JP2000238611A - Tension reducer in seat belt retractor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To simplify and compactify structure, and to facilitate moving of an occupant in a condition fastened with a seat belt. SOLUTION: A reducer shaft 8 is moved vertically by a switch lever 11 when the lever 11 is set in an upper limit position, an internal tooth 8a thereof is meshed with an external tooth 7a of a bush shaft 7 connected to a reel, an external tooth 8b thereof is separated from an internal tooth 11b of the lever 11 to be freely rotatable, and spring force of the second spring connected to the reducer shaft 8 is applied to the reel. When the lever 11 is set in the upper position, the internal tooth 11b of the lever 11 is meshed with the external tooth 8b of the shaft 8 to make rotation of the reducer shaft 8 incapable, the shaft 8 is moved downward by the lever 11 to separate the internal tooth 8a from the external tooth 7a of the bush shaft 7, and the spring force of the second spring is not applied to the reel.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention belongs to the technical field of a seatbelt retractor mounted on a seat of an automobile or other transportation, and in particular, reduces the feeling of pressure felt by an occupant due to the seatbelt when the seatbelt is worn. for,
The present invention belongs to the technical field of a tension reducer in a seat belt retractor for reducing a belt tension.

[0002]

2. Description of the Related Art Seat belt devices are mounted on seats of various transportation means such as automobiles in order to protect occupants in the event of an emergency such as a collision. When not wearing the seat belt completely retracted and stored, and in an emergency with the seat belt fastened, prevent the seat belt from being pulled out,
A seat belt retractor is provided to prevent the occupant from moving forward due to inertia.

In such a seat belt retractor,
The reel for winding the seat belt is constantly urged in the belt winding direction by the spring force of the spring. In this case, it is necessary to set the spring force of the spring large in order to wind up the seat belt reliably and quickly when winding up the seat belt. However, if the spring force of the spring is set to be large, the belt tension becomes large even when the seat belt is worn, which may give an occupant an uncomfortable feeling of pressure. Thus, the seat belt retractor is required to satisfy the contradictory conditions of increasing the spring force of the spring when the seat belt is wound, and decreasing the spring force of the spring when the seat belt is worn. Can be

Therefore, a conventional seatbelt retractor is often provided with a tension reducer for reducing the belt tension in order to reduce the feeling of pressure felt by an occupant when the seatbelt is worn. As an example of such a conventional tension reducer, there is a tension reducer disclosed in Japanese Patent Publication No. Hei 7-8639.

In the tension reducer disclosed in this publication, the spring force of the main spring is always applied to the main shaft via the connecting shaft, and the spring force of the additional spring is applied to the main shaft when the belt is not mounted. By increasing the belt take-up force of the main shaft, connecting the clutch device when the belt is mounted, and by not applying the spring force of the additional spring to the main shaft, the belt take-up force of the main shaft is reduced. I have. In this case, the clutch claws of the clutch device are not directly engaged with the stoppers when the clutch is connected, so that no squealing sound is generated when the phase difference of the spring is released when the clutch is released.

According to this tension reducer, a large belt winding force can be obtained by the spring force of the main spring and the spring force of the additional spring except when the belt is mounted.
Also, when the occupant is stationary with the belt attached, a small belt winding force with only the spring force of the main spring is obtained,
The above-mentioned conflicting conditions required for the seat belt retractor can be almost satisfied.

[0007]

By the way, in the above-mentioned tension reducer, when the clutch is connected, the clutch claw of the clutch device is not directly engaged with the stopper. A phase difference occurs. This phase difference increases each time the seat belt is pulled out and taken up repeatedly, and it is difficult to control the tension reducer accurately. Therefore, a memory mechanism is required that keeps the phase difference between the winding amounts of the two springs, which increases when the seat belt is repeatedly pulled out and taken up repeatedly, to eliminate the increase in the phase difference. This memory mechanism is constituted by a tape memory unit having a drawer adjusting tape of a fixed length wound spirally.

[0008] However, such a memory mechanism is not only large in size but also requires a long memory tape, and the mechanism is complicated. Also,
In the above-described tension reducer, when the electromagnetic solenoid is operated, that is, when the occupant moves while the seat belt is worn and the seat belt is pulled out to some extent, both the main spring and the additional spring are applied to the main shaft. As a result, the fastening force of the seat belt to the occupant is increased, and the occupant becomes difficult to move.
Thereafter, the belt is wound up, at which time a phase difference occurs between the two springs. Due to this phase difference, the winding force of the belt is reduced. That is, once the belt is pulled out with a heavy winding force, the belt is pulled out with a light winding force at the time of the next belt pulling out.
As described above, in the above-described tension reducer, when the electromagnetic solenoid is operated, each spring force of the main spring and the additional spring is applied to the main shaft in relation to the amount of rotation, so that the occupant feels a feeling of pressure when moving. Become.

