It's a giant erector set, or, a series of tubes

January 31, 2010 3:47 PM

Giant erector set filter: I am looking for fittings used to join square tubular steel in various ways without welding, for purposes similar to a large erector set. This is for self-fabrication of a large solar panel mount. The pipes I am using are square in cross section measuring 4 inch x 4 inch or 3.5 inch x 3.5 inch. I'm looking for straight splice joints, T, L, X, and 45 degree angles.

If any mechanical engineers want to take pity on a computer geek attempting to build a huge erector set, please point me in the right direction. First I'll say that I'm not attempting to build anything that will be inhabited by people or animals, individual pieces can be lifted into place by two people, and it won't rise to a great height off the ground, so the risks should be negligible.

I am working with square cross section 4" x 4" (4 mm wall) and 3.5" x 3.5" (3 mm wall) tubular steel in pieces not longer than 3 meters. It generally resembles the products found on the second page of this PDF file

To keep welding to a minimum and because the structure may need to be assembled and disassembled occasionally, I am working with M12 size bolts/washers and drilled holes for attachment of the tubes. The 3.5" tubes fit slide inside the 4" pipes like a sleeve, to create legs that can raise or lower in height by moving the position of the holes and bolts.

I know something like this exists but am not sure where to find it: Imagine I have three 4" square cross section hollow tubes and want to join them in a T shape using only drilled holes in the bolts, and bolts/washers/nuts. I am searching for the prefabricated welded "T" piece (other shapes) to accomplish this sort of thing ( in the same general way as this joint for round pipes ), and hopefully some basic CAD schematics for it...

posted by thewalrus to Technology (6 answers total)

I think what you want is generally known as a "structural framing system". McMaster, savior of all in need of obscure parts and materials, has T-slotted framing, which I've seen used for a number of precision applications (like welding jigs); they have a few other framing systems, including one that lets you use standard 1" pipe.
posted by adamrice at 4:42 PM on January 31, 2010

Are you sure this is legal? My uncle recently put in a decent sized solar system like this and he had to have everything designed and then checked over by a certified public structural engineer. It was also inspected by the state and utilites company before being connected to the grid.
posted by DJWeezy at 7:31 PM on January 31, 2010

Have you ever heard of Unistrut? I've only had a bit of experience with it but the catalogue is massive. I've used it mostly to make small frames for holding pipes and once we build a frame for a small wall using it... the wall was in a hospital so it'd probably be approved for structural uses in your area. It's neat stuff.
posted by glip at 8:09 PM on January 31, 2010

Like adamrice, I think Mcmaster is a godsend, and eventually everyone who ventures any distance down the project path takes a trip through their catalog. However, after re-reading your question, I think their structural framing pieces are probably closer to what you are looking for...though still not perfect.

The thing is, even though your stuff isn't going to be high in the air, these joints are going to subjected to considerable stresses. You really need someone with engineering experience to crunch the numbers for you and make sure you aren't asking your home-made joints to do more than they are capable of doing. Weather -- especially heat and cold cycles, wind, and snow (if your area gets it) can play havoc with this sort of thing.
posted by mosk at 8:53 PM on January 31, 2010

How many of these connectors do you need? It would be a fairly simple task for a welder to make the connectors given a length of suitably sized tubing. The cost of having this done would be reduced if you laid out and cut all the pieces so they only needed to be aligned and welded.

PS: I don't think you necessarily need an engineer. A single story or less of construction, even of essentially a sail, isn't any more difficult than say a regular garden shed or tall solid panel fence. No one gets an engineer for those kinds of projects. Just make sure you have plane triangulation in your construction to prevent buckling/twisting rather than relying on the strength of the connectors.
posted by Mitheral at 9:36 AM on February 1, 2010

see also the 80 20 company or just google "T-slot aluminum extrusion"
posted by at at 11:40 PM on February 1, 2010