Patent Publication Number: US-2009226280-A1

Title: Screw

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a screw having a stem carrying at least in one of its region, a thread and having a stem radium, a tip provided at one end of the stem, a head provided at an opposite end of the stem and a transition region located between the head and the stem. 
     2. Description of the Prior Art 
     Screws of the type described above are formed, e.g., as thread-tapping screws and are used, e.g., for screwing of metal sheets. Often, such screws have a head with a diameter that is noticeably greater than the stem diameter. This is the case, e.g., in screws the head of which has polygonal geometry for transmission of rotation from a wrench or a lock mechanism of a power tool. A large difference in diameters between the head and the stem is associated with large loads applied during screwing in of such screws for obtaining a necessary rotational or turning force. The large loads results in very large notch fatigue in the transitional region between the head and the stem. With such screws, there is an increased danger of the head being broken off the stem. 
     U.S. Pat. No. 3,661,046 discloses a screw of the type discussed above, with a head having a hexagonal geometry as rotation-transmitting means, and a stem adjoining the head. The stem circumferentially carries a thread. In the transition region from the head to the stem, there is provided, from the stem toward the head, a short, extending radially outwardly zone in form of a trumpet. The largest diameter of the trumpet-like zone, in the region closed to the head, only slightly exceeds the stem diameter, whereas the head outer diameter is noticeably greater than the stem diameter. 
     A drawback of the screw, which is described in the above-mentioned U.S. Patent, consists in that a big jump in the diameter between the head and the stem is associated, as discussed above, with large loads applied, during screwing in of the screw, for obtaining the necessary turning force, which leads to a very large notch fatigue in the transition region between the head and the stem. In this screw, the danger of the head being broken off the stem is rather high. 
     Accordingly, an object of the present invention is to provide a screw of the type described above in which the above-mentioned drawback of known screws is eliminated. 
     Another object of the present invention is to provide a screw having a high resistance to breaking of the head of the stem even with a screw head having circumferential rotation-transmitting means the diameter of which exceeds that of the stem. 
     SUMMARY OF THE INVENTION 
     These and other objects of the present invention, which will become apparent hereinafter are achieved, according to the present invention, by providing a screw of the type discussed above and in which the transition region has its circumference formed of from three to ten truncated cones following each other in an axial direction and surface lines of which have different inclination angles toward the axis which diminish from the head in direction of the tip, with a first inclination angle of the surface line of a truncated cone located closest to the head, amounting to from 40° to 50°. 
     The transition region between the head and stem of a screw and which is formed according to the present invention permits to noticeably reduce the notch stress in this region, whereby the resistance to the head breaking off the stem is greatly increased. 
     In the inventive screw, preferably, the first inclination angle amounts to 45°. 
     Advantageously, an inclination angle of a surface line of a truncated cone following, in the axial direction toward the tip, an immediately adjacent, toward the head, truncated cone, amounts to n-times of the inclination angle of the surface line of the immediately adjacent truncated cone, and wherein n is equal to a number between 0.45 and 0.55. It is particularly advantageous when n is equal to 0.5. 
     Advantageously, the first truncated cone has, at an end thereof adjacent to the head, a maximal first radial width with respect to the stem that is smaller or equal to a difference between the minimal head radius and the stem radius according to equation 
       first radial width≦Head Radius−Stem Radius 
     With the first radial width according to this equation, a noticeable reduction of the notch fatigue is achieved. 
     It is advantageous, when the first radial width of the first truncated cone corresponds to from 40% to 80% of the difference between the minimal head radius and the stem radius. With this, a certain underhead surface will remain and which both brakes the screw against the sheet metal to actuate the safety clutch of the screw driving tool, and prevents the pulling of the screw through a sheet(s). 
     The novel features of the present invention, which are considered as characteristic for the invention, are set forth in the appended claims. The invention itself, however, both as to its construction and its mode of operation, together with additional advantages and objects thereof, will be best understood from the following detailed description of preferred embodiment, when read with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the Drawings: 
       Single FIGURE shows a front elevational view of a thread-tapping screw according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A thread-tapping screw  10  according to the present invention, which is shown in the drawings, has a stem  11  carrying a thread  15  and provided with a tip  13  at one of its end and a head  14  at the opposite end. An axis A defines an axial direction of the screw  10 . The head  14  has a rotation-transmitting element  17  for a screw-driving tool and having its outer surface formed as a polygon, in particular, as a hexagon that can be engaged by a wrench. 
