Device to control the feeding of the strap in a strapping machine

Abstract: Device to control the feeding of the strap in a strapping machine, of the type wherein the strap (R) is guided on the periphery of at least one driving wheel (1) and at least one pressure wheel (4) is provided to press the strap (R) against the periphery of said driving wheel (1) so as to guarantee its feeding. An arm (8) is provided to oscillate between a guiding position, in which it at least partly surrounds the periphery of the driving wheel (1) so as to form a guiding channel (C3) for the strap (R), and an opening position of release, away from the driving wheel (1), into which it is moved by said strap when it stops, and in which it controls the removal of said pressure wheel (4) so as to release the feeding pressure. Said oscillating arm (8) extends, beyond its part surrounding the periphery of the driving wheel (1), into a counter-bent portion which determines a saddle path (C4) for the strap (R).

Description: The present invention concerns a strapping machine and, more particularly, a device to control the feeding of the strap, allowing a more precise and rapid control of the strap stopping or speed-changing function. As known, machines of this type comprise drive means which draw the strap from a reel, launch it into a special guide track around the product to be packed, and then recover it - after its leading end has been blocked or clamped - so as to wind it around the product with a preset tension. Further means then provide to tie and cut the strap after its windup. It is also known that modern packing technique requires, on one hand, short operating times and thus high strap launching and recovery speeds and, on the other hand, the possibility to regulate the tensioning of the strap to a wide extent. However, these two requirements are often scarcely compatible. A strapping machine which tries to satisfy both these requirements is described, for example, in the Italian Patent No. 1.135.722, filed on March 24, 1981, by the same Applicant, to which reference is made herein for a better understanding of the present invention. In this machine the strap is drawn between a couple of wheels, i.e. a driving wheel and a pressure wheel, this latter mounted idle on its support shaft. A control motor acts, through an epicycloidal mechanism, on one hand, on the driving wheel and, on the other hand, on a release wheel held up by elastic means. In the initial phase of the recovery stroke of the strap, this release wheel is stopped by the elastic means and the action of the motor is transferred only to the driving wheel; however in the final phase of the recovery stroke, when the strap is tightened over the load and the tightening strength becomes higher than the action of said elastic means, these latter allow the release wheel to move and to control the shifting movement of the pressure wheel away from the driving wheel, thus releasing the strap, which may stop. This machine has turned out to be more than satisfactory for many years, but it is no longer apt to fully satisfy the present requirements of packing speed. One of the problems arising with the increase of the strap feeding speed is its instant and prompt stopping at the end of the launching stroke and, respectively, at the end of the recovery stroke. As known, the strap is moved forward between a driving wheel and an idle pressure wheel. During launching, when the leading end of the strap reaches the stop, it is blocked in correspondence of the clamping gripper where it operates a microswitch which controls the stopping of the drive motor. Although there are known to be several arrangements which try to guarantee an instant stopping, in practice, the inertia of the rotating masses of the driving unit causes - at the high speeds currently requested - irregular behaviours. In particular, as a result of its residual drag, the strap tends to curl up or form a loop along its path between the driving wheel and its blocked leading end. According to the teaching of FR-A-2.621.555, or of US-A-3949662, said loop formed by the strap is used to cause the lifting of an oscillating guide, which partly surrounds the driving wheel and which in turn operates a switch to stop the main drive motor. The actual advantage of these known arrangements is that the strap does not wedge into the feeding path and does not hence create any jamming, in that it freely extends forming said loop. Nevertheless, said arrangements provide no advantages as far as the prompt, instant, stopping of the strap feeding stroke, since it is evident that the switch operated by the oscillating guide - when this latter is moved by the loop being formed by the strap - anyhow operates at a moment which is subsequent to that of operation of the microswitch controlled by the leading end of the strap reaching the clamping gripper. Furthermore, both of the arrangements known from the aforecited documents merely operate during launching of the strap. A similar problem arises however during the fast recovery of the strap: in fact, the residual drag causes an undesired tightening of the strap around the product to be packed, at a tension higher than that expected, which can often cause damage to the product being packed. The object of the present invention is to realize a device allowing to instantly stop the strap, both during launching and or during recovery thereof. In a strapping machine - of the type described in US-A-3949662 and as pointed out in the introductory