A screwed bolt is subjected to two forces, one is downward and the other is upward. The bolt rotates down along the thread, and a downward axial force is generated in the process of contact with the thread.
At the same time, there is an upward force corresponding to the downward axial force. This force comes from the upward distributed force generated by the contact between the nut and the lower plane. A tightened bolt should ensure that the upper and lower forces are in balance with each other. Due to the function of bolts, the material of bolts has two requirements, one is rigidity and the other is toughness, both of which are indispensable.
But all ductile materials have one thing in common, that is, there is a limit to the stress tolerance. In mechanics, this limit is represented by a stress-strain curve. We don't need to be too esoteric. We only need to know that in the process of screwing the bolt, the bolt is a kind of elastic part, and its elasticity will take effect.
As the bolt is tightened gradually, the elastic effect of the bolt will approach a limit. Once this limit is broken, the bolt will enter the yielding stage from the elastic stage.
Once the bolt enters the yield stage, it will produce permanent deformation. This deformation is irreversible, which means that the bolt will fail. In real life, the phenomenon of tripping when we continue to tighten the bolt is caused by this reason, and once it 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 screwed properly. The tighter the better.
In this way, when tightening the bolt, do you really have to tighten it and then back out a half turn? Otherwise. We have said that once the limit is broken when the bolt is screwed, and the yielding stage is entered, the bolt will be permanently deformed. At this time, it will be useless to retreat half a turn. On the contrary, the returned bolt will lose part of its toughness, but will break more easily.
If the degree of bolt tightening does not exceed the limit, it is not necessary to return half a turn, which will only loosen the bolt.
Then when can the bolt return a half turn after tightening?
Be aware that bolts that are not tightened will become looser due to vibration during use. So no matter whether the bolt is tightened and breaks the limit, returning half a turn is harmful and useless. Of course, there is one exception, and that is when there are gaskets. Some things will equip the bolts with washers.
So what exactly is the function of this thin ring-shaped metal piece? In fact, the function of this metal piece is equivalent to a spring, and the purpose is to increase elasticity. The purpose of increasing elasticity is to make the entire assembly stronger. However, as a spring component, the gasket is also limited in its bearing capacity. The so-called spring is that it will deform after being squeezed, and due to its own elasticity, it will recover after the pressure is removed, but the restoring force of the spring has a limit.
When the pressure of the gasket is too large to exceed the limit, its spring action will disappear, so the existence of the gasket is redundant.
In order to maintain the elastic effect of the gasket and make the entire assembly stronger, after tightening the bolts, we can retract a half turn to maintain the elasticity of the gasket, which is more conducive to the firmness of the entire assembly. But the prerequisite is to ensure that the bolts are not deformed due to over-tightening.
So how can we ensure that the bolts are not tightened too tightly? When manufacturing precision instruments, we have to use torque wrenches that can measure the force, but in daily life, this is a bit redundant. In daily life, screwing bolts mainly depends on experience and feel. In addition, don't be swayed by obsessive-compulsive disorder. Don't keep screwing and screwing all the time. With or without shims, the bolts are not as tight as possible. For bolts without washers, it is wrong to retract a half turn after tightening.