FIXINGS INCLUDING SCREWS AND CLAMP PLATES

Abstract: A fixing for fixing a waterproof membrane 6 and an insu­ lation material 4 to a steel sheet 2 consists of a clamp plate 8 and a screw 10. The screw 10 comprises a head 11, a shank, and a drilling tip 14. The head 11 has a clamping face 12 which tapers towards the shank, and which has external ang­ les 30 in cross section. The shank has, in succession from the head 11 to the drilling tip 14, a first portion 16 which is unthreaded, a second portion 18 which is threaded, and a third portion 22 which is threaded. The diameter of the first portion 16 is approximately the root diameter of the second portion 18, and the crest diameter of the second portion 18 substantially exceeds the crest diameter of the third portion 22. The clamp plate 8 is formed from sheet metal and has a central hole 24. The periphery of the clamp plate 8 is of elongate symmertrical shape having a minor dimension which is several times the diameter of the hole 24 and a major dimension substantially greater than the minor dimension. The clamp plate is generally flat with a countersink 28 around the hole 24, and with two endless gutter-like deformations 34, 36, the first one 34 adjacent the periphery, and the second one 36 approximately midway between the hole 24 and the first one 34.

Description: Fixings Including Screws and Clamp Plates In a form of construction used currently for roofs which are flat, or nearly flat, a supporting structure is erected on which is carried corrugated steel sheet, usually with trapezoidal corrugations. On top of the sheet is laid a layer of insulation, and on top of that is laid a waterproof membrane. The membrane and insulation are held in position by fixings comprising a combination of a screw having a head, a shank and a drilling tip, the head including a clamping face which tapers towards the shank, and a clamp plate formed from sheet metal and including a central hole through which the shank of the screw passes and engages the sheet whilst the head of the screw engages the clamp plate to distribute the fixing force over the insulation. In one method of assembly, the insulation is laid first, and then the membrane is laid in a series of strips side-by-side, with each strip overlapping the previous one. As each strip is laid, the screws of the fixings are driven through the membrane and insulation adjacent the free edge of the strip, and then these screws, and their clamp plates, are covered by the overlapping edge of the next strip of membrane. In another method, the insulation is laid, and then screws of the fixings are driven through the insulation, each clamp plate having on its upper surface a material compatible with the material of the membrane. Just before each strip of membrane is laid, a solvent is applied to the upper surface of the relvant clamp plates, and the consequence is that the membrane becomes united with the upper surface of the clamp plates. A number of problems are being encountered with such constructions. When wind blows over the roof, it can cause vibration, and the effect of this vibration is to cause the screws to turn in a retrograde direction, so that they become loose, or even disconnected from the underlying sheet. Also, wind can cause movement of the membrane in the form of a train of waves passing across the roof. Such wave motion can cause current clamp plates to bend. With the first described method of assembly, a bent clamp plate can enable an opening in the membrane, formed by the screw, to stretch until the membrane pulls over the clamp plate and is thus totally released from the screw. From time to time, people walk on a flat roof, and the fixings should be capable of withstanding this. What can happen is that, if a person stands on part of a clamp plate, then the clamp plate is pressed downwards relative to the head of its associated screw. The material of the insulation is such that it does not restore the clamp plate elastically to its original position. Subsequently, therefore, the screw is no longer under tension, and thus it can more easily rotate in a retrograde direction, and also rock from side to side. Furthermore, since the head of the screw stands pround from the upper surface of the clamp plate it may punch a hole through the overlapped strip of membrane, whereupon local water leakage can occur. It is also desirable that the clamp plate should be so constructed that the tightening of the screw to the desired tension does not cause significant bending of the clamp plate. The present invention relates to fixings, screws and to clamp plates which have been developed principally for use in roofing structures of the kind described. They may be used in both the methods of assembly described in this specification. Thus a user needs to keep only a smaller variety of screws in stock. According to a first aspect of this invention such a fixing is characterised in that the clamping face of the screw includes external angles in cross section, and the shank has, in succession from the head to the drilling tip, a first portion, which is unthreaded, a second portion which is threaded, and a third portion which is threaded, the diameter of the first portion being substantially equal to the root diameter of the second portion, and the crest diameter of the second portion substantially exceeding the crest diameter of the third portion; and in that the periphery of the clamp plate has an elongate symmetrical shape with a minor dimension which is several times the diameter of the hole and a major dimension substantially greater than the. minor dimension, and in that the clamp plate is generally flat with a countersink around the hole, and with two endless gutter-like deformations a first one adjacent the periphery, and a second one approximately midway between the hole and the first one. According to a second aspect of this invention the clamping face of a screw includes external angles in cross section, and the shank has, in succesion from the head to the drilling tip, a first portion which is unthreaded, a second portion which is threaded, and a third portion which is threaded, the diameter of the first portion being substantially equal to the root diameter of the second portion, and the crest diameter