Unwinding device for wrapping machine

Abstract

A winding device (1) for delivering a film (50) comprises first and second pre-stretching rollers (2, 3) and a motion transmission system (4) for transmitting rotational motion from the first pre-stretching roller (2) to the second pre-stretching roller (3) at a defined gear ratio, wherein the motion transmission system (4) comprises two toothed driving wheels (5, 6) rotated by the pre-stretching roller (2), two toothed driven wheels (7, 8) mounted on the second pre-stretching roller (3) and optionally connectable, and a selection means (10) operable to connect the toothed driven wheels (7, 8) to the second pre-stretching roller (3) at once, wherein each of the toothed driven wheels (5, 6) engages with each of the toothed driving wheels (5, 6), and the selection means (10) is connected to the second pre-stretching roller (3), and the toothed driven wheels (7, 8) is rotatably supported and the output shaft (11) is provided with a longitudinal internal cavity (12) having at least one first lateral through hole (13) and at least one second lateral through hole (14) suitable for receiving each locking member (20), and the selection pin (15) is provided with first and second transverse protrusions (16, 17), wherein the selection pin is provided with a first operating position (A) in which the first transverse protrusion (16) pushes the locking member (20) to engage with the first seat (22) of the first toothed driven wheel (7) within the internal cavity (12) and to engage the second seat (22) of the second toothed driven wheel (8) and to engage the second toothed driven wheel to engage the output shaft (11). The transverse protrusion (17) can move between a second operating position (B) that pushes out the locking member (20).

Description

Technology Field

[0001] The present invention relates to a machine and apparatus for wrapping a load with a film of a stretchable plastic material, and in particular to a unwinding apparatus associated with a wrapping machine. Background Technology

[0002] In the packaging industry, it is disclosed that a load formed as a single product or a number of products grouped on a pallet is wrapped with a film of a low-temperature stretchable plastic material.

[0003] In particular, the disclosed wrapping machine includes a unwinding device that is movable along and / or around a wrapping axis and supports a film reel, and a film is unwound from the reel to form a series of strips or bands having a spiral arrangement, generally due to a combination of relative linear and rotational motion between the unwinding device and the load.

[0004] In a wrapping machine equipped with a rotary table to support a load, the load rotates together with the table around a vertical wrapping axis during the wrapping process, while the unwinding device moves alternately along a fixed vertical bar of the wrapping machine, moving parallel to the wrapping axis.

[0005] In a self-propelled wrapping machine, the unwinding device moves alternately along a vertical bar fixed to a movable carriage of the self-propelled wrapping machine, which rotates around a load that is kept in a fixed state during wrapping.

[0006] In a wrapping machine with a horizontal rotating ring or rotating arm, the load remains fixed during wrapping, while the unwinding device moves relative to the load by rotating around a vertical wrapping axis or translating along the vertical wrapping axis. To this end, the unwinding device is fixed to a ring structure or arm that is rotatably supported by a machine frame and can rotate around the load.

[0007] In a wrapping machine with a vertical rotating ring, the load moves horizontally through the ring, while the unwinding device fixed to the ring rotates around the horizontal axis.

[0008] The unwinding device supports a film reel and is generally provided with a pair of pre-stretching rollers having a slow roller and a fast roller upstream and downstream, respectively, in relation to film movement to stretch and unwind the film, and one or more guide rollers that deflect the film toward the load during unwinding.

[0009] By appropriately adjusting the difference in rotational speed between the two pre-stretch rollers, it is possible to stretch or extend the film by a predetermined amount according to a predetermined pre-stretch rate or elongation rate when the film leaves the unwinding device before wrapping around the perimeter of the load.

[0010] Plastic films are stretched before being wrapped around the perimeter of a load, as stretching or elongation imparts physical and mechanical properties to the film that make it more suitable for use and better withstand the forces acting on the load during subsequent handling and transport. In particular, when the stretching force is discontinued, the film's elastic recovery generates a clamping force around the load, ensuring a seal.

[0011] In general, the elongation rate of a film is expressed in percentage units as the ratio between the original length of the film and the final length of the stretched film. Generally, the elongation rate imparted to a film is 50 to 300%.

[0012] The elongation or pre-stretching force applied by the pre-stretching roller significantly reduces the thickness of the film and proportionally increases the length of the film, thereby allowing a longer perimeter of the load to be wrapped with the same initial amount of unwound film, and also changing the mechanical properties of the film.

