My high hopes for the UHMW bushing supporting the impeller lasted the better part of a day, because direct contact between the impeller and the motor bearing produced an absurdly loud and slowly pulsating rumble:
My hope that the UHMW would wear into a quieter configuration lasted a week …
Back in the Basement Shop, some free-air tinkering showed the impeller produced enough suction to pull itself downward along the shaft and jam itself firmly against the motor frame. My initial thought of putting a lock ring around the shaft to support the impeller turned out to be absolutely right.
So, make a small ring:
With a 4-40 setscrew in its side, perched atop the impeller for scale:
It just barely fits between the impeller and the motor frame:
This reduced the noise, but the hole in the impeller has worn enough to let it rotate on the shaft and the rumble continued unabated. The correct way to fix this evidently requires a mount clamped to both the shaft and the impeller.
Fast-forward a day …
A careful look at the impeller shows seven radial ribs, probably to reduce the likelihood of harmonic vibrations. After a bit of dithering, I decided not to worry about an off-balance layout, so the screws sit on a 9 mm radius at ±102.9° = 2 × 360°/7 from a screw directly across from the setscrew in another slice from the 1 inch aluminum rod:
Centered on the disk and using LinuxCNC’s polar notation, the hole positions are:
G0 @9.0 ^-90 G0 @9.0 ^[-90+102.9] G0 @9.0 ^[-90-102.9]
As usual, I jogged the drill downward while slobbering cutting fluid. I loves me some good manual CNC action.
Put the mount on a 1/4 inch tube, stick it into the impeller, and transfer-punch the screw holes:
Apparently, some years ago I’d cut three screws to just about exactly the correct length:
I knew I kept them around for some good reason!
The 9 mm radius just barely fits the screw heads between the ribs:
Some Dremel cutoff wheel action extended the motor shaft flat to let the setscrew rest on the bottom end:
Then it all fit together:
The fan now emits a constant whoosh, rather than a pulsating rumble, minus all the annoying overtones. It could be quieter, but it never was, so we can declare victory and move on.
Dropping fifty bucks on a replacement fan + impeller unit
would might also solve the problem, but it just seems wrong to throw all that hardware in the trash.
And, despite making two passes at the problem before coming up with a workable solution, I think that’s the only way (for me, anyhow) to get from “not working” to “good as it ever was”, given that I didn’t quite understand the whole problem or believe the solution at the start.
But it should be painfully obvious why I don’t do Repair Cafe gigs …