pre-tensioner is not actuated, the above described webbing winding device is continually urging the webbing winding shaft so as to reduce play in the webbing. However, the winding force for the webbing from the winding shaft cannot be made very strong in order to avoid imparting a feeling of strong pressure to a passenger from the webbing. AS a result, sometimes there is a slight play in the webbing and the amount of winding until the webbing becomes tense during an emergency such as a vehicle collision varies depending on the magnitude of this play. Therefore, in a conventional webbing winding device, in order to ensure that the webbing remains tense during an emergency such as a vehicle collision, the play in the webbing is assumed to be substantially at the maximum so that the webbing winding amount, which is the amount the webbing can be wound up by the pre-tensioner, is set as a large amount. 
     To increase the winding amount of the webbing by the pre-tensioner in the above described webbing winding device, it is necessary to increase the travelling stroke of the rack by the piston driving apparatus or increase the acceleration ratio from the gear transmission means so as to increase the distance rotated by the belt drum with respect to the distance traveled by the rack. However, if the movement stroke of the rack by the piston apparatus is increased, the lengths of the piston driving apparatus and rack are extended so that the size of the apparatus increases. Further, because the stages of the gears constituting the gear transmitting means need to be multiplied in order to increase the acceleration ratio of the gear transmission means, the size of the apparatus is enlarged. 
     SUMMARY OF THE INVENTION 
     Accordingly, in view of the above problems, an object of the present invention is to provide a small webbing winding device capable of reliably winding the webbing onto a winding shaft until the webbing becomes tense when the pre-tensioner is actuated, even if play in the webbing is large. 
     According to a first aspect of the present invention, there is provided a webbing winding device having a pre-tensioner for tensioning a webbing for restraining a passenger in a passenger restraining direction in an emergency deceleration of a vehicle, the webbing winding device comprising: a winding shaft to which an end of the webbing is fixed; and a gear train mechanism comprising a plurality of gears arranged in such a manner that adjacent gears mesh with each other, wherein a driving gear which receives an external driving force when the pre-tensioner is actuated is driven from a starting end position to a terminating position, which are set in advance, so as to rotate the winding shaft in a direction for retracting the webbing, and when a driving of the driving gear is completed, at least a pair of adjacent gears are separated so as to enable the winding shaft to be rotated in the direction for retracting the webbing by inertial force. 
     In the gear train mechanism of the webbing winding device having the above structure, when the pre-tensioner is actuated, a driving gear is driven from a starting end position set in advance to a terminating end position set in advance so as to rotate the winding shaft in the webbing winding direction. When the driving of the driving gear is completed, at least a pair of adjacent gears are separated so as to enable the winding shaft to rotate in the webbing retracting direction due to an inertial force. As a result, when the pre-tensioner is actuated, the winding shaft is rotated by torque transmitted from the gear train mechanism enabling the webbing to be wound around the winding shaft. When the driving of the driving gear is completed, at least a pair of adjacent gears are separated so that a rotation of the winding shaft in the webbing retracting direction is not blocked. Thus, if there is play in the webbing after the driving of the driving gear is completed, the tension of the webbing to be applied to the winding shaft as a rotation load decreases. As a result, the winding shaft rotates in the webbing retracting direction due to inertial force. Thus, even if the driving gear arrives at the terminal end position before the webbing becomes tense, the winding shaft continues to rotate in the webbing retracting direction until a tension balancing the inertial force of the winding shaft is generated in the webbing. 
     Therefore, if the inertial force generated in the winding shaft when the driving of the driving gear is completed is set large enough, even if the play in the webbing is large, the winding shaft can be rotated in the webbing retracting direction until the play in the webbing fitted around a passenger is removed. 
     The term rotation of the winding shaft due to inertial force used here means a rotating motion of the winding shaft generated by inertial force applied to the winding shaft. The inertial force to be applied to the winding shaft includes inertial force based on the mass of a gear or the like rotating through a link with the winding shaft as well as inertial force based on the mass of the winding shaft itself. 
     In the first aspect of the webbing winding device of the present invention, the gear train mechanism is preferably provided at a position at least corresponding to the terminal end position of one gear and along a pitch line, and has a toothless portion forming a gap between the driving gear and another adjacent gear when the driving of the driving gear is completed. 
     In the webbing winding device having the above structure, when the driving of the driving gear is completed, a gap is formed between at least one gear and a gear adjacent to this gear by the toothless portion. Thus, when the driving of the driving gear is completed, at least one pair of adjacent gears in the gear train mechanism can be separated. Therefore, blocking of the rotation of the winding shaft by the gear train mechanism can be prevented so that the winding shaft can be rotated in the webbing retracting direction by the inertial force. 
     According to a second aspect of the present invention, there is provided a webbing winding device having a pre-tensioner for tensioning a webbing for restraining a passenger in a passenger restraining direction in an emergency deceleration of a vehicle, the webbing winding device comprising: a winding shaft to which an end of the webbing is fixed; a gear train mechanism comprising a plurality of gears arranged in such a manner that adjacent gears mesh with each other, wherein a driving gear which receives an driving force when the pre-tensioner is actuated is driven from a starting end position to a terminating end position, which are set in advance, so as to rotate the winding shaft in a direction for retracting the webbing; and engagement release means which, when the driving of the driving gear is completed, moves at least one gear of the gear train mechanism to a position any from another adjacent gear so as to enable the winding shaft to be rotated in the webbing retracting direction by inertial force. 
     In the webbing winding device having the above structure, when the driving of the driving gear is completed, the engagement release means moves at least one gear in a gear train mechanism to a position away from another adjacent gear so as to enable the winding shaft to be rotated in the webbing retracting direction by inertial force. As a result, when the driving gear is driven, the winding shaft is rotated in the webbing retracting direction by torque transmitted from an output gear so that the webbing can be wound around the winding shaft. Further, when the driving of the driving gear is completed, at least one gear in the gear train mechanism is separated from another adjacent gear so that a rotation of the winding shaft in the webbing retracting direction is not blocked. Therefore, if there is play in the webbing when the driving of the driving gear is completed, the tension of the webbing acting or the winding shaft as a rotation load decreases so that the winding shaft is rotated in the webbing retracting direction by inertial force. Therefore, even if the driving gear arrives at the terminating end position before the webbing becomes tense, the winding shaft rotates in the webbing retracting direction until a tension balancing the inertial force of the winding shaft is generated. 
     In the webbing winding device according to the second aspect of the present invention, preferably the gear train mechanism comprises a pinion and a rack formed as the driving gear which meshes with the pinion, and the engagement releasing means has a guide member for changing a direction of movement of the rack moving along the pitch line with respect to the pinion at an intermediate position between the starting end position and terminating end position so as to release the rack from the pinion near the terminating end position. 
     In the webbing winding device having the above structure, the guiding member changes the direction of the moment of the rack moving along the pitch line relative to the pinion at an intermediate position and releases the rack from the pinion in the vicinity of the terminating end position. Therefore, because the rack can be released from the pinion when the driving is completed, blocking of a rotation of the winding shaft by the gear train mechanism can be prevented so that the winding shaft can be rotated in the webbing retracting direction by inertial force. 
     The pitch line mentioned here includes a pitch circle having a fixed curvature radius as well as a pitch line running in a straight line. 
     In the webbing winding device according to the second aspect of the present invention, preferably the gear train mechanism comprises a pinion and a rack constituted formed as the driving gear which meshes with the pinion and whose rear end tooth portion which corresponds to the terminating end position is separable from other remaining portions, and the engagement releasing means has a release guide member for changing a direction of movement of the rear end tooth portion moving along the pitch line with respect to the pinion at an intermediate position between the starting end position and terminating end position so as to release the rear end tooth portion from the pinion near the terminating end position. 
     In the rack of the webbing winding device having the structure, the rear end tooth which portion corresponds to the terminating end position is separable from the other remaining portions, and the release guiding member changes the direction of movement of the rear end tooth portion moving along the pitch line with respect to the pinion at an intermediate position. Further, the rear end tooth portion is released from the pinion near terminating end position. Thus, when the driving is completed, the rear end tooth portion of the rack can be separated from the pinion. Therefore, blocking of the rotation of the winding shaft by the gear train mechanism can be prevented so that the winding shaft can be rotated in the webbing retracting direction by inertial force. 
     In the webbing winding device according to the second aspect of the present invention, preferably the engagement release means comprises; a gear supporting body supporting at least one gear in the gear train mechanism movably along an axis; and a release driving member for moving a gear supported by the gear supporting body along the axis when the driving of the driving gear is completed along the axis so as to release the gear from other adjacent gears. 
     In the webbing winding device having the above structure, when the driving of the driving gear is completed, the release driving member moves the gear supported movably along the axis by the gear supporting body along the axis so as to release the gear from other adjacent gears. As a result, the blocking of the rotation of the winding shaft by the gear train mechanism can be prevented, so that the winding shaft can be rotated in the webbing retracting direction by inertial force. 
     According to a third aspect of the present invention, there is provided a webbing winding device having a pre-tensioner for a tensioning webbing for restraining a passenger in a passenger restraining direction in an emergency deceleration of a vehicle, the webbing winding device comprising: a winding shaft to which an end of the webbing is fixed; and a gear train mechanism comprising a plurality of gears including an internal gear, a sun gear and a planet gear which meshes with these gears, wherein a driving gear which receives an external driving force when the pre-tensioner is actuated is driven from a starting end position to a terminating end position, which are set in advance, so as to rotate the winding shaft in a direction for retracting the webbing, and when a driving of the driving gear is completed, the internal gear is supported movably along a pitch line of the internal gear so that the planet gear is capable of rotating in a direction corresponding to the webbing retracting direction. 
     In the webbing winding device having the above structure, when the pre-tensioner is actuated, the driving gear is driven from the starting end position to the terminating end position, which are set in advance, and when the driving of the driving gear is completed, a part or all of the internal gear moves along the pitch line of the internal gear so that the planet gear can rotate in a direction corresponding to the webbing retracting direction. Thus, when the pre-tensioner is actuated, the winding shaft is rotated in the webbing retracting direction by the torque transmitted from the gear train mechanism so that the webbing can be wound around the winding shaft. Further, when the driving of the driving gear is completed, the rotation of the winding shaft in the webbing retracting direction by the gear train mechanism is not blocked. Therefore, if there is a play in the webbing when the driving of the driving gear is completed, the tension in the webbing acting as a rotation load on the winding shaft decreases so that the winding shaft is rotated in the webbing retracting direction by inertial force. Thus, even if the driving gear arrives at the terminating end position before the webbing is tensed, the winding shaft rotates in the webbing retracting direction until a tension balancing the inertial force of the winding shaft is generated. 
     According to a fourth aspect of the present invention, there is provided a webbing winding device having a pre-tensioner for tensioning a webbing for restraining a passenger in a passenger restraining direction in an emergency deceleration of a vehicle, the webbing winding device comprising: a winding shaft to which an end of the webbing is fixed; and a gear train mechanism comprising a plurality of gears including a pinion and a rack which meshes with the pinion, wherein when the pre-tensioner is actuated, the rack which receives an external driving force when the pre-tensioner is activated is driven from a starting end position to a terminating end position, which are set in advance, so as to rotate the winding shaft in a webbing retracting direction, and a part or all of the rack is supported movably along the pitch line of the rack so that the pinion is able to be rotated in a direction corresponding to the webbing retracting direction when the driving of the rack is completed. 
     In the webbing winding device having the above structure, when the pre-tensioner is actuated, the rack meshing with the pinion is driven from the starting end position to the terminating end position, which are set in advance, and when the driving of the rack is completed, a part or all of the rack moves along the pitch line of the rack so that the pinion can rotate in a direction corresponding to the webbing retracting direction. Thus, when the pre-tensioner is actuated, the winding shaft is rotated in the webbing retracting direction by the torque transmitted from the gear train mechanism so that the webbing can be wound around the winding shaft. Further, when the driving of the rack is completed, the rotation of the winding shaft in the webbing retracting direction by the gear train mechanism is not blocked. Therefore, if there is play in the webbing when the driving of the driving gear is completed, the tension in the webbing acting as a rotation load on the winding shaft decreases so that the winding shaft is rotated in the webbing retracting direction by inertial force. Thus, even if the driving gear arrives at the terminating end position before the webbing is tensed, the winding shaft rotates in the webbing retracting direction until a tension balancing the inertial force of the winding shaft is generated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded perspective view showing a structure of a webbing winding device according to a first embodiment of the present invention. 
     FIG. 2 is an axial plan view showing a state prior to an actuation of the pre-tensioner of the webbing winding device according to the first embodiment of the present invention. 
     FIG. 3 is an axial plan view in axial direction showing a state after the actuation of the pre-tensioner of the webbing winding device according to the first embodiment of the present invention. 
     FIG. 4 is an axial sectional view showing a state prior to an actuation of the pre-tensioner of a webbing winding device to which a modification  1  of the gear train mechanism and guide member of the first embodiment is applied. 
     FIG. 5 is an axial sectional view showing a state after an actuation of the pre-tensioner of a webbing winding device to which a modification  1  of the gear train mechanism and guide member of the first embodiment is applied. 
     FIG. 6 is an axial sectional view showing a state prior to an actuation of the pre-tensioner of a webbing winding device to which a modification  2  of the gear train mechanism and guide member of the first embodiment is applied. 
