Size a round-wire cylindrical helical torsion spring per GB/T 1239. Torsion springs deflect by bending, so the formula uses Young's modulus E (not shear G). Formula: kt = Ed⁴/(64Dn); θ = T/kt; σ = Kb·32T/(πd³).
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Torsion springs deflect by wire bending — uses Young's E (~206000 MPa), not shear G.
kt = Ed⁴/(64Dn) [N·mm/rad] — torque per radian of twist.
θ = T/kt converts applied torque to angular deflection.
σ = Kb·32T/(πd³) at the wire surface; Kb = Wahl bending form.
kt·θ(rad) = T by construction — built-in arithmetic check.
Inner diameter must clear the guide mandrel under twist; check separately.
Wire d, mean D, active coils n.
Torque T, Young's E, allowable [σ].
Torsional rate, twist angle, bending stress.
Stress within allowable, pass/fail.