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Why do loose screws only get looser and not the other way round, even if there seems to be no resistance to the latter?

Geoffrey Clark
Douglas, Isle of Man, UK

I can’t agree that screws always work loose. As a mechanic and general odd-job person, I can assure you that screws and bolts do become tighter – that is why penetrating oils, notablyWD-40, were developed.

I have damaged screw heads and bolts in efforts to release them and had to drill them out. I have had more problems with too-tight bolts and screws than I ever had with loose ones. I think it is a perception problem.

Neil Sutcliffe
Glossop, Derbyshire, UK

Screws don’t always get looser! Years ago I had a car with wheel bolts rather than nuts, which progressively tightened and were extraordinarily hard to remove. They required a length of scaffold pipe placed over the wheel wrench to give me more leverage – which caused the wrench to bend.

Rod Sykes
Boudrac, France

How would you know if screws that are still tight hadn’t simply loosened then retightened themselves?

Eric Kvaalen
Les Essarts-le-Roi, France

I question the premise of this question. A screw could sometimes be screwing in and sometimes be screwing out. How would you know? You would have to check its position at different times, and I doubt that the question-poser has done that.

I think the screw probably gets jostled around by vibrations and whatnot, and moves slightly each time, going one way or the other by chance. This is called a random walk. If a screw starts totally screwed in, then obviously when you come and see it some time later, it will be more screwed out.

Geoff Sharpe
Lazonby, Cumbria, UK

A screw or bolt requires more angular force and therefore more energy per degree of rotation the further it is screwed in. This is because there is more thread in contact and more friction. This applies to micro degrees of rotation, each requiring incrementally more force and energy than the last.

Vibration will rotate the screw out because this requires less angular force, and less energy, for each micro degree of rotation compared with rotating it in.

John Davies
Lancaster, UK

One word: entropy. To tighten the screw puts energy into the assembly, which is released to the environment as heat if it loosens.

Pat French
Telford, Shropshire, UK

You can think of a bolt as a spiral, like a helter-skelter. When you tighten the bolt, you are putting energy into the system; you are lifting the load higher and higher up the helter-skelter and increasing its potential energy.

You are also increasing the pressure with which the nut and bolt are forced together, so increasing the friction between them.

This friction holds back the potential energy, just as rubber-soled trainers can hold back someone on a helter-skelter. However, by forcing one face of the bolt thread against the nut, you are also widening the gap on the opposite face. Vibration can move the bolt in this space and release some of the potential energy, and the load slides a tiny way down the helter-skelter.

Once started, this repeats until there is no energy differential between the nut and the bolt, meaning the joint is loose. Even the vibration of a 50 cycle AC electric current can have this effect on the screws in plugs and wall sockets, hence the advice to have them checked occasionally.

You might think that, once the forces required for clockwise and anticlockwise rotation are equal, the direction should be as often one way as the other, but this isn’t always so. The very smallest of factors, such as the microscopic variations of the two threaded surfaces, or the nature of any contamination or lubricant present, could bias the rotation to go one way or the other.

The direction that these factors affect rotation will vary. It is all luck and entropy.

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