DEVICE FOR WORKING PIPES

Abstract

A device for working pipes, in particular to carry out a belling working or operation on one end of a pipe, including a belling element configured to produce with a thrust a bell end with a substantially circular shape by means of axial advance into said end of said pipe.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national stage application of International Application PCT/IT2021/050164, filed May 26, 2021, which international application was published on Dec. 2, 2021, as International Publication WO 2021/240566 A1 in the English language. The international application is incorporated herein by reference, in entirety. The international application claims priority to IT Patent Application No. 202000012451, filed May 26, 2020, which is incorporated herein by reference, in entirety.

FIELD OF APPLICATION

Embodiments disclosed herein relate to a device for working pipes, in particular a belling working. The invention finds application for working pipes used for the creation of heat exchangers.

BACKGROUND ART

It is known to carry out workings at the ends of pipes to make the latter suitable to associate with another tubular element such as pipes or manifolds, fittings, etc.

Among the possible workings known in the art, the belling operation or, in short, belling, consists, by means of a suitable forming member, of expanding the end of the pipe from the inside to make it possible to associate it by insertion with another tubular element and provide for a connection between the two parts.

The belling operation is known to be used in creating pipe circuits used in the production of heat exchangers.

Finned heat exchangers generally consist of a finned package crossed by one or more benches of overlapping pipes in which the heat exchange fluid circulates.

Finned heat exchangers are used in a wide variety of applications, and can be small heat exchangers used in the automotive sector, or large heat exchangers used in industrial air conditioning or refrigeration machines.

For the creation of large heat exchangers, straight pipes are also used, of variable length between 4 and 15 meters or more, which are connected together two by two in their end parts to create a circuit, by means of the association of curves that invert the flow by 180°.

The known method with which a heat exchanger is made comprises working the pipes with a machine, or several machines, sequentially, which substantially provide to straighten them, cut them to the desired length and bell them at one end.

The method further comprises inserting the thus worked pipes into predefined seats of a finned package, carrying out a belling operation at the unworked end of the pipes, and carrying out a spinning operation on the pipes to firmly associate the pipes with the finned package.

To bell the end of the pipe once it has been inserted into the finned package, it is necessary to lock the pipe in its seat so as to keep it stable, making it consistent with the fins of the finned package.

To do this, the known belling machines are provided with cushioning members to cushion the end of the pipe protruding from the finned package from the outside to keep it held and locked while carrying out the belling.

Since the cushioning must not interfere with the belling, the pipe must protrude by a certain length whose amount is at least equal to the length necessary for the cushioning members to properly cushion a relative section of pipe plus the length of the bell end to be formed.

In addition, the aforementioned spinning operation of the pipe in the finned package notoriously causes its shortening, varying between about 2% and about 5% of the total length of the pipe, which must therefore be compensated by increasing the length of the pipe in a coordinated manner.

Therefore, the part of the pipe that must be made to protrude in order to be able to be belled with a known belling machine must take into account the variables described above and is, for example, equal to a length between 15 mm and 40 mm corresponding to about three times the diameter of the pipe.

The protruding length, even if minimal with respect to the total length of the pipe, becomes relevant when multiplied by the tens or hundreds of pipes that can be present in a single finned package.

A drawback is therefore that the use of the known belling machines to work pipes inserted in finned packages causes the waste of raw material and an increase in production costs.

Therefore, one drawback is that the portion of pipe used by the cushioning members to cushion the pipe is subtracted from the finned package.

Another drawback is that the cushioning tool causes wear to the pipe especially when the cushioning force is concentrated on a small portion of the pipe in an attempt to limit the protrusion of the pipe (and increase the size of the finned package).

In other known embodiments of heat exchangers, the end of the unworked pipe can be belled following the spinning operation.

As described above, since the spinning causes a shortening of the length of the pipe, it may not be possible to cushion the protruding pipe portion from the finned package except by using a longer pipe that takes into account such shortening but against an increase in production costs.

Therefore, carrying out the spinning operation before the belling operation allows, for the same length of pipes, the use of larger heat exchangers but does not allow the use of the known belling machines which would not be able to cushion the reduced protruding portion of the pipe and the belling for the advance of the belling element would cause the partial exit of the pipe from its seat.

Therefore, in this case the belling is carried out by an operator using portable belling instruments.