     Between the head  14  and the stem  11 , there is provided a transition section  12  having a reinforced geometry in comparison with the stem  11 . The transition section  12  is formed by three truncated cones  21 ,  22 ,  23  following each other in the axial direction. 
     A surface line of the first truncated cone  21 , which is most adjacent to the head  14 , has an inclination angle α toward the axis A in a range between 40° and 50° and, preferably of 45° (for clarity sake, in the drawing, the inclination angle α is shown with respect to a projection  16  of the stem  11  and which extends parallel to the axis A). The surface line is a line on a cone outer surface of a respective truncated cone extending in the axial direction. The inclination angle β, γ of the surface lines of the truncated cones  22 ,  23 , which follow the first truncated cone  21  in the axial direction, is reduced toward the tip  13 . The truncated cone  22 ,  23 , which is closer to the tip  13 , has the inclination angle β, γ of the surface line that amounts to n-times of the inclination angle α, β of the surface line of the truncated cone  21 ,  22  adjacent to that truncated cone  22 ,  23  on the side of the head  14 . n is a number between 0.45 and 0.55. In the embodiment shown in the drawings, the inclination angle β of the surface line of the second truncated cone toward the axis A, i.e., toward the projection  16  amounts to 0.5 of the inclination angle α of the first truncated cone  21 . Thus, the inclination angle β amounts to between 20° and 25°, preferably, to 22.5°. The inclination angle γ of the surface line of the third truncated cone  23  toward the axis A or the projection  16  amounts to 0.5 of the inclination angle β of the surface line of the second truncated cone  22  or to 0.25 of the inclination angle α of the surface line of the first truncated cone  21 . Thus, the inclination angle γ amounts to between 10 and 12.5°, preferably, to 11.5°. The inclination angle α, β, γ are, thus, reduced from the region adjacent to the head  14  in the direction of the tip  13  in a step-like manner. 
     The first truncated cone  21  has at its end adjacent to the head  14  a maximal first radial width R 1  (starting from the projection  16  of the stem  11 ) which smaller or equal to the difference between a minimal head radius R k  (i.e., head radius between the longitudinal axis of the screw and a closest, to this longitudinal axis, outer surface of the head) and the stem radius R s , according to an equation 
         R 1 ≦R   k   −R   s    
     The first radial width R 1  of the first truncated cone  21  corresponds to from 40% to 80% (or to 0.4-0.8) of the difference between the minimal head radius R k  and the stem radius R s . The maximal radial width R 2  of the second truncated cone  22  (in the transition region to the first truncated cone  21 ) corresponds to from 0.4 to 0.8, preferably, to 0.5 of the first radial width R 1 . A maximal third radial width R 3  of the third truncated cone  23  (in the transition region to the second truncated cone  22 ) corresponds to from 0.45 to 0.55, preferably, 0.5 of the second radial width R 2 . 
     The second truncated cone  22  is so arranged with respect to the first truncated cone  21  that it branches from the first truncated cone  21  at a point  18  that lies on a half of an interpolated length H 1  of the surface line of the first truncated cone  21  toward the projection  16  of the stem  11 . The third truncated cone  23  is likewise so arranged with respect to the second truncated cone  22  that it branches from the second truncated cone  22  at a point  19  that lies on a half of an interpolated length H 2  of the surface line of the second truncated cone  22  toward the projection  16  of the stem  11 . 
     The transitional region  12 , which is formed by the truncated cover  21 ,  22 ,  23 , extends axially over a length L. The length L can be calculated for an ideal inclination angle, with the inclination angle α of the surface line of the first truncated cone  21  being equal 45%, according to formula 
         L=R 1 *X    
     where X is a variable constant dependent on the number of truncated cones and an have the following values:
 
X=1 for one truncated cone, X=1.70711 for two truncated cones, X=2.36039 for three truncated cones, X=3.00112 for four truncated cones, X=3.63876; X=4.27564 for six truncated cones, X=4.91232 for seven truncated cones, X=5.54896 for eight truncated cones, X=6.18558 for nine truncated cones, and X=6.82220 for ten truncated cones.
 
     In order to prevent friction, the screw  10  can be provided, at least in the region of the thread  15  and/or the tip  13  with a friction-preventing coating such as, e.g., organic (e.g., wax or cutting oil) or inorganic lubricant (e.g., zinc). 
     Though the present invention was shown and described with references to the preferred embodiment, such is merely illustrative of the present invention and is not to be construed as a limitation thereof and various modifications of the present invention will be apparent to those skilled in the art. It is therefore not intended that the present invention be limited to the disclosed embodiment or details thereof, and the present invention includes all variations and/or alternative embodiments within the spirit and scope of the present invention as defined by the appended claims.