Note: Descriptions are shown in the official language in which they were submitted.
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WELDING TOOL FOR A STRAPPING APPARATUS
The present invention relates to a friction welding tool used in conjunction
with
a strapping machine, said welding tool being fitted with a drive of which the
output
motion can be converted by a mechanism into an oscillating motion of a welding
shoe. Moreover the present invention relates to a strapping apparatus fitted
with a
friction welding tool.
Such friction welding tools are widely used in conjunction with mobile
strapping apparatus that are used at arbitrary stations to strap items to be
packed
with a superficially fusing strap. In principle however such friction welding
tools also
may be used with stationary strapping apparatus. In general such strapping
apparatus comprises a tensioner applying sufficient tension to the strap loop
placed
around the particular item to be packed. Thereupon the strap loop may be
affixed by
a clamping unit of the strapping apparatus to the item to be packed to prepare
it for
the ensuing lacing procedure. As regards strapping apparatus of this kind, the
lacing
procedure can be implemented using a friction welding tool. In such a process
the
strap is depressed by an oscillating friction shoe in the zone between the two
ends of
the strap loop, This pressure and the heat generated locally and for a short
time by
the motion superficially fuse the strap, which typically is made of plastic.
As a result
a connection between the two superposed plies of strap is formed which is
practically permanent and may be undone at best only applying a large force.
A plurality of such friction welding tools uses a cam or a connecting rod
drive
to convert a rotary driving motion into the oscillating motion of the welding
shoe. It
has been found however that the friction welds so made are not wholly
satisfactory.
In order to attain optimally welded links, a given amount of heat must be
applied
within a given time window into a locally bounded segment of the two mutually
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CONFIRMATION COPY
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overlapping strap plies. As regards conventional plastic straps, strap
temperatures
of about 250 C are required.
This feature entails given operational rates of the friction welding shoes
while simultaneously given frictions must be applied to the strap, where both
the
said rates and the friction forces may depend on the nature of the strap.
Furthermore the operational rate should be as constant as possible. In general
problems will be encountered to attain optimal operational rates and friction
forces
of the welding shoes using the forces and torques generated by a manual lever
and
the conventional linkrod drive within the required superficial welding time
window. This difficulty is enhanced moreover where manually operated friction
welding tools are used - of which the weight should be as low as possible - to
enjoy good handling of the strapping apparatus. Consequently very long drive
levers used to apply large torques, or gear units with many transmission
stages, are
usually not used because of handling difficulties.
Even when electrical motors are used to deliver the drive motion as regards
welding tools not actuated manually, similar problems do arise. Accordingly to-
date tradeoffs had to be accepted between the attainable welding parameters.
Accordingly the present invention seeks to create a friction welding tool
of the above cited kind that improves welding.
For a welding tool of the above species used in conjunction with a
strapping apparatus, this problem is solved by the invention by means of an
approximately polygonal drive element which is configured as seen in the
direction
of drive transmission between the drive and the welding shoe and which
comprises
a peripheral surface having at least three zone that peripherally are a larger
distance from the axis of rotation than are other peripheral zones.
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It was discovered that the approximately polygonal drive element of the
present invention allows multiplying the stroke that would otherwise be
available
from the linkrod design, provided that the approximately polygonal drive
element
shall comprise at least three peripheral zones that are a larger distance from
said
unit's axis of rotation. Every two zones of increased distance may enclose a
peripheral zone of which the maximum distance to the axis of rotation is less
than
the least distance from the peripheral surface in one of the adjoining zones.
This
geometry may be used to attain more advantageous welding parameters than
heretofore. The invention allows significant improvement in the welding tool's
efficiency even while simultaneously making possible a more economical welding
tool design than heretofore.
Alternatively the angular speed may be reduced by a factor of three using
manual levers in the preferred application, said angular speed being applied
to a
shaft connected directly or indirectly to the manual lever. Because of the
increased
number of strokes per rotation of the drive element, it is thus feasible to
attain the
same welding parameters as heretofore. Such a feature may be exploited as
leading to a simpler design of the friction welding tool, making possible in
turn a
reduction in weight of the strapping apparatus.
In the present invention, a polygonal or near-polygonal drive element in the
preferred application may be construed being a single or multi-element
component
connected to a rotary shaft and fitted with a peripheral surface deviating
from a
circular shape. An approximately polygonal drive element is further
characterized in
that the peripheral surface may comprise as seen in the peripheral direction
several
mutually adjoining zones that differ in their particular distances from their
particular
maximum distances from the axis of rotation. The distance between the
peripheral
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surface and the axis of rotation also may vary among the particular zones
themselves. Also and as regards their topographies, identical zones may be
employed. Illustratively the zones of greater distance to the axis of rotation
may be
at least approximately congruent. The same design also applies to the zones of
lesser maximum distance from the axis of rotation.
