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Old Apr 14, 2012, 03:26 PM
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Propeller Balancing question

Good day, all. Still very much new (again) at this hobby, with a Top Flight Pro magnetic propeller balancer. Last night I spent hours of zen with the balancer and a 12-by propeller. I'd get the blade balance just perfect, get the hub balance just perfect, then it would suddenly change and go off-kilter.

I discovered that the balancer shaft, although perfectly flat/straight, appears to "detent" or lock into one position in the balancer. I marked the spot with a felt-tip pen and spun the shaft rapidly. As it slows, it appears to "grab" the marked point and hold it securely.

Since this is a magnetic balancer, could there be a magnetic field preventing the shaft from truly spinning free as it approaches this matching point?

I think the reason my perfect balance would go away is that the shaft and centering cones would rotate during the sanding process, causing a chasing of the balance point, even though it wasn't out of balance.

How does one deal with this "magnetic snap" and the tendency to lock to a certain point on the shaft?

thanks

fred
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Old Apr 14, 2012, 05:50 PM
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If you're getting down to such fine levels of balancing, chances are the prop will run pretty smooth. Balancing is important, but don't waste a whole lot of extra time on it. That prop could be broken before the plane even takes off if you have a noseover or something.

Also, even if the prop is perfectly balanced there's a chance that the motor and/or adapter won't be. Ideally, you need to balance everything dynamically at the same time if you want it all to run as smoothly as possible. The prop is only one piece of the equation.

I've got a small magnetic balancer that I made from a fingertip balancer and a wooden stand with magnets that I whipped up. It does the same thing you're describing. I've stopped using it almost entirely and have taken to just dynamically balacing my setups by trial and error; it rarely takes more than a couple minutes and the smoothness is undeniable. I run the motor with the prop installed, then apply clear tape to one blade and re-check. After a couple cycles I've usually got it pegged pretty well.
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Old Apr 14, 2012, 06:17 PM
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Thanks for the reply. It's not so much that I'm concerned about the degree of balance, although I know the difference between "good enough" and "good". I'm more concerned with what appears to be a design flaw, if it is indeed a magnetic field causing the "notch" or "detent" feel to the spinning of the balancer shaft.

I'm going to try rotating the magnets in the mount to see if it changes the location of the notch.

I'm hopeful that the props will last longer than you describe! It's for an Oktokopter, so I have eight to balance, plus eight spares.

One big factor in this effort is the genuine zen feeling I receive by performing a perfect balance. Sometimes it's the journey, sometimes it's the destination.
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Old Apr 14, 2012, 06:54 PM
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Hehehe, I know what you mean about the Zen thing. I do the same thing with sandpaper and wood.

I'll be interested to hear what you find. I've wondered about this and I've never really seen anything conclusive.
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Old Apr 14, 2012, 07:22 PM
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It could just be small irregularities in the surface of the magnet or the shaft contacting it, or just stiction between the metals. My magnetic balancer came with a thin, puncture-resistant plastic tape to put on the magnets, which provides a lower friction surface, and no metal-to-metal interaction.
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Old Apr 14, 2012, 07:36 PM
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My fingertip balancer does well, except for the pin holes I get in my fingers!
Don
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Old Apr 14, 2012, 09:18 PM
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Quote:
Originally Posted by Wintr View Post
It could just be small irregularities in the surface of the magnet or the shaft contacting it, or just stiction between the metals. My magnetic balancer came with a thin, puncture-resistant plastic tape to put on the magnets, which provides a lower friction surface, and no metal-to-metal interaction.
This balancer also came with thin plastic disks, which are attached with CA to the surface of the magnets. It's suggested as an alternative to sand the magnet surface with 400 grit, followed by 600 grit, followed by polishing compound. I may try that method on the reverse ends of the magnets, now that you've mentioned this aspect of the balancer.

Because the spinning shaft stops at precisely the same location each time, it made me think it was related to magnetic fields. I realize now I can check that by rotating both magnets in the same direction and see if the stopping point changes by the same degree.

