Abstract:
A seat belt retractor includes a spool for winding a seat belt, a spring device for always biasing the spool in the belt winding direction, and a locking device which allows rotation of the spool when it is not activated and locks the spool from rotating in the seat belt withdrawing direction when it is activated. The spring device has a first spring always acting on the spool to bias the spool in the seat belt winding direction, and a second spring capable of acting on the spool to bias the spool in the seat belt winding direction. The seat belt retractor has a clutch for switching a mode of the second spring between an effective mode and an ineffective mode. The seat belt retractor further includes a control device which changes the winding torque during the seat belt winding operation.

Description:
BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT  
       [0001]    The present invention relates to a seat belt retractor and a seat belt apparatus employing the same which is installed in a vehicle such as an automobile and is adapted to restrain an occupant with a seat belt thereof, wherein the seat belt retractor has a function of preventing an occurrence of end lock and at least an emergency locking function. 
         [0002]    Conventionally, seat belt apparatuses are installed in vehicles such as automobiles. In the event of an emergency such as a vehicle collision where a large deceleration acts on the vehicle, such a seat belt apparatus restrains an occupant with a seat belt thereof so as to prevent the occupant from jumping out of the seat. 
         [0003]      FIG. 7  is an illustration schematically showing an example of such seat belt apparatuses with conventional seat belt retractors. 
         [0004]    As shown in  FIG. 7 , a seat belt apparatus  1  of this example comprises a seat belt retractor  3  fixed to a vehicle body such as a B pillar  2 , a seat belt  6  which can be withdrawn from the seat belt retractor  3  and is provided at its tip end with a belt anchor  4  fixed to a vehicle floor or a vehicle seat  5 , a deflective fitting  7  which is fixed to a vehicle body such as a center pillar to guide the seat belt  6  withdrawn from the seat belt retractor  3  to a shoulder of an occupant C, a tongue  8  which is slidably attached to the seat belt  6  guided by the deflective fitting  7 , and a buckle  9  which is fixed to the vehicle floor or the vehicle seat and to which the tongue  8  can be detachably latched. 
         [0005]    In the seat belt apparatus  1 , the occupant C wears the seat belt  6  by withdrawing the seat belt  6  from the seat belt retractor  3  and latching the tongue  8  attached to the seat belt  6  to the buckle  9 . 
         [0006]    Generally, the seat belt apparatus is provided with a seat belt retractor. As the seat belt retractor, a seat belt retractor having a function as an emergency locking type seat belt retractor (ELR) capable of locking the seat belt from withdrawn has been known (see, for example, Patent document 1: JP-A-2001-058559). 
         [0007]      FIG. 8  is a vertical sectional view showing an example of the seat belt retractor disclosed in Patent document 1. In this drawing, a numeral  10  designates a spool for winding up the seat belt  6 , a numeral  11  designates a U-like frame, a numeral  12  designates a deceleration sensing device (vehicle sensor) which senses a large vehicle deceleration, generated at an emergency as mentioned above, and thus activates, a numeral  13  designates a locking device which is activated by the deceleration sensing device  12  to lock at least the spool  10  from rotating in the seat belt withdrawing direction, a numeral  14  designates a torsion bar which is fitted and inserted into the center of the spool  10  in the axial direction and connects the spool  10  and the locking device  13  to be rotated together, a numeral  15  designates a spring device which always biases the spool  10  in the seat belt winding direction via a bush  17  by a spiral spring  16 , a numeral  18  designates a pretensioner which is activated when extremely large vehicle deceleration is generated among the aforementioned emergency situations to produce a seat belt winding torque, and a numeral  19  designates a bush for transmitting the seat belt winding torque of the pretensioner  18  to the spool  10 . 
         [0008]    The locking device  13  is provided with a locking base (corresponding to a locking member of the present invention)  22  which can rotate together with a first torque transmitting shaft  20  of the torsion bar  14  and holds a pawl  21  pivotally. The locking device  13  is also provided with a lock gear  13   a  which normally rotates together with the torsion bar  14  but, in an emergency, stops by the operation of the deceleration sensing device  12  to produce a rotational difference relative to the torsion bar  14  so that the pawl  21  is engaged with internal teeth  23  formed in a side wall of a frame  11  to lock the locking base  22  from rotating in the seat belt withdrawing direction. The locking base  22  is provided with an external threaded shaft portion  24  on which a nut-like stopper member  25  rotating together with the spool  10  is screwed. 
