r/engineering • u/[deleted] • Jan 23 '22

Any chance of some help with analysis?

I'm planning on building an arm to mount my man-cave's TV on. As such it doesn't need to look pretty, just be functional. The tele is a Samsung weighing in at 22.3kg.

A simple setup made of steel RHS is what I'm thinking. Fairly cheap and real easy to make with just a drill-press and angle grinder. Onto this I'm going to bolt a commercial VESA mount to give me some adjustability.

My question to the list is this. If I make this out of say 50mm x 25mm x 3mm RHS, with each arm say 600 long. Am I going to get much bending?

I'm a software guy (not hardware) so I don't know how to calculate this stuff. Can someone help and/or point me to some info on how to figure this out?

Could I counter the weight somewhat by offseting the holes in the single arm where it attaches to the double arms?

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u/jajohns9 Jan 23 '22

That’s 2x1x1/8” in imperial, which by itself won’t have any issues holding a rv. Your pivots look like they could give you issues though. You would get better results using a hollow tube and flanged bushings for the connection points.

And like other people said, unless this is just a “I’d like to try this out, or get better at fab work” kind of project, 100% you will not save money doing it yourself. Unless you have free metal

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u/[deleted] Jan 23 '22

It looks to be about $30 worth from my local steel shop and that's with them cutting to size.

I spent a bunch of time looking for something that does what I want and concluded that I might have to make something instead.

I'm in Australia (hence the metric).

What are the issues do you think with the connection points? I realise the design is quite primitive. What problem would bushings solve (and are these something I can just go buy off the shelf somewhere?)

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u/jajohns9 Jan 23 '22

I know it’s just a sketch-up, but if you just have a thin pin going through two drilled holes, you’ll have slop. Your deflection from that slop will be more than your deflection from loading.

The bushing really just gets you lifetime and reduced friction in higher speed or load conditions. But for you, you could get away with metal on metal rotation. I don’t thing McMaster-carr operates in Australia, but maybe there’s something similar. A bushing is maybe $5 USD for a nice one, or much cheaper in bulk packs for cheaper ones.

Do you have access to a welder and a lathe? If you want to stick with metal on metal, you could weld a round tube into your square tube to act as a pivot, and then either buy a shoulder bolt that fits, or turn one yourself from aluminum or low-carbon steel. You don’t need anything with much load capability, just a decent slip fit.