FIELD OF THE INVENTION The present invention relates to the production of helically-wound lock-seam tubing, and more specifically concerns a method and an apparatus for such production with a reduced need of lubrication. BACKGROUND ART Known machines for producing helically-wound lock- seam tubing are disclosed by, for example, British Patent Publication GB 2,213,748 and US Patent No. 4,706,481. These machines are well-known to the person skilled in the art and thus need not to be described in any detail here. Several further developments of such machines are avail¬ able on the market, but these machines still suffer from certain drawbacks, as will be discussed in the following. The metal strip used for the production in question is normally coated with zinc in order to be able to with- stand corrosion. The metal strip is thus galvanised, which leads to problems that are well-known to the skilled per¬ son.

When the metal strip is edge-formed in the roller assembly preceding the forming head, small fragments of zinc are released from the metal strip and adhere to the edge-forming rollers. Owing to these zinc fragments, the diameter of the edge-forming rollers increases, especially in the last pair of rollers. Since these rollers are not resiliently journalled, the clamping force in the nip bet- ween two rollers is considerably increased as a result of the build-up of zinc on the rollers. In severe cases, these undesirable forces are so significant that the rollers are broken, which causes stoppages of production. Another problem is that zinc fragments are released as a result of the sliding of the metal strip between the guiding plates before the forming head, which leads to a build-up of zinc on the opposite faces of these guiding

plates. If the build-up of zinc on the guiding plates is too considerable, the feeding of the metal strip between the guiding plates can be disturbed.

Another disadvantage is that the zinc coating on the metal strip is softened and adheres to the inside of the forming head owing to significant frictional forces and heat development in the forming head. This renders more difficult the forming of the tubing in the forming head. Hitherto, the problems related to the released zinc fragments have been solved by supplying lubricant, normal¬ ly oil, to the metal strip on different places in the ma¬ chine. For example, such lubrication is discussed in US Patent No. 2,136,943. Lubrication of metal strips is also discussed in US Patent No. 3,750,439 and the Russian Pub- Iication No. 835,558.

In most machines available on today's market, the oil is, however, supplied to the metal strip in a very simple manner. Traditionally, the lower roller of the first pair of rollers in the edge-forming assembly is permanently i - mersed in oil in a trough, by means of which oil is sup¬ plied over the total width of the metal strip. However, this arrangement suffers from several drawbacks, as will be discussed below.

As the roller immersed in oil in the oil trough rotates, oil is spread to the upper surface of the metal strip, which later will form the inside of the tube. The finished tubes are thus covered with oil on both the in¬ side and the outside, which has unfavourable consequences. If the tubes are used in ventilation duct systems, dust and other particles accumulate on the inside of the tube, which in turn becomes the breeding-ground of bac¬ teria, virus, mould and the like, which may be dangerous to people's health. Further, accumulated dirt on the inside of the tube causes reduction of the tube inner cross-section with the consequence that the air flow rates and ventilation efficiencies for which the tube was originally designed are no longer upheld.

Similarly, dust and other particles accumulate on the outside of the tubes, making them unpleasant to handle for the personnel. Further, the tubes have to be cleaned on the outside if they are to be painted. During transportation, tubes of smaller diameter are often inserted in tubes of greater diameter in order to save space. Oil on the outside of the smaller tubes is then transferred to the inside of the bigger tubes.

A special disadvantage is that an excess of oil in the lock seam may cause the tube to "spin" or slide during transportation and handling. As a result of such a spin, the tube diameter increases and consequently the sealing is impaired and standard components cannot be fitted on the tube. In recent years, the demand for oil-free, helically- wound lock-seam tubing has increased, and the finding of a solution to the above-mentioned problems is therefore of great interest. SUMMARY OF THE INVENTION It is therefore an object of the present invention to remedy the above-mentioned drawbacks by providing a method and an apparatus for producing helically-wound lock-seam tubing using a minimum of lubricants.

A further object of the invention is to provide the production of helically-wound lock-seam tubes which are pleasant to handle for the personnel.

Another object of the invention is to provide a method and an apparatus which are easily adapted to existing machines. The tubing produced in accordance with the invention may, for example, be used in ventilation duct systems.

