CA1159519A - Process and apparatus for multiple-station resistance welding - Google Patents

Abstract

PROCESS AND APPARATUS FOR MULTIPLE-STATION
RESISTANCE WELDING

ABSTRACT

Several electrodes (6) are mounted, electrically insulated, to a joint power transmission plate (5). Each electrode (6) is associated with an isolating switch (30) by means of which the electrode can be galvanically connected to the plate (5). The plate (5) and a second power trans-mission plate (10) carrying the counter electrodes (11) are connected to the terminals of a single source of welding current. The workpieces are clamped between all of the electrodes (6) and counter electrodes (11). Thereupon, preselected groups of the electrodes (6) are supplied in succession with the welding current.
By means of the apparatus, very many, closely adjacent sites can be welded in a small space, while the workpieces, secure against deformation, are clamped between all of the electrodes and counter electrodes.

Description

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 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 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 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 ~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 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).

~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.

Claims (20)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Apparatus for multiple point resistance welding having a plurality of electrodes and at least one counter electrode comprising at least one electrically conductive pressure and cur-rent transmission plate, all of the electrodes being mounted in an electrically insulated manner to said electrically conductive pressure and current transmission plate, a source of welding cur-rent having one terminal connected to said electrically conductive pressure and current transmission plate, a pressure generating means connected to move said electrically conductive pressure and current transmission plate for the simultaneous actuation of all of the electrodes, and a switch associated with each electrode of the plurality of electrodes, by means of which such electrode can be galvanically connected with the pressure and current trans-mission plate.

2. Apparatus according to Claim 1, including resistors connected in individual circuits with the switches between the electrodes and the power transmission plate, whereby, when pre-selected electrodes are connected in succession to said source of welding current differing amperages in dependence on the select-ed resistors flow through the individual electrodes which are respectively inserted in parallel to one another in the welding current circuit.

3. Apparatus according to Claim 1, characterized in that each switch has two contact members, contact surfaces on said contact members flatly in contact with each other in the turned-on position of the switch and separated by an air gap in the turned-off position of the switch.

4. Apparatus according to Claim 3, characterized in that a protective envelope connected in surrounding relation to the contact surfaces of the contact members to protect the same from dust and weld spatter.

5. Apparatus according to Claim 3, including an individ-ual fluid activatable piston-cylinder unit connected to each switch to operate said switch.

6. Apparatus according to Claim 5, characterized in that one contact member of said two contact members is connected to said pressure and current transmission plate, and the other contact member of said two contact members is formed at or mounted to an end of a flexible conductor displaceable by means of the piston-cylinder unit in parallel to the contact surface of the one con-tact member , the other end of this conductor being connected firmly and conductively to the electrode holder or a connecting element joined thereto.

7. Apparatus according to Claim 6, characterized in that said one contact member is formed by a leg of a U-shaped holder extending around said one end of the flexible conductor, said leg being mounted to said pressure and current transmission plate, a compression spring mounted in said one leg, said U-shaped holder having another leg carrying the piston-cylinder unit, the piston of said piston-cylinder unit engaging in an insulated fashion said one end of the flexible conductor and extending through said one end and engaging said compression spring, the piston being fluid actuatable against the force of said compres-sion spring.

8. Apparatus according to Claim 1, in which each elec-trode is connected to a guide element, a guide for each guide element vertically projecting from the pressure and current trans-mission plate in which the respective guide element is connected for displacement in the direction toward the pressure and current transmission plate, a compression spring connected between said guide and said guide element against the force of which said guide element is displaceable, and means for adjusting the bias of said compression spring.

9. Apparatus according to Claim 6, characterized in that the electrode holders and/or the electrodes as well as the switches are provided with cooling ducts.

10. Apparatus according to Claim 1, characterized in that said source of welding current includes a high-current rectifier connected into the secondary circuit of transformer means, and said pressure and current transmission plate connected to one terminal of said high-current rectifier.

11. Apparatus according to Claim 1, characterized in that each electrode is associated with a counter electrode, a second pressure and current transmission plate, and all of the counter electrodes conductively connected to said second pressure and current transmission plate in parallel with the first mentioned pressure and current transmission plate, said second pressure and current transmission plate being connected to the other terminal of the source of welding current.

12. Apparatus according to Claim 1, characterized in that all of the electrodes are attached to a joint power trans-mission plate.

13. Apparatus according to Claim 1, characterized in that the electrodes are attached to several electrically conductive pressure and current transmission plates, which are together connected to the one terminal of the source of welding current, and a separate pressure generating means connected to move each of said several pressure and current transmission plates.

14. Apparatus for multiple-point resistance welding having a plurality of electrodes and at least one counter electrode, comprising at least one electrically conductive supporting plate, the electrodes being mounted in an electrically insulated manner to said electrically conductive supporting plate, a source of welding current having one terminal connected to said electrically conductive supporting plate, a flexible conductor for each elec-trode, a piston-cylinder unit for each conductor, a switch assoc-iated with each electrode having two contact members, each of said electrodes being connected to an end of a said respective flexible conductor, the other end of said respective flexible conductor being mounted to the piston of said respective piston-cylinder unit, said other end of said flexible conductor forming one contact member of said two contact members of switch, the other contact member of said two contact members of said switch mounted to said electrically conductive supporting plate, and said piston of said piston-cylinder unit being actuatable for operating said switch.

15. Apparatus according to Claim 14, in which said other contact member is formed by a leg of a U-shaped holder extending around said other end of said flexible conductor, said leg being mounted to said electrically conductive supporting plate, a compression spring mounted in said one leg, said U-shaped holder having another leg carrying the piston-cylinder unit, said piston engaging in an insulated fashion said other end of the flexible conductor and extending through said other end and en-gaging said compression spring, and the piston being fluid actuatable against the force of said compression spring.

16. Apparatus according to Claim 14, in which each electrode is connected to a guide element, a guide for each guide element vertically projecting from the electrically conductive supporting plate in which the respective guide element is connect-ed for displacement in the direction toward the electrically conductive supporting plate, a compression spring connected between said guide and said guide element against the force of which said guide element is displaceable, and means for adjusting the bias of said compression spring.

17. Apparatus according to Claim 14, including electrode holders for said electrodes, and the electrode holders and/or the electrodes as well as the switches are provided with cooling ducts.

18. Apparatus according to Claim 14, in which said source of welding current includes a high-current rectifier connected into the secondary circuit of transformer means, and said electrically conductive supporting plate connected to one terminal of said high-current rectifier.

19. Apparatus according to Claim 14, in which each elec-trode is associated with a counter electrode, a second electrically conductive supporting plate, and all of the counter electrodes conductively connected to said second electrically conductive supporting plate in parallel with the first mentioned electrically conductive supporting plate, said second electrically conductive supporting plate being connected to the other terminal of the source of welding current.

20. Apparatus according to Claim 14, in which all of the electrodes are attached to a joint electrically conductive support plate.