Patent Description:
Mechanical fastening devices are well known in building, construction and other industries. Some of them are limited to special types of material in which they are applied. Others request quite demanding operations in order to install the fastening device. Yet others offer only very limited bearing capability.

<CIT> discloses a plug with a tapered core and a central bore for receiving a fastener. A tapered trapezoidal thread and a tip give the plug a self-tapping effect. The plug has a head that is larger than the largest thread to stop the penetration into the underlying material. The plug is made of plastic by injection molding and has a recess which is arranged to receive a head for the fastening means.

<CIT> discloses an expansion plug made of plastic with a central hole. Such expansion plugs are known in many different shapes and embodiments. The expansion plug can have external trapezoidal threads and be used in pre-drilled holes to fasten a screw in the central hole. The central hole is arranged with threads that taper towards the tip of the expansion plug.

<CIT> relates to a plate anchor used to mount and attach various materials to a wall consisting of a fragile material. The plate anchor is self-tapping and is screwed directly into the wall without the need for a borehole. The plate anchor is designed as a screw with external threads and has a drilling part at one end and a flange part at the other end. The plate anchor is arranged with a central hole with internal threads and with a slot. A fastener can be bolted into the plate anchor.

<CIT> discloses a hole screw which is tapered from the head to the tip and which has external threads. The hole screw has a slot in the middle that allows the fastening of a bolt or the like.

<CIT> discloses a hollow screw with tapered threads which are arranged at an acute angle around the main shaft. The screw is not tapered from the flange part towards the end which first meets the underlying material and into which the screw is to be inserted. The hole is arranged centrally in the screw and allows the fastening of a bolt or the like.

<CIT> relates to a holding pin of plastic material which resembles a dowel and which is particularly suitable for attaching furniture fittings to furniture articles. The holding pin can be mounted by pressing or driving it into a bore. The holding pin has an elongate design and from its outer surface extends radially a number of projecting ribs which are arranged in the longitudinal direction of the holding pin. The diameter of the ribs increases from the head of the holding pin to the tip. A hole is arranged centrally in the holding pin and allows the fastening of a screw or the like. The hole is tapered from the head towards the tip of the retaining pin.

<CIT> relates to a hanger made of a strand of material formed into a plurality of coils of stepwise decreasing diameter to make a conical hollow spiral. The hanger is designed with a hook or other engagement at one end of the spiral. The coil is connected by shape or frictional connection to an opening in a selected substrate to provide a receptacle for securing a selected object.

<CIT> discloses a fastener comprising an anchor bolt and a nut. The nut comprises a rearward nut section, a forward nut section located closer to an anchor portion of the anchor bolt than the rearward nut section, and an outside shoulder for load transfer into the nut, which is located between the forward nut section and the rearward nut section and which faces the anchor portion of the anchor bolt, wherein an internal thread of the nut extends at least in the forward nut section and the internal thread of the nut threadedly engages an external thread of a threaded rod of the anchor bolt at least in the forward nut section.

<CIT> discloses a self-tapping threaded insert for threadedly mounting another component by a threaded central connector. An assembly of the insert and a mounting member for threadedly mounting the insert are assembled by a method for mounting the self-tapping threaded insert. The insert has a threaded shank that is threaded into a partially tapped hole and has a shank end with flutes of a construction that provides self-tapping. Upon final torqueing, a locking flange of the insert locks against a surface of the member as threads of the insert shank are completely engaged with the hole.

<CIT> discloses a threaded bushing for fiber-reinforced plastic.

There is a need for fastening devices which can be easily installed and which offer high bearing capability. There is particularly a need for fastening devices which can be installed from one side of an arrangement without the requirement of action such as fastening nut locks on the opposite side of the arrangement.

It is therefore an objective of the present invention to provide an improved mechanical fastening device for plastic materials.

It is a further objective to provide a method for applying an improved mechanical fastening device in plastic materials.

It is still a further objective to provide articles comprising plastic material with improved fastening device.

The above mentioned objectives are achieved by a fastening device as defined by claim <NUM>.

According to another aspect, the present invention concerns a method as defined by claim <NUM>.

Yet another aspect the present invention concerns articles as defined by claim <NUM>.

Preferred embodiments of the different aspects of the invention are disclosed by the dependent claims.