Manually or automatically actuated, such portable belling instruments are provided with a belling tool configured to be inserted into the end of the pipe to be worked to create an expansion of the end or bell end and can be a rotating belling tool driven by an electric motor, or a percussion belling tool (not ergonomic and noisy) driven pneumatically or other drive means suitable to drive the movement of the belling tool.

These instruments do not provide to externally cushion the pipe as they create the bell end by expansion and not thrust.

An example of an instrument known in the art utilizes a substantially cylindrical forming member divided into parts into forming elements or portions capable of radially expanding to create the bell end.

The radial expansion of the forming portions leads to the creation of a bell end with substantially polygonal section with a number of sides corresponding to the number of portions of the forming member.

A drawback is that welding bell ends of polygonal conformation with substantially circular ends of other tubular elements is laborious.

The insertion of the forming member being in the end of the manual pipe, a drawback is that the latter may not be properly aligned, causing the creation of an imperfect bell end and which can be the cause of leaks.

Another drawback is the fact that being an operation carried out by hand, it is not always possible to ensure a standardization of the belling operations and therefore of the belling itself.

Another drawback is that these well-known instruments are hardly ergonomic to use and also produce stresses on the operator that can cause morbidity in the long term.

A further drawback is that the known instruments have such a noise level that, according to the safety regulations, they oblige the users of the instrument and the people present in the area to wear personal acoustic insulation devices.

The drawbacks listed, but also others, are amplified if a high number of pipe ends are required, for example the several dozen with which heat exchangers are provided, in particular those intended for industrial use.

Therefore, there is a need to improve a belling device which can overcome at least one of the drawbacks of the prior art.

In particular, an object of the present invention is to provide a belling device which reduces the intervention of an operator when in use.

Another object of the invention is to provide a belling device which allows the creation of a bell end which that can be easily associated and welded to one end of a tubular element with geometric precision.

A further object of the present invention is to provide a belling device which carries out the belling operations silently.

Another object of the invention is to provide a compact belling device that is easy to handle.

The Applicant has studied, tested and realized the present invention to overcome the drawbacks of the prior art and to obtain these and further objects and advantages.

EXPOSURE OF THE INVENTION

The present invention is expressed and characterized in the independent claims. The dependent claims show other features of the present invention or variants of the main solution idea.

In accordance with the aforementioned objects, a device for working pipes is provided, in particular for carrying out a belling working or operation, or belling, on one end of a pipe, protruding from the side or structure S of a finned package of a heat exchanger, which overcomes the limits of the prior art and eliminates the defects therein.

The device comprises a belling element configured to produce with a thrust a bell end of substantially circular shape by means of axial advance into said end of said pipe.

According to an aspect of the invention, the device for working pipes further comprises a gripping member, oblong in shape and coaxial with the belling element.

The gripping member is configured to pass from a first or contracted conformation, suitable to allow the insertion of the gripping member into the pipe, to a second or expanded conformation in which it contacts, at least partly, the internal wall of the pipe and applies a force suitable to create a friction that allows the belling element to advance inside the end of the pipe, carrying out the aforementioned working.

Unlike what occurs in the prior art, the invention allows the device to engage and lock the pipe from the inside instead of from the outside.

This allows to reduce the length of the protruding part of the pipe by at least 1 time the diameter of the pipe. The protruding length of the pipe saved with the use of the present invention can be used to correspondingly increase the dimensions of the finned package, or to keep the dimensions of the latter unchanged but in consideration of the use of shorter pipes.

In the first case, the length of the pipe is more effectively exploited, allowing the dissipation area to be increased since the finned package can be brought close to the bell end, reducing or even eliminating the dead space of unused pipe therebetween.

In the second case, the length can be kept unchanged but against a saving in the cost of the raw material.

The present invention further eliminates defects that can be caused to the pipe when externally cushioned.

According to an aspect of the present invention, the gripping member is of a length such that it can be inserted into the pipe for a length between 15 and 40 times the diameter of the pipe itself.

According to another aspect of the present invention, said gripping member comprises a first side element, a second side element and a central element interposed therebetween and associated thanks to the complementarity of their shape in correspondence with respective contact surfaces in a sliding manner, in addition, said contact surfaces have a sawtooth profile, whereby as a result of the sliding of said central element the first side element and the second side element expand radially from the central element.

