Anyone who has worked on mechanical equipment knows there is a specific number of foot pounds required to appropriately tighten a bolt. Is this amount determined by physics, trial and error or ?
In: Engineering
So, bolts are usually made from metal, generally steel. Steel is stretchy and elastic, and that’s a good thing because when the steel is slightly pulled out of shape, it wants to go back into shape like a stretched rubber band. And just like a rubber band, that stretchy force can be used to hold things in place.
But steel gets deformed if you stretch it too far. You can bend a paper clip a little and it’ll snap back, but bend it much further and it holds the new shape. This is bad.
We know a lot about steel. We know exactly how stretchy it is, at exactly what point that elastic stretchiness turns into permanent bending. We know how much friction it will have against another piece of steel. There are many types of steel and we have this information on all of them.
Given that information, there are equations where you plug in a bunch of numbers (the carbon content, thickness, diameter, thread size, etc etc etc of the parts used) and it will tell you the ideal amount of torque.
For critical applications this will then be tested destructively (tighten until the steel bends – note that as too much) and non-destructively (tighten the prescribed amount, stress-test the joint, loosen and re-tighten and stress-test again).
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