How Much Do You Know About Bolts Tightening

.The common types of bolts according to materials, including stainless steel bolts, iron bolts, alloy steel bolts, plastic bolts, etc. The bolts shall be tightened at the important joint position. About bolts tightening, how much do you know? Are the bolts should be tightened as tight as possible? This article will introduce some useful knowledge about bolts tightening.


What Affect The Bolts Tightening

When choosing a bolt, you can look at the performance grade mark of the head. These numbers indicate the yield strength and tensile strength of the bolt. The larger the value, the possibility of fracture and sliding wire when the bolts are tightened is low. When tightening the bolt, the bolt will be in elastic deformation, when the stress value exceeds the yield strength of the bolt material, the bolt will be deformed and cannot be recovered. The tensile strength exceeding the bolt material will start to tighten, generate centralized deformation, and even break. Therefore, the bolts should not be tightened as tightly as possible. The tightening operation should be performed according to its specified torque when it is fastened.

The higher the performance grade of the bolts, the better, the higher the yield strength and the tensile strength of the bolt, but the toughness and impact of the bolts will deteriorate at the same time. If the bolt strength is too large, it may also damage the fastened workpiece and equipment, so we should not simply pay attention to its performance grade and should select suitable bolts according to the use scenario.

In other words, when using a strong bolt, the tighter the blot is tightened, the component may be deformed. For example, it is used in a bearing scene, tighter bolts may directly compress the bearing to deformation. Therefore, the bolts that use for the activity component can not be too tight.


Benefits of The Bolts Tightening

The bolt is tightened to get an initial pre-tightening force. What are the effects of this pre-tightening ?

1. Transmitting lateral force or torque
The pre-tightening force of the bolt can cause pressure between the contact surface of the connector. The greater the pressure between the contact surfaces, the greater the transmissible lateral force. When the contact surface pressure is large enough, the horizontal force is transmitted by friction. The bolts only bear pre-tightening force, no longer subject to shear stress and compressive stress, and the safety of the bolts has been increased greatly.

2. Sealing
When the bolt connection is used for pressure pipes or pressure vessels, the bolt needs to have sufficient pre-tightening force to play a sealing role. The total bolt pre-tightening force cannot be smaller than the pressure of the fluid between the contact surfaces.

3. Prevention of structural slit
Generally, the bolted connection subject to fatigue load is not allowed to be seamed,  Once the gap between the connecting parts is generated, the bolt pulling force will increase rapidly with the increase of the external tension load. There is no seam between the connecting parts, and the bolt tensile force increases slowly. After that the joints are slit, the bolt tensile force increases rapidly.


Why Fatigue Life Of The Bolts Can Be Improved Through Being Tightened

In the design of the bolts, to get the pre-tightening force, the tightening bolt is only one method to realize the pre-tightening. Applying the pre-tightening force is an important purpose of improving the fatigue life of the bolt.

Material fatigue strength, amplitude of alternating stress on bolt, average stress, and frequency of stress cycles are the factors affecting the fatigue life of bolt. For the same specific application occasion, if the bolt specifications are not changed, if you want to extend the fatigue life of the bolt, you can only reduce the amplitude value of alternating stress or the average stress.


When the bolt is tightened by the design torque, bolted connection under the same variable load, the stress range of the bolts is small, which has a greater impact on the fatigue life of the bolt. The smaller the stress change range, the longer the fatigue life is. In order to reduce the range of bolt force variation, it is often necessary to ensure that the bolt is connected to the external load and will not be separated. Reasonable design of the  pre-tightening force can maximize the utilization rate of bolt materials.


Bolt tightening is analyzed from three aspects in this article, which includes the factors affecting bolt tightening, advantages and how to improve fatigue life of the bolts though being tightened.

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