Bearing selection for vertical shaft

[SophieLiz]

Hi,

I have a chair with a vertical shaft that is rotated by an electric motor (low speed <100 RPM) Both shafts are small, mounted vertically, and connected by a flexible coupling, as in the attachment. I intend to use a bearing to support the weight of the chair. Although the thrust load is small (chair weight + person), there might be an eccentricity between the axis of rotation and the center of gravity of the set. Would you recommend me to use a thrust bearing or a angular contact bearing (or another type)? For this short shaft application, would a single bearing be enough?
Another question: once gravity keeps the chair in place, is there a need for a locking nut to hold the chair shaft to the structure?

I would very much appreciate your tips.

 

[MikeHalloran]

Push yourself back from your desk and spin your chair around as fast as you can.

~100 rpm is _way_ too fast to spin a person.
They will become dizzy and disoriented very quickly.
Next, they will grip the chair, and probably exert a large overturning moment on it as they are/not falling off of the chair.

You should have learned well before graduation that shafts with three bearings are a generally bad idea.
Similarly for shafts with one bearing.
And that locking devices should be standard for anything that moves.

In your case, you didn't provide enough detail about your coupling. It is sometimes possible (but not good IMHO) to use, e.g., a rigid shaft coupling, such that the motor/gearbox bearings take some of the load applied. ... but most default motor/gearbox bearings are not suitable for much thrust or much external radial load. Check the thrust rating and the 'chain pull' rating of the gearbox's output shaft.





Mike Halloran
Pembroke Pines, FL, USA

 

[Tmoose]

RE: Mike's suggestion about a rigid coupling.

There are radially stiff couplings that are flexible in bending, some that are axially flexible as well.
Thomas/Rexnord disc couplings are one type.

http://www.rexnord.com/ContentItems...omas-Flexible-Disc-Couplings_Catalog-pdf.aspx

They can be used to transfer load to the motor front bearing and restrain the shaft, serving as the "second" bearing for a motorized bar stool's upper shaft.

 

[SophieLiz]

Good mornign Mike,

Thank you for your reply.

I said below 100 rpm, not 100. I know that is too fast for a person, I have used it myself. I just wanted to say it was low rpm My mistake using such a high boundary for low RPM...

I am not a mechanical engineer so bearing and shaft design, apart from load calculation and distribution, is not my area. I am studying them now in more detail because of this project. That is why I am seeking the advice of more experienced people in the field
My question about the single bearing comes from some study I have made on the subject where some references mention that solution for a short shaft. My first thought was using a two-bearing solution but in this case the space is limited and it would mean having the two really close together, which I think is not the most effective solution right? That is why I thought using a thicker bearing that would restrain the shaft more.

The bearing is a Rexnord Elastomeric Viva. The thrust load is all taken by a support structure.

Tmoose: thank you for the suggestion! I will check it.

 

[Tmoose]

"The bearing is a Rexnord Elastomeric Viva. The thrust load is all taken by a support structure."

I believe Rexnord Elastomeric Viva is a flexible elastomeric coupling.
It won't offer any useful radial support to upper shaft. The upper shaft's bearing will need fairly significant moment capacity.
What bearing were you planning on using?

 

[gruntguru]

If you need to manage with a single bearing you could try a turntable bearing as used in "Lazy Susans". Link

je suis charlie

 

[SophieLiz]

Tmoose,

I thought about NKIA 5904 or 7304-B.

gruntguru: I had checked the Lazy Susan in my research. I thought about it as a thrust bearing for the chair and then the shaft (20 mm diameter)would go through the middle of it and would still need another bearing for radial support. Am I thinking wrong?

 

[gruntguru]

A lazy susan bearing would offer some radial load and location as well. The radial load capacity will increase in proportion to the axial load applied.

je suis charlie