METHOD OF PREVENTING SAG OF PANEL AND APPARATUS THEREOF

Abstract: A method and apparatus for preventing a sag of a panel, in which a plate (2) to be attached to a skeleton is located on the skeleton supported by a supporter, both end parts of the plate (2) are gripped by grasp electrodes (3) of a resistance heating device (l), current is caused to flow through the plate (2) by the grasp electrodes (3) to preheat the plate 92) to a predetermined temperature and to thermally expand it, the plate (2) is welded to the skeleton by a welding machine, and a tensile residual stress is developed in the plate by a thermal shrinkage deformation attendant upon cooling of the plate (2) after the welding. In a case where the plate (2) has a cut-away part (B Fig 6) electric conduction members (4) for short-circuiting the cut-away part are interposed in this part so as to form electric conduction paths for the preheating of the plate (2).

Description: BACKGROUND OF THE INVENTION The present invention relates to a method of prevent­ ing the occurrence of the "sag" of a panel which is used for assembling the roof structures, side wall structures etc. of vehicles, ships, buildings etc., and also to an apparatus therefor. In recent years, especially lightening has been eagerly required of the structures mentioned above by way of example, and thinning the structures has accordingly been promoted. Soft steel, stainless steel, aluminium alloys etc. are usually applied to the plates of the panels. With the promotion of the thinning, the "sag" of the plate after welding and assemblage arises inevitably, and problems are posed as to the local strength of a "sagging" part and the external appearance of the panel. With the intention of eliminating the welding distor­ tion, accordingly, there have been disclosed a measure wherein a large number of sheets of standard shape are formed into an elongate plate by welding, whereupon under the state under which the plate is loaded with an over­ stretch, it is attached to a skeleton (refer to, for example, the official gazette of Japanese Patent Applica­ tion Publication No. 53-3926l), and a measure wherein the above plate formed to be elongate is loaded with the overstretch and is also preheated to thermallly expand, whereupon the plate is attached to the skeleton (refer to, for example, the official gazzette of Japanese Patent Application Publication No. 54-20l85). Both the measures, however, require a large-scale stretching device and heating device and involve enormous installation costs as well as large numbers of steps. As another drawback, it is not easy to delicately control a set heating tempera­ ture. There has also been provided a measure wherein a plate of comparatively small size is put in a heating box, and the whole plate is heated to a predetermined tempera­ ture and is thereafter welded to a skeleton (refer to, for example, the official gazette of Japanese Patent Applica­ tion Laid-open No. 60-6479l). However, the heating box is complicated in structure. Moreover, as the plate becomes larger in size, the heating box needs to be larger in scale in proportion, to incur the problems that a large floor space and a workshop of large area are required and that the installation cost becomes high. Further, there has been disclosed a measure wherein a plate having an opening for a window or a door is placed on a heat insulating material laid on a surface table, and heating means such as a heater or a hot iron plate is put on the plate, while the heating means is compressed by pressure application means such as a press and heats the plate in contact, the plate being thereafter welded to a skeleton (refer to, for example, the official gazzette of Japanese Patent Application Publication No. 53-39262). This measure, however, has the drawback that the heating means and the pressing means need to be larger in scale in proportion to the size of the plate and are not easy of handling. SUMMARY OF THE INVENTION In view of the problems of the prior arts, the present invention has for its object to provide a method and apparatus for preventing the sag of a panel according to which the occurrence of the "sag" is prevented during the assemblage of a thin-walled structure. As expedients for accomplishing the object of the present invention, the first aspect of performance of the invention is characterized by comprising the step of locating a plate on a skeleton to which said plate is to be attached, the step of gripping both end parts of said plate with grasp electrodes of resistance heating means, the step of causing current to flow through said plate across said grasp electrodes, thereby to preheat said plate to a predetermined temperature and to thermally expand it, the step of welding said plate to said skeleton, and the step of developing a tensile residual stress in said plate owing to a thermal shrinkage deforma­ tion attendant upon cooling of said plate after the welding. The second aspect of preformance of the inven­ tion is characterized by comprising the step of locating a plate on a skeleton to which said plate is to be attached, the step of gripping both end parts of said plate with grasp electrodes of resistance heating means and inter­ posing electric conduction members for short-circuiting a cut-away part of said plate, in said cut-away part so as to form electric conduction paths, the step of causing current to flow through said plate across said grasp electrodes, thereby to preheat said plate to a prede­ termined temperature and to thermally expand it, the step of welding said plate to said skeleton, and the step of developing a tensile residual stress in said plate owing to a thermal shrinkage