Why do loose screws only get looser and not the other way round, even if there seems to be no resistance to the latter? (continued)
Richard Brent
Tinamba, Australia
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Some cars and trucks have left-handed threads on their left-hand wheels. This is so that, when the brakes are applied suddenly and the wheels stop or slow their rotation, angular momentum will try to tighten rather than loosen the wheel nuts.
If you need to change a tyre on such a vehicle, you will have difficulty unless you remember to turn the wheel brace in the correct direction.
Stuart Jameson
Solihull, West Midlands, UK
Threads can be designed to self tighten, for example on bicycle pedals. The left-hand pedal has a left-handed thread so that the pedalling action turns the pedal in the tightening direction.
David Kernick
Exeter, Devon, UK
Your correspondence on why nuts tend to loosen has stimulated me to solve my long-standing and frustrating problem of why shoe laces untangle and don鈥檛 tighten.
A nut is held in place by the elastic expansion forces of the metal reacting against the friction of the helical thread. The elasticity can be felt in the final 鈥渂ite鈥� of torque when tightening a bolt. When the system vibrates, friction between bolt and thread is reduced and the bolt expands outwards. Once elastic contact has been lost with the metal, it is as likely to tighten as loosen. With shoe laces, the friction of the lace resists the elastic expansion of the opposing shoe eyelets, but unravels in a similar fashion.
Andrew Vevers
York, UK
I suffered from extremely tight wheel bolts on my Volkswagen Beetle and, like one of your previous correspondents, had to resort to a length of scaffold pipe to undo them. However, it wasn鈥檛 because the bolts had inexplicably tightened. It was the apprentice in the tyre shop who had turned the torque setting on his power wrench up to maximum.
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