Why this one

What a 4040 is actually good at

40 × 40 mm sits at a point where several constraints happen to meet at once, and that is the real reason it dominates industrial framing rather than any structural advantage over its neighbours.

• It is the largest common section with an 8 mm slot. That slot takes an M8 nut with a torque ceiling around 15 N·m — enough for a joint that will not work loose under vibration, and small enough that a standard hex key still fits between the members.
• One slot geometry, three walls. Our 4040 four-slot section keeps the same 8 mm mouth and the same 30 mm pitch in light, standard and heavy wall, so a design can be re-specified for stiffness without changing a single accessory part number.
• A 6 m length is a one-person lift. At about 1.7 kg/m a 6 m bar weighs roughly 10 kg. The equivalent length in 45 series is about 15 kg and in 60 series about 26 kg. Handling is a real constraint on site, and it is the constraint that most often decides the section when two options are structurally equivalent.
• The accessory range is the deepest in the industry. Brackets, joining plates, hinges, panel fixings, feet, castors, covers and end caps all exist in 40 series in every catalogue. Choosing a section with a thin accessory range means designing parts that already exist.

The ladder

4040 against 2020, 3030 and everything above it

This is the comparison that a “4040 versus 2020 versus 3030” question is really asking, extended up the range so you can see the shape of the whole curve. The allowable loads are central point loads on a simply supported span at L/300, using the same method throughout. Every section in the table is one we extrude — the 40 series, 45 series and 60 series pages carry the drawings and full specifications.

How much each step actually buys

Read the table as ratios rather than as numbers, and the ladder stops looking like a ladder.

The 20 to 30 step looks enormous because a 20 series section is a very small beam. The 40 to 45 step is the one that disappoints: a 12.5 per cent increase in depth gives a 60 per cent increase in stiffness, and you pay roughly a 50 per cent increase in weight and price for it. That asymmetry is the fourth-power rule working against you, and it is why 45 series is a specialist choice rather than a general upgrade from 40.

The last line is the one to remember. Changing the shape of a 40 mm member — keeping the 40 mm envelope but going to 40 × 60 and standing it on edge — gives 2.5 times the stiffness of a 4040 for the same weight per metre as a 45 series section. Depth in the bending direction is cheaper than any amount of section area.

The hidden half of the price

Stepping up a series costs more than the profile

When a designer steps from 40 series to 50 or 60 series, the quotation that comes back is for aluminium by the metre. The cost that shows up six weeks later is the accessory system, and on a frame with a lot of joints it can exceed the profile cost itself.

Three of those rows have nothing to do with strength and everything to do with the cost of the change. A 10 mm slot needs a different nut, which needs a different bracket, which is drilled for a different pitch. If the frame has forty joints, you have just re-specified forty brackets and forty nut part numbers. And the members are 20 mm wider, which changes the envelope of the machine and may not fit where the old one did.

The test

Run the downgrade test before you buy the upgrade

Before accepting a larger section, ask one question: can the smaller section do this job with a shorter span? Work through this table first. Every row in it is a real substitution that saves material without losing performance. The rectangular sections referred to here are the 40 × 60 and the 50 × 100.

Notice that five of the six rows are not about going smaller at all. They are about changing the shape or the span instead of the series. That is the pattern worth taking away: on a square section, stiffness tracks weight almost exactly, so upsizing a square section is a fair purchase but never a bargain. The bargains are in the shape and in the geometry.

Worked example

A 4040 machine base, checked in four lines

A machine base 1,500 × 1,000 mm carries 400 kg distributed across the top. Two long members span the 1,500 mm direction; the load is shared between them, so each carries 200 kg spread along its length.

The cross-member wins on material and on every downstream consequence: no new bracket size, no new nut, no wider frame, no heavier bundle. The only case for the larger section is if the middle of the base has to remain clear, and if it does, the right answer is the 40 × 60 used on edge rather than a larger square.

Ordering

What to write on the 4040 order

Because the name “4040” does not define the section, a purchase order that says “4040 aluminium profile” is an instruction to the supplier to choose for you. Six lines remove the ambiguity, and they take two minutes to write.

• Series, slot count and wall — “40 × 40 mm, four slot, standard wall”. Not “4040”.
• Second moment of area, if the design depends on it — the one number that makes a stiffness-critical order unambiguous.
• Alloy and temper — 6063-T5 for our standard range, with a certified mill certificate if the design is strength-limited.
• Length and tolerance — stock 6 m or cut to a list, with the tolerance stated. Plus or minus 0.5 mm is normal for a framing cut.
• Finish — mill finish, clear anodised or black anodised, and whether the cut ends may be bare.
• Slot geometry, if it must match an existing frame — the mouth, the cavity and the pitch, because those are what the accessories fit, not the envelope.

Questions

Frequently asked questions