A METHOD OF FASTENING A STRIP OF THIN SHEET METAL ON A CONICAL SEPARATION DISC OF THIN SHEET METAL, AND A SEPARATION DISC PROVIDED IN THIS MANNER WITH A STRIP OF THIN SHEET METAL

Abstract: Conical separation discs of thin sheet metal intended for centrifugal separators have, as a rule, spacing members in the form of thin strips welded onto one of their sides. For avoiding local overheating of a separation disc (1), when a strip (2; 13) is to be welded onto the separation disc by roll welding, the strip is formed with at least one ridge (3, 4; 14) extending in the longitudinal direction of the strip, the width of the ridge being substantially smaller than that of the strip (2; 13). Hereby the contact area between the strip and the separation disc will become very small in the welding area, and only a limited amount of energy need to be used for the welding.

Description: A method of fastening a strip of thin sheet metal on a conical separation disc of thin sheet metal, and a separation disc provided in this manner with a strip of thin sheet metal. Conical separation discs for centrifugal separators are traditionally produced by thin sheet metal and are usually provided with elongated strips also produced from thin sheet metal and fastened onto one sides of the separation discs by welding. When several separation discs are stacked onto each other in the rotor of a centrifugal separator the' -strips serve as spacing members for forming flow spaces between the separation discs. The welding method normally used in connection with fastening of strips of this kind onto separation discs is spot welding. This welding method is cheap and has been considered sufficient from different starting points. Thus, brazing has been abandoned since very long as a connection method, and welding of the strips along all of the edges thereof has been considered unnecessarily expensive and not required. However, in certain connections new requirements have lately been set up on the connection of the strips with the separation discs, meaning that the edges of each strip have to be sealingly connected with its separation disc, so that no pockets are formed, which are difficulty to clean. These new requirements have brought up the question how, in the cheapest and most practical way, a continuous weld joint can be provided through resistence welding between a thin strip of sheet metal of this kind and a conical separation disc also produced from thin sheet metal so that heat, developed during the welding, will not cause deformation of the separation disc. A conical separation disc is usually produced by pressure turning of a circular or annular plane piece of thin sheet metal and it obtains, as a concequence of this treatment, stresses built into the sheet material, which may easily deform the separation disc upon local heating thereof. A problem in connection with use of conventional roller welding for obtaining a continuos weld joint between a strip and a separation disc is that a relatively large contact surface comes up between the strip and the separation disc, when a roll welding electrode is rolled along the strip and presses it against the separation disc. This contact surface comes up in a welding area extending a cross the whole of the strip and over a distance in the longitudinal direction of the strip. In order to obtain melting in this relatively large contact area relatively much energy has to be used, which leads to an undesired strong heating of the separation disc. Theoretically it would be possible to limit the welding to only a small part of the width of the strip for minimising the heat development during a welding operation. However, this would require that both the weld electrode applied and pressed against the strip and the weld elctrode applied against the separation disc were isolated electrically over certain parts of their surfaces intended for contact with the strip and the separation disc, respectively, and further that the uninsulated area or areas of the respective contact surfaces of the weld electrodes were to be placed extremely accurately in relation to both each other and the strip all the way along which a weld joint should be obtained. Considering that the strips here in question have a width of about 3-6 mm, that a weld joint of maximum 1 mm width is deemed desirable, that the strip is to be applied onto a curved (conical) surface and that the strip (weld joint) should not always be straigt and extend along, but sometimes at an angle, to a generatrix of the separation disc, an apparatus for obtaining this would become very complicated and expensive. For resolving the problem of undesired local overheating of a conical separation disc, when a narrow strip of the kind here in question is to be connected by resistance welding with a separation disc through two parallel weld joints along the edges of the strip, it has been suggested in EP 0 444 271 - A3 that a weld electrode in the form of a relatively narrow roller should be rolled across the strip in its cross direction and that welding current should be switched on only when the roller passes over the edges of the strip, where a weld joint should be obtained, and be switched off while the roller rolls across the central part of the strip between the edges. Thus, the roller has to be rolled across the strip several times before two continous weld joints have been obtained along the whole of the strip. It is also suggested in EP 0 444 271 - A3 that the weld roller should be rolled along the entire circumference of the separation disc and that welding should thus occur only when the roller passes across the edges of the strips in question, which in the case disclosed are placed such that they extend along the generatrices of the conical separation disc. From various starting points it may be questioned whether the welding method suggested in EP 0 444 271 - A3 is really performable without obtainment of an undesired local overheating of the separation disc. If it would be possible, after all, the weld roller would probably have to be very narrow, which makes it appearent that a welding operation performed in this manner would take far too long time to be useful in practice. With this backround it is an object of the present invention to provide a new method useful in practice to establish by resistance welding a continuous weld joint between a strip of the above discussed kind and a conical separation disc produced from thin sheet metal, which weld joint extends in the longitudinal direction of the strip. The method should thus make it possible to weld the strip onto the separation disc without undesired local overheating of the latter. Futhermore, the method should make possible a rapid welding operation at an acceptabel cost. This object can be obtained in a way such that a strip of thin sheet metal having a certain width is placed on a conical separation disc and is pressed thereagainst between a first weld electrode in contact with the strip and a second weld electrode in contact with the separation disc, the invention being characterized by choosing a strip of a kind having on one of its sides at least one ridge, which extends along the strip and has a width substantially smaller than the width of the strip, placing the strip on the separation disc with said ridge facing the separation disc, causing a roll welding electrode, constituting said first welding electrode, to roll under pressure against and along the strip such that in a welding area the ridge is pressed against the separation disc, and conducting in the welding area a current between the welding electrodes,which is so adapted that at least a part of said ridge melts in the welding area and gradually forms a continuous weld joint between the strip and the separation disc. During a welding operation performd in this way a very small contact area is obtained in said welding area between the strip and the separation disc as a consequence of said ridge being substantially narrower than the strip. To accomplish melting of material in the strip in such a small contact area there is required a correspondingly small amount of energy. Hereby it is possible to create a continiuous weld joint without undesired overheating of the separation disc. By the invention it is possible during the welding operation to use a minimum of energy. Thus, it is sufficient to use only so much energy that is required for partial melting of the ridge in the welding area. After such a partial melting of the ridge the contact area between the ridge and the separation disc is enlarged, and the size of the current conducted between the electrodes can be adapted such that the cur¬ rent will become insufficient for continued melting of the ridge, when said contact area has reached a certain size. The heating of the strip and the separation disc by the current as used is prevented in this way to lead automatically to undesired local overheating. For obtainment of a desired spacing function of the strip it is suitable, however, that a current conducted between the electrodes is adapted such that the ridge in the welding area melts substantially completely, whereby there is formed a contact area between the strip and the separation disc so large that, when the strip gets into contact with the separation disc across the whole of its width, the current will become insufficient for con¬ tinued melting of the strip. Melting of said ridge to this extent is not necessary if the strip should not operate as a spacing member but have some other function in an interspace between two separation discs. If there is a need to provide tight weld joints between a strip and a separation disc along both of the edges of the strip, a strip is chosen according to the invention which on one of its sides has two spaced ridges exten- ding along the strip; one at each of the strip edges. If a strip of this kind should serve as a spacing mem¬ ber, it is suitable to conduct a current between the weld electrodes adapted such that it will be insuffici¬ ent for continued melting of the strip only when a por- tion of the strip situated between the ridges gets in contact with the separation disc. The invention is described in the following with refe¬ rence to the accompanying drawings, in which figure 1 shows a frusto-conical separation disc of thin sheet metal, seen from its most narrow end and provided with several narrow strips of thin sheet metal welded onto it, figure 2 shows a cross section through part of the sepa¬ ration disc in figure 1, taken along the line II-II in figure 1, figure 3 shows a strip of thin sheet metal formed in accordance with the invention, figures 4 and 5 show alternative embodiments of an end portion of a strip according to figure 3, figure 6 shows a cross section through a separation disc and a strip according to figure 3, applied onto the separation disc between two weld electrodes, before welding has