According to a further aspect of the present invention, said gripping member is provided with one or more circumferential seats, each suitable to allow the housing of at least one elastic element to associate said side elements with said central element.

The belling element can preferably be configured to create the bell end by means of insertion into the end of the pipe and subsequent thrust and not by rotation, thereby allowing the friction between the belling element and the end of the pipe to be minimized. This results in an advantageous elimination of heat and the production of any metal chips which, moreover, due to the heat itself produced by the friction, would tend to stick inside the pipe which must necessarily be cleaned.

With respect to the use of radially expanding forming members, the belling element of the present invention allows to obtain a substantially circular bell end which makes it easy to weld to other tubular elements.

However, with respect to the use of percussion organs, the belling element of the present invention allows to obtain one or more bell ends, also simultaneously, silently and avoiding stresses on an operator, known to cause morbidity.

According to another aspect of the invention, the belling element is provided along its longitudinal development with a channel into which the gripping member is coaxially inserted.

This therefore allows to considerably reduce the overall dimensions of the device since the gripping member has a radial dimension contained in the belling element.

Therefore, the invention is suitable to also bell heat exchangers provided with pipes very close to each other.

According to an aspect of the invention, the device for working pipes lends itself to be configured individually or in series on a belling machine to provide the simultaneous and automatic belling of a plurality of pipes.

Furthermore, the device for working pipes is configured to automatically carry out the belling operations, reducing the intervention of personnel.

The invention further relates to a method for working pipes, in particular to carry out a belling working or operation on one end of a pipe by means of axial advance of a belling element into the end of the pipe.

The method provides to apply, by means of a gripping member, in correspondence with the internal wall of the pipe, a force suitable to create a friction which allows the advance of the belling element inside the end of said pipe.

ILLUSTRATION OF THE DRAWINGS

These and other aspects, features and advantages of the present invention will become clear from the following embodiment description, given as a non-limiting example, with reference to the attached drawings in which:

FIG. 1 is a perspective view of a device for working pipes in accordance with embodiments described herein;

FIGS. 2 and 3 are sectional views of a device for working pipes in accordance with embodiments described herein;

FIGS. 4 and 5 are sectional views of a device for working pipes in accordance with other embodiments described herein

FIG. 6 is a detail of FIG. 4 in accordance with a further embodiment of the present invention;

To facilitate understanding, identical reference numbers have been used, where possible, to identify identical common elements in the figures. It is to be understood that elements and features of an embodiment can be conveniently combined or incorporated into other embodiments without further clarification.

DESCRIPTION OF EMBODIMENTS

Reference will now be made in detail to the possible embodiments of the invention, one or more examples of which are shown in the attached figures by way of non-limiting example. The phraseology and terminology used herein is also for non-limiting exemplary purposes.

In accordance with the objects of the invention, a device for working pipes is provided, in particular to carry out a belling working or operation, or belling, on one end of a pipe P.

With reference to the attached figures, a device for working pipes is described, identified as a whole by the reference number 10. The device 10 is advantageously usable for belling ends of pipes P protruding from the side or structure S of a finned package of a heat exchanger (not shown).

The device for working pipes comprises a belling element 12 configured to produce with a thrust a bell end of substantially circular shape by means of axial advance into the end of the pipe P.

According to an aspect of the invention, the device for working pipes also comprises a gripping member 14, of oblong shape and coaxial with the belling element 12.

The gripping member 14 is configured to pass from a first or contracted shape, suitable to allow the insertion of the gripping member 14 into the pipe P, to a second or expanded shape in which it contacts, at least partly, the internal wall of the pipe P and applies a force suitable to create a friction that allows the belling element 12 to advance inside the end of the pipe P.

The invention therefore allows the pipe P to be locked from the inside, therefore the portion made to protrude from the structure S can only be that necessary to create the bell end (FIGS. 2 and 3) and which eventually takes into account the shortening of the pipe caused by its spinning.

This allows to save on the cost of pipes, which can be shorter at the same finned package size, or allows, at the same length of pipes, to use a larger finned package.

Furthermore, the device for working pipes is configured to automatically carry out the belling operation without the intervention of an operator.

The gripping member 14 can comprise a first portion 16 configured to be associated with operation means 18 and a second portion 20 configured to be inserted inside the pipe P, in which at least the second portion 20 is configured to pass from the contracted conformation to the expanded conformation.

