EP2189375B1

EP2189375B1 - Packaging unit for producing sealed packages of a food product pourable into a tube of packaging material

Info

Publication number: EP2189375B1
Application number: EP08169824A
Authority: EP
Inventors: Daniele Anzaldi; Andrea Nicoli
Original assignee: Tetra Laval Holdings and Finance SA
Current assignee: Tetra Laval Holdings and Finance SA
Priority date: 2008-11-24
Filing date: 2008-11-24
Publication date: 2011-11-09
Anticipated expiration: 2028-11-24
Also published as: EP2189375A1; CN101734383B; JP2010120699A; JP5525176B2; ES2377235T3; CN101734383A; ATE532710T1

Abstract

A packaging unit (1) for producing sealed packages of a food product pourable into a tube (10), comprising a duct (20) for pouring food product into tube (10), a flange (25) housed within tube (10) and having holes (22) through which food product flows, and connecting means (35) for constraining duct (20) to flange (25) and comprising two coupling elements (40a, 40b) fixed to and detachable from duct (20) and flange (25) in a first and in a second configuration, and locking element (50) movable with respect to coupling elements (40a, 40b) in the first configuration along a path (P) between a first position (P 1 ) in which coupling elements (40a, 40b) are kept in first configuration, and a second position (P 2 ) in which coupling elements (40a, 40b) can reach the second configuration; unit (1) has constraining means (49, 59) interposed between coupling elements (40a, 40b) and locking element (50) at a third position (P 3 ); locking element (50) is movable with respect to coupling elements (40a, 40b) crosswise to path (P) at third position (P 3 ) between a fourth position in which first constraining means (49; 59) prevent locking element (50) from moving along path (P), and a fifth position (Q 2 ) in which locking element (50) can move along path (P).

Description

The present invention relates to a packaging unit for producing sealed packages of a food product pourable into a tube of packaging material.
Many food products, such as fruit juice, pasteurized or UHT (ultra-high-temperature treated) milk, wine, tomato sauce, etc., are sold in packages made of sterilized packaging material.
A typical example of this type of package is the parallelepiped-shaped package for liquid or pourable food products known as Tetra Brik Aseptic (registered trademark), which is made by folding and sealing laminated strip packaging material.
The packaging material has a multilayer structure substantially comprising a base layer for stiffness and strength, which may be defined by a layer of fibrous material, e.g. paper, or mineral-filled polypropylene material; and a number of layers of heat-seal plastic material, e.g. polyethylene film, covering both sides of the base layer.
In the case of aseptic packages for long-storage products, such as UHT milk, the packaging material also comprises a layer of gas- and light-barrier material, e.g. aluminium foil or ethyl vinyl alcohol (EVOH) film, which is superimposed on a layer of heat-seal plastic material, and is in turn covered with another layer of heat-seal plastic material forming the inner face of the package eventually contacting the food product.
As is known, packages of this sort are produced on fully automatic packaging units, on which a continuous tube is formed from the web-fed packaging material; the web of packaging material is sterilized in the packaging unit, e.g. by applying a chemical sterilizing agent such as a hydrogen peroxide solution, which is subsequently removed, e.g. by heating and evaporation, from the surfaces of the packaging material.
The sterilized web is maintained in a closed, sterile environment, and is folded into a cylinder and sealed longitudinally to form a tube.
The tube is fed in a vertical direction parallel to its axis, is filled continuously with the sterilized or sterile-processed food product and is heat-sealed at equally spaced cross sections to form pillow packs connected to the tube by transverse sealing strips.
Packaging unit are known which comprise a duct for conveying downwards the food product into the tube, and a flange coaxially housed within the tube and provided with a plurality of through holes.
More precisely, the heat-sealing of each cross-section pumps upwards an amount of food product, which passes through the holes defined by the flange. In this way, the flange controls the flow of food product pumped upwards by the heat-sealing of each cross section.
The flange and the duct are connected one another via locking means.
A need is felt within the industry to avoid that the vibrations and stresses due to the upwards flow of the food product unlock the flange from the duct, so rendering the amount of food product filled into packages uncontrollable.
A need is also felt within the industry to avoid that such vibrations and stresses break the locking means, so generating the risk that some parts of such locking means may fall into the tube and remain into the sealed packages.
It is an object of the present invention to provide a packaging unit for producing sealed packages of a food product pourable into a tube of packaging material, designed to meet at least one of the above requirements in a straightforward, low-cost manner.
According to the present invention, there is provided a packaging unit for producing sealed packages of a food product pourable into a tube of packaging material, as claimed in Claim 1.
A preferred, non-limiting embodiment of the present invention will be described by way of example with reference to the accompanying drawings, in which:

Figure 1 shows a view in perspective, with parts removed for clarity, of a packaging unit for producing packages of a food product pourable into a tube of packaging material according to the present invention;
Figure 2 to 5 show in perspective view respective steps of an operation for connecting a filling duct and a flange of the unit of Figure 1 by using a connecting group;
Figures 6 and 7 show a larger scale view of the connecting group of Figures 2 to 5 respectively in a first and a second operative position;
Figure 8 is a section along line VIII-VIII in Figure 7;
Figure 9 shows the connecting group of Figures 2 and 7 in a third operative position;
Figure 10 is section along line X-X in Figure 9;
Figure 11 shows the connecting group of Figures 2 to 10 in a fourth operative position;
Figure 12 is a section along line XII-XII in Figure 11;
Figure 13 shows the connecting group of Figures 2 to 12 in a fifth operative position;
Figure 14 is a section along line XIV-XIV in Figure 13;
Figure 15 shows the connecting group of Figures 2 to 14 in a sixth operative position; and
Figure 16 is a bottom view of the connecting group of Figure 2 to 15, with parts removed for clarity.

Number 1 in Figure 1 indicates as a whole a packaging unit for continuously producing sealed packages of a food product from a web 3 of packaging material, which is unwound off a reel and fed along a forming path.
Unit 1 preferably produces sealed packages (shown in Figure 1) of a pourable food product, such as pasteurized or UHT milk, fruit juice, wine, etc.
Unit 1 may also produce sealed packages of a food product that is pourable when producing packages, and sets after packages are sealed. One example of such a food product is a portion of cheese, that is melted when producing packages, and sets after packages are sealed.
The packaging material has a multilayer structure substantially comprising a base layer for stiffness and strength, which may be defined by a layer of fibrous material, e.g. paper, or mineral-filled polypropylene material; and a number of layers of heat-seal plastic material, e.g. polyethylene film, covering both sides of the base layer.
More specifically, web 3 is fed along forming path by guide members (not shown), e.g. rollers or similar, and successively through a first station (not shown) for applying a sealing strip to web 3; a second station (not shown) for forming a tube 10 of packaging material; and a third station (not shown) for heat sealing a longitudinal seal along tube 10. In particular, tube 10 has an axis A.
Unit 1 also comprises a fill device 12 for pouring the sterilized or sterile-processed food product continuously downwards into tube 10 of packaging material; and a jaw-type forming assembly (not shown) for gripping, sealing, and cutting tube 10 along equally spaced cross sections to form a succession of packages.
More specifically, fill device 12 comprises a duct 20, a flange 25 and connecting means 35 for connecting duct 20 and flange 25.
Duct 20 has a main portion 21 housed within tube 10 and extending along axis A, an end portion 24 protruding from main portion 21 outside tube 10 and defining a food product inlet opening 23.
With reference to Figures 2 to 5, flange 25 extends symmetrically about axis A, is coaxially housed within tube 10 and coaxially surrounds main portion 21 of duct 20.
In particular, flange 25 comprises two axial ends 26, 27 and a main portion 28 axially interposed between ends 26, 27 with respect to axis A.
End 26 is located above main portion 28 and is cylindrical.
End 27 is located below main portion 28 and defines a couple of grooves 29, which are opposite one another with respect to axis A and mainly extend along respective horizontal axes orthogonal to axis A.
Each groove 29 substantially comprise a main cylindrical portion and an inlet end portion, which is curved and crosswise the main portion.
Furthermore, each groove 29 is open at its side opposite to axis A.
Moving from end 26 to end 27, main portion 28 comprises a top section 30 shaped as a truncated cone and a cylindrical bottom section 31.
Top section 30 is tapered towards end portion 26 and comprises a plurality of through holes 22 angularly equally spaced about axis A.
The diameter of bottom section 31 is greater than the diameter of ends 26, 28.
The heat sealing of the cross-sections of tube 10 pumps an amount of food product upwards and across holes 22 of flange 25. In this way, holes 22 control the flow of such amount of food product pumped upwards.
Connecting means 35 are releasable and advantageously comprise:

