BIZOMAT MAKEOS · Strength
Torque Tube and End Weld Check (Shear, Twist, Weld Throat Stress)
Is this round or rectangular tube strong and stiff enough to carry the torque, and is the fillet weld that joins it to its end plate big enough?
Check of a closed thin-walled tube carrying torque and the fillet weld around its end: shear stress, torsional constant and angle of twist of the tube (as the thin-walled tube torsion tool), the shear flow at the end weld from the area it encloses, the weld throat stress and the leg size needed (as the fillet weld strength tool, with the shear flow around the full perimeter), against the allowable stresses and your twist limit.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Section | choice | Shape of the tube section. |
| Outside diameter or width | mm | Outside diameter (round) or outside width (rectangular). |
| Outside height (rectangular only) | mm | Outside height of a rectangular tube. |
| Wall thickness | mm | Wall thickness. |
| Torque | N·m | Torque carried. |
| Length | mm | Length of the tube between the torque points. |
| Shear modulus | GPa | Shear modulus of the tube. |
| Allowable tube shear stress | MPa | Allowable shear stress of the tube. |
| Largest twist | °/m | Largest angle of twist per metre you accept. |
| End weld | ||
| Fillet weld leg | mm | Leg size of the fillet weld all round the tube end. |
| Allowable weld throat shear stress | MPa | Allowable shear stress on the weld throat. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Section
- What it is
- The cross-section of the tube: a rectangular hollow section or a round tube.
- Units and format
- Choose one of 2 options from the list: Rectangular tube (RHS); Round tube.
- Example
- The form opens with “Rectangular tube (RHS)” selected as a worked example. Choose the option that matches your case.
- Get it right
- For a round tube the outside height input is ignored.
What this means: Outside diameter or width
- What it is
- The outside diameter of a round tube, or the outside width of a rectangular tube, measured over the walls.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 100000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 100 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Outside height (rectangular only)
- What it is
- The outside height of a rectangular tube, at right angles to the width; ignored for a round tube.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 100000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 50 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Wall thickness
- What it is
- The wall thickness of the tube, taken as uniform all round the section.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 10000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 4 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Tube shear stress falls roughly in inverse proportion to the wall thickness.
What this means: Torque
- What it is
- The torque carried along the tube between its ends, which also passes through the end weld.
- Units and format
- Enter a number in N·m: newton-metres. The form accepts 0.000001 N·m or more. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1500 N·m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Tube shear stress, twist and weld throat stress all rise in proportion to torque.
What this means: Length
- What it is
- The length of tube between the points where the torque is applied and resisted, over which it twists.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 1000000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1000 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Twist per metre does not depend on length; the length only sets the total twist.
What this means: Shear modulus
- What it is
- The shear modulus of the tube material, from its data sheet; it sets how much the tube twists.
- Units and format
- Enter a number in GPa: gigapascals (1 GPa = 1000 MPa). The form accepts 0.000001 GPa to 500 GPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 79.3 GPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter GPa, not MPa: the tool multiplies by 1000 internally.
What this means: Allowable tube shear stress
- What it is
- The allowable shear stress of the tube material, from your design code or material data.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 100000 MPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 100 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Largest twist
- What it is
- The largest angle of twist per metre of tube length you accept for the function of the part.
- Units and format
- Enter a number in °/m: °/m. The form accepts 0.01 °/m to 100 °/m. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.5 °/m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- This is per metre of length, not the total twist over the tube.
End weld
What this means: Fillet weld leg
- What it is
- The leg size of the fillet weld running all round the end of the tube; the throat is taken as 0.707 times the leg.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 100 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 5 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Weld throat stress is inversely proportional to the leg size.
What this means: Allowable weld throat shear stress
- What it is
- The allowable shear stress on the fillet weld throat, from your welding code for the filler metal and weld category.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 10000 MPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 145 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Method and formulas
- Shear stress = T ÷ (2 A_m t); J = 4 A_m² t ÷ s; twist = T L ÷ (G J) with A_m and s on the wall mid-line (as the thin-walled tube torsion tool).
- Shear flow at the weld q = T ÷ (2 A_w), A_w = area enclosed by the outside of the tube.
- Weld load = q × perimeter, carried by one weld of that length (as the fillet weld strength tool); throat stress = q ÷ (0.707 × leg).
- Example: the tube sees 42.459 MPa and twists 0.98646°/m; the weld throat sees 42.426 MPa (leg needed 1.463 mm).
Limitations
- Closed thin walls; corner radii and stress raisers at the weld toe are ignored.
- Pure torque; bending at the end plate needs adding separately.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check a torque tube, drive arm or frame member in torsion
- Size the weld of a tube to its flange
- Compare round and box sections for twist
Who it is for
- Design engineers
- Fabrication engineers
- Structural detailers
What you get
- Tube shear stress, torsional constant and twist
- Shear flow at the weld and weld throat stress
- Weld leg needed
Interpreting the result
- Closed tubes are many times stiffer in torsion than open channels of the same weight.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 295–303, 1476–1486. Topic reference; no source tables or recommended settings are reproduced.
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Open the Torque Tube and End Weld Check (Shear, Twist, Weld Throat Stress) calculator