The gripping member 14 can be, by way of example, a rod, a plug, a bar, suitable to be at least partly inserted into the pipe and provided with a longitudinal axis lying on a working axis X.

The operation means 18 can be, for example, electric, hydraulic, pneumatic or other motors, and are configured to provide for the operation of the gripping member 14, i.e., at least to provide for its change of conformation.

In embodiments, the gripping member 14 can be provided with an internal channel (not shown) in fluid communication with the operation means 18, configured, for example, to operate the change in conformation of the second portion 20.

The operation means 18 are configured to ensure that the pressure exerted by the gripping member 14 on the internal wall of the pipe P is such as not to cause deformations of the structure of the pipe P itself.

According to embodiments, the device for working pipes 10 further comprises a machine body 25 inside which a first chamber 26 is created.

The first chamber 26 is provided with an opening 31 from which is inserted, to be housed, a thrust member 27 configured mobile to slide on the working axis X to pass from a retracted position (FIG. 2) to an extracted position (FIG. 3).

A second chamber 28 provided with an opening 35 in which the belling element 12 is housed is created in the thrust member 27.

The stroke of the thrust member 27 between the retracted position and the extracted position allows, positioning at a predefined distance the device for working 10 the pipe P, to insert the belling element 12.

A locking element 32, or more, can be provided to fix the belling element 12 in the second chamber 28 by inserting into suitable seats 33 created in the locking element 32. Removing the locking element 32, the belling element 12 can be removed and replaced with one of different sizes and shapes based on the pipe to be worked and the bell end to be obtained.

According to another aspect of the invention, the belling element 12 is provided along its longitudinal development with a first channel 29 into which the gripping member 14 is coaxially inserted.

The channel 29 is provided with a first opening 29a which in use is in correspondence with the pipe P and with an opposite second opening 29b.

The gripping member 14 is inserted into the first channel 29 so that the second portion 20 is cantilevered with respect to the belling element 12 in correspondence with the first opening 29a.

The thrust member 27 is in turn provided with a second channel 30 coaxial and communicating with the first channel 29 suitable to allow the passage of the gripping member 14. The second channel 30 can be associated with an outlet 34 created on a rear panel 36 that allows the first portion 16 of the gripping member 14 to exit the machine body 25 and associate with the operation means 18.

According to embodiments, the diameter of the first channel 29 and the second channel 30 is such as not to interfere with the movement of the gripping member 14.

According to embodiments, the belling element 12 and the gripping member 14 can move independently of each other.

It is thereby possible in a first moment to insert the gripping member 14 into the pipe to provide for its locking and in a second moment, keeping the gripping member 14 in an enlarged shape, to provide for the advance of the belling element 12.

The thrust member 27 is associated with operation means (not shown) that control its movement and are suitable to provide the necessary thrust to the belling element 12 to create the bell end.

According to embodiments, the operation means can be configured to introduce a pressurized fluid, for example air or oil, into the chamber 26 and provide for the extraction of the thrust member 27.

In other embodiments, the operation means associated with the thrust member 27 can be a hydraulic or pneumatic piston or other, suitable to allow the thrust member 27 to thrust the belling element 12 to create the bell end.

Such operation means have a noise level below the threshold which imposes the use of personal acoustic insulation devices.

Possibly, the machine body 25 can be provided with sound-absorbing elements to further reduce the noise emission.

According to an embodiment, the gripping member 14 comprises a first element 22 and a second element 24, sliding with each other. The first element 22 and the second element 24 are associated with each other thanks to the complementarity of their shape along at least their partial longitudinal development in correspondence with respective contact surfaces 22a, 24a.

The first element 22 is associated with the operation means 18 that slide it on the second element 24 that is fixed, for example associated with the rear panel 36 of the machine body 25.

The rear panel 36 can be associated in a removable manner by means of a plurality of fixing means 38, for example screws.

Preferably, the contact surfaces 22a, 24a have a sawtooth profile whereby, when the first element 22 is slid on the second element 24, the sawtooth profile of the contact surfaces 22a, 24a causes the first element 22 to partly rise from the second element 24.

Thus, as a result of the mutual sliding between the first element 22 and the second element 24, the gripping member 14 passes from the first position (FIG. 2) to the second position (FIG. 3).

The raising causes an expansion of the gripping member 14 that increases its diameter at least in correspondence with the axis in which the raising direction occurs by providing to associate with the internal wall of the pipe P applying the suitable friction force.