         [0009]    The torsion bar  14  is provided with a second torque transmitting portion  26  which is engaged with the spool  10  not to allow the relative rotation therebetween. 
         [0010]    By the spring force of the spring device  15 , the spool  10  is always biased in the seat belt winding direction via the bush  17 , the torsion bar  14 , the second torque transmitting portion  26  of the torsion bar  14 , and the bush  19 . In addition, during the operation of the pretensioner  18 , the seat belt winding torque produced by the pretensioner  18  is transmitted to the spool  10  via the bush  19 , whereby the spool  10  winds up a predetermined amount of the seat belt  6 . 
         [0011]    In the conventional seat belt retractor  3  having the aforementioned structure, the seat belt  6  is wound up by the biasing force of the spring device  15  completely (fully) when the seat belt is not worn. As the seat belt  6  is withdrawn at a normal speed from this state, the spool  10  rotates in the seat belt withdrawing direction, thereby withdrawing the seat belt  6 . After the tongue  8  slidably attached to the seat belt  6  is inserted into and latched to the buckle  9  fixed to the vehicle body, an excessively withdrawn part of the seat belt is wound onto the spool  10  by the biasing force of the spring device  15 . 
         [0012]    In an emergency as mentioned above, the deceleration sensing device  12  senses a large vehicle deceleration and then activates the locking device  13 . That is, by the activation of the deceleration sensing device  12 , the lock gear  13   a  is locked from rotating in the seat belt withdrawing direction so that the pawl  21  of the locking device  13  pivots to engage the internal teeth  23  formed in the side wall of the frame  11 . Then, the locking base  22  is locked from rotating in the seat belt withdrawing direction so that the torsion bar  14  is twisted so as to allow only rotation of the spool  10  relative to the locking base  22  in the seat belt withdrawing direction. After this, the spool  10  rotates in the seat belt withdrawing direction while twisting the torsion bar  14 . According to the torsion torque of the torsion bar  14 , the load applied to the seat belt  6  is limited, thereby absorbing energy applied to the occupant. 
         [0013]    Also as for the conventional seat belt retractor  3 , when the seat belt is rapidly withdrawn, the rotation of the lock gear  13   a  is stopped by an inertia member (webbing sensor) (not shown) supported by the lock gear  13   a.  Accordingly, the locking base  22  of the locking device  13  rotates in the seat belt withdrawing direction relative to the rock gear  13   a.  Therefore, the pawl  21  of the locking device  13  engages the internal teeth  23  formed in the side wall of the frame  11  in the same manner as mentioned above so as to lock the locking base  22  from rotating, thereby locking the spool  10  from rotating in the belt withdrawing direction via the torsion bar  14  and thus locking the seat belt from being withdrawn. 
         [0014]    After the occupant C withdraws the seat belt  6 , inserts and engages the tongue  8  into the buckle  9  when sitting in a vehicle seat, an excessively withdrawn part of the seat belt  6  is wound so as to achieve normal wearing state. In this normal wearing state, it is preferable not to give undesired oppression on a chest of the occupant C. However, since, generally in the seat belt retractor  3 , the spool  10  is always biased in the seat belt winding direction with the winding torque of the spring device  15 , the occupant C may feel oppression when normally wearing the seat belt. To avoid this, it may be considered to employ a spring  16  with weak biasing force for reducing the oppression against the occupant. However, the spring  16  with weak biasing force provides small winding torque for winding up the seat belt so that the seat belt  6  is hardly wound up completely (fully) when the occupant C cancels the wearing of the seat belt  6 . 
         [0015]    For this, it has been known that a seat belt retractor is provided with an ELR function and also a tension reducer for reducing the biasing force of a spring device for winding up a seat belt while an occupant wears the seat belt under the normal wearing condition (for example, see Patent document 2: JP-B-7-008639). 
         [0016]    The tension reducer disclosed in Patent document 2 is provided to a spring device for biasing a spool in the seat belt winding direction. That is, the tension reducer employs a main spring and a sub spring which are arranged in parallel in the spring device. The total spring force of the main spring and the sub spring is set to be substantially the same as the spring force of a case employing one spring in the spring device. Therefore, each of the spring forces of the main spring and the sub spring is set to be smaller than the spring force of the case employing one spring in the spring device. 
