Field of the Invention
The present invention refers to a tension adjustment device in a strapping head for strapping machines.
As known, a very popular wrapping technique is that of wrapping tightly a load to be transported with one or more strapping loops. The strap is a thin ribbon, normally made of plastic material (but in some cases also of metal material), which is tightly wound loopwise around a load, closing it permanently through welding points between the two terminal edges.
The welding prevents an accidental reopening of the strap loop, which must then be severed to free the wrapping.
All strapping machines substantially resort to a strapping head which has two complementary and integrated functions: on the one hand, the launch and subsequent recovery and tightening of the strap around the product to be wrapped and, on the other, the welding of the two terminal edges.
Normally a single strapping head, mounted below the transit plane upon which the load to be packaged is running, incorporates a series of movement devices and members which perform all the above-said functions, i.e. they feed and launch the strap around the load, starting from a strap storage reel, block the free end and recovery or draw back the base portion, until they choke a strap loop around the load; finally, they perform the welding in the loop closing area and severe it from the rest of the strap which comes from the reel.
One of the critical steps of the strapping process is that of the strap recovery or draw back, after the launch thereof around the load to be wrapped, which also determines the tightening degree of the wrapping. Typically, a free end of a strap is launched onto a guiding track around the load to be wrapped, until it ends in correspondence of a welding head, where it is gripped and retained; once a wide loop has been performed around the load (the annular guiding track is normally far wider than the section bulk of the load) it is necessary to close and choke the strap loop to ensure a suitable tight wrapping; to perform this operation, since the terminal end of the strap is kept gripped, the strap portion coming from the storage reel is typically recovered or rewound backwards, causing the strap-launching members to work backwards.
Due to productivity requirements, it is important that the launch and the recovery of the strap occur at high speeds, the launch typically occurring at speeds ranging between 2.5 and 5.5 m/sec. However, at the same time, the strap acceleration/deceleration timing must be managed adequately, especially during the recovery operation. In this last step, moreover, it is important to impart the adequate tension to the strap ribbon, i.e. a force of a preset amount which, despite closely tightening the strap ribbon on the load, does not impair the integrity of the load (which event might occur, for example, with cardboard boxes or other stacks of collapsible material).
For these reasons, strapping heads comprising a complicated mechanism for the launch and recovery of the strap have already been offered on the market, which mechanism is provided with transmission wheels, mounted oscillating through elastic means. For the final recovery step, various systems have been proposed, suited to detect the tensioning of the strap so as to interrupt operation upon reaching of the desired tension value.
Typically, an actuator acts on a transmission wheel whereon the base portion of the strap is kept in engagement, so as to recovery the strap from the launching track and obtain a tightening around the load. In order to obtain the desired tension, it is necessary to interrupt the intervention of the actuator - which, per se, is capable of providing a tension far higher than the one desired for the specific type of strap and wrapping - upon reaching the desired tension.
The most intuitive adjustment systems resort to loading cells, which are capable of providing a signal directly proportional to the tension detected in the traction members: the signal is supplied to a control unit which provides to interrupt the tensioning action of the actuator upon reaching a preset value. The adjustment of the tension value may be obtained simply through a potentiometer suitably arranged on an operator's control panel.
However, these systems are expensive, require delicate tuning and are subject to electromagnetic interference.
Other, more economic and sturdier systems, resort to a yielding control device, wherein a moving control member moves in opposition to a spring upon the varying of the traction load applied to the strap. Thereby, since the stress imparted by a spring is proportional to the linear deformation thereof, it is sufficient to determine by how much the spring is shortened (or extended, depending on the configuration) by the actuator control, to have a determination of the tension which the actuator is imparting in that condition. In these cases, it is experimentally determined what the spring travel must be to obtain the desired strap tension and then a cam or an intervention tooth is arranged, integral in its movement with the spring system, which is capable of activating a micro-switch upon reaching the desired travel. Thereby a travel end signal may be mechanically generated by the micro-switch, upon reaching the desired tension, by which it is possible to control the operation of the actuator. An example of such device is disclosed in EP61620
This second system, despite being sturdier and simpler, is cumbersome in case frequent adjustments are required, because the tension variation may be obtained by physically displacing the micro-switch with respect to the spring system. In the cases in which the tension must be varied often, for example because the loads to be packaged are highly changeable, it is not acceptable to continue to move the micro-switch position to change the intervention position of the spring system and hence the tension expressed by the strapping head. This complication is felt more in large strapping machines, wherein the micro-switch position might also be difficult to reach.
