Why do screws have a 'righty tighty, lefty loosy' convention?

[pi-r8]

"righty tighty, lefty loosy"

Has anyone else heard that expression? It's supposed to be a way to remember which way to screw/unscrew things. It means that you turn the top of the screw to the right to tighten it, and you turn the top to the left to loosen it.

What I'm wondering is, is this just an arbitrary convention that all screwmakers have decided on, or is there a physics principle behind it? I've never, ever, found something that screws in the other way, and it seems suspiciously like a vector cross product, which would seem to indicate that it's a physics principle. On the other hand, it also seems like all the threads in a screw simply go that direction, so I would think that one could reverse it simply by reversing the threads. Can anyone explain this to me?

 

[Janus]

Has anyone else heard that expression? It's supposed to be a way to remember which way to screw/unscrew things. It means that you turn the top of the screw to the right to tighten it, and you turn the top to the left to loosen it.

What I'm wondering is, is this just an arbitrary convention that all screwmakers have decided on, or is there a physics principle behind it? I've never, ever, found something that screws in the other way, and it seems suspiciously like a vector cross product, which would seem to indicate that it's a physics principle. On the other hand, it also seems like all the threads in a screw simply go that direction, so I would think that one could reverse it simply by reversing the threads. Can anyone explain this to me?

Generally, it is convention. I have run across reverse threaded bolts and nuts. They are used on devices where the normal action of the device would tend to loosen the nut or bolt. Such an bolt that holds a pulley onto a shaft that rotates in a clockwise direction. Reverse threading the bolt means that the torque it puts on the bolt will tend to tighen it, not loosen it.

 

[Pengwuino]

Yes as Janus said, it is simple convention. I've never run into them, but i know there are screws that tighten counter clockwise for applications that the use of the object will loosen the screws through its own torque.

 

[brewnog]

An interesting question!

I suppose it's fairly arbitrary. Left-hand threads are often used in rotary devices where the rotation could otherwise cause loosening of the part, or in things like gas cylinders (RH thread for butane, LH for propane) to avoid use of the wrong regulator, but yes, why is a 'typical' thread right handed?

One of my heroes, Joseph Whitworth was the first to standardise screw threads, but whether or not there was convention in terms of handedness before this I don't know.

One thing which crossed my mind was whether, since most people are right handed, it's easier to tighten a screw in the clockwise direction, but intuition seems to say that the opposite is true!

I'll be interested to see whether anyone can shed some more light on this.

 

[Pengwuino]

Maybe it has something to do with the very first manufactoring processes?

 

[quark]

There are both left hand and right hand threads in existence and use. An easier way to remember how to screw and unscrew a RH thread is by using a thumb rule. Fold your RH fingers with a pointing thumb. Direction of rotation of four fingers show the direction you use and the thumb points of in the direction of bolt linear movement.

 

[Pengwuino]

There are both left hand and right hand threads in existence and use. An easier way to remember how to screw and unscrew a RH thread is by using a thumb rule. Fold your RH fingers with a pointing thumb. Direction of rotation of four fingers show the direction you use and the thumb points of in the direction of bolt linear movement.

Yah but we're tryen to figure out why they outnumber counterclockwise-tightening by so many...

 

[Jelfish]

I've never come across a non-conventional (right hand) screw before. And quark - I use that right hand rule too! You're the first person I've known that uses that method as well. I always found the righty-tighty rule kind of ambiguous.

 

[Zorodius]

To me, at least, tightening a screw with my right hand feels more natural than loosening it. Most people are right handed, and screwing something in is more frequently a critical operation than unscrewing it, so it would make sense that screws would normally be threaded to take advantage of this more natural-feeling motion.

Let's not prematurely reject the possibility that the threading direction was chosen by random chance, either.

 

[Pengwuino]

Maybe it feels natural because you've always done it that way before :).

Maybe 1 company was the only company to make threaded screws for a long time and they just always went clockwise-twist. Who knows... we need modern marvels in this discussion.

 

[Danger]

Threads go way the hell back before any machining processes. Wooden screws were used in things like wine-presses a couple of thousand years ago. After all, it's just a modified inclined-plane. My best guess as to the handedness of it would be that originally, holes were bored in (whatever) with awls or such-like and to a right-handed person the power stroke of that would be a right-twist push. It would follow that when screw threads were first devised (probably as a modification of a nail), the threaded receptacle would have been created the same way. The screw itself would therefore be built to fit it.
I know that in the early days of automobiles, the wheel nuts on the left side were left-hand threaded to prevent their loosening by the wheel rotation. Even in my teens, lawn mowers were set up the same way.
Notice how astutely I am refraining from bringing up the Legend of Joe MacLoch...