These objects are achieved by a method as set forth in the appended independent claim 1, preferred variants thereof being defined in subclaims 2-4. The above objects are also achieved by an apparatus as set forth in the appended independent claim 5, pre¬ ferred embodiments thereof being defined in sublcaims 6-11.

Practical tests of the invention using oil as lub¬ ricant have yielded excellent results. Compared with con¬ ventional production in accordance with the above descrip¬ tion of the background art, the oil on the outside of the finished tube has been reduced by at least 25%, and the inside of the tube is almost free from oil. Since no oil trough is used and much less oil is spread, the surround¬ ings of the machine are much more pleasant for the opera¬ tor. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail with reference to the accompanying drawings showing pre¬ ferred embodiments of the invention. In the drawings,

Fig. 1 schematically illustrates an embodiment of the apparatus according to the invention,

Fig. 2 shows a cross-section of the metal strip before the edge-forming assembly,

Fig. 3 is a view similar to Fig. 2 but shows another embodiment of the lubricant-supplying means, and Fig. 4 shows a cross-section of the metal strip after the edge-forming assembly. DESCRIPTION OF PREFERRED EMBODIMENTS

As shown in Fig. 1, drive rollers 11 feed a metal strip 12 (arrow A) through an edge-forming assembly having rolls 13, by which longitudinal edge portions 16a, 16b are formed. The metal strip 12 is formed and helically wound in a forming head 14 in a known manner for producing a tube 15. The forming head 14 has clinching rollers 17-19 for forming a lock seam 25 on the tube 15 fed out of the forming head 14 (arrow B). A pair of guiding plates 20 are arranged between the drive rollers 11 and the forming head 14, as is conventional.

A conduit 30 for supplying a lubricant, for example oil, to the metal strip 12 is arranged under the edge- forming assembly 13. The conduit 30 is connected to an oil supply 31. Optionally, a similar oil conduit 32 and an oil supply 33 may be arranged after the drive rollers 11, as will be discussed below.

As can be seen in Fig. 2, the oil conduit 30 is dis¬ posed at the edge portion 16a, which in the edge-forming assembly 13 is given the shape of a Z (see Fig. 4). The oil conduit 30 has a supply opening 34, which is either applied to the strip 12 or disposed at a short distance therefrom.

In the embodiment shown in Fig. 3, the supply opening

34 of the oil conduit 30 is disposed at a greater distance from the metal strip 12, but there is a brush member 35 provided at the end of the conduit 30. The brush member 35 is in permanent engagement with the metal strip 12 and compensates for any vertical movements of the strip 12 in production.

Fig. 4 shows the Z-shaped edge portion 16a and the L-shaped edge portion 16b, which are formed in the edge- forming assembly.

Since oil is supplied locally and only on that side of the strip 12 which forms the outside of the tube 15, the finished tube 15 is almost free from oil on the in- side. As is clearly understood from Figs 2 and 3, oil is applied to the strip 12 over only a very limited portion of the width of the strip 12. No oil is spread to the upper side of the strip 12.

In order to ensure that no zinc fragment build-up takes place on the inside of the guiding plates 20, a corresponding oil supply assembly 32, 33 may optionally be provided after the drive rollers 11. The oil conduit 32 has an opening 35 disposed in a corresponding opening

35 in the lower guiding plate 20. In order to further reduce the amount of oil on the strip 12, scraper means 40-43 can be provided before, in connection with and after the forming head 14. In prac¬ tice, the scraper 40 comprises rubber lips (not shown) gripping around the edge portions 16a, 16b of the strip 12. Preferably, the scraper 41 on the forming head 14 also has rubber lips (not shown), whereas the scraper 42 arranged in an opening (not shown) of the forming head 14

has means (for example of paper) for absorbing oil. The scraper 43 arranged on the outside of the tube 15 can be of any suitable type, preferably a rubber lip member.

Finally, it should be mentioned that the invention is by no means restricted to the embodiments described here¬ in, but that several modifications are conceivable within the inventive concept and the scope of the appended claims. For example, the apparatus can be provided with further scraper and wiper means for the removal of oil. Further, the brush member can be of any other suitable design.