The present invention relates to an assembly of a fastening device and a plastic material, wherein the fastening device is intended to be introduced into the plastic material, where it as an anchor provides excellent fastening for a utility. The fastening device comprises a threaded bush, typically metallic, which is provided with a coarse thread on the outside of the bush. The bush will have a flexibility less than the flexibility of the plastic material. On the hollow inside of the bush a fine thread in the shape of a standard metric thread is arranged. Any kind of utility may be screwed into the standard metric thread. In order to introduce the fastening device into the plastic material threads are cut into the plastic material with a tapered tool which preferably comprises tapered threads. A cavity in the plastic material comprising internal threads is created. During introduction of the fastening device into the plastic material, the elasticity of the plastic material is of the essence. The coarse thread on the outside of the bush, which is introduced into the cavity in the plastic material, is complementary to the thread in the cavity of the plastic material. The distance within the groove of the thread on the outside of the bush is comparable to the distance within the groove of the thread on the inside of the cavity, which has about the same size and preferably a slightly smaller size than the distance within the groove of the thread on the outside of the bush. When the fastening device, the bush is introduced into the cavity the distance within the groove of the complementary thread is increased. Once the bush is located screwed into the cavity of the plastic material the coarse thread on the outside of the bush and the complementary thread of the cavity of the plastic material are engaged by compression and/or friction. The distance within the thread groove in the cavity is increased by the thread grooves on the outside of the bush engaging the thread grooves in the cavity. The increase in the distance then screwing the bush must take place within the elasticity of the plastic material in order to avoid the plastic material being exposed to crack formation. Elasticity of different plastic materials according to the invention can be compared, among other things, by measuring the elongation at break of the plastic materials with standardized test methods.

From the following discussion, it may seem that the fastening device and the bush is one and the same thing. This is more or less correct, the bush may be considered to be the physical form of the present invention, but its interaction with the plastic material is also significant.

The desired immobility of the fastening device, when attached, is provided by friction forces, which apply when the bush is introduced into the cavity and when it is tried to be loosen or removed. It is an advantage of the present invention that the introduction of the bush is facilitated by the elasticity of the plastic material, whereas loosening is hindered by forces which originate from said elasticity, too.

In a first embodiment, a trapezoidal thread is chosen as the coarse thread. Tools for creating trapezoidal threads are readily available. Trapezoidal threads may with their flat top provide more uniform compression and/or friction forces between the thread on the outside of the bush and the complementary thread of the cavity of the plastic material than threads with different shapes.

In another embodiment, the distance within the groove of the complementary thread is increased by introduction of the bush into the cavity of the plastic material preferably by <NUM>% to <NUM>%, more preferred <NUM>% to <NUM>% and most preferred <NUM>% to <NUM>%. It is important that the increase of the distance within the groove of the thread on the inside of the cavity, caused by the bush, does not exceed the limit of elasticity of the plastic material.

In another embodiment, high density polyethylene (HDPE) is chosen as the plastic material. HDPE is known for its high strength-to-density ratio. The density of HDPE can typically range from <NUM> to <NUM>/m<NUM>. Friction depends on the shape of the coarse thread and the physical properties such as elasticity and surface tension of the plastic material. It is an advantage to combine trapezoidal thread and high density polyethylene in the present innovation.

In yet another embodiment, the friction engaging the coarse thread on the outside of the bush and the complementary thread of the cavity of the plastic material is increased by structural modification of the coarse thread on the outside of the bush. The structure of the coarse thread on the outside of the bush is instrumental for providing friction between bush and plastic material. A modification of this structure, which may increase the friction to be provided in order to loosen or remove the bush is particularly relevant, if the friction to be provided in order to introduce the bush into the cavity, is not increased at the same level. In order to provide an efficient increase of friction the coarse thread on the outside of the bush has after the structural modification to comprise at least one modified contact area which is engaged with the complementary thread.

In another embodiment, the modified contact area comprises a structure selected from the group consisting of random unevenness, random roughness, regular unevenness and regular roughness which represent physical structural modifications.

In another embodiment, the modified contact area comprises a structure selected from the group consisting a changed chemical structure obtained by means selected from the group consisting of oxidation and etching or selected from the group consisting of coating, gluing and impregnation.

