Connecting motor shaft to thrust bearing

[aed]

Hi there -

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), but the problem is still there.

Do you have any idea what could be going on here, and a good way to fix this?

 

[Q_Goest]

Hi aed, welcome to the board,

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.

Sounds like the center of your motor shaft is not perfectly aligned with the center of your bearing. When you attached these acrylic pieces to the thrust bearing, how did you align them so the bearing and motor shaft were concentric?

 

[aed]

Thanks Q_Goest for your answer!

I had the pieces of acrylic laser cut so that the hub would be as concentric as possible to the thrust bearing. I did this by measuring the thrust bearing, hub, and then plotting them in inkscape so that the shaft is at the center of rotation. (I had previously tried drilling them myself but that also resulted in it getting stuck. To do that I drilled the 4 points of the thrust bearing onto the acrylic sheet, found the center using a ruler and drilled around that.)

Is there a better way to do this?

Also, I am guessing that even with accurate cuts there will be some displacement from either the hub not being exactly orthogonal to the shaft, or it not being in the exact center of the thrust bearing. Is there something I can do to mitigate this (e.g. some component that I can put in between that will be more forgiving to these displacements?)

I've also attached a quick schematic I drew up of the system.

Thanks again.

 

[256bits]

You do realize that these turntables are not precission rotating instrumnets. They are fabricated out of pressed sheet metal and the bearing ball races are not machined, and will have hills and valleys in the races. They do not have 0,001 tolerances, I bet more like 0.1 offhand.

Besides the concentricity issue, there is also the fact that by your design you have made a solid connection of the shaft to the hub, and subsequently any play that needs to be there for the balls to rotate in the races is not there any more and the 2 mating parts cannot wobble or play around with one another as they rotate.
Try to replace the set screw connection to the shaft with a flexible rubber tube connection and see how that goes. Put some rubber washers between motor and hub and acycrilic and see how that goes. My point being that you have a rigid connection and you need to make it more flexable.

 

[aed]

Thanks for the advice 256bits. I did some research after reading your reply for flexible couplings and ordered a helical beam coupler as well as an oldham coupler to try them out. I'll also put some rubber washers as you recommend. For flexible rubber tube connection, did you mean buy some rubber tube, cut it, then glue it or are there premade components for this?

thanks

 

[256bits]

If making your own flexible coupling would work, than that is just as fine as buying one.
A short section of rubber air hose that you see at a garage to pump up your car tires might have the required inner diameter to fit nicely and snug on your shaft diameter to try out at first to see what it does, but it won't accept much torgue being just plain rubber with not that much re-inforcement. I was mentioning some quick fixes so you could experiment a bit on how flexible the connection has to be before deciding on a more permanent solution for your project.
With glue it would br kinda difficult to diassasemble unless you would be sure that would never have to happen.
A helical beam coupler acts just like a stiff spring so depending on its stiffness should work fine.
I am sure there are other options as well.

 

[aed]

Thanks again 256bits. I'll take a shot at it.