Understanding bending, moving material in roll forming-4

Rotational Forces

Figure 3
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Two rotational axes are through the center of a section (A) and at the end, or edge, of a section (B). Rotating the section through the center results in the least resistance and the least likelihood of deforming the part.

The axis at the beam's center has a smaller polar moment of inertia than the axis at the beam's bottom. This means the axis at the center has less rotational resistance than the axis at the bottom of the beam. Roll form designers can get better forming results by rotating a section about the center instead of the end.

Note: This illustrates one section, or leg, of a profile, not an entire profile.

Figure 3 illustrates a section rotated on two axes, center and bottom. The polar moment of inertia with an axis at the center of the beam (Figure 3A) is:²

The polar moment of inertia with an axis at the bottom of the beam (Figure 3B) is:

Preventing Longitudinal Elongation. Elongation in the longitudinal direction is another consideration regarding rotational movement. Longitudinal elongation is not an intentional part of the linear roll forming process, in this case rotational movement. Therefore, it is necessary to minimize the longitudinal elongation.