[0013] The pre-stretching force applied to the film to achieve a set elongation ratio or elongation rate depends on a number of factors, such as the type of load to be wrapped (e.g., a somewhat stable product), the initial thickness of the film, the physical and mechanical characteristics of the film's plastic material (material type, composition, amount and distribution of internal impurities and non-uniform substances), and the environmental conditions under which the load is wrapped (temperature, humidity).

[0014] A winding device is disclosed that includes an electric motor that rotates one of two pre-stretching rollers acting as a driving roller (typically a fast roller) and rotates the other pre-stretching roller acting as a driven roller (typically a slow roller) through a mechanical motion transmission assembly to drive a pre-stretching roller. In this way, a gear ratio set between the fast roller and the slow roller is applied according to the desired pre-stretching or elongation value or ratio in the film.

[0015] A mechanical transmission assembly is a reduction gear that substantially connects the shaft of a fast roller to the shaft of a slow roller to reduce the rotational angular velocity of the slow roller relative to the rotational angular velocity of the fast roller.

[0016] A transmission assembly generally consists of gears comprising a series of gear wheels having different numbers of teeth and meshing with each other. Alternatively, a transmission assembly may be formed of pairs of gear pulleys having different numbers of teeth that are coupled to each other by a belt, i.e., tooth-coupled.

[0017] Therefore, to change the film pre-stretch value or ratio while using the wrapping machine depending on the characteristics of the load to be wrapped, the characteristics of the film, environmental conditions, etc., the speed or gear ratio of the transmission assembly must be changed. In other words, one or more gears or pulleys must be replaced.

[0018] Although the gears and pulleys of some disclosed unwinding devices are configured to be quickly assembled and disassembled, replacement is difficult and not only stops the wrapping machine for a considerable period but also requires an operating expert. The problem to be solved

[0019] The objective of the present invention is to improve a disclosed unwinding device associated with a wrapping machine for wrapping a load with a film of a stretchable plastic material.

[0020] Another task is to provide a winding device that can quickly and easily change the preliminary elongation or elongation rate of the film wrapping the load.

[0021] An additional task is to implement an unwinding device that has a low-complexity and economical structure and can operate accurately and reliably. means of solving the problem

[0022] These and additional problems are solved by an extraction device according to one or more of the claims specified below.

[0023] A unwinding device according to the present invention, which may be associated with a wrapping machine and is arranged to deliver a film of a stretchable plastic material, comprises a first pre-stretching roller and a second pre-stretching roller that are rotatably rotatably about a longitudinal axis of rotation and arranged to unwind and pre-stretch the film, and a motion transmission system configured to connect the pre-stretching rollers and transmit rotational motion from the first pre-stretching roller to the second pre-stretching roller at at least one defined gear ratio. The motion transmission system comprises at least two toothed driving wheels that are mounted integrally with each other coaxially and rotated by a pre-stretching roller and each have a different number of teeth; at least two toothed driven wheels that are mounted coaxially with the second pre-stretching roller and can optionally be coupled to the second pre-stretching roller, and each have a different number of teeth and mesh with the respective toothed driving wheels to transmit the rotational motion of the first pre-stretching roller to the second pre-stretching roller at a corresponding gear ratio when the toothed driven wheels are coupled to the second pre-stretching roller; and a selection means that can be operated to couple the toothed driven wheels to the second pre-stretching roller at once.

[0024] The selection means comprises an output shaft connected to the second pre-stretching roller and rotatably supporting a toothed driven wheel, and provided with a longitudinal internal cavity having at least one first lateral through hole and at least one second lateral through hole suitable for receiving each locking member, and a selection pin that slides into the internal cavity and is provided with one first transverse protrusion and one second transverse protrusion, wherein the selection pin has a first operating position in which the first transverse protrusion is pushed adjacent to at least one locking member so as to protrude outward from the first lateral through hole to engage with a first seat of the first toothed driven wheel and to engage the first toothed driven wheel with the output shaft, and a second transverse protrusion in which the second transverse protrusion is pushed adjacent to at least one locking member so as to protrude outward from the second lateral through hole to engage with a second seat of the second toothed driven wheel and to engage the second toothed driven wheel with the output shaft. It can be moved between operating positions. Effects of the invention

[0025] Accordingly, the gear ratio of the pre-stretching roller can be changed quickly and easily by the unwinding device according to the present invention. In practice, by engaging each toothed driving wheel, each toothed driven wheel is coupled and integrated with the output shaft connected to the second pre-stretching roller, thereby allowing the rotational motion of the first pre-stretching roller to be transmitted to the second pre-stretching roller according to the corresponding gear ratio depending on the number of teeth of the toothed driving and driven wheels. By acting on a selection pin of a selection means provided with a transverse protrusion acting on a locking member capable of locking the desired toothed driven wheel to the output shaft, it is possible to actually combine and integrate the desired driven wheel with the output shaft and thus with the second pre-stretching roller.

