SCREW FOR USE IN CONCRETE

FIELD OF THE INVENTION

The present invention relates generally to a screw fastener, which is adapted to be secured in a hole drilled in a masonry structure such as concrete but it may be used in other masonry materials such as brick or ceramic The screw is of a type, sometimes called Hi-Lo, having a major thread of a major crest diameter, and an intermediate minor thread of minor crest diameter

BACKGROUND OF THE INVENTION

Concrete screws, namely screws adapted for use in a concrete or other masonry substrate, typically are screwed, either by hand or power tools, into an appropriately sized bore hole in which the diameter of the bore hole is slightly greater than the diameter of the shank of the screw, while the crest diameter of the threads of the screw exceed the diameter of the bore hole

Typically, the threads of concrete screws cut into the sidewalls of the borehole Heretofore, screws with a fine pitch have been utilized, allowing multiple convolutions of threads in contact with the wall of the borehole, typically with a thread height which provided shallow penetration of the threads into the walls of the boreholes With many shallow threads in close proximity, pullout of concrete screws is a problem With increased diameter of threads while maintaining the same pitch and same shank and borehole diameters, greater pullout resistance can be obtained However, such a screw structure increases the risk of over-cutting the sidewalls and even "drilling out" the concrete on an over-torqued screw In any event, the result of increasing the number of thread convolutions and consequent length of thread contacting the sidewalls, resistance to rotation increases Unfortunately, many concrete screws suffer from this increased resistance, whereby the torque required to sink the screw to the desired depth results in over-torquing of the screw and shearing of the screw shank

In order to reduce the shear resistance, screws having two different shank diameters have been disclosed, as in United States Patent 5,061 ,136 Other screws have employed a thread of gradually increasing diameter as in US Patent 4,842,467 A Hi-Lo screw with a line of symmetrical thread notches is disclosed in US Patent 3,937,1 19

US Patent 877,131 discloses a wood screw with drill & countersinking flutes extending into the head of the screw US Patent 1 ,235,626 discloses a wood screw with a drill flute therein US Patent 4,697,969 also discloses a wood screw with one or more spiral drill flutes, of the same rotational hand as the thread

The present screw provides a Hi-Lo structure, with a counter rotational groove to absorb cuttings from the borehole and reduce frictional resistance while inserting the screw

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the present invention to provide a concrete screw having reduced rotational resistance, decreased incidence of shearing, and increased resistance to pullout According to the invention, the screw has a set of major and minor co-helical threads of a pitch generally at least equal to the shank diameter Furthermore the screw embodies one or more counter-helical V-shaped grooves extending through the full height of the major and minor threads and into the shank of the screw Preferably, the counter-rotational helical grooves are of a very coarse pitch, typically about two to six times the opposite pitch of the threads, preferably about five times The screws also have secondary partial length grooves intermediate the V-shaped grooves

BRIEF DESCRIPTION OF THE DRAWINGS

Brief descriptions of the drawings of the invention will now be described by way of preferred and exemplary embodiments with reference to the accompanying diagrammatic and not to scale drawings

Fig 1 is a side view of a concrete screw according to the present invention,

Fig 2 is a cross-sectional view taken along the lines Il - Il of Fig 1 showing a single convolute of major and minor threads and the counter-rotational grooves,

Fig 3 is a sectional view taken along section line III - III of Fig 1 , and showing the geometry of the asymmetrical W-shaped cut-outs in the tip portion of the screw in a further embodiment of the invention

Fig 4 is a side view of another embodiment of the screw according to the present invention

Fig 5 is a side view of an enlarged view of the tip portion of the screw of Fig 4

Fig 6 is a cross-sectional view taken along the lines Vl-Vl of Fig 5 showing two sets of grooves

DETAILED DESCRIPTION OF THE PREFERRED AND EXAMPLARY EMBODIMENTS In the following description, the same reference numbers are used for equivalent parts, although different embodiments of the invention may be shown and described

In Fig 1 is illustrated an exemplary type of concrete screw incorporating the present invention The screw is integrally formed, preferably of nickel, and comprises a shank portion 10, having at one end a tip 11 The opposite, second end of the shank portion 10, has a head end portion 12, which is unthreaded Preferably the head end portion 12 has a somewhat increased diameter to provide a smooth transition to a head 13, for instance, by a frustoconical transitional section 14 The enlarged transitional section also enables reception of a larger or a more powerful screw driving means

Between the tip 1 1 and head portion 12, the shank 10 has multiple convolutions of at least two helical threads A first thread 15 has a crest diameter (the diameter of the virtual cylinder enclosing the outer periphery of the threads) greater than the shank diameter Preferably, such crest diameter of major thread 15 exceeds the shank diameter from about 20 to 30 percent Intermediate the major helical thread 15 is a minor helical thread 16 The crest diameter of minor helical thread 16 exceeds the diameter of the shank to a far lesser extent, in the order of 5 to 10 percent

In contrast to conventional "Hi-Lo" screws, which have a relatively fine pitch, the pitch of the present screw is much greater Preferably, the thread pitch in the present screw has a major crest-to-crest interval at least as large as the diameter of the shank 10 of the screw Applicant believes that a thread pitch of less than the shank diameter, when threads as disclosed herein are used, results in excessive undercutting of the borehole walls, with resulting reduction in pull-out resistance

