Bearing for "art" thing?

[julianf]

The explanation -

I've got a bit of old cast iron well pipe.

My plan is to turn up a block of somthing to fit in the end of it that will have some sort of bearing in it.

On the upper side I want to attach some bent rebar to an offset object.

The job will only ever be turned by hand and probably only ever 1/4 of a rotation a day. It's not an engine.

The load, at an estimate, may be 15-20kg. So somthing but not loads.

Offset (which is, I realise, the bit that creates the moment) maybe 40cm.

I can't remember how to calculate moments. But that sort of twisting levering force isn't what most bearings are supposed to sustain. But either way, we are not talking huge forces.

What's my best plan? Cutting down a wheel hub will be a bit on the large side. I don't really want to spend a million pounds either otherwise it will never get done.

Thank you.

 

[wyn]

A couple of "oilite" bushes top and bottom.

 

[James1979]

I was going to suggest a piece of delrin or nylatron or similar about 100mm long with a snug ish hole bored in it.

It won’t rust then.

Moment =force x distance
=200N x 0.4m
= 80Nm

Edited because i forgot to convert to Newtons

 

[merryman]

Doesn't matter what the bearings are, but I recon you need at least a 25mm dia shaft up the middle to dangle your "thing" from. So if pipe isn't much bigger, use bushes or needle roller bearings. If you need to take up more space, ball races. Cheapo ones will do for that job, plastic is OK but if the shaft is plain steel and outside, it will rust and turn into a round file. Incidentally roughly 80nm (kg x gravity = N).

 

[julianf]

Pipe is cast iron, will be done in some sort of curing oil, so rust look but no extra rust.

It's probably about 100mm dia.

 

[merryman]

It isn't the cast pipe that is the rusting problem. If it is going outside, the shaft and bearings will need to be protected against rusting or made of stainless. This can be as simple as a cap that keeps the rain off the shaft and bearings. Something to take the difference between the pipe bore and the bearings OD is going to be a problem too.

 

[James1979]

It isn't the cast pipe that is the rusting problem. If it is going outside, the shaft and bearings will need to be protected against rusting or made of stainless. This can be as simple as a cap that keeps the rain off the shaft and bearings. Something to take the difference between the pipe bore and the bearings OD is going to be a problem too.

That was my thinking with just a non metallic bush.

It shouldn’t need much more than that. If you were keen you could drill and tap for grease nipples i suppose.

 

[merryman]

I work on quads and find the polybushes used in some bike A arms don't last, because despite rubber seals to keep water out, it gets in and turns the inner race of the bush into the aforementioned round file.

 

[Johnnybravo]

I make windsock rotators for a company that exports them worldwide. I’ve got an early prototype that was further developed into to what they are today. 100mm stainless steel bearings in an all stainless housing. £25 incl postage and it’s all yours.It’s shielded from the weather and tested in Artic conditions…errr…no …not an artic in a Tescos car park It’s been painted,but that’s easy enough to remove…

 

[julianf]

It's indoor only.

 

[optima21]

glad to see this thread isn't about making naked calendars for charity

 

[Maker]

But that sort of twisting levering force isn't what most bearings are supposed to sustain. But either way, we are not talking huge forces.

A single bearing won't be happy with a moment load like that, but if you have two bearings with some space between them the force they see is just a radial load, like they're designed to take.

With two bearings 6" apart, that 80Nm puts a load of about 1KN on each bearing, a 608 skateboard bearing would handle that.

 

[julianf]

glad to see this thread isn't about making naked calendars for charity

You assume the worst - it may not have been me doing the baring!

 

[julianf]

A single bearing won't be happy with a moment load like that, but if you have two bearings with some space between them the force they see is just a radial load, like they're designed to take.

With two bearings 6" apart, that 80Nm puts a load of about 1KN on each bearing, a 608 skateboard bearing would handle that.

Would it work like that? I'd think the top bearing would take more of the force? I mean I'm not saying that matters, but just that I'm not sure it's a simple "divide by two"?

 

[Maker]

Would it work like that? I'd think the top bearing would take more of the force? I mean I'm not saying that matters, but just that I'm not sure it's a simple "divide by two"?

Yeah it's split evenly, if you consider the top bearing to be pushing the shaft to the left, and the bottom bearing pushing right, there's no other horizontal loads on the shaft so the forces from the two bearings have to be in equilibrium.

My number was wrong though, it's a load of 500N on each bearing.

 

[bricol]

At that rate of rotation, and indoors, it doesn't need to actually be a bearing does it?

A bit of bar/pipe/tube for your shaft, sat in a couple of clearance holes through blocks of anything, steel, iron, wood shoved in the top and part way down your well pipe?

 

[julianf]

Yeah it's split evenly, if you consider the top bearing to be pushing the shaft to the left, and the bottom bearing pushing right, there's no other horizontal loads on the shaft so the forces from the two bearings have to be in equilibrium.

My number was wrong though, it's a load of 500N on each bearing.

I'm not saying your wrong, I just don't understand -

Wouldn't the bottom bearing be the pivot point, so the top bearing be taking more force?

Rather like if I stick a crowbar under a paving slab - the force between the slab and the bar is greatest on the edge of the slab, isn't it?

 

[julianf]

At that rate of rotation, and indoors, it doesn't need to actually be a bearing does it?

A bit of bar/pipe/tube for your shaft, sat in a couple of clearance holes through blocks of anything, steel, iron, wood shoved in the top and part way down your well pipe?

Probably. But bearings are fancy and it gives an impression of engineering..

Or somthing like that!

 

[Maker]

Wouldn't the bottom bearing be the pivot point, so the top bearing be taking more force?

Why shouldn't the top bearing be the pivot point and the bottom one take the force in the other direction?

Rather like if I stick a crowbar under a paving slab - the force between the slab and the bar is greatest on the edge of the slab, isn't it?

Yes, but the force between the kink in the crowbar and the ground is the same as the force between the bar and the edge of the slab