         [0017]    As shown in  FIG. 9 , in the tension reducer, the spool is biased in the seat belt winding direction with a relatively large winding torque by both the spring force of the main spring and the spring force of the sub spring during the seat belt withdrawing operation, while a winding torque only by the spring force of the main spring acts on the spool at the completion of the seat belt wearing operation by the occupant C. Accordingly, when the occupant C wears the seat belt, the winding torque on the spool is relatively small so as to lower the tension on the seat belt. Therefore, the seat belt is lightly pulled only by the main spring so as to lightly fit the occupant C. 
         [0018]    As the occupant C disengages the tongue from the buckle and releases the tongue for cancelling the wearing of the seat belt, the spool winds up the seat belt completely (fully) by the relatively large torque of the total spring forces of the main spring and the sub spring. During this winding action, the winding torque is constant. 
         [0019]    By the way, in the general seat belt retractor having the ELR function, the rotation of the spool  10  is rapidly stopped at the completion of seat belt winding by the spring force of the spring device  15  when cancelling the wearing of the seat belt  6 . Accordingly, a phenomenon may occur in which at least one of the vehicle sensor  12  and a webbing sensor is activated due to impact of the rapid stoppage of the spool  10  so that the seat belt  6  is hardly withdrawn next time. This phenomenon is called “end lock”. 
         [0020]    For preventing the end lock, it may be considered to employ a spring  16  with weak biasing force for winding up the seat belt. However, if employing the spring with weak biasing force, the seat belt is hardly wound up completely (fully) as mentioned above. 
         [0021]    The present invention has been made under the aforementioned circumstances, and an object of the present invention is to provide a low-cost seat belt retractor and a seat belt apparatus comprising the same which can wind up a seat belt completely when an occupant cancels the wearing of the seat belt and still can prevent the end lock. 
         [0022]    Further objects and advantages of the invention will be apparent from the following description of the invention. 
       SUMMARY OF THE INVENTION  
       [0023]    To solve the aforementioned problems, a seat belt retractor according to the invention of a first aspect comprises at least: a spool for winding up a seat belt; a spring device for always biasing said spool in the belt winding direction; and a locking device which allows rotation of said spool when it is not activated and locks said spool from rotating in the seat belt withdrawing direction when it is activated, wherein said spring device has a first spring with a spring force always acting on said spool to bias said spool in the seat belt winding direction, and a second spring with a spring force capable of acting on said spool to bias said spool in the seat belt winding direction. The seat belt retractor is provided with a clutch for switching the mode of said second spring between an effective mode where its spring force acts on said spool and an ineffective mode where its spring force does not act on said spool. The seat belt retractor further comprises a control device which changes the winding torque during the seat belt winding operation after the occupant cancels the wearing of the seat belt by controlling said clutch so as to switch the mode of said second spring between the effective mode and the ineffective mode. 
         [0024]    As to a seat belt retractor according to a second aspect of the invention, said winding torque is changed by controlling said clutch to make only the spring force of said first spring to act on said spool and, after a lapse of time, make both the spring forces of said first and second springs to act on said spool. 
         [0025]    As to a seat belt retractor according to a third aspect of the invention, said winding torque is changed by controlling said clutch to make both the spring forces of said first and second springs to act on said spool and, after a lapse of time, make only the spring force of said first spring to act on said spool. 
         [0026]    A seat belt retractor according to a fourth aspect of the invention comprises at least: a spool for winding up a seat belt; a spring device for always biasing said spool in the belt winding direction; and a locking device which allows rotation of said spool when it is not activated and locks said spool from rotating in the seat belt withdrawing direction when it is activated, wherein said spring device has a first spring with a spring force capable of acting on said spool to bias said spool in the seat belt winding direction, and a second spring with a spring force smaller than that of the first spring and capable of acting on said spool to bias said spool in the seat belt winding direction. The seat belt retractor is provided with a clutch for switching the respective modes of said first and second springs between an effective mode where the spring force acts on said spool and an ineffective mode where the spring force does not act on said spool, and said seat belt retractor further comprises a control device which changes the winding torque during the seat belt winding operation after the occupant cancels the wearing of the seat belt by controlling said clutch so as to switch the respective modes of said first and second springs between the effective mode and the ineffective mode. 