Basically however the approximately polygonal drive element may comprise a
number of zones farther away from the axis of rotation and preferably said
number is
odd and larger than 3. A zone of lesser distance to the axis of rotation of
the
approximately polygonal drive element may be configured between every two
preferably peripherally sequential zones that are farther away.
The approximately polygonal drive element may be operationally connected
with at least one contacting element in order to transmit to this element a
motion
resulting from the drive element rotation. Preferably the drive element is
configured
between two contacting elements and displaces latter to and fro during a
stroke in
synchronous, translational manner and in the same direction. Said contacting
elements are designed to rest against the drive element's peripheral surface.
Advantageously too, the contact between the drive element and the two
contacting
elements shall be as play-free as possible. A preferred joint straight path of
the
contacting elements may preferably intersect the axis of rotation of the
approximately
polygonal drive element and be perpendicular to it.
Sliding friction may exist between the drive element and the minimum of one
contacting element. However advantageously the mechanical loading of the
minimum of one contacting element and of the drive element shall be in the
form of
rolling friction. This feature may be implemented for instance by imparting a
degree
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of freedom to the contacting elements in the form of rotation about their
bearing
axis in addition to their ability to translate.
Advantageously, regardless of the number of zones, the diameter lines of
the approximately polygonal drive element intersecting the axis of rotation at
least
approximately shall be of the same length. Accordingly the drive element may
be
a structure of the same thickness or similar.
The present invention is elucidated by the illustrative embodiments shown
in purely schematic manner in the Figures.
Fig. 1 is a strapping apparatus integrating a friction welding tool of the
invention,
Fig. 2 is a highly schematic representation of part of a friction welding tool
of the invention,
Fig. 3 is a friction welding tool of the invention, and
Fig. 4 is a lever mechanism of the friction welding tool of Fig. 3 driven
by the rotational motion of an approximately polygonal drive element.
The exclusively manually operated strapping apparatus 1 of the invention
shown in Fig. 1 comprises a base plate 2 of which the lower side is designed
to
be configured on an item to be packed. All operating units of the strapping
apparatus 1 are mounted on the base plate 2. The various designs of the
individual operational units of such strapping apparatus are known in their
many
ways in the state of the art. Accordingly, following a short discussion of the
basic
design of said apparatus, the next discussion shall relate only to the welding
tool.
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Using the strapping apparatus 1, a loop not shown in further detail in Fig. I
of
a plastic strap (for instance made of polypropylene [PP] or of polyethylene
terephthalate [PET], previously deposited on the item to be packed, may be
tensioned by said apparatus' tensioner 3. The tensioner comprises a drum 4
holding
the strap to be tensioned. In this manner the loop may be sufficiently
tensioned for
good strapping.
Next the strap loop shall be affixed by a clamp 5 of the strapping apparatus
to
this apparatus. For that purpose the clamp comprises two strap jaws 6, 7.
Thereupon welding may be carried out, at a site of said loop where two strap
plies
are superposed, by means of the strapping apparatus' friction welding tool 8.
In this
manner the strap loop can be closed on itself permanently. For that purpose
the
friction welding tool 8 is fitted with a welding shoe 9 which, by applying
pressure to
the strap and by carrying out a simultaneously oscillation motion, does
superficially
melt the two superposed strap plies onto each other. The plasticized or
superficially
molten segments flow into each other so that, following strap cooling, a
connection
has been established between the two strap plies.
To the extent required, the strap loop may severed from a strap roll by a
cutter
10 of the strapping apparatus 1.
The tensioner 3, the clamp 5, the friction welding tool 8 and the cutter 10
are
actuated by manually powered actuation elements of the strapping apparatus.
The
strapping apparatus of Fig. 1 does not receive external, additional power such
as
electricity for instance. However such might be the case for other strapping
apparatus of the invention (omitted). In the embodiment mode shown, the said
actuation elements include at least one manual lever 15 rotatable about a
pivot axis
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14 that, by means of omitted connections and/or freewheeling elements, can act
on
the various operational units.
In the welding unit of the invention, the manual lever 15 may be actuated
several times to move the welding shoe 9 until welding is complete. The
pivoting
motion of the manual lever 9 in this process is transmitted in a manner not
shown in
detail to a planetary gear 16 (Fig. 3) . The planetary gear 16 converts the
motion of
the shaft of the manual lever 15. The drive motion so converted then will
rotate a
planetary gear output shaft 18. An approximately polygonal drive element 20 is
shown in Fig. 2, of which the rotational motion is transmitted by a further
mechanism
21 to the welding shoe, is mounted on said output shaft. This mechanism 21 is
shown in highly schematic manner in Fig. 2 by a rectangle and it may in
principle be
designed in very many ways.
