Fastener bolt tightening torque standard and calculation formula

2021-10-13

In the assembly and production of rolling stock, mechanical equipment and automobile, the connection between parts is usually realized by bolts. The connection quality of bolts in key parts determines the reliability of assembly. When using fasteners for bolt connection, attention should be paid to the fastening axial force and fatigue limit of bolts.

·When the tightening axial force of the fastening bolt is calculated by the torque method, the elastic range is calculated with 70% of the specified endurance as the upper limit in the torque method

·The bolt fatigue strength caused by repeated load shall not exceed the allowable value

·The seating surface of bolts and nuts shall not fall into the surface of the object to be fastened

·Do not damage the fastened object during fastening

What is the specific calculation formula for the tightening axial force and tightening torque of bolts?

The relationship between tightening axial force FF is as follows: FF = 0.7 ×σ y × As

Tightening torque TFA: TFA ＝ 0.35k (1 + 1 / Q) σ y・As・d

k: Torque coefficient

d: Nominal diameter of bolt [cm]

Q: Fastening factor

σ y: Endurance (112kgf / mm2 when the strength classification is 12.9)

As: effective sectional area of bolt [mm2]

Although the bolt is small part, it plays a great role, so its selection and design still need to strictly comply with the requirements.

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