Screw threaded fastener and fastening system

Abstract: A screw threaded fastener (103) has an elongate shank (122) with a drilling tip (104) at one end and a head (123) at the other end. A screw thread extends along at least a part of the length of the shank. The trailing flank (116) of the thread subtends an angle relative to the axis of the fastener which approaches 90° and is greater than the angle subtended by the leading flank (115). The screw is for use in fixing sheets of material to thin metal battens (5) and the angle between the trailing flank (116) and the adjacent thin metal section (18) of the batten, is reduced in comparison to a conventional thread form, which lessens the distorting effect of the thread on the batten under relatively high pull-out loads.

Description: This invention relates to the fastening of material to thin metal structures. It will be convenient to hereinafter describe the invention with particular reference to the fixing of roofing or cladding material to battens of thin material section, but the invention has wider application. The invention is concerned with fastening systems of the foregoing kind and is also concerned with a fastening screw for use in such a system. In the context of this specification the expression "thin metal" is to be understood as referring to metal having a thickness not significantly greater than one millimetre, and more generally in the range 0.4 to 0.8 mm. It is common practice to secure metal roofing material to thin metal battens by means of self-tapping threaded fasteners of the kind having a drilling tip. Selection of such a fastener from the available range of such fasteners is largely determined by the speed with which the fastening operation can be achieved, but the security or strength of the fastened assembly may be compromised in the process. Fastenings of the aforementioned kind have been found to fail particularly in situations involving relatively high wind loading. It has been discovered that such failures are due at least in part to the distorting effect of the thread form of conventional fasteners on the thin metal battens. The article "Schraube mit Spezialgewinde zum Verbinden von Kunststoffbauteilen" by Richard Bergner (Kunststoffe vol. 74, no. 7, July 1984, page 413) discloses a screw having a thread profiled to minimise tearing of plastic when installed. Each flank of the thread has two unequal radii of curvature such that, near the peak of the thread, the flank is at approximately 30° to the radial direction of the screw, and the angle increases to 90° where the thread joins the shank of the screw. EP-A-0142037 discloses a screw having a thread wherein, near the head, each flank of the thread is formed from two differently angled surfaces, the surface adjoining the shank being at a lesser angle to the longitudinal axis of the screw than the surface adjoining the peak of the thread. According to a first aspect of the invention a screw threaded fastener includes, an elongate shank having a longitudinal axis, a drilling tip at one end of the shank and a head at the opposite end, and a screw thread (X) extending over at least a part of the length of the shank, wherein the screw thread (X) includes a leading flank having an angular disposition relative to the longitudinal axis whereby it slopes outwardly relative to the longitudinal axis in a direction away from the drilling tip and a trailing flank directed outwardly relative to the longitudinal axis; characterised in that the trailing flank is at a greater angular disposition relative to the longitudinal axis to that of the leading flank, and in that the angular disposition of the trailing flank is in a range from 80 °to 90°. A screw threaded fastener according to the invention has the angle subtended between the longitudinal axis of the screw and the trailing flank of the screw thread greater than the corresponding angle subtended between said longitudinal axis and the leading flank of the screw thread and wherein the trailing flank angle is closer to 90° than is the leading flank angle. This angular disposition of the trailing flank relative to the leading flank of the screw thread, in use of the fastener, reduces the angle between the trailing flank and the adjacent thin metal section of a batten surrounding the fastener. This has a lesser distorting effect on the thin metal batten under pull out loads on the fastener to that produced by the screw thread of a conventional fastener wherein the leading and trailing flanks slope at substantially the same angle relative to the longitudinal axis of the fastener. In an example arrangement according to the invention the trailing flank angle is substantially 83° as compared with substantially 60° for the leading flank angle. In each case the angle is the inner subtended angle of the relevant flank so that in the example referred to the angle subtended between the two flanks is substantially 37°. It will be appreciated from the foregoing that in the example referred to the trailing flank slopes outwardly from the fastener axis in a direction towards the tip of the fastener shank. It is preferred that the fastener includes a drilling portion which terminates at the drilling tip and which has two flutes formed therein on diametrically opposite sides of the fastener axis, wherein each flute extends generally in the direction of the longitudinal axis of the fastener. The terminal end of the fastener shank is formed by the tip of the drilling portion, and that tip preferably comprises two terminal end faces each of which slopes outwardly from the fastener axis in a direction towards the head of the fastener. The maximum cross sectional dimension of the drilling portion, which portion may be