Wood better than aluminum for outrunner motor mount balancing out vibrations

Lately I played with outrunner motors, and am still impressed of their power and their roaring loudness.

I learned how to measure current and measured voltage in parallel on 2nd small voltmeter: |500x376

3.42A with 10.4V is 35.5W, a lot ow power for a single motor. This screenshot shows a run with a perfectly balanced out wheel (im am not interested in propellers).

In trying to balancing out a second wheel (which I need for 2nd outrunner motor for circular running robot in my Motor Test Station) I did break most legs from two aluminum motor mounts and one leg from third aluminum mount. See quite some movies about that on HermannSW youtube channel

I already bought a 5pcs set of aluminum mounts 6 days ago, but will have to wait 21-47 days for delivery. And they will not be a real help for balancing out, because more aluminm legs will break: https://www.aliexpress.com/item/4PCS-2204-2212-2208-2216-2218-22-Brushless-Outrunner-Motor-Mount-X-type-Cross-holder/1951976788.html

Today I built my own motor mount from 4mm thick wood (click for detailed view): |500x273

The distances between two opposite screws of A2212 type motor are 19mm and 16mm. And in the middle a whole is needed for center moving motor part. The screws I had were too long, so I shortened them with adding screw nuts. This construction is really firm, and works better than aluminum mounts especially for high vibration runs as this one: https://www.youtube.com/watch?v=DX7E-nckZFY&feature=youtu.be

I did drill the holes with different diameter screwdrivers, and cut the wood with the saw like kitchen knife shown above.

Hermann.

If your system is so imbalanced as to produce enough radial force to damage the mount, you are overloading the motor ball-bearings massively.

You can't get away with grossly imbalanced high speed rotor. I presume you are using wheels at much higher rpm than they are designed for. This can even lead to violent mechanical explosion if the wheel isn't strong enough.

Which wheels are you trying to spin at what speed?

If the wheels can cope with the speed, you'll need some sort of universal joint linkage to remove the imbalance forces and vibration from the motor shaft and thus keep it happy. The bearings for the wheel itself need to be rugged enough for the forces involved, ie large.

What are you trying to do that you have to spin the wheels that fast?

As an engineer I can see your problem is all down to your wheel being out of balance. The plastic tyre on the rim is quite heavy compared to the rim, and it would only need to be a fraction of a gram heavier on one side to produce your problem of vibration. If at all possible - find another type of wheel if you need it to go at that speed, otherwise you can use a gearbox to reduce the speeds. Find a wheel that is totally machined and not cast or moulded as that is likely to be better balanced.

Failing that and you must use this wheel at the high speeds, then buy some small roller bearings and use a bearing on each side of the wheel, then mount the bearings in something big and heavy. Then try adding a small weight inside the rim like they do on car tyres to balance it better. You may only need a small nut or length of solder glued on. It's trial and error. Some places will make it a lot worse, some places better.

The aluminium brackets are failing because the holes in them form a weak spot, and the flexing caused by the vibration first hardens the metal and then cracks it. With using wood it can flex without work hardening so is actually pretty good for motor mounts because it is easy to work, add holes and shape.

A note about woodwork - USE THE RIGHT TOOLS! Buy a junior hacksaw at least as that will cope with most materials for model making. Don't use a breadknife. Then you can buy specific tools as you go along - like a small wood saw, drills, hand drill etc. Makes life so much easier - safer for you and you get better results quicker.

Thanks for all your comments on this.

If your system is so imbalanced as to produce enough radial force to damage the mount, you are overloading the motor ball-bearings massively.

Agreed.

You can’t get away with grossly imbalanced high speed rotor.

The imbalance will only be existing in the process of outbalancing a new wheel.

I presume you are using wheels at much higher rpm than they are designed for.

Definitely :wink: The wheel I use is is from my children Lego parts they do not use anymore (youngest is 17yo).

This can even lead to violent mechanical explosion if the wheel isn’t strong enough.

That is the reason why I do the experiments in my Motor Test Station (MTS) which is surrounded by safety glass. I have seen complete destruction in the past :wink:

Which wheels are you trying to spin at what speed?

Lego 13x24 wheel and Lego 13x24 tire.

What are you trying to do that you have to spin the wheels that fast?

Try to see whether my A2212 outrunner motors can show new speed record in Motor Test Station:

The thread started with slow speeds and people telling me that gears are a must. I never used gears in that thread and superglued wheels directly onto motor shaft. It ended with speed record of 17.79m/s or 64.04km/h or 39.80mph sofar, with 24mm diameter Lego wheels. Lately I measure rpm/rps by 90fps Raspberry camera slowmo videos from above MTS (instead of fork type infrared speed sensor), here is the record run:

For the forces heard and seen with A2212 2700KV outrunner motors I will not get away with lightweight wooden beam in motor test station as in the past. I already built a (159g) 315x34x34mm wooden beam, that is in place on MTS vertical center nail already. The 34x34mm will allow to fixate the 30x30mm aluminum mounting crosses:

As an engineer I can see your problem is all down to your wheel being out of balance. The plastic tyre on the rim is quite heavy compared to the rim, and it would only need to be a fraction of a gram heavier on one side to produce your problem of vibration. If at all possible - find another type of wheel if you need it to go at that speed,

Will do if I cannot outbalance the wheels. As you can see here I got the right wheel perfectly outbalanced already, just need to get a 2nd wheel that is outbalanced as well.

otherwise you can use a gearbox to reduce the speeds.

