Note: Descriptions are shown in the official language in which they were submitted.
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"DEVICE TO CONTROL THE FEEDING OF THE STRAP IN A STR~PPING MACHINE"
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The present invention concerns a strapping machine and more parti-
cularly, a device to control the feeding of the strap, allowing a more
precise and rapid control of the strap stopping or speed-chan~ing func-
tion during its recovery. that is, closing and tightening of the strap
around the packed product to be fastened.
As known, machines of this type comprise drive means which draw the
strap from a reel, 1aunch it into a special guide track around the pro-
]0 duct 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 the 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, these two requirements often being scarce-
ly compatible.
A strapping machine which tries to satisfy 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. 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 to obtain its instant and prompt stopping at the end of the
launching stroke and, respectively, at the end of the recovery stroke.
Especially during fast recovery of the strap, the inertia of the
machine parts and thus the effect of residual drag of the strap resulting
therefrom, causes an undesired tightening thereof around the product to
be packed, at a tension higher than that expected, which may often cause
damage to said product.
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These problems have already been solved in the strapping machine
described in the Italian Patent Application No. MI92A 002957 filed on
23rd December 1992, and in the EP-A1-0603868 filed on 22nd December 1993
both in the name of the same Applicant. This strapping machine provides
very high performances with a perfectly regular and very precise working
but it also involves very high production costs.
~ther strapping machines are known from DE-4014307, lrs-3526187
US-5379576 and Us-45]6488.
The object of the present invention is to thus realize a strapping
]0 machine which enables, with a simpler and more economic structure, to
perfectly control the tensioning of the strap, during its recovery, and
then the stopping thereof, so that the packed product may be fastened
with a perfectly registered tightening. In a strapping machine of the
general type described heretofore, said object is reached by means of the
characteristics indicated in Claim 1.
Further characteristics and advantages of the strapping machine
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 shows, very diagrammatically, a strapping machine according
to the invention;
Fig. 2 shows the strap driving unit of the strapping machine,
during launching of the strap;
Fig. 3 shows the same unit of fig. 2, during fast recovery of the
strap;
Fig. 4 shows the same driving unit, during the speed-changing
steps; and
Fig. 5 shows the same driving unit, during slow recovery. that is,
during tightening of the strap around the product being packed.
As shown diagrammatically in fig. 1, a strapping machine generally
comprises a guiding channel C for the strap, forming a closed-ring path -
through which is moved the product P to be fastened (shown only by a
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dash-and-dot line) - and a strapping head T, which mainly includes a
strap driving unit T1 (to launch the strap along the path C and recover
the same) and a strap welding unit T2 to weld the ends of the strap.
The working of a machine of this type includes - as described more
clearly in the patents cited further above - the follo-~ing steps: a strap
launching step, in which the strap is caused to pass through the welding
unit T2, and slide along the channel C (for example, in the anticlockwise
direction indicated by the arrows in fig. ~) up to reaching again the
welding unit T2; at this point, the end of the strap is temporarily
blocked by a gripper; a strap recovery step then starts and continues
until the strap has been properly tightened around the product P; the
final step consists in welding the two ends of the strap and in cutting
the same.
In particular, the object of the invention is the strap launching
and recovery unit T1 which, as pointed out further above, forms the most
critical part of the machine. According to the invention, and as shown in
fig. 2, said unit comprises a drive shaft 1, rotating clockwise at low
speed - hereinafter, the clockwise direction will be that indicated by
the rotation arrow referring to the shaft 1 in fig. 2 - and being active
during slow recovery of the strap. On the drive shaft 1 there is mounted
a driving wheel 2, by way of a so-called "freewheel" mechanism, which
allows to draw the strap only in an anticlockwise direction.
A second drive shaft 3 is rotated at high speed and carries, keyed
thereon, a fast driving wheel 4; said shaft 3 is active both during the
launching step, with a clockwise rotation (see arrow in fig. 2), and
during the fast recovery step, with an anticlockwise rotation (see arrow
in figs. 3 and 4).
Preferably, the shafts 1 and 3 are driven by a single motor, which
drives the shaft 1 through a reduction gear, and thus at low speed but
with a high drawing force, and which directly drives the shaft 3, whose
wheel 4 rotates at high speed but with less drawing force.
A pressure wheel 5 cooperates moreover with the driving wheel 4, to
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ensure drawing of the strap R interposed between said wheels.
The pressure wheel 5 is mounted freely rotating on a shaft support-
ed by an arm 6 which is mounted oscillating on the same axis of the shaft
1; a spring 7 imparts onto the arm 6 a thrust, which can be regulated by
way of an adjusting element 7a, and which determines the pressure of the
wheel 5 against the wheel 4.
To the side of the arm 6 and of the wheel 2 there is mounted oscil-
lating an arched sector 8; said oscillation is obtained (between two
guiding sides, not shown), on one side, by pivoting the lower end of the
sector 8 onto the axis of a roller 9, which is in turn guided so as to
slide horizontalwise into a groove lOa of a fixed support 10 of the unit
T], and on the other side, by anchoring the upper end of the sector 8
onto a bracket 11, also sliding horizontalwise along the support 10, in
opposition to the spring 1la. A link rod 12 forms an articulated connec-
tion between the axis of the roller 9 and the oscillating arm 6.