Summary of the Invention
The object of the present invention is hence to provide a strapping head which, despite having a tension adjustment device of the latter type - i.e. relying on the displacement of a spring system - is also simple to adjust, overcoming the disadvantages of the prior art.
Such object is achieved through a strapping head and a corresponding strapping machine as described in its essential features in the attached main claim.
In particular, according to a first aspect of the invention, a tension adjustment device is provided in a strapping head for a strapping machine, of the type comprising at least a plurality of transmission pulleys apt to perform the launch and recovery of a strapping ribbon, actuated by an actuation motor, as well as a yielding system moving in opposition to spring means according to the tension imparted to the strap by said transmission pulleys, which furthermore comprises a star-shaped wheel the rotation of which is proportional to the displacement of said yielding system moving in opposition to spring means, said star-shaped wheel comprising a plurality of equally-distanced marks detectable by a sensor/transducer apt to issue in correspondence a counting signal of the number of marks detected during the rotation of said star-shaped wheel, recovery of said strap ribbon performed by said actuation motor being stopped upon reaching a preset threshold of said counting signal.
According to a further aspect, the yielding control system moving in opposition to spring means comprises at least one linearly sliding rack which a rotation mechanism controlling said star-shaped wheel engages with.
According to an additional aspect, the rack is controlled in displacement by an external wheel of a double epicyclic rotation mechanism, one input of which receives torque from said actuation motor and one output of which is a wheel for the recovery of said strap.
Preferably, the cited marks are in the form of cut outs at the periphery of the star-shaped wheel and the sensor is an optical sensor.
According to another aspect of the invention, a strapping head is provided, and a corresponding strapping machine, comprising at least one strap launch and recovery assembly which comprises a tension adjustment device of the strap as indicated above.
Further inventive aspects of the device are described in the dependent claims.
Brief Description of the Drawings
Further features and advantages of the strapping machine and of the strapping head device according to the invention will in any case be more evident from the following detailed description of a preferred embodiment of the same, given by way of example and illustrated in the enclosed drawings, wherein:
fig. 1 is an elevation front view of an exemplifying strapping head device;
fig. 2 is a front and side perspective view of the strapping head device of fig. 1;
fig. 3 is a perspective view similar to that of fig. 2 in an attitude which highlights some components of the tensioning device according to the invention;
fig. 4 is a longitudinal-section view of the arrangement according to the invention, seen from the front;
fig. 5 is an elevation rear view of the arrangement according to the invention; and
fig. 6 is a crosswise section view taken along line VI-VI of fig. 5.
Detailed Description of a Preferred Embodiment
A strapping machine (not shown) consists, in a manner known per se, of a frame whereon a strap-launching track is arranged, arranged around a support and transit plane of a load to be packaged. Below the support plane, in correspondence of the entry to the launching track, a strapping head is arranged, conceptually known per se.
The strapping head typically comprises in sequence, in the launch direction of the strap, a launch and recovery assembly of the strap, provided with suitable actuation motors, and then a welding assembly, apt to block the free end of the strap (once launched and wound around the load to be packaged) against a remaining base portion of the strap and joining together these two portions, so as to define a closed strap loop tightened around the load to be packaged.
Fig. 1 shows a strapping head divided into two independent assemblies, as better described in a co-pending application by the same Applicant.
The strap launch and recovery assembly 1 is illustrated in the left portion of the drawing. This launch and recovery assembly generally consists of a single-body 11 which includes and supports a series of transmission members, among which a pair of drawing pulleys 12 and 13, between which a third cooperation pulley 14 is arranged. In the upper portion of the single-body a sliding track 15 is provided whereon a strap ribbon (not shown) is apt to slide, both in the launching step and in the recovery step. The strap ribbon, coming from a storage reel, enters the single-body assembly from the bottom (in relation to the attitude the strapping head has during operation), runs through transmission pulleys 12 and 13 and runs above on track 15.
