Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
5~ .
Background of The Invention -
The invention relates to a process and an apparatus
for multiple-station resistance welding.
Description of The Prior Art -
Multiple-station resistance welding devices with
a plurality of electrodes have been known for a long time.
Yet, it has been customary heretofore to weld workpieces, for
example those for automobile doors wherein a large number of
relatively closely juxtaposed weld spots had to be welded,
on several multiple-station welding machines arranged along
a welding line, rather than using a single machine. This is
so, because it was previously thought to be impossible to weld
very many, closely adjaoent spots on a single machine, for
constructional reasons, since each eIectrode pair was corre-
lated with a welding trans~ormer occupying a considerable
amount of space. The transport of the workpieces on the
welding line and the positioning and welding on various machines
was not only cumbersome and complicated, but also entailed the
danger that the workpieces would warp during the individual
welding steps and the subsequent transport, on the one hand,
and that the welding operations would not take place at exactly
the intended locations, on the other hand.
Summary of The Invention -
The invehtion is based on the obJect of conducting
at a single operating site a large number of welding bonds
in close mutual proximity, in a precise fashion, avoiding de-
formations of the workpieces during thls operation. For this
purpose, the process and apparatus are to be simple and also
operable from a power supply which can tolerate only low loads.
-2 ~
~S9~19
According to the invention, the individual elec-
trodes or electrode groups are connected in succession into
the same welding circuit. This means that all of the suc-
; cessively connected electrodes or electrode groups are fed
by the same source of welding current, e.g. a transformergroup. The amperage and the duration or pulse sequence of
the welding current can be varied in this procedure from
one welding operation to the next by a corresponding, e.g.
program-controlled, switching of the current source. If
the current density in the welding spots is to be constant
in all welding steps, the voltage of the current source must
normally be adjusted differently from one welding step to
the next, because the resistance of the welding spots can
vary-(for example, due to difference in thickness of work-
piece portions) and because the previously welded sites con-
stitute shunts.
.
One embodiment of the invention wlll be described
in greater detail hereinafter following with reference to
the appended drawings.
Brlef Description of The Drawings -
Figure 1 is a schematic lateral view of a resistance
welding device accord~ng to the invention;
Figure 2 is a schematic bottom view of the elctrode
units, each including a switch~ and with two workpieces to be
welded indioated in dot-dash lines;
5 ~3
Fiyure 3 is a vertical sectio~ through on~ of the
electrode units and the counter electrode unit along line III
of Figure 1 and on an enlarged scale, without the arms of the
welding machine,
Figure 4 is a lateral view in the orientation
direction of arrow IV in Figure 3,
Figure 5 shows the switch of Figure 3 on an enlarged
scale, and
Figure 6 shows a schematic illustration of very
closely juxtaposed electrode units.
DESCRIPTION OF THE PREFERRED ~3~. BOI~IMENT
The resistance welding device illustrated, which
serves ~or projection or spot ~eldiny, comprises an upper arm
and a lower arm 1 and 2. The upper arm 1 carries a pneumatically
operable piston-cylinder unit 3, a power transmission plate 5
being attached to the piston rod 4 thereo~. To this plate
are mounted, in total, sixteen electrode units each comprising
an electrically insulated electrode and a switch, in the arrange-
ment illustrated in Figure 2. In the example for the process,
described hereinbelow, respectively one aroup of four electrodes
is simultaneously supplied with the ~elding current. The
electrodes of the electrode groups supplied in succession
- with the welding current are denoted, in Figure 2, by 6, 7, 8,
and 9. In this connection, the electrodes of the same group
are in each case characterized by the same symbol, different
from the remaining groups. The lower arm 2 carries a second
powar transmission plate 10 to which is mounted one counter
~ ~L5~5~l9
electrode unit for each electrode; the counter electrode of
this unit i5 electrically conductively connected to the plate.
The two electrically conductive power transmission plates 5
and 10 are connected to the terminals of a six-phase rectifier
consisting of six high-current diodes 12. This rectifier is
connected to the three secondary windings 13 of a transformer
group 14, which latter is connected on the primary side via a
thyristor switching device 15 to the three-phase supply
network R, S, T.
The electrode units of the electrode groups 6-9
are all of identical structure and mounted in the same way;to
the plate 5. Figures 3 and 4 show, as an example, the
arrangement of one of the electrode units as well as the
counter electrode unit, likewise corresponding exactly to the
remaining counter electrodes mounted likewisè:to the
plate 10. The electrode 6 and a cooling pipe 18 are seated
in an electrode holder 20 provided with cooling ducts 19.
