Abstract:
A screw ( 10 ) having a screw-threaded shank ( 14 ) and a circular head ( 12 ), the head being provided with three radially extending screwdriver tip receiving portions ( 16; 18; 20 ) defined between three first, off-central integrally formed projections ( 16   a;    18   a;    20   a ) designed to become shaved-off the screw head by a pre-set shear force applied during tightening of the screw, and three second, integrally formed projections ( 16   c;    18   c;    20   c ) designed to withstand a shear force greater than the said pre-set force applied during slackening of the screw.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates to screws, and more specifically to restricted torque fastening screws.  
       BACKGROUND OF THE INVENTION  
       [0002]     In the relevant industries, it is often required that fastening screws be tightened by a limited, controlled force, in order to avoid damage to the equipment or to fulfill other conditions, e.g. in case of a cover with a rubber seal which must not become over-squeezed.  
         [0003]     Other, related examples, are cases where it is requested that once tightened, the screw cannot be slackened by ordinary tools such as a screwdriver, or at all.  
         [0004]     Alternatively, a demand may be posed that it would be possible to unscrew the screw, but there will be clear and unmistakable indication that the screw has been tampered with by an unauthorized person.  
         [0005]     The invention aims at proposing a solution satisfying any and all of these requisites.  
         [0006]     It is a further object of the invention that the screw heads be produced by forging.  
       SUMMARY OF THE INVENTION  
       [0007]     Thus provided according to the invention is a screw having a screw-threaded shank and a circular head, the head being provided with at least one radially extending screwdriver tip receiving portion defined between a first off-central, integrally formed projection designed to become shaved-off the screw head by a pre-set shear force applied during tightening of the screw, and a second, integrally formed projection designed to withstand a shear force greater than the said pre-set force applied during slackening of the screw.  
         [0008]     There are preferably provided three pairs of the first and the second projections equi-angularly located on the head of the screw wherein the first projections are of a generally right-angled triangular cross-section seen in a plane normal to the radial direction of the screw head, and the second protrusions are of a generally isosceles triangle cross-section seen in a plane normal to the axis of the screw head.  
         [0009]     A tool is provided for operating screws according to the invention, comprising a circular handle and three, equi-angularly arranged and radially extending ribs fitting into the said screwdriver tip receiving portions. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     These and additional constructional features and advantages of the invention will become more clearly understood in the light of the ensuing description of few preferred embodiments thereof, given by way of example only with reference to the accompanying drawings, wherein  
         [0011]      FIG. 1  is a three-dimensional view of a screw according to a first preferred embodiment of the invention;  
         [0012]      FIG. 2  is a top view of the screw of  FIG. 1 ;  
         [0013]      FIG. 3  is a section taken along line III-III of  FIG. 2 ;  
         [0014]      FIG. 4  is a side view of the screw of  FIG. 1 ;  
         [0015]      FIG. 5  illustrates a tool for tightening the screw of  FIG. 1   
         [0016]      FIG. 6  is a three dimensional view of a screw according to a second preferred embodiment of the invention;  
         [0017]      FIG. 7  is a top view of the screw of  FIG. 6 ;  
         [0018]      FIG. 8  is a side view of the screw of  FIG. 6 ;  
         [0019]      FIG. 9  is a three-dimensional view of a screw according to a third preferred embodiment of the invention;  
         [0020]      FIG. 10  is a top view of the screw of  FIG. 9 ;  
         [0021]      FIG. 11  is a side view of the screw of  FIG. 9 ;  
         [0022]      FIG. 12  is a partial cross-sectional view taken along line XII-XII of  FIG. 10 ;  
         [0023]      FIG. 13   a  schematically shows the tightening operation of the screw of  FIG. 9  by a designated tool;  
         [0024]      FIG. 13   b  shows the unscrewing operation of the screw using the same tool;  
         [0025]      FIG. 14  is a three dimensional view of a screw according to a fourth preferred embodiment of the invention;  
         [0026]      FIG. 15  is a top view of the screw of  FIG. 14 ;  
         [0027]      FIG. 16  is a side view of the screw of  FIG. 14 ;  
         [0028]      FIG. 17   a  shows the tightening operation of the screw at  FIG. 14  by a designated tool; and  
         [0029]      FIG. 17   b  shows the unscrewing of the same screw. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]     The embodiment of  FIGS. 1-4  relates to a screw with limited tightening torque and indication if opened without authorization. The indication is given in such a manner that if the screw has been unscrewed—it cannot be closed again whatsoever.  
         [0031]     Hence, the screw generally denoted  10  comprises a shoulder (machine-screw) head  12  and a shank  14 . As will be made clear, the screw can be of any kind, either of the standard or of special design.  
         [0032]     A pattern of cavities or depressions is applied to the top of the head  12 , most conveniently, and as normally applied in the production of bolts and screws, by forging, in a one-time operation, as will be now described with respect to the present example. Variations and modifications of the pattern will be readily apprehended after understanding the design principles governing this embodiment.  
