Why it became the default

The 40×40 module is a compatibility decision, not a strength one

40 × 40 mm won the aluminium framing market for reasons that have almost nothing to do with structural performance. It sits at the point where a slot is big enough to take a robust fastener and a panel fixing, the section is stiff enough for the spans most machines actually use, and the accessories are stocked by everyone.

Our 4040 four-slot section is a 40 × 40 mm envelope in 6063-T5 with an M6 slot, an 8 mm slot mouth and a 30 mm slot pitch on all four faces, plus a Ø12 mm centre bore tapped M14 for through-bolting. Wall thickness is held to ±0.15 mm to EN 755-9, the same die family runs in light, standard and heavy wall from 1.30 to 2.35 kg per metre, and cut lengths are held to ±0.5 mm with machined features at ±0.2 mm.

That combination — one envelope, three walls, one slot geometry, one accessory range — is the real reason to standardise on it. A frame built entirely from one section lets you hold one bracket part number, one nut part number and one torque figure. On a production line building ten frames a month, that saves more money than any structural optimisation.

The judgement

Nine applications, and what each one actually asks of the section

The useful question is not “is 40×40 strong enough” but “what span and what load does this application produce”. Here is that translation, application by application.

The strength question

What “strength” actually means for a 40×40 section

Ask a supplier about the strength of a 4040 and you will usually get a load figure with no span attached. That figure is meaningless, because deflection rises with the cube of the span. The section property that governs everything is the second moment of area, I, and for a standard-wall 4040 it is about 9.8 cm⁴ — consistent with our own 45 × 45 mm section, which publishes 15.5 cm⁴, since scaling by the fourth power of the envelope gives 9.7 cm⁴ for a 40 mm square.

Look at the fourth column, and notice the pattern in the order the rows are written. The three square sections — 4040, 4545 and 6060 heavy — all land at roughly 6 cm⁴ of stiffness per kilogram, even though their weights differ by a factor of nearly four. On a square section, going bigger adds stiffness and weight in almost exactly the same proportion.

The three rectangular sections break the pattern. Mounted on edge, the 4060 and 4560 deliver about 9.4 cm⁴ per kg/m, and the 100 × 50 delivers 24.2 — four times the square-section figure. That is the fourth-power rule at work: I rises with the cube of the section depth while weight rises roughly with its square, so depth is the only lever that genuinely improves the stiffness-per-kilogram ratio.

What goes wrong

The three failure modes that actually happen in the field

Worked case

An automation cell frame, 2,000 × 1,500 × 2,200 mm

A six-axis robot on a separate pedestal, a 1,800 mm wide conveyor running through the cell, and polycarbonate panels on three sides with a sliding door on the front. The frame is 40-series throughout. Three decisions have to be made, and each one follows from the span rather than from the load.

The interesting thing about that table is how little it depends on load. Every decision came from span, alignment requirement or load direction. That is characteristic of aluminium framing: the strength is almost always there, and the stiffness and the bracing are what you are really designing.

For the accessories, the profile range is on the 40 series aluminium profiles page, and the section itself is 4040 4-slot strut profile. If you need a deeper section for a rail bed, the 4060 is the next step up in the same family, and the 45 series covers the heavier automation frames.

Frequently asked questions