substantially cylindrical, may be selected to suit requirements, but it is preferably substantially equal to the root diameter of the thread of the fastener. In some circumstances the aforementioned dimension could be greater or less than the root diameter. The threaded portion of the screw fastener extends over a suitable part of the shank length and is located between the drilling portion and the head of the fastener. It is preferred that a section of the threaded portion adjacent the drilling portion is tapered so that the crest diameter of the thread progressively increases from a minimum at the end of the threaded portion adjacent the tip portion, to a maximum at a location towards the head of the fastener. According to a second aspect of the invention a fastening system includes, at least one thin metal batten having a support wall, a sheet of roofing or cladding material, and at least one screw threaded fastener in accordance with the first aspect of this invention, wherein said fastener extends through said sheet and its screw thread is threadably engaged in the support wall of the batten thereby to attach the sheet to the batten, wherein the trailing flank of the thread extends outwardly approximately parallel to an adjacent surface of the support wall of the batten surrounding the fastener, whereby said trailing flank subtends an angle relative to said adjacent surface of no more than about 10°. The screw threaded fastener having a modified thread which enables relatively rapid fastening of one member to another and provides a secure fastening between those members which is highly resistant to failure. A particular example of the invention will now be described and contrasted with the prior art with reference to the accompanying drawings in which:- Figure 1 illustrates a typical prior art fastening system; Figure 2 is an end view of two overlapped thin metal battens for explaining a problem which fasteners according to the invention can overcome; Figure 3 diagrammatically illustrates the distortion in a thin metal batten of a failed assembly of a prior art fastener; Figure 4 is a partial sectional view of a prior art fastener thread form in a thin metal batten; Figure 5 is a similar view to Figure 4 illustrating how the prior art threadform can cause the distortion that leads to failure under high pull-out loads; Figure 6 is a partial sectional view of an example fastener according to the invention in a thin metal batten; Figure 7 is an elevational cross sectional view of an example thread form according to the invention; Figure 8 shows an example fastener according to the invention; and, Figure 9 is a plan view from the drilling tip end of an example fastener according to the invention. Figure 1 illustrates a typical fastening system in which a sheet 1 of roofing material is secured to a thin metal batten 2 by means of a conventional self-tapping screw threaded fastener 3 having a drilling tip 4. The batten 2 is of a typical construction having a body of inverted channel configuration comprising a top wall 5 and a pair of spaced side walls 6. A laterally extending flange 7 is provided along the lower edge of each side wall 6 and, as shown, it is usual to turn back the outer edge 8 of the flange 7 for strengthening purposes. As is well known, sheets of roofing material are generally corrugated or otherwise formed to have alternating ribs and valleys of various shapes. The roofing sheet 1 shown in Figure 1 is of that construction and as shown the fastener 3 passes through the upper wall 9 of a rib 10 whereas the lower wall 11 of a valley bears against the batten top (support) wall 5. Battens 2 are generally supplied in standard lengths and it will occur that two or more battens are required to span a required distance. In such cases it is usual practice to overlap adjacent ends of two battens 2 as shown diagrammatically by Figure 2. As will be apparent from Figure 2 the double thickness of the flanges 7 prevents the top wall 5 of the upper most batten 2 engaging the top wall 5 of the lowermost batten 2, and a gap G is thereby formed between the walls 5. The gap G can create a problem when a fastener 3 is Being driven into the battens 2 at the region of the overlap. It will sometimes happen that the fastener 3 enters into threaded engagement with the uppermost wall 5 before the tip 4 penetrates into or through the lowermost wall 5, and in that event the uppermost wall 5 will be forced upwards relative to the lowermost wall 5 thereby increasing the size of the gap G and adversely affecting the appearance of the roof construction. Quite separate from the foregoing problem however, there is a problem of inadequate pull-out resistance in conventional assemblies of the general kind shown by Figure 1, and that problem exists both outside and inside overlaps as referred to above. Inspection of failed assemblies has revealed a condition of the kind diagrammatically illustrated by Figure 3. That figure shows part of the top or support wall 5 of a batten 2 and the hole 12 which is formed through the wall 5 by the drilling tip 4 of a fastener 3. The hole 12 is formed so as to have a diameter which is related to the thread of the fastener 3, and is typically of substantially the same diameter at either the root diameter or the pitch diameter of the fastener thread. As shown by Figure 3 the section 13 of the wall 5 which surrounds the hole 12 tends to lift and fracture at a number of zones 14 with the result that the fastener 3 no longer secures the roofing material 1 to the batten 2. Experiments have revealed that such failure of the fastening is due, at least in part, to the thread form used with conventional fasteners 3. An example of that thread form is shown by Figures 