KEVOS® MAKEOS · Springs
Torsion Bar Spring Design (Diameter for Stress and Twist)
What diameter must this torsion bar be to carry the torque within the allowable shear stress and twist, and what rate and stroke will it give?
Design of a solid round torsion bar spring: torque from load and arm, the least diameter for the allowable shear stress and the least for the largest twist, the governing diameter, and with the diameter you choose the shear stress, twist, torsional rate, arm-tip travel (as the torsion bar tool) and energy stored.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Load at the arm | N | Load applied at the end of the arm. |
| Arm length | mm | Length of the lever arm. |
| Active bar length | mm | Active length of the bar between the fixed end and the arm. |
| Shear modulus | GPa | Shear modulus of the bar. |
| Allowable shear stress | MPa | Allowable shear stress for the duty (your value; presetting raises it). |
| Largest twist allowed | ° | Largest twist you allow (for travel or stops). |
| Bar diameter chosen | mm | Diameter you intend to use. |
Method and formulas
- Torque T = F × arm.
- Diameter for stress d = (16T ÷ π τ_allow)^⅓; for twist d = (32 T L ÷ π G θ_max)^¼.
- τ = 16T ÷ π d³; θ = T L ÷ G J with J = π d⁴ ÷ 32; rate = G J ÷ L; tip travel = arm × θ (as the torsion bar tool); energy = T θ ÷ 2.
- Example: 1000 N on a 200 mm arm needs 18.524 mm; a 20 mm bar twists 7.3595° at 127.32 MPa.
Limitations
- Solid round bar; the splined or hexagon ends need a larger diameter and their own stress check.
- Static stress; fatigue governs repeated loading.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Size a torsion bar for a counterbalance or suspension
- Check a bar for a higher load
- Trade length against diameter
Who it is for
- Design engineers
- Vehicle engineers
- Machine designers
What you get
- Diameter needed for stress and for twist
- Stress, twist, rate and tip travel with the chosen diameter
- Energy stored
Interpreting the result
- A longer bar twists more for the same stress: use length, not diameter, to soften the rate.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 295–303, 344. Topic reference; no source tables or recommended settings are reproduced.
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Open the Torsion Bar Spring Design (Diameter for Stress and Twist) calculator
