Bolt preload explained

Preload is the tension — the clamp force — created in a bolt the moment you tighten it. As the nut advances, the bolt stretches like a very stiff spring and pulls the joint members together. That stored tension, not the bolt's shear or the threads themselves, is what makes a bolted joint work.

Why preload matters more than the bolt

A properly preloaded joint resists both fatigue and self-loosening. The reason is load sharing: when an external tensile load is applied to a clamped joint, the bolt and the compressed members share it in proportion to their stiffnesses. The members are usually far stiffer than the bolt, so the bolt sees only a small fraction of the cyclic load — until the external load approaches the preload and the joint starts to separate. Keep the preload high enough that the joint never reaches separation, and the bolt's stress range stays tiny, which is exactly what fatigue life depends on. The same residual tension keeps friction high at the threads and under the head, which is what stops vibration-driven loosening.

Too little preload and the joint can gap, slip, or fatigue. Too much and you yield the bolt or crush the members. The whole game is hitting a target preload — and the usual tool for that is a torque wrench.

The torque–tension relation

Torque and preload are linked by the short-form relation:

T = K · d · F

where:

• T — the tightening torque applied to the nut or head.
• K — the nut factor, a dimensionless lumped coefficient that captures all the thread and underhead friction. As-received/dry steel is about 0.20; lightly lubricated joints run nearer 0.15; anti-seize and some coatings push it lower still.
• d — the nominal bolt diameter.
• F — the preload (clamp force) you actually achieve.

Rearranged to get preload from a torque spec:

F = T / (K · d)

Most of the applied torque does not turn into tension — it is burned overcoming friction in the threads and under the bearing face. That is why K dominates the result, and why anything that changes friction (lube, plating, dirt, reuse) changes the preload you get for the same torque.

Choosing a target preload

Preload is normally set as a fraction of the bolt's proof load — the largest tension the bolt can carry with no measurable permanent set. A common rule of thumb is 75% of proof load for reusable joints and up to ~90% for permanent ones. Proof load is:

Proof load = Sₚ · Aₜ

where Sₚ is the material's proof strength and Aₜ the tensile stress area of the thread:

Aₜ = (π/4)·(d − 0.9382·P)²

Here P is the thread pitch. The stress area sits between the minor and pitch diameters because that is the effective area carrying tension across the threaded section.

Worked example: M10×1.5, class 8.8

Take an M10×1.5 bolt of property class 8.8 (proof strength Sₚ = 600 MPa). First the stress area:

Aₜ = (π/4)·(10 − 0.9382·1.5)² = (π/4)·8.593² ≈ 58 mm²

Then the proof load and a 75% target:

Proof = 600 · 58 = 34 800 N ≈ 34.8 kN

Target F = 0.75 · 34.8 = 26.1 kN

Finally the torque to reach it, using a dry nut factor K = 0.20 and d = 0.010 m:

T = K · d · F = 0.20 · 0.010 · 26 100 ≈ 52.2 N·m

So roughly 52 N·m on a dry, as-received M10 class 8.8 bolt targets about 26 kN of clamp. Lubricate the same bolt to K = 0.15 and the same 52 N·m would over-tension it past proof — which is exactly why the spec must match the assembly condition. (These are the numbers the calculator returns for these inputs.)

Why torque is the loose end

The torque method is convenient but imprecise. Because K swings with lubrication, plating, surface finish and reuse, the K-method carries about ±25–30% scatter in delivered preload for a given torque. When the preload really matters, tighter methods help: angle control (turn-of-nut, which measures rotation past snug and is largely friction-independent in the plastic region) and direct bolt-stretch measurement both cut the scatter substantially.

Preload is only part of joint design. Once a joint is clamped, you still need to check how it carries shear — see the bolt shear strength calculator — and for quick lookups by size and grade the bolt torque spec chart gives torque values without working the relation by hand.

Frequently asked questions

What is bolt preload?

Preload is the tension (clamp force) locked into a bolt when you tighten it. That tension squeezes the joint members together. A well-preloaded joint resists fatigue and self-loosening because the bolt feels only a small fraction of any cyclic external load until the joint is nearly pulled apart.

How does torque relate to preload?

Through the short-form torque–tension relation T = K·d·F, where T is tightening torque, K the nut factor, d the nominal diameter and F the preload. Rearranged, F = T/(K·d). Most of the torque is spent overcoming thread and underhead friction, so the nut factor K — not the thread geometry alone — sets how much tension you actually get.

How accurate is torque as a way to set preload?

Not very. The nut factor K varies with lubrication, surface finish and assembly conditions, so the torque method typically delivers ±25–30% scatter in preload. Angle (turn-of-nut) control and direct bolt-stretch measurement are far tighter when the preload must be controlled precisely.

Last reviewed: 2026-05-29.