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Frame stress and tie-off points

Q: Is there a way that the amount of pressure or stress on the frame can be measured? It seems that the stress must increase at the tie-off points. I am curious how much additional load this places on the racquet.

A: The easiest way to check the stress on the frame is to measure its dimensions before, during, and after stringing. A tape measure will get you pretty good information, although the USRSA uses a set of special oversized micrometers supplied by Dr. Carl Love, which are accurate to one-thousandth of an inch. This won’t tell you how much stress there is per se, but it will tell you if the stress is unbalanced enough to deform the hoop.

As for stress at the tie-off point, given the huge amount of overall stress to which the hoop is subjected, measuring frame deformation at one point would be difficult. If you think about it, though, you’ll see why the tie-off isn’t a problem.

Consider the case in which the string does not tie off, but continues to the other side of the racquet. In this case, that string will receive full tension, which will impart corresponding stress to the frame. The frame is designed to withstand this stress, of course, so there is no problem.

In the case where the string ties off, on the other hand, the only pull on the string before releasing the clamp is that applied manually during the knotting process, and that is (or should be) a tiny fraction of the reference tension. After the knot is tied and the clamp released, the added stress from the tied-off string is along the side of the hoop, not across the racquet face.

To look at it another way, before you could apply enough tension on the tie-off knot to deform the hoop, the anchor string would fail.

From the January 2008 issue
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