Technical Field

The present invention relates to a threaded device, particularly for self-threading and coupling thermoplastic materials. More specifically, the invention relates to a device that is adapted to form threads and couple various types of technopolymers, in particular technopolymers with carbon or glass fibers or loaded with various inert materials (talc, carbon, glass), for heavy use, wood and agglomerates thereof and with appropriate thermal treatments, composite materials with other hardening elements, with holes made by mechanical processing or injection molding, with exceptionally low screwing torque and high resistance to thread-stripping and with an exceptionally low thread formation load and high thread-stripping and unscrewing load, without generating overheating of the female screw when used on technopolymers.

Background Art

As is known, in order to couple parts made of plastic material, threadings are sometimes used in an improper manner.

However, threadings of the known type have drawbacks which are due to the fact that they cause phenomena of radial precompression owing to their profile having broad angles and high threading pitches, which cause high thread-formation loads, with low resistance to unscrewing and to thread-stripping.

Furthermore, because of the friction produced for the reasons mentioned above, surface melting of the female screw occurs, which is caused by the high speed of modern screwing tools.

The threadings currently used are double-threaded or single-threaded screws, with symmetric profiles at 30°- 40°- 50° which create phenomena of creep, breakage over time of the studs, caused by the ageing of the technopolymers, with consequent easy thread-stripping and unscrewing. Disclosure of the Invention

The aim of the present invention is provide a threaded device, particularly for self-threading and coupling thermoplastic materials and the like, which makes it possible to overcome the drawbacks of the background art, and which makes it possible to have an extremely low screwing torque and a high resistance to thread-stripping.

Within this aim, an object of the present invention is to provide a threaded device, particularly for self-threading and coupling thermoplastic materials, which does not generate overheating of the female screw when used on thermoplastic polymers.

Another object of the present invention is to provide a threaded device that is highly reliable, relatively easy to implement and low-cost.

This aim, as well as these and other objects which will become more apparent hereinafter, are achieved by a threaded device, particularly for self- threading and coupling thermoplastic materials, that comprises a cylindrical body provided, at one end, with a head, said body being providing with a threading, characterized in that the thread of said threading is angled toward said head, with an angle that is comprised between 3° and 6° with respect to an axis perpendicular to the longitudinal axis of said cylindrical body.

Brief description of the drawings

Further characteristics and advantages of the invention will become clearer from an embodiment of the device according to the invention, illustrated by way of non-limiting example in the accompanying drawings, wherein:

Figure 1 is a schematic plan view of the device according to the invention;

Figure 2 is a side view of the device according to the invention;

Figure 3 is a detailed cross-sectional view of the thread of the device according to the invention;

Figure 4 is a view of a rolling comb used to produce the device according to the invention;

Figure 5 is a view of the bending sequence made by the comb in Figure 4.

Ways of carrying out the invention

With reference to the figures, the device according to the invention comprises a cylindrical body 1 made of metal in the natural state or hardened by means of thermal treatment, and provided, at one end, with a head 3, and along the body 1 , with a thread 2. The device is especially adapted for self- threading or for coupling two parts made of plastic material and the like.

Experimental research and studies and technical and practical tests carried out by the applicant have surprisingly ascertained that, with a shape of the profile of the thread 2 having:

- an angle a less than or equal to 18° (see in particular Figure 3, the angle a being the angle defined by the extensions of the two faces 2a and 2b of the thread 2);

- a side toward the head 3, i.e. the side of the thread facing the head 3 of the device, with an inclination of δ -3°-6° with respect to the axis 5 of the device;

- an angle ω, defined as the angle defined between the surface of the point or cusp of the thread 2 and an imaginary line parallel to the axis 5, of 10°

(Figure 3);

- anti-tearing connections 4 at the base of the threading, defining an angle of 45°-50° γ with respect to an imaginary line parallel to the axis 5 of the thread 2 (Figure 3).

With these characteristics of the thread 2 we obtain a low thread formation load, since the friction is considerably reduced and no radial precompression and overheating is generated, which in plastic materials generates phenomena of immediate stress-cracking or stress relaxation, and later on, owing to ageing, creep phenomena with loosening of the screw.

With the side toward the head 3 of the device at a negative angle, the effusion of the material of the female screw is prevented.

With a careful study of the pitches (Figure 2) to give to the threading, it was ascertained after innumerable tests on different types of material that, contrary to what was always believed, i.e. that high pitches with consequently a large amount of material between the spires of the screw improve the hold against thread-stripping, in reality the unscrewing coefficient deteriorates considerably, thus rendering the gained resistance to thread-stripping useless.

On the contrary, by using these suitably reduced pitches the thicknesses of the elements to be connected are reduced, thus saving on raw materials.

With a ratio of the outer diameter of the threading (dl) to the pitch of the screw, dl/p (which in general in threads of the known type is 1.5 - 2 times the diameter of the screw, with an inclination of the thread of 40°-50°) of 3 - 3.5 times dl , with an inclination δ comprised between 3° and 6°, in tests a high unscrewing coefficient was obtained with materials, i.e. technopolymers with glass and carbon fibers in constructions that support considerable forces, with tightening and loosening maneuvers for an undefined number of times, with an improved thread-stripping load. This is due to the compression exerted by the thread with negative inclination δ (Figure 3) which ensures that the female screw behaves like an elastic washer, thus improving the hold against unscrewing and making it possible to tighten elements even if they have thin cross-sections.

With the inclined plane ω (Figure 3) on the summit of the thread, radial compression is also prevented for thin studs (i.e. cast parts where the screw is screwed and which have their own hole so as to allow the insertion of the screw) and the profile of the resistant section is strengthened.

Industrial production of the device according to the invention, once the machine tools with the profile adapted for this purpose have been made, is possible with common machinery (rolling screw-cutting machines, die- stamping machines and the like).

For example, it is possible to use a rolling comb P, Figure 4, which is constituted by two sections, one that threads the device without negative inclination of the thread and a final part A which creates the negative inclination of the thread. From Figure 2 it can be seen that the thread 2 is inclined gradually by the action of the comb P (a, b, c, d, e, f, g). The end part of the comb is negatively inclined and during the threading it transforms the face of the thread, i.e. the part of the thread facing the head 3, from a right angle to a negative angle with an inclination of 3°- 6°.

The technological improvements of the invention over other systems of threading are made clear by the comparative tests between one type of screws currently in use and the threaded device described herein, as illustrated in the table below.

Thread-forming, unscrewing and thread-stripping properties.

In practice it has been found that the device according to the invention fully achieves the intended aim and objects.

Advantageously, the device according to the invention can be used in the manufacturing industry where use is made of technopolymers of various types, glass or carbon fibers, polymers loaded with talc, carbon and other inert elements.

The device, thus conceived, is susceptible of numerous modifications and variations, all of which are within the scope of the appended claims.

In practice the materials employed, as well as the contingent dimensions and shapes, may be any according to requirements and to the state of the art.

The disclosures in Italian Patent Application No. PV2010A000003 from which this application claims priority are incorporated herein by reference.

Where the technical features mentioned in any claim are followed by reference numerals and/or signs, those reference numerals and/or signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly, such reference numerals and/or signs do not have any limiting effect on the interpretation of each element identified by way of example by such reference numerals and/or signs.