two coupling elements 40a, 40b movable with respect to main portion 21 of duct 20 between a first configuration in which they are firmly connected to main portion 21 and to flange 25 (Figure 5), and a second configuration, in which they are detachable from main portion 21 and flange 25 (Figure 3);
a locking element 50 movable with respect coupling elements 40a, 40b arranged in the first configuration along a path P (Figures 6, 7, 9, 11, 13 and 14) between a position P₁ (Figure 6), in which such locking element 50 keeps coupling elements 40a, 40b in the first configuration, and a position P₂ (Figure 15), in which coupling elements 40a, 40b are free to move towards said second configuration; and
first constraining means for constraining locking element 50 to coupling elements 40a, 40b at a position P₃ (Figures 7 and 8), which is arranged between positions P_1, P₂ along path P; locking element 50 is movable, at position P₃, with respect to coupling elements 40a, 40b crosswise to path P and between a plurality of first blocking positions, in which first constraining means prevent locking element 50 from moving along path P towards position P₂, and a position Q₂ (Figure 8), in which locking element 50 is free to move along path P towards position P_2.

In particular, locking element 50 moves with respect to coupling elements 40a, 40b at position P₃ along a line Y crosswise to path P.
The first blocking positions correspond to all the positions that locking element 50 can reach along line Y at position P₃, except position Q_2.
Furthermore, unit 1 comprises second constraining means interposed between coupling elements 40a, 40b and locking element 50 at a position P₄, which is interposed between positions P₃, P₂ along path P.
Locking element 50 is also movable with respect to coupling elements 40a, 40b parallel to line Y at position P₄ between a plurality of second blocking positions, in which second constraining means prevent locking element 50 from moving along path P towards position P₂ and a position Q₄ in which locking element 50 is free to move along path P towards position P₂ (Figure 12).
The second blocking positions correspond to all the positions that locking element 50 can reach along line Y at position P₃, except position Q_4.
In particular, positions Q₂, Q₄ are spaced along a line parallel to line Y.
In greater detail, coupling elements 40a, 40b define respective projections 48, 49, and locking element 50 defines seats 58, 59.
First constraining means comprise projection 49 and seat 59, and second constraining means comprise projection 48 and seat 58.
Projection 49 is housed with clearance within seat 59 when locking element 50 is at position Q₂ along line Y and at position P₃ along path P (Figures 7 and 8).
Differently, projection 49 cooperates with locking element 50 when locking element 50 is at one of first blocking positions along line Y and at position P₃ along path P.
Projection 48 is housed with clearance within seat 58 when locking element 50 is at position Q₄ along line Y and at position P₄ along path P (Figures 11 and 12).
Differently, projection 48 cooperates with locking element 50 when locking element 50 is at one of second blocking positions along line Y and at position P₄ along path P.
Each coupling element 40a, 40b is shaped as a closed annulus, which extends about a respective axis B.
Furthermore, each coupling element 40a, 40b substantially comprises:

a cylindrical portion 41a, 41b, which extends along an axis orthogonal to axis A and relative axis B;
a portion 42a, 42b opposite to the relative portion 41a, 41b with respect to relative axis B; and
two opposite curved C-shaped portions 43a, 44a; 43b, 44b, which are positioned with their concavities facing respective portions 41a, 42a; 41b, 42b.