occured, figure 7 shows a cross section similar to that in figure 6 but showing the separation disc and the strip after welding has occured, figures 8 and 9 correspond to figures 6 and 7,respec¬ tively, the strip having a different shape, figure 10 shows equipment for welding of the strip onto a conical separation disc, figure 11 shows part of the equipment in figure 10 and figure 12 illustrates a weld joint formed by means of an equipment according to figure 10. Figure 1 shows a frusto-conical separation disc 1 pro¬ duced from thin sheet metal and seen from above, and strips 2 also produced from thin sheet metal and welded onto the outside or upperside of the separation disc 1. The strips 2 extend in this case in straight paths along the generatrices of the conical separation disc 1. Each strip can extend, however, along any desired straigt or curved path on the separation disc. Alternatively , the strips could be welded onto the inside or underside of the separation disc 1. Figure 2 shows a cross section through the separation disc 1 taken along the line II-II in figure 1. As can be seen, the inside or underside of the separation disc is entirely smooth. When several separation discs 1 are stacked onto each other the strips 2 serve, among other things, as spacing members between the separation discs. The interspaces thus formed between the separation discs are supposed to serve as separation spaces in the rotor of the centrifugal separator. The thickness of a separation disc 1 of this kind may vary depending on, among other things, the size of the separation disc. For instance the size may be 0,6 mm. Even the thickness of the sheet metal strips 2 may vary depending on the desired thickness of the interspaces between the separation discs stacked onto each other. The thickness of the spacing strips are often of the same magnitude as the thickness of the separation discs. Figure 3 shows a view of a strip 2, which has not yet been welded onto a separation disc. The strip is shown from the one of its sides that is to face the separation disc. Figures 4 and 5 show alternatively formed end portions 2a and 2b, respectively, of the strip. As can be seen, the strip 2 on one of its sides has two ridges 3 and 4 extending along the respective edges of the stip. In fig 5 these ridges are designated 3a and 4a, respectively. Even at the ends of the strip ridges 5 and 6 are formed which interconnect the ridges 3 and 4. In fig 5 a ridge of this kind is designated 5a. As can be seen, each one of the ridges 3-6 has a cross section of a kind such that the ridge is broadest at its base and gets narrower towards its crest. In an area 7 surrounded by the ridges 3-6 the strip 2 is entirely smooth. A corresponding area of the strip 2b in fig 5 is designated 7a. Figur 6 shows in a cross section a strip 2 according to figure 3, placed on the upper side of a separation disc 1 with its ridges 3 and 4 in contact therewith. Between the ridges 3 and 4 there is formed a space 8, in which the strip 2 has no contact with the separation disc 1. On top of the strip 2 in fig 6 there is placed a first welding electrode in the form of a roller 9. The roller 9 is oriented with its rotational axis extending across the longitudinal direction of the strip 2, so that it can be rolled on the strip 2 therealong. On its underside the separation disc 1 is supported by a second welding electrode 10, which may have the form of a solid conical body (see fig 10). The welding electrodes 9 and 10 are connectable to a source of current in a conventional manner (not shown) and arranged for roll welding by meens of conventional equipment. During a welding operation the roller 9 is rolled along the strip 2 while being pressed thereagainst. The comp- ression of the strip 2 and separation disc 1 has to be so strong that a contact pressure required for welding of the strip and the separation disc together comes up in the intended welding area. Since the strip can be pressed to contact with the separation disc only trough the ridges 3 and 4, the crests of which are very narrow, a very small contact area is obtained in the intended welding area between the strip and the separation disc. A possibly pulsed welding current adapted with regard to the size of the contact area is conducted between the welding electrodes 9 and 10, while the welding electrode or roller 9 is rolled along the strip 2. While the roller 9 moves along the strip 2 the ridges 3 and 4 and possibly part of the material in the separa- tion disc 1 melt gradually to a liquid state, whereby the contact area in the welding area between the strip and the separation disc is enlarged every moment. The welding current then need not be stronger than so that its ability of melting material in the welding area automatically ceases, when a desired amount of material has been melted and said contact area has got a desired size. When the central area 7 of the strip 2 gets into contact with the separation disc 1, the size of the con¬ tact area increases strongly, which can be utilized for suitable adaptation of the strength of the welding cur¬ rent, so that the ridges 3 and 4 will safely melt enti¬ rely without the welding current after that having to cause continued melting in the welding area. From figure 7 it can be seen how two weld joints 11 and 12 have been formed between the separation disc 1 and the respective edge portions of the strip 2 after the material in the ridges 3 and 4 and parts of the material in