Conventionally, a car seat for an automobile is made slidable longitudinally of a body thereof by use of slide rails comprising a pair of upper rails fixed to both sides of the lower surface of the seat and a pair of lower rails fixed to the car body at the positions corresponding to the upper rails respectively as disclosed in the Japanese Utility Model Laid-Open Gazette No. Sho 60-43,435. Also, a slide locking device for locking the car seat slidable through the slide rails at a predetermined position is built in the car seat.
The slide locking device, as shown in the aforesaid gazette, is so constructed that a lock plate having locking teeth is fixed to each lower rail, a lock shaft at the driving side having a handle and made controllable thereby is rotatably supported to one of the upper rails, a lock shaft at the driven side cooperative with rotation of the driving side lock shaft is rotatably supported to the other, and lock levers each having an engaging member engageable or disengageable with or from one of the locking teeth are fixed to the lock shafts respectively.
In addition, the lock shafts associate with each other through a wire so that the driving side lock shaft is rotatably operated and the driven side lock shaft rotates following the rotation of driving side lock shaft, thereby engaging or disengaging the engaging member with or from the locking teeth.
At the slide locking unit, between the driving side and driven side lock shafts and the upper rail side members for supporting the lock shafts are separately interposed torsion coil springs for biasing the engaging members in the direction of engaging with the locking teeth, so that the driving side lock shaft is rotatably operated to deflect the torsion coil spring, thereby disengaging the engaging member at each lock lever from the locking teeth, and the same is released to engage the engaging member with the locking teeth by a restoring force of the torsion coil spring.
In the above-mentioned construction, however, since the torsion coil springs for locking must be mounted to both the lock shafts, the number of parts increases and machining work is required to form a retaining groove at each lock shaft for retaining one end of the torsion coil spring to the lock shaft.
Also, a spring load of deflecting the torsion coil spring when the handle is rotated for lock-releasing is notlinear as shown by the dotted line in FIG. 6, thereby having the defect that the handle is not released in a good feeling. The reason for the above will be described in accordance with FIG. 6.
Usually, the slide locking device sets the locking spring and a range of stroke of the handle. At first, the spring is decided on the basis of a maximum spring load W max. not-hindering fatigue strength and release operation, and of a minimum spring load W min. enough to reliably lock the slide of seat when slidably operated, and nextly a stroke range of the handle corresponding to the maximum and minimum spring loads W max. and W min. is decided.
The torsion coil spring, when subjected to a load, deflects not only at an effective coiled portion but also at both ends, thereby having the mon-linear characteristic that the relationship between the displacement and the spring load downwardly curvs as shown by the dotted line in FIG. 6. As a result, an operating stroke S.sub.1 between the maximum spring load W max. and the minimum spring load W min. is smaller than that S.sub.2 for the linear characteristic, so that the smaller stroke must correspond to variation in a large spring load, thereby deteriorating a feeling of release operation.