A Scientific American bolt puzzle

Next post Previous post A week or so ago, Scientific American republished this Martin Gardner puzzle from 1958: Two identical bolts are placed together so that their helical grooves intermesh (see illustration). If you move the bolts around each other as you would twiddle your thumbs, holding each bolt firmly by the head so that it does not rotate, will the heads move inward, move outward or remain the same distance from each other? The problem should, of course, be solved without resorting to an actual test. The “twiddle your thumbs” description is very good. because a key feature of the bolts’ movement is that even though they orbit one another, they don’t rotate. The vertical faces on their heads, for example, remain vertical throughout the motion. Also important is that the bolts are identical, otherwise their threads wouldn’t mesh. Here’s a drawing from ANSI Standard ASME B1.1, Unified Inch Screw Threads, which shows the standard thread profile: If the pitch, P , the major di ... Read full article.