METHOD FOR LEVELLING THIN STRIPS BY STRETCHING AND BENDING

Abstract: The invention describes a method for roller levelling of strips with a maximum thickness of 4 mm by simultaneous stretching and bending by passing the strip (2) through a conventional roller levelling device with only two levelling rolls (1) but with a certain relation between the radiuses of the levelling rolls depending of the characteristics and thickness of the material in the strip (2).

Description: TITLE . Method for levelling thin strips by stretching and bending. FIELD The present invention concerns a method for le¬ velling strips, the thickness of which is maximum 4 mm, by simultaneous stretching and bending and by help of two levelling rolls only. BACKGROUND TECHNICS Known levelling devices, fig 1, for levelling strips with a small thickness and widths, which can be considerable, is constituted in its central part of a number of zigzag-placed rolls 1 between which the strip 2 is passing and where the surface of the strip 2 is bended in two opposite directions. On the same time the strip 2 is subjected to a stretching in order to move the elastic zone from the centre of the strip at the ben jng around the levelling rolls 1. The main principle for the level¬ ling of the strip 2 is to bring the material to exceed the yield point s across the whole of its cross section in such a way that partly eventual buckles and others are removed, partly the strip leaving the last levelling roll without beeing bent. Thus the strip is passing the level¬ ling device, fig 1, under a pull effected by a tractive force of the coiling reel 3 and the braking force of the decoiling reel 4. Breaking rolls 5 are arranged on both sides of the levelling roll assembly to compensate chang¬ es in the pulling force in the strip 2 at changing outer diameters of the coils at coiling and decoiling. To get a good result a constant pull is required in the strip 2. The stretching force in the strip usually is adjusted in that way that a specific pulling force of 20-40 % of the yield point is reached in the strip. The diameter of the levelling rolls usually is dimensioned according to the unlevelty of the strip before levelling and the thickness of the strip. Normally the roller diameter is about 100 times the thickness t of the strip, but can of course vary within wide limits. A varying roller diameter in the assembly of levelling rolls also can be found. The draw¬ backs of conventional roller levelling assemblies is among others that these, because of a great number of levelling rolls, are complicated to adjust to get level strips and rather expensive in manufacturing. Also the plastic area of the strip is utilised into a too high de¬ gree because of a great number of bendings, which reduces the area of finishing of the material before reaching the breaking point and also hardens considerably because of the cold working. THE INVENTION Surprisingly it has been found that strips can be levelled by only two levelling rolls if, besides of an appropiate pull in the strip and an appropiate dimension of the diameter of the breaking rolls, the mutual rela¬ tion between the roller diameters is adjusted. The inven¬ tion will be described in the claims enclosed, in the closer description below and by help of the enclosed drawings on which figure 1 schematically shows an ordi¬ nary straightening machine and figure 2 a tensile test diagram of the actual strip material. CLOSER DESCRIPTION OF THE INVENTION Initially pieces for tensile test are taken from the strip 2, which is to be levelled, tensile test is made and a diagram for stress and elongation is drawn up in a conventional way with the elongation £ as abscissa and the stress O as ordinate, fig 2, (see the Swedish general specification SS 11 21 10). A mainly straight up- ward slooping line extends from the origin of the coordi¬ nates, the elasticity line 7, up to the yield point 8, which after that turns into a more level line, the plas¬ ticity line 9.. The later line 9 whithout disadvantages can be approximated by a straight line. A radius R1 of the first levelling roll 1 is choosen, which because of earlier experiances from roller levelling devices shall be around or somewhat below 50 times the thickness t of the strip 2. A distance a is pointed out from the origin of coordinates along the abscissa, which distance is equal to the thickness t of the strip divided by the ra¬ dius R1 of the first roll, and from that point a first parallel line 10 is drawn up against the plasticity line 9, parallel to the elasticity line 7. By that a first surface 11 is constituted in the diagram, fig 2, between the abscissa, the elasticity line 7, the first parallel line 10 and the plasticity line 9. A new second parallel line 12 is drawn to the right of the first surface 11 in that way that a second surface 13 between the abscissa, the first and the second parallel line 10, 12 and the plastcity line 9 becomes equal to the first surface 11. The distance b along the abscissa between the first and the second parallel line 10, 12 is measured. The thick¬ ness t of the strip 2 is devided with the later distance b which constitutes the radius R2 of the second levelling roll 1 in the rollsr levelling device, fig 1. These two rolls are used in a roller levelling device, fig 1, men- tioned in the preamble, and the strip 2 is levelled in a usual manner by help of an appropriate pull. By that a level strip 2 is reached.

Claims: CLAIMS A method for levelling a thin strip (2) by stretching and bending in a roller levelling device (fig 1) constituted by a braked decoiling reel (4), a stress control device (5), unpowered rolls (1) and a powered coiling reel (3), c h a r a c t e r i z e d in that only two levelling rolls (1) are used and that the relation between their radiuses, R1 and R2, is determined by ma¬ king a tensile test according to the Swedish general spe- cification SS 11 21 10 of the material in the strip (2), a stress elongation diagram (fig 2) is drawn up with the elongation € as abscissa and the stress as ordinate, constituted by an elasticity line (7) starting from the origin of coordinates, followed by a plasticity line (9), that a distance a is set out from the origin of coordina¬ tes along the abscissa, which is equal to the thickness t of the strip (2) devided by the radius R1 of the first roll (1), and a firs parallel line (10) iε -irav.π paral¬ lel to the elasticity line (7) up to the plasticity line (9), that a second parallel line (12) is drawn in a cor¬ responding way on a distance b along the abscissa from the first parallel line (10) in that way that the two surfaces (11, 13) constituted between the abscissa and the plasticity line (9) are equal and the radius R2 of the second levelling roll is determined according to the formula R1 , b R2 a

Inventors: INGVARSSON LARS INGVAR, INGVARSSON, LARS, INGVAR
Applicants: ORTIC AB
Application year: 1990
Main classification: B21D1
IPC codes: B21D1