         [0027]    As to a seat belt retractor according to a fifth aspect of the invention, said first and second springs and said clutch cooperate together to compose a tension reducer for reducing the tension on the seat belt during the wearing of the seat belt, and said control device controls said clutch to make the spring force of said second spring not to act on said spool when the occupant completes the wearing of the seat belt. 
         [0028]    A seat belt apparatus according to a sixth aspect of the invention comprises at least: a seat belt for restraining an occupant; a seat belt retractor which winds up the seat belt and allows the seat belt to be withdrawn; a tongue slidably attached to said seat belt; and a buckle to which said tongue can be detachably latched, wherein said seat belt retractor is a seat belt retractor in any one of the aforementioned seat belt retractors of the present invention. 
         [0029]    According to the seat belt retractor and the seat belt apparatus of the present invention having the aforementioned structure, the clutch is controlled to switch the respective modes of the first and second springs between an effective mode where its spring force acts on said spool and an ineffective mode where its spring force does not act on said spool, thereby changing the winding torque during the seat belt winding operation. This enables the winding speed during the seat belt winding operation to be changed. Accordingly, the winding torque or the winding speed at the completion of the seat belt winding operation can be changed to a suitable value, thereby reducing the impact generated by the stop of the rotation of the spool at the completion of the seat belt winding operation. Therefore, the occurrence of end lock at the completion of the seat belt winding operation is effectively prevented by a simple structure. 
         [0030]    Since the winding torque or the winding speed at the completion of the seat belt winding operation can be adjusted to be suitable value, the winding torque and the winding speed before the completion of the seat belt winding operation can be set to such a value as to wind up the seat belt completely (fully). Therefore, the end lock can be prevented while the seat belt is still enabled to be wound up completely when the seat belt is not worn. 
         [0031]    Especially, the end lock can be prevented by a low cost structure using components of the conventional known tension reducer such as the two springs, the clutch, and the electromagnetic solenoid. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]      FIG. 1  is an illustration schematically showing an embodiment of a seat belt retractor according to the present invention. 
           [0033]      FIGS. 2(   a )- 2 ( c ) are illustrations for explaining the actions of a tension reducer of the embodiment shown in  FIG. 1 . 
           [0034]      FIG. 3  is a flow chart showing a flow for controlling the winding torque to be changed. 
           [0035]      FIG. 4  is an illustration for explaining the winding torque during the seat belt withdrawing operation and the seat belt winding operation of the seat belt retractor according to the flow shown in  FIG. 3 . 
           [0036]      FIG. 5  is a flow chart similar to  FIG. 3  but showing a flow of another embodiment of the seat belt retractor of the present invention. 
           [0037]      FIG. 6  is an illustration for explaining the winding torque during the seat belt withdrawing operation and the seat belt winding operation of the seat belt retractor of the embodiment shown in  FIG. 5 . 
           [0038]      FIG. 7  is an illustration schematically showing an example of a seat belt apparatus with a conventional seat belt retractor. 
           [0039]      FIG. 8  is a vertical sectional view showing an example of a seat belt retractor disclosed in Patent document 1. 
           [0040]      FIG. 9  is an illustration for explaining the winding torque during the seat belt withdrawing operation and the seat belt winding operation of the conventional seat belt retractor. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0041]    Hereinafter, best modes for carrying out the present invention will be described with reference to the attached drawings. 
         [0042]      FIG. 1  is a sectional view schematically showing an embodiment of a seat belt retractor according to the present invention. It should be noted that the same components of the aforementioned seat belt apparatus and the aforementioned seat belt retractor will be marked with the same numerals so as to omit the detail description. 
         [0043]    The seat belt retractor  3  of this embodiment comprises a locking device  13  which is the same as the locking device  13  of the aforementioned seat belt retractor disclosed in Patent document 1 shown in  FIG. 8 . Further, the seat belt retractor  3  of this embodiment comprises a tension reducer which is the same as the tension reducer of the aforementioned seat belt retractor disclosed in Patent document 2. Furthermore, the seat belt retractor  3  of this embodiment is employed in a seat belt apparatus  1  which is the same as the seat belt apparatus  1  shown in  FIG. 7 . 