part of claim 1 - the above object is reached thanks to the characteristics specified in the characterizing part of claim 1. In such a way, according to the fundamental characteristic of the invention, the movement of the oscillating arm is apt to determine the immediate stopping of the strap, not so much because it controls, by way of a microswitch, the stopping of the driving wheel - as described in US-A-3949662 - but due to the fact that the pressure wheel, being mounted on the arm, is thus shifted away, i.e. parted from the driving wheel, when said arm oscillates towards its opening position. Thanks to said parting, which may even be very slight, there is no longer the pressure which keeps the strap adherent to the driving wheel: this latter can thus eventually continue to rotate by inertia, while the strap can be instantly stopped. According to an advantageous characteristic of the present invention, the oscillating arm moves into an opening position of release, at the end of the launching stroke, thanks to a loop being formed by the strap - in known manner - between the periphery of the driving wheel and said oscillating arm. According to a further advantageous characteristic of the invention, said oscillating arm extends, beyond its part surrounding the periphery of the driving wheel, into a counter-bent portion which determines a saddle path for the strap, thanks to which said arm moves into an opening position of release also during strap recovery, in response to the tensioning of the strap along said saddle path . In other words, the arrangement according to the present invention allows an instant and prompt opening of the oscillating arm, with a consequent removal of the pressure wheel and, hence, release of its feeding pressure; furthermore, this occurs both during launching of the strap, due to forming of the loop, and during recovery thereof, due to tensioning of said strap. Characteristics of further developments and advantages of the device according to the invention will anyhow be more evident from the following detailed description of a preferred embodiment thereof, given by way of example and illustrated on the accompanying drawings, in which: Fig. 1 is a front elevation view of the driving head of a strapping machine, comprising the device to control the feeding of the strap according to the invention; Fig. 1a shows the detail of the eccentric shaft supporting the oscillating arm of the control device of fig. 1; Fig. 2 is a top view of the system to lock the oscillating arm of fig. 1; Figs. 3 and 4 are views fully similar to those of figs. 1 and 2, but with the control device in a position of release at the end of strap recovery; Figs. 5 and 6 are also views similar to those of figs. 1 and 2, but with the control device in a reset position. As it appears evident from the drawings, the control device according to the invention comprises a main driving wheel 1 and a secondary wheel 2 - rotating in synchronism but in opposite directions - on the periphery of which is guided the strap R. During launching of the strap, the wheels move according to the arrows F, and during its recovery they move in the opposite sense. The strap slides along a path including a first channel section C1 around the wheel 2, a second straight channel section C2 leading from the wheel 2 to the wheel 1 and being tangent to both, a third channel section C3 around the wheel 1, a fourth saddle-shaped channel section C4 radiused to the outlet of the channel section C3 from the wheel 1, and finally a last channel section C5 leading to the normal guiding track (not shown) around the product to be packed. A pressure wheel 4 bears against the periphery of the driving wheel 1 so as to press the strap R against said wheel and thereby guarantee its dragging and thus the feeding of the strap. The wheel 4 is mounted freely rotating on a shaft 5 fixed onto a plate 6, said shaft being eccentric in respect of the axis 7 about which said plate 6 is rotatable (see fig. 1a). On the same shaft 5 there is also mounted rotating an oscillating arm 8. On the plate 6 there is also fixed a square support bracket 9. A spring 10, whose pressure is adjustable by means of an adjusting screw 11, acts onto a first arm of said bracket 9. Onto the other arm of said bracket 9 there are mounted a thrust spring 12 and a stop finger 13, this latter being adjustable by means of an adjusting screw 13a. The arrangement is such that the pressure spring 10 imparts, by way of the square support bracket 9, a pressure onto the plate 6 which causes its clockwise rotation. Said rotation leads the wheel 4 to bear against the driving wheel 1 and impart a pressure on the strap R, sufficient to ensure the dragging and feeding thereof. The bearing of the wheel 4 against the wheel 1 also determines the working position of the bracket 9 during feeding of the strap. The oscillating arm 8 is apt to form - with a first portion curved with a bending radius substantially corresponding to the radius of the wheel 1 - the section C3 of the guiding channel for the strap R. It then forms - with a second counter-bent portion (i.e. bending in a sense opposite to that of the first portion) and in cooperation with a fixed bottom saddle-shaped seat - the section C4 of the guiding channel for the strap. To allow the strap R to be always correctly guided into the channel sections C3 and C4, taking into account its variable thickness, the arm 8 is provided with two adjustment means: on one hand, it comprises a tailpiece 