[0026] The present invention is not limited to but can be better understood and implemented by referring to the accompanying drawings illustrating exemplary embodiments. Brief explanation of the drawing

[0027] FIG. 1 is a front view of a winding device according to the present invention supporting a film reel. FIG. 2 is a cross-sectional view along line II-II of FIG. 1. FIG. 3 is an enlarged partial perspective view showing the preliminary stretching roller and motion transmission system of the unwinding device of FIG. 1. FIG. 4 is a plan view of the preliminary stretching roller and motion transmission system of FIG. 3. FIG. 5 is an enlarged partial cross-sectional view along line VV of FIG. 4 in which the selection member of the motion transmission system is in the first operating position. FIG. 6 is a detailed enlarged view of the cross-section of FIG. 5. FIG. 7 is a cross-sectional view similar to FIG. 5 in which the selected member is in a non-operating position. FIG. 8 is a drawing similar to FIG. 5 in which the selection member is in a second operating position. Specific details for implementing the invention

[0028] FIG. 1 illustrates a unwinding device (1) according to the present invention, and the unwinding device may be associated with a wrapping machine not illustrated, configured to deliver a film (50) of a flexible plastic material that is unwound from a reel (60) and wrapped around a load formed, for example, by one or more products grouped on a pallet.

[0029] The unwinding device (1) includes a first pre-stretching roller (2) and a second pre-stretching roller (3) that are rotatably rotatably about a longitudinal axis of rotation and are arranged to unwind and pre-stretch the film (50). The unwinding device (1) also further includes a plurality of guide rollers (33, 34, 36) that deflect the film from the reel (60) toward the pre-stretching rollers (2, 3) and deflect it from the pre-stretching rollers toward the load to be wrapped coming out of the unwinding device (1). More precisely, the unwinding device (1) includes a first guide roller (33) that deflects the film (50) unwound from the reel (60) toward the second pre-stretching roller (3), and second and third guide rollers (34, 36) that deflect the film (50) leaving the first pre-stretching roller (2) toward the load to be wrapped.

[0030] The pre-stretch rollers (2, 3) are rotatably mounted on the support frame (30) of the unwinding device (1) which rotatably supports the film reel (60).

[0031] The unwinding device (1) further comprises a motion transmission system (4) configured to mechanically connect the pre-stretching rollers (2, 3) in particular to each other and to transmit rotational motion from the first pre-stretching roller (2) to the second pre-stretching roller (3) at one or more defined gear ratios. More precisely, the motion transmission system (4), which is rotated by the first pre-stretching roller (2) (fast roller), which is a roller that rotates faster, for example at a first angular velocity or rotational speed, rotates the second pre-stretching roller (3) (slow roller) at a second angular velocity or rotational speed lower than the first angular velocity or rotational speed.

[0032] The motion transmission system (4) comprises at least two toothed drive wheels (5, 6) that are mounted coaxially and integrally with each other, rotated by a first pre-stretching roller (2), and each have a different number of teeth. The toothed drive wheels (5, 6) are mounted and fixed on a support shaft (31) which is rotatably supported by a support frame (30) of the unwinding device (1). In the embodiment of the drawing, the motion transmission system (4) comprises a first toothed drive or main wheel (5) and a second toothed drive or main wheel (6).

[0033] The motion transmission system (4) also includes at least two toothed driven wheels (7, 8) that are mounted coaxially and integrally with each other and can be optionally coupled to a second pre-stretching roller (i.e., optionally coupled) and each have a different number of teeth, and each toothed driven wheel (7, 8) is engaged with each toothed driving wheel (5, 6) to transmit rotational motion from the first pre-stretching roller (2) to the second pre-stretching roller (3) at a corresponding gear ratio when the toothed driven wheel (7, 8) is coupled to the second pre-stretching roller (3).

