A Cheap Way to Balance a Turntable

"Spool the disc up to running speed and apply a marker to the edge lightly to find the" high/heavy spot..." --- How do you tell high/heavy spot when spinning?
2nd Q: If you manage to identify the heavy spot, by adding a magnet piece to that spot, will it not make the imbalace even worst?

You add the weight to the opposite side of the mark on the disk.

Although, I would be very weary of any magnet spinning on a disk at 1300 RPM as it could easily become a missile.

1,300 RPMs at a distance of 16 cm from the radius (100 cm circumference) is going to generate a lot of force.

A 5 gm weight at that speed (about 1,307 m/s) will exert about 302 grams of force. Just be sure that the magnet will not slide if 1/3 kg of force is applied to the side of the magnet.

You got me!! thats what I get for doing this kinda stuff when I'm fatigued. DOH!.

Should have said apply the weight to the opposing side from the mark made by the marker.

I humbly apologize for the faux pas, I'll set back and take the beating now.

My problem with your idea is, how to you tell where is the imbalnced point when the disc is spinning?

That is what the marker is for.

When the disk is spinning, slowly bring the marker down until it almost touches the disk, the high points will touch the marker before the low points do, so the marker line will only appear where the high (light) point is.

Once you stop the disc, you will be able to see where the marker has, er, left its mark. those are the high points that need to be weighted down.

I would also repeat the recommendation that you get a different motor, fan motors need considerable air flow to stay cool. Fan motor design takes into account the air the fan will be moving over its own motor, so they are designed with less 'passive cooling' features to save cost/weight (or to cram more power into a particular housing/budget, to look at it from another angle). by replacing the blades with a dolid disc, you have removed the 'active cooling' the motor was relying on.

Thanks for explaining. But you are assuming the high point is the heavy point. On what basis or principle this will be so?

I believe he is making the reasonable assumption that the platter was fabricated from uniform density material and the possibly not reasonable assumption that only the radius from the axis of rotation is the dominant factor that makes this dynamicly unbalanced. Unfortunately this approach does also assumes that the re-purposed bearing tolerances will not produce additional errors. None the less it is the simplest dynamic balancing proposed so far and very worthy of the GA.

Now I get it. I thought he meant approaching the pen tip from top, in perpendicular the disc plane. Silly me.

No, I was going for the marker perpendicular to the 'plane' of the disc, as the disk is unbalanced, the heavy side should be lower, the light side higher, so a light touch with the marker will 'skip off' the disc when it descends and 'crash into' the disc as it rises.

I'm basing my theory on the 'typical' method for balancing ceiling fans: you place the plastic clip on each blade in turn until you find the blade that reduces the wobble when the clip is attached, then you move the clip up and down that blade until you find the 'least wobble' point, and stick one of the tiny weights on the fan at that point.

So you expect the slop in the bearings to allow a warp free turntable to tilt away from the pen because of the extra weight on the other side. No bouncing or other oscillation happens. The table is perfectly perpendicular to the axis of rotation. That the excess mass will be precisely 180 degrees away from the marked region.

I like my misunderstanding better.

Since I don't rebalance turntables here, I'm just giving my 'bue4st guess,' which might be completely wrong.

I've seen similar things done on those 'how they make it' shows, where they cover a crankshaft or flywheel with marker, then lightly grind while spinning. The high points are ground away, and if there's any marker left, it indicates a low point that never got touched by the grinding wheel/lathe bit.

We don't have the full rig to test out, so I need to reduce the unknowns. Yes, the bearings could be uneven, the plate may not by mounted square to the axle, the axle may have some wobble, the motor mounting may not be secure to the test rig, a convoy of 18-wheelers and Sherman tanks may race by the building during testing. Any of these things could be possible, but these are also things that can be tested for and eliminated from consideration, so I would assume (and I know that old song about 'never assume,' my favorite rendition was the one Benny Hill did on his show) that the OP would have at least looked at these before asking for help. It would be a little silly to call for help saying 'my bicycle won't change gears' when the bike chain is lying on the ground next to the bike, everyone will be offering advice about shifter levers and cables, assuming that the bike has the chain installed to shift gears with.