This electrode holder is threadedly joined to the web 21
. of a T-connector 22, the screws 16 being associated with
slotted holes 17 in the web 21, so that the electrode holder
can be adjusted in its ~leva~on~The connecting elements 23, 24
for the cooling water conduits are extended, for the same
reason, through a further siotted hole of the web 21 and
screwed into the holder 20. The head of a setscrew 26
threaded into the flange 25 of the T-connector 22, on the
left-hand side as seen in Figure 3, serves as an adjustable
stop for the positioning of the holder 20 when it is attached
~5~
at the web 21. Between the flange 25 a~d a guide member 27
fixedly joined to this flange, an insulating plate 28 is
disposed so that the welding current can pass only through
the other flange 29 of the connecting element 22 to the
electrode 6; this flange can be electrically connected to the
plate 5 by way of the isolating switch 30 described in greater
detail hereinbelow. The guide member 27, equipped with a
bore, is displaceable in a guide 31 mounted to the plate 5
vertically against the bias of a compression spring tcup
spring) 32 seated in the bore and supported on a screw bolt 33
serving for the adjustment of the spring tension. The head 34
of a screw extended through a bore of a lateral projection
of the guide 31 and threaded into the guide member 27 maintains
the guide member 27 in its illustrated rest position against
the bias of the spring 32 in the guide 31. The screw 34
additionally serves for the fine adjustment of the electrode
level. The fine adjustment takes place after installing the
electrode 6 in the holder 20 fixedly threaded to the web 21,
the counter electrode 11 being correspondingly readjusted.
The isolating switch 30, shown in Figure 5 on a
larger scale than in Figure 3, has two contact surfaces 36, 37
separated, in the illu trated inactivated condition, by an
air gap. The contact surface 36 is formad on the underside
of a leg 38 of a multipartite U-shaped holder 39, which leg
is mounted to the plate 5. The contact surface 37 is formed
on the topside of an end 40 of a so-called current tape 41, i.e. a
flexible conductor consisting of several copper strips.
~ - 6 -
~il ~9 5 ~
The current tape 41 extends in a U~shape around the other
leg 42 of the holder 39 and is connected with its other end 43
conductively with the flange 29; the latter is covered by an
insulating material 44 in the zone where the current tape 41
is only in loose contact therewith, in order to avoid spark-
over. The leg 42 of the holder 39 constitutes (or carries)
the cylinder of a hydraulically operable piston-cylinder unit,
a screw 46 being threaded into the piston 45 thereof. The
screw 46 is extended, with an insulating sleeve 47 disposed
thereon, through the current tape end 40. Insulating disks 57
are arranged on the ends of the insulating sleeve 47, one of
these disks being supported against the head of the screw 46
and the other against the topside of the piston 45; these
disks clamp in place the end 40 of the current tape 41 and
; 1~ an intermediate element 48. The head of the screw 46 abuts
against a compression spring (cup spring) 49 arranged in a
bore of the leg 38 and supported against the plate 5. The
hydraulic fluid for the piston-cylinder unit 42, ~5 is fed
via the connection 50 and the duct 51~ The hydraulic system
makes it possible to produce large forces. These are neces-
sary so that the contact surfaces 36 and 37 are firmly pressed
together so that a perfect passage of the high welding cur-
rents i5 ensured. Numerals 52 and 53 (Figure 4) denote two
cooling water conliections terminating in a cooling duct 54
provided in the leg 42. Due to the ~act that the leg 42 i5
conductively connected to the contact surface 36, the heat
is removed which is generated during current passage at the
contact resistor 36, 37. To increase heat transfer, the
- 7 -
cooling duct 54 can also be extended through the web of the
holder 39 and the leg 38. As shown in Figure 5, a synthetic
resin ring 58 arranged at the lower part of the leg 38 and
urged by springs 62 against the end ~0 of the curreht tape
protects the contact surfaces 36 and 37 from contamination by
weld spatter or dust. A protective bellows could also be
provided for the same purpose.
