Abstract:
The invention has been contrived to solve the above-mentioned problems of the prior grommets and to provide a water-proof grommet, in which insertion load can be reduced further than those of the prior grommets and an entire grommet is not moved axially irrespective of plate thickness of a member being mounted.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The invention relates to an improvement in water-proof grommets of synthetic resin used in mounting small parts or the like to a member being mounted, such as body steel-sheet of automobiles.  
           [0002]    Water-proof grommets of synthetic resin having a body in the form of a bottomed cylinder for prevention of entrance of rain water or the like are disclosed in Japanese Utility Model Publication No. 22659/1987 filed by the applicant of the present application. FIG. 8 is a cross sectional view showing a water-proof grommet described in the above publication, which grommet is provided with short, upward latch pawls  41  on both sides of an upper portion of a grommet body  40 , and a neck  43  below a head  42 . A falling-off preventing effect is obtained by inserting the grommet body  40  into a mount hole  31 , which is formed in a member being mounted  30 , such as body steel-sheet, while flexing the latch pawls  41  inward, and then threading a screw, for example, a tapping screw, into a lumen of the grommet body  40  to expand the grommet body  40  outward to bring the neck  43  into close contact with an inner edge of the mount hole  31  of the member being mounted  30 .  
           [0003]    With such grommet, however, when the grommet body  40  is intensely pulled upward, tips of the latch pawls  41  abut against a back surface of the member being mounted  30  to exhibit the falling-off preventing effect of the entire grommet body  40  but the tips of the latch pawls  41  are in a free state (engagement is not present between the tips and an underside of the member being mounted) when the grommet body  40  is inserted into the mount hole  31  or when a screw is threaded into the lumen of the grommet body  40 . Accordingly, the entire grommet is made axially movable depending upon a thickness of the member being mounted  30 . Also, there is involved a problem that since the latch pawls  41  are short, a force (hereinafter referred to as insertion load) required for insertion of the grommet body  40  into the mount hole  31  is great and so the insertion is not easy.  
         SUMMARY OF THE INVENTION  
         [0004]    The invention has been contrived to solve the above-mentioned problems of the prior grommets and to provide a water-proof grommet, in which insertion load can be reduced further than those of the prior grommets and an entire grommet is not moved axially irrespective of plate thickness of a member being mounted.  
           [0005]    That is, a water-proof grommet according to the invention comprises a head provided with an insertion hole, a grommet body provided below the head to be capable of expanding outward as a screw, such as a tapping screw, adapted to be threaded into a central hole contiguous to the insertion hole advances, at least two latch pawls, which are connected at lower ends thereof to an outside surface of a substantially central portion of the grommet body in a longitudinal direction and at least portions of inside surfaces of tip ends of which can be pushed by the outside surface of the grommet body when the grommet body expands outward, and inclined surfaces formed on upper surfaces of tip ends of the latch pawls such that the upper surfaces lower outward than inward, at least portions of the inclined surfaces pushing a member being mounted in an upward direction and an upper surface of the member being mounted being brought into close contact with a back surface of the head when the latch pawls are pushed outward due to expansion of the grommet body.  
           [0006]    In addition, the inclined surfaces can assume various configurations, for example, generally straight, stepped, steps formed at lower portions thereof, and generally curved.  
           [0007]    With the water-proof grommet according to the invention, since upper surfaces of tip ends of the latch pawls are made inclined surfaces formed such that the upper surfaces thereof lower outward than inward, portions of the inclined surfaces always contact with and push a member being mounted whenever a screw is threaded into a lumen of the grommet body, even if the member being mounted is different in thickness. Therefore, irrespective of thickness of the member being mounted, there is no possibility that the entire grommet can move axially. Also, since lower ends of the latch pawls are connected to a substantially central portion of the grommet body, adequate elasticity is ensured for the latch pawls with the result that load, at which the grommet body is inserted into the member being mounted, can be reduced further than those with the prior grommets. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 is a front view showing an embodiment of a water-proof grommet according to the invention.  
         [0009]    [0009]FIG. 2 is a bottom view showing the water-proof grommet of FIG. 1.  
         [0010]    [0010]FIG. 3 is a cross sectional view taken along the line B-B in FIG. 1.  
         [0011]    [0011]FIG. 4 is a cross sectional view taken along the line A-A in FIG. 3.  
         [0012]    [0012]FIG. 5 is a cross sectional view showing a state, in which the water-proof grommet of FIG. 1 has been completely mounted in the case where a member being mounted is thin.  
         [0013]    [0013]FIG. 6 is a cross sectional view showing a state, in which the water-proof grommet of FIG. 1 has been completely mounted in the case where a member being mounted is thick.  
         [0014]    [0014]FIG. 7 is an enlarged view showing various embodiments of inclined surfaces.  
         [0015]    [0015]FIG. 8 is a cross sectional view showing a prior grommet. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]    The invention will be described below in details with reference to the accompanying drawings, in which an embodiment is shown.  
         [0017]    In these drawings, the reference numeral  1  designates a plate-shaped head, and  2  a grommet body arranged under the head  1  to be square in external form, these elements being generally formed and manufactured with a hard synthetic resin, such as polyamide resin, polypropylene, polyacetal or the like, of a proper elasticity. Formed centrally of the head  1  is an insertion hole  3  (its diameter being smaller by  50-60 % than an outer diameter of a screw) permitting a screw  10 , such as a tapping screw (for example, made of carbon steel) to be inserted therethrough under a proper load, and also formed centrally of the grommet body  2  is a central hole  4  (not extending through the grommet body  2 ) contiguously communicated to the insertion hole  3  to have the same diameter as that of the insertion hole. As shown in FIGS. 5 and 6 (oblique lines indicative of cross section being not drawn on the screw  10  for avoiding difficulty in seeing), reaction forces produced when the screw  10  such as a tapping screw is threaded into the central hole  4  from the insertion hole  3  causes the grommet body  2  to expand radially while being pulled upward toward the head  1  (the reference numeral  11  designates a washer).  