8a, apt to cooperate with the aforecited stop finger 13; on the other hand, it comprises a protuberance having a slot 8b, into which is housed a stop 14 in the form of a cam. By adjusting the cam 14 - against which the arm 8 is pressed by the spring 12 - it is possible to regulate the minimum opening of the channel C3-C4, while the adjustment of the stop finger 13 determines its maximum opening. Between these two minimum and maximum adjustments, the arm 8 can oscillate - contrasted only by the action of the spring 12 - so as to adapt itself to the thickness tolerances of the strap during its feeding, as better specified hereinafter. Such an arrangement of the arm 8 essentially allows a double movement of oscillation: a) the arm 8 is first of all rotatable about the shaft 5, with a possibility of oscillation which, as said, is limited by the interaction, on one side (in the anticlockwise oscillation, with respect to Fig. 1), of its tailpiece 8a with the stop 13 and, on the other side (in the clockwise oscillation), of its slot 8b with the cam-stop 14: said oscillation practically represents a slight slack - substantially free, or merely limited by the weak spring 12 - which the arm 8 has in order to allow the free sliding of the strap R into the channel C3-C4, without any risks of jamming, in spite of the inevitable thickness tolerances of said strap; b) the arm 8 can moreover oscillate, anticlockwise, beyond the slack allowed by the spring 12; in fact, when the tailpiece 8a bears against the stop 13, said arm further oscillates about the axis 7, together with the plate 6 and the bracket 9 and against the action of the spring 10, releasing the contact pressure of the wheel 4 against the wheel 1. A locking system is also provided to complete the aforedescribed device, said system essentially comprising a lever 16 oscillating about a vertical pin 17; a spring 18 presses the core 20a of an electromagnet 20 against an adjustable stop 19, thereby causing the anticlockwise rotation of the lever 16, as indicated by the arrow F' in fig. 2. On the end of the lever 16, opposite to that connected to the core 20a, there is mounted a pawl 21 sliding perpendicularly to the plane of oscillation of the arm 8, under the action of a pressure spring 22. A finger 23 is fixed on the pawl 21, said finger controlling a microswitch 24, as better described hereinafter. The aforedescribed device works as follows: when, at the end of the launching stroke, the strap R reaches with its leading end a fixed stop (not shown), it is blocked at this end; nevertheless, under the thrusting action of the driving means, its intermediate length undergoes a bending - forming a loop - in correspondence of the only yielding point of the guiding channel, that is, along its section C3-C4 in correspondence of the arm 8. This latter is then caused to oscillate anticlockwise, up to the position shown in fig. 3. Likewise when, at the end of the recovery stroke, the strap is put under tension due its tightening around the product to be packed, said strap positions itself along the chord of the saddle path C4, thereby causing again the arm 8 to oscillate anticlockwise towards the position of fig. 3. In both these cases, the oscillation is ample enough for the arm 8 to carry its end cavity 8' in correspondence of the locking pawl 21; this latter, under the thrust of its spring 22, engages into said cavity 8' and locks the arm 8 in the reached position (being its opening position of release shown in fig. 3). The engagement of the pawl 21 also causes the operation of the microswitch 24, which controls the stopping of the drive motor. Said oscillation up to the position of fig. 3 is also ample enough for the arm 8, after having led the tailpiece 8a to bear against the stop 13, to cause the whole unit of the bracket 9, plate 6, shaft 5 and wheel 4, to oscillate anticlockwise about the axis 7, against the action of the spring 10. This determines - as said - the parting of the pressure wheel 4 from the periphery of the driving wheel 1, leaving the strap R substantially free. Thus, even if the wheel 1 continues to rotate by inertia, the removal of the wheel 4 from its periphery prevents a further drawing of the strap. In other words, this arrangement allows to free the strap R from the drawing action of the wheel 1 in the very instant in which said strap undergoes a bending sufficient to allow the oscillation of the arm 8, and this quite independently from how long it takes for the wheel 1 to stop, due to the inertias involved, in respect of the instant in which the microswitch 24 operates to stop the drive motor. To reset the working position of fig. 1, one operates the electromagnet 20, which causes the anticlockwise oscillation of the lever 16 up to releasing the pawl 21 from the cavity 8' of the arm 8 (position of figs. 5 and 6): the arm 8 then returns to its working position, under the thrust of the springs 10 and 12, while the pawl 21, under the thrust of its spring 22, returns to its initial position, releasing the finger 23 from the microswitch 24. The release of the electromagnet 20 also allows the lever 16 to return to its initial position of fig. 2. It is anyhow understood that the invention is not limited to the particular embodiment described heretofore, which merely forms a non-limiting example thereof, but that many modifications can be introduced, all within reach of a technician skilled in the art, without thereby departing from the scope of the invention itself as defined by the claims.

