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Old Feb 01, 2012, 02:00 AM
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modisc's Avatar
United States, MI, Ann Arbor
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Help!
How to balance the bell of Scorpion 40xx

I have rewinded and replaced the shaft and bearings of one of my Scorpion 4035 12N8P. It seems that the bell of the motor is not balanced anymore, and some vibration can be felt during the test run (no-load).

Is there any practical way that I can restore the balance of the bell? Any help appreciated!

xiaochen
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Old Feb 01, 2012, 07:07 AM
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Sofia, Bulgaria, East Europe
Joined Nov 2006
166 Posts
I think those videos could be of any help:
How to balance a brushless motor (1 min 10 sec)

balancing motor rotor and open your ears... (2 min 30 sec)

The other relatively simple method is to use a piezo element like this one and mount it on the firewall the motor is attached to. Then you could monitor the output waveform from the piezo element on an oscilloscope and balance the rotor so the peak-to-peak voltage is minimal.
You can see both methods described here. The page is in Bulgarian, but I guess you could use Google Translate or just get the idea from the pics
Good luck!

Nickolay
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Old Feb 01, 2012, 10:55 AM
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gkamysz's Avatar
Chicagoland
Joined Feb 2000
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When you have a short bell, the above methods will work. When the rotor becomes long, dynamic (two plane) balancing will be necessary. Dual plane balancing requires equipment to do well.

First, make sure the bell is concentric to the shaft. If it was balanced before, and you didn't add, remove, or move any weight in the bell, the only cause can be the shaft is not installed exactly as it was before.

Greg
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Old Feb 01, 2012, 12:46 PM
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modisc's Avatar
United States, MI, Ann Arbor
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Quote:
Originally Posted by dando View Post
I think those videos could be of any help:

The other relatively simple method is to use a piezo element like this one and mount it on the firewall the motor is attached to. Then you could monitor the output waveform from the piezo element on an oscilloscope and balance the rotor so the peak-to-peak voltage is minimal.
You can see both methods described here. The page is in Bulgarian, but I guess you could use Google Translate or just get the idea from the pics
Good luck!

Nickolay
Thanks Nickolay, the use of a piezo to detect vibration level is very interesting and it is surely much more precise than using my bare hand to feel the vibration during adjusting and testing the bell in a test run.
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Old Feb 01, 2012, 12:58 PM
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modisc's Avatar
United States, MI, Ann Arbor
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Quote:
Originally Posted by gkamysz View Post
When you have a short bell, the above methods will work. When the rotor becomes long, dynamic (two plane) balancing will be necessary. Dual plane balancing requires equipment to do well.

First, make sure the bell is concentric to the shaft. If it was balanced before, and you didn't add, remove, or move any weight in the bell, the only cause can be the shaft is not installed exactly as it was before.

Greg
you are right and it is indeed my concern that for a longer bell like 4035, it is really troublesome to figure exactly where to add or lose the weight! I used to have a very hard time dealing with a 9050 sized bell, and actually I gave up finally.

The scorpion bell has roughly 1 cm to fix the shaft, and since the shaft is brand new, i think the shaft and the bell is lined up perfectly as before. There are several epoxy spots inside the bell for balancing, and I am not 100% sure whether i have knocked off one piece when working with the bell.

my guess and my hope is that i have knocked off some epoxy spots inside the bell for balancing, then i may not be in big trouble.

I actually did not understand why scorpion have the inside bearing fixed by the stator itself, but not the stator holder. Since the stator is made of hundreds of thin plates, and if it is somehow damaged or some plates displaced, the position of the inside bearing will be not same as original. I guess it may cause some problems.
However, after some thinking, I think even though one of bearing (probably the inside bearing) is misplaced a little bit, the bell/shaft should also be running nicely if the bell/shaft itself is balanced, right?
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Old Feb 01, 2012, 01:49 PM
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Originally Posted by modisc View Post
However, after some thinking, I think even though one of bearing (probably the inside bearing) is misplaced a little bit, the bell/shaft should also be running nicely if the bell/shaft itself is balanced, right?
Correct, unless the bearing alignment is very bad the rotor should run smooth.

Two things can happen with replacement shafts. First, the new shaft damages the bore and is not concentric with the bell any more, this is usually the case when a sliver of aluminum is pushed out by the new shaft during installation. Second, the fit of the shaft is not tight, either by the bore being too big initially or the new shaft being smaller. And again when reseated, the shaft is not on the same axis as the original shaft when it was balanced.

Even if you lost a piece of balancing epoxy, if you can't find the location where it was you'll never get it balanced. If you know the location, trial and error will get you close.

Greg
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Old Feb 01, 2012, 02:03 PM
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Quote:
Originally Posted by gkamysz View Post
Correct, unless the bearing alignment is very bad the rotor should run smooth.

Two things can happen with replacement shafts. First, the new shaft damages the bore and is not concentric with the bell any more, this is usually the case when a sliver of aluminum is pushed out by the new shaft during installation. Second, the fit of the shaft is not tight, either by the bore being too big initially or the new shaft being smaller. And again when reseated, the shaft is not on the same axis as the original shaft when it was balanced.


Greg
It is lucky that i am not experiencing the two things you mentioned ^^
shaft removal and re-install is perfect, using a drill press. new shaft is tight fit as well.

So i guess i just have to go though the hard way, using tapes to find the jack spot. no short cut right?
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Old Feb 08, 2012, 05:56 PM
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Dennis Everett's Avatar
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you could try to balance it on a prop balancer , might get you close , did you check if it is still round ? if too much preasure was put on the end plate , you could bent it .....
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