     FIG. 7 is an axial sectional view showing a state after an actuation of the pre-tensioner of a webbing winding device to which a modification  2  of the gear train mechanism and guide member of the first embodiment is applied. 
     FIG. 8 is an exploded perspective view showing a structure of a torque transmission mechanism in the webbing winding device according to a second embodiment of the present invention. 
     FIG. 9 is a side sectional view showing state in which a pinion and a rack are engaged prior to an actuation of the pre-tensioner of the webbing winding device according to the second embodiment of the present invention. 
     FIG. 10 is a side sectional view showing a state in which a pinion and a rack are separated after the actuation of the pre-tensioner of the webbing winding device according to the second embodiment of the present invention. 
     FIG. 11 is an exploded perspective view showing a structure of a webbing winding device according to a third embodiment of the present invention. 
     FIG. 12 is an axial plan view showing a state prior to an actuation of the pre-tensioner of the webbing winding device according to the third embodiment of the present invention. 
     FIG. 13 is an axial plan view showing a state which a planet gear and an internal gear are engaged immediately after an actuation of the pre-tensioner of the webbing winding device according to the third embodiment of the present invention. 
     FIG. 14 is an axial plan view showing a state in which a planet gear and an internal gear are separated immediately after an actuation of the pre-tensioner of the webbing winding device according to the third embodiment of the present invention. 
     FIG. 15 is an axial plan view showing a state after an actuation of the pre-tensioner of the webbing winding device to which a modification  1  of the gear train mechanism of the third embodiment of the present invention is applied. 
     FIG. 16 is an exploded perspective view showing a structure of the webbing winding device according to a fourth embodiment of the present invention. 
     FIG. 17 is an exploded perspective view showing a structure of a one-way clutch in the webbing winding apparatus according to the fourth embodiment of the present invention. 
     FIG. 18 is an axial plan view showing a lockup state in a rotation direction of the internal gear prior to an actuation of the pre-tensioner of the webbing winding device according to the fourth embodiment of the present invention. 
     FIG. 19 is an axial plan view showing a release state in a rotation direction of the internal gear after the actuation of the pre-tensioner of the webbing winding device according to the fourth embodiment of the present invention. 
     FIG. 20 is an axial plan view showing a state in which the internal gear is rotated by an inertial force from a spool after the actuation of the pre-tensioner of the webbing winding device according to the fourth embodiment of the present invention. 
     FIG. 21 is an exploded perspective view showing a  1 structure of the webbing winding device according to a fifth embodiment of the present invention. 
     FIG. 22 is an exploded perspective view showing a structure of a rack in the webbing winding device according to the fifth embodiment of the present invention. 
     FIG. 23 is an axial plan view showing a state prior to an actuation of the pre-tensioner of the webbing winding device according to the fifth embodiment of the present invention. 
     FIG. 24 is an axial plan view showing a state after an actuation of the pre-tensioner of the webbing winding device according to the fifth embodiment of the present invention. 
     FIG. 25 is an axial plan view showing a state in which a tooth portion of the rack is advanced by an inertial force from the spool after the actuation of the pre-tensioner of the webbing winding device according to the fifth embodiment of the present invention. 
     FIG. 26 is an exploded perspective view showing a structure of a modification  1  of the rack in the webbing winding device according to the fifth embodiment of the present invention. 
     FIG. 27 is an axial plan view showing a state after an actuation of the pre-tensioner in the webbing winding device to which the modification  1  of the rack according to the fifth embodiment of the present invention is applied. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, the embodiments of the present invention  1 will be described with reference to the accompanying drawings. 
     (First Embodiment) 
     FIGS. 1-3 show a webbing winding device  10  according to the first embodiment of the present invention. As shown in FIG. 1, the webbing winding device  10  has a frame  11  which is fixed to a vehicle body. This frame  11  comprises a plate  12  in which is formed a through hole  12 A through which is inserted a bolt or the like to be fastened to the vehicle, and a pair of side plates  13 ,  14  which are bent from both sides of the plate  12  at a right angle such that they are parallel to each other. The frame  11  rotatably supports a spool  15  which is a winding shaft with the side plates  13 ,  14 . One end of the webbing  16  for restraining a passenger is fixed to the spool  15 . A circular opening  17  is formed in one side plate  14  around an axis A which is a rotation center of the spool  15 . A circular concave engagement hole  18  is formed around the axis A in a side face of the spool  15  facing this circular opening  17 . 
     A one-way clutch  19  is disposed outside the side plate  14  of the frame  11 . This one-way clutch  19 , as shown in FIG. 1, comprises a torque transmission shaft  20 , cylindrical rollers  21 , a holding plate  22  and a rotor  23 . The torque transmission shaft  20  comprises a spline shaft portion  24  and an inner wheel portion  25 , these portions being formed each in the form of a cylinder having a different outside diameter. The spline shaft portion  24  having a smaller diameter and the inner wheel portion  25  having a larger diameter are provided coaxially and integrally with each other. Spline teeth  24 A extending in the same direction as the axis A are formed on an outer peripheral face of the spline shaft portion  24  at a predetermined pitch in the circumferential direction. Spline grooves  18 A having a shape corresponding to the shape of the spline teeth  24 A of the spline shaft portion  24  are formed on an inner peripheral face of the engagement hole  18  at the same pitch as the spline teeth  24 A. As a result, if the spline shaft portion  24  is inserted into the engagement hole  18  so that the spline teeth  24 A engage with the spline grooves  18 A, the torque transmission shaft  20  is supported on the axis A of the spool  15  and connected to the spool  15  such that it rotates integrally with the spool  15 . Further a screw hole  20 A penetrating along the axis A is formed in an outer side face in the axial direction S of the torque transmission shaft  20 . 
     The holding plate  22  of the one-way clutch  19  is formed in the form of a thin circular plate. The outside diameter thereof is slightly smaller than the inside diameter of the circular opening  17  of the side plate  14 . A circular opening portion  22 A having a diameter slightly larger than the outside diameter of the inner wheel portion  25  of the torque transmission shaft  20  is formed in the central portion of the holding plate  22 . Three roller holding portions  22 B are formed by cutting U shapes extending outwards from the inner peripheral face of the opening portion  22   a  in a radial direction. The roller holding portions  22 B are formed slightly wider than the diameters of the roller  21 . These roller holding portions  22 B are provided at equal intervals (at intervals of 120°) in the circumferential direction around the axis A. 
     The opening portion  22 A of the holding plate  22  is fitted around the outside peripheral face of the inner wheel portion  25  and the holding plate  22  is then placed on a side face of the spool  15  via a circular opening  17  of the side plate  14 . As a result, the holding plate  22  is held in a ring-like concave portion formed by the outside peripheral face of the inner wheel portion  25  and the inside peripheral face of the circular opening  17  in the radial direction and is positioned coaxially with the spool  15 . 
     The center axis of each of the three rollers  21  is parallel to the axis A. The three rollers  21  are placed against a side face of the spool  15  passing through each of the roller holding portions  22 B. As a result, the three rollers  21  are positioned at equal interval in the circumferential direction by the roller holding portions  22 B. If the holding plate  22  is rotated around the axis A, they slide on the side face of the spool  20  in the circumferential direction with the equal interval maintained by the holding plate  22 . 
     A rotor  23  is configured as an outside wheel of the one-way clutch  19  and formed in a circular shape having a larger thickness than a height of the inner wheel portion  25  in the axial direction S as shown in FIG.  1 . An inner wheel accommodating chamber  23 A and three roller accommodating chambers  23 B communicating with the inner wheel accommodating chamber  23 A are formed in this rotor  23 . The inner wheel accommodating chamber  23 A and roller accommodating chambers  23 B are open to the side of the spool  15  of the rotor  23 . The inner wheel accommodating chamber  23 A forms a cylindrical space corresponding to the inner wheel portion  25  inside the rotor  23  and each of the roller accommodating chambers  23 B forms a space extending outward from the inner peripheral face of the inner wheel accommodating chamber  23 A. These spaces are provided at equal intervals (120°) in the circumferential direction around the axis A. Each of the roller accommodating chambers  23 B is formed in a substantially elongated ellipse shape extending in a spiral direction around the axis A as shown in FIG.  2 . Partition wall-like stopper protrusions  23 C are formed along an inner peripheral face of the inner wheel accommodating chamber  23 A between an outside end of each roller accommodating chamber  23 B in the spiral direction and the inner wheel accommodating chamber  23 A. The roller accommodating chambers  23 B form substantially wedge-like spaces which gradually narrow in width, when looked at axially, toward the inside in the spiral direction between the roller accommodating chambers  23 B and an outside peripheral face of the inner wheel portion  25  accommodated in the inner wheel accommodating chamber  23 A. Further, the rotor  23  has a pinion  29  provided integrally and coaxially therewith on an outside side face in the axial direction S as shown in FIG. 1. A center hole  31  is made through this pinion  29  and the center hole  31  communicates with the inner wheel accommodating chamber  23 A in the rotor  23 . 
     The rotor  23  is placed against the side face of the spool  15  with the holding plate  22  therebetween and accommodates the inner wheel portion  25  of the torque transmission shaft  20  coaxially within the inner wheel accommodating chamber  23 A. At the same time, the rollers  21  are accommodated in the three roller accommodating chambers  23 B. A pair of curved guide plates  27  and  28  are provided on the side face  14  so as to surround an outside peripheral face of the rotor  23  placed against the side face of the spool  15  as shown in FIG.  1 . Inner peripheral faces of these guide plates  27  and  28  are curved at a curvature radius slightly larger than an outside peripheral face of the rotor  23  around the axis A. This restricts the motion of the rotor  23  and pinion  29  in the radial direction so that they are positioned coaxially with the spool  15 . 
     When the rotor  23  and pinion  29  are positioned coaxially with the spool  15 , a connecting member  32  is inserted into a center hole  31  of the pinion  29 . The connecting member  32  is a thrust bearing comprising a round bar like shaft portion  32 A, a male threaded portion  32 B provided at a front end of this shaft portion  32 A and a head portion  32 C supported at a rear end of the shaft portion  32 A coaxially therewith as shown in FIG.  1 . The head portion  32 C is supported rotatably by the shaft portion  32 A so as to suppress a rotation resistance of the rotor  23  and pinion  29 . 
     The shaft portion  32 A of the connecting member  32  is inserted through the center hole  31  in the pinion  29  and the male threaded portion  32 B is driven into the threaded hole  20 A in the torque transmission shaft  20 . As a result, the connecting member  32  fixes the rotor  23  and the pinion  29  to the torque transmission shaft  20  in the axial direction S and at the same time, supports the rotor  23  and the pinion  29  rotatably around the axis A. Therefore, the rotor  23  and the pinion  29  are capable of rotating relative to the spool  15  and the torque transmission shaft  20 . 
     The webbing winding device  10  having the above structure is mounted on a vehicle body in a state in which the one-way clutch  19  is not transmitting torque to the spool  15 , that is, in an OFF state in which the spool  15  is separated from the pinion  29 . In this OFF state, relative positions of the holding plate  22  and the rotor  23  are adjusted so as to hold each of the rollers  21  within an end portion on the outside of each roller accommodating chamber  23 B as shown in FIG.  2 . At this time, the stopper protrusion  23 C of the roller accommodating chamber  23 B stops the roller  21  and holds it at a position removed from the outside peripheral face of the inner wheel port ion  25 . Consequently, because the torque transmission shaft  20  is placed in an OFF state in which no torque is transmitted between the torque transmission shaft  20  and the rotor  23  even if the pinion  29  is locked so that it is not capable of rotating, the spool  15  is capable of rotating in both the retracting direction W and the feeding out direction R of the webbing  16 . 
     On the other hand, in the frame  11 , as shown in FIG. 2, a rack  34  which meshes with the pinion  29 , a piston driving unit  35  for the pre-tensioner connected to this rack  34  and a guide frame  33  for restricting the direction of movement of the rack  34  are disposed on an outer side face of the side plate  14 . 
     The rack  34  is separable into two parts, as shown in FIG.  1  and comprises a tooth portion  40  disposed at the distal end thereof in the longitudinal direction and a rod portion  41  disposed at the proximal end. The tooth portion  40  and the rod portion  41  are formed in a substantially rectangular thick plate form with the pitch line P L  running in the longitudinal direction, as shown in FIG.  2 . Teeth  40 A which mesh with the teeth  29 A of the pinion  29  are formed continuously in the direction of the pitch line P L  on one side end face in the transverse direction which is perpendicular to the pitch line P L , as shown in FIG.  2 . Further, a pair of guide pin s  42  is provided on an outer side face of the tooth portion  40  in the axial direction S so that they project in the axial direction S. This pair of guide pins  42  are disposed at the distal and proximal end portions of the tooth portion  40  in the longitudinal direction thereof and are positioned on a straight line parallel to the pitch line P L . Further, an insertion hole  40 B is formed the side face of the tooth portion  40  on which the guide pins  42  are provided, such that it is located slightly closer towards the proximal end than the guide pin  42  at the distal end and is offset towards the teeth  40 A as shown in FIG. 1. A pressure receiving face  40 C which is a flat surface orthogonal to the longitudinal direction of the tooth portion  40  is formed on the proximal end face thereof. On the other hand, the proximal end in the longitudinal direction of the rod portion  41  of the rack  34  is connected to the piston driving unit  35 . A pressure applying face  41 A which is a flat surface orthogonal to the longitudinal direction is formed on the distal end face of the rod portion  41  in the longitudinal direction. 