[0034] The gear ratio is related to the number of teeth of the aforementioned toothed driven and driving wheels.

[0035] In the embodiment of the drawing, the motion transmission system (4) includes a first toothed drive wheel (5) and a second toothed drive wheel (6).

[0036] The number of toothed driven wheels (7, 8) is the same as the number of toothed driving wheels (5, 6).

[0037] The first drive wheel (5) and the first driven wheel (7) meshing with each other perform a first gear ratio between the preliminary stretching rollers (2, 3), and the second drive wheel (6) and the second driven wheel (8) meshing with each other perform a second gear ratio between the preliminary stretching rollers (2, 3).

[0038] The motion transmission system (4) further includes a selection means (10) that can be manually operated, for example by an operator, to selectively combine and connect the toothed driven wheels (7, 8) to the second pre-stretching roller (3) at once.

[0039] More precisely, the selection means (10) is configured to combine the first toothed driven wheel (7) or the second toothed driven wheel (8) with the second pre-stretching roller (3) to form a single unit, depending on the operation requirement.

[0040] The selection means (10) includes a driven or auxiliary shaft (11) connected to a second pre-stretching roller (3) to rotate together with a second pre-stretching roller around a longitudinal axis and rotatably supporting a toothed driven wheel (7, 8). The output shaft (11) has a blind internal cavity (12) having one or more first lateral through holes (13) and one or more second lateral through holes (14) suitable for receiving each locking member (20). The first through hole (13) and the second through hole (14) are openings orthogonal to the central longitudinal axis (X) of the output shaft (11), particularly coinciding with the rotation axis of the output shaft. The first through hole (13) and the second through hole (14) are, for example, circular radial openings formed in the lateral tubular wall (11b) of the output shaft (11). The first through hole (13) and the second through hole (14) are spaced apart from each other along a direction parallel to the longitudinal axis (X) of the output shaft (11).

[0041] In the embodiment illustrated in the drawing, the internal cavity (12) of the output shaft (11) includes a plurality of first through holes (13) spaced apart at a certain angle with respect to the longitudinal axis (X) of the output shaft (11), said through holes are placed in each plane particularly orthogonal to the longitudinal axis (X) and are suitable for receiving each locking member (20). The internal cavity (12) also includes a plurality of second through holes (14) spaced apart at a certain angle with respect to the longitudinal axis (X), said through holes are placed in each plane particularly orthogonal to the longitudinal axis (X) and are suitable for receiving each locking member (20). The locking members (20) each include a spherical member.

[0042] The selection means (10) includes a selection pin (15) that slides into an internal cavity (12) and is provided with a first transverse projection (16) and a second transverse projection (17). The transverse projections (16, 17) are positioned in respective planes that are nearly orthogonal to the longitudinal axis (X) of the output shaft (11).

[0043] The selection pin (15) includes a gripping end (24) in the shape of a handle, which protrudes from the access opening (11a) of the internal cavity (11), said access opening (11a) is located on the opposite side of the second pre-stretching roller (3). More precisely, the access opening (11a) is formed at the first end (11c) of the output shaft (11) opposite to the second end (11c) of the output shaft (11) connected to the second pre-stretching roller (3).

[0044] The selection pin (15) can be moved, particularly manually, between at least a first operating position (A) and a second operating position (B).

[0045] In the first operating position (A), the first transverse protrusion (16) of the selection pin (15) is pushed adjacent to one or more locking members (20) to protrude outward from each first through hole (13) to engage with each first seat (21) of the first toothed driven wheel (7) and to combine and integrate the first toothed driven wheel (7) with the output shaft (11), thereby preventing relative rotation between the first toothed driven wheel (7) and the output shaft (11).

[0046] In the second operating position (B), the second transverse protrusion (17) of the selection pin (15) is pushed adjacent to one or more locking members (20) to protrude outward from the second through hole (14) to engage with the second seat (22) of the second toothed driven wheel (8) and to combine and integrate the second toothed driven wheel (8) with the output shaft (11), thereby preventing relative rotation between the second toothed driven wheel (8) and the output shaft (11).

[0047] The first lateral through-hole (13) and the second lateral through-hole (14) of the output shaft (11) face the first seat (21) of the first toothed driven wheel (7) and the second seat (22) of the second toothed driven wheel (8), respectively.

