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

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

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

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.

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

first mistake … "but when glued to the shaft do not sit concentrically". Did you guys glue them or a vendor ? if a vendor, get them to fix it, on their dime.

second mistake … "technicians have been pulling the wheels off". Maybe it's just words, but things that are glued together shouldn't be "pulled" apart, the glue should be "disabled" is some way (like using dry ice to metal-metal bonds).

A loose fit would work, if you had a tool to establish the correct geometry, then to hold the pieces whilst the glue set up. Though it'd be an interesting glue to survive the temperature extremes of space. A better joining would be to hold the pieces in the tool, fill the gap between them with maybe epoxy, then cross bolt.

Surely (don't call me Shirley) this has been done many times before. A couple ideas …
1) make the joint as accurate as you can then dynamically balance (like a tire) and correct with balance weights,
2) the opposite extreme is to mount them on a spherical bearing so they'll spin about the "right" axis naturally.

another day in paradise, or is paradise one day closer ?

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

Thanks rb1957,

Yeah, the wheels are being glued in house, I suggested sending them to the vendor, but it seems that isn't an option. I'm not sure how they're pulling them off, I've mentioned that any kind of pulling is a bad idea and highly likely to damage the motor bearings unless they have a suitable gear puller that can press against the shaft. I'm only around here for a month and this isn't part of the project I'm here to work on, otherwise I'd be pushing to redesign the entire assembly as there looks to be a lot of issues with the design.

I assume by cross bolting you mean through the axle? The assembly is pretty tiny, 31mm diameter 3mm thickness for the wheel only 1.5mm diameter for the axle, so no way of doing that other than a grub screw, though access would be difficult and would require rebalancing. (the wheels come balanced from the vendor).

I'll have a look into balancing after assembly, I think this is likely a good option, I'll also recommend designing some new wheels that will allow assembly after the flywheel has been attached to the motor.

Thanks for the suggestions.

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

Just in case anybody else wants to waste their time this is why we don't cross post. There is a longer thread in Mech Eng on exactly the same subject.


Greg Locock

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