deformation attendant upon cooling of said plate after the welding. The third aspect of performance of the invention is characterized by comprising support means for supporting a skeleton, resistance heating means for preheating a plate, and weld means for welding said plate to said skeleton, said resistance heating means being made of grasp electrodes which detachably grip both end parts of said plate and which cause current to flow through said plate and includ­ ing electric conduction members which are interposed in a cut-away part of said plate and which short-circuit said cut-away part. As functions based on the expedients described above, the plate is preheated to the predetermined temperature to be thermally expanded by the grasp electrodes and electric conduction members of the resistance heating means before being attached to the skeleton, and it is thereafter welded, so that the tensile residual stress capable of checking the occurrence of the "sag" is developed in the plate by the thermal shrinkage deformation in the cooling process. BRIEF DESCRIPTION OF THE DRAWINGS The drawings exemplify the aspects of performance of the present invention, in which: Fig. l is an outward perspective view showing a state in which both the end parts of a plate are gripped by the grasp electrodes of resistance heating means; Fig. 2 is an outward perspective view showing the grasp electrodes in an exploded state; Fig. 3 is an outward perspective view of an electric conduction member; Fig. 4 is a diagram showing the cooling character­ istic of a plate at the stage of natural cooling after heating; Fig. 5 is a diagram showing a temperature distribu­ tion in the heating test of a plate specimen; and Figs. 6 and 7 are diagrams each showing a temperature distribution in the heating test of a plate specimen which has a cut-away part. PREFERRED EMBODIMENT OF THE INVENTION: Now, embodiments of the present invention will be described with reference to the drawings. Resistance heating means l shown in Figs. l thru 3 is applied in case of fabricating a panel in which a plate 2 is coupled with a skeleton (not shown) having the plate attached thereto. An apparatus for preventing the sag of the panel (not generally shown) is constructed of a known supporting rest (not shown) made up of sleepers, bearers etc. and arranged on a floor in order to support the skeleton, the resistance heating means l for preheating the plate 2, the known welding means (not shown) made up of a spot welding machine or the like. The resistance heating means and the welding means are disposed so as to be movable relative to the skeletion which is held horizontal at a proper height by the supporting means, and the plate which is located on the skeleton. The resistance heating means l consists of grasp electrodes 3 and one or more electric conduction members 4. The grasp electrodes detachably grip both the ends of the plate 2 (refer to Fig. l) and directly cause current to flow, thereby to heat the plate 2. As shown in Fig. 2, each grasp electrode comprises an upper electrode 5 and a lower electrode bar 6 which are made of a copper alloy material or the like, and clamps 7 which are made of a steel material or the like. In this embodiment, the upper electrode bar 5 and the lower electrode bar 6 are formed in the shapes of straight bars of rectangular sections, and a terminal piece 8 for connecting an electric wiring lead is fastened on the top surface of the upper electrode bar 5. The clamps 7 each having a locking bolt 9 and a bracket l0 are arranged at positions of equal intervals in the lengthwise directions of both the electrode bars 5, 6 in order to ensure a uniform conduction performance. The upper electrode bar 5, the plate 2 and the lower electrode bar 6 are fixed and bound in pressed contact by the lock­ ing bolts and the brackets. In a case where prompt detachment is expected, known hand clamps may well be mounted on the upper electrode bar instead of the locking bolts. The electric conduction member 4 serves to form a conduction path by short-circuiting the cut-away part or space of the plate 2 at the state of preheating this plate owing to the direct conduction. As shown in Fig. 3, this electric conduction member is formed in the shape of letter I which has fitting portions 4b, 4b at both the ends of a strip portion 4a, by the use of an aluminum alloy plate l.6 mm thick by way of example. It has the strip portion 4a interposed in the cut-away part (B in a heating test to be described later) and has the fitting portions 4b, 4b detachably fixed to the peripheral edge parts of the cut-away part B by known fixation and binding means such as hand clamps or vices, whereby the conduction path for short-circuiting the cut-away part is formed. In order to readily cope with changes in the shape of the cut-away part B, the electric conduction members 4 are prepared with various lengths, or they are sometimes formed of electric wires having appropriate cross-sec­ tional areas. The direction of arrangement of the electric conduction menbers 4 is set to be parallel or slant to the facing direction of the grasp electrodes 3, 3 (the direction in which the current flows) as may be needed. Next, there will be described a method of preventing the sag of a panel to which the resistance heating means l is applied. First, the plate 2 is located on the skeleton held horizontal at the proper height by the supporting means arranged on the floor. Then, at a preceding heating position to be first preheated, both the end parts 2a, 2a of the plate 2 in the widthwise direction thereof are gripped by the grasp electrodes 3, 3 and are fixed and bound by the locking bolts 9, 9 ....., while