[0048] The selection pin (15) has, for example, a cylindrical shape, and the shape of the inner cavity (12) of the output shaft (11) is also cylindrical, and the transverse protrusions (16, 17) are annular protrusions, each including a conical portion (16a, 17a) and a cylindrical portion (16b, 17b) respectively to push and guide the locking member (20) to protrude outward from the through hole (13, 14) and to maintain the locking member (20) protruding outward from the through hole (13, 14) in an operating position (A, B). The conical portion (16a, 17a) of each transverse protrusion (16, 17) further guides the locking member into each through hole (13, 14) when the selection pin (15) is moved.

[0049] The first toothed driven wheel (7) and the second toothed driven wheel (8) each include a first through hole or hub (18) and a second through hole or hub (19) suitable for rotatable coupling with the output shaft (11), particularly by the insertion of a rolling bearing (35). The first through hole (18) and the second through hole (19) each have a plurality of first seats (21) and second seats (22) suitable for receiving a locking member (20). The first seats (21) and second seats (22) each include a recess formed radially in the inner cylindrical wall of the through hole (18, 19).

[0050] The selection pin (15) can also be moved to a non-operating position (C) where the first transverse protrusion (16) and the second transverse protrusion (17) are not adjacent to the locking member (20), so that the toothed driven wheels (7, 8) can idle on the output shaft (11). In this non-operating position (C) of the selection pin (15), the second pre-stretching roller (3) is not rotated by the first pre-stretching roller (2).

[0051] The selection means (10) acts on the selection pin (15) and includes a locking means (25) configured to reversibly lock at least a first operating position (A) or a second operating position (B) and to avoid unwanted axial displacement of the selection pin (15) inside the inner cavity (12) during operation of the wrapping device (1).

[0052] The locking means (25) comprises one or more elastic pressure members (26), for example, two spring pressure members fixed to the output shaft (11) and a first annular groove (27) and a second annular groove (28) formed in the selection pin (15). The elastic pressure members (26) are configured to engage with the first annular groove (27) or the second annular groove (28) at the first operating position (A) and the second operating position (B) of the selection pin (15), respectively. An elastic pressure member (26) of the disclosed type not described in detail includes a stop member that slides into a support member and is held in a protruding position by a spring.

[0053] The locking means (25) further includes a third annular groove (29) inserted between the first annular groove (27) formed on the selection pin (15) and the annular groove (28) which is engaged by an elastic pressing member (26) when the selection pin (15) is placed in a non-operating position (C). In this way, the locking means (25) reversibly locks the selection pin (15) even in a non-operating position (C).

[0054] The unwinding device (1) includes a motor (38), specifically a rotary electric motor positioned to rotate the first pre-stretch roller (2). More precisely, the motor (38) is connected to the first pre-stretch roller (2) by a drive or main shaft (32).

[0055] The motion transmission system (4) is fixed coaxially to the first pre-stretching roller (2) and includes a third toothed drive wheel (9) that acts on one of the toothed drive wheels (5, 6), for example, the second toothed drive wheel (6), by means of an inserted idle toothed wheel (23). The third toothed drive wheel (9) is fixed and becomes integral with the drive shaft (32).

[0056] The operation of the unwinding device (1) according to the present invention provides for selecting a gear ratio between the pre-stretching rollers (2, 3) required according to the pre-stretching rate or elongation rate to be achieved in the film (50) during wrapping, by acting on the selection means (10) of the motion transmission system (4) during the initial adjustment stage of the wrapping machine in which the unwinding device is installed.

[0057] More precisely, when a first gear ratio is required between the pre-stretching rollers (2, 3), the operator can move the selection pin (15) to a first operating position (A) to be coupled and integrated with the output shaft (11), and then when the first pre-stretching roller (2) is rotated by the motor (38), it can be coupled and integrated with the first toothed driven wheel (7), i.e., the second pre-stretching roller (3), which is rotated by engaging with the first toothed driving wheel (5).

[0058] At the first operating position (A) of the selection pin (5), the first transverse protrusion (16) is pushed adjacent to the locking member (20) so as to protrude outward from the first lateral through hole (13) of the inner cavity (12) of the output shaft (11), and engages with the first seat (21) of the first toothed driven wheel (7) so that the first toothed driven wheel (7) is combined with the output shaft (11) and becomes an integral part.