The arrangement of the electrode groups 6-9 shown
in Figure 2 is suitable, for example, for the welding of the
two workpieces 55 and 56, indicated in dot-dash lines, of
which the first has the shape of a cross and the second is
square~ The electrodes 6-9, sixteen in total, which are
required for the multiple-station resistance welding operation,
are mounted to the power transmission plate 5 with their
units indicated in contour by respectively one rectangle
and comprising the components 20 through 53, in accordance
with the required welding site arrangement.
After the two workpieces 55 and 56 have been placed
into the correct position between the electrodes 6-9 and counter
electrodes 11, the piston-cylinder unit 3 is activated whereby
the workpieces 55 and 56 are clamped between all of the
electrodes 6-9 and counter electrodes ll. The unit 3 provides
the pressure force required for the simultansous actuation of
all electrodes (i.e. the sum total of the necessary single-
` 25 electrode forces). This force can range, for example, from
500 to 5000 N. Thereafter, the four electrodes of the inner- -
most electrode group 6 are first of all galvanically connected
8 --
with the plate 5 by hydraulic activation of the pistons 45 of
their switches 30. The switches of all other electrode
groups 7-9 xemain in the rest position (turned-off position)
so that the electrodes of these groups 7-9 are insulated from
the plate 5. After the electrodes 6 have been connected with
the plate 5 and thus have been connected, respectively in
a row, in parallel with their cownter electrode 11, the
welding current is turned on by means of the switching device 15.
(This current can be, for example, up to 22 kA.) After
cutting off the welding current, the switches 30 of the
electrode group 6 are opened by relieving their pistons 45, and
the switches of the electrode group 7 are closed by activating
their pistons 45. Thereupon, during the reactivation of the
welding current, only the electrodes 7 are supplied with the
welding current. In the.same way, the electrodes of group 8
and then those of group 9 are thereafter supplied with the
welding current. After the time re~uired for the cooling of
the welding sites, the plate 5 with the electrodes of all
groups 6-9 is lifted into the starting position by a corres-
ponding a:ctivation of the piston-cylinder unit 3, so that
the workpieces 55, 56, welded at sixteen sitest can be
- withdrawn from the welding device.
As mentioned in the introduction, the amperage and
the current application period can be varied by means of a
program control device (not shown), provided in place of the
switching device 15, from one electrode grOUp to the next.
In this connection, it is also possible to interrupt the
welding current for a short time, i.e. to conduct the welding
operation in short impulses. The amperage can furthermore
be selected to be different not only from one electrode group
to the next but also from one electrode of one group ~o the
next electrode of the same group. To attain this objective,
resistance disks of an alloy having the trade name constantan
with appropriately,selected resistance values can be inserted
between the power transmission plate 5 and the leg 38 or between
the flsnge 29 and the current tape en~ 43 of the individual,
parallel-connected electrode units. In Figure 3, such a
resistanc~ disk 59 of constantan is inserted between the
current tape end 43 and the flange 29.
The aforedescribed electrode units, each comprising
one electrode and one isolating switch, are canstructed of
a very small size so that,the electrodes can be arranged in
spacings of a few centimeters from each other. One example
for such a closely spaced arrangement of electrode units is
shown in Figure 6. The electrode units 61, comprising respectively
one electrode 60 and the associated components 20-53, are
illustrated by their rectangular contourO Nine electrode
; units 61 are disposed in mutual spacings of 2 mm in one row
side-by-side; two further units are arranged at the end ace
or at a right angle to the two outer electrode units of the
row. All electrodes 60 could have the configuration of the
electrodes 6; their axial spacing in such a case would be
42 mm. The electrodes, however, can be arranged in an even
closer mutual spacing -- if this is necessary due to the
arrangement of the welding sites -- by selectlng an elbow
shape so that the ele~trode tip is laterslly ofset with respect
- 10 -
5~
to the axis of the cooling pipe 18. This is the case in
several of the electrode units 60. This produces a minimum
spacing of the electrode tips of a~= 30 mm. The two other
spacings illustrated are b = 54 mm and c = 42 mm. The spacing
of the two outermost electrodes of the nine electrodes 60 dis-
posed in a row is d = 37.8 cm.
The forces with which the individual electrodes with
their counter electrodes compress the workpiece sections at
the welding site depend not only on the force of the piston-
cylinder unit 3, but also on the tensions and/or forces of thecompression springs 32. By adjusting the screw ~olts 33,
the individual electrode forces can thus be adapted individually
to one another, which is of importancè especially if the
welding sites are of a varying character, for example if the
workpieces exhibit differently thick portions. As explained
above, a fine adjustment can be additional performed by means
of the screw 34.