of the second portion substantially exceeding the crest diameter of the third portion. According to a third aspect of this invention such a clamp plate is characterised in that the periphery of the clamp plate has an elongate symmetrical shape with a minor dimension which is several times the diameter of the hole and a major dimension substantially greater than the minor dimension, and in that the clamp plate is generally flat with a countersink around the hole, and with two endless gutter-like deformations a first one adjacent the periphery, and a second one approximately midway between the hole and the first one. Preferably the external angles are present by virtue of the clamping face of the screw having a basic frustoconical shape, from which project a plurality of ribs which are elongate in directions from the top of the head to the shank. As one alternative, the basic shape of the clamping face may be a truncated pyramid for example one with six angles. The countersink in the clamp plate may be of frustoconcial shape or may be non-circular in shape, i.e. be prepared with a shape complementary to a screw head which tapers towards the screw shank, and which has external angles in cross section. In the first case when the clamping face of the screw includes external angles these dig into the countersink to prevent retrograde rotation of the screw with respect to the clamp plate and in the second case since the clamping face of the screw and the countersink of the clamp plate are both non-circular again retrograde rotation of the screw with respect to the clamp plate is prevented One example of a fixing, a screw and a clamp plate, in accordance with this invention, together with some modifications, will now be described with reference to the accompanying drawings, in which: : Figure 1 is a vertical section through the axis of a screw and clamp plate in position in a fragment of roofing structure; Figure 2 is a side elevation of the screw on a larger scale; Figure 3 is an axial view of the screw from below; Figure 4 is a plan of the clamp plate on the same scale as Figure 1; and, Figures 5 and 6 are sections on the lines V-V and VI-VI in Figure 4; and, Figure 7 is a section similar to Figure 6 but of a modified clamp plate. Figure 1 shows at 2 the crest of one corrugation of a steel sheet forming part of a roofing structure. 4 indicates a layer of insulation and 6 indicates a portion of a waterproof membrane. The membrane 6 and insulation 4 are clamped to the sheet 2 by means of a large clamp plate 8, and screw 10 which is in tension between the clamp plate 8 and the sheet 2. As shown on a larger scale in Figure 2, the screw 10 has a countersunk head 11 having a frustoconical clamping face 12, a shank, and a drilling tip 14. The shank has, in succession from the head 11 to the drilling tip 14, a first portion 16 which is unthreaded, a second portion 18 which is threaded, a third portion 20 which is unthreaded, and a fourth portion 22 which is threaded. There is a central hole 24 in the clamp plate 8, through which the crests of the threaded portion 22 can pass with slight clearance. In use, after the insulation and a strip of membrane have been laid, the clamp plate 8 is placed in the desired position, and the tip 14 of the screw is inserted in the hole 24 in the clamp plate. Then the screw is driven downwards through the membrane and the insulation until the drilling tip 14 reaches the metal sheet 2. Continued driving of the screw first causes the tip 14 to drill a hole through the sheet 2, and then causes the tapered lower end of the threaded portion 22 to enter the hole in the sheet 2, and form a co-operating thread, so that the screw is then drawn downwards until the head 11 engages the clamp plate 8, as described more fully below. The screw can be driven by a known driver having means for disengaging when the screw has attained the desired position, being then in tension between the clamp plate 8 and the sheet 2, so that the membrane and insulation are clamped. The portion 22 has, as shown, a buttress thread, which gives a high pull-out load in the sheet 2. The unthreaded portion 20 is simply present to avoid the expense of forming unnecessary turns of thread on the screw, while nevertheless the portion 22 is made of sufficient length to enable the screw to be used with more than one thickness of insulation 4. For use with different thicknesses of insulation 4, screws can be made of different total lengths, but the lengths of the threaded portions 18 and 22 may be unchanged. The threaded portion 18 has a crest diameter which is substantially larger than the internal diameter of the hole 24 in the clamp plate. The thread 18 has a tapered entering turn 26, which serves to form a co-operating internal thread in the clamp plate, and the final stage of driving the screw causes the threaded portion 18 to pass entirely through the clamp plate, until the periphery of the hole in the washer is level with the unthreaded portion 16. The axial extent of the unthreaded portion 16 is preferably matched to the dimension of the clamp plate, so that, in the final condition, the clamp plate is trapped between the head 12 and the thread 18, with substantially no freedom of relative axial movement. In this final condition, the crest diameter of the thread 18 is sufficient to provide a support to the clamp plate against axially-directed forces. Consequently, if a person stands on the clamp plate, the centre of the clamp plate is not displaced relatively to the screw. As shown in Figure 1, and also in Figure 5, the hole 24 in the clamp plate is surrounded by a countersink 28, to match the shape of the countersunk head 12 of the screw. As shown in Figure 5, the upper surface of the countersink is smooth. On the co-operating surface of the head 12, there are six radially extending, downwardly projecting ribs 30, of V-section. During the final tightening of the screw, these ribs bite into the surface of the countersink 28. Consequently, in the assembled condition, there is interengagement between the ribs 30 and the deformations in the surface of the countersink 28, this interengagement resisting any tendency for the screw to turn in a retrograde direction when the roof is subject to vibration by wind or other