The present invention has been made in view of such circumstances, and has as its object to satisfy the above-mentioned conflicting conditions required for a seatbelt retractor and to eliminate an impact sound while simplifying the mechanism. It is an object of the present invention to provide a tension reducer in a seat belt retractor which can be made compact and compact. Another object of the present invention is to provide a tension reducer in a seatbelt retractor that can make the occupant less likely to feel a great deal of oppression when the occupant moves while wearing the seatbelt.

[0010]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the invention of claim 1 includes at least a reel for winding a seat belt and a spring means for applying a spring force to the reel in a belt winding direction. In the seat belt retractor provided, the spring means includes a first spring that constantly applies a spring force to the reel in a belt winding direction, and a second spring that selectively applies a spring force to the reel in a belt winding direction. Switching means for selectively setting a state in which the spring force of the second spring is applied to the reel and a state in which the spring force of the second spring is not applied to the reel regardless of the rotation amount of the reel. It is characterized by.

According to a second aspect of the present invention, the switching means is provided so as to rotate integrally with the reel, and the first shaft having external teeth on the outer periphery and the spring force of the second spring are constantly maintained. A cylindrical second shaft having an inner tooth that can be meshed with the outer tooth of the first shaft on the inner periphery while being added in the belt winding direction, and moving the second shaft to move the outer periphery of the first shaft And a switching lever for controlling engagement and disengagement between the teeth and the internal teeth of the second shaft.

Further, according to a third aspect of the present invention, the second shaft has external teeth on the outer periphery, and the switching lever has teeth that can mesh with the external teeth of the second shaft, and the switching lever is provided with the switching lever. When the engagement between the external teeth of the first shaft and the internal teeth of the second shaft is released, the teeth of the switching lever mesh with the external teeth of the second shaft to prevent the rotation of the second shaft. It is characterized by becoming.
Further, the invention according to claim 4 is characterized in that the switching means is actuated by electromagnetic force when the tongue and the buckle are engaged, and
The mesh between the external teeth of the shaft and the internal teeth of the second shaft is released.

Further, in the invention according to claim 5, the second spring is a spiral spring, an inner end of the spiral spring is connected to the second shaft, and an outer end of the spiral spring is connected to a spring case. The spiral spring is removably connected, and the spiral spring is wound and tightened when the reel rotates in the belt withdrawing direction. Further, an outer end of the spiral spring is wound around the spiral spring. When the tightening amount exceeds a certain amount, the connection with the spring case is released, and after a predetermined amount of rotation, the connection is releasably connected to the spring case again.

Further, according to the present invention, a predetermined number of grooves are formed in the spring case in the rotation direction of the outer end of the spiral spring, and the outer end of the spiral spring is formed in the groove of the spring case. It is characterized in that it is adapted to be detachably connected to the vehicle.

[0015]

In the tension reducer according to the present invention constructed as described above, when the switching means is not operated, that is, when the belt is not mounted, the spring force of the first spring and the spring force of the second spring increase the belt winding force. Is obtained,
When the switching means is activated, that is, when the belt is attached, the first
As a result, a small belt winding force of only the spring force of the spring can be obtained, and the above-mentioned contradictory conditions required for the seat belt retractor can be reliably satisfied.

Further, in the tension reducer of the present invention, since a memory mechanism comprising a drawer adjusting tape wound in a spiral shape as in the conventional case described above is not required, the number of parts is reduced, and the tension reducer can be reduced in size. In addition to being compact, the mechanism is simpler and operation is more reliable. Furthermore, when the switching means is activated, that is, when the belt is worn, regardless of the operation amount after the belt is worn,
Since a small belt winding force only by the spring force of the first spring is obtained at all times, the occupant can easily move without having a feeling of pressure when moving.

[0017]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a sectional view partially showing an example of an embodiment of a tension reducer according to the present invention.

