Question 1

What is bolt tightening torque?

Accepted Answer

Bolt tightening torque is the rotational force applied to a fastener to generate a controlled clamping preload in the joint. It is calculated using T = K × F_i × d, where K is the nut factor, F_i is the bolt preload, and d is the nominal diameter.

Question 2

What is a nut factor (K-factor)?

Accepted Answer

The nut factor (K) is a dimensionless coefficient that accounts for thread friction, under-head friction, and thread lead angle. Typical values are K ≈ 0.20 for dry steel, K ≈ 0.15 for lubricated, and K ≈ 0.13 for MoS₂-coated fasteners, per VDI 2230.

Question 3

What is bolt preload and why does it matter?

Accepted Answer

Bolt preload is the tensile force generated in a bolt when it is tightened. It creates the clamping force that holds the joint together. Standard practice is to target 75–90% of proof load to ensure reliable joint performance without yielding the bolt.

Question 4

What is the difference between ISO Class 8.8 and Class 10.9?

Accepted Answer

ISO Class 8.8 has a yield strength of 640 MPa and is the most common structural bolt. ISO Class 10.9 has a higher yield strength of 940 MPa and is used in high-strength joints. Class 12.9 is the strongest at 1080 MPa yield, used in critical applications.

Bolt Torque Calculator