In particular, portions 41a, 41b engage respective grooves 29 of end 27 of flange 25 and comprise respective stop ends 45a, 45b.
Stop ends 45a, 45b abut against corresponding surfaces of end 27 when coupling elements 40a, 40b engage respective grooves 29.
Moving from each portion 43a, 43b to respective portion 44a, 44b, each portion 42a, 42b substantially comprises a tooth 46a, 46b and a concave cavity 47a, 47b, which is delimited by an arc-shaped profile.
In particular, teeth 46a, 46b protrude from respective portions 42a, 42b substantially parallel to relative axis B.
Furthermore, each portion 43a, 43b comprises a groove 39a, 39b (Figures 2, 7, 8, 11, 13 and 15), in order to avoid that portions 41a, 41b interfere with relative grooves 29, when coupling elements 40a, 40b engage relative grooves 29.
Projections 48 protrudes from portion 44a of coupling element 40a on the opposite side of axis B.
Projections 49 protrudes from a section which divides portions 42b, 44b of coupling element 40b on the opposite side of axis B.
Preferably, projections 48, 49 are tapered towards respective sides opposite to respective axes B.
Therefore, projections 48, 49 are staggered along path P.
In particular, projection 49 is located upstream from projection 48 along path P.
Coupling elements 40a, 40b are substantially vertical in the first configuration (Figures 5, 6, 7, 9, 11, 13 and 15).
With regard to such first configuration, axes B are orthogonal to axis A, and portions 42a, 42b are located under respective portions 41a, 41b.
Moving from portions 41a, 41b to respective portions 42a, 42b, portions 43a, 43b are descending.
Moving from portions 42a, 42b to relative portions 41a, 41b, portions 44a, 44b are ascending.
Due to the fact that coupling elements 40a, 40b are substantially vertical in first configuration, the abutment of portions 43a, 43b against end 27 prevents respective coupling elements 40a, 40b from being detached from end 27 of flange 15.
Furthermore, teeth 46a, 46b and cavities 47a, 47b face main portion 21 of duct 20 and define a seat 38 (figure 16) firmly engaged by main portion 21 of duct 20.
More precisely, seat 38 is shaped as an arc of circumference symmetric with respect to axis A, and is defined, at its first side, by tooth 46a and cavity 47a, and at its second side, opposite to first side with respect to axis A, by tooth 46b and cavity 47b.
Coupling elements 40a, 40b are substantially horizontal in the second configuration (Figures 3).
With regard to second configuration, axes B are parallel to axis A, portions 41a, 41b are interposed between portions 42a, 42b, and portions 43a, 43b are crosswise to axis A.
In this way, when coupling elements 40a, 40b move relative to end 27 along a direction opposite to the one indicated by the arrows in Figure 2, portions 43a, 43b do not interfere with end 27 and coupling elements 40a, 40b can be freely detached from end 27.
Locking element 50 is shaped as an elongated closed annulus and integrally comprises:

a rectilinear portion 51 sliding on first profiles nearer to relative axes B of respective coupling elements 40a, 40b arranged in the first configuration;
a portion 52 opposite to portion 51 and sliding on second profiles, opposite to the respective first profiles, of respective coupling elements 40a, 40b arranged in the first configuration; and
two opposite curved C-shaped portions 53a, 53b, which are positioned with their concavities facing respective portions 51, 52.

