Spring rate, initial tension, working load and shear stress per GB/T 1239 / GB/T 23935
Formulas per GB/T 1239: k = G·d⁴/(8·D³·n), F0 = π·d³·τ0/(8·D) (initial tension), F = k·δ + F0, τ = K·8FD/(π·d³) with Wahl K = (4C−1)/(4C−4)+0.615/C, C=D/d. Initial stress τ0 typically 0.2–0.3 of tensile strength; 0 to ignore initial tension.