Why Are Bolts Tightened?

Recently, there has been a lot of publicity about the spirit of craftsmen, 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 assembly, the method of tightening bolts has been defined in the design, even with what tools? Screw bolt so a simple thing, how to still decide on 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 pre-tightening force. What role does this pre-tightening play?

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

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Transfer Lateral Force or Torque

The bolt-tightening force generates pressure between the contact surfaces of the connected parts. When the pressure on the contact surface is large enough, the transverse force is transferred entirely by the friction force, and the tightening bolt only bears the pre-tightening force, not the shear stress and compressive stress, and the safety of the tightened nuts and bolts are greatly increased, as shown in Figure on the left.

Sealing Function

When the tightening bolts connection is used for a pressure pipe or pressure vessel, the tightening bolts should have enough pre-tightening force to play a sealing role, and the total pre-tightening force of the bolt should not be less than the pressure of the fluid between the contact surface.

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Prevent the Structure from Opening

Generally under fatigue load of tightening bolts connection is not allowed to be fitting slotted, once produce a gap between fittings, nut bolt tightening tension will be increased with the increase of the tensile load increase quickly, the image below for the tightened nuts and bolts tensile force and the load curve, 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 tightening bolts will increase rapidly, and the fatigue strength of the bolt will be greatly shortened.

Bolt tightening is analyzed from three aspects,

  • Nut bolt tightening can prevent the opening separation of the connecting piece, and improve the fatigue strength of the bolt tightening 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 nut bolt tightening subjected to shear load;
  • Prevent internal pressure leakage of pressure vessels.

The above three points are exactly the three parts of the VDI2230 standard to calculate the tightening 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 tightening bolt preload.

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