The invention relates- to a method defined in the preamble of claim 1 and to an apparatus defined in the preamble of claim 3 for tacking intended for the assembly of a girder casing of a crane. Background of the invention

According to a known technique, a girder casing of a crane is welded during assembly by means of large clamp gantries running on separate rails. During assembly welding, the side plates and the cover and bottom plate of the girder casing are welded in place. The side plates are welded to the internal structures of the casing, and lastly, the side plates are welded to the cover and bottom plate. The clamp gantries press the side plates and the cover and bottom plate against the internal structures of the casing during the welding. In a similar manner, the clamp gantries press the side plates against the cover and bottom plate during the welding.

The problem with the prior art arrangement is the large clamp gantry that runs on its own rails and that is expensive and bound to one place. Small subcontractors cannot usually acquire such an apparatus either due to the space available or the price of the apparatus. Furthermore, it is not worthwhile to move the apparatus presently used for example closer to the place of assembly during the manufacture. Disclosure of the invention

The purpose of the present invention is to eliminate the aforementioned drawbacks and to provide a new and advantageous arrangement for the assembly welding of the crane girder casing. The method according to the invention is characterized by what is disclosed

in the characterizing portion of claim 1. Correspondingly, the apparatus according to the invention is characterized by what is disclosed in the characterizing portion of claim 3. The other embodiments of the invention are characterized by what is disclosed in the other claims.

The invention provides the advantage that the assembly welding of the girder can be performed with a small and inexpensive apparatus which can be transported closer to the place of assembly of the crane at a very low cost, or which can be given to the subcontractor to be used for a predetermined period. List of figures

In the following, the invention will be described in greater detail by means of an embodiment and with reference to the drawings, in which

Figure 1 is a front view of the clamping apparatus according to the invention, and

Figure 2 is a side view of the apparatus of Figure 1.

Detailed description of the invention

In order to illustrate the invention, an apparatus, called below a clamp gantry, implementing the method according to the invention will be described first. Figure 1 shows the clamp gantry in a side view with solid lines. A crane girder casing 16 to be assembled is depicted in turn from the end by broken lines. The casing is usually welded when it is upside down, wherefore in the figure the upper plate of the girder casing is called a bottom plate 17 and the lower plate is called a cover plate 18. The clamp gantry consists of two sheet-like elongated lever arms 5 that are interconnected from their upper ends by means of a hydraulic cylinder 1 that acts as a power means. Two attachment brackets 4 are fastened to the upper end of

each lever arm 5 in such a way that there remains a space between the attachment brackets for the fastening of the hydraulic cylinder, which is articulated from its piston to the attachment brackets of the first lever arm by means of a joint 2, and from the cylinder end to the attachment brackets of the second lever arm by means of a joint 3.

The lower end of each lever arm comprises, in the manner of the attachment brackets, two claw plates 14, the claw-like projecting part of which is turned inwards with respect to the clamp gantry, i.e. the claw plates of both lever arms are positioned opposite one another. A horizontal support plate is fastened over the projecting part and it joins together the claw plates of one lever arm. The support plates rest on the cover plate 18 of the girder casing to be welded. The claw plates of each lever arm are fastened to both sides of the lever arm by means of bolts. The elevation of the claw plates in the lever arms can be altered for the purpose of casings of different heights.

At the lower end of each lever arm, there is also a horizontal side press 15 that is attached to the inner edge of the lever arm by means of fixing screws 20 in such a way that the elevation of the side press can be altered. The side press consists of, for example, a rectangular hollow section and a fastening member connected thereto, the side press being tightened to the inner edge of the lever arm by means of the fastening member. The purpose of the side press is to press the side plates of the casing against the internal structures of the casing for the purpose of welding.

The frame of the clamp gantry is formed of a carriage 6 the body of which consists of two vertical frame plates 9 that have substantially the same length as the carriage, that are spaced from one another at a

distance of substantially the width of the carriage, and that are mutually parallel. The frame plates are interconnected by means of transverse support plates that are not shown in the drawings. Each frame plate also comprises two associated vertical lug plates 7 protruding sideways from the carriage. The lug plates on each side are horizontally spaced from one another at a distance corresponding approximately to the thickness of the lever arm 5, so that the lever arm 5 can be placed between the lug plates 7. The lug plates 7, as well as the lever arms 5, comprise an opening for a joint pin 8 by means of which the lever arms are articulated to the lug plates 7 of the carriage. This point of articulation is located closer to the upper end than the lower end of the lever arm in the longitudinal direction of the lever arm.