4 and 5, and it is a characteristic of that thread form that the leading and trailing flanks 15 and 16 respectively of the thread slope at substantially the same angle relative to the fastener axis 17. When an upward pull is applied to the fastener 3, for example as a result of high wind loading, the trailing flank 16 presses against the under surface 18 of the wall 5 and that is thought to be the cause of the upward distortion of the wall section 13. Tests have indicated that such upward distortion promotes the fracture at the zones 14 as shown by Figure 3, and also indicate that the relatively large angle A subtended between the thread flank 16 and the wall surface 18 is a major cause of the problem. It is generally the case that the angle A is approximately 30°. It is against the foregoing background that the thread forms shown by Figures 6 to 8 have been developed. For convenience of comparison parts of a fastener 103 shown by Figures 6 to 9 which correspond to parts of the conventional fastener 3 will be given like reference numerals except that those numerals will be in the number series 100 to 199. A significant feature of the thread forms shown by Figures 6 to 8 is the relatively "flat" (approximately 90°) disposition of the trailing flank 116 by comparison with the leading flank 115. In the particular example shown in Figure 7 the angle B subtended between the trailing flank 116 and the fastener axis 117 is approximately 83°, whereas the corresponding angle C subtended by the leading flank 115 is approximately 60°. That particular thread form is suited to manufacture by means of a thread rolling technique, and other different but nevertheless satisfactory thread forms could be produced by use of other techniques, such as thread cutting. Regardless of how the thread is formed it is preferred that the angle B is closer to 90° than is the angle C, and in some circumstances angle B could be 90°. In general however, it is preferred that angle B is in the range 80° to 90° so as to minimise angle A as shown by Figure 4. Tests conducted with fasteners having the thread form of Figure 7 reveal a substantial improvement over fasteners having a conventional thread form, such as the thread form shown by Figures 4 and 5. Resistance to pullout under a static load is 30% greater than that of conventional fasteners, and the improvement is as high as 200% in the case of a cyclic load intended to simulate strong or cyclonic wind conditions. It is preferred that the drilling portion 104 of the fastener 103 is of the kind having a substantially cylindrical cross sectional shape and having two diametrically opposed flutes 119, each of which extends generally in the axial direction of the fastener 103. The terminal end of the tip 104 may be formed by two sloping end faces 120 and a substantially straight cutting edge 121 is formed at the junction between each face 120 and a respective one of the flutes 119. Drill tips of that general configuration are known. In the particular example shown by Figure 8 the fastener 103 has an elongate shank 122 and a head 123 at the end of that shank remote from the drill tip 104. It will be appreciated that the head 123 can be of a form distinctly different to that shown by Figure 8. The shank 122 as shown is formed of three major portions, a threaded portion X, a drilling portion Y and a head portion Z, but it is to be understood that the head portion Z could be omitted so that the thread extends up to the head 123. Assuming the unthreaded head portion Z is provided it can be of any suitable length, and will usually have a diameter substantially the same as the root diameter 124 of the thread (see Figure 7). It is preferred, as shown, that the threaded portion X is tapered over part of its length adjacent the drilling portion Y. That is, the thread crest diameter in the section marked "T" in Figure 8 progressively increases from a minimum adjacent the drilling portion Y to a maximum remote from the drilling portion Y. The minimum diameter could be substantially zero thread height, and the maximum is preferably the standard crest diameter of the thread in the remainder of the shank portion X. Such a tapered arrangement promotes proper co-action between the threaded portion X and the hole 12 formed by the drill tip 104. It is further preferred that the drilling portion Y is relatively long so as to cope with the existence of a relatively large gap G as previously discussed with reference to Figure 2. A long drilling portion Y increases the probability that a hole will be formed through the lowermost wall 5 of overlapped battens 2 before the threaded portion X enters the hole previously formed in the uppermost wall 5 (Figure 2). By way of example, a fastener having a thread crest diameter (standard or maximum) of approximately 6 millimetres may have a drilling portion approximately 12 millimetres in length, but other crest diameter to length relationships could be adopted. The flutes 119 need not extend the full length of the drilling portion Y. In an example fastener according to the invention each flute 119 has a length in the range of two thirds to three quarters the length of the drilling portion Y. It will be appreciated from the foregoing description that a fastener incorporating the features of the invention provides a major advantage over conventional fasteners of the same general kind. The significantly improved resistance to pull-out is a very important benefit, and that benefit is augmented by the fact that the particular fastener described is easy to use and enables rapid fastening of the members with which it is associated. A fastening system involving use of the fastener is accordingly a major improvement over conventional systems.