The launch and recovery of the strap occurs according to principles known per se, through driving the pulleys 12 and 13 in one direction or in the other, through the drive made available by a motor M fastened to single-body assembly 11. In particular, during the recovery step, the traction imparted on the strap by lower pulley 13 and by an inner wheel (described further on) reflects into a greater or smaller tensioning of the strap which produces the displacement of a yielding member in opposition to an elastic element, according to principles already known also from EP-A1-0795475
in the name of the same Applicant.
The tensioning applied to the strap by transmission pulleys 12 and 13 and the inner recovery wheel, produces a progressive rotation of a pinion 20 which meshes with a rack 21: said rack is mounted sliding in the body 11 in opposition to elastic means, such as a preloaded spring 22, along the axis a-a' (fig. 6) .
In particular, pinion gear 20 represents the outer wheel of a double rotation epicyclical mechanism deputed to the strap tensioning, according to a principle already described in EP-A2-61620
Such epicyclical rotation mechanism, according to the embodiment shown here, consists of a series of gears clearly shown in the section view of fig. 6. A control wheel 30 is driven into rotation by a belt 31 wound around a motion pulley 32 of an electric motor M (fig. 3). Control wheel 30 is mounted integral in rotation with a drive shaft 33, which in turn meshes with satellite gears 34 of a first epicyclical rotation mechanism. Said satellites are mounted on rotation pins integral with a satellite-carrier 35 which transmits the motion to a strap recovery mechanism. In particular, in a second rotation mechanism, satellite-carrier 35 makes up the inner wheel: the rotation of said wheel transmits alternately - through a second series of satellites 36 - either to a strap-driving wheel 37 (satellite-carrier of the second epicyclic rotation mechanism) or to the wheel 20 driving the rack 21 (outer wheel of the second epicyclic rotation mechanism).
This construction allows to distribute the torque coming from control wheel 30 to the inner wheel 37 for strap recovery and to pinion gear 20, according to the way of operation which will be described further on.
According to the invention, pinion gear 20, the rotation of which is opposed by spring 22 through rack mechanism 21, is furthermore meshed with an idle transmission wheel 23 which, in turn, drives into rotation a star-shaped wheel 24. In particular, to aid accessibility, transmission wheel 23 engages with a terminal wheel 25 the shaft of which comes out from the body 11 with an outer pin 25a whereon star-shaped wheel 24 is keyed.
Star-shaped wheel 24 has a plurality of marks, preferably cutouts 24a, along the circumferential periphery thereof, for example 16 cutouts, which define a series of interruptions in the continuity of the material of wheel 24. In correspondence of the periphery of wheel 24, on body 11 an optical sensor/transducer 26 is fastened, apt to detect the presence or absence of cutouts 24a.
In substance, the optical sensor detects the passage of cutouts 24a according to a visual axis orthogonal to the rotation plane of wheel 24.
Sensor 26 is act to issue and output an electric signal upon the detection of each cutout or upon the detection of the closed portion of wheel 24, between one cutout and the other. Thereby, during the rotation of star-shaped wheel 24, sensor/transducer 26 produces a step signal by which it is possible to count how many cutouts have passed in front of sensor 26. Since the rotation of star-shaped wheel 24 is integral with the rectilinear movement of rack 21, through sensor 26 it is possible to obtain a signal proportional to the displacement of rack 21, in terms of a counting of pulses starting from the home position (or travel end) of rack 21 (the home position is the one shown in fig. 4 wherein the rack is pushed towards the right travel end).
The pulse signal derived from sensor 26 is fed to a central control unit (not shown), wherein the tension adjustment by the operator occurs. In other words, on the electronic unit the operator sets the number of pulses (equal to a certain travel of rack 21) at which the desired tension is reached on the strap, and beyond which the torque application of motor M for strap recovery must be discontinued to interrupt tensioning.
Preferably, the setting of the desired tensioning is obtained through a step potentiometer, by which the number of pulses (possibly readable on a digital display) is set on the control unit corresponding to the desired tension. The use of a step potentiometer makes adjustment by the operator more intuitive also to the touch, which operator can develop the necessary adjustment sensitivity without necessarily having to read a numeric display.
Advantageously, it is possible to provide more than one adjustment potentiometer, to be able to establish two strap tensioning levels which the machine may use in two distinct steps of an automatic strapping process of a complex load.
