1) Field of the Invention
This invention relates to a webbing retractor for a seat belt system which serves to protect an occupant of a vehicle in the event of an emergency such as collision of the vehicle, and especially to a webbing retractor suitable for use with a lap webbing.
2) Description of the Related Art
It has conventionally been attempted to control the movements of various parts of an occupant, such as the head and lap, upon collision with a view toward protecting the body of the occupant from an impact of the collision. FIG. 25 illustrates, by way of example, a seat belt system making use of a 2-point electric shoulder webbing and a lap webbing in combination. The seat belt system is suitable as a 3-point seat belt system to which the present invention is applied. A shoulder webbing 201 is taken up from one end thereof by an emergency locking retractor 203 mounted inboard a seat of a vehicle, whereas the other end of the shoulder webbing 201 is fastened to a movable anchor 205. The movable anchor 205 is guided by a guide rail 206 provided along a roof side of a vehicle body, so that the movable anchor 205 is caused to move forward or rearward along the length of the vehicle by an electric drive means (not illustrated) when an associated door of the vehicle is opened or closed.
In addition, one end of a lap webbing 202 is fastened to a tongue 208 which is releasably latched in a buckle 207 also mounted inboard the seat. The lap webbing 202 is taken up from the other end thereof into an emergency locking or automatic locking retractor 209 provided on a side sill. It has generally be known that in a dynamic test of such a seat belt system, the forward tilting of the upper torso of an occupant is delayed, the degree of movement of his head is reduced and better influence is therefore given to the severity of injury as the extendable lap webbing length (the possible increment of the restraining length of the lap webbing) by elongation of the lap webbing and tightening of windings of the lap webbing increases.
As a method for increasing the extendable webbing length of a seat belt system, it has heretofore been practised to make the elongation of the webbing itself greater so as to increase the outward extendability of the webbing or to provide a fuse, namely, to fold back and sew a portion of the webbing so that the stitches may be successively broken off upon application of an impact.
It has also been practised to leave more windings of a lap webbing on a webbing take-up spindle even after the lap webbing has been worn, so that the lap webbing would be allowed to extend longer owing to greater tightening of the windings of the lap webbing upon application of an impact.
It is however by no means possible to obtain any sufficient extended webbing length by simply increasing the elongation of a webbing itself. The provision of a fuse is also accompanied by such drawbacks that the manufacturing cost becomes high due to the need for a longer webbing and the external appearance is poor due to the exposure of the folded fuse to the outside. The method of leaving more windings of a webbing to achieve greater tightening of the windings requires a much longer webbing, leading to a problem that an increased manufacturing cost is unavoidable.