         [0018]    Lower ends of latch pawls  5  are connected to substantially central, outer sides of the grommet body  2 . In addition, while the latch pawls  5  in the embodiment shown are two in number, they may correspondingly be provided on all the sides, that is, four in number in this case. Also, an external shape of the grommet body shown is square but may be polygonal including a pentagonal shape. However, as the external shape resembles a circle, there is increased the possibility that the grommet rotates on its axis. So, there is caused the need of an arrangement for prevention of rotation, in which concave and convex portions engageable with each other are provided on upper surfaces of tip ends of the latch pawls  5  and on a member being mounted to be disposed in corresponding positions, respectively. It suffices to determine an appropriate one of polygonal shapes in view of manufacturing cost. The latch pawls  5  extend toward the back surface of the head  1  from substantially central, outer sides of the grommet body  2 , and upper halves of the latch pawls  5  expand outward as shown in the figures. Also, upper surfaces of tip ends of the latch pawls  5  are made inclined surfaces  6 , which lower outward (the inclination is not specifically limitative but may be sufficient to enable permitting at least portions of the inclined surfaces to contact with and press against the member  30  being mounted, upward. In addition, contact positions between the both are determined in accordance with a thickness of the member being mounted  30 ). In addition, inner sides and sides of the latch pawls  5  form a continuous cavity  8  in order to provide the latch pawls  5  with elasticity, which is required when the grommet is inserted through the member being mounted  30 . Besides, in the embodiment shown, a flat-plate shaped seal rubber  9  is provided on an underside of the head  1  to provide the grommet with the water-proof function.  
         [0019]    Also, in the embodiment shown, the inclined surfaces  6  includes small steps  7  (see a type C in FIG. 7) on lower portions thereof, but the inclined surfaces  6  may be generally straight (a type A in FIG. 7), generally stepped (a type B in FIG. 7), and generally curved (a type D in FIG. 7).  
         [0020]    With the water-proof grommet of the invention thus formed, the grommet body  2  is expanded radially outward by inserting the grommet into the mount hole  31 , which is formed in the member being mounted  30 , until upper ends of the latch pawls  5  reach the back surface of the member being mounted  30 , and inserting the screw  10 , such as a tapping screw, which is inserted through a mount hole  21  of a mounting member  20  and on an underside of a head of which a washer  11  is mounted, into the central hole  4  from the insertion hole  3 . As a result, being pushed by the grommet body  2  having been expanded as shown in FIGS. 5 and 6, the latch pawls  5  is caused to deform outward to rigidly engage the grommet body  2  with the member being mounted  30 , and simultaneously the mounting member  20  is mounted to the member being mounted  30 , with an upper surface of the head  1  of the grommet body  2  therebetween. In addition, since lower ends of the latch pawls  5  are connected to the grommet body  2  at a substantially central portion of an outer surface of the grommet body  2  and the latch pawls  5  are large in length (length from the lower ends to the tip ends) as compared with the prior grommets, elasticity is adequately ensured with the result that it is possible to reduce the insertion load more than those of the prior grommets.  
         [0021]    In this manner, in a state, in which the water-proof grommet of the invention is inserted into the mount hole  31  of the member being mounted  30  and the screw is threaded into the central hole  4 , an edge of the mount hole  31  on a back surface abuts against upper portions of the inclined surfaces  6  formed on upper surfaces of tip ends of the latch pawls  5  as shown in FIG. 5 in the case where the member being mounted  30  is small in plate thickness, while the edge of the mount hole  31  on the back surface abuts against the lower portions of the inclined surfaces  6  formed on the upper surfaces of the tip ends of the latch pawls  5  as shown in FIG. 6 in the case where the member being mounted  30  is large in plate thickness, in either of which the latch pawls  5  push up the member being mounted  30 , to bring the same into close contact with the back surface of the head  1  of the grommet (actually, reaction forces produced when the latch pawls push up the member being mounted  30  deform the grommet itself downward, so that the back surface of the head is intensely brought into close contact with an upper surface of the member being mounted  30 ).  
         [0022]    In this manner, the grommet of the invention always enables portions of the inclined surfaces  6  formed on the upper ends of the latch pawls  5  to contact with the edge of the mount hole  31  on the back surface of the member being mounted  30  even when the member being mounted  30  is varied in plate or sheet thickness. In a state, in which threading of the screw has been completed, the latch pawls  5  are intensely pushed outward to have the inclined surfaces  6  thereof pushed against the member being mounted  30 , so that it is not possible that the water-proof grommet moves axially relative to the member being mounted  30  unlike prior grommets.  
         [0023]    Besides, sure water-proof performance can be obtained since the bottomed configuration of the grommet body  2  eliminates the possibility of invasion of water therethrough and the seal rubber  9  provided on the underside of the head  1  contacts closely with the member being mounted  30  to seal a periphery of the mount hole  31  of the member being mounted  30 .  
         [0024]    As described above, the water-proof grommet of the invention has advantages that the grommet is not moved axially once being engaged since the portions of the inclined surfaces formed on the tip ends of the latch pawls always contact with and push against the member being mounted, and that load upon insertion of the grommet into the mount hole of the member being mounted can be reduced since the latch pawls posses adequate elasticity.