Hi John it is obvious that if you used two RH threads on cycle crank ends one of the pedals would keep coming unscrewed. As you pedal and if the correct way installed with LH and RH threads they will tend to tighten as you pedal.

David

I don't know if it is the case with all modern cranks but I agree with David on this, when the pedals were the screwed in type they came marked LH and RH on both the pedals and cranks on ones I changed out.

I recently re-did the preload on my mill's spindle bearings and I have a written note on the mill saying LH threads and an arrow to the top nuts. I remember the first time I tackled the job I was puzzled over my inability to get the top locknut loose. There was no visible thread to give a clue so when it finally dawned on me that it was LH I put the note on to make sure I didn't go through the same head scratching in the future. Once again this makes sense from the point of is it tightened in normal use? With the potential for intermittent cuts causing vibration it seems to me that the importance of the nuts tightening in use rather than loosening is quite high.

Martin C

Before the advent of spigot-located roadwheels on large goods vehicles, the securing nuts on the left hand side of these vehicles were always left hand thread, but now with spigots they are all right hand. We were always told that the reason for the LH threads was to stop the tendency for the nuts to unscrew as the wheels rotated, but this no longer seems valid with spigot wheels. Initially with these wheels there was a rash of cases of wheels coming loose and sometimes falling off, but this was overcome by improved materials for wheel studs and nuts and the now extensive use of torque-limiting wrenches to tighten the wheel securing nuts to the correct tension.

Corrosion will also add to the force required to remove pedals. Pedals are a sort of "fit & forget" component.

The only way to explain that book is the book itself. Good luck. You are trying to understand it in the classical way, like the first half. Try the Romantic way. Its all about how the book makes you feel.

Even Pirsig struggled to define Quality as he meant it. Something to do with subject and object becoming one. Maybe. EG when you become "totally involved" in machining a piece and forget about the surrounding world. Your hands are operatiing the lathe control handles, your mind is absorbed in the cutting of the metal by the tool. So you are probably doing Quality every day, just didnt know it.

Edited By Hopper on 14/03/2021 10:35:57

re that bearing retaining nut – why are the spanner flats on them rounded off so they are impossible to grip?

Left Hand threads are relatively more difficult to make, so are only used where really needed, but do have their uses.

Left Hand threads are also used in conjunction with right hand threads in such places as scissor jacks for cars, and turnbuckles to adjust the length of straining wires or links..

In other places, (Such as pulleys in vacuum cleaners, or one end of a bench grinder, as on bicycles ) they are used to prevent the item unscrewing during normal use.

The nut that retains the pulley on top of the quill on my Mill/Drill uses a Left Hand Thread.

Howard

And the little yellow plastic pointer tell tales.

Hi, or better still, use anti-seizing compound, as grease can dry out over time and go quite hard.

Regards Nick.

Either that or you don't understand it. Bit like the Romantic with the left hand thread that SOD mentioned.

Lots of older vans and large cars had LH threads on the UK nearside. Rolls Royce are brass, and easily stripped with an air gun, as favoured by tyre fitters. The general explanation is that rotational panting of a steel wheel centre applied a differential torque to the conical seat of the nut, which would unscrew a RH thread on the nearside; the larger the wheel, the greater the potential problem. Now we have spigot mountings with flat faced nuts, so no need for LH threads.

I'm a bit surprised that this thread hasn't wandered off into abstruse explanations of the forces involved when a pedal unscrews itself, but I'm sure that our ancestors, being in the main classical thinkers, simply observed which bits dropped off and reversed the threads accordingly.

I've read Zen and the … several times over the last forty-odd years, and always get something new and worthwhile out of it. It must be time to read it again. A friend who had studied philosophy at university dismissed it as lightweight, homespun philosophy, but I find it a lot more accessible than the classic philosophers. A bit like finding my way into jazz through listening to Chris Barber, when my serious (and scornful) jazz friend was into Charlie Parker and Thelonius Monk.

My 'hack' bike has a fixed wheel (not literally – it's the opposite of a freewheel) and the sprocket is screwed to the hub with a RH thread. There is then a lock ring on a slightly smaller LH thread which prevents the sprocket from unscrewing when I press back against the pedals to slow down. No abstruse physics there, but I do have a mangled hub from when I forgot that the lock ring comes off in the opposite dirtection.

George B.