Portion 52 comprises two curved C-shaped sections 54 which are positioned with their concavities facing portions 51 and the second profiles of respective coupling elements 40a, 40b.
Furthermore, portion 52 comprises a rectilinear section 55 interposed between sections 54, and sections 54 define seats 58, 59.
In particular, coupling elements 40a, 40b are forged, so reducing the risk that cracks may arise in curved portions 43a, 43b, 44a, 44b.
Operation of unit 1 will be now described starting with coupling elements 40a, 40b in the first configuration (Figure 5) and with locking element 50 in position P₁ (figure 6) along path P.
In this situation, connecting means 35 firmly connect main portion 21 of duct 20 with flange 25.
Very briefly, web 3 is fed along the forming path, and tube 10 is formed and filled with the food product by duct 20.
Subsequently, jaw-type forming assembly grips and seals tube 10 along equally spaced cross sections to form a succession of packages.
The heat sealing of each cross section of tube 10 pumps upwards an amount of food product towards flange 25.
Such amount of food product goes across holes 22 and generates stresses and vibrations on coupling elements 40a, 40b and locking element 50.
Such vibrations and stresses move locking element 50 with respect to coupling elements 40a, 40b both along path P and parallel to line Y.
Due to the fact that these vibrations and stresses are random, it is very unlikely that such vibrations and stresses move, at position P₃, at the same time locking element 50 at position Q₂ parallel to line Y and towards position P₄ along path P.
On the contrary, these vibrations and stresses are likely to move locking element 50 along line Y in one of first blocking positions, at which projection 49 abuts against C-shaped portions 53a, so preventing locking element 50 from moving towards position P₃ along path P.
Consequently, locking element 50 remains in position P₁ and coupling elements 40a, 40b remain in first configuration (Figure 5).
In case vibrations and stresses move, at position P₃, at the same time locking element 50 both in position Q₂ along line Y and locking element towards position P₂ along path P, projection 49 is housed with clearance within seat 59 moving along path P, and locking element 50 remains between positions P₃, P₄ along path P.
Vibrations and stresses continue to move locking element 50 with respect to coupling elements 40a, 40b both along path P between positions P₃, P₄ and parallel to line Y.
Due to the fact that these vibrations and stresses are random, it is very unlikely that such vibrations and stresses move, at position P₄, at the same time locking element 50 in position Q₄ parallel to line Y and towards position P₂ along path P.
On the contrary, these vibrations and stresses are likely to move locking element 50 parallel to line Y in one of second blocking positions, at which projection 48 abuts against C-shaped portion 53b (figure 10), so preventing locking element 50 from moving towards position P₂ along path P.
When unit 1 is stopped, an operator can easily detach connecting means 35 form main portion 21 of duct 20 and flange 25.
In fact, starting with coupling elements 40a, 40b in the first configuration (Figure 5) and with locking element 50 in position P₁ (figure 6), the operator pushes at first locking element 50 in position P₃ and then moves locking element 50 along line Y in position 0₂.
At this stage, the operator further pushes locking element 50 towards position P₄, so that projection 49 is housed with clearance within seat 59 moving along path P.
When locking element 50 is near to position P₄, the operator moves locking element 50 along line Y in position Q₄ and pushes locking element 50 towards position P₂ along path P.
In this way, projection 48 is housed with clearance within seat 58 moving along path P and locking element 50 can easily pushed to position P₂.
Once locking element 50 is at position P₂, coupling elements 40a, 40b can reach the second configuration (Figure 3) and can be easily detached from main portion 21 of duct 20 and flange 25.
Analogously, the operator can easily connect main portion 21 of duct 20 with flange 25 by using connecting means 35.
In fact, starting with coupling elements 40a, 40b in second configuration (Figure 2) and detached from main portion 21 of duct 20 and flange 25, the operator at first engages portions 41a, 41b within respective grooves 29 up to stop ends 45a, 45b abut against the surface of end 27 surrounding respective grooves 29 (Figure 3), then rotate coupling elements 40a, 40b with respect to relative portions 41a, 41b up to coupling elements 40a, 40b reaches the first configuration (Figure 4).
At this stage, locking element 50 is in position P₂ (figure 4).
Then, the operator moves locking element 50 towards position P₄, moves locking element 50 parallel to line Y up to position Q₄, and moves locking element 50 from position P₄ to position P₃.
Finally, the operator moves locking element 50 at first parallel to line Y up to position Q₂ and then up to position P₁ (Figure 5).
At this stage, coupling elements 40a, 40b are firmly locked by locking elements 50 in the first configuration and unit 1 can be operated.
The advantages of unit 1 according to the present invention will be clear from the foregoing description.
In particular, locking element 50 can move along path P from position P₁ to position P₂ only if it is at the same time at position Q₂ along line Y and at position P₃ along path P, and if it is at the same time at position Q₄ parallel to line Y and at position P₄ along path P.
Since the vibrations and stresses generated by the upwards flow of the food product and acting on connecting means 35 are very unlikely to cause such combination of movements of locking element 50, connecting means 35 are very unlikely to be accidentally detached from main portion 21 of duct 20 and flange 25.
Analogously, such vibrations and stresses are very unlikely to break locking element 50 or coupling elements 40a, 40b, so drastically reducing the risk that some parts of them may fall into tube 10 and remain into the packages.
In particular, due to the fact that coupling elements 40a, 40b are shaped as respective closed annuluses, potential cracks in such coupling elements 40a, 40b are prevented from causing the fall of pieces of them into tube 10.
Finally, locking element 50 can be easily moved along path P by an operator from position P₁ to position P₂ and vice versa when unit 1 is stopped, so that coupling elements 40a, 40b can be easily moved by the operator between first and second configuration.
Clearly, changes may be made to unit 1 without, however, departing from the scope as defined in the accompanying Claims.
In particular, coupling elements 40a, 40b could comprise a plurality of respective projections 48, 49.