An applicable embodiment of said mechanism is shown in Figs. 3 and 4. As
may be inferred from these Figures, the approximately polygonal drive element
20 is
situated between two contact elements designed as radially cylindrical roller
bearings 22, 23 (hereafter roller bearings).The two roller bearings 22, 23
rest at a
mutual offset of about 180 against a peripheral surface 24 of the
approximately
polygonal drive element 20. Said roller bearings are configured in a manner
that
their axes of rotations run parallel to the axis of rotation 18a of the
approximately
polygonal drive element. All three axes of rotation intersect a straight
displacement
path 25 along which the roller bearings 22, 23 may translate. The roller
bearings 22,
23 are mounted respectively by their inner rings on bolts 27, 28, the two
bolts
resting in a fork-shaped guide bracket 29. Both the roller bearings 22, 23 and
the
drive element 20 are situated within the fork subtended by the guide bracket.
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The translations of the roller bearings 22, 23 are determined by a given
geometry of the peripheral surface 24 of the approximately polygonal drive
element
20, namely said drive element comprises three zones 30a, 30b, 30c at its
peripheral
surface that are farther away from its axis of rotation. These three zones are
apart
by 120 from each other. A zone 31a, 31b, 31c of the peripheral surface 24 is
situated in each case between two of those zones 30a, 30b, 30c at
comparatively
smaller radial distances from the drive element's axis of rotation 18a. The
zones
31 a, 31 b, 31 c situated at lesser radial distances also are equidistant by
about 120 .
The contact sites between a particular roller bearing and the drive element
may shift in relation to the translational path and they may be situated on
the
displacement path 25 as well as above or below. In this manner tangential
contact
may be attained at the particular contact site, that is a contact site at
which a
common tangent exists for both contacting partners. This constraint should
affect
the shaping of the individual zones of the approximately polygonal drive
element.
The fork-shaped guide bracket 29 is linked at its left end, as seen in Fig. 3,
by
means of a rotatably supported connecting lever 33 to an upper lever 34. A
welding
shoe lever 35 is linked to the right end of the guide bracket 29 and also by
its upper
end to the upper lever 35. The upper lever 34 rests on the base plate 2 at the
linkage site 36 of the connecting lever 33 in a manner not shown in further
detail in
Fig. 3.
The welding shoe 9 is linked in tipping manner to the lower end of the
welding shoe lever 35. The guide bracket 29, the connection lever 33, the
upper
lever 34 and the welding shoe lever 34 constitute a parallelogram. An omitted
tension spring affixed both to the upper lever 34 and in the region of the
base plate 2
forces the upper lever 34 and hence the welding shoe 9 toward the base plate
2.
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In order to arrive at an optimal length of stroke of the welding shoe 9 in
relation to operational rate and compression, advantageously the distance from
the
linkage site 41 of the guide bracket 29 at the welding shoe lever 35 to the
linkage
site art the upper lever shall be 1/2 or less than the distance between the
linkage
site 41 of the guide bracket 29 and the linkage site 43 of the welding shoe 9.
In this
manner the linkage site 41 of the guide bracket 29 divides the welding shoe
lever 35
into segments which are 1/3 to 2/3 the lever's length.
Because of the topography of the peripheral surface 24 and the substantially
playless rest of the roller bearings 22, 23 against said peripheral surface,
the two
roller bearings 22, 23 both resting in the fork-like guide bracket 29
synchronously
and jointly carry out three complete double strokes (along the double arrow
44) of
identical lengths along the translation displacement path 25 for one
revolution of the
drive element 20.
The translations of the roller bearings 22, 23 are transmitted by their
support
bolts 27, 28 to the guide bracket 29. The site 41 linking the welding shoe
lever 35 to
the guide bracket 29 then entails a swivel motion of the welding shoe lever 35
of
which the motion furthermore is furthermore determined by the linkage of the
welding
shoe lever 35 at the upper lever 34. In turn the motion of the upper lever 34
is
determined by the linkage of the guide bracket 29 via the connection lever 33
to the
upper lever 34 and also by the tipping support of the upper lever about the
axis of
the linkage site 36. Moreover the motion of said lever 34 is affected by the
force
exerted on it by the omitted tension spring.
The rotary motion of the drive element 20 is transmitted in this manner by
means of the translating stroke of the roller bearings 22, 23 to the lever
mechanism
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beyond the drive element, resulting in a reciprocating, pure translating
motion of the
welding shoe 9 on a plastic strap configured underneath it.
Compared to the already known linkrod drives, the above shown embodiment
of the drive element allows carrying out, differently from conventional
designs, not
one double stroke, but instead a total of three double strokes of the welding
shoe in
the direction of the double arrow 11 for each drive shaft rotation. Assuming
that the
shaft of the drive element 18 runs at the same angular speed as in known
designs,
then the invention offers a tripling of the rate at which the welding shoe is
reciprocating. This feature considerably increases the efficiency of the
welding tool
of the invention compared to conventional welding tools used in strapping
apparatus.