The carriage 6 is provided with four wheels 10 that are positioned in pairs on shafts 11 that are transverse with respect to the longitudinal direction of the girder casing to be welded. The shafts are in turn attached to coupling straps provided at each end of the frame plates 9. The diameter of the wheels and the locations of the shafts are selected in such a way that the carriage can run on its wheels on top of the bottom plate 17 of the girder casing to be welded.

The carriage also comprises a hydraulic cylinder 12 that acts as a pressing means and that is arranged to vertically press, by means of a press plate 13, the casing to be welded. The press plate 13 presses against the bottom plate 17 of the casing, the bottom plate in turn pressing against the internal structures of the casing. Simultaneously, the pressing means pulls the entire clamp gantry upwards, so that the cover plate 18 of the casing also presses against the internal structures of the casing as it tends to rise upwards.

Depending on the dimensioning, the cover and bottom plate press simultaneously against the edges of the side plates or, alternatively, there remains a desired opening for welding between the cover and bottom plate and the edges of the side plates. The carriage further comprises a control unit with levers 19 for controlling the hydraulic cylinders. The carriage may be either manually movable or provided with a separate small transfer mechanism. With the method according to the invention, the assembly welding of the girder is performed for example in the following manner: the casings are assembled when they are upside down, with the cover plate 18 being the lowermost. Firstly, the cover plate is positioned on a simple base having the radius of curvature of 2000 m. The base is curved since the side plates are cut to be curved due to the bending down caused by the weight of the casing and the load. The base comprises, transversely at suitable distances, square hollow sections that are slightly shorter than the width of the cover plate, so that the support plates provided in the claw plates 14 of the clamp gantry can move freely from one end of the casing to the other below the cover plate. Next, spacers having a width that is suitably smaller than the cover and bottom plate and a height that is slightly less than the height of the side plates are tack-welded to the inner surface of the cover plate transversely at calculated intervals. Similarly, connecting plates required by the end joint are tack- welded on each end of the casing.

The cover and bottom plate, which are also called boom sheets, comprise longitudinal backings against which the side plates, i.e. web plates, are pressed in order to obtain a straight edge. The side

plates are hoisted on both edges of the cover plate against the spacers, and the plates are locked to the spacers, in order to prevent the plates from falling, by means of screw fasteners that are provided for this purpose and that are later removed as the compression and tacking proceed.

The clamp gantry is now hoisted on the other end of the casing on the edges of the side plates by utilizing sheaves as the wheels 10 at this stage, so that the clamp gantry would remain on the edges. The seams between the cover plate and the side plates are first pressed tightly together and tack-welded from the inside of the casing. The pressing stage and the tacking are performed from one end of the casing to the other. Next, the side plates are pressed to the spacers. The side presses 15 are moved to the required height, which depends on the cavity that is situated between the spacer and the side plate and that is visible from the inside of the casing. The opening between the side plate and the spacer is closed by activating the hydraulic cylinder 1, whereupon the upper ends of the lever arms 5 are pushed apart by means of the hydraulic cylinder 1 in the transverse direction of the girder casing, so that the lower ends of the lever arms turn towards each other also in the transverse direction of the casing and the side presses simultaneously press the side plates against the internal structures of the casing. During the pressing, the spacers are tack-welded to the side plates. All parts in the spacers are treated in a similar manner.

Next, the clamp gantry is removed and the bottom plate 17 is hoisted on the casing blank. Traversing wheels 10 having a smaller diameter are changed to the clamp gantry since the height of the casing is now greater by the thickness provided by the

bottom plate. The suitable diameters of the traversing wheels help avoid lost motion in the movement of the hydraulic cylinder 1. Furthermore, the wheels do not have to be grooved then. The clamp is hoisted on the bottom plate, the side presses are lifted to about 50 mm from the upper edge of the side plates, and the pressure of the hydraulic cylinder 1 is decreased from an adjustable valve so that the casing would not be compressed with too great a force resulting from the altered lengths of the lever arms. The seams between the bottom plate and the side plates are pressed together and tack-welded from the outside, beginning from one ehd of the casing and continuing through the entire casing. The casing is then ready for the final assembly welding. The clamp gantry comprises a fixed frame and a hydraulic unit. Casings of different sizes may be manufactured by changing the size and shape of the lever arms.

It is clear for a person skilled in the art that the different embodiments of the invention are not restricted to what is described above by way of example, but they may vary within the scope of the appended claims. Thus, for example other kinds of power means may be used instead of the hydraulic cylinders 1 and 12, and the working order during welding may vary depending on the case.