The tighter the bolt, the better? Or half a turn back?

When many people fix the mechanical equipment, they will screw the bolt to the point where it can't be screwed. Is this a good way? Not exactly. It's also said that bolts should be tightened and then back half a circle. Is that right? Is there any scientific basis? In fact, there are many believers in life who think so. What's the scientific basis?

The tighter the bolt, the better? Or half a turn back?
A screwed bolt is affected by two forces, one is downward and the other is upward. The bolt rotates down the thread, creating a downward axial force in contact with the thread.
At the same time, there is an upward force corresponding to the downward axial force, which comes from the upward distribution force generated by the contact between the nut and the lower plane. A tightened bolt should ensure that the upper and lower forces are balanced with each other. Due to the function of bolts, the materials for manufacturing bolts have two requirements, one is rigidity, the other is toughness, both of which are indispensable.
But all ductile materials have a common feature, that is, there is a limit to the stress bearing. In mechanics, the limit is expressed by stress-strain curve. We don't need to be too deep. We only need to know that in the process of screwing the bolt, as an elastic part, its elasticity will take effect.

As the bolt is tightened gradually, the elastic action of the bolt will approach a limit. Once the limit is broken, the bolt will enter the yield stage from the elastic stage.
Once the bolt enters the yield stage, it will produce permanent deformation, which is irreversible, that is to say, the bolt will fail. In real life, when we tighten the bolt continuously, the phenomenon of tripping will occur. Once the bolt is tripped, it is impossible to screw it on again, because the bolt has been permanently deformed. It can be seen that the bolt is not tightened as tightly as possible.
So, do you really need to tighten the bolt and then back half a turn? Otherwise. We have said that once the bolt breaks through the limit and enters the yield stage, the bolt will be permanently deformed. At this time, another half turn back has no effect. On the contrary, the returned bolt will lose part of its toughness and be more likely to break.
If the tightening degree of the bolt does not exceed the limit, it is unnecessary to return half a turn, which will only loosen the bolt.
When can the bolt be returned half a turn after tightening?
It is necessary to know that the bolts that are not tightened will become looser and looser due to vibration during use. So whether or not the bolt is tightened beyond the limit, half a turn back is harmful and useless. Of course, there is an exception, that is, when there is gasket. Some things will gasket the bolts.
So what's the function of the thin ring-shaped small metal sheet? Actually, the function of the metal sheet is equivalent to a spring, and the purpose is to increase the elasticity. The purpose of increasing flexibility is to make the whole component more robust. However, as a spring component, the bearing force of gasket is also limited. The so-called spring, after extrusion, will deform, and because of its own elastic relationship, it will recover after the pressure is eliminated, but the recovery force of the spring is limited.
When the pressure on the gasket is too large to exceed the limit, the spring effect will disappear, so the existence of the gasket is redundant.
In order to maintain the elastic function of the gasket and make the whole assembly more solid, after tightening the bolt, we can return half a circle to maintain the elasticity of the gasket, which is more conducive to the firmness of the whole assembly. However, the premise is to ensure that the bolt is not deformed due to over tightening.
So how can we make sure that the bolts are not too tight? When making precision instruments, we need to use a torque wrench that can measure the torque. In daily life, this is a little redundant. In daily life, screw bolt mainly depends on experience and hand feeling. In addition, don't be around obsessive-compulsive disorder, and don't screw bolt all the time. No matter whether there is gasket or not, the tighter the bolt is, the better. For the bolt without gasket, it is wrong to screw it down for another half turn.