Left-hand threads
Why they're so screwy
Think, for a minute, about all the threads you've turned today. The caps on your toothpaste and mouthwash. Perhaps a jar lid or two on something yummy as you made breakfast or lunch. Replaced a lightbulb? Maybe you even picked up a screwdriver to tighten the screws in a loose door hinge. All of these items and countless others have threads. And every last one is known as a right-hand thread - they turn right to screw in and left to screw out.
Righty tightie; lefty loosie.
It's so ingrained into our behavior that we seldom think about it. This is simply the way it's done. That is, except when it comes to left-hand threads.
As one might expect, left-hand threads are the opposite of regular old right-hand threads. Left-hands, sometimes called "reverse threads," are righty loosie and lefty tightie. And operating one feels like the natural world order has been tilted upside down.
Imagine stepping off a short drop where you go up instead of down as you put your foot forward. Turning a left-hand thread feels that strange. When you have one you are trying to loosen that is tight, and you start turning it right to get it out - because you know it's a left-hand thread - there is always that moment of doubt. Doing this feels so wrong. I should be turning it the other way! And then, the screw breaks free and begins to come out. Still weird as heck, but at least it's working.
Left-hand threads exist not to perplex humanity but to keep other spinning things from coming loose. The most common example is the left side pedal on a bicycle - yep, left-hand threads. If they weren't lefties, the left pedal's forward rotation would potentially loosen it as the rider pedals.
Another one occasionally encountered is the locking screw inside a drill chuck.
The chuck itself threads on righty tightie. And drilling forward with the drill will not loosen it. But drills go forward and reverse, so there's a screw in there that is left-hand threaded to lock it all in place.
There are many other applications where left-hand screws function to keep other parts in place, all counter to their expected movement.
With the functional aspects of left-hand screws out of the way, there's still the intriguing psychology behind why they feel so weird.
I'm more of a professional screw-turner than a practicing head shrink, so I won't drill too deep into this one. But the "strangeness" of left-hand threads is that they go against our intuition - turning one and getting the opposite result conflicts with how we expect the physical world to work.
Human intuition is a thought process where knowledge appears in consciousness without deliberate thought. It's a product of experience and cumulative learning. We know what we know, and when life throws us curve balls, we question them.
A good example where intuition butts heads with reality is while watching a magic show. Seeing the magician pull off an impossible feat is unsettling - like when she draws a rabbit from a formerly empty hat. We intuitively know the performer can't actually materialize a living animal out of nothing. But it sure looks like she did. And it bothers (and entertains) us.
As children, we don't know the magician is pulling a trick - it looks real, so we believe such carnival magic is real. But as we grow, learn, and see how things are, we doubt events that conflict with what we understand to be possible. Here, we listen to our intuition, our truth-vetting faculty built on facts, and we question what is really going on.
The left-hand thread, while being very real and totally un-magical, bothers us much like the magic trick. After all, we learn early on in life to get lids off, we must turn them left. (How else would we get our little fingers into that jelly jar?) Intuition tells us to turn left to loosen and right to tighten, so instinctively that we seldom think about it when we pick up a jar or a screwdriver and go to work.
But when it works the opposite way, our brains object.
Indeed, those left-hand threads are strange. And fun, not unlike that magic show. If you have a chance to turn one, do it, if only to feel the odd sensation it is.
Left-hand threads are certainly screwy, in the best sort of way.
Until next time.
JRC
Oh yes, those lefty screws. You don't encounter them often, but when you do, they throw you for a little loop. When I read the title of your essay, I thought you were going to talk about left-handedness, not left-handed screws. So many tools in a shop are designed for right-handed people (consider tin snips, most scissors, and calipers).
After an accident in my teen years where I fractured my right elbow, the fracture went undetected for a month after my doctor put my arm in a splint with a tight bandage to hold it more or less motionless. It didn't work. The slight but painful movement caused new bone to grow on the joint, immobilizing my elbow permanently. After detecting the mistake, they had to surgically open the elbow, remove the undesired bone growth (basically break the elbow again and saw away), and let it heal it properly in a cast, followed by physical therapy for months to regain movement (the mouthful pronation/supination movements). Long story short, for a whole year and essentially forever, I had to become a lefty.
As you explained clearly, my brain, used to turn things left to open and right to close or tighten, had to relearn this lefty loosely, righty tightie all over again but flipped. The lefties out there may know what I'm talking about.
If you are right-handed, grab a screwdriver with your left hand and quickly start working on a screw. There will be a moment of pause where you consciously have to work against your intuition and think about how you should turn that screw to loosen or tighten it. Even today, after so many years, I still have to override my intuition when using a screwdriver. I am a lousy mechanic, and IKEA furniture assembly drives me a bit crazy. It takes a little while of conscious retraining to get all the screws going smoothly in the right direction. Worse, I've become ambidextrous over the years and use my left and right hands interchangeably. Handy when screws are in odd positions but crazy when there are many of them, and I switch from left to right hand often. I guess it is a form of brain exercise.
Thank you, John, for writing these essays that make me think and write.
Mnium: I really liked it. It was cool talking about magic and how it’s basically instinct to do righty-tighty lefty-loosy. And good job papa!