         [0044]    In  FIG. 1 , a mark “Sm” designates a main spring (corresponding to the first spring of the present invention), as one of springs in the spring device  15 , for always biasing the spool  10  in the winding direction, a mark “Ss” designates a sub spring (corresponding to the second spring of the present invention), also as one of the springs for giving biasing force in the winding direction to the spool  10  intermittently. A numeral  27  designates a tape which is disposed in a clutch gear  28  with a boss  28   a  connected to one end of the sub spring Ss and is wound in a spiral shape to connect the main spring Sm and the sub spring Ss, a numeral  29  is a clutch pawl, a numeral  30  designates a pivot lever which is pivotally supported by a frame  11  and has the clutch pawl  29 , a numeral  31  designates an electromagnetic solenoid for driving the pivot lever to pivot, a numeral  32  is a connecting shaft, a numeral  33  designates a main shaft, a numeral  34  designates a spring housing, a numeral  35  designates a buckle switch for detecting the latching of the tongue  8  with the buckle  9 , and a numeral  36  designates a cover. It should be noted that the seat belt retractor  3  of this embodiment has no torsion bar  7  (shown in  FIG. 8 ) and has the simple main shaft  33 . The main shaft  33  is connected to the spool  10  to rotate together with the spool  10  and is connected to the locking base  22  of the locking device  13  to allow rotation relative to the locking base  22 . 
         [0045]    The main spring Sm and the sub spring Ss are arranged in parallel in a plane perpendicular to the axial direction of the spool  10 . The main spring Sm always biases the spool  10 , on which the seat belt  6  is wound, in the seat belt winding direction via the connecting shaft  32  and the main shaft  33 . The inner end of the main spring Sm is connected to an end portion of the connecting shaft  32  and the outer end of the main spring Sm is connected to a portion of the inner surface of the spring housing  34 . 
         [0046]    On the other hand, the inner end of the sub spring Ss is fixed to the boss  28   a  formed integrally with the clutch gear  28  having ratchet teeth. The clutch pawl  29  is disposed to prevent the clutch gear  28  from return-moving in a direction toward the non-operational position of the clutch gear  28  (leftward in  FIG. 2 ). The prevention of the return-movement of the clutch gear  28  by the clutch pawl  29  can be cancelled by the pivot lever  30  and the electromagnetic solenoid  31 . 
         [0047]    The tape  27  for memory is disposed between the main spring Sm and the sub spring. 
         [0048]      FIGS. 2(   a )- 2 ( c ) are illustrations for explaining the operation of the seat belt retractor, wherein  FIG. 2(   a ) is an illustration showing the non-operational state of the seat belt retractor,  FIG. 2(   b ) is an illustration showing a state where the seat belt is withdrawn from the seat belt retractor for wearing the seat belt, and  FIG. 2(   c ) is an illustration showing the normally wearing state of the seat belt after latching the tongue with the buckle. 
         [0049]    When the seat belt retractor is in the non-operational state as shown in  FIG. 2(   a ), the main spring Sm and the sub spring Ss are compressed to the maximum so that the tape  27  is in the tensioned state. In this state, the seat belt  6  is wound completely. In addition, the clutch pawl  29  is set in its non-operational position where the clutch pawl  29  can engage the clutch gear  28  in the seat belt winding direction. 
         [0050]    As the seat belt  6  is withdrawn from the seat belt retractor  3  (rightward in  FIG. 2(   a )) from the non-operational state of the seat belt retractor  3 , the clutch gear  28  and the tape  27  move rightward together with the seat belt  6  as shown in  FIG. 2(   b ) (rotate in the seat belt withdrawing direction). Then, both the main spring Sm and the sub spring Ss expand, and the clutch pawl  29  engages the clutch gear  28  to prevent the movement in the belt winding direction, thereby preventing the return movement of the clutch gear  28 . 
         [0051]    As the occupant C wears the seat belt  6  by latching the tongue  8  with the buckle  9 , the seat belt  6  excessively withdrawn is slightly wound leftward in  FIG. 2(   b ). Since the return movement of the clutch gear  28  is prevented, only the main spring Sm compresses so that the tape  27  sags as shown in  FIG. 2(   c ). Accordingly, the seat belt  6  is not biased by the sub spring Ss and is biased only by the main spring Sm. Therefore, in the normally wearing state in which an excessively withdrawn part of the seat belt  6  is wound after the tongue is latched with the buckle, the seat belt  6  is pulled lightly only by the main spring Sm as shown in  FIG. 2(   c ) so that the seat belt  6  lightly fits the occupant C. 