causes. If desired, as indicated in Figures 4 and 6, the countersink 28 may be prepared with a series of shallow radial grooves, to co-operate with the ribs 30 on the screw head. As other alternatives, the screw head 11 may have another non-circular shape, such as hexagonal, with or without a corresponding prepared shape in the countersink in the clamp plate. The clamp plate may be modified to include a short cylindrical portion 32 below the countersink 28, as shown in Figure 7. This can serve to strengthen the periphery of the hole in the clamp plate. In this case the axial extent of the unthreaded portion 16 of the screw should be chosen having regard to the axial extent of the portion 32 of the clamp plate. In the example shown, the screw has an overall length of 65 mm, and the crest diameters of the portions 22 and 18 are 5.33 mm and 6.35 mm respectively. The clamp plate is of metal with a thickness 0.8 mm, and the diameter of the hole 24 is 5.1 mm. As shown in Figure 4, the clamp plate is of elongate symmetrical shape having a minor dimension of 40 mm and a major dimension of 80 mm. The ends are eliptical, while the sides are straight. The clamp plate is generally flat save for the countersink 28, and two endless gutter-like deformations 34 and 36. The deformation 34 runs adjacent to the periphery of the clamp plate, while the deformation 36 runs approximately midway between the hole 24 and the deformation 34. In cross section, as shown in Figures 5 and 6, the deformations are of equal depth, but the deformation 36 is narrower than the deformation 34. Both cross sections have rounded bottoms. These deformations impart considerable rigidity to the clamp plate, and ensure that the downward clamping force exerted by the screw head 12 on the countersink 28 of the clamp plate is distributed to the membrane 6 and insulation 4 over the whole area of the clamp plate. Furthermore, the clamp plate is resistant to forces applied to it by wave motion of the membrane. If there is ever need to remove the screw intentionally, this can be done by engaging a driver with the screw head, and applying anti-clockwise rotation.

Claims: CLA IMS 1. A fixing comprising a screw (10) including a head (11), a shank (10), and a drilling tip (14), the head having a clamping face (12) which tapers towards the shank, and a clamp plate (8) formed from sheet natal and having a central hole (24) characterised in that the clamping face (12) of the screw includes external angles (30) in cross section, and the shank has, in succession from the head (11) to the drilling tip (14), a first portion (16), which is unthreaded, a second portion (18) which is threaded, and a third portion (22) which is threaded, the diameter of the first portion (16) being substantially equal to the root diameter of the second portion (18), and the crest diameter of the second portion (18) substantially exceeding the crest diameter of the third portion (22); and in that the periphery of the clamp plate (8) has an elongate symmetrical shape with a minor dimension which is several times the diameter of the hole (24) and a major dimension substantially greater than the minor dimension, and in that the clamp plate is generally flat with a countersink (28) around the hole (24), and with two endless gutter-like deformations (34, 36), a first one (34) adjacent the periphery, and a second one (36) approximately midway between the hole (24) and the first one (34). 2. A fixing according to claim 1, in which the external angles are present on the screw by virtue of the clamping face (12) having a basic frustoconical shape, from which project a plurality of ribs (30) which are elongate in directions from the top of the head (11) to the shank of the screw, and in which the countersink (28) is frustoconical in shape. 3. A fixing according to claim 1, in which the basic shape of the clamping face (12) of the screw is a truncated pyramid with six angles, and in which the clamp plate has a countersink (28) which is non-circular in shape and is prepared with a shape which complements that of the screw head (ill). 4. A fixing according to any one of the preceding claims, in which the clamp plate (8) includes a cylindrical portion (32) surrounding the hole (24). 5. A screw comprising a head (11), a shank, a drilling tip (14), the head having a clamping face (12) which tapers towards the shank, characterised in that the clamping face (12) includes external angles (30) in cross section, and the shank has, in succesion from the head (11) to the drilling tip (14), a first portion (16) which is unthreaded, a second portion (18) which is threaded, and a third portion (22) which is threaded, the diameter of the first portion (16) being substantially equal to the root diameter of the second portion (18), and the crest diameter of the second portion (18) substantially exceeding the crest diameter of the third portion (22). 6. A screw according to claim 5, in which the external angles are present by virtue of the clamping face (12) having a basic frustoconical shape, from which project a plurality of ribs (30) which are elongate in directions from the top of the head (11) to the shank. 7. A screw according to claim 5, in which the basic shape of the clamping face (12) is a truncated pyramid with six angles. 8. A clamp plate (8) formed from sheet metal and having a central hole (24), characterised in that the periphery of the clamp plate has an elongate symmetrical shape with a minor dimension which is several times the diameter of the hole (24) and a major dimension substantially greater than the minor dimension, and in that the clamp plate is generally flat with a countersink (28) around the hole (24), and with two endless gutter-like deformations (34, 36) a first one (34) adjacent the periphery, and a second one (36) approximately midway between the hole (24) and the first one (34). 9. A clamp plate according to claim 8, in which the countersink (28) is. non-circular in shape. 10. A clamp plate according to claim 8 or 9, in which the clamp plate (8) includes a cylindrical portion (32) surrounding the hole (24).

Inventors: HEWISON, GEORGE DAVID
Applicants: ITW LIMITED
Application year: 1984
Main classification: F16B35
IPC codes: E04D5, F16B35, F16B5, E04D3, F16B43, F16B39