No, I am assuming that the LOW point is the heavy point, and that the high point is the light point.

My reason for this is as follows: when an item is unbalanced, the heavy end falls and the light end rises. this is basic physics. With this disc, the difference may be too small to see with the naked eye, but will be made visible by the marker tracks (as the high point will also be pushing the marker and hand up when it passes underneath, the technique of lowering the marker slowly is to insure that you are not applying pressure with your hand, as that would cause the marker to 'ride' the disc as it goes up and down, instead of 'jumping off' the high point as it should).

Thanks for the assist, I sometimes don't get to see the OPs response in time to explain/correct myself.

I have no idea what you are doing with this but the overheating of your motor worries me. Most desktop fan motors are cooled by the air the fan is moving. The shaded pole winding frequently produce significant amount of heat that must be dissipated or damage can happen. Also the mechanical load of a fan (rotational inertia) requires very little initial torque but as higher amounts as air is moved the torque demand increases, an ideal mechanical load for a shaded pole motor.

As for balancing this turntable, a simple static balance of the platter will be very simple to test but this may not be your balancing problem. A momentary imbalance from the drag of that arm may make this moot. I'm not sure the bearings in this motor will keep the center of rotation and center of mass properly aligned. The fan blade with many a plastic or sheet metal fan will almost do a self alignment.

You are most probably correct, the overheating does not come from imblance but from lack of air flow due to fan blade.

The vibration is there with or without the arm dragging the disc. I am going to try running it with an external fan blowing at the motor. Thanks for your good post.

That's an excellent method for balancing the disk.

It does have one drawback, the rotational bearing needs to be virtually frictionless to work.

This is why they spin balance tires. It is more accurate and bearing friction is not an issue.

A tire or a disk could be in perfect static balance by adding weights to the light side, but not dynamically balanced.

Take, for example, a perfectly balanced disk that is not mounted perpendicularly to the axis. As it spins, it will have a tendency to wobble.

http://en.wikipedia.org/wiki/Balancing_of_rotating_masses

You are absolutely correct. The devil is in the details. A 32 cm platter spinning more than 20 times a second will have an edge velocity of about 78 kph (48 mph). I really doubt that this fan motor spun this fast when the original blade assembly was attached. (Interesting bit of trivia, a perfect 32 cm diameter disk has almost exactly a 1 meter circumference. If you didn't have a picture, I'd suspect a homework problem with that combination.) This is fast enough that all of the mechanical details (bearings, wobble, true, outside loads, mass on platter) can become significant, risky and possibly lethal. [OK, the last was hyperbole but it does make my point.]

Spinning a perfectly balanced platter this fast is fairly easily done. Needing to spin a platter for a few thousand continuous(?) hours begs the question, why bother? There must be other critical details to this task. These details can easily become the fatal flaw when spinning something this fast.

1300 rpm is not that fast ? I will do a few hundred hour first, see if it fail.

Sounds like you have done this before.

GA for a static balance. As for as dynamically balancing, hoe far does the OP want to go with it.

Yep, I once designed a thick spinning polygon mirror for a laser scanner. It required 2-plane dynamic balancing.

The OP's disk looks thin enough that static balance might be all that's needed.

That's interesting.

I like to share something too.

But I have to go Going off topic here.

Balancing a saw blade on a saw mill isn't that critical, but they do have to tension it, because the blade can be 60 diameter and only 7 or 8 ?? gauge thick.. How they do it to a saw blade on a saw mill, is that they tension the blade to the working RPM.. Making it concave when its idle, and when its running at its working RPM it straightens out. As far as balanced..... the arbor is big enough for compensation.

It does not work in my case. As AH rightly pointed out, the bearing friction is greater than any imbalanced force.

Which is why the disk and its shaft needs to be a separate component, on good bearings, so that it can be balanced on its own.....

