# Wire sag calculation...Catenary?

by Machinist1
Tags: calculationcatenary, wire
 Sci Advisor HW Helper PF Gold P: 6,049 Wire sag calculation...Catenary? Hey, that Millwright website sag calculator was based on info I had provided in response to a question posted some time ago on these forums! It was based on the parabolic approximation to the catenary curve, which is pretty darn accurate when sag to span ratios are small. Anyway, the two charts are just about identical. For .016 inch diameter steel cable strung to 30 pounds in a 30 foot span, the sag is , per the Millwright site, .031 inches. And per Mr. Simpson's spaceage site, the sag is .002551 feet, which is also .031 inches to 3 sig figures. I think you forgot to convert feet to inches. When sag-span ratios are less than about 10 percent, the parabolic approximation $sag = wL^2/8T_H$ is very good to 3 significant figures, where L is the horizontal span in feet, w is the weight of the cable in pounds per foot, $T_H$ is the horizontal tension at the lowpoint of the cable, in pounds, and the sag is in feet. Otherwise, use the spaceage site.