Good Bearing for a Low Friction Turntable?

[runningman19]

Hi Everyone,

I am working on a project which involves building an extremely low friction turntable. To give you an idea of how low, we're placing a tank full of water on top of the turntable, placing a stirrer into the tank, turning the stirrer on and trying to get the tank to rotate on the bearing. A good estimate for the torque in the stirrer rod is about 10^-1 N-m, so the torque is extremely small.

I had initially purchased a 3mm thrust bearing for this purpose, but the tank is about 1 ft in diameter. When I attached the thrust bearing to the plywood disk which the tank was to be placed on top of, there was too much wobble for the turntable to be usable. We also tried a "lazy susan" bearing from Walmart, which was a good size but way too high friction to be useful.

Any suggestions for axial load bearings which are large enough to accommodate a 1 ft diameter tank, but are under $150? I considered something called a flange bearing, but have no idea how appropriate they would be for axial loads.

I have attached a diagram below to give you an idea of what we're doing.

Thanks for the help,
Nick

 

[trurle]

I do not think the problem is solvable with solid bearing. You are off the specs by 10-100 times.
You need something like "air bearing rotary table"

 

[berkeman]

+1 on the air bearing. It should include a circular lip just a bit wider in diameter than the circular bearing/table, to keep the horizontal motion of the turntable contained. If you maintain fairly close tolerances in the construction, you should be able to get away with a small compressor as the source of the compressed air...

 

[runningman19]

I do not think the problem is solvable with solid bearing. You are off the specs by 10-100 times.
You need something like "air bearing rotary table"

I looked into it but they are very expensive (along the lines of $4000). From what I’ve heard you can build them, but how? Is there literature where I can learn more about this?

 

[berkeman]

Using a 3-D printer and a good drill press in your university's ME shop...?

 

[Dullard]

If I had a $150 budget, I'd consider just 'floating' the tank in a slightly larger bucket.

 

[berkeman]

If I had a $150 budget, I'd consider just 'floating' the tank in a slightly larger bucket.

What would be a good fluid (other than air) to float the tank in? Some fluid with a very low viscosity, or water with some viscosity-lowering additive like soap?

 

[Dullard]

I was thinking water. I was assuming (incorrectly, perhaps) very low speeds.

 

[runningman19]

Actually the tank wouldn’t be rotating at all. We’re attaching a fish scale to the turntable to measure the impeller torque. The tank should just be able to rotate freely, it doesn’t actually need to move.

That is a good suggestion. I’m going to give it a shot.

 

[trurle]

What would be a good fluid (other than air) to float the tank in? Some fluid with a very low viscosity, or water with some viscosity-lowering additive like soap?

To my experience, water as support medium is possible, but is very unforgiving for even slight dimension mismatches, including mismatch of placement of central stirrer. Unlike compressed air in lipped table it does not provide self-centering forces (lipped air table is stabilized in XY plane by Bernoulli`s pressure drop). Also, water-supported tank may be unstable against tilt.

 

[Dullard]

I agree that a well-executed air bearing would be superior. I could probably even build one for $150 (if I ignore the cost of my CNC lathe, mill, etc). A 'precise-enough' wooden bearing is also possible, but would require uncommon tools/skills. I think that a bucket with sloppy centering guides (in-line skate wheels, maybe) could be made to work for < $150. Some fiddling required.

 

[Baluncore]

You do not need a bearing under the tank if you hang the tank from a support above. The lowest torque suspension employs a thin tape or ribbon.

Alternatively, if you hang the tank from three long vertical threads that attach to the rim, any torque induced in the tank will move those threads circumferentially out of the vertical. Weigh the tank first to get the vertical component of thread tension. The horizontal component in the threads will be the induced torque. Measure the circumferential movement of the thread attachment points once the stirrer is turned on. Knowing the length of the threads will then give you the angles. Don't forget to multiply the torque by three if you use three threads.

 

[Dullard]

I like that. The system may be 'ranged' by lengthening/shortening the threads A/R.

 

[CWatters]

Actually the tank wouldn’t be rotating at all. We’re attaching a fish scale to the turntable to measure the impeller torque. The tank should just be able to rotate freely, it doesn’t actually need to move.

That is a good suggestion. I’m going to give it a shot.

Will the fish scale pull the tank sideways? You might need two?

 

[runningman19]

To my experience, water as support medium is possible, but is very unforgiving for even slight dimension mismatches, including mismatch of placement of central stirrer. Unlike compressed air in lipped table it does not provide self-centering forces (lipped air table is stabilized in XY plane by Bernoulli`s pressure drop). Also, water-supported tank may be unstable against tilt.

What do you mean by stabilized in XY plane by Bernoulli’s pressure drop? How does this act as a stabilizing force?

Sorry if it’s a dumb question; I’m not familiar with air bearings.

 

[runningman19]

Alternatively, if you hang the tank from three long vertical threads that attach to the rim, any torque induced in the tank will move those threads circumferentially out of the vertical. Weigh the tank first to get the vertical component of thread tension. The horizontal component in the threads will be the induced torque. Measure the circumferential movement of the thread attachment points once the stirrer is turned on. Knowing the length of the threads will then give you the angles. Don't forget to multiply the torque by three if you use three threads.

Placing a thread in the center of the tank will not work, as the impeller and impeller shaft are there. The three thread idea, however, would work. I think you’re saying we should determine the vector of the vertical component without the motor spinning, then start the motor spinning, measure the change in distance between where the tank started and where it ended (due to the fact that it will rotate slightly when a torque is applied) and then calculate the torque from the known angle and vertical component.

The problem is that our tank weighs about 100 lbs. Will the horizontal component be so small that it is impossible to measure because of this?

 

[Baluncore]

You understand the principle. A single tape suspension would need a spreader above the stirrer to support the tank below.

The problem is that our tank weighs about 100 lbs. Will the horizontal component be so small that it is impossible to measure because of this?

No problem. Do not use threads, use longer parallel wires or tapes on a smaller radius circle to get a significant rotation angle.
Fish scales will be much less sensitive and more eccentric than the parallel thread suspension method.