     As is shown in FIG. 1, the guide frame  33  to be disposed on the side plate  14  comprises a rectangular plate portion  43  which is to be disposed on the side plate  14  with the longitudinal direction thereof parallel with the direction of the pitch line P L  (the pitch line direction) and leg portions  44  and  45  (see FIG. 2) which are bent at right angles from the distal end of the plate portion  43  in the longitudinal direction thereof and the outside end of the plate portion  43  in the transverse direction thereof respectively. These leg portions  44  and  45  have the same dimensions as each other in the axial direction S. The guide frame  33  is disposed on the side plate  14  such that the end faces of the leg portions  44  and  45  in the axial direction S abut the outer side face of the side plate  14 . Consequently, the plate portion  43  is supported parallel to the side plate  14  by the leg portions  44 ,  45 . 
     As shown in FIG. 2, a pair of elongated guide grooves  46  is formed running along the direction of the pitch line P L  in the plate portion  43 . Here, one guide groove  46  is disposed at the distal end portion in the longitudinal direction corresponding to the guide pin  42  at the distal end of the tooth portion  40 . The other guide groove  46  is disposed at the proximal end portion in the longitudinal direction corresponding to the guide pin  42  at the proximal end of the tooth portion  40 . In each of these guide grooves  46 , a range from a starting end  46 A at the proximal end in the longitudinal direction of the guide grooves  46  as far as an intermediate point  46 C just before a terminal end  46 B at the distal end in the longitudinal direction of the guide grooves  46  is a parallel portion  46 D formed parallel to the pitch line P L , while a range from the intermediate point  46 C as far as the terminal end  46 B is an inclined portion  46 E which slopes away from the pitch line P L . A through hole  43 A is formed in the plate portion  43  at a position corresponding to the insertion hole  40 B of the tooth portion  40 , as shown in FIG.  1 . 
     A pair of positioning plates  47  for positioning the guide frame  33  is disposed on an outer side face of the side plate  14 . These positioning plates  47 , as shown in FIG. 2, are in firm contact with the outside faces of the leg portions  44  and  45  of the guide fame  33  placed on the side plate  14  so as to position the guide frame  33 . At this time, fixing screws  48  are screwed into each of the leg portions  44  and  45  of the guide frame  33  through each of the positioning plates  47  enabling the guide frame  33  to be fixed to the side plate  14 . 
     In the guide frame  33  placed on the side plate  14 , the pair of guide pins  42  on the rack  34  are inserted into the pair of guide grooves  46 . A shear pin  49  made of resin inserted through the plate portion  43  is inserted through the insertion hole  40 B of the tooth portion  40   50  that movement of the tooth portion  40  in the direction of the pitch line is blocked. At this time, the tooth portion  40  is held at a position (starting end position) in which, as shown in FIG. 2, the pair of guide pins  42  are in contact with the starting end  46 A of guide groove  46  and the pressure receiving face  40 C is in firm contact with the pressure applying face  41 A of the rod portion  41 . 
     The piston driving apparatus  35  supports the rod portion  41  of the rack  34  movably along the pitch line P L  which is contiguous contact with a pitch circle P C  of the pinion  29 . The piston driving apparatus  35  comprises a gas generator  36 , a piston cylinder  37  into which high pressure gas generated by this gas generator  36  is introduced and a piston  38  disposed within this piston cylinder  37 . The piston  38  is supported slidably along an inner wall of the piston cylinder  37 , forming an air chamber of a variable volume sealed from the outside together with the piston cylinder  37 . A proximal end face of the rod portion  41  of the rack  34  is fixed to the face of the piston  38  open to the air, i.e. on the opposite side to the air chamber. When the webbing winding device  35  is assembled in the frame  11 , the piston  38  is held at a position at which the volume of an internal air chamber formed within the piston cylinder  37  is at the minimum, as shown in FIG.  2 . 
     When the webbing winding unit  10  is mounted on to the vehicle body, the rotor  23  is temporarily blocked from rotation by a shear pin  39  (see FIG. 1) inserted through a temporary blocking hole  23 F provided on an outer peripheral face of the rotor  23  through the guide plate  27 . At this time, a tooth face of the distal end side of the tooth  40 A at the distal end of the rack  34  is brought into contact with a single tooth of the pinion  29  located on the pitch line P L . 
     Next, the action and operation of the webbing winding device  10  of the present embodiment having the above described structure will be described. The gas generator  36  of the piston driving apparatus  35  is connected to an ignition power supply (not shown). If an emergency deceleration detecting sensor (not shown) detects an emergency deceleration of the vehicle due to a collision or the like, this ignition power supply is actuated so as to supply an ignition current to the gas generator  36 . When this ignition current is supplied, the gas generator  36  generates a high pressure gas and supplies this high pressure gas to the piston cylinder  37 . As a result, the piston  38  is made to slide in a direction for expanding the volume in the air chamber of the piston cylinder  37  by the high pressure gas so that the rod portion  41  of the rack  34  is moved linearly along the pitch line P L . At this time, a pressing force in the pitch line direction is transmitted to the tooth portion  40  of the rack  34  via the rod portion  41 . When this pressing force is received, the tooth portion  40  shears the shear pin  49  and starts to move. During the period of initial movement, the direction in which the tooth portion  40  moves is restricted by the parallel portion  46 D in the guide groove  46  of the plate portion  43 . The tooth portion  40  moves along the pitch line P L  with the teeth  40 A meshing with the teeth  29 A of the pinion  29  from the starting end position up to a release starting position which corresponds to the intermediate point  46 C of the guide groove  46 . If the tooth  40 A at a front end of the tooth portion  40  which is moved linearly by a driving force from the driving apparatus  35  presses the tooth  29 A of the pinion  29 , the pinion  29  converts the pressing force from the tooth portion  40  to a rotation force. The shear pin  39  is sheared by this rotation force so that the pinion  29  and the rotor  23  begin to rotate in the reacting direction W. 
     At this time, the rotor  23  rotates in the reacting direction W relative to the holding plate  22 . As a result, as shown in FIG. 2, each of the rollers  21  in the roller accommodating chambers  23 B is moved from the position (OFF position) in which the roller  21  is stopped by the stopper protrusion  23 C to an ON position in which the roller  21  is nipped by the a pressure from the outer peripheral face of the inner wheel portion  25  and the inner peripheral face of the roller accommodating chamber  23 B as shown in FIG.  3 . 
     When the roller  21  is moved up to the ON position, the rotor  23  is connected to the torque transmission shaft  20  in the rotation direction so that the one-way clutch  19  enters into an ON state in which it is able to of transmit torque from the pinion  29  to the spool  15 . Consequently, the spool  15  is rotated in the retracting direction W integrally with the rotor  23  and pinion  29 . 
     Then, when the tooth portion  40  moves up to the release starting position together with the rod portion  41 , the pair of guide pins  42  of the tooth portion  40  enter from the parallel portion  46 D of the guide groove  46  into the inclined portion  46 E. As a result, the tooth portion  40  is inclined with respect to the pitch line P L  and the direction of movement thereof is restricted to a direction in which it moves away from the piston driving unit  35 . Therefore, the tooth portion  40  is advanced in the pitch line direction by a driving force from the piston driving unit  35  and moves away from the pinion in the radial direction relative to the axis A with the pressure receiving face  40 C sliding along the pressure applying face  40 A of the rod portion  41 . In the tooth portion  40 , when the guide pin  42  of the rack  34  is moved by a driving force from the piston driving apparatus  35  to a position (terminal position) at which it reaches the terminal end  46 B of the guide groove  46 , the engagement between the teeth  40 A and the teeth  29 A is released at a position just before this terminal position. Therefore, the tooth portion  40  is held at the terminal position away from the pinion  29 , as shown in FIG. 3, when the operation of the piston driving apparatus  35  is terminated. 
     In the webbing winding device  10  of this embodiment, if the pre-tensioner is actuated or the piston driving apparatus  35  is actuated so that the one-way clutch  19  is placed in an ON state, the rack  34  which is a driving gear is driven from the beginning position to the release starting position. As a result, torque in the retracting direction W is transmitted from the pinion  29 , which constitutes the gear train mechanism together with this rack  34 , to the spool  15 . If there is play in the webbing  16  worn by a passenger, the spool  15  is rotated quickly in the retracting direction W by the torque, so that the webbing  16  is wound onto the spool  15 . At this time, if the play of the webbing  16  is slight, the piston driving apparatus  35  rotates the spool  15  in the retracting direction until the play of the webbing  16  is eliminated. Even if the rack  16  has moved only partway to the terminal position and has not separated from the pinion  29 , the piston  38  is stopped when a tension balancing a gas pressure in the piston cylinder  37  is generated in the webbing  16 . 
     On the other hand, if the rack  34  moves away from the pinion  29  just before the terminal end position, an inertial force based on the mass of the rotor  23 , pinion  29  and the like which rotate integrally with the spool  15  as well as an inertial force based on weight of the spool  15  is applied to the spool  15  as a rotation force in the retracting direction W. Thus, if there is still play in the webbing  16  when the rack  34  moves away from the pinion  29 , because the tension of the webbing  16  acting on the spool  15  as a rotation load is small and a large inertial force acts on the spool  15  which is rotating at high speed, the spool  15  continues its rotation in the retracting direction W. Then, if a tension balancing the inertial force of the spool  15  is generated in the webbing  16 , the rotation of the spool  15  is stopped. Therefore, even if the play in the webbing  16  is large just before the piston driving apparatus  35  is actuated, the rack  34  does not block the rotations of the pinion  29  and the spool  15  but the rotation of the spool  15  in the retracting winding direction W can be continued until the play of the webbing  16  is removed completely. 
     (Modification  1  of the First Embodiment) 
     Modification  1  of the gear train mechanism and guide member according to the first embodiment of the present invention will be described with reference to FIGS. 4 and 5. A rack  51  is structured so that a tooth portion  52  and a rod portion  53  are integrated as shown in FIG.  4 . Teeth  52 A which mesh with teeth  29 A of the pinion  29  are formed continuously on one side end face in the transverse direction of the tooth portion  52 . The other side end face which is on the opposite side to the teeth  52 A acts as a guide face  54  of the tooth portion  52 . This guide face  54  is formed over a range corresponding to the traveling stroke of the rack  51  from the distal end of a side end face of the tooth portion  52  to the proximal end face thereof. A range corresponding to the distance from the starting end position of the rack  51  to the release starting position is a parallel portion  54 A comprising a flat surface parallel to the pitch line P L  and a range corresponding to the distance from the release starting position of the rack  51  to the terminal end position is an inclined portion  54 B comprising an inclined flat surface which slope towards the pitch line P L  as it approaches the proximal end. Further, in the guide face  54 , a stopper portion  54 C comprising a flat surface parallel to the transverse side direction of the rack  51  is formed at the proximal end of the inclined portion  54 B. 
     A guide frame  55  disposed on the side plate  14  comprises a rectangular plate portion  56  whose longitudinal direction is parallel to the pitch line direction and leg portions  57  and  58  which are bent at right angles from the distal end in the longitudinal direction of the plate  56  and an outer side end in transverse direction. These leg portions  57  and  58  have the dimensions as each other same in the axial direction S. The guide frame  55  is mounted on the side plate  14  such that the end faces of the leg portions  57  and  58  in the axial direction S abut an outer side face of the side plate  14 . The guide frame  55  is positioned at a predetermined position by positioning plates  47  on the side plate  14  and fastened thereto with fastening screws  48 . 
     The plate portion  56  has a guide pin  59  provided on an inner side face thereof facing the side plate  14  so as to project in the axial direction S. This guide pin  59  is supported so as to be in contact with the guide face  54  of the rack  51 . When the piston driving unit  35  is not actuated, i.e. when the rack  51  is located at the beginning position, the guide pin  59  is in contact with the vicinity of the distal end of the parallel portion  54 A of the guide face  54 , as shown in FIG.  4 . Further, a guide piece  60  is provided in the side end portion of the leg portion  57  at the distal end of the guide frame  55  on the side thereof closest to the pitch line P L . This guide piece  60  extends from a side end of the leg portion  57  to a position intersecting the pitch line P L  and is substantially parallel to the inclined portion  54 B of the guide face  54  in the rack  51 . 
     A guide piece  61  extending towards the distal end of the rack  51  is provided at an end of the piston cylinder  38 . This guide piece  61  is inclined with respect to the pitch line P L  such that it approaches the teeth  51 A of the rack  51  from the distal end of the piston cylinder  38  the closer to the distal end of the rack  51 . The guide piece  61  and guide frame  55  together form the guide member. 
     Next, the action and operation of the webbing winding device  10  according to the first embodiment having the gear train mechanism and guide member having the above structures will be described. If the piston driving apparatus  35  is actuated, the gas generator  36  generates a high pressure gas and supplies it to the piston cylinder  37 . The piston  38  is made to slide in a direction for expanding the volume of the air chamber in the piston cylinder  37  by this high pressure gas thereby moving the rack  51  located at the starting end position along the pitch line P L . During in the initial period of traveling of the rack  51 , the guide pin  59  of the plate portion  56  is in contact with the parallel portion  54 A of the guide faces  54  so as to restrict the traveling of the rack  51  to liner along the pitch line P L . As a result, the rack  51  moves along the pitch line P L  with the teeth  51 A thereof meshing with the teeth  29 A of the pinion  29  from the starting end position to the release start position. The pinion  29  converts pressure from the rack  51  moving linearly along the pitch line P L  into rotation force. This rotation force rotates the pinion  29  and the rotor  23  in the retracting direction relative to the holding plate  22 . As a result, as shown in FIG. 4, each of the rollers  21  in the roller accommodating chambers  23 B is moved from a position (OFF position) in which the roller  21  is stopped by the stopper protrusion  23 C to an ON position in which the roller  21  is nipped by the a pressure from the outer peripheral face of the inner wheel portion  25  and the inner peripheral face of the roller accommodating chamber  23 B, as shown in FIG.  5 . 