Claims: A screw threaded fastener including, an elongate shank (122) having a longitudinal axis, a drilling tip (104) at one end of the shank (122) and a head (123) at the opposite end, and a screw thread (X) extending over at least a part of the length of the shank (122), wherein the screw thread (X) includes a leading flank (115) having an angular disposition relative to the longitudinal axis whereby it slopes outwardly relative to the longitudinal axis in a direction away from the drilling tip (104) and a trailing flank (116) directed outwardly relative to the longitudinal axis; characterised in that the trailing flank (116) is at a greater angular disposition relative to the longitudinal axis to that of the leading flank (115), and in that the angular disposition of the trailing flank (116) is in a range from 80 °to 90°. A screw threaded fastener according to claim 1 wherein the angular disposition of the trailing flank (116) relative to the longitudinal axis is at an angle in a range from 83° to 90°. A screw threaded fastener according to claim 2, wherein the angular disposition of the trailing flank (116) relative to the longitudinal axis is substantially 90° or wherein the angular disposition of the trailing flank (116) relative to the longitudinal axis is substantially 83° and the angular disposition of the leading flank (115) relative to the longitudinal axis is substantially 60°. A screw threaded fastener according to any one of the preceding claims, wherein the shank (122) includes a drilling portion which terminates at the drilling tip (107), wherein the drilling portion has two flutes (119) formed therein on diametrically opposite sides of the longitudinal axis, wherein each flute (119) extends from the drilling tip (104) generally in the direction of the longitudinal axis, and wherein the drilling tip comprises two end faces (120), and, wherein each end face (120) slopes outwardly from the longitudinal axis in a direction towards the head (127) of the fastener. A screw threaded fastener according to claim 4, wherein the drilling portion has a maximum cross sectional dimension which is substantially equal to a root diameter of the screw thread (X) of the fastener and preferably is substantially cylindrical. A screw threaded fastener according to claim 4 or 5, wherein the flutes (119) in the drilling portion have a length which is in the range of two thirds to three quarters of the length of the drilling portion. A screw threaded fastener according to any one of claims 4, 5 or 6, wherein the drilling portion has a length which is up to substantially twice the maximum crest diameter of the thread (X). A screw threaded fastener according to any one of the preceding claims, wherein the screw thread (X) is tapered over a part of the length of the shank (122) adjacent the drilling tip (104) whereby a crest diameter of the screw thread progressively increases from a minimum to a maximum value towards the head of the fastener and preferably the minimum crest diameter is of substantially zero thread height. A fastening system including, at least one thin metal batten (2) having a support wall (5), a sheet of roofing or cladding material (1), and at least one screw threaded fastener (103) in accordance with any of the preceding claims, wherein said fastener (103) extends through said sheet (1) and its screw thread (X) is threadably engaged in the support wall (5) of the batten (2) thereby to attach the sheet (1) to the batten (5), wherein the trailing flank (116) of the thread extends outwardly approximately parallel to an adjacent surface of the support wall (5) of the batten (2) surrounding the fastener (103), and whereby said trailing flank (116) subtends an angle relative to said adjacent surface of no more than about 10°, or is substantially 7° or is between 0° and 7°. A fastening system as claimed in claim 9, which includes a second thin metal batten (2) having an end which overlaps an end of said at least one batten (2) whereby a gap (G) exists between corresponding support walls (5) of the overlapped battens (2), wherein said screw threaded fastener (103) is threadably engaged in each said support wall (5) of the overlapped battens (2) and wherein the shank (122) of the screw threaded fastener (103) includes a drilling portion which terminates at the drilling tip (104), wherein the drilling portion has a length which is greater than said gap (G).

Inventors: DONEDDU MARK, ROBERTS DAVID JAMES ALEXANDER, DONEDDU, MARK, ROBERTS, DAVID JAMES ALEXANDER
Applicants: DEUTSHER PTY LTD, W.A. DEUTSHER PTY. LTD.
Application year: 1998
Main classification: F16B25
IPC codes: E04D3, E04B1, F16B5, F16B25