A bending roll is a rotating roll (pulley) on which the product to be straightened is plastically deformed during a partial or complete wrap. The important point is that the product should only be bent in the direction of its initial curvature. In other words: the initial curvature, the bend and the final curvature should all lie in one plane. A change of plane along this course of curvature and bending during the straightening process will cause helicities (twisting of the product to be straightened).
Subjecting the curvature to several changes of plane has a particularly negative effect. It leads not only to curvature fluctuations, which are very difficult to eliminate, but also to extremely disadvantageous right/left helicities.
The magnitude of plastic deformation suffered by the product to be straightened depends e.g. on its material parameters and on the diameter of the bending roll.
See Also
Adapted from “"We do it straight" — Wire Straightening”, p. 31 (ISBN 3-00-005897-4).