 

[brewnog]

Notice how astutely I am refraining from bringing up the Legend of Joe MacLoch...

Urrrm, yet again, something I've obviously missed out on!

Tell me the legend, uncle Danger, tell me!

 

[FredGarvin]

I've never, ever, found something that screws in the other way...

If you have a desk fan or floor fan in your house, I'd suggest a quick peek at the nut holding the fan to the motor shaft. More than not, I have noticed them held on with a L/H nut.

Like Brew said, we use L/H threaded nuts to maintain clamp groups on shafts depending on rotation. Also, in the US the compressed gas cylinder fittings (CGA fittings) have some L/H threads.

 

[pi-r8]

Interesting. I had no idea they made L/H threated screws and nuts. I did as FredGarvin suggested and opened up my fan, and just like he said, it was a L/H nut. All the other screws in the fan were the regular kind though.

At any rate, from what Danger was saying, it sounds as though the R/H screw thing is sort of an endless cycle. The wine press makers thousands of years ago needed screws that were easy to screw in by hand, so they used R/H screws. Then when they started making screws by maching, they already had all these R/H screw holes that they needed screws for. Then they started making R/H screw holes to fit all these new R/H screws... and the cycle repeats.

 

[pi-r8]

Oh, and to Quard and Jellyfish- I've been taught that "fold your fingers, point your thumb" in almost every one of my physics/calculus classes as an easy way to remember the vector cross product rule- you point your fingers (on your right hand) in the direction of the first vector, curl them towards the second vector, and your thumb will point in the direction of the product. That's why I said that screws reminded me of vector cross products.

 

[Integral]

There are both left hand and right hand threads in existence and use. An easier way to remember how to screw and unscrew a RH thread is by using a thumb rule. Fold your RH fingers with a pointing thumb. Direction of rotation of four fingers show the direction you use and the thumb points of in the direction of bolt linear movement.

This is the way I do it also.

 

[DaveC426913]

Righties are better at tightening screws and jars. Lefties are better at loosening them.


Lefties unscrew thing that righties SCREW UP!

 

[TOKAMAK]

Another example of left-handed threads can be seen in scythes (the thing that death carries around). Since it is swung from right to left, the grips on the handle would have a tendency to loosen if a right-handed thread were used.

Screws remind me of cross-products as well

 

[Dr Transport]

Left hand threads are also used on explosive gas cylinders. The way you can tell is that a left handed nuts will be scored, I learned that many years ago when I was learning how to weld.

 

[wolram]

How about a tie rod with a rose joint at each end, one is l/h the other r/h
to change the length of the tie you simply have to slacken the lock nuts
and turn the rod, if both ends were r/h you would have to remove the rod.

 

[Hybird]

Lefthand thread is used on brake layths where the layth turns clockwise. This is so the machines rotation and vibrations do not interfere with the nut on the end. They are also used on some semi-uck wheel lugs, for the same reason. As well as inside transmissions on the "driven" gear shaft. I suppose It is entirely convention. Just as which way an engine rotates is convention.

 

[quark]

I think Wolram is speaking about a turn buckle, a classic conjunction of RH and LH threads.

 

[brewnog]

How about a tie rod with a rose joint at each end, one is l/h the other r/h
to change the length of the tie you simply have to slacken the lock nuts
and turn the rod, if both ends were r/h you would have to remove the rod.

Good one!

 

[Galileo]

I use the 'right-hand rule' all the time as well. I also have encountered left-handed devices like a watertap which opens if you turn it clockwise. There is one on a pressurized heliumtank in my lab which I found out the hard way. We use it for external pressure to fill a cryostat with liquid helium. Once that thing was nearly full I had to stop the cryostat from overflowing and close off pipe to the tank, so I quickly ran over to the tap and turned it clockwise to close it only to fully open the tap .

 

[Danger]

from what Danger was saying, it sounds as though the R/H screw thing is sort of an endless cycle. The wine press makers thousands of years ago needed screws that were easy to screw in by hand, so they used R/H screws. Then when they started making screws by maching, they already had all these R/H screw holes that they needed screws for. Then they started making R/H screw holes to fit all these new R/H screws... and the cycle repeats.

I should have clarified in the first post that I wasn't actually linking the two types of threads historically. The early wooden ones were huge (by modern standards) and usually turned with a wheel attached to one end. They were active components of the machines rather than fastening devices. I don't have any idea what handedness they were, or even if it would make any difference. It might have just been up to the whim of whoever built it, although it might have been more natural to do it one way or the other on a lathe. The bit about forming the receptacles was intended in reference to the smaller metallic fastener types.