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How Do You Make a Round Steel Bar Straighter?

To answer this, we need to have in mind two concepts that apply to metals: stress and strain. Stress is the internal force exerted by either of two adjacent parts of a body upon the other across an imagined plane of separation. Strain is any forced change in the dimensions of a body.

Metal strain in a body will rise and fall proportionally with the stress within limits. As a result the body will deform under load and then spring back to its original shape when the load is removed. This is referred to as elastic behavior. If the body is deformed so that the stress is higher than a limit called the yield strength, it will still spring back, but not to its original shape. This is referred to as plastic behavior.

To improve the straightness of metal bar then, we must increase the stress level of some portion of its cross section above the yield strength by applying a sufficient load. When the bar springs back after the load is removed, it will have improved straightness provided that the proper load is applied and sufficient plastic behavior has occurred.

In the case of a two roll straightener applied to a round metal bar, stresses above the yield strength can be induced by compressing the bar between the concave and straight rolls, by the action of the concave roll forcing the bar over the straight roll, and by the guides both above and below the rolls keeping the bar on a horizontal centerline. The higher the yield strength of the material and the smaller the cross section, the more curvature is required on the concave roll to induce plastic behavior by the action of roll bending a bar. For most applications, it is desirable to achieve plastic behavior through the bend action of the rolls.

Generally you will attain consistent straightness improvement in round bars with a two roll straighener provided that the processes used to manufacture the bar are under control and your material quality does not vary. This of course assumes that your straightener set ups are consistent for each individual bar product straightened, and that your straightener is properly maintained.





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