Datum
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FEARev. Q3 2026

Bolted joint sizing

Size and verify a bolted joint end to end, covering preload, torque spec, separation and slip margins, thread engagement, and fatigue on the alternating load share. Use whenever the user asks what bolt size or grade to use, what torque to apply, whether a joint will slip or separate, how many bolts a pattern needs, or mentions preload, clamp load, or gasket compression.

Lars Ekstrom portraitLars EkstromBolted Joint EngineerNord-Lock Group logoNord-Lock Group

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Bolted Joint Sizing

Runs a VDI-2230-style joint verification at hand-calc fidelity: preload from torque, load sharing through joint stiffness, then explicit margins against separation, slip, yield, thread strip, and fatigue.

Required inputs

  1. External loads on the joint: tensile (axial) force per bolt or on the pattern, shear force, and whether loads are static or cyclic. If cyclic, min and max.
  2. Bolt candidate: size, pitch, property class (e.g., M8x1.25 class 8.8, or 5/16-18 Grade 5). If unknown, ask for the load and propose one.
  3. Clamped material and joint geometry: material of clamped parts, grip length, whether there is a gasket, through-hole or tapped hole, and internal thread material if tapped.
  4. Lubrication condition: dry, oiled, anti-seize, or a thread locker. This sets the nut factor.
  5. Friction interface: coefficient of friction between clamped parts if shear is carried by friction.

Method

  1. Preload target. Default to 65 to 75 percent of proof load for reusable joints, up to 90 percent for permanent joints with controlled tightening. State the choice.
  2. Torque: T = K x F x d. Use K = 0.20 dry steel, 0.15 to 0.18 lightly oiled, 0.10 to 0.12 anti-seize, and say the assumed value. Report preload scatter: plus/minus 25 percent minimum for torque control with a hand tool, tighter only with angle control or measured methods.
  3. Load sharing: estimate bolt stiffness (AE/L on the effective sections) and joint stiffness (frustum approximation is acceptable; state it). Compute the load factor phi = kb / (kb + kj). Only phi times the external tensile load goes into the bolt; the rest unloads the clamp.
  4. Margins, each computed at minimum preload (nominal minus scatter):
    • Separation: residual clamp = Fmin minus (1 minus phi) x external load. Must stay positive with margin.
    • Slip: friction capacity = mu x residual clamp x number of interfaces vs. applied shear. If shear exceeds friction capacity, the joint works in bearing; check bolt shear and hole bearing, and flag that friction-grip was lost.
    • Bolt stress at maximum preload plus phi times max load vs. proof and yield, including torsion during tightening (von Mises with thread torsion, roughly 1.13 factor on axial stress for standard threads).
    • Thread engagement: for tapped holes, required engagement so the bolt breaks before threads strip. Rule of thumb 1x nominal diameter in steel, 1.5x in aluminum, 2x in plastics, then verify with shear area if geometry is given.
    • Fatigue: alternating bolt stress = phi x load amplitude / stress area, compared against roughly 50 to 60 MPa endurance for rolled threads (class 8.8 to 10.9). Cut threads: half that. If alternating stress exceeds the limit, increase preload or joint stiffness before increasing bolt size.
  5. Write the torque spec line the way it goes on a drawing: fastener, torque with tolerance, lubrication condition, and any thread locker, e.g., "M8x1.25 class 8.8, torque 22 Nm plus/minus 10 percent, threads lightly oiled."

Output format

  • Assumptions table (K, mu, preload target, stiffness model)
  • Preload nominal / min / max
  • Margin table: separation, slip, static strength, thread strip, fatigue, each with PASS or FAIL and the governing number
  • Torque spec line ready for the drawing
  • If FAIL: the single most efficient fix, stated first

Guardrails

  • Never give a torque number without stating the lubrication assumption. K uncertainty dominates everything.
  • If the joint is safety-critical (lifting, pressure boundary, vehicle structure), recommend verification testing or a measured-preload method and note applicable standards (VDI 2230, NASA-STD-5020) rather than treating the hand calc as final.
  • Gaskets change everything: flag that soft joints have high phi and poor fatigue behavior, and creep relaxation must be checked.
  • If load per bolt was derived from a pattern, show the pattern math including moment-induced prying, or ask for it.

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