Why do eye nuts need to be tightened three and a half turns?

In most high-end mechanical equipment factories, when assembling special parts, there are strict operating manuals for screw tightening, and there are clear specifications for how much torque to apply. In fact, after the screw is tightened, in order to prevent loosening, an additional pre-tightening force should be applied, so the pre-tightening force will be eliminated after loosening half a turn, and the screw is in elastic deformation after tightening, especially under high temperature and vibration load. Such continuous pressure for a long time will produce creep, and after the screw becomes plastically deformed, its strength will be greatly reduced or even fail. Returning half a circle is to restore some elastic deformation and eliminate the pre-tightening stress at the same time. In the future, the deformation of the screw under continuous pressure is still in elastic deformation, and the probability of plastic strain and failure is greatly reduced, so that the screw can maintain continuous high-strength pressure. But directly twisting two and a half turns will not achieve this effect.