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
A bolt works like a spring. When you tighten the bolt you are stretching the bolt, like a spring, which then pulls the plates together. The more you tighten the bolt the more it stretches and the more it pulls the plates together. If you tighten too much the bolt will yield and permanently become too long.
As for how big of a bolt is needed that depends on what you are fastening together. If the bolts are just to prevent the two plates from separating you need to calculate how much force might be pulling them apart and how much the bolt can flex. You need to be weary of cyclic loads though, if the bolt tightens and relaxes over and over it will suffer metal fatigue and snap. For example in a car when you turn a corner you are stretching the wheel bolts or studs on the inside wheels and compressing them on the outside wheel. If they are already tightened then this does not matter much but if they are lose they end up stretching and relaxing over and over until they snap.
Normally you do not want much forces through the bolt. The bolt keeps two plates together creating lots of friction between these which can handle huge forces. Again wheels are a good example, the forces of the acceleration and braking does not go through the bolts or studs but directly from the brake disk to the rim. So in order to get the torque specification on the bolt you need to calculate how much forces needs to go through and then based on the friction between the plates you find out how much normal force is needed which gives you the forces of the bolt which gives you the torque spec. And now remember that steel plates bend so only the area around the bolt is going to be in much contact so you might need multiple bolts. And then do all this but for all the cyclical loads. You do not want to be turning around a corner stomping the throttle and have the rim come off your brake disk snapping all the bolts on its way.
If you think this is a lot of physics and math then welcome to engineering.
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