in the presence of the cut-away part or space B (refer to Fig. 6 or 7) in the plate 2, the electric conduction members 4 are fixed to the required parts of the cut-away part so as to form the conduction paths. Subsequently, current is fed through the plate 2 by the grasp electrodes 3, 3 to preheat this plate to a predetermined temperature T described later and to thermally expand it. Thereafter, the grasp electrodes 3, 3 are removed from the preceding heating position, and they are moved to the next heating position and are caused to grip both the widthwise end parts of a plate 2 in this position. Meantime, the plate 2 in the preceding heating position is quickly tacked and welded to the skeleton. Further, the plate 2 in and behind the next heating position are heated, tacked and finish-welded by repeating the same operations as describ­ ed above. It has been experimentally verified that, when the tack welding is promptly and reliably carried out, the plate 2 where heat still remains will not develop a considerable tensile stress, and that the plate can generate a sufficient residual stress when reliably finish-welded meantime. If the parts gripped by the grasp electrodes can be avoided as positions to-be-welded, it will pose quite no problem to perform the finish welding without the tacking. In addition, a measure in which the grasp electrodes are partly cut away or holed for the purpose of the tack welding is very convenient because the plate can be tack-welded before the detachment of the electrodes. At the stage of the finish welding,, spot welding is suitable for the panel of this type, and continuous welding such as arc welding is unsuitable because it gives rise to distortions. The plate 2 fixed to the skeleton by the finish welding develops a tensile residual stress in the plate 2 owing to a thermal shrinkage deformation attendant upon the subsequent natural cooling, whereby the fabrication of the panel free from any "sag" is completed. In the case of heating the plate 2, the heating temperature of the plate 2 can be controlled in such a way that temperature sensors are previously disposed at the proper positions of the plate 2 as needed and that the detection signals of the tempera­ ture sensors during the heating are associated with the electric circuit of the grasp electrodes 3. Concretely, one sensor may be set at the required lowest temperature and the other at the required highest temperature so as to control the heating temperature between time. The predetermined temperature T at the step of heating the plate 2 is set so that the plate 2 may give rise to a required tensile residual stress, in considera­ tion of the material of the plate 2, the ambient tempera­ ture, the strength of the skeleton, the handling of the plate 2 after the heating, the period of time taken till the start of the finish welding since the tacking, and so forth. An example of the predetermined temperature T will be calculated as to a case where a stainless steel plate l.5 mm thick is employed for the plate 2, the ambient temperature is about 20°C, and the period of time taken till the start of the finish welding since the outer plate heated has been taken out and tacked is about l minute. It is known that a tensile residual stress necessary for preventing the occurrence of the welding distortion of the plate 2 after welding may be l5 kg/mm² or so. The temperature difference X between the plate 2 and the skeleton on which this plate is to be mounted, for developing the above tensile residual stress is evaluated with the following formula: X = k . σ/σy σ : tensile stress, σy: yield stress or proof stress value, k : temperature difference at which a tensile stress corresponding to σy develops. When σ = l5 kg/mm², σy = 52.7 kg/mm² and k = l54°C are respectively substituted into the above formula, the temperature difference X becomes about 44°C. Mean­ while, a cooling rate near, e.g., 90°C is read to be: about 6°C/min from the diagram of the cooling curve of the stainless steel plate after the heating thereof as shown in Fig. 4. The temperature difference X is needed at the point of time at which the welding fixation of the plate 2 has ended. Therefore, assuming that the period of time taken since the end of the heating till the completion of the tack welding be approximately l minute and that the ambient temperature, namely, the temperature of the skeleton be 20°C, the heating target temperature of the plate 2 becomes 20°C + 44°C + 6°C = 70°C. Accordingly, the predetermined temperature T of the plate 2 during the production of the product should desirably be set at approximately 80°C with some allowance for the aforementioned heating target temperature. In this connection, there will now be described the essential points of heating tests which were conducted as to conditions for uniformly heating at 80°C stainless steel plates of various shapes to be applied to the plates 2. Fig. 5 shows the surface temperatures (in °C) of a plate measured when a test piece P₁ made of the stainless steel plate l245 mm wide, 2400 mm long and l.5 mm thick and having no cut-away part was subjected to a heating test by the direct conduction heating based on grasp electrodes A, A. This heating test was carried out under the conditions of a current value of 4000 amperes and a total conduction time (conduction with pulses at 20 Hz) of 3 minutes, and produced the favorable results of the highest temperature of 88°C and the lowest temperature of 7l°C. Besides, Fig. 6, shows the surface temperatures (in °C) of a plate measured when a test piece P₂ made of the stainless steel plate, which was l245 mm wide, 3l55 mm long and l.5 mm thick and which had a cut-away part