         [0033]     At least one—but preferably three—radially extending slots  16 ,  18 ,  20  are present, equi-angularly arranged as shown. The slot  16  (see  FIG. 3 ) is delimited at one side thereof by a first, off-centrally located projection  16   a  of a generally right-angled triangular cross-section (seen in a plane parallel to the axis of the head  12 ). The surface of the hypotenuse  16   b  slopes from the root of the projection  16   a  up to the original top level of the head  12  (projection  18   c —see below).  
         [0034]     At the other side of the slot  16  there is formed a solid projection  16   c  of a generally isosceles triangle cross-section as seen in a plane normal to the axis of the head  12 .  
         [0035]     Similarly, slot  18  has triangular projection  18   a  and curved surface  18   b  at one side, and projection  18   c  at the other side; and the same applies to the slot  20 . Preferably, a circular depression (or boss)  22  is made at the center (the apexes of the triangles  16   c ,  18   c , and  20   c ).  
         [0036]     An operating tool or driver for the screw  10  is shown in  FIG. 5 . The tool denoted  24  has a hand gripping handle (or electric screwdriver standard coupling tip)  26 , three elongated radially extending ribs  26   a ,  26   b ,  26   c  and a central pin  28 , designed to fit the slots  16 ,  18 ,  20 , and the depression (or boss)  22 , respectively.  
         [0037]     In use, namely fastening a workpiece by the screw  10 , the screw is turned by the tool  24  and tightened by an increased torque until the projections  16   a ,  18   a , and  20   a  are simultaneously torn or shaved off by the shear force applied to the respective roots thereof.  
         [0038]     The amount of the maximum applicable torque can be pre-determined by correctly calculating the collective shear strength that will cause the breaking of the projections, the relevant parameters being the cross-sectional area of the roots and the shear strength of the material the screw is made of.  
         [0039]     Once broken, further rotation of the tool would simply “throw” the tool out of the respective slots to merely idle over the top of the screw head  12 .  
         [0040]     However, and in that respect uniquely different from prior art auto-torque screws, although opening of the screw  10  remains possible, by turning the head  12  in the opposite (counter-clockwise) direction by the tool  24 , the re-fastening thereof is not possible.  
         [0041]     Hence, let us take the following frequent example where a manufacturer of certain equipment makes it a condition to the validity of its warranty that no one except authorized personnel would be allowed to dismantle any part of it. Once a purchaser attempts to release the screws by himself, he will be compelled to replace them by “ordinary” screws when bringing the equipment to repair under the warranty, which will immediately indicate that the above condition has been violated and free the manufacturer of its obligation.  
         [0042]      FIGS. 6-8  exemplify application of the invention to a flat head (wood) screw and is otherwise analogous to the preceding embodiment and therefore similar reference numerals are used.  
         [0043]     Hence, provided at the screw  110  are three slots  116 ,  118 ,  120  and their related shearable projections  116   a - 120   a , etc.  
         [0044]     In the modified embodiment of  FIGS. 9-13 , no screwdriver slots in the normal sense of the term are present, but rather two sets of unidirectional projections are formed as will now be described. Machine screw  210  with shoulder head  212  is formed with a first set of projections—three in the described example—denoted  230 ,  232 , and  234  which are functional for closing the screws; a second set of projections  240 ,  242 ,  244  for unscrewing; and central bore (or projection, at the option of the designer)  250 .  
         [0045]     Again emphasized is that this pattern is easily attained by press-forging so that, as in the preceding embodiments, the projections are integrally formed with the head  212 .  
         [0046]     The first set of projections  230 - 234  have a right-angled triangle cross-section. The hypotenuses of the triangles extend radially facing the same, clockwise direction.  
         [0047]     The second set of projections  240 - 244  are spaced from the projections of the first set both in radial and in the angular directions, with hypotenuses facing the opposite, counter-clockwise direction.  
         [0048]     The total roots area of the first set is calculated to yield (become shaved off) by shear, under a pre-determined force applied by a driving tool (see  FIG. 5 ) represented by the shadowed surface depicted in  FIG. 13   a.    
         [0049]     The total roots area of the second projections would preferably be made to withstand a greater force, for unscrewing (in the appropriate cases—if at all) by turning the screw-driving tool in the counter-clockwise direction as depicted in  FIG. 13   b.    
         [0050]     Once torn away by the shear force applied during the tightening stage ( FIG. 13   a ), the second set of projections neutralizes the operation of the tool in the closing direction due to the sloping sides of the projections  240 - 244 .  
         [0051]     The embodiment of  FIGS. 14-17  is a modification of the preceding embodiment. As evident, the shear-controlled projections are those denoted  340 ,  342 , and  344 , namely located around the outer circumference of the head  310 , whereas the unscrewing projection  330 - 334  are deployed along an inner circle.  
         [0052]     Also noteworthy is the fact that in this example the driving tool (seen as shadowed areas in  FIGS. 17   a  and  17   b ) is not identical for closing and for opening the screw (one being a mirror-image of the other).  
         [0053]     The invention thus offers a novel and efficient solution fulfilling the objectives as specified in the preamble paragraphs above.  
         [0054]     Those skilled in the art to which this invention pertains will readily appreciate that numerous changes, variations and modifications can be effectuated without departing from the true spirit and scope of the invention as defined in and by the appended claims.