         [0052]    In the normally wearing state of the seat belt  6  as shown in  FIG. 2(   c ), as the electromagnetic solenoid  31  is excited by rotating the pivot lever  30  in the clockwise direction so as to release the clutch pawl  29  from the clutch gear  28 , the clutch gear  28  is allowed to move (rotate) leftward in  FIG. 2(   c ) so that the clutch gear  28  moves (rotates) rapidly in a direction of winding (direction of tensioning) the tape  27  by the biasing force of the sub spring Ss. As the tape  27  is tensioned, the seat belt  6  is biased by both the main spring Sm and the sub spring Ss so that the seat belt  6  is strongly wound onto the spool  10 . That is, the tape  27  is structured as a memory device for storing the rotating amount of the spool  10  relative to the clutch gear  28 . 
         [0053]    As the winding of the seat belt  6  onto the spool  10  is completed, the electromagnetic solenoid  31  is not excited, the pivot lever  30  rotates in the counter-clockwise direction so that the clutch pawl  29  becomes the non-operational position shown in  FIG. 2(   a ) where the clutch pawl  29  can engage the clutch gear  28 . In addition, the seat belt  6 , the main spring Sm, the sub spring Ss, and the tape  27  become their non-operational states shown in  FIG. 2(   a ). 
         [0054]    The electromagnetic solenoid  31  is connected to an electronic control unit (ECU)  37  (corresponding to the control device of the present invention) and is thus controlled by the ECU. In this case, the ECU  37  controls the electromagnetic solenoid  31  according to a control flow shown in  FIG. 3 . 
         [0055]    That is, in step S 1 , the electromagnetic solenoid  31  is in the OFF state and the buckle switch  35  is in the ON state because the seat belt is worn as shown in  FIG. 3 . Because of the OFF state of the electromagnetic solenoid  31 , the clutch pawl  29  engages the clutch gear  28  to prevent the movement in the seat belt winding direction. In step S 2 , it is determined whether or not the buckle switch  35  is turned OFF. That is, it is determined whether or not the tongue  8  is released from the buckle  9 . When the buckle switch  35  is in the ON state, the tongue  8  is latched with the buckle  8 . When the buckle witch  35  is in the OFF state, the tongue  8  is released from the buckle  9 . Until it is determined that the buckle switch  35  is turned OFF, the process of the step S 2  is repeated. During this operation, the tongue  8  is latched with the buckle  9  so that the seat belt  6  is worn by the occupant C. 
         [0056]    If it is determined that the buckle switch  35  is turned OFF in step S 2 , it is determined whether or not the time elapsing from the OFF of the buckle switch  35  reaches a preset time. The preset time is set to be smaller than a time elapsing from “buckle release” to “completion of belt winding”. The “buckle release” means that the buckle switch  35  is turned OFF, i.e. the tongue  8  is released from the buckle  9 , and the “completion of belt winding” means that the spool  10  winds up the seat belt  6  completely, i.e. the seat belt winding operation is completed. 
         [0057]    As the tongue  8  is released from the buckle  9  so that the buckle switch  35  is turned OFF, spool  10  starts to wind up the seat belt. At this point, the clutch pawl  29  of the tension reducer engages the clutch gear  28  to prevent the movement in the seat belt winding direction so that the seat belt  6  is wound up by the spool  10  with only the spring force of the main spring Sm. That is, the winding torque of the spool  10  is relatively small. Until it is determined that the time elapsing from the OFF of the buckle switch  35  reaches the preset time, the process of the step S 3  is repeated. In this case, the winding torque of the spool  10  is kept at a small torque by the main spring Sm. 
         [0058]    If it is determined that the time elapsing from the OFF of the buckle switch  35  reaches a preset time, the electromagnetic solenoid  31  is turned ON in step S 4 . Accordingly, the pivot lever  30  pivots to release the clutch pawl  29  from the clutch gear  28 . Then, the seat belt  6  is wound up by the spool  10  with the total of the spring force of the main spring Sm and the spring force of the sub spring Ss. That is, the winding torque of the spool  10  is relatively large because of the total of the spring force of the main spring Sm and the spring force of the sub spring Ss. 
         [0059]    Then, in step S 5 , it is determined whether or not the seat belt winding operation is completed. Until it is determined that the seat belt winding operation is completed, the process of the step S 5  is repeated. At this point, the seat belt winding operation is conducted with the large winding torque by the total of the spring force of the main spring Sm and the spring force of the sub spring Ss. 