As shown in FIG. 1, the seat belt retractor 1 of this embodiment has two side walls (one side) of a U-shaped frame 2 in the same manner as a conventional seat belt retractor disclosed in the above-mentioned publication. A reel 3 rotatably supported by the side wall 2a (shown) and winding the seat belt; and a spring means 4 attached to one of the side walls 2a and applying a spring force to the reel 3 to generate a winding force. ing.
Although not shown, on the other side wall of the frame, there is provided a conventionally known locking means which operates in an emergency such as a vehicle collision or the like to prevent rotation of the reel 3 in the belt withdrawing direction, and a large locking means such as a vehicle collision. Conventionally known deceleration sensing means for operating the locking means when deceleration occurs is provided.

The spring means 4 includes a cover case 5 attached to one side wall 2a, a spring case 6 fixed to the cover case 5, and a spring case 6
A cylindrical reducer shaft 8 rotatably and vertically movable and selectively engageable with and disengageable from a bush shaft 7 connected to the reel 3; an outer end connected to the cover case 5; A first spiral spring 9 which is connected to the bush shaft 7 and constantly applies a spring force to the reel 3 in the belt winding direction. The first spiral spring 9 corresponds to the main spring of the above-mentioned publication, and the outer end can be disengaged from the spring case 6. A second spiral spring 10 which is connected to the reducer shaft 8 and whose inner end is connected to the reducer shaft 8 to constantly apply a spring force to the reducer shaft 8 in the belt winding direction. A switching lever 11 provided to be vertically movable between a lower limit position and selectively controlling engagement and disengagement of the bush shaft 7 and the reducer shaft 8; And an electromagnetic solenoid 12 which controls the operation of the switching lever 11.

As shown in FIGS. 2A and 2B, the bush shaft 7 is formed with external teeth 7a, and the cylindrical reducer shaft 8 is engaged with the external teeth 7a of the bush shaft 7. Possible internal teeth 8a and external teeth 8b are respectively formed. The switching lever 11 has an elliptical hole 11a, and a reducer shaft 8 is provided in the elliptical hole 11a so as to be rotatable and slidable in the vertical direction. The hole 11 of this switching lever 11
At the upper end of a, a semicircular arc-shaped internal tooth 11b that can mesh with the external tooth 8b of the reducer shaft 8 is formed.

When the reducer shaft 8 is moved upward by the switching lever 11 as shown in FIG. 2A, the internal teeth 1 of the switching lever 11 of the switching lever 11 are moved.
1b is detached from the external teeth 8b of the reducer shaft 8,
The reducer shaft 8 becomes rotatable, and the internal teeth 8 a of the reducer shaft 8 mesh with the external teeth 7 a of the bush shaft 7. At this time, since the spring force of the second spiral spring 10 is applied to the reel 3, both spring forces of the first and second spiral springs 9 and 10 are applied to the reel 3.

On the other hand, when the reducer shaft 8 is moved downward by the switching lever 11 as shown in FIG. 2B, the internal teeth 11b of the switching lever 11 mesh with the external teeth 8b of the reducer shaft 8, and Shaft 8
Becomes unrotatable, and the internal teeth 8a of the reducer shaft 8 separate from the external teeth 7a of the bush shaft 7,
The reducer shaft 8 cannot rotate, and only the bush shaft 7 can rotate. At this time, the spring force of the second spiral spring 10 is not applied to the reel 3, and only the spring force of the first spiral spring 9 is applied to the reel 3. At this time, before the internal teeth 8a of the reducer shaft 8 separate from the external teeth 7a of the bush shaft 7, the internal teeth 11b
Meshes with the external teeth 8b of the reducer shaft 8, and the reducer shaft 8 cannot rotate. Therefore, even if the connection between the reducer shaft 8 and the bush shaft 7 is released,
The winding of the second spiral spring 10 hardly loosens, and the spring force thereof does not substantially change.

The switching lever 11 is moved up and down by an electromagnetic solenoid 12. Electromagnetic solenoid 1
2 is not excited, the plunger 12a of the electromagnetic solenoid 12 projects by the spring force of a spring (not shown), and the switching lever 11 is set to the upper limit position shown in FIG. Also, when the electromagnetic solenoid 12 is excited, the plunger 12a of the electromagnetic solenoid 12 is retracted by the electromagnetic force of the solenoid, and the switching lever 11 is turned on as shown in FIG.
Is set to the lower limit position shown in FIG. The electromagnetic solenoid 12
As in the prior art, when a not-shown tongue is engaged with a not-shown buckle, a buckle switch (not shown) provided in the buckle is turned on to be excited.