The operation of the recovery and tightening of the strap occurs in the following way. In the first part of the fast recovery of the strap, no significant resistant-torque is detected on outer wheel 37 of the second epicyclic rotation mechanism: hence the torque which enters the system through shaft 33, transfers entirely to wheel 37, because wheel 20 is kept stationary by the action of rack 21 pushed to travel end (right end in fig. 4) by spring 22.
When the strap has been fully recovered and adheres to the load to be packaged, a significant tensioning on the strap begins to occur, and hence a resistant torque is felt by the wheel 37. The motion coming from shaft 33 hence tends to distribute at least partly -through the two epicyclic rotation mechanisms - also to wheel 20 which starts to rotate slightly overcoming the resistant torque offered by spring 22. Upon the increase of the tension on the strap, the torque also increases which is transferred towards wheel 20 and hence increases the travel of rack 21 against the bias of the spring means 22, which causes the rotation of star 24. The star rotation causes the desired signal to be issued by the sensor 26, until reaching the desired tension which causes the detachment of the driving force produced by motor M.
As can be evinced from the above-reported description, the device according to the invention, which comprises a rotation sensor connected to the rack movement, allows to achieve the desired results.
As a matter of fact, the rotation continuity of the star-shaped wheel does not require to move any reference or switch with respect to the rack, this removes any need to act on the machine upon changing the desired tension. As a matter of fact, tension adjustment is obtained simply by acting on a control unit, indicating the number of pulses (received from the sensor/transducer) which must determine the detachment of the supply to the strap-recovery motor M. Advantageously, using a step potentiometer, to achieve the adjustment of the number of pulses, a pleasant sensitivity is provided to the operator.
However, it is nevertheless understood that the invention is not limited to the particular embodiment illustrated above, which represents only a non-limiting example of the scope of the invention, but that a number of variants are possible, all within the reach of a person skilled in the field, without departing from the scope of the invention.
For example, although in the description reference has always been made to a particular star-shaped wheel with 16 cutouts, it is possible to change size and step of the cutouts, to obtain greater or lesser precision of action, depending on requirements. Moreover, instead of the star-shaped wheel, which has proved to be particularly economic, the impulse detection system may be replace by a conventional, but more expensive, angular encoder system. In such respect, it must be understood that cutouts 24a are equivalent to marks detectable by a sensor and said sensor is not necessarily of the optical type, but could also be of a magnetic type, or else. Accordingly, the term star-shaped wheels simply means a wheel having a number of detectable marks.
Moreover, the yielding drive system moving in opposition to the spring, following the tension on the strap, may also take up different shapes, provided they are suitable to obtain a proportional rotation of the star-shaped wheel.
1. Adjusting strap tensioning device of a strapping head for a strapping machine, of the type comprising at least a plurality of transmission pulleys apt to perform the launch and recovery of a strap ribbon, actuated by an actuation motor (M), as well as a yielding system moving in opposition to spring means according to a tension imparted on the strap by said transmission pulleys, characterised in that it furthermore comprises a star-shaped wheel (24) the rotation of which is proportional to the displacement of said yielding system movable in opposition to spring means (22), said star-shaped wheel (24) comprising a plurality of equally distanced marks detectable by a sensor/transducer (26) apt to issue in correspondence a counting signal of the number of marks detected during the rotation of said star-shaped wheel (24), recovery of said strap ribbon performed by said actuation motor (M) being stopped upon reaching a preset threshold of said counting signal.
2. The device as claimed in claim 1, wherein said yielding system moving in opposition to spring means comprises at least one linearly sliding rack (21) with which a rotation mechanism driving said star-shaped wheel (24) meshes.
3. The device as claimed in claim 2, wherein said rack (21) is driven in displacement by an outer wheel (20) of a double epicyclic rotation mechanism an entry of which receives torque from said actuation motor (M) and an exit of which represents a recovery wheel (37) of said strap.
4. The device as claimed in any one of the preceding claims, wherein said marks are in the shape of cutouts (24a) at the periphery of said star-shaped wheel (24) and said sensor is an optical sensor (26).
5. A strapping head for a strapping machine comprising at least a strap launch and recovery assembly, characterised in that said assembly includes a strap tension adjustment device as claimed in any one of the preceding claims.
6. A strapping machine comprising a support plane whereon a load to be wrapped is intended to lie, a strap-launching track arranged above said support plane and a strapping head mounted below said support plane, characterised in that said strapping head is as claimed in claim 5.