The single most important thing to understand about a bolted joint is that the bolt's job is not to "hold the load" — it's to clamp the parts together hard enough that the load never moves them. That clamp is called preload, and getting it right is where most joint failures are won or lost.
Torque is a terrible proxy for preload
You tighten to a torque, but you care about tension. The two are linked by T = K·D·F, where K is
the nut factor — and K bundles up all the friction under the head and in the threads. Only about
10–15% of your torque becomes clamp force; the rest is lost to friction. Worse, K scatters ±25%
or more with lubrication, plating and surface finish, so torque control alone gives you a wide band of actual
preload. That's why critical joints use angle control or direct tension measurement. Estimate torque for a target
preload on the Bolt Torque tool.
Why preload beats the external load
Here's the magic: in a properly preloaded joint, an external tensile load is shared between the bolt
and the clamped members according to their relative stiffness. The bolt is usually far less stiff than the members
it clamps, so it only picks up 10–20% of the external load — the rest just unloads the clamped
interface. This is the whole reason preloaded joints survive fatigue: the bolt barely sees the fluctuation. The
load factor C = k_b/(k_b + k_m) quantifies it; the Bolted-Joint tool runs
the numbers.
The number that actually matters: separation
The joint stays healthy right up until the external load overcomes the preload and the faces
separate. After that, the bolt takes the full fluctuating load directly and fails fast. So design
the preload so the joint never separates under the worst-case load, with margin — then the bolt lives an easy
life. Separation load = F_i / (1 − C).
Practical checklist
Set preload at 65–90% of proof load (see bolt grades). Use enough thread engagement — about 1×D into steel, 2×D into aluminium. Watch for gasketed or "soft" joints, which relax. And for anything that matters, follow VDI 2230 — the definitive bolted-joint standard.