According to the known art, described in US patent No. 3,784,081 which has already expired, use is made of a plant comprising a forming machine which bends a trapezoidal metal sheet until it pre-forms a frustoconical pipe in which a longitudinal slit is left open at the top, said slit being intended to be subsequently welded while the pipe is fed continuously in the plant, at a speed of about 18 metres per minute, gripped between two calibrating rollers having a profile shaped in a manner complementing that of the pipe so as to embrace the pipe and keep the free edges thereof correctly positioned during welding.

The active profile of the calibrating rollers therefore has the same frustoconical progression as the pipe which is to be calibrated pnor to welding.

The pre-formed pipe is inserted with its smaller cross-section between the rollers, rotation of which causes feeding and simultaneous calibration of the profiled part, which is transformed into a welded frustoconical pipe in a welding station located immediately downstream of the calibration zone and intended to perform fusion of the edges of the pipe.

Drawing means are also provided for feeding the pipe during welding, as well as pipe pushing means consisting of a motor-driven carriage which is movable on rails and pushes the pipe towards the calibrating rollers.

Pipe guiding means ensure that the pipe is kept correctly aligned and positioned

during feeding in the welding station.

The caiibrating rollers rotate in a horizontal plane about their vertical axis. One of the two rollers is movable away from or towards the other one so as to be adapted to pipes of different dimensions and so as to adjust the pressure of the rollers on the pipe.

The profile of the calibrating rollers is such that it reproduces the conicity of the pipe and, therefore, viewing the calibrating roller from above, has a spiral-shaped progression.

The diameter D of the calibrating rollers Is such that the measurement of the circumference, equal to zD, is equivalent to the maximum length L of the pipe which can be produced.

The feed speed of the pipe between the calibrating rollers Is perfectly equal to the tangential speed of the calibrating rollers in the zone of tangency with the pipe.

The pipe, during the welding phase, keeps the upper profile constant, so as to be positioned in a horizontal plane, while the axis of the pipe obviously is inclined with respect to a horizontal plane.

A first drawback of the plant described above consists in the considerable dimensions. In fact, in order to produce pipes of about 15-20 m length, calibrating rollers each with a diameter of about 4.5 - 6.5 m are necessary.

Moreover, since the extension of the profiles of the rollers Imitates exactly the conicity of the pipe, whenever it is required to manufacture pipes of different conicity the profiles of the rollers must be replaced, with a considerable loss of time.

Therefore the abovementioned system has notable limitations in terms of flexibility and high costs owing to the range of profiles or dies required and the

stoppage times of the plant when there is a variation in the conicity.

A further drawback conslsts in the need for precise synchronism of the feed speed of the pipe and rotation of the rollers in order to avoid intense friction between pipe and rollers.

An object of the present invention is that of eliminating the abovementioned drawbacks and providing a method and a plant for the production of frustoconical pipes, as a result of which smaller dimensions of the plant are possible and the production of pipes of varying conicity using the same profile of the rollers can be achieved.

A further object is that of obtaining savings in terms of time and hence greater productivity owing to reduced stoppage times of the plant.

Said objects are fully achieved by the method and the plant which form the subject of the present invention and are characterized by the contents of the claims indicated below.

According to the method. the teed speed of the pipe between the calibrating rollers is different from, In particular greater than, the tangential speed of said calibrating rollers.

The central axis of the pipe is kept at a constant level during feeding of the pipe, whereas the height at which welding Is performed varies.

The plant comprises two calibrating rollers having an extension of the active profile which is less than the length of the pipe to be formed and welded.

The ratio between the extension of the active profile of the calibrating rollers and the length of the pipe to be formed and welded is variable between 0.05 and 0.5, and preferably between 0. 13 and 0.4.

These and other characteristic features will emerge more clearly from the

following description of two preferred embodiments illustrated, purely by way of a non-limiting example, in the accompanying plates of drawings, in which: Figures 1, 2 and 3 show the present plant respectively in a side view, plan view and section along the axis A-A of Figure 2; Figures 4, 5 and 6 show a variation of embodiment of the plant respectively in a side view, plan view and section along the axis B-B of Figure 5.

With reference to the Figures, 1 denotes a metal profiled part pre-formed in the shape of a frustoconical pipe by a bending press of the known type and not illustrated, situated upstream of the plant according to the present invention.

The pipe 1 has at the top a longitudinal slit which must be welded so as to provide the final pipe or pole.

The pipe 1 is supplied to a welding station 2, only schematically shown and of the known type, by means of a movable carriage 3 provided with wheels 5 and travelling on rails 4.

The movable camage 3 carnes a die 6 which supports the bottom part of the pipe I and is shaped like the pipe, I.e. has the same conicity as the pipe so that the central axis of the pipe, indicated by 7 in Figure 1, is at a constant level during feeding of the pipe and is preferably horizontal.

In this way the centre of the pipe remains stationary, while the working height of the welding station varies.