The screw of the present invention also includes a plurality of helical grooves, having a rotation of opposite hand to the rotation of the helical threads In one embodiment, illustrated in Fig 1 and 2, a total of three counter-helical grooves 17 are illustrated Grooves 17 are generally V-shaped, having a narrow apex angle of 30° or less The grooves may be symmetrical (as illustrated in Fig 2), or asymmetrical in the direction of the screw rotation The grooves extend completely through the full height of both

the major thread 15 and the minor thread 16, and penetrate into the shank 10 Preferably, the groove has a depth into the shank 10 which exceeds the height of the minor thread, whereby the screw diameter at the base of the groove is in the order of 5 to 10 percent less than the diameter of shank 10 The grooves 17 have a pitch of at least twice the opposite pitch of the threads In the screw illustrated in Fig 1 , the pitch of the groove forms an angle α in the order of 30° with the axis of the screw In comparison, the pitch may be significantly greater as illustrated in Figs 4 and 5 The steepest pitch exhibiting the benefits of the present invention forms an angle α of about 10° with the vertical axis of the screw

In still a further aspect of the invention, illustrated in Fig 3, a limited number of threads, such as the first two or three convolutions, depending on the pitch, and preferably at least 25 mm of threads, are cut not only by the counter-helical groove 17, but by an additional rotationally adjacent groove 18 which, in combination with groove 17, forms a W-shaped cut-out An asymmetrical W cut-out of combined groove 17 and 18 is shown in Fig 3, and provides a saw tooth aspect to the leading few convolutions of major and minor threads 15 and 16 This saw effect aids in the initial scarifying or cutting of a thread into the surface walls of the borehole into which the screw is driven

Referring to Fig 4, a further embodiment of the present screw is illustrated The screw has a set of counter helical grooves of much steeper pitch, having an angle α of about 10° with the axis of the screw In another embodiment of the invention, illustrated in Figs 5 and 6, a second set of grooves 19 is provided intermediate the previously described set of grooves Those grooves, also of a V-shape, may extend over a limited portion from the tip end of the screw For example, where there are three major grooves 17 extending the length of the screw, the three shorter intermediate grooves 19 may be provided for approximately 1/3 of the threaded length of the screw and are preferably at least 25 mm in length Grooves 19 may also have an asymmetrical V-shape, inclined in the direction of rotation of the screw

Of course, the intermediate grooves 19 can be positioned adjacent the major grooves 17 to effect a W-shaped combination

In operation, the screw is positioned in a borehole having a diameter slightly larger than the diameter of shank portion 10 and head end portion 12 By means of an appropriate interface, such as a Phillips, Robertson or Torques pattern driver hole or wrench flats, rotational forces are applied to head 13 of the screw to rotate the screw into the hole The lead threads and cut-outs created by grooves 17 and 18 (19)

carve an initial thread pattern, which is followed by major thread 15 and minor thread 16 Major thread 15 cuts significant thread tracks in the wall and provides the greatest resistance against putlout Minor thread 16 penetrates the sidewall to a lesser extent, and provides a solid second bearing for the body of the screw, but without significant cutting or weakening of the wall

It is believed that, with prior art fine pitch concrete screws, the concrete powder scored from the walls of the bore hole by the screw threads remains in the vicinity of the screw tip, and itself binds and compacts between the threads and the bore hole wall, significantly increasing resistance to the driving of the screw In fact, in many designs of screw, this resistance in fine pitch screws is so high that the screw is incapable of transmitting the rotational forces from the head to the tip, and shearing of the screw body occurs in a disproportionately high number of instances

In contrast, it is believed that the screw of the present invention does not retain the abraded concrete powder in the vicinity of the tip of the screw, but rather the series of helical groove allows the concrete powder to disperse along the length of the screw, and to the extent necessary, to fill in the grooves 17 (and 18, 19) This dispersion of concrete powder avoids the increased resistance at the tip of the screw, thereby permitting full transmission of torque throughout the screw length, and full penetration to the design depth, with vastly improved shear resistance and pullout resistance Furthermore, in view of the coarse pitch of the major and minor threads, there is a significant decrease in the tendency, as exhibited by prior art screws, to "drill out" or break away the concrete structure between the thread tracks

In one embodiment of a concrete screw exhibiting the foregoing features, the screw has a length of 92 mm, a threaded length of 60 mm, a shank diameter of 5 7 mm, a major thread crest diameter of 7 5 mm, and a minor thread crest diameter of 5 85 mm, and a thread pitch of 5 7 mm The screw also had three counter-helical grooves, having a groove depth of 5 25 mm and a groove pitch of 30 mm (about 30°) Asymmetrical W shaped cut-outs are provided on the first three convolutions (initial 25 mm) of the major and minor threads In another embodiment, the screw has the same length, shank and thread diameters and thread pitch However, the pitch of the counter helical groove is about 120 mm In comparison with traditional concrete screws of the same shank diameter, but thread pitch of one-half the shank diameter, and without the helical grooves of the present invention, the screw driving torque required for the screw of the present invention was only 60 percent of that of

prior art screws while pullout resistance was 70 percent higher than with the traditional screws

The screw as described above exhibits greater pull-out resistance, decreased torque requirements and consequently greatly reduced failure from shearing of the screw during installation This significantly speeds installation, as it avoids re-drilling of pilot boreholes for installation of a replacement screw, and possibly total replacement of the anchor system involved

While the embodiments described above have a variation in the number of convolutions with W-cut grooves, the symmetrical or asymmetrical shape of the grooves and in the degree of pitch of the grooves and length of the intermediate grooves, those skilled in the art will readily appreciate that it is within the scope of the present invention to modify the numbers of W-cut convolutions, the geometry of the cuts and the degree of pitch or length, without departing from the scope of the invention as recited in the accompanying claims Substitution of elements from one described embodiment to another are also fully intended and contemplated It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto