Bolt Torque and Friction Coefficient Reference

Torque is commonly used to tighten a bolted joint, but it is only an indirect estimate of clamp load. A nut factor K is one simplified way to represent the torque-to-load relationship for a stated fastener condition. The examples below come from Fastenal’s torque-tension guide. They are advisory estimates, not universal installation torques or substitutes for a joint-specific specification.

Estimated nut factor K by condition

MECHANITEC / ENGINEERING REFERENCE

Estimated torque coefficient K by fastener condition

Fastenal's stated coefficient examples for its torque-tension charts. K is a nut factor, not a direct coefficient of friction or a universal installation specification.

1–5 of 5 matching rows (5 total)

Estimated torque coefficient K by fastener condition — page 1 of 1
Fastener condition	Estimated K factor — low	Estimated K factor — high	Torque relationship	Source scope
Fastenal EcoGuard coated nut, bolt, and washer	0.12	0.12	T = K × D × F	Fastenal-listed estimate for the named coating system; verify the actual product and assembly.
Lubricated (including some oils and tapping fluids)	0.15	0.15	T = K × D × F	Fastenal-listed estimate; lubricant type, amount, and application alter friction.
Some anti-seize, thread lockers, and some plain conditions	0.17	0.17	T = K × D × F	Fastenal's condition wording is broad; do not assume this value for every product or finish.
Zinc plated and dry	0.2	0.2	T = K × D × F	Fastenal-listed estimate for its stated dry zinc condition; surface treatment details matter.
Zinc plated, wax-lubricated prevailing-torque lock nut	0.12	0.18	T = K × D × F	Fastenal states K can vary from 0.12 to 0.18 for this condition.

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What K means

The simplified relationship is T = K × D × F, where T is torque, D is nominal fastener diameter, F is clamp load, and K is an estimated nut factor. Use compatible units: for example, a diameter in inches and clamp load in pounds produce torque in inch-pounds when K is dimensionless.

K is not the same as a coefficient of friction μ. It is an empirical factor that reflects friction and other losses in the torque-to-preload relationship. Fastenal’s guide lists example K values for conditions such as lubricated, zinc-plated dry, and EcoGuard-coated assemblies. A wax-lubricated prevailing-torque lock nut is shown as a range because its K value can vary.

Example calculation — not an installation value

Suppose a hypothetical joint has a 0.5 in nominal fastener diameter, a target clamp load of 10,000 lbf, and an assumed K of 0.20. The simplified calculation gives T = 0.20 × 0.5 × 10,000 = 1,000 in·lbf, or about 83.3 ft·lbf. This arithmetic only demonstrates the formula. It does not verify that 10,000 lbf is appropriate for a particular bolt, nut, joint, or application.

Why surface condition matters

Changing a coating, lubricant, washer, thread fit, installation speed, reused fastener, or tightening tool changes the friction conditions. The same applied torque can therefore produce different clamp loads. Fastenal warns that torque provides only an indirect indication of tension and that over-tightening or under-tightening can lead to failure or injury.

Do not transfer a K value between conditions without evidence. A lubricated thread may require a different torque than a dry or coated one for the same target clamp load. A joint that requires controlled preload may need a tested tightening procedure, direct tension measurement, or a method specified by the responsible engineer.

K-factor values and historical coefficient formulas are different

This table covers the K-factor relationship T = KDF. Some historical fastener references use a separate empirical equation with coefficients m and b and a specific diameter convention. Those coefficients are not interchangeable with K and should not be substituted into this formula. Use the method and conditions stated by the source that supplied the data.

Source

The listed K examples and condition notes are from Fastenal’s Torque-Tension Reference Guide. The document labels its information advisory, notes that torque values depend on the fastener and joint conditions, and urges caution when using torque-tension relationships.