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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.
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.
|
Text from
straight-to-medart.com,
October 2004 newsletter |
www.acegrinding.com
www.centerlessgrinding.com
mschram@acegrinding.com
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