the separation disc 1 have melted to liquid form and after that solidified again. As can be seen, the strip 2 has got into contact with the separation disc 1 across its central area 7 (see fig.3 and 4) without any weld joint having been formed in this area. Some liquid mate¬ rial has instead moved outwardly to the areas outside the edges of the strip. The figures 8 and 9 illustrate how a metal sheet strip 13 having only one centrally situated ridge 14, exten¬ ding along the strip 13, is welded onto a separation disc 1. The same welding electrodes 9 and 10 as in figu¬ res 6 and 7 may be used in this case. One single weld joint 15 is formed in this case, which in many connec¬ tions can be sufficient. If a strip of this kind is to extend along a generatrix of the separation disc 1, the strip 13 may be given a small curvature, seen in a cross section trough the strip, which is of the same magnitude as or insignificantly stronger than the curvature of the upper side of the separation disc 1 onto which its shall be welded. (The curvature of the separation disc is not shown in the figures 8 and 9 ) . Hereby it can be avoided that gaps come up along the edges of the strip or bet¬ ween the strip and the separation disc. Figure 10 shows schematically certain parts of an appa- ratus for welding metal sheet strips onto frusto-conical separation discs. On a frusto-conical solid body forming a supporting welding electrode 10 there is placed a separation disc 1, which is produced from thin sheet metal and has a form substantially coinciding with that of the welding electrode 10. The welding electrode 10 is rotatable around an inclined axis 16, constituting also the centre axis of the electrode 10, and is arranged upon rotation around this axis to entrain the separation disc 1 in its rotational movement. The axis 16 is inclined in a way such that a generatrix 17 of the separation disc 1 al¬ ways extends horizontally and simultaneously is situa¬ ted in the same vertical plane as the inclined axis 16. A roller forming a roll welding electrode 9 is suspended from a frame 18, which is movable horizontally from the left to the right with respect to figure 10 in parallel with said generatrix 17. The roll welding electrode 9 is rotatable relative to the frame 18 around its centre axis 19, which extends horizontally, whereas the frame 18 together with the roller 9 is rotatable around a ver¬ tical axis 20. The frame 18 is suspended in a way such that the vertical axis 20 during movement of the frame horizontally is all the time situated in the same verti¬ cal plane as the axis 16 and the generatrix 17. As can further be seen from figure 10 the frame 18 is equipped with a device 21 for guiding a sheet metal strip 2 into an interspace between the roll welding electrode 9 and the separation disc 1 in an area where welding is to take place. The metal sheet strip 2 has previously been part of a continuous long strip, which during a welding operation is supplied towards the gui- ding device 21. Before the strip reaches the guiding device 21 it is divided into pieces having a desired length by means of a cutting machine (not shown) after which the pieces, i.e. the strips 2, are directed into the guiding deviced 21. Whereas the ridges 3 and 4 ( see fig 3) are formed in the strip already before it is divided, the ridges 5 and 6 are formed in said cut¬ ting machine. The welding electrodes 9 and 10 are con¬ nected in a known manner (not shown) to a source of cur¬ rent. The welding operation is started with the frame 18 and the roller 9 being situated in the vicinity of the wi¬ dest end of the separation disc 1, i.e. to the left in figure 10. Here the frame 18 is adjusted to a desired rotational position around the axis 20, so that the device 21 can quide the strip 2 in a desired direction relative to the generatrix 17 and so that the rotational axis 19 of the roller 9 itself will extend across the longitudinal direction of the strip. Thereafter one end of the strip 2 is inserted into the gap between the rol¬ ler 9 and the separation disc 1 and the welding current circuit is closed. Simultaneously the electrode 10 and the separation disc 1 are caused to rotate around the axis 16 and the roller 9 is caused to move along the generatrix 17 while it is allowed to roll on and be pressed against the strip 2. At the beginning, i.e. close to the widest end of the separation disc, the strip 2 forms in every point a relatively large angle to the generatrix of the separa¬ tion disc through the point in question, but this angle decreases gradually as new parts of the strip 2 are welded onto the separation disc. Thus, the strip 2 when it is fastened to the separation disc will extend along a path that is curved in several planes. At the welding place where the roller 9 is situated in figur 10 the strip 2 extends substantially in parallel with the gene¬ ratrix 17. The rotation of the electrode 10, as well as the hori¬ zontal movement and rotation around the axis 20 of the frame 18, can be performed automatically according to a program supplied into a computer. Also the welding might be performed automatically. Thus, the strength of the welding current and/or possibly the pulsation of the welding current may have to be given certain values, when the welding electrode 9 is rolling along the main part of the strip 2, but other values when the roller passes across the end portions of the strip 2, when the size of the contact area between the strip 2 