HOW DO YOU UNBEND A BAR?
What is it about a bent bar that we need to correct to make it straight? To answer this, we need to have in mind two concepts that apply to metals: stress and strain. Stress is the force carried by a specific cross sectional area of the metal, usually expressed in Pounds per Square Inch (for example, 40,000PSI). Strain is the amount the metal has been stretched, stated as a decimal proportion of its original length (a strain of .02 means that the metal has been stretched 2% of its original length).
The processing history of a metal bar can leave it with residual stresses and strains, even after external forces are removed. If we were able to examine the stresses on the cross section of a bent bar, we would find that they vary from place to place. The same would be true of the strains. Under the influence of non-uniform stress, the bar assumes a bent shape.
Metal strain rises and falls proportionally with the stress, within limits. This means that a bar will stretch under load, and then spring back to its original length when the load is removed. We call this elastic behavior. If stress is taken higher than a limit, called the yield stress, it springs back, but not to its original length - it becomes longer. This is known as plastic behavior.
To remove the bend in a bar, we must re-configure the pattern of residual stresses so that enough uniformity exists to hold the bar straight. If the bar is bent in one direction, we can re-configure the stress by flexing the opposite way-just enough so that it springs back straight. To counteract residual stresses permanently, we must bend the bar in the opposite way, enough to exceed the yield stress. If we repeatedly flex the bar, rotate it slightly and flex again, we will eventually produce a uniform stress pattern around the whole circumference of the bar. A two-roll straightener works just this way.
We use a concave shaped roll as an anvil to control the amount of flexure we place in the bar. A straight roll forces the bar to mold itself to the contour of the concave roll. The two rolls turn to make the bar rotate while it is flexed, so that the bending stresses are uniform around the circumference. By skewing the rolls to the bar slightly, we cause the bar to advance through the rolls - giving the bar a number of revolutions while flexed. Each successive revolution improves the residual stress uniformity, producing a straight bar.
In a two-roll straightener, the bar is:
FLEXED enough to counteract non-uniform stresses
SPUN so the residual stresses are uniform
ADVANCED to straighten from end to end.
October 2004 newsletter
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