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Fastener engineering reference — calculators, geometry, and standards.

Preload and tightening torque

Thread

Property classISO 898-1:2013, Table 3. 8.8 changes its values above M16, and 9.8 is specified only up to M16.

K lumps thread and underhead friction into one number, and is the largest uncertainty in the result. For a measured value, see ISO 16047.

Assumed, not derived. 0.20 is common shop practice for dry, unplated steel, not standardized.

A percentage of the proof load Fp. The slider covers 10 to 100 %; type a larger value in the field to see the exceeds case.

T Tightening torque
62.6N·m
Fi Preload
31.3kN
Fp Proof load
48.1kN

Within proof load

As and Sp, ISO 898-1:2013 · K = 0.20, assumed · 65 % of proof load · Not hot-dip galvanized or structural · T = K·Fi·d

Show the formula

Fp = As·Sp; Fi = (target / 100)·Fp; T = K·Fi·d

As
Tensile stress area, as ISO 898-1:2013 defines it, computed from the ISO 68-1:2023 basic profile, mm²
Sp
Nominal stress under proof load for the property class, ISO 898-1:2013, at the thread's diameter, MPa
Fp
Proof load, N
target
Target preload as a percentage of proof load, %
Fi
Preload, N
K
Nut factor. Assumed, not derived; common shop practice, not standardized
d
Nominal diameter, mm
T
Tightening torque. With d in mm, K·Fi·d is in N·mm and is divided by 1000, N·m

The derivation, the full assumption set and a worked example