What bearing possibilities?

[BrowserUk]

What methods are possible for "bearings" for a 120mm diameter x 50mm rim-driven, HUBLESS rotor?

Mass: ~128.5g
RPM: 2000-3000
Forces:
Radial: its own (well-balanced) mass x angular velocity.
Axial: ~500g in one direction.

Note: Besides the ludicrous costs, stuff like this is excluded because of weight.

Looking for ideas, possibilities...

 

[SwinnyGG]

Give us a sketch of what you're trying to accomplish here and you'll get better answers. I'm not picturing it.

 

[BrowserUk]

Here you go:

The rotor is driven via the external gear teeth.

It need to be supported by some external arrangment to bear rotation and axial forces in one direction.

Ideas?

 

[SwinnyGG]

Oof. You've designed yourself into a corner here.

Thrust is easy at 500g. A relatively inexpensive thrust bearing and washer set will do that easily.

From what I can tell your real problem is that your bearing needs to not only retain this impeller-looking-thing but also provide for rotation. For this arrangement you can choose from any two of: simple, light, cheap.

What exactly is this impeller doing? Driving a device like this, from the OD of a spinning part, is not conventional for a reason.

 

[SwinnyGG]

How much power are you designing this impeller looking thing to transmit

 

[BrowserUk]

They are more common than you think: Rim driven thrusters

 

[SwinnyGG]

Big difference between building one to transmit 500kW and one to transmit power on the milliwatt scale.

If this is going to be immersed in water, I'd start with lubrication-free solid state bearings (ie.. plastics) and see if that gets you close. Immersion in water basically eliminates any off-the-shelf sealed bearing in this size range.

 

[BrowserUk]

Not water, air.

 

[GregLocock]

How about an air bearing?

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[SwinnyGG]

If you're not immersed in water, can you provide lubrication?

The only way to do this cheaply will be a solid bearing of some type, which won't last for all that long. But if proof-of-concept is all you need, you don't need long life.

 

[3DDave]

Why not include the inner race as part of the rotor and create your own outer race? Pack in some ball bearings; the loads for that size are very low; 6-8 mm balls will be more than enough.

 

[BrowserUk]

A couple of ideas I've already had:

The pink balls are loose Delrin 3mm balls. Needs a cage or low-friction spacers of some kind.

The 3 'gear bearings' @ 120° have (say) 5x16x5 minature deep groove bearings top and bottom.
Two are fixed position, 1 spring loaded or adjustable. (1 drives the rotor).

The flat cylinders above and below the gearteeth are pitch circle diameter.
The teeth are half inset half proud:

I didn't want to mention these ideas, because I wanted to ellicit others.

 

[BrowserUk]

GregLocock:"How about an air bearing?"

The rotor is to be 3D printed (UV resin mLCD), which has a basic pixel size of 0.05mm. Air bearings require much finer detail.

Greg Locock:"New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies

http://eng-tips.com/market.cfm?

"

Did I infringe some rule?

 

[BrowserUk]

3DDave:"Why not include the inner race as part of the rotor and create your own outer race? Pack in some ball bearings; the loads for that size are very low; 6-8 mm balls will be more than enough."
If I went for a full compliment ~66 per race, of 6mm steel balls, it would weight roughly 0.25kg, which would kill the weight allowance for batteries.

Using a sparse arrangement -- (say) 18/race -- requires a cage or spacers and its difficult to see how to make that work mechanically.

 

[3DDave]

So, how fast will those be rotating?

Don't use steel or use fewer with a generous separator. There are other ball bearing materials. And it's too bad you didn't mention the weight problem earlier.

 

[SwinnyGG]

Are you trying to build a drone using this arrangement instead of normal shaft driven rotors?

All you've told us so far is that this thing will move air and has some relationship with a battery

The more detailed your problem statement, the more detailed help you're going to get

 

[GregLocock]

@OP, no that's my standard sig


Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[BrowserUk]

3DDave:"So, how fast will those be rotating? "

2000-3000rpm (as in OP)

3DDave:"Don't use steel or use fewer with a generous separator. There are other ball bearing materials."

I've looked at Delrin balls, which is 1/7th weight; but about 10x the price

Using spacers is problematic because of the extra friction.

3DDave:"And it's too bad you didn't mention the weight problem earlier. "

Sorry. I implied it with the mention that 120mm crossed-roller bearing were too heavy.

 

[BrowserUk]

SwinnyGG:"Are you trying to build a drone using this arrangement instead of normal shaft driven rotors?"

I specifically do not want to get into the overall project. The design is coming along nicely, and isn't open to change.

If you'd like to see some stuff about a related design (not mine), then you could seek out the paper
"Aerodynamic Analysis and Design of a Rim Driven Fan for Fast Flight" [Bolam et al.]

But this bearing issue could use more consideration and a wider audience.

 

[3DDave]

"How fast will THOSE be rotating?", not "How fast is THAT rotating? " THOSE, as in plural, as in the multiple small bearings.

Yes - the metal races of a metal bearing are heavy. Not saying that no weight at all is allowable is different.

Now you have a major problem with cost? Welcome to engineering.

There's not a lot of extra friction from spacers, which is why they are used. Unless you also have no remaining budget for that either.