The pipe 1 which is loaded onto the support die 6, oriented with its bottom section towards the welding station, reaches a pair of vertical-axis calibrating rollers 8 rotating in a horizontal plane.

Said calibrating rollers 8 have an active profile interacting with the substantially spiral-shaped pipe.

Said profile, illustrated in Figure 3, in the situation relating to the bottom or initial section of the pipe (left-hand side in the Figure) and bigger or end section of the pipe (right-hand side in the Figure), extends over about 90" for each calibrating roller, so that the profiles 9 of the two calibrating rollers together with the bottom profile of the die allow all the sections of the pipe, from the smallest to the largest, to be obtained.

According to a variation of embodiment illustrated in Figures 4 to 6, the active profile of each calibrating roller extends over about 1800, embracing half of the pipe which does not rest on a support mould, but is supported only at its ends, as illustrated in Figure 4, by support carriages 10 and I 1.

Unlike the known art, in which zD-L or the maximum length of the pipe is equal to the extension of the active profile of the calibrating rollers, in the present plant, in an original manner, zD<L. Preferably TcD is comprised between 0.05 L and 0.5 L and advantageously between 0.13 L and 0.4 L, so that, for example. with calibrating rollers of 0.7 m diameter, operations mav be performed on poles of length between about 16 m and 5.5 m, respectively.

For the same conicity, and with the variation in the length of the pipe the ratio between the feed speed of the pipe and the tangential speed of the calibrating roller is constant.

With the variation in the conicity, said ratio changes proportionally to the ratio between the length of the pipe and the extension part of the profile of the calibrating rollers interacting with the pipe (which in the case of "short" pipes is less than nod).

With the variation in the conicity, the speed ratios between the pipe and the calibrating roller vary so as to be able to use the same calibrating roller for operations on pipes of varying length L and varying conicity. When the reduction in the length of the pipe is accompanied by an increase in the conicity, there is an increase in the speed of the calibrating rollers.

Since it is always the case that zD<L, the feed speed of the pipe between the calibrating rollers Is greater than the peripheral speed of the rollers themselves and is adjusted so as to reduce the friction and ensure that the profiles of the rollers (if necessary completed by the profile of the base die 6) offer a sequence of sections corresponding to those of the pipe to be produced.

In the example of embodiment illustrated n Figures 1 to 3, the movable carriage 3 advances towards the calibrating rollers, moved by a drive unit 12 which causes rotation of a pinion meshing on a rack mounted underneath the movable carriage In the example of embodiment illustrated in Figures 4 to 6, feeding of the pipe Is achieved by a drive unit which pushes the carriage 10 or pulls the carriage 11.

According to present method, in an original manner feeding of the pipe 1 is performed at a controlled speed far greater than the peripheral speed of the active profiles of the calibrating rollers.

The ratio between the feed speed of the pipe and the peripheral speed of the profiles of the calibrating rollers is a function of the conicity of the pipe for the same calibrating rollers used and for the same pipe length.

Immediately after calibration, welding of the edges of the pipe and finallv so- called de-beading are performed, said de-beading operation consisting in the mechanical removal of the excess material arising during the previous welding phase.

Since the centre of the pipe is kept constant, the position of the upper profile of

the pipe is continuously detected so as to effect subsequentiy the variation in level of the welding system which must be adapted to the variable level of the pipe and hence the welding zone.

Also the working heights of the de-beading means are varied.

After de-beading, the pipe is unloaded and the movable carriage or carriages return into the starting position for the loading of another profiled part to be calibrated and welded.

A plurality of electronic regulators and actuators of a substantially known type and not illustrated ensure perfect circularity in the closing section of the pipe performed by the calibrating rollers.

With the process according to the present invention it is possible to use only one <BR> <BR> <BR> <BR> pair of calibrating rollers for different conicities (for example from 1 °/o 10% to 2° O O and in particular 1%, 1.05%, 1.1%, 1.15%, 1.2%, 1.1%, 1.15%, 1.2%, 1.25°o ,1.250%, 1.25°o ,1.250%, 1.3%, 1.35% 1.4%, 1.45%, 1.50%, 1.55°/ F 1.6so, 1.6so, 1 1.65so, 1.65so, 1.7° , 1.75so, 1.75so, 1.8°,/o, 1.859it, 1.859it, 1.9°/o,1.9%, 1.9°/o,1.9%, 1.95%, 9so). 9so).

In the case of feeding of the pipe onto the die 6 (Figures 1, 2 and 3), said die need only be replaced when the conicity must be changed whereas in the case where the pipe Is supported only in the front end zone and in the terminal zone (Figures A, 5 and 6), no element need be replaced.

The application to the calibrating rollers of a peripheral speed which is different from (lower than) the feed speed of the pipe makes it possible to use calibrating rollers of small diameter and to adapt them to pipes of varying conicitv and varying lengths.