     When the roller  21  is moved to the ON position, the rotor  23  is connected to the torque transmission shaft  20  in the rotation direction so that the one-way clutch  19  is placed in an ON state in which it is able to transmit torque from the pinion  29  to the spool  15 . Consequently, the spool  15  is rotated at high speed in the retracting direction W integrally with the rotor  23  and pinion  29  so that the webbing  16  is retracted by the rotor  23 . 
     When, if the rack  51  is moved up to the release start position, the piston  38  is discharged from the piston cylinder  37 . At the same time, a tooth face of the first tooth  51 A at the distal end of the rack  51  moved along the pitch line P L  by the inertial force comes into contact with the guide piece  60  while an outer peripheral end portion of the piston  38  comes into contact with the guide piece  61 . A component of force acts on the piston  38  and rack  51  so as to move the piston  38  and the rack  51  away from the pinion  29 . The teeth  51 A of the rack  51  are in contact with the teeth  29 A of the pinion  29  at a pressure angle of about 20°. As a result, when the pinion  29  is about begins to rotate in the retracting direction W due to the inertial force, a component of force is applied to the rack  51  in a direction perpendicular to the pitch line P L  and moving away from the pinion  29 . 
     Therefore, the rack  51  is advanced in the pitch line direction from the release start position by the inertial force and at the same time moved in a direction away from the pinion in the radial direction relative to the axis A with the inclined portion  54 B of the guide face  54  sliding on the guide pin  59 . The rack  51  is moved to the terminal position where the distal face thereof comes into contact with the leg portion  57  because of the inertial force and at the same time the guide pin  59  is in contact with the stopper portion  54 C of the guide face  54 . The engagement between the teeth S 1 A and the teeth  29 A is released just before this terminal position. Therefore, when the driving of the rack  51  by the piston driving unit  35  is completed, the rack  51  is held at the terminal position away from the pinion  29 , as shown in FIG.  5 . 
     If, in the webbing winding device  10  of the first embodiment to which the modification  1  of the gear train mechanism and guide member is applied, there is a play in the webbing  16  when the driving of the rack  51  by the piston driving apparatus  35  is completed, the rack  51  does not lock the rotations of the pinion  29  and the spool  15  and the rotation of the spool  15  can be continued in the retracting direction W due to the inertial force until the play of the webbing  16  is removed completely. 
     (Modification  2  of the First Embodiment) 
     Modification  2  of the gear train mechanism and guide member according to the first embodiment will now be described with reference to FIGS. 6 and 7. As shown in FIG. 6, a rack  65  for meshing with the pinion  29 , the piston driving apparatus  35  connected to this rack  65  and a guide frame  66  for restricting the direction of travel of the piston  34  are disposed on an outer side face of the side plate  14 . 
     The rack  65  is separable into three parts, as shown in FIG. 7, and comprises a distal end tooth portion  67  disposed at the distal end in the longitudinal direction, a proximal end tooth portion  68  disposed in the intermediate portion and a rod portion  69  disposed at the proximal end. The tooth portions  67  and  68  and the rod portion  69  are each formed as a substantially rectangular thick plate with the pitch line P L  as the longitudinal direction thereof. Teeth  67 A and  68 A which mesh with the teeth  29 A of the pinion  29  are formed continuously on one side face in the transverse direction of the tooth portions  67  and  68 , as shown in FIG. 6. A guide pin  70  and an insertion hole  67 B are provided on an outer side face of the proximal end tooth portion  68  in the axial direction S, as shown in FIG.  7 . 
     A pressure receiving face  67 C made of a flat surface orthogonal to the longitudinal direction thereof is formed on the proximal end face of the distal end tooth portion  67 . An engaging concave portion  67 D which is indented in a step shape towards the distal end of the distal end tooth portion is formed in the outer side end portion in the transverse direction of the pressure receiving face  67 C. On the other hand, a pressure applying face  68 C made of a flat surface orthogonal to the longitudinal direction thereof is formed on the distal end face of the proximal end tooth portion  68 . This pressure applying face  68 C has an engaging convex portion  68 D provided at an outer end portion in the transverse direction corresponding to the engaging concave portion  67 D of the distal end tooth portion  67 . 
     An insertion hole is formed in a side face in the transverse direction of the engaging concave portion  67 D and an insertion hole which is a through hole in the transverse direction is also formed in the engaging convex portion  68 D. The pressure receiving face  67 C and the pressure applying face  68 C are brought into contact with each other, as shown in FIG.  6  and the engaging concave portion  67 D is fit to the engaging convex portion  68 D. A connecting pin  71  is then passed through the through hole and inserted into insertion hole, so that the tooth portions  67  and  68  are supported with their respective pitch lines P L  on the same line. 
     The proximal end face of the proximal end tooth portion  68  and the distal end face of the rod portion  69  act as the pressure applying face  68 F and the pressure receiving face  69 A, respectively, each comprising a flat surface orthogonal to the longitudinal direction. Moreover, the proximal end tooth portion  68  and the rod portion  69  are supported with the pressure applying face  68 F and the pressure receiving face  69 A being in firm contact with each other. A proximal end face of the rod portion  69  is fixed to the piston  38  in the piston cylinder  37 . 
     A guide frame  66  mounted on the side plate  14  comprises a rectangular plate portion  72  disposed on the side plate  14  so as to have the direction (pitch line direction) of the pitch line P L  as the longitudinal direction thereof, as shown in FIG. 6, and leg portions  73  and  74  bent at right angles from the distal end portion in the longitudinal direction of this plate portion  72  and an outer side end in the transverse direction respectively, towards the side plate  14 . These leg portions  73  and  74  have the same dimensions as each other in the axial direction S. The guide frame  66  is disposed on the side plate  14  such that the end faces in the axial direction S of the leg portions  73  and  74  are in contact with the outer side face of the side plate  14 . As a result, the plate portion  72  is supported so as to be parallel to the side plate  14  by the leg portions  73  and  74 . 
     The plate portion  72  has a guide groove  75  formed so as to extend along the pitch line P L . Here, the guide groove  75  is disposed at a position corresponding to the guide pin  70  on the proximal end tooth portion  68 . In this guide groove  75 , a proximal end portion in the longitudinal direction thereof corresponding to the distance from the starting end position to the release starting position of the traveling range of the rack  65  is a parallel portion  75 A formed parallel to the pitch line P L , and a distal end portion in the longitudinal direction thereof corresponding to the distance from the release starting position to the terminal position is an inclined portion  75 B which slopes away from the pitch line P L . 
     The guide frame  66  is positioned at a predetermined position by a pair of positioning plates  47  on the side plate  14  and fixed with fixing screws  48 . The plate portion  72  has a through hole (not shown) at a position corresponding to the insertion hole  67 B in the distal end tooth portion  67 . A distal end portion of a shear pin  76  made of resin is inserted into this insertion hole  67 B via this through hole. This blocks movement of the distal end tooth portion  67  and the proximal end tooth portion  68  in the pitch line direction. In the guide frame  66  placed on the side plate  14 , the guide pin  70  of the proximal end tooth portion  68  is inserted slidably into the guide groove  75  of the plate portion  72 . 
     Next, the action and operation of the webbing winding device  10  of the first embodiment, in which the gear train mechanism and guide member having the above structure are employed will be described. When the piston driving apparatus  35  is actuated, high pressure gas is supplied to the piston cylinder  37  so that the piston  38  is slid in a direction for expanding the volume of the air chamber in the piston cylinder  37 . As a result, the piston  38  moves the rod portion  69  of the rack  65  linearly along the pitch line P L . At this time, a pressing force in the pitch line direction is transmitted to the tooth portions  67  and  68  of the rack  65  via the rod portion  69 . After receiving this pressing force, the tooth portions  67  and  68  shear the shear pin  76  and begin to move. In the initial period of traveling, the direction of moment the proximal end tooth portion  68  and the distal end tooth portion  67  connected thereto are restricted by the parallel portion  75 A of the guide groove  75  in the plate portion  72 . From the starting end position to the release start position, the proximal end tooth portion  68  and the proximal end tooth portion  67  move along the pitch line P L  with the teeth  67 A,  68 A meshing with the teeth  29 A of the pinion  29 . When the teeth  67 A and  68 A of the tooth portions  67  and  68  moving linearly due to the driving force from the driving apparatus  35  apply pressure to the teeth  29 A, the pinion  29  converts the pressure from the tooth portion  40  into a rotation force. This rotation force shears the shear pin  39  which had been temporarily halting the rotor  23 , so that the pinion  29  and rotor  23  begin to rotate in the retracting direction W. 
     At this time, the rotor  23  rotates in the retracting direction W relative to the holding plate  22 . As a result, as shown in FIG. 6, each of the rollers  21  in the roller accommodating chambers  23 B is moved from a position (OFF position) in which the roller  21  is stopped by the stopper protrusion  23 C to an ON position in which each roller  21  is nipped by pressure from the outer peripheral face of the inner wheel portion  25  and the inner peripheral face of the roller accommodating chamber  23 B, as shown in FIG.  7 . 
     When the roller  21  is moved to the ON position, the rotor  23  is connected to the torque transmission shaft  20  in the rotation direction so that the one-way clutch  19  is placed in an ON state in which it is able to transmit torque from the pinion  29  to the spool  15 . Consequently, the spool  15  is rotated at high speed in the retracting direction W integrally with the rotor  23  and pinion  29 . 
     When the tooth portions  67  and  68  move up to the release start position together with the rod portion  41 , the guide pin  70  of the proximal end tooth portion  68  enters from the parallel portion  75 A of the guide groove  75  in the plate portion  72  into the inclined portion  75 B. Consequently, the proximal end tooth portion  78  is inclined with respect to the pitch line P L  and the direction of movement of the proximal end tooth portion  78  is restricted to a direction moving away from the pinion  29 . Thus, the proximal end tooth portion  68  is advanced in the pitch line direction by a driving force from the piston driving apparatus  35  and moves away from the pinion in the radial direction with respect to the axis A with the pressure applying face  68 C sliding relative to the pressure receiving face  67 C of the distal end tooth portion  67  and, at the same time, with the pressure receiving face  68 F sliding relative to the pressure applying face  69 A of the rod portion  69 . At t he same time, the engaging convex portion  68 D of the proximal end tooth portion  68  is released from the engaging concave portion  67 D of the distal end tooth portion  67 . Further, the shear pin  71  is pulled out of the insertion hole  67 E so that the connection between the tooth portions  67  and  68  is released. At this time, the teeth  68 A of the proximal end tool portion  68  disengage from the teeth  29 A of the pin ion  29  at a position just before the terminal position. When the operation of the piston driving apparatus  35  terminates, the proximal end tooth portion  68  is held at the terminal position away from the pinion  29 , as shown in FIG.  7 . 
     If, in the webbing winding device  10  of the first embodiment to which modification  2  of the gear train mechanism and guide member is applied, there is a play in the webbing  16  when the driving of the rack  51  by the piston driving apparatus  35  is completed, the rack  51  does not lock the rotations of the pinion  29  and the spool  15  and then the rotation of the spool  15  can be continued in the retracting direction W due to the inertial force until the play of the webbing  16  is removed completely. 
     (Second Embodiment) 
     FIGS. 8-10 show a webbing winding device  80  according to a second embodiment of the present invention. In a description of the second embodiment, the same reference numerals are attached to components having basically the same structure and function as in the first embodiment and a description thereof is omitted. Moreover, components common to the webbing winding device  10  of the first embodiment will be described as required with reference to FIGS. 2 and 3. 
     A circular opening  81  is formed in one side plate  14  of the frame  11  around an axis A which is a rotation center of the spool  15 . A circular concave engagement hole  82  is formed around the axis A in a side face of the spool  15  facing this circular opening  81 . 
     A coil spring  83 , a supporting circular plate  84  and a one-way clutch  85  are disposed on the outside of the side plate  14  of the frame  11  coaxially. The supporting circular plate  84  has a through hole  86  formed in the axial direction S in the center thereof and a guide rib  87  provided around the entire circumference thereof in the axial direction S such that it is bent outwards in the axial direction S. A circular concave portion  90  is formed around the axis A in an inner side face in the axial direction of the supporting circular plate  84  as shown in FIG.  9 . An end of the coil spring  83  is inserted into this concave portion  90 . 
     The one-way clutch  85  is mounted on the side plate  14  via the coil spring  83  and the supporting circular plate  84 , as shown in FIG.  8 . This one-way clutch  85  comprises a torque transmission shaft  88 , cylindrical rollers  21 , a holding plate  22  and a rotor  23 . The torque transmission shaft  88  comprises a spline shaft portion  89 , an inner wheel portion  25 , and a supporting shaft portion  91 , these portions each being formed in the form of a cylinder and having a different outside diameter. The spline shaft portion  89 , the inner wheel portion  25  and the supporting shaft portion  91  are provided coaxially and integrally. 