[0059] If a second gear ratio is required between the pre-stretching rollers (2, 3), the operator can move the selection pin (15) to a second operating position (B) to be coupled and integrated with the output shaft (11), and then when the first pre-stretching roller (2) is rotated by the motor (38), it can be coupled and integrated with the second toothed driven wheel (8), i.e., the second pre-stretching roller (3), which is rotated by engaging with the second toothed driving wheel (6).

[0060] At the second operating position (B) of the selection pin (5), the second lateral protrusion (17) is pushed adjacent to the locking member (20) so as to protrude outward from the second lateral through hole (14) of the inner cavity (12) of the output shaft (11), and engages with the second seat (22) of the second toothed driven wheel (8) so that the second toothed driven wheel (8) is coupled and integrated with the output shaft (11).

[0061] In a modified example of the unwinding device (1) according to the present invention not shown in the drawing, the motion transmission system (4) may include three or more toothed driving wheels integrated and mounted coaxially on a support shaft (31), and a corresponding number of toothed driven wheels that are mounted coaxially but optionally or alternatively connected to an output shaft (11) connected to a second pre-stretching roller (3).

[0062] In this variation, the selection means (10) includes three or more lateral through holes spaced apart along a direction parallel to the longitudinal axis (X) of the output shaft (11). Each of the lateral through holes is spaced apart at a constant angle with respect to the longitudinal axis (X) and faces the seat of each toothed drive wheel.

[0063] Similarly, the selection pin (15) includes a plurality of transverse protrusions equal to the number of lateral through-holes, i.e., the number of toothed driven wheels. Each transverse protrusion may be associated with each of the plurality of lateral through-holes and is configured to be pushed adjacent to the locking member (20) to protrude outward from each of the aforementioned through-holes, and to engage with the seat (22) of the corresponding toothed driven wheel to integrate the toothed driven wheel with the output shaft (11).

[0064] A unwinding device (1) according to the present invention, which may be associated with a wrapping machine for delivering a film of a stretchable plastic material, enables the rapid and easy change of the film's pre-stretch rate or elongation rate or value, that is, the change of the gear ratio between two pre-stretch rollers (2, 3), in particular, the change of the second pre-stretch roller (3) while maintaining the rotational speed of the first pre-stretch roller (2) without changing the rotational speed of the first pre-stretch roller (2).

[0065] By means of a motion transmission system (4) of a winding device (1) comprising at least one pair of driving wheels (5, 6) with different tooth counts and driven by a first pre-stretching roller (2), and a pair of toothed driven wheels (7, 8) with different tooth counts and which can be selectively coupled to a second pre-stretching roller (3) by a selection means (10), it is actually possible to quickly and easily change the gear ratio between the pre-stretching rollers (2, 3).

[0066] Each toothed driven wheel (7, 8) is coupled with an output shaft (11) connected to the second pre-stretching roller (3) and integrated with each toothed driving wheel (5, 6), thereby making it possible to transmit the rotational motion of the first pre-stretching roller (2) to the second pre-stretching roller (3) according to a corresponding gear ratio related to the number of teeth of the toothed driving and driven wheels.

[0067] By acting on the selection pin (15) of the selection means (10), which is provided with a lateral protrusion (16, 17) acting on the locking member (20) capable of locking the desired toothed driven wheel (7, 8) to the output shaft (11), it is possible to actually connect and integrate the desired driven wheel (7, 8) to the output shaft (11) and thus to the second pre-stretching roller (3).

[0068] The accurate positioning and stability of the selection pin (15) at the operating positions (A, B) are ensured by a locking means (25) comprising a plurality of elastic pressure members (26) that can reversibly engage with annular grooves (27, 28) corresponding to each operating position (A, B) that are formed and maintained on the selection pin (15).