         [0060]    If it is determined that the seat belt winding operation is completed in step S 5 , the electromagnetic solenoid  31  is finally turned OFF in step S 6 . 
         [0061]    As shown in  FIG. 4 , in the tension reducer in the seat belt retractor  3  of this embodiment similarly to the conventional tension reducer as mentioned above, the spool is biased in the seat belt winding direction with a relatively large winding torque by both the spring force of the main spring Sm and the spring force of the sub spring Ss during the seat belt withdrawing operation, while a winding torque only by the spring force of the main spring acts on the spool at the completion of the seat belt wearing operation by the occupant C. Accordingly, when the occupant C wears the seat belt, the winding torque on the spool is relatively small so as to lower the tension on the seat belt. Therefore, the seat belt is lightly pulled only by the main spring so as to lightly fit the occupant C. 
         [0062]    As the occupant C disengages the tongue  8  from the buckle  9  and takes his hand off the tongue  8  or the seat belt  6  in order to cancel the wearing of the seat belt, the spool  10  first winds up the seat belt  6  with the relatively small torque only by the spring force of the main spring Sm. As the preset time elapses from the disengagement of the tongue  8  from the buckle  9 , the spool  10  winds up the seat belt  6  completely (fully) with the relatively large torque by the total of the spring force of the main spring Sm and the spring force of the sub spring Ss. In this manner, the winding torque at the start of the winding operation is set to be small torque, and the winding torque after a lapse of the preset time is set to be large torque. Therefore, at the completion of the winding of the seat belt  6 , the seat belt winding speed is low so that the impact generated by the stop of the rotation of the spool  10  is reduced. Therefore, the occurrence of end lock at the completion of the winding of the seat belt  6  is prevented. 
         [0063]    According to the seat belt retractor  3  and the seat belt apparatus  1  of this embodiment, the winding torque is changed such that the winding torque at the start of the seat belt winding operation just after the occupant cancels the wearing of the seat belt is set to be small and the winding torque after a lapse of the preset time is set to be large, whereby the seat belt winding speed can be set to be low at the completion of the winding of the seat belt  6  as compared to the conventional seat belt winding with a constant large winding torque. Therefore, the impact generated by the stop of the rotation of the spool  10  is reduced, thereby effectively preventing the occurrence of end lock at the completion of the winding of the seat belt  6  by a simple structure. 
         [0064]    Further, since the seat belt  6  is wound up with the large winding torque by the total spring force of the main spring Sm and the sub spring Ss and at a relatively high winding speed, the seat belt  6  can be wound up completely (fully). 
         [0065]    In this manner, the end lock can be prevented while the seat belt  6  still can be wound up completely when the occupant cancels the wearing of the seat belt. 
         [0066]    Especially, a low-cost structure for preventing the end lock is achieved by using components of the conventional tension reducer such as the two springs Sm, Ss, the clutch  29 , the electromagnetic solenoid  31 . 
         [0067]    Since the other components and the other works and effects of the seat belt retractor  3  of this embodiment are the same as those of the seat belt retractor disclosed in the aforementioned Patent document 2, the detailed description about those will be omitted. 
         [0068]      FIG. 5  is a flow chart similar to  FIG. 3  but showing another embodiment of the seat belt retractor of the present invention and  FIG. 6  is an illustration for explaining the winding torque during the seat belt withdrawing operation and the seat belt winding operation of the seat belt retractor of this embodiment. 
         [0069]    Though the winding torque at the start of the seat belt winding operation is set to be a small winding torque and the winding torque after that is set to be a large winding torque in the aforementioned embodiment, the winding torque at the start of the seat belt winding operation is set to be a large winding torque and the winding torque after that is set to be a small winding torque in the seat belt retractor  3  of this embodiment. 
         [0070]    That is, as shown in  FIG. 5 , in step S 7  and step S 8 , the same processes in step S 1  and step S 2  of the aforementioned embodiment are conducted. Then, if it is determined that the buckle switch  35  is turned OFF in step S 8 , the electromagnetic solenoid  31  is turned ON in step S 9 . As the tongue  8  is released from the buckle  9  so that the buckle switch  35  is turned OFF, spool  10  starts to wind up the seat belt. In addition, the pivot lever  30  pivots so as to disengage the clutch pawl  28  from the clutch gear  28  so that the seat belt  6  is wound up by the spool  10  with the total of the spring force of the main spring Sm and the spring force of the sub spring Ss. That is, the winding torque of the spool  10  is relatively large because of the total of the spring force of the main spring Sm and the spring force of the sub spring Ss. 
