Tightening bolts

A few weekends ago I embarked on some house maintenance, which basically entailed trying to fix things! As I was tightening up a set of bolts holding a chair together it crossed my mind – why do we tighten most screws by turning them in a clockwise direction?

Apparently it is to do with the rotation of our forearm. Scientific American estimates that 70-95% of the population are right-handed. Rotating the arm inwards, called supination, is a stronger movement then rotating it outwards (pronation). So for the right-handed group, it is easier to twist something in a clockwise direction.

If you imagine forming a “thumbs-up” gesture with your right hand, your thumb will point in the direction you wish the screw to move, whist your curled fingers show the direction you will need to twist it.

But, whilst the clockwise to tighten rule holds a lot of the time, there are occasions where screws are designed to be tightened in an anticlockwise direction. As a keen (and pretty average) cyclist I had never noticed that the left pedal on a bike is reverse threaded. A simple design modification but one which prevents it from becoming loose when used. Apparently fan parts and bolts on power saws join the anticlockwise list. Also present are some gas fittings; oxygen bolts tighten to the right but flammable gases are distinguished by their reverse threads, the left-handed fittings helping to guard against accident.

So whilst the saying “righty-tighty, lefty-loosey” will work most of the time, every now and then you might just need a “lefty-tighty, righty-loosey”.

However, what I really need is a new set of chairs that actually stay together…