Claims: Device to control the feeding of the strap in a strapping machine - of the type wherein the strap (R) is guided on the periphery of at least one driving wheel (1) and at least one pressure wheel (4) is provided to press the strap (R) against the periphery of said driving wheel (1) so as to guarantee its feeding, and comprising moreover an arm (8) oscillating between a guiding position, in which it at least partly surrounds the periphery of the driving wheel (1) so as to form a guiding channel (C3) for the strap (R), and an opening position of release, away from the driving wheel (1), in which it controls the stopping of said driving wheel (1), said arm being movable towards the opening position directly by said strap - characterized in that said pressure wheel (4) is associated to said oscillating arm (8) and is moved away from the driving wheel (1), so as to release the feeding pressure, when the arm (8) is moved into said opening position. Control device as in claim 1), wherein said oscillating arm (8) moves into said opening position of release, at the end of the strap launching stroke, thanks to a loop being formed by the strap (R) between the periphery of the driving wheel (1) and said oscillating arm (8). Control device as in claim 1), wherein said oscillating arm (8) extends, beyond its part surrounding the periphery of the driving wheel (1), into a counter-bent portion which determines a saddle path (C4) for the strap (R). Control device as in claim 1), wherein said pressure wheel (4) is mounted rotating about a shaft (5), carried by the oscillating arm (8) in a position close to the pivoting axis (7) of said arm (8). Control device as in claim 4), wherein the oscillating arm (8) and the pressure wheel (4) are mounted onto a bracket (9) rotating about a fixed axis (7), the oscillation of said arm (8) towards the opening position of release causing the rotation of said bracket (9) and the parting of the pressure wheel (4) from the driving wheel (1). Control device as in claim 5), wherein to said bracket (9) there are associated spring means (10) which determine the feeding pressure of said pressure wheel (4). Control device as in claim 6), wherein to said spring means (10) there are associated means (11) to adjust said feeding pressure. Control device as in claim 5), wherein said bracket (9) carries said shaft (5) eccentric in respect of its fixed rotation axis (7), onto which there are rotatably mounted said oscillating arm (8) and said pressure wheel (4). Control device as in claim 8), wherein said oscillating arm (8) is mounted on said eccentric shaft (5) with a slight oscillation amplitude, apt to determine the minimum and, respectively, the maximum opening of the guiding path for the strap (R) along said driving wheel (1), said amplitude being defined between a minimum opening stop, anchored to a fixed part of the machine, and a maximum opening stop anchored to said bracket (9). Control device as in claim 9), wherein said minimum opening stop consists of a cam (14) engaged into a wide slot (8b) of said oscillating arm (8), the position of said cam (14) being adjustable so as to regulate said minimum opening. Control device as in claim 9), wherein said maximum opening stop consists of a finger (13) cooperating with a tailpiece (8a) of said oscillating arm (8), the position of said finger (13) being adjustable on said bracket (9) so as to regulate said maximum opening. Control device as in claim 11), wherein to said tailpiece (8a) of the oscillating arm (8) there is moreover associated a spring (12) to return said arm to the position of minimum opening. Control device as in claim 1), comprising moreover a locking pawl (21) to stop the oscillating arm (8) into the opening position. Control device as in claim 13), wherein said locking pawl (21) is mounted on an oscillating lever (16) with the interposition of spring means (22) which push said pawl into a position apt to stop said oscillating arm (8), means being associated to said oscillating lever (16) to return said pawl (21) into a position of release. Control device as in claim 14), wherein said return means associated to the oscillating lever (16) consist of an electromagnet (20).

Inventors: MAGGIONI CESARINO, MAGGIONI, CESARINO
Applicants: SESTESE OFF MEC, OFFICINA MECCANICA SESTESE S.P.A.
Application year: 1993
Main classification: B65B13
IPC codes: B65B13