A transmission wheel 13 is mounted freely rotating at the upper end
of the sector 8. Furthermore, substantially at the centre of the sector 8
there is fixed a pressure pad 14, whose arc-shaped surface 14a is centred
on the axis of the shaft 1 and thus runs parallel to the periphery of the
wheel 2; the surface 14a facing the wheel 2 is perfectly smooth, to act
as an anti-friction surface for the purpose better specified hereunder. A
guide 15a-15b, is moreover fixed to the sector 8 for the function indica-
ted hereinafter.
Finally, a microswitch 16 is mounted on the fixed support 10, said
microswitch being operated by a rod 17, 17a, moved by the roller 9 in
opposition to the adjustable spring 18.
The aforedescribed strapping head works as follows:
a) During the launching step (fig. 2), the strap R - driven by the
wheels 4 and 5 - is drawn in the direction of the arrow F towards the
surface 14a of the pressure pad 14 fixed to the sector 8; from here, it
is deviated towards the guide 15a-15b and then towards the channel C
(fig. 1), in a substantially known manner. During this step, the strap
always remains properly spaced both from the wheel 2 and from the wheel
13.
b) During the fast recovery step (fig. 3), the strap R - still
driven by the wheels 4 and 5, but this time in a direction F' opposite to
the previous one - gets in contact with the wheels 2 and 13, which are
driven into a fast rotation due to friction of the strap R. Thanks to the
freewheel mechanism, the wheel 2 is apt to rotate at high speed, even if
its shaft I rotates at low speed.
c) During the speed-changing step, followed by slow tightening?
that is, at the moment in which the strap starts to get in contact with
the surface of the product having to be fastened, it is almost stopped
against the product to be packed and it instantly becomes tensioned; by
reaction, this produces a pressure on the only yielding surface, which is
the wheel 13, causing this latter to move in the direction of the arrow S
(fig. 4) and thereby causing the clockwise oscillation of the sector 8.
One may easily imagine a sequence wherein:
- to start with, a very slight clockwise oscillation of the sector
8 is produced about the fulcrum formed by the axis of the roller 9, with
compression of the spring lla, until the surface 14a of the pad 14 bears
against the periphery of the wheel 2. This produces the only effect to
guarantee a perfect coupling between the strap R and the surface of the
wheel 2, seen that the strap - which still moves fast, driven by the
wheels 4 and 5 - is free to slide against the smooth surface 14a, with no
friction, while it clings onto the friction surface of the wheel 2. Said
oscillation - as can be easily understood - is very slight since, as the
wheel 13 moves further in the direction S, under the pressure of the
strap which is tightening around the product P to be fastened,
- a stronger oscillation is produced just after; in fact, as shown
in fig. 5, as the pad 14 presses against the wheel 2, the sector 8 rota-
3Q tes about a new fulcrum formed by the actual pad 14. This leads the
roller 9 to slide in a direction S', always guided into the groove 10a,
and to thus also draw the oscillating arm 6 by way of the link rod 12,
against the action of the spring 7, into a clockwise rotation (arrow S')
which leads the wheel 5 to move apart from the driving wheel 4. As the
pressure wheel 5 moves away from the wheel 4, the drawing action on this
latter ceases, while the pad 14 simultaneously keeps the strap R always
perfectly adherent to the wheel 2; this latter, which is now driven by
the shaft I through the operative freewheel mechanism, draws the strap at
low speed up to tightening the product P with the desired stren~th;
- said strength is regulated by adjusting the resistance opposed by
the spring 7 and by the contrast spring 18 of the microswitch 16; in
fact, as said resistance is overcome, the sector 8 further oscillates in
a clockwise direction, always about the fulcrum formed by the pad 14,
causing the roller 9 to move backward and press the rod 17-17a, which
controls the closing of the microswitch and thereby stops the working of
the machine.
It is appropriate to place in evidence a further important advanta-
ge of the control device according to the invention, determined by the
fact that the strap path on the driving and transmission wheels is fairly
"smooth"; in other words, the strap is not forced - as it happens instead
in prior art, especially US-4516488 and US-5379576 - to follow arc-shaped
paths developing through wide angles, which normally create the inconve-
nience that, in the event of a prolonged stop, the strap remains perma-
nently deformed, with subsequent risk of jamming when the machine starts
working again.
Another considerable advantage lies in the fact that the rotation
axes of the wheels 2 and 4 are fixed, whereby the motors driving the
shafts 1 and 3 do not have to be mounted on oscillating or otherwise
movable structures, to the full advantage of structural simplicity.
It is anyhow understood that the invention is not limited to the
particular embodiment described heretofore, which merely forms a non-li-
miting example of its scope, but that many modifications can be introdu-
ced, all within reach of a technician skilled in the art, without thereby
departing from the protection scope of the present invention.