     Spline teeth  89 A extending along the axis A are formed on an outer peripheral face of the spline shaft portion  82  at a predetermined pitch in the circumferential direction. Spline grooves  82 A having a shape corresponding to the shape of the spline teeth  89 A of the spline shaft portion  24  are formed on an inner peripheral face of the engagement hole  89  at the same pitch as the spline teeth  89 A. As a result, when the spline shaft portion  89  is inserted into the engagement hole  82  so that the spline teeth  89 A engage the spline groove  82 A, the torque transmission shaft  88  is supported so that it can move coaxially with the spool  15  and is connected to the spool  15  such that it can rotate integrally with the spool  15 . The supporting shaft portion  91  of the torque transmission shaft  88  protrudes from an outer side face of the inner wheel portion  26  in the axial direction S. 
     The opening portion  22 A of the holding plate  22  is placed on an outside peripheral face of the inner wheel portion  25  and then the holding plate  22  is placed on the supporting circular plate  84 . The center axis of each of the three rollers  21  is parallel to the axis A. The three rollers  21  are placed inside the guide rib  87  of the supporting circular plate  84  passing through the roller holding portions  22 B of the holding plate  22 . As a result, the three rollers  21  are positioned at equal intervals in the circumferential direction by the roller holding portions  22 B. If the holding plate  22  is rotated around the axis A, the rollers  22  slide on the supporting circular plate  84  in the circumferential direction with the equal interval maintained by the holding plate  22 . 
     As shown in FIG. 8, the rotor  23  has a release driving cam  92  projecting in a radial direction thereof from an outer circumferential face thereof. This release driving cam  92  has a cam face  93  on outer side faces in the axial direction. The cam face  93  comprises a driving face  93 A on the side of the retracting direction W and a holding face  93 B on the side of a feed-out direction R relative to the driving face  93 A. The driving face  93 A is composed of a flat surface which slopes towards the spool  15  in the retracting direction W. The holding face  93 B is composed of a flat surface orthogonal to the axial direction S. Here, it is assumed that the outside diameters of the supporting circular plate  84 , the holding plate  22  and the rotor  23  are the same. 
     The pinion  29  provided integrally and coaxially with the rotor  23  has a center hole  31  along the axis A as shown in FIG.  8 . The center hole  31  communicates with the inner wheel accommodating chamber  23 A in the rotor  23 . The rotor  23  is placed on the supporting circular plate  84  with the holding plate  22  sandwiched therebetween and accommodates the inner wheel portion  25  of the torque transmission shaft  88  coaxially within the inner wheel accommodating chamber  23 A. At the same time, the rollers  21  are accommodated in the three roller accommodating chambers  23 B. At this time, the supporting shaft portion  91  of the torque transmission shaft  88  passes through the center hole  31 . The rotor  23  and the pinion  29  are supported by this supporting shaft portion  91  rotatably around the axis A and supported movably in the axial direction S. 
     A guide plate  95  which is curved cylindrically is provided on the side face  14  so as to surround an outside peripheral face of the supporting circular plate  84  placed on the side face of the spool  15 , as shown in FIG.  8 . An inner peripheral face of the guide plate  95  is curved at a curvature radius slightly larger than an outside peripheral face of the supporting circular plate  84  around the axis A. 
     The webbing winding device  80  has a casing partially formed from a plate  97  shown in FIG.  8 . This casing is entirely formed in a box shape and accommodates the frame  11  supporting the spool  15 , the one-way clutch  85 , the piston driving apparatus  35  shown in FIG.  2  and the like. The plate  97  forming an end of the casing contains a ring-like bearing  98  embedded therein. The distal end portion of the supporting shaft portion  91  protruding from the center hole  31  of the pinion  29  is inserted through this bearing rotatably and slidably. The plate  97  is supported parallel to the side plate  14 . A driving protrusion  99  is provided on an inner side face of the plate  97  such that it projects in the axial direction S. The driving protrusion  99  corresponds to the release driving cam  92  on the rotor  23 . The distance from the axial line A to the driving protrusion  99  is substantially the same as the distance from the axial line A to the release driving cam  92 . The distal end face of the driving protrusion  99  in the axial direction S is a sliding face  99 A which slides against the cam face  93 . This sliding face  99 A is supported at substantially the same position in the axial direction as the driving face  93 A of the cam face  93  before the piston driving unit  35  is actuated, as shown in FIG.  9 . 
     The rotor  23  and the pinion  29  are always urged outward in the axial direction S by the coil spring  83  via the supporting circular plate  84 . Before the piston driving apparatus  35  is actuated, as shown in FIG. 9, the pinion  29  is held at a position in which an outer side face thereof in the axial direction S is in pressure contact with an inner side face of the plate  97 , and the supporting circular plate  84  is held such that an inner end thereof in the axial direction S is inserted slightly inside the guide plate  95 . At this time, as shown in FIG. 9, a position of the rotor  23  in the rotation direction thereof is such that a side end face of the release driving cam  92  in the pay-out direction R is in contact with a side face of the driving protrusion in the retracting direction W, and the rotor  23  is temporarily halted in the rotation direction by a shear pin (not shown) as in the first embodiment. 
     The webbing winding device  80  having the above structure is mounted to a vehicle body in a condition in which the one-way clutch  85  is not transmitting any torque to the spool  15 , that is, in an OFF state in which the spool  15  is separated from the pinion  29 . 
     On the other hand, in the frame  11 , as shown in FIG. 8, a rack  100  which meshes with the pinion  29  and the piston driving apparatus  35  connected to this rack  100  are disposed on an outer side face of the side plate  14 . 
     The rack  100  is formed as an integrated member from a tooth portion  101  in which teeth  101 A are formed on one side face in the transverse direction and a rod portion  102 . In the rod portion  102 , as shown in FIG. 8, a side end face of one side in the transverse direction thereof is extended beyond the pitch line P L  of the teeth  101 A toward the pinion  29 . A proximal end face of the rod portion  102  is fixed to a piston  38 . Before the piston driving apparatus  35  is actuated, as shown in FIG. 9, the rack  100  is sandwiched between the rotor  23  and the plate  97  of the casing so that movement thereof in the axial direction S is restricted. 
     Next, the action and operation of the webbing winding device  10  according to the present embodiment having the above structure will be described. When the piston driving apparatus  35  shown in FIG. 2 is actuated and high pressure gas is supplied to the piston cylinder  37 , the piston  38  slides in a direction for expanding the volume of the air chamber in the piston cylinder  37 . Then, the rack  100  moves along the pitch line P L  with the teeth  101 A of the tooth portion  101  meshing with the teeth  29 A of the pinion  29  so as to rotate the pinion  29  and rotor  23  in the retracting direction W. At this time, when the rotor  23  rotates in the retracting direction W relative to the holding plate  22 , each of the rollers  21  in the roller accommodating chambers  23 B is moved from a position (OFF position) in which the roller  21  is stopped by the stopper protrusion  23 C to a position (ON position) in which the roller  21  is nipped by the pressure from the outer peripheral face of the inner wheel portion  25  and the inner peripheral face of the roller accommodating chamber  23 B. As a result, the one-way clutch  85  is placed in an ON state in which it is capable of transmitting torque from the pinion  29  to the spool  15 . Consequently, the spool  15  is rotated in the retracting direction W integrally with the rotor  23  and pinion  29 . 
     When the rotor  23  rotates in the retracting direction by about one turn, the rotor  23  brings the driving face  93 A of the release driving cam  92  into contact with the sliding face  99 A of the driving protrusion  99  and continues to rotate in the retracting direction with the driving face  92 A sliding on the sliding face  99 A of the driving protrusion  99 . As a result, a part of the rotation force from the pinion  29  is converted to a component of force (thrust force) in a direction which is parallel to the axis A and moving away from the plate  97 A. This thrust force moves the supporting circular plate  84 , rotor  23  and pinion  29  supported movably in the axial direction  5 , towards the spool  15  while resisting an urging force of the coil spring  83 . At this time, the supporting circular plate  84  moves in the axial direction S a long the inner peripheral face o f the guide plate  95  while compressing the coil spring  83 . 
     If the rotor  23  rotates in the retracting direction W to where the sliding face  99 A of the driving protrusion  99  comes into contact with the holding face  93 B of the cam face  93 , the pinion  29  is moved in the axial direction S to where the teeth  29 A thereof disengage from the teeth  101 A of the rack  100 . When the sliding face  99 A of the driving protrusion  99  is in contact with the holding face  93 B of the cam face  93 , the piston driving apparatus  35  moves the rod portion  102  of the rack  100  over an outer side face in the axial direction S of the pinion  29 . As a result, even after the sliding face  99 A of the driving protrusion  99  is released from the holding face  93 B of the cam face  93 , the pinion  29  is held by a side face of the rod portion  102  at a position where it does not mesh with the teeth  101 A of the rack  100 , as shown in FIG. 10, and is also capable of rotating in the retracting direction W without being restricted by the rack  100 . 
     In the webbing winding device  80  of this embodiment, if the piston driving apparatus  35  is actuated so that the one-way clutch  85  is placed in the ON state, the rack  100  which is a driving gear is driven and the torque in the retracting direction W is transmitted from the pinion  29  which constitutes the gear train mechanism together with this rack  100  to the spool  15 . If there is play in the webbing  16  worn by a passenger, the spool  15  is rotated rapidly in the retracting direction W by the torque transmitted from the pinion  29 , so that the webbing  16  is retracted by the spool  15 . 
     On the other hand, when the driving of the rack  100  is completed, the pinion  29  is moved in the axial direction S and held by the rod portion  102  at a position away from the rack  100 . At this time, an inertial force based on the masses of the rotor  23 , pinion  29  and the like rotating integrally with the spool  15  as well as an inertial force based on the mass of the spool  15  is applied to the spool  15  as a rotation force in the retracting direction W. Thus, if there is still play in the webbing  16  fitting to a passenger when the driving of the rack  34  is completed, because the tension of the webbing  16  acting on the spool  15  as a rotation load is small and a large inertial force acts on the spool  15  rotating at high speed, the spool  15  continues its rotation in the retracting direction W. Then, if a tension balancing the inertial force of the spool  15  is generated in the webbing  16 , the rotation of the spool  15  is stopped. Therefore, even if the play in the webbing  16  is large just before the piston driving apparatus  35  is actuated, the rack  100  does not block the rotations of the pinion  29  and the spool  15  and the rotation of the spool  15  can be continued in the retracting direction W until the play in the webbing  16  is removed completely. 
     (Third Embodiment) 
     FIG. 11 shows a webbing winding device  112  according to a third embodiment of the present invention. This webbing winding device  112  comprises a plate  114  to be fixed to a vehicle body and a pair of side plates  116 A and  116 B which are bent from both sides of the plate  114  at right angles and parallel to each other. The side plates  116 A and  116 B rotatably support a spool  118 . The distal end of the webbing to be fitted to a passenger is anchored to this spool  118  and the webbing  120  is wound around the spool  118  in layers. 
     A clutch  122  is disposed outside one side plate  116 A at an end of the spool  118 . This clutch  122  comprises a sun gear  124  projecting outward from one side plate  116 A, a planet gear  126  which is always meshing with this sun gear  124 , a slider gear  128  disposed so as to be able to mesh with the planet gear  126  and a cam gear  132  disposed so as to be able to mesh with the slider gear  128 . 
     The sun gear  124  is disposed coaxially with the spool  118  and rotates integrally with the spool  118 . The cam gear  132  is comprised of a semi-circular pinion  133  and a thin ring-shaped plate  134  and this plate  134  is disposed on the side of one side plate  116 A. A circular opening  132 A is formed around the axis of the cam gear  132 . The sun gear  124  is inserted into this circular opening  132 A. That is, the sun gear  124  is located inside the pinion  133  and meshes with the planet gear  126  located on an opposite face of the plate  134  to the side plate  116 A. 
     A starting end gear  133 A of the pinion  133  is disposed so as to be in contact with a rack  136  connected to a piston  140  of the piston driving apparatus (not shown) via a connecting rod  138 . The piston  140  is accommodated movably in a piston cylinder (not shown). Note that, a driving member for rotating the cam gear  132  is not restricted to the rack  136 , and, for example, a driving gear such as a fan-shaped gear (sector gear) having no teeth over the entire periphery thereof may be used. 
     As shown in FIG. 12, an elongated hole  134 A is formed in a portion of the plate  134  of the cam gear  132 . The elongated hole  134 A is disposed so that the longitudinal direction thereof is formed in an arc shape centered around the axis of the cam gear  132 . This is used for positioning the planet gear  126  so that the teeth of the slider gear  128  can be lined up with the teeth of the planet gear  126  and engaged therewith when the slider gear  128  moves in a direction orthogonal to the axis of the sun gear  124 . 
     In the plate  134 , a triangular engaging protrusion  135 A forming a part of the cam portion is provided so as to protrude outward at a position corresponding to the elongated hole  134 A. Moreover in the plate  134 , a triangular engaging cutout portion  135 B forming a part of the cam portion is formed at a position opposite the engaging protrusion  135 A across the center of the cam gear  132 . 
     As shown in FIG. 11, the planet gear  126  is disposed rotatably above the cam gear  132 . A supporting shaft  127  protrudes from the axis of each of both side faces of the planet gear  126  and a small-diameter pin  127 A also protrudes from each of the supporting shafts  127 . One end of the supporting shaft  127  is inserted into the elongated hole  134 A and the other end of the supporting shaft  127  is inserted into an elongated hole (not shown) formed in the same way as the elongated hole  134  in a cover (not shown). 
