A method of forming ribs in bimetallic tubes and a ribbed tube formed by the method

Abstract

 © A method of forming ribs on bimetallic tubes, in which initial compressive stress is introduced in the external tube (2), the roughness of the ends of the internal tubes (1) is increased, and after placing the internal tube (1) in the external tube (2), the ends of the external tube are clamped on segments (3, 4) of the internal tube (1) of increased roughness with simultaneous axial displacement of a part of the material of the external tube of a quantity adequate to introduce compressive stresses of a required quantity. In the tube thus prepared ribs are formed. It is preferable if at least one end of the external tube (2) is formed conically during clamping.

Description

[0001] This invention relates to a method of forming ribs in bimetallic-tubes, in which the rib is pressed from the outer tube into which compressive pre-stress is previously introduced.

[0002] It is known that ribs may be formed in tubes by the action of rotary tools comprising profiled rollers. From the Polish Patent Specification, Application No. P-241323, a method of pressing ribs on tubes is known, in which before placing the tube in the zone of the action of the rollers initial compressive stress is introduced into the tube. In this way relief is achieved during the operation of the tool from axial loads bending out the roll-formed disks practically from the first stage of the working contact of the tool with the tube. For the purpose of inducing compressive stresses in the tube in the Application No. P-241323, a screw-clamp is provided.

[0003] In the case of pressing ribs in bimetallic tubes such a solution is practically useless because it is difficult to meet the requirement of accurately setting both tubes in relation to each other with simultaneous application of a compressive force on the external tube.

[0004] According to the invention, there is provided a method of forming ribs in bimetallic tubes in which, by the action of roller tools, ribs are pressed from the external tube and initial compressive stress is applied to the external tube, roughness of the external surface of the internal tube is increased along two segments, each of length P in the range from 0.8d to 3d, where d denotes the outer diameter of the internal tube, and the said segments are spaced from each other by a distance L - 2P, where L denotes the length of the external tube. Next, the internal tube is placed in the external tube so that the ends of the external tube are in line with the outer ends of the segments of the internal tube of increased roughness. Next, the ends of the external tube are clamped - over a length of from 0.5 to 1.5 of the length P of the segment of the internal tube of increased roughness - on the internal tube with a simultaneous axial displacement of the deformed volume of the material of the external tube in the direction of the centre of the tube. The axial deformation has the quantity corresponding to introducing to the material of the external tube compressive stresses of the required quantity. Next, the merged tubes are placed in the zone of the action of tools and ribs are pressed on the external tube. It is preferable to form at least one end of the external tube in a conical shape during clamping.

[0005] The method according to the invention enables durable connection of the external tube with the internal tube in an accurately established position of the ends of the tubes with introduction to the external tube of the necessary compressive stresses. An advantage of the method is also that merged tubes do not displace in relation to each other during rolling, and the use of instruments for the maintenance of initial stresses in the tube is eliminated to reduce labour costs of operations in the production of ribbed bimetallic tubes. Moreover, production of ribbed bimetallic tubes by this method enables considerable material savings to be achieved in relation to so-called free rolling.

[0006] The subject of the invention will be now explained in more detail with reference to the single Figure of the. drawing illustrating a merged bimetallic tube with introduced initial compressive stress, prepared for forming ribs.

[0007] A bimetallic tube consists 'of an internal tube 1 and an external tube 2. At distances from the ends of the internal tube 1 determined by design considerations are provided segments 3 and 4 of increased roughness of the surface of the tube. The increase of roughness can be effected, for example, by knurling, grooving or thread- cutting. The length P of the segment of increased roughness is from 0.8 to 3 of the magnitude of the diameter d of the internal tube. At least one of the deformed ends of the external tube 2 is deformed so that a part of the deformed end of the tube is displaced in the axial direction, towards the centre A of the external tube 2. The quantity of the axially displaced material of the external tube 2 corresponds to the quantity of the deformation induced by introducing compressive stresses necessary for drawing the ribs.

Claims

1. A method of forming ribs on bimetallic tubes (1,2), in which ribs are formed mechanically in the external tube (2), and initial compressive stress is introduced in the external tube (2), characterised in that the roughness of the outer surface of the internal tube (2) is increased along two segments (3,4), each of length P from 0.8d to 3d, where d is the outer diameter of the internal tube (1), and the said segments are spaced from each other by a distance L - 2P, where L denotes the length of the external tube (2), the internal tube (1) being placed in the external tube (2) so that the ends of the external tube (2) are substantially coplanar with the outer ends of segments of the internal tube (1) of increased roughness, and then the ends of the external tube (2) are clamped onto the internal tube (1) along a length of from 0.5 to 1.5 of the length P of the segment of the internal tube of increased roughness with axial displacement of a part of the material of the external tube in the direction of the centre (A) of the tubes, the axial deformation having a value sufficient for introducing in the material of the external tube (2) sufficient compressive stress, and the merged tubes are subjected to a forming operation to provide the external tube with ribs.

2. A method according to claim 1, characterised in that at least one end of the external tube (2) is conically formed during clamping on the internal tube (1).

3. A method according to claim 1 or 2 characterised in that said forming operation is carried out with rollers.

4. A ribbed bimetallic tube produced by the method of any one of the preceding claims.

Drawing