Abstract:
An integrally formed screw, such as a wood screw has a threaded cylindric stem and a head ( 9 ). At least a part ( 25 ) of the head ( 9 ) is shallowly convexly curved ( 25 ) and is provided with a screwdriver recess ( 26 ). The head ( 9 ) has an annular, shallowly concave underside ( 22 ) adapted to accommodate a relatively soft, polymeric washer ( 27 ).

Description:
FIELD OF INVENTION  
         [0001]    The present invention relates to an integrally formed screw such as a woodscrew. Wood screws are used in many different applications. Some applications require a wood screw combined with a soft, polymeric washer. In such applications, it is often required that when the screw and the polymeric washer is in place, the screw head be as unobtrusive as possible in order to avoid damage to the screw head or other problems when working on the surface from which the screw heads project, e.g. when shoveling snow from the roof.  
           [0002]    The known screws have a flat underside which engages the polymeric washer. The flat underside often causes a non-uniform deformation of the soft washer whereby the sealing or locking performance of the washer is lost or reduced.  
           [0003]    My U.S. Pat. No. 5,919,020 describes a wood screw in which the specific pressure exerted by the screw head on a workpiece is relatively low, the fastening force high and the top surface of the head relatively unobstructive. This is achieved by combining the shape of a round head screw with an integrally formed radial extension resembling a washer having a flat underside. While this type of screw enjoys a considerable commercial success, in use with a polymeric, e.g. rubber washer the disadvantages mentioned in the preceding paragraph could be encountered.  
         SUMMARY OF INVENTION  
         [0004]    It is an object of the present invention to provide a screw, particularly, but not exclusively, a wood screw, which would avoid or at least reduce the problem of undesired lack of uniformity of compression of a soft washer while securing the unobtrusive state in a fully applied condition and providing strength sufficient for use with power screwdrivers.  
           [0005]    In broad terms, the invention provides an integrally formed screw having a circular head defining a peripheral rim, and an integrally formed stem which includes a thread formed in at least a part thereof. The stem has a root end at the head of the screw, and a free end portion. The head includes an underside from which the stem projects. The top surface portion of the head, facing away from the stem, includes a screwdriver engaging recess. An annular underside of the head is disposed about the root end of the stem. In accordance with the invention, the underside of the head defines a shallowly concave annular portion co-axial with the stem and decreasing in depth in the radial direction toward the circumference of the head. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    The invention will now be described by way of a preferred and an exemplary embodiments, it being understood that many other embodiments may exist which differ from those described, without departing from the present invention.  
         [0007]    In the drawings:  
         [0008]    [0008]FIG. 1 is a diagrammatic, not-to-scale enlarged side view, partly in section, of a wood screw including the combination of features according to the present invention;  
         [0009]    [0009]FIG. 2 is a cross-sectional view of a polymeric washer used with the screw of  
         [0010]    [0010]FIG. 3 is a view similar to that of FIG. 1 but showing a modified, exemplary embodiment of the invention; and  
         [0011]    [0011]FIG. 4 is a diagrammatic, simplified view similar to that of FIG. 1 showing approximate measurements of different sizes of the screw of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0012]    The screw shown in the drawings is an integrally formed product usually made from suitable steel, it being understood that certain applications may warrant or require different material, e.g. nylon.  
         [0013]    The screw shown in the drawings comprises a stem having, in the embodiment shown, a cylindric portion  10  (also referred to as “main section”). As is known, the stem has a free end portion  11  which, in the embodiment shown, is pointed. The opposite end of the stem is designated as a root end  12 . In the embodiment shown, a major part of the stem is cylindric except for a reinforcement at the head and at the pointed free end portion  11  where the shape is conical. A helical thread  13  projects from the surface of the stem. The thread extends from the pointed free end portion  11  and terminates at a root end portion  14  of the thread  13 , which is spaced from the reinforced root end  12  of the stem.  
         [0014]    Preferably, but not necessarily, the reinforcement of the stem includes a minor base end  15  of a frustoconical portion  16 . The opposite end of the frustoconical portion  16  defines a major base end  18 . In the embodiment shown, the major base end  18  coincides with, and thus has the same diameter as a cylindric part  20 . The reference numbers  15 ,  16 ,  18  and  20  thus combine to form one embodiment of what is generally referred to as a reinforcement portion of the stem.  
         [0015]    The cylindric part  20  merges, at a merger  21 , with a shallowly concavely formed, downwardly facing underside  22 , also referred to as a “shallowly concave annular portion  22 ” which includes the underside of a washer section  23 .  