     In one side plate  116 A corresponding to the pin  127 A and the cover (not shown) are punched coaxial holes  116 C. The pin  127 A is inserted into these holes  116 C so that the planet gear  126  is rotatable. That is, the planet gear  126  is journaled rotatably at a predetermined position of the side plate  116 A. If a predetermined load is applied to the pin  127 A, the pin  127 A is sheared and the planet gear  126  rotates along the slider gear  128 . That is, the sun gear  124 , the planet gear  126  and the slider gear  128  form a planet gear mechanism in which the planet gear  126  is rotated around its own axis in a clockwise direction by the slider gear  128  at the same time as it is being rotated counterclockwise around the axis A of the sun gear  124 . 
     As shown in FIG. 11, the slider gear  128  is an integral member comprising a substantially semi-circular internal tooth type internal gear  129  and a thin plate-like cam  130 . This internal gear  129  is as long in a pitch circle direction along the pitch circle P C  around the axis A of the spool  118 , as a traveling distance of the rack  136  moving linearly from a starting end position shown in FIG. 11 to a terminal position shown in FIG. 14 when the piston driving apparatus is actuated. Although, in the internal gear  129 , teeth  129 A are formed continuously on an inner peripheral face thereof along a pitch circle P C , no tooth  129 A is provided at a portion corresponding to the terminal position of the rack  136  on the inner peripheral face of the internal gear  129 . Instead, a toothless portion  129 B is formed which comprises a curved face running along the deddendum circle of the teeth  129 A. Thus, if the rack  136  is moved to the terminal position, as shown in FIG. 14, a gap is formed between the tips of the teeth of the planet gear  126  and the toothless portion  129 B of the internal gear  129 , so that the planet gear  126  is disengaged from the internal gear  129 . Thus, the planet gear  126  and the sun gear  118  which meshes with the planet gear  126  are capable of rotating without being restricted by the internal gear fixed to the side plate  116 A. Therefore, the spool  118  connected to the sun gear  124  is capable of rotating in the retracting direction W. 
     The cam  130  is disposed on the side of one side plate  116 A and pins  131  which protrudes towards the side plate  116 A are provided on each of both ends of the cam  130 . An elongated hole  116 D is formed at a position corresponding to each of these pins  131  in the side plate  116 A. These elongated holes  116 D are disposed such that the longitudinal directions thereof are orthogonal to the axis of the sun gear  124 , i.e. the longitudinal directions thereof match a radial direction of the sun gear  24 . Further, triangle shaped protrusions  130 A and  130 B are formed at positions corresponding to the pins  131  of the cam  130  as engaging portions. These protrusions  130 A and  130 B protrude toward the axis of the sun gear  124 . 
     The sun gear  124  and the cam gear  132  are disposed such that a vertex of the protrusion  130 A faces and is in contact with a vertex of the engaging protrusion  135 A, and a vertex of the protrusion  130 B faces and is in contact with a valley portion of the engaging cutout portion  135 B. As shown in FIG. 12, the supporting shaft  127  of the planet gear  126  is located at an end  134 B of the elongated hole  134 A in the cam gear  132 . In this initial state, the pin  131  of the slider gear  128  is located on an upper side (direction indicated by the arrow UP in FIG. 12) of the side plate  116 A so that the planet gear  126  does not mesh with the slider gear  128 . Thus, the slider gear does not obstruct the rotations of the planet gear  126  and the spool  118  so that the webbing  120  can be retracted or fed out freely. That is, at this initial position, the vertex of the protrusion  130 A of the sun gear  124  opposes the vertex of the engaging protrusion  135 A of the cam gear  132  and the vertex of the protrusion  130 B opposes the vertex of the engaging cutout portion  135 B. 
     On the other hand, if the engagements between the engaging protrusion  135 A and the protrusion  130 A and between the engaging cutout portion  135 B and the protrusion  130 B are released and the pins  131  of the slider gear  128  drops down to the right along the elongated hole  116 D as shown in FIG. 13, the planet gear  126  meshes with the internal gear  129  of the slider gear  128 . That is, the gear mechanism of this embodiment is a planet gear mechanism comprising a sun gear  124 , a planet gear  126 , a slider gear  128  and a cam gear  132 . 
     In the initial state shown in FIG. 12, the starting end gear  133  of the pinion  133  of the cam gear  132  is kept in contact with the rack  136 . With the movement of the piston  140 , the rack  136  moves upward rotating the cam gear  132 . 
     The piston driving apparatus includes a gas generator (not shown), which ignites in an emergency deceleration of a vehicle to generate a large amount of gas. This gas generator communicates with a space behind the piston  140  in a piston cylinder (not shown). Thus, if the gas generator ignites to generate a large amount of gas, this large amount of gas is supplied to the backside of the piston  140  in the cylinder. 
     The other end portion of the spool  118  extends out of the other side plate  116 B to reach an emergency deceleration locking mechanism (not shown). This locking mechanism is identical to a conventionally known locking mechanism for stopping the feed-out rotation of the spool  118  in response to an emergency deceleration of the vehicle or an acceleration in the feeding out of the webbing  120 . Thus, a description thereof in detail is omitted. 
     Next, the action and operation of the webbing winding device  112  of this embodiment having the above structure will be described. 
     First, an action of the webbing winding device  112  of the present embodiment when the pre-tensioner is actuated will be described. When the vehicle is traveling normally, as shown in FIG. 12, the clutch  122  is maintained in its initial state, namely, the vertex of the protrusion  130 A the slider gear  128  opposes the vertex of the engaging protrusion  135 A of the cam gear  132  and the vertex of the protrusion  130 B opposes the vertex of the engaging cutout portion  135 B. Thus, the planet gear  126  is held away from the slider gear  128  (internal gear  129 ). Therefore, a rotation of the spool  118  is not obstructed by the slider gear  128  so that the spool  118  is capable of rotating freely while rotating the planet gear so as to achieve retracting and feed-out of the webbing. 
     If the vehicle decelerates in an emergency, the piston driving apparatus (not shown) is actuated, so that a high pressure gas flows to the back face of the piston  140  in the piston cylinder. As a result, the piston  140  disposed in the 
     cylinder moves the rack upwards in a straight line from the starting end position (the direction shown by the arrow UP in FIG.  12 ). As a result, the rack  136  presses the starting end gear portion  133  with which it is in contact in the initial state upward, thereby rotating the cam gear  132  counterclockwise (the direction indicated by the arrow F) shown in FIG.  12 . When the cam gear  132  is rotated, the web winding device is no longer in the initial state shown in FIG. 12, but has entered the ON state shown in FIG.  13 . 
     That is, the protrusion  130 A moves away from the engaging protrusion  135 A and the cam  130 B moves away from the engaging cutout portion  135 B. At the same time, the lower pin  131  receives a downward pressing force from the inclined face of the engaging cutout portion  135 B and is guided along the elongated hole  116 D so that the slider gear  128  slides in the direction of the indicated by the arrow N in FIG.  12 . As the slider gear  128  slides, as shown in FIG. 13, the teeth of the internal gear  129  of the slider gear  128  fit between and mesh with the teeth of the planet gear  126 . Thus, because according to this embodiment, the slider gear  128  slides along the elongated hole  116 D, the teeth of the internal gear  129  of the slider gear  128  engage the teeth of the planet gear  126  smoothly. Note that after the slider gear  128  engages the planet gear  126 , the planet gear  126  revolves around. 
     As shown in FIG. 13, if the cam gear  132  rotates counterclockwise, the pin  127 A of the planet gear  126  is moved relatively from one end  134 B to the other end  134 C of the elongated hole  134 A relatively and comes into contact with the other end  134 C. If the rack  136  is moved further upward from this state, the cam gear  132  is rotated counterclockwise. Then, if the rotation force exceeds a predetermined value, the pin  127 A of the planet gear  126  is sheared. Thus, the planet gear  126  revolves as around the cam gear  132  rotates, with the pin  127 A in contact with the other end  134 C of the elongated hole  134 A. 
     That is, when the cam gear  132  is rotated counterclockwise from the state shown in FIG. 13, the planet gear  126  revolves around counterclockwise while it is itself rotated clockwise by the internal gear  129  of the slider gear  128 . Therefore, the sun gear  124  meshing with the planet gear  126  rotates counterclockwise. Because of this, the spool  118  is forced to rotate at high speed in the retracting direction (see FIG. 11) along with the rotation of the sun gear  124 . As a result, the webbing  120  is wound around the spool  118  rapidly. 
     When the rack  126  is moved up to the terminal end, as shown in FIG. 14, so that the rotation of the cam gear  132  stops, the planet gear  126  is released from the internal gear  129  due to the tooth missing portion  129 B, enabling that the sun gear  124  and the spool  118  to be rotated in the retracting direction W. 
     In the webbing winding device  112  according to this embodiment described above, when the piston driving unit is actuated, the rotation of the cam gear  132  is accelerated and transmitted to the sun gear  124 . Thus, when the piston  140  is moved in the piston cylinder by gas pressure when there is play in the webbing  120  fitted around a passenger, the spool  118  is rotated at high speed in the retracting direction W by the torque transmitted from the sun gear  124  to the spool  118  resulting in the webbing  120  being wound around the spool  118 . If the play in the webbing  120  is slight at this time, the pre-tensioner rotates the spool  118  in the retracting direction until the play in the webbing  120  is removed. When a tension balancing the gas pressure in the piston cylinder is generated, then, even if the piston  140  has traveled partway to the terminal end on its traveling stroke, the piston  140  is halted. On the other hand, because the planet gear  126  is released from the internal gear  129  when the driving of the rack  136  by the piston driving apparatus is completed, if there is play left in the webbing  120 , the tension of the webbing  120  acting as a rotation load on the spool  118  is small and a large inertial force acts on the spool  118  rotating at high speed. Thus, the spool  118  continues to be rotated in the retracting direction by its inertial force. If a tension balancing the inertial force on the spool  118  is generated, the rotation of the spool stops. Therefore, even if the play in the webbing  120  is large just before the piston driving apparatus is actuated, the spool may be rotated in the retracting direction until the play in the webbing  120  is removed completely. 
     (Modification  1  of the Third Embodiment) 
     Modification  1  of the gear train mechanism according to the third embodiment of the present invention will be described with reference to FIG.  15 . The webbing winding device  150  shown in FIG. 15 includes a gear train mechanism comprised of a rack  151  and a pinion  29 . The same reference numerals are attached to components having basically the same structure and function as the webbing winding device  10  of the first embodiment, and a description thereof is omitted. 
     The rack  151  has a structure in which a tooth portion  152  and a rod portion  153  are integrated. This tooth portion  152  is substantially as long in the pitch line direction as a traveling distance of the rack  151  when it is moved from its starting end position to its terminal end position by the piston driving apparatus  35 . 
     In the tooth portion  152 , teeth  152 A which mesh with teeth  29 A of the pinion  29  are formed continuously on a side end face in the transverse direction. A toothless portion  152 B, which has a slightly larger diameter than the circle formed by the tips of the teeth of the pinion  29  when the rack  151  is stopped at the terminal end position and is made of a concave curved face which curves in a circular arc form around the axis A, is formed at a position of the proximal end corresponding to the terminal end position on one side end face of the tooth portion  152 . As a result, if the rack  151  moves up to the terminal end position, as shown in FIG. 15, a gap is generated between the tooth tip of the pinion  29  and the toothless portion  152 B of the rack  151 ,  50  that the pinion  29  moves away from the rack  151 . Therefore, the spool  15  connected to the pinion  29  is capable of rotating in the retracting direction without being restricted by the rack  151 . 
     The action of this webbing winding device when the piston driving apparatus  35  is actuated is basically the same as that of the webbing winding device of the first embodiment and a description thereof is omitted. In the webbing winding device  150  to which modification  1  of the gear train mechanism is applied also, if there is play in the webbing  16  when the driving of the rack  151  by the piston driving apparatus  35  is completed, the rack  151  does not restrict the rotations of the pinion  29  and spool  15  but the spool  15  is able to rotate in the retracting direction W due to its inertial force until the play in the webbing  16  is removed completely. 
     (Fourth Embodiment) 
     FIGS. 16-20 show a webbing winding device  160  according to the fourth embodiment of the present invention. The same reference numerals are attached to components having basically the same structure and function as in the third embodiment and a description thereof is omitted. The webbing winding device  160  of this embodiment comprises a well known planetary gear mechanism  161  and one-way clutch  162  as a torque transmission mechanism for transmitting torque to the spool  118 . 
     In the webbing winding device  160 , a sun gear  124  is connected to a spool  118  which is a winding shaft via the one-way clutch  162 . This one-way clutch  162 , as shown in FIG. 17, comprises an inner wheel portion  162  provided integrally with the spool  118 , three cylindrical rollers  164 , a thin circular holding plate  165  and a rotor  166  provided integrally with the sun gear  124 . 
     The inner wheel portion  163  is provided so as to protrude from a side face of the spool  118  and is formed in a circular convex form around the axis A of the spool  118 . Circular opening (not shown) slightly larger than an outside diameter of the holding plate  165  is formed centering on the axis A in the side plate  116 A of the plate  114 . The inner wheel portion  163  of the spool  118  supported by the side plates  116 A and  116 B protrudes outside the side plate  116 A via the circular opening in the side plate  116 A. 