and the separation disc 1 through the ridges 3-6 is changed. The values for the strength of the current and possible pulsation of the current is to be determined empirically beforehand for each separate case with regard to, among other things, the material in and the thickness of the separation disc 1 and the strips 2. By means of an equipment of the described kind strips may be welded onto a separation disc along almost any paths. The sidewise bending of a strip then needed is facilitated by the fact that the strip, during the wel¬ ding, is locally heated and becomes soft in the welding area. Figure 11 shows the roll welding electrode 9 seen from the side during a welding operation. A welding area 22 shown between two arrows is the zone in which melting occurs during the welding. Figure 12 illustrates the extension of a weld joint 23 accomplished at an end portion of a strip 2 of the kind shown in figures 3 and 4. The roll welding electrode 9 is shown by dashdot lines. Even this figur shows the welding area 22 in which melting occurs during the wel- ding. Some curved dashdot lines 23a show an alternative extension of a weld joint. Both of the electrodes 9 and 10 may need to be subjected to cooling during welding so that their temperature is kept at a suitable level. In the examples shown in the drawing the ridges 3,4 and 14 have been given a height of the same magnitude as the thickness of the rest of the strip 2, e.g. 0,4 mm. This relation can of course be different if this would be suitable. For obtaining as little heating as possible of the separation disc the ridges are preferably given the smallest possible height required for accomplishing satisfactory weld joints. However, the ridges 3 and 4 must not be too low, since the central portion of the strip between the ridges 3 and 4 may in this case get into contact with the separation disc 1, before the wel¬ ding operation has started. This depends on the fact that the upper side of the separation disc is somewhat convex, which has not been shown in the figures 6-9.

Claims: Claims 1. A method of fastening by resistance welding a strip (2;13) of thin sheet metal, having a certain width, onto a substantially conical separation disc ( 1 ) of thin sheet metal, the strip being applied onto the separation disc and pressed thereagainst between a first welding electrode (9) in contact with the strip (2; 13) and a second welding electrode (10) in contact with the sepa- ration disc (1), c h a r a c t e r i z e d b y choosing a strip (2;13) of a kind having on one of its sides at least one ridge (3,4; 14), which extends along the strip and has a width substantially smaller than the width of the strip, - placing the strip (2; 13) on the separation disc (1) with the ridge (3,4; 14) facing the separation disc, causing a roll welding electrode constituting said first welding electrode (9) to roll under pressure against and along the strip (2;13), so that in a welding area the ridge (3,4; 14) is pressed against the separation disc ( 1 ) , and - conducting in the welding area a current bet¬ ween the welding electrodes (9,10) which is so adapted that at least a part of the ridge (3,4; 14) melts in the welding area and gradually forms a continuous weld joint (11,12; 15) bet- ween the strip (2; 13) and the separation disc (1). 2. A method according to claim 1, including conducting a current between the welding electrodes (9,10) which is adapted such that the ridge (3,4; 14) melts only partly in the welding area, a contact area being created bet¬ ween the strip (2; 13) and the separation disc (1) ha¬ ving a size such that the current is insufficient for continued melting of the ridge (3,4; 14). 3. A method according to claim 1, including conducting a current between the welding electrodes (9, 10) which is adapted such that the ridge (3,4; 14) melts substan- tially completely in the welding area, a contact area being created between the strip (2; 13) and the sepa¬ ration disc (1 ) having a size such that the current is insufficient for continued melting of the strip. 4. A method according to any of the preceding claims, including choosing a strip (2 ) of a kind which has on one of its sides two ridges (3,4) extending along the strip spaced from each other, one at each longitudinal edge of the strip, a current being conducted between the welding electrodes (9,10) and adapted such that tight welding joints (11,12) are formed along both of the edges of the strip between the strip and the separation disc. 5. A method according to claim 4, including conducting a current between the welding electrodes and adapting it such that the two ridges (3, 4) melt completely in the welding area, a contact area being created between the strip (2) and the separation disc ( 1 ) , when a portion (7) of the strip (2) situated between the ridges (3,4) gets into contact with the separation disc ( 1 ) , which contact area has a size such that the current is insuf¬ ficient for continued melting of the strip (2) . 6. A conical separation disc of thin sheet metal, which is formed such that it can be mounted in a rotor of a centrifugal separator and which is provided with one or more strips (2; 13) welded onto it in accordance with the method of anyone of the preceding claims.

Inventors: ELOFSON JOHN, ELOFSON, JOHN
Applicants: ELOFSON JOHN, TETRA LAVAL HOLDINGS & FINANCE, ELOFSON, JOHN, TETRA LAVAL HOLDINGS & FINANCE S.A.
Application year: 1996
Main classification: B04B7
IPC codes: B04B7, B23K11