Providing firm and secure screw fastenings in various materials may often present a problem. In particular, some screw fastenings weaken if it becomes necessary to unscrew or pull the screw out of the original fastening and reuse the same screw hole.

This particularly applies to chipboard materials, in which small wood chips have been glued and pressed together to form a relatively hard, compact material much used in house building. In addition, some furniture, and particularly bookshelves are made from various chip materials. The weak screw fastenings are a common problem when the piece of furniture or the bookshelf is to be reassembled after first having been disassembled. Such screw holes become seriously weakened and often useless after being used several times.

As regards screw fastenings in concrete and more porous building materials such as breeze blocks, the fastening is often based solely on friction forces between the screw thread and the actual screw hole. The fastening may be weakened if the screw is subjected to high loads.

There are many known designs of screws and fastening devices, but what many of them have in common is the fact that they do not provide proper fastening.

The present invention provides a device for fastening in various building materials such as wood, chipboard, plaster, brick, breeze blocks, concrete and similar, where use is made of a predrilled hole, characterised in that it comprises a screw with a screw thread and a head, a cylindrical taper section and a nut with fastening devices.

The invention will be explained in greater detail by means of drawings, in which Fig. 1 shows a common drill for drilling a hole;

Fig. 2 shows the drilled hole with the fastening device of the invention; Fig. 3 shows the fastening device screwed into the hole; and Fig. 4 shows the use of an alternative screw head.

Figure 1 shows a section through a material 13 in which is drilled a hole 14. The material 13 may be wood, chip material, brick, concrete, plaster, breeze blocks or another building material. The hole 14 has been made by means of a conventional twist drill 13 suitable for drilling in one of the above materials.

Figure 2 shows the fastening device 10 according to the invention prior to mounting in the hole 14. The fastening device consists of a screw section 15 with a screw thread 16.

The thread may extend along the entire screw section, but preferably, the thread covers approximately half the length. The fastening device has a head 14 that may be in the form of a nut, a hexagon key or screw driver head. A loose washer 18, preferably circular, has been inserted between the head and the material 13 to be fastened, which washer provides a larger fitting surface around the actual screw hole 14. Further, the fastening device consists of a taper tube 17 that is threaded onto the screw section 15 and possibly partly over the screw thread 16. The taper tube may be made from a suitable material such as electrogalvanised steel, galvanised brass or similar. The taper tube 17 is threaded onto the fastening device before the entire assembly is placed in the drilled hole. The actual taper is found along the lower edge of the taper tube.

The fastening device further comprises a nut 20 with wings 21 that are gradually fastened and anchored near the bottom of the drilled hole 14. The nut 20 is circular and has a plurality of wings 21 extending from an upper edge of the nut. As shown, the wings 21 are split, and the number of wings can range from four to eight. The wings 21 are preferably of a length that corresponds to the height of the nut 20; however nuts that are to be used in porous or loose materials may have longer wings. The wings 21 are graded or sharpened at the top 24 in order to facilitate engagement with the material 13.

The wings have been deflected slightly outward at the top 24, and provide a slight pressure against the wall of the drilled hole. This deflection ensures that the nut is not rotated before the wings have been screwed into the material. The transition between the wings and the nut contains a weakening or bending zone 22 in order to further

ensure that the wings 21 are screwed into the material. The nut 20 with the wings 21 has normally been screwed onto the screw thread 16 when the fastening device is inserted into the hole.

Figure 3 shows the complete fastening device 10 screwed into a hole. The nut 20 has been tightened, and the wings 21 will, as a result of the screwing and the pressure from the taper tube 17, be forced into the material and thereby provide a solid fastening and a durable anchor in the material. Due to their pointed shape, the wings 21 will, upon rotation of the screw, gradually be forced up and outward and cut into the material 13.

Experiments have shown that the wings penetrate the material without destroying it.

The taper tube 17 may as shown in the figure remain on the screw 15,16 in order to provide additional stability. However, depending on the material in which the hole has been drilled, the taper tube may be removed after the nut 20 has been fastened. This is accomplished by unscrewing the device, removing the taper tube and refastening the screw by making use of the nut 20 that is anchored to the material walls by its wings 21.

Figure 4 shows the fastening device with an alternative embodiment of the actual screw head 14. Here, a plank or board 25 is fixed by means of the device of the invention. The screw head 14 is rounded, and is fastened by means of a hexagon key. Alternatively, the screw head may have a fastening or slot for various types of screwdrivers. The taper tube 17 has been removed from the screw after the nut with the wings has been anchored in the hole.

The device according to the invention may be manufactured in different materials and sizes according to the area of use.