     The holding plate  165  is formed in a thin circular plate and the outside diameter thereof is substantially the same as the outside diameter of the rotor  166 . An opening portion  165 A whose diameter is slightly larger than the outside diameter of the inner wheel portion  163  is formed in the center of the holding plate  165 . Three roller holding portions  165 B are formed by cutting U-shaped portions from the inner periphery face of the opening portion  165 A in a radial direction. The roller holding portions  165 B are formed slightly wider than the diameter of the rollers  164 . These roller holding portions are provided at equal intervals (120°) in a circumferential direction around the axis A. 
     The opening portion  165 A of the holding plate  165  is placed on an outside peripheral face of the inner wheel portion  163  and then the holding plate  165  is placed on a side face of the spool  118  via a circular opening of the side plate  116 A. As a result, the holding plate  165  is held in a ring-like concave portion formed by the outside peripheral face of the inner wheel portion  163  and the inside peripheral face of the circular opening in the radial direction and is positioned coaxially with the spool  15 . 
     The center axis of each of the three rollers  164  is parallel to the axis A. The three rollers  164  are placed on a side face of the spool  118  via each of the roller holding portions  165 B in the holding plate  165 . As a result, the three rollers  164  are positioned at equal intervals in the circumferential direction by the roller holding portions  165 B. If the holding plate  165  is rotated around the axis A, they slide on the side face of the spool  118  in the circumferential direction with the equal intervals maintained by the holding plate  165 . 
     A rotor  166  is configured as an outside wheel of the one-way clutch  162  and formed in a circular shape having a large thickness, as shown in FIG.  17 . An inner wheel accommodating chamber  166 A and three roller accommodating chambers  166 B communicating with this inner wheel accommodating chamber  166 A are formed in this rotor  166 . The inner wheel accommodating chamber  166 A forms a cylindrical space corresponding to the inner wheel portion  163  inside the rotor  166  and each of the roller accommodating chambers  166 B forms a space extending outward from the inner peripheral face of the inner wheel accommodating chamber  166 A. These spaces are provided at equal intervals (120°) in the circumferential direction around the axis A. Each of the roller accommodating chambers  166 B is formed in a substantially elliptic shape extending in a spiral direction around the axis A. Partition wall-like stopper protrusions  166 C are formed along an inner peripheral face of the inner wheel accommodating chamber  166 A between an outside end of each roller accommodating chamber  166 B in the spiral direction and the inner wheel accommodating chamber  166 A. Each roller accommodating chamber  166 B forms a substantially wedge-like space which narrows in width gradually toward the inside in the spiral direction between the roller accommodating chamber and an outside peripheral face of the inner wheel portion  163  accommodated in the inner wheel accommodating chamber  166 A. The rotor  23  has a sun gear  124  provided integrally and coaxially therewith on an outside side face in the axial direction S, as shown in FIG. 17. A center hole  124 A is made through this sun gear  124  and the center hole  124 A communicates with the inner wheel accommodating chamber  166 A in the rotor  166 . 
     The rotor  166  is placed on the side face of the spool  118  with the holding plate  165  sandwiched and accommodates the inner wheel portion  163  within the inner wheel accommodating chamber  166 A. At the same time, the rollers  164  are accommodated in the three roller accommodating chambers  166 B. If the rotor  166  and the sun gear  124  are placed on the side face of the spool  118 , a connecting member  168  is inserted into the center hole  124 A in the sun gear  124 . The connecting member  168  is comprised of a round bar-like shaft portion  168 A, a male threaded portion  168 B provided at a front end of this shaft portion  168 A, and a head portion  168 C supported at a rear end of the shaft portion  168 A coaxially therewith as shown in FIG.  17 . In the connecting member  168 , the shaft portion  168 A is inserted through the center hole  124 A in the sun gear  124  and the inner wheel accommodating chamber  166 A in the rotor  166 . The male threaded portion  168 B is screwed into the threaded hole  163 A in the inner wheel portion  163 . As a result, the sun gear  124  and the rotor  166  are supported rotatably coaxially with the spool  118 . 
     The one-way clutch  162  is mounted to a vehicle in an OFF state in which the spool  118  is separated from the sun gear  124 . This OFF state is basically the same as the OFF state of the one-way clutch  19 , and therefore a description thereof is omitted. When the one-way clutch  162  is in the OFF state, even if the sun gear  124  is blocked from rotation, the spool  118  may be rotated both in the retracting direction and feed-out direction of the webbing  120 . If torque is transmitted to the sun gear  124  in the retracting direction W and the rotor  166  rotates in the retracting direction W with respect to the holding plate  165 , the one-way clutch  162  changes from an the OFF state to an ON state, so that the torque can be transmitted from the sun gear  124  to the spool  118 . 
     As shown in FIG. 16, the planetary gear mechanism  161  comprises a slider gear  169  and a ring gear  170 . The sun gear  124  which meshes with the planet gear  126  is disposed coaxially with the spool  118  via the one-way clutch  162  as described above. Only when a torque is transmitted in the retracting direction W, is the sun gear  124  connected to the spool  118  and rotated integrally with the spool  118 . 
     The slider gear  169  is formed integrally from comprised of an internal tooth type internal gear  171  and a thin ring-like slider  172 . The slider  172  is disposed at an end portion on the side of the spool  118  in the axial direction S. A circular opening  172 A is formed around the axis A. The inside diameter of this circular opening is slightly larger than the outside diameter of the rotor  166 . Teeth  171 A are formed continuously on an inner peripheral face of the internal gear  171  along a pitch circle P C  (see FIG. 18) corresponding to a traveling range of the rack  136  which moves linearly from its starting end position to the terminal end position when the piston driving apparatus is actuated. In the internal gear  171 , as shown in FIG. 18, an engaging plate  173  extends from a bottom of a starting end tooth  171 A. This engaging plate  173  is formed as a curved plate running along the dedendum circle of the teeth  171 A. As shown in FIG. 18, an engaging groove  173 A which is concave towards an outer peripheral face thereof is provided on an inner peripheral face of an end of the extending portion of this engaging plate  173 . 
     In the slider gear  169 , the slider  172  is placed on the side plate  116 A with the circular opening  172 A placed around the outer peripheral face of the rotor  166 . As a result, the slider gear  169  is journaled by the rotor  166  so that it is rotatable around the axis A. On the other hand, a supporting shaft  177  is provided at an outer side face of the side plate  116 A so as to protrude in the axial direction S. One end of this supporting shaft  177  corresponds to the engaging groove  173 A in the engaging plate  173 , as shown in FIG. 18, and the other end thereof supports stopper lever  176  supported on the pitch line P L  of the rack  136 . This stopper lever  176  is always urged by an urging member such as a twisted coil spring (not shown) in one direction (clockwise in FIG.  18 ). Before the piston driving unit is actuated, an end of this stopper lever  176  is inserted into the engaging groove  173 A of the engaging plate  173  so as to restrict the slider gear  169  at a predetermined position in the rotation direction. 
     The ring gear  170  is comprised of a pinion  174  which is a semi-circular sector gear and a thin plate-like ring  175 . This ring  175  is disposed on the side plate  116 A. A circular opening  175 A is formed around the axis in the ring  175 . The inside diameter of this circular opening is equal to the inside diameter of the circular opening  172 A in the slider gear  166 . In the ring gear  170 , the ring  175  is placed on the slider  172  with the circular opening  175 A fitted around the outer peripheral face of the rotor  166  projecting through the slider  172 . As a result, the ring gear  170  is journaled by the rotor  166  so that it is rotatable around the axis A. The sun gear  124  is located inside of the ring gear  170  in the radial direction. 
     A supporting shaft  175 B is provided at a position opposite to the pinion  174  in the circumferential direction on an outer side face of the ring  175  so as to project in the axial directions. A hole made in the direction of the axis of the planet gear  126  is fitted around this supporting shaft  175 B so that the planet gear  126  is supported rotatably. At the same time, the planet gear  126  meshes with the sun gear  124  inside the pinion  174 . The initial position of the ring gear  70  is adjusted so that the starting end tooth  174 A of the pinion  174  is in contact with the rack  136  connected to the piston  140  via the connecting rod  138  in the rotation direction and the teeth  126 A of the planet gear  126  mesh with the starting end tooth  171 A of the internal gear  171 . 
     Note that a cover (not shown) for accommodating together with the side plate  116 A the planetary gear mechanism  161  is attached to an outer side face of the side plate  116 A. An inner side face of this cover is in contact with an outer side face of the pinion  174  so as to prevent the slider gear  169  and ring gear  170  from slipping off the rotor  166 . 
     In the initial state before the piston driving apparatus shown in FIG. 18 is actuated, the starting end tooth  174 A of the pinion  174  of the ring gear  170  is maintained so as to be in contact with the rack  136 . If the rack  136  moves upward from the starting end position together with the movement of the piston  140 , the ring gear  170  rotates counterclockwise. 
     Next, the action and operation of the webbing winding device of the present embodiment having the above structure will be described. 
     First, an action of the webbing winding device  160  of this embodiment when the pre-tensioner is actuated will be described. When the vehicle is ordinarily traveling normally, the planetary gear mechanism  161  is maintained in its initial state as shown in FIG.  18 . That is, the rotation of the slider gear  169  is restricted by the stopper lever  176 . The planet gear  126  meshes with the starting end tooth  171 A of the internal gear  171  and the sun gear  124 . In this initial state, the one-way clutch  162  is maintained in an OFF state so that the spool  118  can rotate in both directions. 
     If the vehicle decelerates in an emergency, the rack  136  is moved linearly from the starting end position to the terminal end position by the piston driving apparatus. As a result, the rack  136  rotates the ring gear  170  counterclockwise. At the same time, the one-way clutch  162  is place in an ON state, so that the sun gear  124  and spool  118  rotate integrally. When the ring gear  170  rotates counterclockwise, the planet gear  126  revolves around the axis A together with the rotation of the ring gear  170  and the sun gear  124  meshing with the planet gear  126  rotates counterclockwise. Thus, the spool  118  is rotated at high speed in the retracting direction along with the rotation of the sun gear  124 . Consequently, the webbing  120  is retracted rapidly by the spool  118 . 
     When the rack  136  is moved up to the terminal end position, as shown in FIG. 19, the front end of the rack  136  presses the other end of the stopper lever  176  and rotates it counterclockwise, thereby releasing the stopper lever  176  from the engaging plate  173  of the slider gear  169 . As a result, the slider gear  169  may slide on the side plate  116 A and rotate around the axis A. That is, the internal gear  171  of the slider gear  169  becomes able to move along the pitch circle P C . 
     In the webbing winding device  160  according to this embodiment described above, if the piston driving apparatus is actuated, the rotation of the ring gear  170  is speeded up and transmitted to the sun gear  124 . Thus, when the piston  140  is moved in the piston cylinder by receiving gas pressure when there is play in the webbing  120  fitted around a passenger, the spool  118  is rotated quickly in the retracting direction by the torque transmitted from the sun gear  124  to the spool  118  and the webbing  120  is wound around the spool  118 . If the play in the webbing  120  is slight at this time, the pre-tensioner rotates the spool  118  in the retracting direction W until the play in the webbing  120  is removed. Accordingly, when a tension balancing the gas pressure in the piston cylinder is generated, then even if the piston has traveled partway to the terminal end on its traveling stroke, the piston  140  is halted. On the other hand, because the internal gear  171  becomes able to move along the pitch circle P C  when the driving of the rack  136  by the piston driving apparatus is completed, the planet gear  126  meshing with the internal gear  171  becomes capable of being itself rotated counterclockwise by rotating the internal gear  171  counterclockwise along the pitch circle P C  from the initial position, as shown in FIG.  20 . As a result, when the driving of the rack  136  is completed, the sun gear  124  meshing with the planet gear  126  also becomes able to rotate together with the spool  118 . Therefore, if there is play in the webbing  120  when the driving of the rack  136  by the piston driving apparatus is completed, the tension of the webbing  120  acting as a rotation load on the spool  118  is small and a large inertial force acts on the spool  118  rotating at a high speed. Thus, the internal gear  171  is rotated along the pitch circle P L  by the torque transmitted from the sun gear  124  to the planet gear  126  so that the spool  118  may be rotated in the retracting direction W until the play in the webbing  120  is removed completely. 
     (Fifth Embodiment) 
     FIGS. 21-25 show a webbing winding device  180  according to the fifth embodiment of the present invention. The same reference numerals are attached to components having basically the same structure and function as in the third and fourth embodiments and a description thereof is omitted. The webbing winding device  180  of this embodiment comprises a well known planetary gear mechanism  181  and one-way clutch  162  as a torque transmission mechanism for transmitting torque to the spool  118 . 
     In the webbing winding device  180 , the sun gear  124  is connected to the spool  118  via the one-way clutch  162 . This one-way clutch  162  is provided with an inner wheel portion, rollers and the like (not shown) accommodated in the rotor  166  provided integrally with the sun gear  124  shown in FIG.  21 . 
     The one-way clutch  162  is mounted to a vehicle in the OFF state in which the spool  118  is separated from the sun gear  124 . If the one-way clutch  162  is in the OFF state, even if the sun gear  124  is blocked from being able to rotate, the spool  118  can be rotated both in the retracting direction W and the feed-out direction R of the webbing  120 . If torque is transmitted to the sun gear  124  in the retracting direction W and the rotor  166  rotates relatively in the retracting direction W with respect to the holding plate  165 , the one-way clutch  162  taken out of the OFF state and placed in an ON state, so that the torque can be transmitted from the sun gear  124  to the spool  118 . 