Yet another embodiment refers to a method for bolting a fastening device for plastics into a plastic material of known elasticity. The fastening device comprises a threaded bush, typically metallic, which is arranged with a coarse thread on the outside of the bush. On the hollow inside of the bush, a standard metric thread is arranged. A cavity with coarse threads is formed in the plastic material, said threads being complementary to the threads on the outside of the bush. The bush is screwed into the threaded cavity of a plastic material and, as a consequence, the distance within the groove of the complementary thread is increased. The coarse thread on the outside of the bush and the complementary thread of the cavity of the plastic material are engaged by compression and/or friction, which keep the fastening device in place.

Yet another embodiment refers to an article comprising an assembly of a fastening device and a plastic material of known elasticity according to the present invention. The fastening device is to the limit screwed into the plastic material and a utility is screwed into the standard metric thread on the hollow inside of the fastening device and full momentum can be applied on the utility without moving the fastening device. By full momentum is meant that the fastening device does not come loose when the standardized metric screw inserted inside the fastening device is loaded with maximum momentum. If the load exceeds the maximum momentum for the internal screw, it must be assumed that the internal screw would be damaged and that the fastening device would come loose for that reason.

Below a more detailed explanation of the present invention is provided in the form of exemplary embodiments with reference to enclosed drawings.

<FIG> shows a side sectional view of a piece of plastic material <NUM> into which the fastening device has to be introduced. The plastic material does not show significant inhomogeneity within the range of the typical size of the fastening.

<FIG> shows a side sectional view of a tool <NUM> which is used in order to provide a cavity in the plastic material. The tool is equipped with cutting edges <NUM> and means for creating coarse threads <NUM>), in this case trapezoidal threads. A connecting area <NUM> for a drilling machine or the like, by which a momentum may be applied to the tool, is provided at its rear end.

<FIG> shows a side sectional view of the tool <NUM> from <FIG> in operation. The cavity is created in the plastic material <NUM> and arranged with coarse threads, in this case trapezoidal threads. The distance Dp between the grooves created in the plastic material is also indicated.

<FIG> shows a side sectional view of a bush <NUM> according to the present invention. The bush is arranged with coarse trapezoidal threads <NUM> which are complementary to the cavity created by the coarse threads <NUM> shown in <FIG>. The groove width or distance Db of the coarse threads is indicated. On the hollow inside of the bush <NUM>, a fine thread <NUM> in the shape of a standard metric thread is arranged. Any kind of utility may be screwed into the standard metric thread (not shown in <FIG>). At one end of the bush <NUM>, a hexagonal nut shaped extension <NUM> or the like may optionally be provided for the bush to be screwed into the plastic material. When the top of the nut shaped extension <NUM> reaches the plastic material, the introduction of the bush <NUM> comes to completion. Alternatively, the bush <NUM> may be completely introduced into the plastic material.

<FIG> shows the correlation between the force FS which is applied on the bush <NUM> during introduction into the cavity and the force FH which acts on a tool with which the momentum is applied onto the top <NUM> of the bush <NUM>. The distance r is the radius of the circular movement, d is the moved distance of the bush. Without taking into account any friction, the correlation can be expressed as shown in equation <NUM>.

By considering a contribution from friction, the correlation can be expressed as shown in equation <NUM>. <MAT> where µ is a real number larger than <NUM> representing contributions to friction from the structure of the involved threads on the outside of the bush and on the inside of the cavity. The contribution of friction is directly proportional to the force, which is needed to move the bush.

Claim 1:
An assembly of a fastening device and a plastic material (<NUM>) of known elasticity, the fastening device comprising a threaded bush (<NUM>), arranged with a coarse thread (<NUM>) on the outside of the bush and a standard metric thread (<NUM>) on the hollow inside of the bush, wherein the coarse thread on the outside of the bush is arranged for screwing into a cavity of the plastic material (<NUM>), thereby said cavity of the plastic material is arranged with a complementary thread with a chosen distance within the groove, characterized in that the distance within the groove of the complementary thread is increased within the limit of the plastic materials elasticity, when the fastening device is fastened into the plastic material, wherein the coarse thread (<NUM>) on the outside of the bush and the complementary thread of the cavity of the plastic material are engaged by at least one of compression and friction.