Projectile Weaving
Bar diameter enters to the fourth power. Two millimetres more steel is a different loom, and a different fatigue life.
— m/s
From the stored energy after transfer losses
Linear elastic torsion, so it holds only while the bar stays well inside its elastic limit — and the bar sees this stress on every pick, which makes fatigue rather than yield the governing criterion. Check the peak shear stress against the material's endurance limit, not its ultimate strength, and treat bar replacement as a scheduled item.
Projectile Loom Torsion Bar Twist & Firing Energy Calculator — free, with the formula and a worked example, at Textile School.