Many good suggestions on how to balance but I suggest the motor you are using is not designed for a horizontal load such as this. That may have more to do with the overheating than the balance. You might want to try a different motor, such as a ceiling fan motor, or rig a pulley and belt system.

Professional balancing is not that expensive given the time frame that you want your system to work.

Imbalance could be caused by many things. Most fan shafts are a D cut. If the platter bore is too big the set screw will push it off center. Not much of a problem with a fan blade. Have you put an indicator on it to check that in it's turning that the run out is not too great.

Also how old is the fan. May have sleeve bearing that are worn.

I find that these motors will be hot to touch even with the fan blade on it. Most have thermal over loads to shut down if over heated. But they were designed to use the blades air over to cool.

You are expecting a forced air cooled motor to survive kilohours without forced air cooling? It's not going to happen. Now for the balancing, not long ago I used my scope, a disassembled piezoelectric spark igniter from an cigarette lighter as acceleration sensor and for reference pulse used a led reflection to a sticker attached to the blade of that stubborn chinese ceiling fan. Of course if I didn't have the scope the "method" would not be cheap. Now about static balancing suggested by others. A trick to compensate for bearing friction is to subject the whole thing to external vibration. The heavy side will come down. Static balancing has limitations for complex items, but in your system we can assume motor was balanced at the factory. Now since you're into mods, in the back of the motor you can possibly remove the fan swivel gearing and make room to attach a small fan, or at least a thermal cutoff. The alternative is a smoke detector. S.M.

For the friction issue you can balance it on razor blades and almost no friction. We use to balance small armatures that way before we found a company to balance on computer balancer then drill holes for balance. It took our armatures up almost an additional 10 thousand rpm. but considering that they were running 70-80 thousand. Without balancing they did not last long, before we got them balanced on the Hoffman balancer it was a crap shoot if you got them close to balanced. After we found a shop to balance them we could race them for several weeks before they shifted and went out of balance from the heat and rpm. But once they were balanced a couple of times they tended to stay in balance as the epoxy got harder with the heat and stayed put. The only thing that would throw them off after that was cutting the commutator too much. AHH all those miss-spent years as a youth racing slot cars. Duke

Thanks for your suggestion. Could you draw a picture to show how do you use a razor blade to balance the disc?

This method was used to balance TACAN antennas (rotating) that were to be placed in radomes for naval and ground service: If the out of balance is caused by an uneven radius from center---->

Put a white mark on the disk that will be used to actuate an argon timing light through a suitable electric eye and triggering device between the two. this causes the timing light to flash once for every full rotation. Turn on the motor/turn table and shine the light on the surface of the disk. Slowly move the light around the circumference and the pulsing light will make the turntable appear to stand still. By looking at the varying diameter when seen with the strobe light you can see the longer and shorter points of the radius. Make appropriate radius changes by grinding or placing weights (use at least two weights) on the turntable. Retry till you get it right. If combined with a tachometer you can find out the rotational speed when the out of balance is worse. Watch out for the harmonics of this. They can be confusing if you are not paying attention, not a lot just some attention. A good method to do primary adjustments with weights before doing this is to take an auto tire "center bubble balancer" (the old way to balance tires) and balance with it first. This will cut down on the number of times with the strobe.

Good Luck, Old Salt

If you can sacrifice a little real-estate, you can put this device in the center of your plate.

So, here it is - take a hockey puck shaped piece of metal large enough to make the center cup about 2" across.

Firstly, drill 4 equally spaced holes just in from the outer portion of the circle for mounting

Then cut it horizontally, but the top portion can be relatively thin, as it will serve as a lid.

Then machine out the center of the thicker portion creating a circular cup. Leave enough around the edge for a groove, and adequate structural material for the mounting holes. Just in from the inner rim of the cup mill out that circular groove for an 'O'-ring.

