9C out of balance covers

[John in CR]

Tonight I was trying to check whether I should replace the bearings in my 9C covers before I close it up, so I stuck the end of my finger in the hole of the bearing and gave the first cover a spin. Instead of a smooth feeling I can feel the bearings rolling and almost a tick on each revolution. That answered that question in a hurry...new bearing for sure since this will run out of wheel at 147V off the charger, so probably 1krpm or more.

That's not the biggie though. As the cover slows I feel my finger forced back and forth by the spinning cover. Uh oh, so I turn the cover near vertical and spin again, and when it slowed to a stop, it turned back the other way before stopping. I do this a few times and it always came to rest at the same rotational position, sometimes it rotated back the other way nearly 180° to come to a rest with the heavy point down. I figured the hole for the bearings was off center, but nope the edges and lip that locks into the magnet retaining ring are nice and true.

Then I start thinking maybe I drilled my ventilation holes inconsistently, but I look closely and it appears that I actually took more material out of the side that's heavy and there's even a ridge that was cut when the machine cut the retaining lip that doesn't exist on the light side. I don't have the kind of caliper for measuring a thickness in the position, but from the best I can tell with the caliper I have, the aluminum at the curve in the cover is well over 1mm thicker at the heaviest point than it is at the lightest.

The cover for the other side has the same issue. I wonder if this could be part of the singing 9C covers issue.

Here's are pictures of the inside edges of the cover at the lightest point and then turned 180° to the heaviest point. Notice the edge cut where the material is thicker that occurred when the machine cut the retaining lip and axle hole in the cast blank motor side cover.


Now that I've identified that the cover is significantly out of balance, though the stator will still be centered for a consistent magnetic gap, what do I do to fix it? Could I get accurate enough for 1000-1200rpm by taking a grinding wheel to it and remove material from the heavy side, and then test for balance by clamping onto the inner race of the bearing with two nuts and a threaded rod clamped in a vice and spin the cover up using the wire brush wheel on my angle grinder?

I welcome any ideas for a fix.

John

 

[docnjoj]

Hey John
Maybe glue weight to the light side like the mag wheel folks. It is easier to do than grinding stuff away. might only be 1/2 oz or less. Or just balance the whole wheel with motorcycle weights. Your rims are M/C rims, right?
otherDoc

 

[BIG BEAM]

Maybe the hub was balanced as a unit and the cover is compensating for the motor being out of balance.What you are looking at is independent from the hub.

 

[John in CR]

Maybe the hub was balanced as a unit and the cover is compensating for the motor being out of balance.What you are looking at is independent from the hub.

The side cover issue is just a cover built to very poor tolerances in the casting stage. It can't possibly be an intentional adjustment. I will, however check the rest of the rotor to see if they did something to balance the rotating assembly during a quality control step, but I doubt it. I did mark the original positioning of both covers, so I guess it's possible that the 2 out of balance covers were positioned to cancel each other. That too is doubtful to me, since both bearings have a hitch or tick in them when rotating. Time/money spent on high QC would not permit the use of that low quality of bearing, the only 2 moving parts in the system. The out of balance may not even be an issue at typical ebike rpms.

Doc,
This motor will run out of wheel and at 1krpm or higher, so I can't balance at the wheel, and I wouldn't even want to try something on the exterior, except maybe a bolt and washers through a spoke hole or two. Hmmm, that actually sounds like a pretty good and easily adjustable way to balance it, since I know the position. Once I get it the added weight right, I can just put the strongest locktite available to secure it.
Thanks for making me think about it. I'll just grind a bit, and assemble. Then easily balance it at the end that way.

John

 

[docnjoj]

Ah this is the Stokemonkey type of thing, John. I see why U want the motor balanced. Good luck with that project and show pictures.
otherDoc

 

[John in CR]

Ah this is the Stokemonkey type of thing, John. I see why U want the motor balanced. Good luck with that project and show pictures.
otherDoc

Yep, over 140V off the charger with sag + a conservative 65A limit = over 9000W through a ventilated 9C bolted directly to an extended swingarm. 3 speeds through the chain, each with the chain in perfect alignment with front and rear sprockets should keep it quiet, as well as make it both a mountain goat and a road rocket in a lightweight and easy to build package. If I pull it off as planned, I think it will be a good compromise combining the advantages of hubbies and non-hubbies.

John