analogy of CW/CCW threads (R&L)

[A.K. Boomer]

Wondering if anyone knows how R&L threads keep things from loosening.

[MikeL46]

Left hand threads keep things from loosening because everybody tries to tighten them to get them off.

You can easily demonstrate this effect with the keyless chuck on your drill. As you turn forward the chuck tightens its grip on the bit. When in reverse the chuck loosens and the bit spins free.

Mike

[TGTool]

They probably work rather like Congress. Whatever direction the one wants to go, the other absolutely refuses, so things are fixed right where they are.

[Toolguy]

Post #3 is a classic explanation which must be preserved for the edification of future generations.

[A.K. Boomer]

Left hand threads keep things from loosening because everybody tries to tighten them to get them off.
Mike

hah never thought about using human error/habit to achieve positive results lol


Seriously - all different kinds of applications for them so differences in design will apply - but the centrifuge thread (no pun intended) got me thinking

I use R&L hand threads all the time with my bicycle pedals, yet they are somewhat counter intuitive in the way they work, in fact at first thought they are opposite of what you would expect, the right pedal is right hand thread and left is left - this means that any spindle drag from pedal bearings would be a driving force to immediately loosen up the pedal,

Yet this past summer I took out a $3,600 Mt. bike on a demo,,, all's I had to do was show up with my own pedals to be bolted onto the cranks, show up - meet the sales rep. - real nice guy - he's busy lining everyone up, gets to me and just takes my pedals and hand threads them into the crank - someone asks him a question and it seems like he's distracted and missed final tightening them with a wrench,
he's onto the next bike, I quickly remind him that he did not tighten my pedals, he says they will be just fine as they will tighten up while I pedal, I said "really?" you just let them go like that? he says yeah no worries they will self tighten as you pedal ----- while I was very uncomfortable with this I said uh - ok,
Did about a 1 1/2 to 2 hour ride - no problem - get back and he has to put a wrench on them to get them back off - and use a fair amount of torque to boot.

having had a pedal come loose as a kid I was amazed and also have been cranking down extra hard to tighten on my pedals ever since,

this opened my eyes to the mystery that was going on - at least with the bicycle pedal there's only one force that could be getting this done - not only overriding the spindle bearing drag that's actually trying to loosen the pedal but have such a surplus of torsion that it actually makes the pedal tight enough to not only have to use a wrench to get it off - but apply a fair amount of torque to do so, in fact the harder you pedal and the more force your capable of applying the more the pedal will tighten

this tightening phenomenon comes in the form of "oscillation" and the very "drive mechanism" is between the inboard and outboard threads - for one is getting loaded on the downforce (the inboard) whilst the other is applying an upward thrust into the crank threads, and due to the threads angle of pitch it simply "walks" the spindle arm into the crank during rotation oscillation,,, it's really pretty amazing when you think about it, The left hand size with the left hand thread is mirror opposite and does the exact same thing,,,

again this application might be different than a keyed gear on a shaft or whatever, but it does provide the details needed to understand the forces at work here - and why at first it seems opposite of what should be happening, till you dig a little deeper...


there's only one real mystery to me left --- how come my pedal came loose as a kid? as old as the bike was L&R threads have been a standard for several decades and this bike was no different.

the reason being must have been poor maintenance with my pedal bearings - perhaps even a slight seizure, there's really no other explanation that i have for that anomaly...

[enl]

If you use the opposite threading, the pedals will loosen. The action is due to the male part rolling in the female (similar to Tony's Hilch tube followup video) and the rolling is in the opposite direction relative to the female that frictional drag would turn it. If there is enough frictional drag to loosen even an only hand snug pedal, then you will not like riding that bad boy.

In something like a sprocket, the threading is set to tighten directly under load.

For a centrifuge, I would guess that the threads are chosen to snug during spin up, being driven by friction between the nut and the carrier, rather than during spin down. If there is a hard brake, if might be set the other way, but in that case I would suspect that a keyed or splined drive would be used with a locking retainer, rather than a nut.

[EddyCurr]

... having had a pedal come loose as a kid ...

I seem to remember a similar experience.

I'll offer that not all bikes were created with direction-appropriate threads on the cranks/pedals - very likely I and perhaps you both had bikes fitted with RH threads on both sides ...

.

[A.K. Boomer]

I seem to remember a similar experience.

I'll offer that not all bikes were created with direction-appropriate threads on the cranks/pedals - very likely I and perhaps you both had bikes fitted with RH threads on both sides ...

.

Eddy that may very well be so - in fact I do remember kind of a transitional stage or at least one for really cheap bikes where you did have to make sure you had the right pedals in fact they were possibly not only different thread direction (R&R) but a lot smaller thread ID also... too far back to be sure of myself but helps to hear someone kinda verify, could be the pedal just plain worked itself loose due to it being a right hand thread on a left hand pedal.

[wierdscience]

This reminds me of a buddy that built his own circular sawmill.He had collected various bits and pieces of various different mills and put them together.His crucial mistake was running a left hand threaded saw mandrel in a left handed setup.Meaning the first log through the mill backed the thread off mid-way in the cut,not good.
He was too far into it to back up and rebuild everything,so he asked me to build him a new mandrel with right hand threads.This worked well and he had no more problems.

[Arcane]

...........having had a pedal come loose as a kid I was amazed and also have been cranking down extra hard to tighten on my pedals ever since..........

I had a pedal break off the crankarm once. I was a young man and bought (cheap) an old single speed fat tired bike and rather than walking it up the footpath over the RR tracks one day I tried to ride it up. Halfway up it failed and to this day I consider myself supremely fortunate that I didn't nut myself.

[wdtom]

Twenty years ago I worked part time in a bike shop for 10 or so years. I can never remember not seeing right and left threads on pedals. Cheap one piece steel cranks have 1/2" threads, but still L&R, most bikes have 9/16" threads, the tpi is not common either. Possibly steel cottered cranks have 1/2" threads, I don't remember. Just try and buy a tap for the 9/16". They were available from bike tool suppliers, but a couple of years ago I looked in MSC and couldn't find them. Maybe a right, but not the left. I was making an adapter to put clipless pedals on a rowing machine exerciser. I finally cut the thread in the lathe and welded the threaded part onto the bracket/adapter I made. Saved money this way anyway but I wouldn't have minded owning the taps as I am into mountain bikes.

[kc5ezc]

9/16x20 according to Jet.com who has a pair of taps for a little less than $50 USD.

[Edwin Dirnbeck]

THIS ACTUALLY HAPPENED
A friend of mine had an old hot rod pickup truck with a heavy big block engine. He decided to cobble together a power steering system.Evertthing was a tight squeeze, He finally took a test drive, It worked perfect BUT when he turned left the truck went right.Edwin Dirnbeck