That is not an option, at least for new speed record.

Find a wheel that is totally machined and not cast or moulded as that is likely to be better balanced.

Will search for that. On the other hand the previos speed record was done with 24mm diameter Lego wheel with tire superglued on wheel superglued on motor shaft, with more than 233rps/14000rpm at motor shaft.

The aluminium brackets are failing because the holes in them form a weak spot, and the flexing caused by the vibration first hardens the metal and then cracks it.

I think I will be able to use aluminum mounting for outbalanced wheels.

With using wood it can flex without work hardening so is actually pretty good for motor mounts because it is easy to work, add holes and shape.

So wood is what helps in the process of outbalancing a wheel.

Hermann.

At least you are taking safety precautions... If you want a well balanced wheel consider machining one on a lathe from delrin or aluminium alloy - should be close to balanced after machining, and because it is solid won't change shape and become worse. Look at how jet turbine discs are shaped to see how a high speed wheel should be made for maximum strength.

There are ways to make a system self balancing by introducing a compliant section that allows the centre of mass to adjust itself on-axis automatically, which I suspect takes some tuning and experimentation

I dismantled the outrunner motor testing in Motor Test Station, and assembled wheels and outrunner motors with wooden beam, and then assembled the 30A ESCs and 3S LiPo, as well as multifunction tester:

Did a first (dangerous) test where I had to drop tester after some short time to avoid coiling of tester cables too much:

Even though I made potentiometer just to make motors move, counting frames of 90fps slowmo video showed 3.6m/s or 12.9km/h already … more to come next.

Hermann.

Hi,

So wood is what helps in the process of outbalancing a wheel.

Sorry to sound vague, what is OUTBALANCING ?

Do you mean you are trying to BALANCE an OUT OF BALANCE wheel?

Thanks.. Tom.. :)

Hi Tom,

Do you mean you are trying to BALANCE an OUT OF BALANCE wheel?

yes and no, trying to get to a wheel that is outbalanced.

In the past (Motor Test Station thread (MTS)) I did superglue the wheels directly onto motor shaft, and center wheel hole diameter was bigger than motor shaft diameter. So besides making sure that rubber is firmly superglued to wheel rim, I had a lot of influence on where to drill a small hole into the center rim hole, that I completely filled with superglue before: |500x353

With the DC motors I used at that time testing whether wheel is outbalanced was much more easy than with the outrunner motors here, just superglue wheel onto motor shaft, wait until firm, connect battery and just see: |500x418

With the outrunner motor and the 43mm diameter wheels my possibilities of influence are smaller. Yes, I can make sure that rubber is firmly superglued to rim (testing with 40W showed even small areas not superglued good enough). But the perfectly outbalanced wheel (right in picture of 3 wheels above) perfectly fitted on the propeller adapter holder, no influence possible from me nor needed. The other three 43mm diameter wheels with problems have a little more rim center hole diameter. So here I had to fill in a little superglue, but much less than for previous MTS wheels.

I now did the next step because the 2nd wheel is not perfectly outbalanced as the 1st, but hopefully good enough as can be seen in this 90fps slowmo video: https://www.youtube.com/watch?v=gYTz7SoCupM

Interesting about this video is, that even with 90fps the video shows right wheel much better outbalanced than I was able to see directly with my eyes.

Hermann.

P.S: As can be seen in big photo above, I mounted one outrunner motor with aluminum cross mount, the other with my selfmade wooden mount this thread described initially (for the not so good wheel). Because of the different mounts the radius of that wheel (around center vertical nail through wooden beam) is 0.5cm more than that of the other wheel, In the past such small imperfectness was never a problem.

Hi, What is a perfect OUTBALANCED wheel, how does it perform with respect to a BALANCED wheel?

I'm sorry I know it s only terminology, but I would assume you are looking to get a BALANCED wheel, so it spins true and without vibration...

BALANCED or UNBALANCED wheel...

Tom... :)

Hi,

you are right, I want to get a BALANCED wheel. I am not a native speaker, so perhaps I did not get the right verb. What I meant with "outbalancing" was German "auswuchten", which I now looked up as "to balance out". Sorry for confusion by my use of "outbalancing".

Hermann.

P.S: I am pretty sure there is no "to superglue" in English, but I hope it can be understood what I mean.