         [0071]    Then, in step S 10 , it is determined whether or not the time elapsing from the OFF of the buckle switch  35  reaches a preset time (which is different from the preset time of the aforementioned embodiment). The preset time is set to be shorter than a time elapsing from “buckle release” to “completion of belt winding”. The “buckle release” means that the buckle switch  35  is turned OFF, i.e. the tongue  8  is released from the buckle  9 , and the “completion of belt winding” means that the spool  10  winds up the seat belt  6  completely, i.e. the seat belt winding operation is completed. 
         [0072]    Until it is determined that the time elapsing from the OFF of the buckle switch  35  reaches the preset time, the process of the step S 10  is repeated. In this case, the winding torque of the spool  10  is kept at a large torque by both the main spring Sm and the sub spring Ss. 
         [0073]    If, in step S 10 , it is determined that the time elapsing from the OFF of the buckle switch  35  reaches a preset time, the electromagnetic solenoid  31  is turned OFF in step S 11 . Accordingly, the pivot lever  30  pivots by the spring force of the electromagnetic solenoid  31  so that the clutch pawl  29  engages the clutch gear  28 . By this engagement, the winding torque of the spool  10  is changed to be relatively small because of only the spring force of the main spring Sm. That is, the seat belt  6  is wound up by the spool  10  with the small winding torque through to completion. Therefore, the seat belt winding speed is low at completion of the seat belt winding operation so that the impact at the stop of the rotation of the spool  10  is small, thereby preventing the occurrence of end lock at the completion of the seat belt winding operation. 
         [0074]    According to the seat belt retractor  3  and the seat belt apparatus  1  of this embodiment, the winding torque is changed such that the winding torque at the start of the seat belt winding operation just after the occupant cancels the wearing of the seat belt is set to be large and the winding torque after a lapse of the preset time is set to be small as shown in  FIG. 6 , whereby the seat belt winding speed can be set to be low at the completion of the winding of the seat belt  6  as compared to the conventional seat belt winding with a constant large winding torque. Therefore, the impact generated by the stop of the rotation of the spool  10  is reduced, thereby effectively preventing the occurrence of end lock at the completion of the winding of the seat belt  6  by a simple structure. 
         [0075]    Further, the seat belt  6  is wound up with the large winding torque by the total spring forces of the main spring Sm and the sub spring Ss and at a relatively high winding speed during the preset time after the start of the seat belt winding operation. Though the winding speed just before the completion of the seat belt winding operation of this case is low as compared to the conventional winding speed by the large constant winding torque because of the small winding torque, the winding speed is kept to be not lower than a certain speed so as to wind up the seat belt  6  completely (fully). 
         [0076]    The other components and the other works and effects of the seat belt retractor  3  of this embodiment are the same as those of the seat belt retractor of the aforementioned embodiment. 
         [0077]    Though the present invention is applied to a seat belt retractor  3  having an ELR function with a tension reducer in any one of the aforementioned embodiments, i.e. the present invention is achieved by using components of the tension reducer, the present invention is not limited thereto and can be applied to a seat belt retractor  3  having an ELR function without a tension reducer. In this case, the seat belt retractor comprises, instead of the main spring Sm and the sub spring Sm, a first spring capable of biasing the spool  10  in the seat belt winding direction and a second spring capable of biasing the spool  10  in the seat belt winding direction with a spring force smaller than that of the first spring and also comprises a spring device in which the first and second springs are arranged in parallel, a clutch for controlling the spring forces of the first and second springs to act or not to act on the spool  10  during the seat belt winding operation after the occupant cancels the wearing of the seat belt, and an electromagnetic solenoid for driving the clutch. 
         [0078]    The seat belt retractor  3  and the seat belt apparatus employing the same of the present invention are suitably used as a seat belt retractor  3  which is provided with a tension reducer and a seat belt apparatus employing the same, wherein the tension reducer reduces the biasing force of a spring device for winding the seat belt while the seat belt is normally worn. 
         [0079]    The disclosure of Japanese Patent Application No. 2006-285982 filed on Oct. 20, 2006 is incorporated as a reference. 
         [0080]    While the invention has been explained with reference to the specific embodiments of the invention, the explanation is illustrative and the invention is limited only by the appended claims.