Aluminium Lean Pipe Joint System 28 mm for Kaizen Cart Frames

01When is a pipe frame the right answer instead of an aluminium profile?

A 28 mm lean pipe joint system and a T-slot aluminium profile frame solve the same problem — a structure that is built from stock parts — and they are good at opposite things. The pipe system is good at being changed; the profile system is good at staying where you put it.

The decision is usually made by one question: will this thing be modified? A kaizen cart, a supermarket flow rack, a temporary workstation for a new product line — all of these get changed, and a pipe frame is the cheaper answer over its life even though the parts cost more per metre. A machine frame, a guard, a fixture, a gantry — these are dimensioned once and must not move, and a pipe frame is the wrong tool.

02Aluminium tube or steel-core lean tube? The numbers

Most lean pipe in the market is a steel tube with a plastic coating, and the coating makes it look like plastic. What we supply for this system is an aluminium tube, 28 mm outside diameter with a 1.0 mm wall, and the trade is straightforward once you put the numbers side by side.

Aluminium 28 x 1.0 mm

0.23 kg/m, EI about 5.4 x 10^8 N·mm2

Steel core 28 x 1.2 mm

0.79 kg/m, EI about 1.9 x 10^9 N·mm2

Ratio, mass

aluminium is about 71 percent lighter

Ratio, stiffness

steel is about 3.5 times as stiff in bending

Conclusion

aluminium buys weight and corrosion resistance, steel buys stiffness

Those figures come from the section geometry and the elastic moduli — E of about 70 000 N/mm2 for aluminium and 210 000 N/mm2 for steel — so they are a calculation rather than a rating. Read them honestly: the aluminium tube is not a like-for-like substitute for a steel lean tube. It is a third of the stiffness, for a quarter of the weight.

That makes it right for one job and wrong for another. Right for a cart that is pushed by hand over short spans, where the bending load is small and the weight is felt every time somebody moves it. Wrong for a rack that spans a metre and a half with 200 kg on it, where the steel tube is doing real structural work. The weight saving is only a benefit if the structure still does its job without it.

03Which joint types exist, and what does each cost you?

The joint is where a pipe system’s stiffness actually lives. Every joint type has a different failure mode, and mixing them without thinking produces a frame that racks in one direction only.

Two practical rules come out of that table. First, a pipe frame is only as square as its diagonals — put a diagonal in the back plane, or the frame will parallelogram the first time it is pushed. Second, do not mix swivel joints into a load path; use them where something has to be adjustable, and fix the geometry with a rigid joint elsewhere.

04What does the set screw actually do to the tube?

It makes a point contact and a local dent, and that is by design — the dent is what stops the joint rotating. The failure is over-tightening: a set screw driven hard into a thin wall deforms the tube enough that the joint body no longer sits square, and the frame goes out of true at that corner.

• Tighten to contact plus a quarter turn, not to a torque figure. The set screw is a position lock, not a preloaded fastener — and on a 1.0 mm wall it will dent, which is how it grips, but also how the tube gets used up.
• Put the set screws on opposite sides of the tube, at 90 degrees to each other in plan, so the tube is held rather than pushed to one side.
• Use a through bolt on any joint that carries load. Drill through both walls and use a bolt with a washer on the far side; that converts a point contact into a bearing contact.
• Do not re-use a joint on a different tube position without checking the old dent. A joint that lands on a previous set screw dent will not sit square.
• On a cart that gets rebuilt, expect to replace joints rather than tubes. The joint body and its set screw are the wear parts.

05Can the tube be bent, and where should it not be?

Yes, with a rotary draw bender, to a centreline radius of about 150 mm minimum for a 28 mm tube — roughly five times the diameter. Tighter than that and the wall thins on the outside of the bend and the tube ovalises, and a joint will no longer clamp on it properly.

Bending is worth it for two cases and not for a third. It is right for handle loops and for guard rails on a cart, where a bend removes two joints and makes the part stronger and safer. It is wrong for structural corners — a bent corner is a cantilever, not a joint, and it cannot be adjusted later. If the corner may ever need to change, use a joint.

06How do you join a pipe frame to an aluminium profile frame?

This is the question that comes up on almost every mixed project, and the answer is that you join them at a plate, not at a joint. There is no standard part that clamps a 28 mm tube to an 8 mm T-slot, and trying to improvise one is where mixed frames get loose.

1. Bolt a small plate to the profile slot using two M6 T-nuts, so the plate cannot rotate.
2. Fix the pipe joint to the plate, using the joint’s own through-bolt holes rather than a set screw.
3. Keep the interface in one plane. A mixed frame that connects in two planes will fight itself as soon as the two materials move differently.
4. Put the stiff material where the accuracy is needed. Profile for the machine datum, pipe for the handling and storage around it.

A common and sensible arrangement is a profile machine frame with pipe flow racks feeding it. The profile holds the process, the pipe holds the material, and the interface is a plate at one or two points rather than a continuous connection.

07What to check before ordering

08Questions from lean and continuous improvement teams

Related sections and categories

Browse Lean Pipe Profiles, Pipe and Joint Systems, or see the 28 mm aluminium pipe for the tube data, and the 40 x 40 mm four slot section if the same project also needs a rigid machine frame.