TECHNICAL FIELD

5 The present invention relates to a device for interconnecting tubes, comprising a first and a second tube and an inner tube disposed in the interconnecting region, the inner tube being secured in the one tube and on which the other tube is slidable.

1 0 BACKGROUND ART

In the manufacture of spiral stairs, a central column is employed which consists of tubes and which is normally divided up into sections with interconnections between the different sections included in the central

1 5 column. As far as possible, this central column should have a smooth outer surface, since the staircase steps are screwed in place via anchorage plates which abut against the outer surface of the central column. Furthermore, it is crucial that the central column is straight despite the division into sections.

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Those loadings which the central column (or the entire spiral staircase) must be capable of withstanding are primarily loadings from normal use, i.e. essentially axial pressure loadings towards the central column. For safety reasons, the central column must also be capable of withstanding such

25 axial upward draught as might, for example, be caused by the shock wave from an explosion on a lower storey level. Thus, the staircase must mot fail even in the event of such reverse loading.

Spiral staircases of the type under consideration here are often used 30 outdoors or in an industrial environment which may be corrosive. For this

4. reason, the components of the spiral staircase are surface treated, often by hot galvanizing.

In prior art devices for interconnecting central columns, holes are made in 35 the circumferential surface of the outer tube, whereafter the inner tube is welded in place through these holes.

Since the tubes employed in the central column do not possess particularly good tolerances, there is a risk that the above-described welding operation results in the centre axes of the outer and the inner tube not coinciding with one another. Thus, problems relating to precision have commonly occurred in this prior art technique.

The welding of the inner tube in the above-described manner has also entailed demands for considerable after-treatment if the outer surface of the outer tube is to be smooth. Furthermore, it has not proved possible to employ tubes hot galvanized beforehand, but it has been necessary instead to carry out the cutting, welding and finishing work before performing the hot galvanizing. This entails requirements on costly transport to and from the hot galvanizing plant.

PROBLEM STRUCTURE

The present invention has for its object to realize a device of the type mentioned by way of introduction, the device being designed in such a manner as to obviate the drawbacks inherent in prior art techniques. In particular, the present invention has for its object to realize a device which offers an automatic and accurate centering between the inner and outer tube, irrespective of defects in surface quality thereof, and irrespective of defects in their tolerances. The present invention also has for its object to realize a device which permits interconnecting of previously surface- treated, principally hot galvanized, tubes without damaging the surface treatment. Finally, the present invention also has for its object to realize a device which gives an extremely durable and economical interconnection.

SOLUTION

The objects forming the basis of the present invention will be attained if the device intimated by way of introduction is characterized in that the inner tube has a number of embossed portions which force against the inner surface of the one tube.

According to one preferred embodiment of the present invention, the embossed portions are suitably also disposed with uniform distribution in the circumferential direction.

This embodiment is suitably also characterized in that the embossed portions are disposed in rows parallel with the longitudinal axis of the inner tube.

These characterizing features afford an interconnection which automatically aligns the two tubes in relation to one another so that the centre axes of the tubes will coincide.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

The present invention will now be described in greater detail hereinbelow, with particular reference to the accompanying Drawings. In th e accompanying Drawings:

Fig. 1 shows a section of such tubes, for example a central column for a spiral staircase, as are to be interconnected according to the invention; and

Fig. 2 shows an axial and diametric cross section through an interconnection region.

DESCRIPTION OF PREFERRED EMBODIMENT

Fig. 1 shows a section from a central column for a spiral staircase, and it is apparent that this central column is divided into sections in which the left- hand section in the Figure is shown with its upper end, while the right-hand section in the Figure is shown with its lower end. The sections comprise a first and second outer tube 1 and 2, respectively, and further an inner tube 3 which is located in the interconnection region between the sections and which, prior to the interconnection, is inserted a certain distance in the first outer tube. The tolerances between the inner tube 3 and the outer pipe 2 are such that the inner tube may substantially closely be slid into the outer

tube 2. Furthermore, the inner tube is permanently secured in the outer tube 1.

