Connecting motor shaft to thrust bearings

[aed]

Hi there -

As part of a newbie project, I built a small electronic turntable with the following components: 1- Arduino 2- Sparkfun easy stepper driver 3- Stepper motor 4- Servocity set screw hub. 5- Thrust bearing from mcmaster-carr (square turntable, found here http://www.mcmaster.com/#swivel-plates/=eltxix) 6- Perforated pieces of acrylic attached to both sides of the thrust bearing. I attached the set screw hub to one side and tied down the motor to the other side. The set screw hub is attached to the shaft of the stepper.

The problem I've been seeing is that even with nothing on it, after a quarter of a rotation or so the thrust bearing will start making noise and get stuck. I am not sure but it looks as if the bearings may be getting "squished", probably from some misalignment. The motion overall is not nearly as smooth as when I make the same connection with no thrust bearing. I tried getting a much more powerful stepper motor as well (this one: http://search.digikey.com/us/en/products/PG20L-D20-HHC0/P14334-ND/2417058, 450mNm torque), and I also rebuilt the project from scratch by taking great care in measurements, but the problem is still there.

I believe it may be due to the way the shaft is connected to the bearing (i.e. using a set screw). Do you have any ideas as to what would be a good way to do this?

Thanks for your help!

[Soeren]

Hi,

5- Thrust bearing from mcmaster-carr (square turntable, found here http://www.mcmaster.com/#swivel-plates/=eltxix)

That is not a thrust bearing, it's an (unprecise) swivel plate with no real control over ball position.

The problem I've been seeing is that even with nothing on it, after a quarter of a rotation or so the thrust bearing will start making noise and get stuck.

Oh, they should be better than this. How is it if you turn it by hand?
You might have gotten one with the plates a tiny bit non-parallel. If it was like that when you got it, return it for a replacement.

I am not sure but it looks as if the bearings may be getting "squished", probably from some misalignment. The motion overall is not nearly as smooth as when I make the same connection with no thrust bearing.

Personally, I'd get a real thrust bearing where each ball (or needle) is caged.

I tried getting a much more powerful stepper motor as well [...] but the problem is still there.

When a bearing won't work, exchange the bearing. Powering up won't repair a faulty bearing.

I believe it may be due to the way the shaft is connected to the bearing (i.e. using a set screw). Do you have any ideas as to what would be a good way to do this?

Hard to say without an idea of how it's connected (a photo/sketch would help), but if you need a slightly flexible connection, a short piece of tube/hose may help.

Any bearing should be very easy to turn by a single finger. Don't connect anything up until you have a bearing that turns smoothly and with minimal force.

What is supposed to go on top of it and what is the weight of this load?

[aed]

Hi Soeren,

Thanks so much for your reply. To answer your questions:

How is it if you turn it by hand?

I am able to turn it by hand when it's not connected to anything and the motion is pretty smooth.

Personally, I'd get a real thrust bearing where each ball (or needle) is caged.

Can you please point me to what you'd consider a real thrust bearing so that I give it a shot? I picked this one because it was easy to screw onto acrylic sheets.

a photo/sketch would help

I've attached a quick schematic I made in paint (excuse the newbieness). My gut feeling is that the motor is either not spinning perfectly orthogonal to the sheets, or that it is not exactly at the center of the swivel plate. Also, adding a load might be exaggerating this. I did get those acrylic sheets laser cut to what I think are accurate dimensions (since I was having the same problem when I drilled them myself). What would be a good way to fix this design?

What is supposed to go on top of it and what is the weight of this load?

The load should be able to go up to 50lbs. I've seen the stepper motor I have spin that amount over a quarter circle or so before getting stuck (but then again it also gets stuck on no load). Also, this should be able to turn in small angle increments (i.e. it's a very low speed thing) - for example the stepper motor I have has a 1.8degree stepping angle and it would be ideal if the system as a consequence was able to turn in these increments.

Thanks again for taking the time to answer this.

[waltr]

From what I see a guess is that the stepper shaft is not exactly centered and/or that the turntable bearings are not exactly circular/concentric.

Try what Soeren suggested and use a flexible coupler between the stepper's shaft the the upper plexiglass disk.

[Soeren]

Hi,

I am able to turn it by hand when it's not connected to anything and the motion is pretty smooth.

Then it's probably allright.

Can you please point me to what you'd consider a real thrust bearing so that I give it a shot? I picked this one because it was easy to screw onto acrylic sheets.

You can see some here, but they will be harder to mount and , as mentioned, I think the one you have will do.

I've attached a quick schematic I made in paint (excuse the newbieness). My gut feeling is that the motor is either not spinning perfectly orthogonal to the sheets, or that it is not exactly at the center of the swivel plate. Also, adding a load might be exaggerating this. I did get those acrylic sheets laser cut to what I think are accurate dimensions (since I was having the same problem when I drilled them myself). What would be a good way to fix this design?

Either something flexible, or you could mount an internal gear on the upper part and get a further speed reduction if you'd like that - you need a fairly substantial motor to turn lbs50 without losing steps, unless you gear down (to increase torque.

The load should be able to go up to 50lbs. I've seen the stepper motor I have spin that amount over a quarter circle or so before getting stuck

And it didn't loose any steps?

(but then again it also gets stuck on no load). Also, this should be able to turn in small angle increments (i.e. it's a very low speed thing) - for example the stepper motor I have has a 1.8degree stepping angle and it would be ideal if the system as a consequence was able to turn in these increments.

Gearing down will give you even finer resolution and more power.
I sounds like a TV stand rotator, but I cannot imagine anyone needing that kind of precision angling a boob tube 

If you don't like the idea of making a coupler of a bit of hose, you could use a timing belt to transfer the motor power to the upper "plate", timing belts are quite forgiving about small misalignments.

[aed]

Thanks for the very helpful answers Soeren and Walter.

you need a fairly substantial motor to turn lbs50 without losing steps, unless you gear down (to increase torque).

The stepper I got has a gearbox on it already mounted (got it from digikey). When spinning by itself it doesn't seem to be losing steps (not that I can tell for sure at this resolution ), but when mounted to the bearings it does lose plenty of steps.

For a hose made coupler, is the attached what you had in mind? From a search on the internet for this it looks like "helical beam couplings" somewhat meet this criterion. Are there any "standard" flexible couplers that do this or were you thinking more of actually cutting a piece of tube and then gluing it to the surface and shaft?

Thanks again!

[Soeren]

Hi,

For a hose made coupler, is the attached what you had in mind? From a search on the internet for this it looks like "helical beam couplings" somewhat meet this criterion. Are there any "standard" flexible couplers that do this or were you thinking more of actually cutting a piece of tube and then gluing it to the surface and shaft?

Around an inch of rubber (or similar) hose with a suitable flexibility should do, but if you wanna by something ready made, it's up to you of course.
A short length of steel wire (depending on diameter) will flex as well, but in RC hobby, a bit of tubing is/was often used to couple eg. a boat motor to it's propellor, to counter for the usual offset in the axles.

If you find some hose that fits very tight on the motors output axle (add some glue and perhaps a Tie-Wrap when all is tested and ready) and a piece of metal of the same diameter to go into the other end (leave the middle part "empty" and able to flex), you can tighten the set screw in the hub without the hose just collapsing.
By adjusting the amount/length of "empty middle", you adjust the amount of flex in the system.

[aed]

Thanks for this insight Soren!! I'll give it a shot