Getting closer to an answer...
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Old Apr 14, 2012, 09:54 PM
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The zen thing made me laugh, every time I pull out the magnetic balancer my wife rolls her eyes and says, you gonna stare at thing for the next hour or two? I can usually get the prop balanced in any direction but I have noticed the shaft seems out of balance when I free spin it. I never thought much of it but maybe the shaft is getting magnetized, I will put mine through the demagnetizer and see what happens.
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Old Apr 14, 2012, 11:00 PM
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I tried the demagnetizer and it didn't do anything different that I could tell. I think maybe the hubs are slightly off balanced. As soon as I pulled the balancer out my wife rolled her eyes lol!
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Old Apr 15, 2012, 05:15 AM
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I don't have a demagnetizer but it's a good thought. Since the instructions say to maintain polarity and to have the magnets attract, perhaps the field from one magnet passes through the rod to the other, still maintaining some magnetic field that keeps things aligned.

With brushless motors, there's a strong cogging effect and I'd liken this issue to that, of a much smaller magnitude.

I think I'll spring for a non-magnetic model and compare balance results and also friction factors vs. magnetic detent.
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Old Apr 15, 2012, 07:28 PM
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The magnets face each other, so that the field passes straight from one to the other, assuming they are oriented so that they attract. When a magnetic material (the rod) is placed in this field, it draws the field to its center, as a lens focuses light, effectively 'shorting' the magnets together, so that the rod is suspended between them. If there is a small gap at one end of the rod, friction is minimized to only the one end. There is no way that there is a cogging effect, as the field is constant and direct. Any tendency of the rod to stop at one location is either an imbalance in it, or what is on it, or pitting in the contacting surfaces.
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Old Apr 15, 2012, 07:49 PM
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That's good to hear. I tried the setup with the magnets opposing each other and there was no difference there, so it fits with your description.

The assembly instructions suggest to use the plastic disks or to polish the magnet ends. It is written that only one magnet needs to be polished, but I did both just today. I went a bit overboard too, using 400/800/2000 grit followed by rubbing compound. It's the first time I've ever been able to see a reflection in a non-rare-earth magnet or non-plated magnet surface. No pits now, to be sure!

I've ordered a replacement shaft, as well as a different type of balancer, along with the fingertip balancer, just to have more reference points. The stock rod rolls cleanly on a glass plate, no bobbles or snags, so the rod is likely to be true. I suppose a small-diameter steel rod could have an imbalance by weight, but the cogging effect is there.

The rod itself will not attract light steel parts, as my screwdriver tips will do, or even the tip of my pocket knife, so there's no indication that the rod is magnetized. It's a mystery so far.

I put my first (and only) prop to be balanced in the tool, and used a marked reference on the shaft to maintain a known "heavy" point. I was able to remove some of the CA I used on the hub and keep the blades in balance, by rotating the shaft mark 180 degrees and re-assessing the balance.

Time will tell, and the delivery man may have new answers.
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Old Apr 15, 2012, 08:11 PM
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Hmmm...

A few things strike me. First, are you balancing your props in the middle of the shaft? There could be an extremely fine bend in it that doesn't even show up on glass, but the weight of the cone-nuts is enough to make the bend be noticed by gravity (since they'd be offset by just that tiny amount). Try moving the cones and prop far to one side or the other and see if anything changes.

Second, the points on the shaft's ends could perhaps be ground with enough imprecision that they're just a hair off center, which would also affect balance and again be magnified by the weight of the cones. This could potentially be working in combination with the above. Maybe chuck them up in a drill and polish them to needle-sharp tips?

Third, the cones themselves could be slightly off-balance. Have you tried balancing them one at a time in different positions relative to the shaft (say, 45* increments)?
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Old Apr 15, 2012, 08:27 PM
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All good questions. I am balancing the props in the center of the shaft, eyeballed position.

The shaft alone, no cones, has a dead-stop point, which does not change when I rotate the cones in 90 degree intervals, which makes me think the cones alone are well enough balanced.

The balancer instructions suggest to not have needle sharp points and again, by eyeball, the tiny domes appear to be centered, but I'll use a lens to examine them a bit more closely. I don't trust my own power tools to be so precise as to permit a perfectly centered tip, but I have a client who is a machinist and can verify to 0.0001 inches in the next few days. He would also be able to verify the precision of straightness of the overall rod, too, I expect.

I can see that a slightly off-center point would provide for a settling point of the assembly, and I'm thinking that's the most likely answer. It fits all the physical behaviors I've experienced so far.

Thanks for a great suggestion.
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Old Apr 15, 2012, 08:48 PM
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Is there any way the earths magnetic field might have a slight pull like a compass points to the north? Just a random thought.
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