As shown in FIG. 3, the second spiral spring 10 is housed in the annular portion 6a of the spring case 6. A predetermined number (eight in the illustrated example) of notch grooves 6b are formed on the inner wall of the annular portion 6a at equal intervals in the circumferential direction. These notches groove 6b is composed of two side walls, with the side wall 6b ₁ of the side facing the belt withdrawing direction α is an inclined surface that is curved, the belt winding direction β
Side walls 6b ₂ opposite sides there is a perpendicular vertical plane on the inner wall surface of the annular portion 6a to.

An outer end 10a of the second spiral spring 10 is fitted and connected to these notched grooves 6b so as to be able to be engaged and disengaged. When the reducer shaft 8 rotates in the belt withdrawing direction α, the second spiral spring 1
However, when the second spiral spring 10 is further tightened, the outer end 10a is pulled inward, so that the engagement between the outer end 10a and the groove 6b becomes smaller, and the outer end 10a is eventually formed. The end 10a comes off the groove 6b. When the outer end 10a separates from the groove 6b, the second spiral spring 10 is slightly extended back, and the outer end 10a rotates in the belt withdrawing direction α while sliding on the inner wall of the annular portion 6a. When the outer end 10a comes to the next groove 6b,
The outer end 6b is fitted into the groove 6b by the elastic force of the second spiral spring 10, and the annular portion 6a of the spring case 6 is again
Linked to Further, when the reducer shaft 8 rotates in the belt withdrawing direction α, the second spiral spring 10 is again tightened, and the outer end 10a comes off the groove 6b again. As described above, when the second spiral spring 10 is wound by a certain amount, the outer end 10a is detached from the groove 6b and repeatedly engaged with the next groove 6b, so that the second spiral spring 10 is not wound beyond a certain amount. It has become.

The operation of the tension reducer 4 of the thus constructed seat belt retractor 1 of this embodiment will be described. (1) State in which the belt is stored In the state in which the belt is not mounted, the electromagnetic solenoid 12 is not actuated by non-excitation, and as shown in FIG.
1 and the switching lever 11 and the reducer shaft 8 are set at the upper limit position as shown in FIGS. 1 and 2A. In this state, the internal teeth 8a of the reducer shaft 8 mesh with the external teeth 7a of the bush shaft 7, and the internal teeth 11b of the switching lever 11 are separated from the external teeth 8b of the reducer shaft 8. As a result, the reducer shaft 8 becomes rotatable, the reducer shaft 8 is connected to the bush shaft 7, and the spring force of the second spiral spring 10 is applied to the reel 3 via the bush shaft 7. Therefore, the spring force of both the first and second spiral springs 9, 10 is applied to the reel 3, and a large belt winding force is generated.

(2) At the time of pulling out the belt for wearing the seat belt After the occupant sits on the vehicle seat in the normal posture while the belt is stowed, when the occupant pulls out the seat belt to wear the seat belt, the reels 3 Since the first spiral spring 9 rotates in the drawing direction α, the first spiral spring 9 is tightened. At the same time, the rotation of the reel 3 is transmitted via the bush shaft 7 to the reducer shaft 8 in which the reducer shaft 8 is rotatable, so that the reducer shaft 8 also rotates in the same belt withdrawing direction α, and the second spiral The spring 10 is also tightened. At this time, when the second spiral spring 10 is wound beyond a certain amount, its outer end 10a is separated from the groove 6b, and this outer end 10a also rotates in the same direction α and fits into the next groove 6b. Therefore, the second spiral spring 10 is not wound beyond a certain amount.

(3) When the tongue and the buckle are engaged After the seat belt is pulled out by a predetermined amount, when the tongue is engaged with the buckle for the occupant to wear the seat belt, the buckle switch is turned on, and the electromagnetic solenoid is turned on. 12 is excited. Then, the plunger 12 of the electromagnetic solenoid 12
a is retracted downward in FIG. 1 by the electromagnetic force of the solenoid, and the switching lever 11 moves downward. Therefore, the internal teeth 11b of the switching lever 11 mesh with the external teeth 11b of the reducer shaft 8 and the reducer shaft 8
Is moved downward, the inner teeth 8a of the reducer shaft 8 are disengaged from the outer teeth 7a of the bush shaft 7, and both the switching lever 11 and the reducer shaft 8
It is set to the lower limit position shown in FIG. As a result, the reducer shaft 8 is separated from the bush shaft 7, so that the spring force of the second spiral spring 10 is not applied to the reel 3. Accordingly, only the spring force of the first spiral spring 9 is applied to the reel 3, and the belt winding force of the reel 3 is reduced.

