## Tuesday, June 15, 2021

### 270: Which Way To Turn

Welcome to Math Mutation, the podcast where we discuss fun, interesting, or weird corners of mathematics that you would not have heard in school.   Recording from our new headquarters in Wichita, Kansas, this is Erik Seligman, your host.

Apologies for the long gap since the last episode— as you just heard, we have been relocating Math Mutation headquarters across the country.   You can probably guess that this involves lots of details to handle.   We should soon be back on our almost-regular monthly schedule.

Anyway, with all the moving stuff going on, I recalled a topic I had been considering a while back.   It’s a pretty simple geometry question, yet one with a major (if subtle) effect on all our lives:  why are screws generally right-handed?    If you’ve ever had to screw something together, you probably remember the saying “lefty loosey righty tighty’, which reflects the common design of these basic and universal tools.   Screws are typically considered one of the basic six “simple machines”, along with inclined planes, levers, pulleys, wedges, and wheels.   So why are they always designed to rotate in one direction?

With a quick web search, it seems almost unanimous that this convention simply derives from the typical handedness of humans:  if you’re right-handed, then a right-handed screw is easiest to screw in.    It’s just as easy to manufacture screws in either direction, but when they first became standardized (with the Whitworth design in 1841), a single design became ubiquitious.   I guess my poor left-handed daughter will forever be a victim of this society-wide conspiracy.

But the more surprising fact I discovered when researching this topic is that left-handed screw threads do exist, and are used for a variety of specialized applications.    Perhaps the most obvious is for situations where the natural rotation of an object would tend to loosen a right-handed screw:  for example, the left-side pedals on a bicycle, certain lug nuts on the left side of cars, or connections that secure other machine parts that are rotating the wrong way.      There are also cases where it’s useful to couple a left-handed and a right-handed connection, in a pipe fitting for example, so rotation in a single direction helps connect at both ends.    A less obvious usage is for safety:   in many applications involving flammable gas lines, the connections for the gas line use left-handed rather than right-handed threads, so nobody connects the wrong pipe by accident.

On the other hand, a few of the uses described just seemed kind of silly, though I suppose they were for valid reasons.    In the early 20th century, many lamps used in subways were specially designed to have bulbs that screw in with left-handed threads, so nobody could steal them and use them at home.   It’s probably a sign of our society’s growing prosperity over the last century that most people don’t steal public light bulbs anymore.    More bafflingly, I found a few references to the use of left-hand threads in early ballpoint pens, to provide a “secret method” of disassembly.   I guess business meetings were a lot less boring back then— given the amount of idle fiddling I’ve typically done with my pens on a normal day at work, I can’t imagine that secret lasting very long.

And this has been your math mutation for today.

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