B 950 mm wide and l420 mm long centrally of the plate, was subjected to a heating test by the direct conduction heat­ ing based on grasp electrodes A, A. This heating test was carried out under the conditions of a current value of 3l00 amperes and a total conduction time (conduction with pulses at 20 Hz) of 3 minutes. Very intense actions of generating heat arose in both the sideward parts of the cut-away part B; and the highest temperature of 207°C and the lowest temperature of 27°C were measured. Shown in Fig. 7 are the surface temperatures (in °C) of the plate in the case of a heating test in which, in order to eliminate such a conspicuous deviation in the temperature distribution, electric conduction members C each being made of an aluminum alloy plate l.6 mm thick were interposed in the cut-away part B. This heating test was carried out under the conditions of a current value of 4l00 amperes and a total conduction time (conduction with pulses at 20 Hz). The deviation of the temperature distribution was sharply improved, and results close to the target heating temperature of 80°C mentioned above were obtained. In order to raise the temperatures still more, the conduction time or the current value may be set greater. In the embodiments, only the flat plates have been exemplified. However, if the skeleton exhibits a suffi­ cient strength and can satisfactorily endure a tensile residual stress, even a panel which is, per se, in the shape of a curved surface will be producible. On that occasion, desirably the grasp electrodes are formed into a shape which conforms to the curved surface. In a case where the tack welding is executed without detaching the grasp electrodes, the total time between the period of time taken for detaching the electrodes and the period of time taken for moving the electrodes is saved, and in a case where the finish welding is executed immediately after the heating, the time with the period of time for the tack welding added to the above total time can be saved. Thus, the heating temperature can of course be lowered in correspondence with the saved time. The present invention is constructed as described above. Thus, the first aspect of performance of the invention attains the effect that input energy can be readily adjusted in accordance with the material and dimensions of a plate and is very easily controlled with sensors. The second aspect of performance of the inven­ tion attains, besides the effect of the first aspect, the effect that a tensile residual stress for preventing the occurrence of a sag can be developed for a plate having a cut-away part, so the "sag" is conspicuously reduced. In addition, according to the third aspect of performance of the invention, resistance heating means for preheating a plate can be readily adapted to variations in the material, size and thickness of the plate and the geometries of a cut-away part by the use of grasp elec­ trodes and electric conduction members, to attain the excellent effects that the quality of a panel product is enhanced, that the limitation of the size of a panel in the prior-art methods and apparatuses preventive of a sag is not involved and that a temperature control can be performed very easily. All the aspects of performance are suited to flow processes and can render operations efficient.

Claims: 1. A method of preventing a sag of a panel, comprising the step of locating a plate on a skeleton to which said plate is to be attached, the step of gripping both end parts of said plate with grasp electrodes of resistance heating means, the step of causing current to flow through said plate across said grasp electrodes, thereby to preheat said plate to a predetermined temperature and to thermally expand it, the step of welding said plate to said skeleton, and the step of developing a tensile residual stress in said plate owing to a thermal shrinkage deformation attendant upon cooling of said plate after the welding. 2. A method of preventing a sag of a panel, comprising the step of locating a plate on a skeleton to which said plate is to be attached, the step of gripping both end parts of said plate with grasp electrodes of resistance heating means and interposing electric conduction members for short-circuiting a cut-away part of said plate, in said cut-away part so as to form electric conduction paths, the step of causing current to flow through said plate across said grasp electrodes, thereby to preheat said plate to a predetermined temperature and to thermally expand it, the step of welding said plate to said skeleton, and the step of developing a tensile residual stress in said plate owing to a thermal shrinkage deforma­ tion attendant upon cooling of said plate after the welding. 3. A method of preventing a sag of a panel as defined in Claim l or Claim 2, wherein a temperature control of the resistance heating is performed by the use of temperature sensors. 4. An apparatus for preventing a sag of a panel, comprising support means for supporting a skeleton, resistance heating means for preheating a plate, and weld means for welding said plate to said skeleton, said resistance heating means including grasp electrodes which detachably grip both end parts of said plate and which cause current to flow through said plate. 5. An apparatus for preventing a sag of a panel as defined in Claim 4, wherein said reistance heating means includes electric conduction members which are detachably interposed in a cut-away part of said plate and which short-circuit said cut-away part.

Inventors: KINOSHITA, TSUNEO, SAKAGUCHI, AKIRA
Applicants: KAWASAKI JUKOGYO KABUSHIKI KAISHA
Application year: 1986
Main classification: B23K31
IPC codes: B21D1, B23K11