Field of the Invention
[0001] The present invention refers to a tension adjustment device in a strapping head for
strapping machines.
Background Art
[0002] 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.
[0003] The welding prevents an accidental reopening of the strap loop, which must then be
severed to free the wrapping.
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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).
[0008] 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.
[0009] 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.
[0010] 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.
[0011] However, these systems are expensive, require delicate tuning and are subject to
electromagnetic interference.
[0012] 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.
[0013] 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
[0014] 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.
[0015] Such object is achieved through a strapping head and a corresponding strapping machine
as described in its essential features in the attached main claim.
[0016] 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.
[0017] 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.
[0018] 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.
[0019] 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.
[0020] 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.
[0021] Further inventive aspects of the device are described in the dependent claims.
Brief Description of the Drawings
[0022] 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
[0023] 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.
[0024] 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.
[0025] Fig. 1 shows a strapping head divided into two independent assemblies, as better
described in a co-pending application by the same Applicant.
[0026] 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.
[0027] 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 o
EP-A1-0603868 in the name of the same Applicant.
[0028] 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) .
[0029] 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.
[0030] 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).
[0031] 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.
[0032] 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.
[0033] 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.
[0034] In substance, the optical sensor detects the passage of cutouts 24a according to
a visual axis orthogonal to the rotation plane of wheel 24.
[0035] 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).
[0036] 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.
[0037] 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.
[0038] 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.
[0039] 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.
[0040] 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.
[0041] 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.
[0042] 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.
[0043] 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.
[0044] 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.
[0045] 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.