- Round tube suits torsion and multi-axis bending; rectangular tube orientation matters.
- Higher D/t raises thin-wall bending and local-buckling risk.
- Holes, clamps and welds are often governed by local strength.
- Validate telescopic fit against actual tolerance, seam and coating.
Values in this guide are practical starting points for RFQ, DFM and prototype validation. They do not replace load calculations, regulations, material certificates, qualified procedures or approved production drawings.
Select the Section for the Load Direction
Round tube bends equally in all directions and is torsion-efficient, suiting handles, guards and curved frames. Square tube provides flat mounting faces. Rectangular tube is stiffer about its deep axis, so orientation must be controlled on the drawing. Moving material away from the neutral axis is efficient: a larger thin-wall section may be lighter and stiffer than a smaller thick-wall tube, but local buckling and dent resistance need separate checks.
Use Diameter-to-wall Ratio to Screen Local Risk
For round tube, outside diameter D divided by wall t is a useful thin-wall indicator. Higher D/t increases sensitivity to wrinkling, flattening and local buckling. It is not an acceptance criterion by itself, but above roughly 25–30, discuss mandrel/wiper tooling and allowable ovality early, especially for tight bends. Square and rectangular tube also require corner-radius and face-concavity checks.
Local Connections Often Govern
Bolt preload can crush a thin wall, welding can burn through or pull the face inward, and direct tapping may not provide enough threads. Consider an internal sleeve, crush tube, weld nut, rivet nut, end plate or local doubler. Do not thicken the entire tube to solve one connection when local reinforcement is lighter.
Telescope from Actual OD/ID Tolerances
A nominal 30×30×2 square tube has a theoretical 26 mm inside dimension, but seam bead, corner radius, wall variation and straightness can obstruct a long insert. Obtain actual mill tolerances or sample tube and test the fit after coating. Long manual telescoping needs more total clearance than a short splice and may require plastic guides, hole-alignment allowance and contamination clearance.
Prefer Available Standard Sizes
Ask for available outside sizes, walls, grade, welded versus seamless route, corner radius and stock length. Non-standard sizes may trigger mill-bundle MOQ, roll tooling and long lead time. Specify the full tube definition—for example, welded rectangular tube 50×30×2.0, S355, 50 mm side vertical—rather than “50 tube.”
SELECTION AND VALIDATION
- Load direction, moment, torque, impact and allowable deflection
- Section inertia/modulus and local buckling
- Bend radius, D/t, tangents and section distortion
- Local reinforcement at holes, threads, welds and clamps
- Actual telescopic tolerances, seam and coating allowance
- Stock size, MOQ, certificates and substitution rule