Then, in the center cup, put about 20-30 tiny bird shot lead pellets. There should be more than enough in an average shotgun shell. Put enough motor oil in the cup to cover the shot. Put your 'O'-ring in place, put the top on, and secure it to the plate as close to 'center' as you can make it. Though even if you are "slightly off", this device should compensate for both the imbalance of the plate, and the centrifugal balancing device, that you have just created.

If it is still off balance, then you might try more shot.

IF, you are unable to put it on top, you could put it underneath, by attaching it with counter sunk screws; AND, IF you make it a donut shape to accommodate your center shaft, and leave adequate material around the center for another groove for a second 'O'-ring.

If you can't visualize this, then I will draw you a picture, but not tonight, as it is late, and I have to sleep.

Yes, please do make a picture, much appreciate your help.

Here is a quick and dirty 2-D rendering of the hockey puck balancer.

For the squared off donut shape that would go on the bottom, you just need to make the place where you put the shot a large deep circular groove, and then place another 'O'-ring groove next to the center to keep the oil from leaking out.

Hope this helps.

I would say the easiest thing to do for the balancing would be to do as Wrench Twirler had suggested early on and try to get some markings made. Only you might want to consider drilling some divots or possibly grinding off a little material on the underside where it shows the balance marks and then test again.

As far as the fan handling it goes... If you were really trying to save cost and time- being that you already have this thing setup, I was thinking you could submerge the motor in some oil to keep it cool. But that suggestion is a little on the wild side. And it doesn't really look like something like that would appeal to you from the picture.

As several people have already mentioned, fan bearings are usually "OilLite" or similar and will not handle this large disk. They were designed for light loads and usage only....if you are still using the original bearings that is....

You need to separate the motor from the disk (assuming they are on the same shaft) and have the disk on at least two cheap ball bearings. I use skateboard bearings as they have an 8mm hole and fit almost perfectly on an 8mm bolt shaft!!! Easily bought on ebay for very little money....an axial bearing at the bottom of the plate spindle will be a good idea too, also very reasonable are.....

There are many possible ways to connect motor and disk to each other, maybe with some speed reducing gearing of some sort to "help" the motor.....

Best of luck.

I did not get a sense if this was a simple one-time test that was not going to be repeated or a test fixture that will be used again and again.

If its a one-time test, then I would use bailing wire and bubble gum if it got the job done and not over engineer the problem. If the fan is trashed at the end, so be it.

As for the heat, set a muffin fan to blow on the motor.

Try post 5 first. Think post 6 next. Then post 1 (but put the weights on the oppostie side).

Obviously, as pointed out by other posters, this is a totally inappropriate use of the motor, and the bearing is not a thrust bearing (load-taking bearing). Also, as pointed out by others you need to separate the motor and the disc, installing the disc on a driven hub similar to a small lathe, but with a thrust taking bearing (in other words a fine tolerance shaft). You can use drive sheaves with an O-ring or small belt (such as on a micro-lathe) to couple motor and driven shaft. I expect the machine shop probably did a fine job of centering up the disc, and probably turned the OD, so there should be a near balance already, but other posters made two excellent suggestions: (1) hub (harmonic) balancer hollow cup with shot, and (2) horizontal axle symmetric balancing using counter weights shifted around the circumference.

One thing to consider that no one else seems to have mentioned: The actual wear test may be producing "chirping" that may be the entire source of noise and vibration.

Have you actually run up the device without the load arm present? That load out near the disc periphery may be inducing an off-axis torque to the weak motor bearing, that results in the motor shaft oscillating (and the bearing heating up even more).

Your design may be OK for a one of experiment, but you could easily improve on this if you wanted to, and soon you may have to when the motor wears out. Good luck.

There are some fairly expensive 100% duty motors out there, and 90 degree gear boxes to convert a horizontal shaft rotation to vertical if you need that. Obviously, check with the OEM of the gear box to see what thrust load any bearings can take, or else install a separate shaft mounted on a thrust bearing and use a belt coupling.

Without the arm loading, the vibration and noise is already there.

