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Jan 31, 2006, 11:26 AM
Thread OP
Careful!

Outrunner Rotor Runout


For greater motor efficiency, one of the things I do is check the runout of the bell. The airgap is critical in the design of a brushless motor, yet how many bells on thick stator motors wobble? Or worse, have magnets hit the stator?

To check the runout, mount you motor on something solid and use a dial indicator on a stand to measure the variation near the end of the flux ring as you slowly turn the motor.

If the runout exceeds .2 mm, you're probably in big trouble in terms of efficiency. The runout should be more like something less than .025 mm or .001 in.

I found that the shaft should almost always be press fit into the bell end. The set screws just don't hack it, if you're into performance and efficiency. A .0004 in. interference fit between the hardened steel shaft and the aluminum end bell is good. I also press the shaft into the ball bearings, and there, a .0002 interference fit is good.

How important is this? Depends on how bad the runout is, but a 5% improvement in efficiency is not uncommon.

Newt
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Jan 31, 2006, 12:54 PM
Genetic Aeromodeller
aeronut66's Avatar
Hi Newt,

Although I don't have any test experience I agree with your run out comments ~ makes good sense

After some years of F2A C/L speed I know that it is not easy to control all of the bearing fits in a motor Much better to nullify some of the fit tolerance by clamping both the inners and outers against shoulders ~ obviously no radial play allowed. Since the outers seem to be glued in these CD ROM style motors controlling the spacing of the inners is where to start ~ I'd go for a slight biase to separate the inners since the outers will move apart when their housing heats up with all these losses you've been talking about Some careful considered design could yield benefits ~ but you could get a good headache in the process
Jan 31, 2006, 03:25 PM
jj1
jj1
Registered User
jj1's Avatar
Some good machining tips to eliminate runout in the manufacturing stage:

http://www.flyelectric.ukgateway.net/machin.htm

Great site.

-JJ
Feb 02, 2006, 06:20 AM
Registered User
podavis's Avatar
Maybe after all these years I finally have a solid reason to buy a dial indicator!
Feb 02, 2006, 07:35 AM
So what do you do when your kit parts have runout?

Good Luck!
Feb 02, 2006, 12:12 PM
Quote:
Originally Posted by BeavrdamElectric
So what do you do when your kit parts have runout?

Good Luck!

Beaver, I think that you've already demonstrated what you do when your kit parts have run out. Smashing them is about all you can do.

latrans
Feb 02, 2006, 04:02 PM
Thread OP
Quote:
Originally Posted by BeavrdamElectric
So what do you do when your kit parts have runout?

Good Luck!
To fix your runout:

You can bore out the aluminum bell end if it's the problem (use dial indicator to determine) and press in an aluminum plug. Alternatively, you could use your TIG (heliArc) welder and fill in the out of round hole.

In either case rebore the hole (do not drill or ream it) to .1250, +.0000, -.0004 in. Replace the shaft with a 2" hardened steel alloy .1252 in. dia. dowel. Everything will then be a press fit including the ball bearings. Only relatively light force is needed to press everything together.
Feb 02, 2006, 05:19 PM
Registered User
radfordc's Avatar
How do you bore a 1/8" hole? Do they make boring bars that small?
Latest blog entry: Assembling a Spandau kit.
Feb 02, 2006, 05:25 PM
Thread OP
Quote:
Originally Posted by radfordc
How do you bore a 1/8" hole? Do they make boring bars that small?
I thought one just grinds 1/4" square tool steel to meet their needs.
Feb 02, 2006, 06:36 PM
Registered User
radfordc's Avatar
You would have to grind the boring bar to a size of approx 1/16" square with the proper rake. I'm sure I couldn't do that. Have you done it?
Latest blog entry: Assembling a Spandau kit.
Feb 02, 2006, 07:35 PM
Registered User
trashbug's Avatar
If by run out we are talking about wobble there is a cure. If we are talking about the center hole being drilled off center causing an eccentric type problem.......your up the old creek.

The stamped cans that come with some kits are not tough to straighten if there is wobble but it requires a dial indicator (or a REALLY good eye ) and some way to hold them steady. Chuck the motor in drill press if you have one. I can't comment about the turned front end on some cans but I suspect you could achieve the same good results if wobble is the problem..

LOL
Feb 02, 2006, 08:39 PM
Registered User
MorrisM's Avatar
Quote:
Originally Posted by radfordc
How do you bore a 1/8" hole? Do they make boring bars that small?
They do make boring bars that small, but I use a reamer. This one works well for 1/8" shafts.
Feb 02, 2006, 09:06 PM
Quote:
Originally Posted by INewton
To fix your runout:

You can bore out the aluminum bell end if it's the problem (use dial indicator to determine) and press in an aluminum plug. Alternatively, you could use your TIG (heliArc) welder and fill in the out of round hole.

In either case rebore the hole (do not drill or ream it) to .1250, +.0000, -.0004 in. Replace the shaft with a 2" hardened steel alloy .1252 in. dia. dowel. Everything will then be a press fit including the ball bearings. Only relatively light force is needed to press everything together.
So now I need a TIG welder and lathe to work on these things too? (I'm working on the lathe, forget the TIG welder!)

I just wanted to assemble a $30 motor...

Good Luck!
Last edited by BeavrdamElectric; Feb 02, 2006 at 09:15 PM.
Feb 02, 2006, 09:15 PM
X-Era Motors
DavidB.'s Avatar
You don't need to buy a reamer to make a perfectly round and sized hole. Just get a piece of 1/8" shafting and grind half of it away at the tip. This forms a "D" shape, and I believe the tool is call a "D" drill. Put whatever you want to bore in the lathe, drill a pilot hole a little less than 0.125. Then go in with the D drill and finish the hole, just don't feed it in fast and it'll cut fine. The hole will be ever so slightly larger than the shaft, maybe 0.0005-.001. If you want the hole a press fit, then sand down the shafting a little at the end before grinding it to shape.
Feb 02, 2006, 10:29 PM
Greediest Suer
Ron H's Avatar
Now I should throw away my homade motors and try again? I use a hand held drill and and a dremel. Never measured anything. I have found that merely twisting or tilting the stator on the mount can mean the difference between rubbing and not using kits.


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