In correspondence with the second portion 20, the first element 22 and/or the second element 24 are provided with respective contact portions 22b and 24b configured to interfere with the internal wall of the pipe P.

The contact portions 22b and 24b can have a worked surface, for example knurled, so as to improve the friction therebetween and the internal wall of the pipe P.

The contact portions 22b and 24b can have a greater radial development with respect to the remainder of the development of the gripping member 14 such as to make the second portion 20 thicker.

Thereby less raising is required to provide for the association of the second portion 20 with the internal wall of the pipe P.

In another embodiment, the device 10 comprises a gripping member 114 (FIGS. 4 and 5) divided into a first rear portion 116 configured to be inserted into the machine body 25 and a second front portion 120 configured to be inserted into the pipe P.

The gripping member 114 comprises a central element 40 which, at least in correspondence with the second portion 120, has a first contact surface 42 and a second contact surface 43 opposite and each provided with a sawtooth shaped profile. The gripping member 114 further comprises a first side element 45 and a second side element 46 each associated in a slidable manner with the first contact surface 42 and the second contact surface 43 with a contact surface 48 thereof which, at least in correspondence with the second portion 120, is provided with a sawtooth profile coordinated with the sawtooth profile of the respective first or second contact surface 42 and 43 to couple therewith.

The side elements 45 and 46 also have an external surface 49, opposite the contact surface 48, which at least in correspondence with the second portion 120 has a contact surface 51, with a curved profile, suitable to contact the internal curved wall of the pipe P.

The central element 40 is associated with the operation means 18 to be mobile along the working axis X and is configured to pass from a first position, or retracted position (FIG. 4) in which the gripping member 114 is in the first conformation i.e., in the position in which the first and the second side element 45 and 46 are substantially adhered to the central member 40, to a second position, or extracted position (FIG. 5), advancing along the working axis X, in which the gripping member 114 is in the second conformation i.e., in the position in which the first and the second side element 45 and 46 are radially distanced from the central element 40, creating a consequent contact inside the pipe P with a force sufficient to provide sufficient friction to allow the insertion of the belling element 12.

The gripping member 114 is provided with four circumferential seats 53 (FIG. 6) each suitable to allow the housing of at least one elastic element 54 of annular shape to associate the side elements 45 and 46 with the central element 40 and suitable not to prevent the passage to the dilated shape. The seats 53 are arranged in correspondence with the second portion 120.

Furthermore, when the central element 40 passes from the extracted position to the retracted position, the elastic elements 54 return the side elements 45 and 46 in adhesion with the central element 40.

In a variant not shown, the side elements 45 and 46 are fixed in correspondence with their first portion 116 in the machine body 25.

The contact surface 51 of the first and the second side element 45 and 46 can have a worked surface, e.g., knurled, so as to improve the friction therebetween and the internal wall of the pipe P.

The invention further relates to a method for working pipes, in particular to carry out a belling working or operation on one end of a pipe P by means of axial advance of a belling element 12 into the end of the pipe P.

The method provides to apply, by means of a gripping member 14, 114, in correspondence with the internal wall of the pipe P, a force suitable to create a friction which allows the advance of the belling element 12 inside the end of said pipe P.

According to embodiments, the gripping member 14, 114 is configured to pass from a first conformation, suitable to allow the insertion of the gripping member 14, 114 into the pipe P, to a second conformation in which it at least partly contacts the internal wall of the pipe P, applying the friction force.

It is clear that modifications and/or additions of parts or steps can be made to the device for working pipes described heretofore and the method for working pipes, without departing from the scope of the present invention as defined by the claims.

In the following claims, the references in parentheses have the sole purpose of facilitating reading and must not be considered as limiting factors as regards the scope of protection underlying the specific claims.

Claims

1. A device for working pipes, in particular for performing a belling working or operation on one end of a pipe protruding from the side or structure of a finned package of a heat exchanger, comprising a belling element configured to produce with a thrust a bell end with a substantially circular shape by means of axial advance into said end of said pipe which further comprises a gripping member, oblong in shape and coaxial with said belling element, said gripping member being configured to pass from a first conformation, suitable to allow the insertion of said gripping member into said pipe, to a second conformation in which it contacts, at least partly, the internal wall of said pipe, and applies a force suitable to create a friction that allows said belling element to advance inside the end of said pipe, carrying out the aforementioned working.

