Inertia Wheel/Reaction Wheel motor to flywheel attachment and fit.

[FIREotter]

x-post from Aerospace

I'm helping out with a little bit of design work at a small aerospace company, they've just lost there full time Mechanical engineer and are having a number of issues with some parts turning up. One of these parts are brass flywheels for an inertia wheel model.

The motors that are being used are faulhaber brushless motors with a 1.5mm shaft (+0/-0.01mm) Which I believe would make this an h7 fit. Our issue is the previous engineer has specified a rather loose fit on the reaction wheels (though unfortunately I can't measure it I'd estimate they're ~1.6mm), these have been balanced but when glued to the shaft do not sit concentrically and have induced a vibration into the system. As they are inertia wheels, (unlike reaction wheels) they spin all the time, so this is quite an issue. To compound the issue the wheels have to be mounted after the motor is in the assembly as no access has been left for motor fasteners, this means jigging is very difficult.

The technicians have been pulling the wheels off and resetting them which has damaged the motor bearings. Awkwardly, even if there was a way to press on the flywheel, there is no way to support the axle as it does not protrude from the back end of the motor (the whole unit is encased in plastic, I believe in the rear is an encoder)

My current thoughts are that redesigning the wheels with a shrink fit and access holes/spokes for the motor mounting would be the best solution for this, but getting the required tolerance on such a small hole may be unachievable for us, the fairly loose tolerance on the shaft may make this especially difficult. Specifying an H7/H8 on the hole creating a fairly close clearance fit and using a loctite 648 may be the most sensible option.

Any thoughts or suggestions would be greatly appreciated.

 

[rothers]

What's the existing drawing specification (Dia & Tol) for the flywheel hole ?

 

[Jboggs]

If you end up redesigning it you might consider a tapered fit. That way the wheel could be easily installed after the motor, and it would be self-centering.

 

[FIREotter]

rothers (Mechanical)28 May 19 11:21
What's the existing drawing specification (Dia & Tol) for the flywheel hole ?

I'm not sure, I have no access to the original drawings, I'd estimate they've been speced at 1.6mm +-0.01 by feel of the fit.

Jboggs (Mechanical)28 May 19 12:00
If you end up redesigning it you might consider a tapered fit. That way the wheel could be easily installed after the motor, and it would be self-centering.

I hadn't considered that. Ideally the motor shaft would be tapered too, any recommendation of how it should be specified h7 wide end m7 small end etc? do you think there would be much risk of it going on out of axis?

 

[electricpete]

these have been balanced but when glued to the shaft do not sit concentrically and have induced a vibration into the system

Assuming the “glue” works well enough and the only problem is the lack of centering, perhaps it is possible to do a post-installation balance? (unfortunately might be more challenging to compensate for wheel tilt depending on whether 2-plane balance is practical).


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(2B)+(2B)' ?

 

[MintJulep]

Maybe a tapered hole in your flywheel and a mating tapered busing that you could squeeze together.

 

[jlnsol]

That' a small shaft for a taper bushing! As an alternative you could split the brass flywheels in two equal pieces right through the shafthole.
Consider using a lasercutter for that to keep the cutting width as small as possible.
Then clamp the two halves together with screws and nuts/threaded holes.

 

[IRstuff]

A tapered shaft doesn't guarantee concentricity, per se, you still need to have good dimensioning, tolerancing, and fabrication. Moreover, a taper can make the flywheel be non-perpendicular to the shaft, which would cause a different type of wobble.

TTFN (ta ta for now)
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[hendersdc]

I don't know the details of your design but I would expect mounting any flywheel directly to the shaft of a small DC motor like these would be well over the allowable forces on the motor shaft. A press fit operation would definitely be too much for the motor to handle as well. Any possibility of redesigning to mount the flywheel on an independent, well supported shaft?

 

[Strong]

It is probably a waste of time and money to balance the inertia wheel on a mandrel at probably low balancing speed, and then mount it eccentrically and run at 15,000 rpm. Have you made sure that the structure is not resonant near shaft speed and amplifying vibrations? If it is, then detuning the structure may allow the vibration from the fit tolerance to be acceptable. It may be possible (a worthy challenge) to balance the unit after partial assembly with inertia wheel attached. Have you considered a thermal shrink fit by heating the inertia wheel and slipping it on the shaft?

Walt