For interconnecting the two sections, the inner tube 3 has threaded through- holes 6 and the outer tube 2 has through-holes 7 in which screws are placed.

Fig.2 shows an axial diametric section through the interconnection region of the outer tube 1 and the inner tube 3. It will be apparent from the Drawing that the inner tube 3 is inserted in a portion of the outer a distance which at least amounts to the diameter of the inner tube, but preferably more.

The inner tube 3 is provided with a number of embossed portions 4 which have been realized by pressure means having been applied against the inner surface of the inner tube and pressed hard against the surface. As a result of this pressing operation, impressions have been formed on the inner surface of the inner tube, which, on the outside of the inner tube, correspond to bulges 5. The bulges 5 engage hard against the inner surface in the outer tube 1 and may possibly have caused minor depressions therein. However, the grip between the outer and the inner tubes is substantially caused via friction.

It will be apparent from Fig. 1 that the abutment between the inner and the outer tubes in the left-hand side of the Figure is effected more or less via a surface contact, while the abutment in the right-hand side of the Figure is realized substantially by means of the bulges 5. This entails that the play which existed between the tubes has been "gathered up" in the region of the bulges.

According to the present invention, several embossed portions are employed, which have regular distribution in the circumferential direction. Fig. 2 shows one embodiment in which three bulges are disposed with 120° distribution in a common diametric plane to the tubes, it will also be apparent that two such groups with bulges are disposed in axial spaced- apart relationship and that the bulges in both groups are in lines which are parallel with the centre axis of the two tubes.

In the production of the bulges, it is crucial that at least the bulges in the same diametric plane to the tubes are created simultaneously. Hereby, accurate centering will be ensured of the tubes so that the risk is avoided that the inner tube rests eccentrically in relation to the outer tube. In practice however, the pressing-out of all six bulges 5 takes place simultaneously.

The embossed portions 4 are achieved in that pin-shaped, ball-shaped or other suitable pressure means are applied against the inner surface of the inner tube 3 and are forced radially outwards. This pressing operation is carried out without any stop, dolly or other configurational stabilization means being employed against the outer surface of the outer tube 1. The pressing operation is executed with such force that the bulges 5 on the inner tube 3 create a certain elastic deformation in the outer tube so that this can, to some extent, become "triangular" if a distribution of 120° is employed between the embossings. However, this deformation mostly recedes when the pressure means are removed so that the outer surface of the outer tube remains unaffected. In such a case, surface or linear abutment between the tube surfaces will take place between the corners in the "triangle shape" while, on the other hand, the clearance or play which existed between the tubes prior to production of the embossings will be gathered up at these portions.

The above-described interconnection technique implies that extremely great forces which are caused by elastic deformations in the tube material - primarily the outer tube material - realize such a high degree of friction between the tubes that these can be considered as permanently interconnected. Possibly, a rigid engagement may, to some extent, also be effected between the bulges 5 and corresponding depressions in the inner surface of the outer tube 1.

DESCRIPTION OF ALTERNATIVE EMBODIMENTS

In the foregoing, it has been presupposed that the embossed portions 4 are six in number and are disposed in threes with a distribution of 120° in the diameter plane. Naturally, four or five, or possibly more embossed portions can also be employed in each diameter plane, and the embossings may

possibly also be disposed in more diameter planes than the two disclosed above.

According to the present invention, it may also be possible to employ a different number of bulges 5 in different diameter planes, and also to turn one group of bulges 5 in one diameter plane in relation to another group.

In the foregoing, it has been presupposed that the tubes 1 , 2 and 3 are cut from tubes which have been previously given their surface treatment, preferably hot galvanizing. There is no risk for damage to the hot galvanization employing the above-described interconnection technique. However, it is fully possible to realize the interconnection between the tubes first and thereafter carry out surface treatment of the tubes.

Embodiments are also possibly conceivable in which the bulges 5 are produced at the wall of the inner tube 3 before this is inserted with its one end portion into the outer tube 1. However, in such an event the mounting operation would require a considerable insertion driving force for the inner tube.

The present invention may be modified further without departing from the spirit and scope of the appended Claims.