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Is it better to tighten the bolts as tightly as possible?

March 1, 2024

The tighter the bolt, the better? Is it better to tighten and reverse half a turn? A tightened bolt is subjected to two forces, one downward and the other upward. The bolt rotates along the thread and generates a downward axial force during contact with the thread. There is also an upward force corresponding to the downward axial force, which comes from the upward distributed force generated by the contact between the nut and the lower plane. A tightened bolt should ensure that the two forces above and below are balanced against each other. Due to the function of bolts, the materials used to manufacture bolts have two requirements: rigidity and toughness, both of which are indispensable. All ductile materials have a commonality, which is that there is a limit to their ability to withstand stress. During the process of tightening the bolt, as an elastic component, the elasticity of the bolt will take effect.


As the bolt is gradually tightened, the elastic action of the bolt will approach a limit. Once this limit is exceeded, the bolt will enter the yield stage from the elastic stage. Once the bolt enters the yield stage, it will undergo permanent deformation, which is irreversible, meaning the bolt will fail. In real life, the phenomenon of detachment occurs when we constantly tighten bolts, and once it is detached, it is impossible to tighten it again because the bolt has permanently deformed. Therefore, it can be seen that the tighter the bolt, the better.


Do you really need to tighten the bolts before retreating half a turn? When tightening the bolt, once the limit is exceeded and it enters the yield stage, the bolt will permanently deform. At this point, retreating another half turn will have no effect. The returned bolt will lose some of its toughness and instead be more prone to breaking.


When can the bolt be pulled back half a turn after tightening? Bolts that are not tightened will become increasingly loose due to vibration during use. So whether the bolt is tightened beyond its limit or not, returning half a turn is harmful and not beneficial. Of course, there is an exception, which is when there is a gasket. Some things will equip bolts with washers. The function of this metal plate is like a spring, with the aim of increasing elasticity. Increasing elasticity is to make the entire component more sturdy. However, as a spring component, the bearing capacity of the gasket is also limited. The so-called spring refers to the deformation that occurs after compression, and due to its own elasticity, it will recover after the pressure is eliminated. However, the restoring force of the spring is limited.


In order to maintain the elasticity of the gasket and make the entire component more sturdy, we can retract half a turn after tightening the bolts to maintain the elasticity of the gasket, which is more conducive to the firmness of the entire component.


In life, tightening bolts mainly relies on experience and hand feel. In addition, do not be influenced by obsessive-compulsive disorder, do not keep tightening, keep tightening. Whether or not there is a gasket, the tighter the bolt, the better. For bolts without washers, tightening and then retreating half a turn is also an error.


What are the control methods for tightening bolts?

1. Torque control method:

Definition: the control method of immediately stopping tightening when the tightening torque reaches a set control torque.

Advantages: the control system is simple, direct, and easy to use torque sensors or high-precision torque wrenches to check the quality of tightening.

Disadvantages: the control accuracy is not high (pre tightening force error is about ± 25%), and the potential of the material cannot be fully utilized.


2. Torque angle control method:

Definition: the control method for tightening a bolt to a specified angle starting from a small torque and then starting from this point.

Advantages: the axial pre tightening force accuracy of the bolt is relatively high (± 15%), which can obtain a larger axial pre tightening force, and the values can be concentrated around the average value.

Disadvantages: the control system is relatively complex, requiring measurement of two parameters: torque and angle. And the quality inspection department also finds it difficult to find appropriate methods to check the tightening results.


3. Yield point control method:

Definition: a method of stopping tightening a bolt after tightening it to the yield point.

Advantages: the tightening accuracy is very high, and the pre tightening force error can be controlled within ± 8%. But its accuracy mainly depends on the yield strength of the bolt itself.

Disadvantages: the tightening process requires dynamic and continuous calculation and judgment of the slope of the torque and angle curves, and the control system has high requirements for real-time performance and calculation speed.