Further, since the rotation of the reducer shaft 8 is prevented by the engagement of the internal teeth 11b of the switching lever 11 with the external teeth 8b of the reducer shaft 8, the winding amount of the second spiral spring 10 is reduced. The second spiral spring 10 is maintained in the state at the time of rotation inhibition, and does not loosen. In particular, in the tension reducer of this example, after the internal teeth 11b of the switching lever 11 mesh with the external teeth 8b of the reducer shaft 8, the internal teeth 8a of the reducer shaft 8 come off from the external teeth 7a of the bush shaft 7. Therefore, loosening of the second spiral spring 10 is more reliably prevented.

(4) When the occupant releases his / her hand from the seat belt after the tongue and the buckle are engaged and the occupant releases their hand from the seat belt after the tongue and the buckle are engaged. Since the seat belt is rotated in the belt winding direction β by the spring force of the spring 9, the extra belt pulled out is wound up by the reel 3 until the seat belt fits the occupant in the normal sitting posture. At this time, since the belt winding force of the reel 3 is reduced only by the spring force of the first spiral spring 9, when the seat belt is fitted to the occupant, the fastening force of the occupant by the seat belt is relatively weak. . That is, the feeling of oppression of the occupant is greatly reduced.

(5) Pulling out the seat belt due to the movement of the occupant with the seat belt fastened When the occupant moves, for example, to take an object, with the seat belt fastened, the seat belt is pulled out by the moving occupant's body. It is. At this time, the reel 3 rotates in the belt withdrawing direction α, but since only the first spiral spring 9 is wound and tightened, the fastening force of the seat belt is small and the occupant can easily move.

(6) When the occupant returns to the normal sitting position after the occupant moves with the seat belt fastened. When the occupant returns to the normal sitting position after the occupant moves with the seat belt fastened, the occupant is pulled out by the movement of the occupant. After the seat belt is released from the seat belt after the engagement of the tongue and the buckle with the aforementioned (4),
The first spiral spring 9 winds up the reel 3 with a small spring force only until it fits the occupant in the normal sitting posture.

(7) When the tongue is disengaged from the buckle for getting off When the occupant removes the tongue from the buckle for getting off, the buckle switch turns off and the electromagnetic solenoid 12 is demagnetized. . Then, the plunger 12 of the electromagnetic solenoid 12
a protrudes upward in FIG. 1 by the spring force of the spring of the electromagnetic solenoid 12, and the switching lever 11 moves upward. For this reason, the internal teeth 11b of the switching lever 11 are disengaged from the external teeth 11b of the reducer shaft 8, and the reducer shaft 8 is moved upward, so that the internal teeth 8a of the reducer shaft 8
a, the switching lever 11 and the reducer shaft 8 are both set again to the upper limit position shown in FIG. As a result, the reducer shaft 8 is connected to the bush shaft 7 again, so that the spring force of the second spiral spring 10 is applied to the reel 3 again. Therefore, the respective spring forces of the first and second spiral springs 9 and 10 are applied to the reel 3, and the belt winding force of the reel 3 increases.

(8) After releasing the engagement between the tongue and the buckle,
When the occupant releases his / her hand from the seat belt after the tongue and the buckle are disengaged from each other when the hand is released from the seat belt, the reel 3 winds the belt with both spring forces of the first and second spiral springs 9 and 10. Since the seat belt rotates in the take-up direction β, the pulled-out seat belt is taken up until it is completely stored. At this time, the belt winding force of the reel 3 is increased by the first and second spiral springs 9,
Since the seat belt is increased by both spring forces of 10, the seat belt can be quickly and reliably wound up. In this manner, the state (1) when the belt is stored is obtained.

As described above, according to the tension reducer of this embodiment, the spring force of the first spiral spring 9 and the spring force of the second spiral spring 10 are larger when the electromagnetic solenoid 12 is not excited, that is, when the belt is not mounted. A belt winding force is obtained, and the electromagnetic solenoid 1
At the time of excitation of 2, that is, in the belt mounted state, a small belt winding force of only the spring force of the first spiral spring 9 can be obtained, and the above-mentioned conflicting conditions required for the seat belt retractor 1 can be reliably satisfied. Become like

Also, since a memory mechanism including a spirally-adjusted drawer adjusting tape as in the conventional case is not required, the number of parts can be reduced, and the tension reducer can be formed small and compact, and the mechanism can be reduced. Simple and more reliable operation.

