Why are Bolts Tightened? What are the Benefits of Tightening Them?

Keywords:bolts,Why are bolts tightened? What are the benefits of tightening it?

Recently, there has been a lot of publicity about the spirit of craftsman, with reports on how rigorous the method of tightening bolts by German technicians is compared with the method of Tightening bolts by Chinese workers. Strictly speaking, in the process of equipment assemblly, the method of tightening bolts has been defined in the design, even with what tools?Screw bolt so simple thing, how to still decide an operation specification? Are designers really looking for trouble? In fact, the screw bolt is not as simple as imagined. The screw bolt is to make the bolt obtain an initial pretightening force. What role does this pretightening play?

Why to make the bolt to obtain an initial preload, this should start from the role of the bolt connection.

Transfer lateral force or torque

The bolt pretightening force generates pressure between the contact surfaces of the connected parts. The greater the pressure between the contact surfaces, the greater the transverse force can be transmitted. When the pressure on the contact surface is large enough, the transverse force is transferred entirely by the friction force, and the bolt only bears the pre-tightening force, not the shear stress and compressive stress, and the safety of the bolt is greatly increased, as shown in Figure 1.

Sealing function

When the bolt connection is used for pressure pipe or pressure vessel, the bolt should have enough pretightening force to play a sealing role, and the total pretightening force of the bolt should not be less than the pressure of the fluid between the contact surface, as shown in Figure 2. 

Prevent the structure from opening

Generally under fatigue load of bolt connection are not allowed to be fitting slotted, once produce gap between fittings, bolt tension will be increased with the increase of the tensile load increase quickly, the image below for the bolt tensile force and the load curve (as shown in figure 3), we can see that in the first half of the curve, by fitting between slit, bolt tensile slow growth, After the joint is slit, the bolt tension increases rapidly.

After the joint is opened, the amplitude of the alternating fatigue load of the bolt will increase rapidly, and the fatigue strength of the bolt will be greatly shortened.

Bolt tightening is analyzed from three aspects, including:

• Bolt pretightening can prevent the opening separation of the connecting piece, improve the fatigue strength of the bolt connection;
• To prevent the sliding of the connecting piece, relying on friction to achieve the role of the connecting piece to resist external load, to avoid the bolt subjected to shear load;
• Prevent internal pressure leakage of pressure vessels.

The above three points are exactly the three parts of VDI2230 standard to calculate the bolt required preload, that is, the minimum preload required by the bolt, so as to design the appropriate bolt specification and performance grade, and finally determine the bolt preload.

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