Claims

Packaging unit (1) for producing sealed packages of a food product pourable into a tube (10) of packaging material, comprising;
- a duct (20) for pouring said food product into said tube (10);

- a flange (25) receivable within said tube (10);

- releasable connecting means (35) for connecting said duct (20) to said flange (25);
characterized in that said connecting means (35) comprise, in turn,:
- two coupling elements (40a, 40b) movable with respect to said duct (20) between a first and second configuration; said coupling elements (40a, 40b) being firmly connected to said duct (20) and to said flange (25) in said first configuration, and being detachable from said duct (20) and said flange (25) in said second configuration;

- a locking element (50) movable with respect to said coupling elements (40a, 40b) arranged in said first configuration along a path (P) between a first position (P₁) in which said coupling elements (40a, 40b) are kept in said first configuration by said locking element (50), and a second position (P₂) in which said coupling elements (40a, 40b) are free to move towards said second configuration; and

- first constraining means (49, 59) for constraining said locking element (50) to said coupling elements (40a, 40b) at least at a third position (P₃), which is arranged between said first and second position (P₁, P₂) along said path (P);
said locking element (50) being movable, at said third position (P₃), with respect to said coupling elements (40a, 40b) crosswise to said path (P) of said path (P) between at least a fourth position in which said first constraining means (49; 59) prevent said locking element (50) from moving along said path (P) towards said second position (P₂), and a fifth position (Q₂) in which said locking element (50) is free to move along said path (P) towards said second position (P₂).
Unit according to claim 1, characterized by comprising second constraining means (48; 58) for constraining said locking element (50) to said coupling elements (40a, 40b) at least at a sixth position (P₄) along said path (P);
said locking element (50) being also movable with respect to said coupling elements (40a, 40b) crosswise to said path (P) at said sixth position (P₄) between at least a seventh position in which said second constraining means (48, 58) prevent said locking element (50) from moving along said path (P) towards said second position (P₂) and an eight position (Q₄) in which said locking element (50) is free to move along said path (P) towards said second position (P₂).
Unit according to claims 1 or 2, characterized in that said first constraining means (49, 59) comprise a first seat (59) defined by said locking element (50), and a first projection (49) defined by one (40b) of said coupling element (40a, 40b) and located at said third position (P₃); said first projection (49) being housed with clearance within said first seat (59) at said fifth position (Q₂) of said locking element (50), and contacting said locking element (50) at said fourth position of said locking element (50) to prevent said locking element (50) from moving towards said second position (P₂).
Unit according to claim 2 or 3 when dependent on claim 2, characterized in that said locking element (50) is movable at said third and sixth positions (P₃, P₄) parallel to a rectilinear line (Y) crosswise to said path (P); said fifth position (Q₂) and said eight position (Q₄) being spaced along said rectilinear line (Y).
Unit according to anyone of claims 2 to 4, characterized in that said second constraining means comprise a second seat (58) defined by said locking element (50), and a second projection (48) defined by the other (40a) of said coupling element (40a, 40b) and located at said sixth position (P₄); said second projection (48) being housed with clearance within said second seat (58) at said eight positions (Q₄) of said locking element (50), and contacting said locking element (50) at said seventh position of said locking element (50) to prevent said locking element (50) from moving towards said second position (P₂).
Unit according anyone of foregoing claims, characterized in that said coupling elements (40a, 40b) are shaped as respective closed annuluses.
Unit according anyone of foregoing claims, characterized in that said coupling elements (40a, 40b) define in said first configuration an arc-shaped third seat (38) which can be engaged by said tube (10).
Unit according to claim 7, characterized in that said coupling elements (40a, 40b) comprise respective first portions (42a, 42b) and second portions (41a, 41b) opposite to corresponding said first portions (42a, 42b).
Unit according to claim 8, characterized in that said first portions (42a, 42b) comprise respective teeth (46a, 46b) and cavities (47a, 47b) facing one another and defining said third seat (38) when said coupling elements (40a, 40b) are in said first configuration.
Unit according to claim 8 or 9, characterized in that said second portions (41a, 41b) may be engaged in respective grooves (29) defined by said flange (25).
Unit according to anyone of claims 8 to 10, characterized in that each coupling element (40a, 40b) comprises a third portion (44a, 44b) interposed between respective first and second portions (42a, 42b; 41a, 41b); said third portions (44a, 44b) being ascending, moving from said respective first portions (42a, 42b) towards said respective second portions (41a, 41b) when said coupling elements (40a, 40b) are in said first configuration.
Unit according to claim 11, characterized in that each coupling element (40a, 40b) comprises a fourth portion (43a, 43b) opposite to respective said third portion (44a, 44b) and provided with a further groove (39a, 39b).