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→How tight should a screw be?https://www.elexp.com/Images/AllAboutScrews.pdf
Additional tightening preload does not buy any additional fatigue protection as long as joint separation does not happen.
=== How to achieve the required tightness? <ref name = "allabout" />===
According to the Machinery's handbook, tightening by feel is only +-35% accurate. Using a torque wrench improves the accuracy to only about +-25%. These uncertainties are huge, and thus, we should not to tighten too close to yield, or too loose. They give a good reason to design a joint so that it will still work with a wide range of possible pre-loads. There's a method called "turn-of-nut" which can supposedly get within +-10% accuracy, but it relies heavily on a reliable starting point from which to start counting turns. (see Machinery's handbook).
See https://www.boltdepot.com/fastener-information/materials-and-grades/bolt-grade-chart.aspx for more information on these marking schemes.
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== References ==