     As shown in FIG. 21, the planetary gear mechanism  181  is provided with an internal gear  182 . This internal gear  182  is fixed to an outer side face of the side plate  116 A coaxially with the spool  118  and is formed in the form of a plate curved at a predetermined curvature around the axis A. Teeth (internal teeth)  182 A having a shape which corresponds to the planet gear  126  are formed on an inner peripheral face of the internal gear  182 . 
     A rack  183  to mesh with a pinion  174  of the planetary gear mechanism  181 , as shown in FIG. 23, is supported by a piston  140  such that the longitudinal direction thereof is parallel to the pitch line P L . As shown in FIG. 23, this rack  183  comprises a rod portion  184  disposed on the proximal end side in the length direction and a tooth portion  185  disposed on the distal end side. The tooth portion  185  comprises a supporting portion  186  and a slide portion  187  supported by this supporting portion  186 , as shown in FIG.  22 . The slide portion  187  is disposed on the side of the pinion  174  the supporting portion  186  closer to the pinion  174 . In this slide portion  187 , three teeth  187 A are formed continuously on one side end face thereof in the transverse direction facing the pinion  174 . A sliding piece  188  is provided on the other side face of the slide portion  187  such that it protrudes in the transverse direction from near the proximal end portion of the slide portion  187 . Round bar-shaped guide pins  189  are provided on both side faces in the axial direction S of the sliding piece  188  so that they protrude in the axial directions. 
     On the other hand, the supporting portion  186  has a main body  190  formed integrally with the rod portion  184 . A sliding groove  191  open to side end faces facing the slide portion  187  and the outer side face in the axial direction S, is formed in this main body  190  such that it extends in the longitudinal of the rack  186  as shown in FIG. 22. A side portion of the supporting portion  186  in the axial direction S is made of a plate-shaped lid plate  192 . This lid plate  192  is fixed to the main body  190  with a plurality of (4) screws  193  so as to close the outer side face in the axial directions of the sliding groove  191 . A guide slot  194  is formed in each of the outer side faces in the axial direction S of the sliding groove  191  and the lid plate  192  corresponding to each of a pair of guide pins  189  of the sliding piece  188 . These guide slots extend in parallel to each other in the longitudinal direction of the rack  183 . 
     When the rack  183  is assembled, the sliding piece  188  of the slide portion  187  is inserted into the sliding groove  191  of the main body  190  and, at the same time, one guide pin  189  of the sliding piece  188  is inserted into the guide slot  194  in the main body  190 . After that, the other guide pin  189  is inserted into the guide slot  194  in the lid plate  192  and the lid plate  192  is fastened by the screws to the main body  190 . Consequently, the slide portion  187  is able to slide in the longitudinal direction in the corresponding range of the guide slot. 
     Before the piston driving apparatus is actuated, the initial position of the ring gear  170  is adjusted in the rotation direction so that, as shown in FIG. 23, the teeth  126 A of the planet gear  126  mesh with the starting end tooth  171 A of the internal gear  171 . On the other hand, the initial position of the rack  183  is adjusted so that the proximal end face of the slide portion  187  is in contact with the distal end face of the rod portion  184  and so that the guide pins  189  are located near the rear end  194 A of the guide slots  194 . Moreover, the rack  183  is supported by the piston  140  so that the starting end tooth  187 A of the slide portion  187  comes into contact with the starting end tooth  174 A of the pinion  174 . 
     Next, the action and operation of the webbing winding device  180  of the present embodiment having the above structure will be described. 
     First, the action of the webbing winding device  180  of the present embodiment when the pre-tensioner is actuated will be described. When the vehicle is traveling normally, as shown in FIG. 23, the planetary gear mechanism  181  is maintained in its initial state. That is, the planet gear  126  meshes with the starting end tooth  171 A of the internal gear  171  and the sun gear  124  and the rack  183  is in contact with the starting end tooth  174 A of the pinion  174 . In this initial state, the one-way clutch  162  is maintained in the OFF state so that the spool  118  can rotate in both directions. 
     If the vehicle performs an emergency deceleration, the rack  183  is moved linearly from the starting end position to the terminal end position by the piston driving apparatus. As a result, the tooth  187 A of the slide portion  187  of the rack  183  presses the tooth  174 A of the pinion  174  along the pitch line P L . This pressing force is converted into a rotation force by the pinion  174  so as to rotate the ring gear  170  counterclockwise. In this condition, the slide portion  187  is confined to a linear motion along the sliding groove  191  by the tooth  174 A of the pinion  174  and the distal end face of the rod portion  184 . At the same time as the ring gear  170  is rotating counterclockwise, the one-way clutch  162  is placed in an ON state and the sun gear  124  and spool  118  rotate integrally. When the ring gear  170  rotates counterclockwise, the planet gear  126  revolves around with the rotation of the ring gear  170 , and the sun gear  124  meshing with the planet gear  126  rotates counterclockwise. Thus, with the rotation of the sun gear  124 , the spool  118  is rotated at a high speed in the retracting direction W. As a result, the webbing  120  is retracted rapidly by the spool  118 . 
     When the rack  183  is moved up to the terminal end position as shown in FIG. 24, the slide portion  187  of the rack  183  is able to move linearly along the sliding groove  191  with the rotation of the pinion  174  in the counterclockwise direction. 
     In the webbing winding device  180  according to this embodiment described above, if the piston driving apparatus is actuated, the rotation of the ring gear  170  is speeded up and transmitted to the sun gear  124 . Thus, when the piston  140  is moved in the piston cylinder by receiving gas pressure when there is play in the webbing  120  fitted around by a passenger, the spool  118  is rotated at high speed in the retracting direction W by torque transmitted from the sun gear  124  to the spool  118 , and the webbing  120  is wound around the spool  118 . If the play in the webbing  120  is slight at this time, the pre-tensioner rotates the spool  118  in the retracting direction W until the play in the webbing  120  is removed. When a tension balancing the gas pressure in the piston cylinder is generated, then, even if the piston has traveled partway to the terminal end on its traveling stroke, the piston  140  is halted. 
     On the other hand, when the driving of the rack  183  by the piston driving apparatus is completed, the slide portion  187  of the rack  183  becomes capable of moving forward along the pitch line P L  along the sliding groove  191 . By advancing the slide portion  187  of the rack  183  up to a position where it does not interfere (contact) with the tooth  174 A of the pinion  174 , the pinion  174  which had been meshed with the rack  183  becomes able to rotate counterclockwise. Further, if the ring gear  170  rotates up to a position where the planet gear  126  is released from the internal gear  182 , the planet gear  126  and the sun gear  124  become able to rotate with the spool  118  without the rotation of the pinion  174 . 
     Therefore, if there is play in the webbing  120  when the driving of the rack  183  is completed, the tension of the webbing  120  acting as a rotation load on the spool  118  is small and a large inertial force acts on the spool  118  rotating at a high speed. Thus, the inertial force (torque) from the spool  118  is transmitted to the slide portion  187  of the rack  183  via the sun gear  124 , planet gear  126  and internal gear  182 . As a result, the slide portion  187  is advanced so that the spool  118  can be rotated in the retracting direction W until the play in the webbing  120  is removed completely. 
     (Modification  1  of the Fifth Embodiment) 
     Modification  1  of the rack according to the fifth embodiment of the present invention will be described with reference to FIGS. 26 and 27. Note that the rack  195  shown in FIG. 26 may be applied to the webbing winding device  180  shown in FIGS. 21 to  25  instead of the rack  183 . 
     As shown in FIG. 27, the rack  195  is supported by the piston  140  such that a longitudinal direction thereof is parallel to the pitch line P L . This rack  195  is comprised of a rod portion  196  disposed on a proximal end side thereof in the longitudinal direction and a tooth portion  197  disposed on the distal end side thereof. In the tooth portion  197 , as shown in FIG. 26, three teeth  198 ,  199  and  200  are formed continuously along the pitch line P L  on one side end face in the transverse direction facing the pinion  174 . 
     The starting end tooth  198  of the tooth portion  197  has a tooth width corresponding to the pinion  174  and the tooth width thereof is larger than the tooth width of the teeth  199  and  200 . The tooth width of the other teeth  199  and  200  are substantially ½ the tooth width of the starting end tooth  198 . The tooth  199  disposed in the center in the longitudinal direction is disposed towards the inside in the axial direction S (towards the spool  118 ). The tooth  200  at the terminal end is disposed further towards the outside in the axial direction the central tooth  199 . 
     The teeth  198  and  199  in the tooth portion  197  and the rod portion  196  are formed integrally. The tooth  200  at the terminal end is a single part which is attached to a predetermined position of the tooth portion  197 . The tooth  200  is integrally provided with a sliding piece  201 , which protrudes in the transverse direction from the bottom of the tooth  200  (a section at right angle in the short side direction of this sliding piece  201  is of the same shape as the tooth bottom of the tooth  200 ). Round bar-shaped guide pins  202  are provided on both sides in the axial direction S of the sliding piece  201  so as to protrude in the axial direction S. Further, a cylindrical supporting protrusion  203  protrudes in the longitudinal direction from the distal end face of the sliding piece  201 . 
     On the other hand, in the tooth portion  197 , as shown in FIG. 26, a sliding groove  204  open to the side face facing the pinion  174  and the outer side face in the axial direction S is formed extending in the longitudinal direction of the rack  186 . This sliding groove  204  is disposed further to the outside in the axial directions than the central tooth. The width of the sliding groove  204  in the axial direction S is slightly larger than the width of the sliding piece  201 . This sliding groove  204  extends in the longitudinal direction over a range from the distal end of the tooth bottom of the central tooth  199  to a rear end of the tooth bottom of the tooth  200  at the terminal end. A lid plate  205  is fixed to an outer side face in the axial directions of the tooth portion  197  by a plurality (4) of screws  206 . This lid plate  205  closes the outer side face in the axial direction S of the sliding groove  204 . 
     Each of the inner side face in the axial direction S of the sliding groove  204  and the lid plate  205  has a guide slot  207  corresponding to each of the pair of guide pins  202  of the sliding piece  201 . These guide slots  207  extend in parallel to each other in the longitudinal direction of the rack  195 . A cylindrical supporting protrusion  208  is provided on the distal end face of the sliding groove  204  corresponding to the supporting protrusion  203  of the sliding piece  201 . Once the sliding piece  201  has been inserted into the sliding groove  204 , one end of a coil spring  209  is fitted over the supporting protrusion  203  while the other end of the coil spring  209  is fitted over the protrusion  208 . The supporting protrusions  203  and  208  support the coil spring  209  in a compressed state between the sliding piece  201  and the distal end face of the sliding groove  204 . 
     When the rack  195  is assembled, first the sliding piece  201  of the tooth  200  at the terminal end is inserted into the sliding groove  204  of the tooth portion  197 . Then, one guide pin  202  of the sliding piece  201  is inserted into the guide slot  207  of the sliding groove  204 . At the same time, the ends of the coil spring  209  are fitted over the supporting protrusions  203  and  208 . After that, while the other guide pin  202  is being inserted into the guide slot  207  of the lid plate  205 , the lid plate  205  is screwed tightly to the side face of the tooth portion  197 . As a result, the tooth  200  at the terminal end is able to move from a position one pitch to the rear of the central tooth  199 , indicated by solid line of FIG. 27, to a position which substantially coincides with the central tooth  199 . Here, the tooth  200  is maintained at the position one pitch to the rear of the central tooth  199  by an urging force of the coil spring  209  before the piston driving apparatus is actuated. 
     The action and operation of the webbing winding device according to the fifth embodiment, to which the rack  195  of the modification  1  having the above structure is applied, will now be described. 
     The operation when the piston driving apparatus is operated to move the rack  195  from the starting end position to the terminal end position is basically the same as in the webbing winding device  180  to which the rack  183  shown in FIGS. 21-25 is applied and therefore, a description thereof is omitted. In the webbing winding device to which the rack  195  according the modification  1  of the present invention is applied, when the driving of the rack  195  by the piston driving apparatus  35  is completed, the tooth  200  at the terminal end of the rack  195  is able to advance along the pitch line P L . Therefore, in the pinion  174 , which had been meshed with the rack  195 , the tooth  200  of the rack  195 , as indicated by the two dot dash line of FIG. 27, is advanced up to a position where it does not interfere with (contact) the tooth  174 A of the pinion  174  so that the pinion  174  is able to rotate counterclockwise. Therefore, if there is play in the webbing  120  when the driving of the rack  183  is completed, the tension of the webbing  120  acting as a rotation load on the spool  118  is small and a large inertial force acts on the spool  118  rotating at a high speed. Thus, the inertial force (torque) from the spool  118  is transmitted to the tooth  200  of the rack  195  via the sun gear  124 , planet gear  126  and internal gear  182 . The tooth  200  is advanced while resisting an urging force of the coil spring  209  so that the spool  118  may be rotated in the retracting direction W until the play in the webbing  120  is removed completely. 
     According to the webbing winding device of the present invention described above, even if the play in the webbing is large, the webbing is able to be retracted reliably until it becomes tense when the pre-tensioner is actuated. Further, because the driving rotation amount of the pinion by the driving gear is increased there is no need to increase the traveling stroke of the driving gear or to and add a speed up gear, and therefore enlargement of the device can be suppressed.