         [0016]    It is emphasized that the primary purpose of the reinforcement portion is to provide the screw with suitable strength for use with a power screwdriver. Therefore, the actual size or shape of the reinforcement portion is optional and may differ from what is shown in the drawings, without departing from the scope of the present invention. In certain embodiments, it may even be entirely eliminated leaving the diameter of the cylindric portion  10  uniform up to the underside  22  of the head.  
         [0017]    The term “shallowly” concavely formed refers to an annular concave underside  22  having the depth D 1  of the order of about 0.5 mm in a screw whose head  9  has the outer diameter B 1  of about 12 mm, the thickness A 2  (FIG. 4) of the washer section  23  being in the order of about 1.5 to about 2 mm. Thus, the depth D 1  of said shallowly concave annular underside  22  is about 6 to 8% of the outer diameter B 1  of the head  9  of the screw. The top of the washer section  23  defines a shallowly generally frustoconical, annular upper face section  24 . The reference to “generally” frustoconical includes embodiments (as shown in the drawings) where the surface may actually be somewhat convexly rounded due to the rolling process in the manufacture of the screw. In the embodiment of FIGS. 1 and 4 the axial height D 2  of the generally frustoconical portion is about 1 mm. It is emphasized that the size figures refer to a typical size of a wood screw and are not to be interpreted as limiting the invention to the specific measures.  
         [0018]    Reference number  17  designates the rim of the washer section  23 . The rim  17  defines the outside diameter B 1  of the head  9  of the screw. The deepest part of the shallowly concave underside  22  is disposed on a first reference plane P 1  perpendicular to the stem of the screw. The lowermost part  19  or edge of the rim  17  presents a part of the head  9  which is located on a second reference plane P 2 , which is parallel with and is axially spaced from the first reference plane P 1 . The second reference plane P 2  is closer to the tip  11  than the first reference plane P 1 . The depth D 1  of about 0.5 mm referred to above is the axial between the reference planes P 1 , P 2 . The depth D 1  is provided, for example, by rolling the top face  24  of the washer section  23  to change its shape from a planar to generally frustoconical.  
         [0019]    A centrally disposed, coaxial, shallowly convex head portion or central portion having outer diameter or boundary B 2  (FIG. 4), projects upwardly from the upper face section  24 . The head portion  25  is provided with centrally disposed screwdriver engagement recess  26 .  
         [0020]    The term “shallowly convex” within the context of the central portion  25 , designates an arrangement, wherein the maximum axial thickness Al of the shallowly convex head portion  25  is only slightly larger than the thickness A 2  of the washer section  23 . In the embodiment shown, the thickness A 1  is about 3 to 4 mm.  
         [0021]    The screw of the present invention is intended for use with a polymeric washer  27  which does not form a part of the invention and is readily commercially available. The size of the washer  27  is selected to have the inner diameter roughly corresponding to the maximum diameter of the reinforcing portion  12 . The washer is preferably bevelled at its end facing the underside  22  of the screw.  
         [0022]    With particular reference to FIGS.  4 , Table 1 presents three examples of different size embodiments of the screw of the present invention, having typical physical measurements. The three embodiments shown are 6, 7 and 8 mm which roughly corresponds to ¼′; {fraction (5/16)}′ and ⅜′ wood screws each having a total length of about 80 mm, respectively. The respective size particulars of the screw.  
                                 TABLE 1                           APPROXIMATE SIZE PARTICULARS OF ADDITIONAL EXAMPLES       OF THE INVENTIVE SCREW (in mm)            Size of the screw   8   7   6               A1 thickness range of the head 25   2.4   2     1.6       A2 thickness range of washer section 23   1.8-1.5   1.6-1.3   1.4-1         A4 the length of frustocon. port. 16   2     2     1.5       B1 diameter range of washer section 23   18.5-16     16.5-14       14-12.5       B2 diameter range of head portion 25   10.5-10      10-9.7     9-8.7       B3 outer dia. range of thread 13     8-7.5   7.2-6.7   6.2-5.7       B4 diameter range of stem 10   5.4-5.1   4.82-4.79   4.24-4.18       D1 depth of the shallowly concave underside 22   1.2   1.1   0.9       D2 axial height of the frustoconical portion 24   1.4   1.2   1.1                  
 
         [0023]    It follows from the above that a wide range of the screws utilizing the present invention can be produced.  
         [0024]    Turning now briefly to the exemplary embodiment shown in FIG. 3, the concavely formed underside  22  is disposed under a continuously convex, rounded head  28  rather than a combined head and washer section portion described with reference to with FIG. 1.  
         [0025]    As mentioned, the invention can be modified to a greater or lesser degree to differ from the embodiments disclosed, without departing from the scope of the present invention as set forth in the accompanying claims.