1. Field of the Invention
The present invention relates in general to a seat assembly of a motor vehicle, and more particularly to a seat slide device which is capable of sliding the seat in fore-and-aft direction to a desired locked position for providing at seat occupant with a comfortable sitting posture.
2. Description of the Prior Art
In order to clarify the task of the present invention, one conventional seat slide device will be described with reference to FIGS. 7 to 12 of the accompanying drawings.
Referring to FIGS. 7 to 12, particularly FIG. 10, there is shown a seat assembly 100 which is equipped with a conventional seat slide device 102 for movement in fore-and-aft direction to a desired locked position relative to a vehicle floor 104. The seat assembly 100 comprises a seat cushion 106 mounted on the seat slide device 102, and a seat back 108 pivotally connected to the seat cushion 106 through a known reclining device 110.
The seat slide device 102 comprises two slide units which are parallelly arranged between the seat cushion 106 and the vehicle floor 104. The two slide units are essentially the same in construction and configulation. As is seen from FIGS. 8 and 9, each slide unit comprises a stationary rail 112 which is secured to the vehicle floor 104 to extend in fore-and-aft direction, and a movable rail 114 which is secured to the seat cushion 106 and slidably disposed on the stationary rail 112.
A plurality of slide assisting units 116, as shown in FIG. 7, are arranged between the stationary and movable rails 112 and 114 in order to provide smooth sliding movement of the movable rail 112 relative to the stationary rail 114.
Each slide assisting unit 116 comprises a generally rectangular base structure 118 which is formed with a pair of curled side portions each having at the lower portion thereof two circular cuts 120 for rotatably receiving therein bearing balls 122. The base structure is further formed at its middle portion with a rectangular opening 124 for rotatably receiving therein a roller 126.
As is seen from FIG. 9, upon assembly, the roller 126 in the rectangular opening 124 contacts both a major flat portion of the movable rail 114 and a bottom portion of the stationary rail 112, while, the bearing balls 122 in the circular cuts 120 contact both the inwardmost flat ends 118a (see FIG. 7) of the curled side portions of the base structure 118 and the inwardly bent flat portions 114a (see FIG. 9) of the movable rail 114. Due to rotation of the rollers 126 and the bearing balls 122, the movement of the movable rail 114 and thus the movement of the seat proper relative to the stationary rails 112 are smoothly made. A plurality of projections (not shown) are formed on the bottom portion of the stationary rail 112 at evenly spaced intervals to suppress excessive displacement of each slide assisting unit 116 relative to the stationary rail 112.
Although not shown in the drawings, a known position locker is incorporated with one of the slide units to lock the associated movable rail 114 and thus the seat proper at a desired position relative to the stationary rail 112.
However, the above-mentioned seat slide device 102 has the following drawback due to its inherent construction. This drawback will be described with reference to FIG. 10.
When, by accident, the associated vehicle is struck from behind, the seat occupant H is violently pressed at his or her back against the seat back 108 thereby producing a moment M which tends to turn the seat assembly 100 in a clockwise direction in FIG. 10. Thus, a forward portion of the seat assembly designated by reference P is subjected to an upwardly urging force F. As is understood from FIG. 11, the force F causes generation of a moment of (F.times.L.sub.1) which is created by multiplying the force F by the length L.sub.1 (which is the distance from the forward portion P to the forefront ball bearing 122), so that the forward portion P of the movable rail 114 of each slide unit tends to be bent upward. As is seen from FIG. 12, the moment (F.times.L.sub.1 ) sometimes breaks the proper engagement between the movable and stationary rails 114 and 112, resulting in desengagement of the movable rail 114 from the stationary rail 112. One method of solving this drawback is to use rails having greater thickness. However, this method induces higher cost production of the seat slide device.