KEVOS® MAKEOS · Strength
Torsion Member Comparison (Solid Bar, Tube or Rectangle)
For this torque, how do a solid round bar, a round tube and a rectangular bar compare in shear stress, twist and weight, and does the one I chose pass?
Side-by-side check of three shaft or link sections under the same torque: a solid round bar and a round tube by the exact polar moment (as the shaft torsion tool), and a solid rectangle by the handbook rectangular-section formulas (as the rectangular bar torsion tool), giving shear stress, twist over the length, twist per metre and mass per metre, with the chosen section checked against your allowable stress and twist limit.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Torque | N·m | Torque applied. |
| Length | mm | Length between the torque inputs. |
| Shear modulus | GPa | Shear modulus of the material. |
| Density | kg/m³ | Density for the mass per metre. |
| Allowable shear stress | MPa | Allowable shear stress (your design value). |
| Largest twist per metre | °/m | Largest twist per metre of length you accept (stiffness requirement). |
| Section chosen | choice | Which section to check. |
| Sections | ||
| Solid bar diameter | mm | Diameter of the solid bar. |
| Tube outside diameter | mm | Outside diameter of the tube. |
| Tube wall thickness | mm | Wall thickness of the tube. |
| Rectangle long side | mm | Longer side of the rectangle. |
| Rectangle short side | mm | Shorter side of the rectangle. |
Method and formulas
- Round sections: J = π (D⁴ − d⁴) ÷ 32; τ = T (D ÷ 2) ÷ J; twist = T L ÷ (G J) (as the shaft torsion tool).
- Rectangle: τ = T (3b + 1.8c) ÷ (b² c²); K = b c³ [⅓ − 0.21 (c ÷ b)(1 − c⁴ ÷ 12b⁴)]; twist = T L ÷ (K G) (as the rectangular bar torsion tool).
- Mass per metre = density × area.
- Example: solid 37.726 MPa, tube 33.296 MPa, rectangle 48.75 MPa; the tube weighs 2.7374 kg/m against 5.5488 for the bar.
Limitations
- Elastic, uniform torsion away from ends and fittings; warping of open sections is not covered.
- Stress concentrations at keyways and holes are not included.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Choose a section for a torque tube or drive link
- Show the weight saved by a tube
- Check a rectangular bar used as a torsion member
Who it is for
- Design engineers
- Machine designers
- Structural engineers
What you get
- Stress, twist and mass for each section
- Chosen section checks
- Lightest section that passes
Interpreting the result
- Open and rectangular sections are poor in torsion; closed round tubes give the most stiffness per kilogram.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 295–303. Topic reference; no source tables or recommended settings are reproduced.
WORKOS tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.
Open the Torsion Member Comparison (Solid Bar, Tube or Rectangle) calculator
