# [eli5] how is the appropriate number of foot pounds originally determined when tightening a bolt?

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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

It means how many pounds of pressure are being pushed against it literally as if it were the ground and you had an object that is x pounds.

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).

It’s a function of the diameter, lubrication, and strength of the bolt material.

Once the metal plies are in contact any further tightening will put tension into the bolt (this is good). This tension makes it harder to further tighten the bolt, so you need to apply more torque to tighten it. The specified torque is to stretch the bolt enough that it loads up with enough tension to reach a defined value relative to the bolt material strength. This exact value varies based on the bolt material properties.

Lots of physics, lots of trial and error, and sometimes even a bit of common sense. Many of the guidelines that engineers use are determined “empirically”, meaning that they were literally tested to compliance instead of just derived from the physics – sometimes it’s just quicker/cheaper to build a scale model.

One of the primary purposes of a bolt is to hold two objects together. Once you hold them together tight enough, they don’t slide anymore and the bolt itself is only held in tension, no longer subjected to shear – this is desirable since most cheap bolts aren’t strong in shear force, but quite strong in tension.

We know from physics how much force a bolt can handle, so we know how much torque to apply before possibly causing the bolt to stretch and fail.

We also know from physics how much force is needed to prevent slipping, so once we’ve got that and the bolt “strength” we can either add more bolts till there’s enough to prevent slipping, or we go up a size in bolt diameter!

For highly specialised applications, we don’t generally like bolts, though, since torque alone isn’t a very reliable way to know if it’s good enough… Rivets, welds, assembly adhesives and interlocking shapes all work significantly better.