Claims

Claim 1 A winding device (1) associated with a wrapping machine and arranged to deliver a film (50) of stretchable plastic material unwound from a reel (60), comprising: a first pre-stretching roller (2) and a second pre-stretching roller (3) rotatably rotatably about a longitudinal axis of rotation and arranged to unwind and pre-stretch the film (50); a motion transmission system (4) configured to connect the pre-stretching rollers (2, 3) to each other and to transmit rotational motion from the first pre-stretching roller (2) to the second pre-stretching roller (3) at least one defined gear ratio, wherein the motion transmission system (4) comprises: at least two toothed drive wheels (5, 6) mounted integrally coaxially with each other and rotated by the first pre-stretching roller (2) and each having a different number of teeth; and a drive wheel mounted coaxially with the second pre-stretching roller (3) and optionally coupled to the second pre-stretching roller (3) and each having a different number of teeth. A winding device comprising: at least two toothed driven wheels (7, 8), wherein the toothed driven wheels (5, 6) engage with each toothed driving wheel (5, 6) to transmit rotational motion of the first pre-stretching roller (2) to the second pre-stretching roller (3) at a corresponding gear ratio when coupled to the second pre-stretching roller (3); and a selection means (10) capable of operating to couple the toothed driven wheels (7, 8) to the second pre-stretching roller (3) at once, wherein the selection means (10) comprises: an output shaft (11) connected to the second pre-stretching roller (3), rotatably supporting the toothed driven wheels (7, 8), and provided with a longitudinal internal cavity (12) having at least one first lateral through hole (13) and at least one second lateral through hole (14) suitable for receiving each locking member (20);- Includes a selection pin (15) that slides into the internal cavity (12) and is provided with at least one first transverse protrusion (16) and at least one second transverse protrusion (17), wherein the selection pin (15) is provided with a first operating position (A) in which the first transverse protrusion (16) is pushed adjacent to at least one locking member (20) so as to protrude outward from the first lateral through hole (13) to engage with the first seat (21) of the first toothed driven wheel (7) and to engage with the second seat (22) of the second toothed driven wheel (8) and to engage with the second toothed driven wheel to engage with the second lateral through hole (14) so ​​as to protrude outward from the second lateral through hole (14). A winding device characterized in that a protrusion (17) is movable between a second operating position (B) that pushes adjacent to at least one locking member (20), and the selection means (10) includes a locking means (25) configured to act on the selection pin (15) and reversibly lock the selection pin (15) at least at the first operating position (A) or the second operating position (B), wherein the locking means (25) includes at least first and second annular grooves (27, 28) of the selection pin (15) and at least one elastic pressing member (26), and the elastic pressing member (26) is configured to engage with the first annular groove (27) at the first operating position (A) of the selection pin (15) or to engage with the second annular groove (28) at the second operating position (B). Claim 2 A winding device according to claim 1, characterized in that the at least one first lateral through hole (13) and the at least one second lateral through hole (14) are spaced apart along a direction parallel to the longitudinal axis (X) of the output shaft (11). Claim 3 A winding device according to claim 1 or 2, wherein the internal cavity (12) of the output shaft (11) comprises a plurality of first lateral through holes (13) spaced apart at a certain angle with respect to the longitudinal axis (X) of the output shaft (11) and suitable for receiving each of the locking members (20), and second lateral through holes (14) spaced apart at a certain angle with respect to the longitudinal axis (X) and suitable for receiving each of the locking members (20). Claim 4 A winding device according to claim 1 or 2, wherein the selection pin (15) has a cylindrical shape and each lateral protrusion (16, 17) comprises a conical portion (16a, 17a) and a cylindrical portion (16b, 17b) configured to push the locking member (20) outwardly protruding from the lateral through hole (13, 14) and maintain the state in which the locking member (20) protrudes outwardly from the lateral through hole (13, 14). Claim 5 A winding device according to claim 1 or 2, wherein the first toothed driven wheel (7) and the second toothed driven wheel (8) each include a central opening (18, 19) provided with a plurality of first sheets (21) and second sheets (22) suitable for receiving each of the locking members (20) and suitable for coupling with the output shaft (11). Claim 6 A winding device according to claim 1 or 2, characterized in that the selection pin (15) is movable to a non-operating position (C) in which the first transverse protrusion (16) and the second transverse protrusion (17) are not adjacent to the locking member (20) and the toothed driven wheel (7, 8) idles on the output shaft (11). Claim 7 delete Claim 8 delete Claim 9 A winding device according to claim 1 or 2, characterized in that the selection pin (15) includes a gripping end (24) protruding from the access opening (11a) of the internal cavity (12) and positioned opposite the second pre-stretch roller (3). Claim 10 A winding device according to claim 1 or 2, characterized by including a motor (38) arranged to rotate the first pre-stretch roller (2). Claim 11 A winding device according to claim 1 or 2, wherein the motion transmission system (4) comprises a number of driven wheels (7, 8) equal to the number of toothed driving wheels (5, 6), and each toothed driven wheel (7, 8) engages with each specific toothed driving wheel (5, 6). Claim 12 In claim 2, the first and second lateral through holes (13, 14) are a winding device perpendicular to the longitudinal axis (X).

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