Due to the fact that the electrodes are.supplied
in groups one after the other with the welding current, the
load on the mains and on the transformer group is not dependent
on the total number, but only on the number of electrodes in
one group. Therefore, the electrodes, in case of a mains (or
transformer group) which can bear only small loads, can be
subdivided into very many, small groups and, if necessary,
it is also possible to supply the individual electrodes one
after the other singly with the welding current. The number
of electrodes simultaneously fed with the welding current
is, however, normally limited less due to the load-bearing
-- 11 --
~15~ 9
ability of the mains but rather on account o~ thermal reasons.
This number, as well as the subdivision of the electrodes
among the individual groups, which do not necessarily have
all the same number of electrodes, and, finally, the sequence
in which the electrode groups are supplied with the -- optionally
program-controlled - welding current, are suitably chosen so
that the workpieces are heated uniformly and as little as
possible, so that stresses therein are avoided. De~ormations
of the workpieces are reliably prevented by clamping the work-
pieces between all of the electrodes and counter electro-des
during the entire ~lelding operation, i.e. from the first to the
last welding step (and suitably ~hereafter for a certain cooling
period).
: As described above, the electrode groups to be
` 15 respectively supplied with the welding current are galvanically
connected to the plate S before the welding current i5 turned
on by means of the switching device 15. Correspondingly, the
electrodes are galvanically separated from the plate 5 and thus
~rom one another only after.the welding current has been turned
off. By the currentless switching of the isolating switches 30,
spark erosions on the contact surfaces 36, 37 are avoided.
The formation of the contact surface 37 at the uppermost
leaf of the current tape end 40 ensures, together with the
large force of the hydraulic unit 42, 45, 46, that the contact
resistance is equally low over the entire contact surace:
Due to the flexihility of the metal leaves constituting the
current tape, the uppermost leaf is urged, by the force of
- 12 -
5~9
the piston 45, 46 transmitted by the intermediate element 48,
over its entire surface tightly against the rigid contact
suxface 36 of the leg 38, which would be ensured in case of
two rigid contact surfaces only under utmost parallelism.
The welding current could also be an alternating
current. The direct current, however, has the advantage as
compared with alternating current that, due to the lack of
skin effect, it is more uniformly distributed not only in the
electrodes but also in the workpieces and remains focused.
In place of the counter electrodes 11 individually
associated with the electrodes 6 through 9, it is also pos-
sible to provide a single counter electrode fashioned as a
~orce transmission plate 10.
The switch for the welding current can furthermore
be connected into the secondary circuit of the transformer
group 14. Basically, the switches 30 could also be power
switches which switch the welding current.
Further, the piston-cylinder unit 3 can also engage
at the plate 10 instead of at the plate 5, wherein the latter
can be rigidly mounted at the machine frame. Also, two
piston-cylinder units can be provided for the two power trans-
mission plates 5 and 10, one of which adjusts the plate
into a position determined by a stop, and the other of which
is activated until the desired welding ~orce has been produced.
Furthermore, the counter electrodes 11 can be supported --
instead of or in the same way as the electrodes 6 -- in a
resilient fashion at the plate 10 ~by means of appropriate
compression springs 32).
- 13 -
~51~
The contact surface 36 can also be formed directly
on the underside of the plate 5; the contact surface 37 can
be formed, for example, at a contact spring attached to the
end 40 of the current tape.
Th~ other end 43 of the current tape 41 can, finally,
- also be directly attached to the electrode holder 20.
Lastly, the individual el~ctrodes of one group can
also be aonductively connected with one another and can be
connectible together with the plate 5 by means of a single
switching element correlated with the entire group. The
illustrated embodiment wherein each electrode is associated
with its own switching member 30 has the advantage, though, that
the individual switching members have a lesser current load
and the subdivision into groups can be readily varied merely
by a correspondingly different hydraulic activation of the
pistons 45.
In the described example, all electrodes are mounted
on a joint power transmission plate. However, it is also
possible to arrange the electrodes in groups at several power
transmission plates, each plate having its own power generating
means. The power transmission plates would then have to be
connected electrically with one another and/or together with
the rectifier 12. The advantage of this version resides in
that the welding forces can be selected to be different from
one electrode group to the next -- by a corresponding adjustment
of the individual power generating means.
14 -