Yes, I also suspect that typical fan motor cannot take axial thrust, I may have no choice but to re built the whole thing with separate shaft and thrust bearing.

Thanks for your input!

OK, gotta explain myself here, from the picture the disc/turntable is STILL mounted in the fan cage so it is not all that stable to begin with (Bravo88 please don't take this as a smack), this unit looks like a cobbled up job that DOES NOT NEED TO BE OVER COMPLICATED/ENGINEERED, thats why I suggested the method I did.

Yes the marker will hit the high spot either parallel or perpendicular to the disc, the heavy side will have a tendency to swing a wider arc in the parallel plane while running and will probably dip away from the marker in the perpendicular plane due to mounting instability indicating the light side. But the parallel check will find the heavy point due to instability.

The fan motor is not designed for this task and will fail, hence the suggestion of a washine machine motor as they have roller bearings in them and are not particularly worried about air flow to cool them AND you can buy 'em cheap

As for taping the magnets to the disc for these tests is just for a very short duration spin and the magnets I suggested are quite capable of hanging on under some pretty significant centrifugal loads, the tape is just added insurance against sling off, this is being done on the cheap, I think, so why not give it a shot?

Thanks for the good input. I thought I can quickly cobble up a simple machine to do a one-off task. But, after initial run, as it is now, I think it will not be able to run a few hundred hours as intended. Balancing is an issue, will try out your method of balancing. I think the bearing is a bigger problem . Your suggestion using washing machine, I intend to use direct drive, do you know what is the usual RPM ? Much appreciate your help!

If you go with indirect drive, then a world of speed possibilities open up. Think small motor driving a lathe. The motor is nothing fancy, but the lathe has an exceptionally fine tolerance bearing, capable of handling quite a mechanical loading without creating too much heat.

Brute force works, too.

You can buy pulleys, belt, and bearing blocks from McMaster-Carr or even a local farm supply store and mount a motor and the bearing blocks on a stout plank of wood.

Unless the disk is wildly out of balance, it won't matter.

Thanks for your many inputs. Yes, I had thought so. But now , I am thinking of using a bicycle wheel hub, it's bearing should be able to take the axial thrust, what do you think ?

NO, bicycle wheel bearing is meant for vertical load, not thrust.

It probably will not be an ideal bearing but it will be better than the bearing found in a fan motor. Consider the lateral loads a bicycle bearing must handle in a turn. Then again I wouldn't be surprised if some exotic bikes might use a tapered roller bearing.

For those interested in bearing designs there is a nice write up from MIT.

Look at the drawing, it can handle both vertical and horizontal load.

"Radial" as against "axial"!

Certainly. Also, thanks for your many posts and suggestions.

No problem....you have posed an interesting problem, it gets our brains up and running!!

Fix a pin (temporarily if needed) through the center of the disk. Make a vertical stand with two small ball bearings with small balls. Mount it on a tabletop and rotate. The disk will stop at the heavier side down.

Throw it down from a height. It will land with the heavy point down.

Gajanan Phadte

"Throw it down from a height. It will land with the heavy point down."

Galileo tried that once and it didn't work as you described.

It still amazes me that after several centuries some people still do not understand this.

One could always try for the next Darwin award and jump off a bridge as they also drop a coin and see who wins.

My call is - Heads I'll win, Tails you'll lose.

I was knowing these comments would come, but never thought that u will not think it is not two objects.

Your comparison is wrong. There were two objects.

Test this for yourself.

Take a incandescent light bulb and throw it from a height with the glass down. It will land on the holder end.

Ok u say aerodynamics makes it do that.

Take a pipe and make one end heavy and drop with the lighter end down. What will happen.

Gajanan Phadte

Take a blade longer than the diameter. Fix this blade with sharp end up. Keep the disk centre on this blade and keep doing it. You can find the heavy point.

This is highly impractical as you will have to balance the disk horizontally and no air moving. As difficult as making a chair stand on its two leg edges.

But it is a concept.

Gajanan Phadte