Why to loosen always by rotating it counterclockwise and to tighten by rotating it clockwise?

Question

For example:

• Screw / bolt
• Bottle design

Why this intuition works for most people? Does it have anything to do with the right-hand rule?

Answer 1

Assumptions to be tested:

1. It appears as if 90% of people are right-handed.
2. It's easier for people to use their dominant hand while doing tasks such as tightening a screw.
3. Our arms are constructed so as there is more power in a clockwise turn.
4. In a muscle powered age this difference would have been noticed very quickly.

So, at first blush it seems that the answer is "Yes." It has to do body mechanics and with the fact that the overwhelming percentage of people are right-handed.

I've been looking around and it seems as if points 3 and 4 discussed above would not have been important in classical times (Archimedes) as they used screws (with levers) to transport water, grain, sewage and to press olives as opposed to fastening pieces of wood.

The first lathe able to cut screws was fashioned in the 16th C by Jaques Besson and the first modern pression lathe was created in the 1790s by Henry Maudsley. In the 1840s Maudsley's apprentice Joseph Whitworth presented a paper advocating a series of standards including the angle of the threads, and number of threads per inch depending upon the diameter of the screw.

Along that note I did find this interesting tidbit about 19th C screw production:

Early screw manufacturing suffered from the absence of accurate and powerful machinery capable of holding minimally accurate tolerances. This was compounded by the lack of accurate inspection methods. For many years screws and nuts were manufactured and used in matched sets, and as a result were not interchangeable. http://www.roton.com/application_engineering.aspx

Nowhere have I found anyone directly stating WHY they chose a right-handed thread so we're back to the assumptions of body mechanics and the fact that the overwhelming percentage of the people are right-handed.

Answer 2

In terms of it being an intuition, I have taught this to a couple of children who were helping me with things including, recently, my own child. In both cases they needed to be taught, had no preconceptions, but soon got the idea and also readily generalised it. There was no underlying spooky intuition, but it was quickly understood.

So my, -- probably somewhat unfashionable -- answer is that we are taught it. As we do a great deal of tightening and loosening of things in the modern world, having a more general form is useful. The symmetry would probably break spontaneously even without any clockwise or anticlockwise biases. If things had fallen differently, we would all have been happy with doing it the other way around.

In terms of history and narrative, as other answers point out, it was standardised by Whitworth when he standardised the screw. It was widely believed at the time (and still is) that supination (turning your palm upwards) is stronger than pronation (turning your palm downwards), which leads to a clockwise motion in the right handed being stronger, and so to be used for the more physically demanding task of screwing in.

While that's the tiny mote that tipped the scales one way and not the other, the scales were ready for the tipping and that this difference is largely irrelevant in most cases of use is itself irrelevant: it broke the symmetry.

Answer 3

In my experience, it's just social norm. I work with many students in our labs who always turn cylinder valves and other knobs randomly. If they don't know from experience, they almost never have any confidence in the correct way, so that hints to me that it's something that's pervasively taught rather than intuitive.

Answer 4

It's convention, just as the thread size & pitch are conventions*. You do find reverse threads in some applications: turnbuckles, the left pedal on bicycles, some gas fittings, &c.

*In the same way that you have coarse & fine thread nuts & bolts in the US, and metric sizes that are compatible with neither. And if you've ever worked on older British sports cars, you'll know of the Whitworth standard, too: http://en.wikipedia.org/wiki/British_Standard_Whitworth Standards are great, everybody should have their own :-)

Answer 5

I see no point to assume it's based on intuition in any way. For that claim, I see several (possibly subjective?) reasons:

• Both loosening and tightening can be very hard and require a great deal of strength on some objects.
• Neither loosening nor tightening can be generally assumed to be the "default" or "more frequent" case:
  • Many objects need to be loosened just as often as they need to be tightened (anything that can be opened and closed),
  • some normally only need to be loosened (things that get opened, emptied, and thrown away), and
  • others normally only need to be tightened (screws etc. that are meant to last "forever").

This leads me to figure there is no point in assuming one direction is inherently more suited for human/right-handed/whatever specific manipulation.

Instead, what seems much more likely is that one particular convention is followed. One convention that is so omnipresent that we learn it early on, as soon as we learn how to open bottles or how to use a water-tap.

In fact, it may even be the case that the convention is so arbitrarily chosen that (as opposed to many other somewhat arbitrary conventions that come in many variations around the world, e.g. power plugs) no-one even bothers to deviate from the convention, as always adhering to the convention is much more beneficial - as it is, a plumber will directly know in what direction to hold their pipe wrench to open or close a valve, and likewise, in what direction to turn a valve in the first place if they want to open or close it, even if the actual effect is not directly visible.