In the seat belt 1 of this embodiment, similarly to a conventionally known seat belt, when a large deceleration occurs at the time of a vehicle collision or the like, the deceleration sensing means and the lock means are operated to move the reel in the belt withdrawing direction. Obviously, by preventing the seat belt from being pulled out by preventing the rotation, the restraint of the occupant can be surely protected.

[0038]

As is apparent from the above description, according to the tension reducer of the present invention, a large belt winding force can be obtained by the spring force of the first spring and the spring force of the second spring except when the belt is mounted. Further, in the belt mounted state, a small belt winding force of only the spring force of the first spring is obtained, so that the above-mentioned conflicting conditions required for the seat belt retractor can be reliably satisfied. Become.

Further, according to the tension reducer of the present invention, a memory mechanism comprising a drawer adjusting tape wound in a spiral as in the conventional case described above is not required, so that the number of parts can be reduced and the tension reducer can be reduced in size and size. In addition to being compact, the mechanism can be simplified and the operation can be made more reliable.

Furthermore, when the belt is mounted, a small belt winding force of only the spring force of the first spring is always obtained irrespective of the amount of rotation of the reel. Can be made easier.

[Brief description of the drawings]

FIG. 1 is a sectional view partially showing an example of an embodiment of a tension reducer according to the present invention.

2A and 2B illustrate the operation of the tension reducer of the example shown in FIG. 1, and FIG. 2A illustrates a state in which both spring forces of a first and a second spring are set to be applied to a reel; () Is a diagram showing a state in which only the spring force of the first spring is set to be applied to the reel.

FIG. 3 is a view showing a connection mechanism between an outer end of a second spiral spring and a spring case in the tension reducer of the example shown in FIG. 1;

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Seat belt retractor, 2 ... Base, 2a ... Side wall, 3 ... Reel, 4 ... Spring means, 5 ... Cover case, 6 ... Spring case, 7 ... Bush shaft, 8
... Reducer shaft, 9 ... First spiral spring, 10 ... Second spiral spring, 11 ... Switching lever, 12 ... Electromagnetic solenoid, 12a ... Plunger

Claims (6)

[Claims] 1. A seat belt retractor comprising at least a reel for winding a seat belt and a spring means for applying a spring force to the reel in a belt winding direction, wherein the spring means constantly winds the belt on the reel. A first spring that applies a spring force in the direction of the belt, a second spring that selectively applies a spring force to the reel in the belt winding direction, and a state in which the spring force of the second spring is applied to the reel. Switching means for selectively setting a state in which a spring force is not applied to the reel irrespective of a rotation amount of the reel, wherein a tension reducer in the seat belt retractor is provided. 2. The switching means is provided so as to rotate integrally with the reel, and a first shaft having outer teeth on an outer periphery and a spring force of the second spring are constantly applied in a belt winding direction. A cylindrical second shaft having inner teeth on its inner periphery capable of meshing with the outer teeth of the first shaft; and moving the second shaft to move the outer teeth of the first shaft and the second shaft. 2. A tension reducer for a seat belt retractor according to claim 1, further comprising a switching lever for controlling engagement and disengagement with the internal teeth. 3. The second shaft has external teeth on its outer periphery, and the switching lever has teeth that can mesh with the external teeth of the second shaft, and the switching lever is in contact with the external teeth of the first shaft. When the meshing with the internal teeth of the second shaft is released, the teeth of the switching lever mesh with the external teeth of the second shaft to prevent the rotation of the second shaft. The tension reducer in the seat belt retractor according to claim 2. 4. The switching means is actuated by electromagnetic force when the tongue and the buckle are engaged to release the mesh between the external teeth of the first shaft and the internal teeth of the second shaft. The tension reducer for a seat belt retractor according to any one of claims 1 to 3, wherein: 5. The second spring is a spiral spring. An inner end of the spiral spring is connected to the second shaft, and an outer end of the spiral spring is removably connected to a spring case. In addition, the spiral spring is adapted to be wound when the reel rotates in the belt withdrawing direction, and further, the outer end of the spiral spring, when the amount of tightening of the spiral spring exceeds a certain amount. 5. The device according to claim 1, wherein the connection with the spring case is released, and after being further rotated by a predetermined amount, the connection is releasably connected to the spring case. Tension reducer for seat belt retractors. 6. The spring case has a predetermined number of grooves formed in a rotation direction of an outer end of the spiral spring, and an outer end of the spiral spring is detachably connected to the groove of the spring case. The tension reducer in a seat belt retractor according to claim 5, wherein