Repairing sheared tube

[clowes_ian]

Hi

I have a strut on a pram that has sheared. It's at a 'springy' point but isn't particularly load bearing. The fracture hasn't caused any distortion to the tube, and the two halves still mate nicely.

Main problem is that the fracture is just at the end of a curved section, so making a suitable plug to tap into each end won't work.

The tube is aluminium (or an alloy), so 'simple' welding doesn't seem to be an option for repairing it. The tube is also one of several welded into a reasonably complex array, and so isn't easily detachable for popping some type of replacement in.

I have thought of two ways of repairing it:

1) Cut the tube off at a lower straigh point, make up a replacement section and join to the original: a) with suitable plug/dowel; b) by welding old to new. From these two I'd prefer 1a as I can just about do all of that myself.

2) Align and secure the two parts, then fill with some suitable epoxy or meltable resin. Once set the resin would need to have the strength and durability under flexing to keep the joint stable.

Can anyone comment on the merits or otherwise of these suggestions, or contribute more?

In case it helps, the tube OD is about 22mm, with a wall thickness of about 3mm.

TIA IanC

[:::Jerry::::]

So why has it fractured ?....

[BigWallop]

A piece of tubing of a similar size with a slot cut along its length to form a wrap round would probably do the job. Fitting jubilee clips (hose clips, elephant clips) or drilling through each end and fixing with self tapper screws or rivets might be a good way of holding it in place tighter.

You could also just place strips of metal along two sides of the tube, drilling through both metal and tube, and then fixing in place with self tapper screws or rivets.

[clowes_ian]

Because its flexed a lot? I can understand that it takes an amount of force to flex it, but that isn't (to my mind) the same as it being under a compressive or tensile load. I'm happy to be corrected on this if I'm misunderstanding the cause of failure.

Cheers IanC

[Ian Stirling]

It's exactly the same. To bend a tube, you've got to compress one side, and stretch the other. Consider that when bending a tube, you are in effect shortening one side and stretching the other. This may be clearer if you think about how much force you'd usually need to use to make the tube stretch/shrink by that much.

If the ends are sort-of held in place by the rest of the structure, then you'll get a fracture at one point, that then opens and shuts, with the crack moving along the diameter of the pipe till it's through.

[Malcolm Race]

Was going to suggest that - did it with a McClaren Buggy 20+years ago using jubilee clips and it worked very well - in the end there were more splints, jubilee clips and new wheels than I could count. The only problems in your example seem to be the curve and the ability to flex, although I cannot imagine how a plain 22mm tube could flex.

Malcolm

[Rob Morley]

How about this: Take a piece of aluminium mesh (the sort you use with car body filler for bridging holes) and roll it up so it's a snug fit inside the tube and several layers thick. Jam one end of the rolled mesh into the curved tube section so it takes the right shape (ish), then remove it. Now lag the inside of the curved tube and the curved half of the mesh with epoxy and jam it back together. Try to get plenty of epoxy through the mesh so it bonds all the layers. Then glue the straight piece of tube and the other end of the mesh. Although the ultimate strength of the repair won't be great it should be adequate, and will hopefully reduce flexing and the chance of a further fatigue failure.

[Cicero]

============== An aluminium tube of 22mm od and a wall thickness of 3mm is quite a strong tube. If such a strong tube breaks it suggests a possibly dangerous design fault. I would ask the manufacturers for a repair and if they are difficult ask Trading Standards to intervene. Children have died in prams which unsafe.

Cic.

[Pete C]

How about a plug of smaller dia tube held in with JB Weld? Or use a polyurethane sealant/adhesive which would absorb some flexing.

cheers, Pete.

[The Natural Philosopher]

If you want it pretty, make a plug of e.g., carbon fibre or galss starnds and car body filler and sover it up bokth while clamoping.

Otherses wrap in fibergalss drenched in epoxy, or solder it with appropiate flux and solder.

[clowes_ian]

Hi

This is the way I'm going at present, using a length of 12mm mild steel stud bent to follow the curve.

This leaves a gap of about 2mm all around that I'd like to pour a suitable glue or resin into, drive out the air pockets and hope it'll all hold.

In the end, the adhesive/resin will (I think) need to flex to transfer the load from the pram chassis into the steel, and not fracture too much itself and hence fall away from filling the void effectively.

Is JB Weld pourable, or at least 'pokeable' into position with a bit of wire to fill the voids? I'm not at all familiar with polyurethane adhesives: would they (or something else) be better?

And finally, I'd like to pop out and get the product tomorrow or Monday. Suggestions for suppliers for more specialist products would be useful.

TIA IanC

[Newshound]

Araldite or similar epoxy won't flow down a gap like this. But if you arrange the joint so that it is vertical, put a fillet of freshly mixed araldite over the gap, then heat the tube below with a hot air gun the araldite will liquify and flow down the gap (rather like end-feed soldering) before setting. You will need to keep feeding fresh mix to get enough volume (i.e. penetration). Better to use 24 hour than 5 minute araldite for this. You will need to do the two "directions" separately. That said, I have never had much success in repairing buggies: the stresses are high at the joints and the alloy tubes very prone to fatigue.

[clowes_ian]

Hi

Did exactly that and it worked like a charm, better than I would have hoped. I warmed the work pieces before hand, but the araldite flowed like oil after a gentle warming so am not even sure this was necessary.

It feels like a very sturdy repair, but time will tell. Thanks for the help.

IanC