2. The device as in claim 1, wherein said belling element is provided along its longitudinal development with a first channel in which said gripping member is coaxially inserted.

3. The device according to claim 1, comprising a thrust member associated with said belling element and configured to thrust it, wherein said thrust member is provided with a second channel connected to said first channel in which said gripping member is positioned.

4. The device as claim 1, wherein said belling element and said gripping member, move independently of each other.

5. The device as in claim 1, wherein said gripping member comprises a first element and a second element, sliding and associated with each other thanks to the complementarity of their shape along at least their partial longitudinal development in correspondence with respective contact surfaces, said contact surfaces have a sawtooth profile whereby, as a result of the sliding of said first element on said second element, said sawtooth profile causes said first element to partly rise from said second element.

6. The device as in claim 1, wherein said gripping member comprises a first side element, a second side element and a central element interposed between said first side element and said second side element, associated thanks to the complementarity of their shape in correspondence with respective contact surfaces and in a sliding manner and wherein said contact surfaces have a sawtooth profile, whereby as a result of the sliding of said central element said first side element and said second side element expand radially from said central element.

7. The device as in claim 6, wherein said gripping member is provided with one or more seats each suitable to allow the housing of at least one elastic element to associate said lateral elements with said central element.

8. A method of working pipes, in particular to carry out a belling working or operation on one end of a pipe protruding from the side or structure of a finned package of a heat exchanger, by means of axial advance of a belling element into said end of said pipe wherein it provides to apply, by means of a gripping member, in correspondence with the internal wall of said pipe, a force suitable to create a friction, without causing deformation, which allows the advance of said belling element inside the end of said pipe, carrying out the aforementioned working.

9. The method as in claim 8, wherein said gripping member is configured to pass from a first conformation, suitable to allow the insertion of said gripping member into said pipe, to a second conformation in which it at least partly contacts the internal wall of said pipe applying said friction force.

10. The belling machine comprising one or more devices as in claim 1.

11. The device according to claim 2, comprising a thrust member associated with said belling element and configured to thrust the belling element, wherein said thrust member is provided with a second channel connected to said first channel in which said gripping member is positioned.

12. The device as in claim 2, wherein said gripping member comprises a first element and a second element, sliding and associated with each other thanks to the complementarity of their shape along at least their partial longitudinal development in correspondence with respective contact surfaces, said contact surfaces have a sawtooth profile whereby, as a result of the sliding of said first element on said second element, said sawtooth profile causes said first element to partly rise from said second element.

13. The device as in claim 3, wherein said gripping member comprises a first element and a second element, sliding and associated with each other thanks to the complementarity of their shape along at least their partial longitudinal development in correspondence with respective contact surfaces, said contact surfaces have a sawtooth profile whereby, as a result of the sliding of said first element on said second element, said sawtooth profile causes said first element to partly rise from said second element.

14. The device as in claim 4, wherein said gripping member comprises a first element and a second element, sliding and associated with each other thanks to the complementarity of their shape along at least their partial longitudinal development in correspondence with respective contact surfaces, said contact surfaces have a sawtooth profile whereby, as a result of the sliding of said first element on said second element, said sawtooth profile causes said first element to partly rise from said second element.

15. The device as in claim 2, wherein said gripping member comprises a first side element, a second side element and a central element interposed between said first side element and said second side element, associated thanks to the complementarity of their shape in correspondence with respective contact surfaces and in a sliding manner and wherein said contact surfaces have a sawtooth profile, whereby as a result of the sliding of said central element said first side element and said second side element expand radially from said central element.

16. The device as in claim 3, wherein said gripping member comprises a first side element, a second side element and a central element interposed between said first side element and said second side element, associated thanks to the complementarity of their shape in correspondence with respective contact surfaces and in a sliding manner and wherein said contact surfaces have a sawtooth profile, whereby as a result of the sliding of said central element said first side element and said second side element expand radially from said central element.

17. The device as in claim 4, wherein said gripping member comprises a first side element, a second side element and a central element interposed between said first side element and said second side element, associated thanks to the complementarity of their shape in correspondence with respective contact surfaces and in a sliding manner and wherein said contact surfaces have a sawtooth profile, whereby as a result of the sliding of said central element said first side element and said second side element expand radially from said central element.