HVAC question…

[Coder]

Our Amana furnace blower fan was making a lot of noise but I never paid attention to how much the gas line was vibrating. I replaced the motor - old motor had about 1mm of play in it along the axis of the shaft. When I put the new motor in I was still getting some (but less) noise and cleaned the fan blades - same issue. So I bought a new fan wheel and installed it… it was at this point I noticed the gas line (we don’t have a flex line) was vibrating a lot with the new fan.

So… new motor and new fan - quieter but nasty vibration.

At this point - do I replace the fan blade/wheel with a new one? If that still has an issue could the new motor be bad? Or could there be something restricting air flow causing the vibration?

I tried posting on my usual places but no one responded

[crfriend]

Clearly something is out of balance with the blower assembly, and the easiest way to test that in the field will be to simply remove the impeller, spin the motor up and see if the motor vibrates when unloaded. If it does, the motor is out of balance somehow; if not, then the issue in entirely likely the impeller. Also, make sure the mounts for the motor aren't too tight; they frequently need a bit of play to damp any vibration.

Is the set-screw on the impeller in the correct position (i.e. engaging the flat section of motor shaft)? If it's not, that could cause an imbalance as well.

You might also want to swap in a length of flexible fuel line to avoid precisely this sort of trouble. You don't want to crack the gas line and wind up with a leak.

(Generally speaking, I leave gas-fitting to professionals.)

[Coder]

The motor looks to be fine but I’ll take a closer look. I think I can run it with the impeller off - though I don’t know if the motor can be run safely with no load?

As for the mounts - I wasn’t sure. The mount consists of a metal bracket with a dampening grommet that is on both sides of the mounting hole. Into this fits a metal washer that has an attached tube - I was under the impression that this washer was to be tightened tightly and the depth it protruded was supposed to bottom out on the sheetmetal. The screws themselves have lock washers, for those to work I’d imagine the screws have to be tight?

And yes - I made sure that the flat and set screw were tightened against each other properly.

[trainspotter48]

As regards running the motor 'light' ie with no load, it will not be a problem with an induction motor, but a series motor with a commutator and brushes can overspeed on no load.

[Coder]

Well... I ended up swapping the fan wheel for another one - vibration is lower but still there. It "pays" to live near repairclinic.com (100% not a commercial - it's just so amazing to be able to pick up a repair part on a whim).

Observing the shaft turn, to my eyes it looked fine, but I didn't use my dial indicator to check if there was any warp to the shaft.

At the moment the system is back together. I've done what I think I can reliably do - am having an hvac technician come over to check the system over (hasn't been checked in over a year) and listen/observe and rate my work

There was one detail about all of this that bugs me. When I aligned the motor bracket against the existing holes, they didn't quite match up. I have a feeling that the rubber mounts are being compressed laterally by the screw, and I probably needed to drill three new holes to make the bracket meet up exactly. So I figure if this is the case there are two things I could have done. Either drilled new holes (which would have been messy) or bent all three brackets in one direction to get them to align perfectly. The metal is pretty thick, however, and spot welded, so bending while tempting seemed pretty dangerous.

This is the motor:

https://www.repairclinic.com/PartDetail ... SP/4959291

Anywhoo - will keep all posted as I started this

[Coder]

Oh, one other thing...

I'm a bit skeptical of how the motor is wired up for heating/cooling anyhow. Whoever worked on it in the past cut the black lead from the motor (high speed) and wire-nut jumped it to Hot H and Cold H. I made a jumper wire with a piggyback connector to have the same configuration so I didn't have to cut the black wire. Regardless, "the internet" seems to indicate heating and cooling fan speeds should be different, or at least tuned. I might bring this up with the tech when he comes by, though I will say we've been mostly happy with how our system is sized and runs. They also cut the quick disconnects off the unused wires and wire nutted them... seems unnecessary and overly cautious? All the quick disconnects on the old motor were fully insulated.

[crfriend]

The motor looks to be fine but I’ll take a closer look. I think I can run it with the impeller off - though I don’t know if the motor can be run safely with no load?

Most household motors are synchronous AC types and can be safely run with no load. Trainspotter48 brings up a good point about series-wound motors which will try to spin up to infinity if run without a load. These, however, tend to be confined to heavy-load short duty-cycle use e.g. as starter motors for gasoline and diesel engines and as DC traction motors in railway locomotives. I have only encountered one AC motor with brushes and a commutator in my life, and that was an antique. Needless to say, if the thing tries to speed up to infinity, the sound alone will make you remove the power. An unloaded AC synchronous motor will spin up to its characteristic speed, usually in the US at some multiple of 60 Hz this will be listed on the motor's label. Only very occasionally will you be confronted with 50 Hz or 400 Hz motors

You sound spot on with your description of the vibration-mounts -- what you're describing is a classic simple setup with washers as bearing surfaces, an elastomeric grommet as the isolator, and a rigid system designed to keep the washers separated by the correct amount to allow for the system to dampen any vibration. Tightening this assembly down, so long as it doesn't crush the elastic component, is the way to work the thing.

And yes - I made sure that the flat and set screw were tightened against each other properly.

The only reason I included that was that I committed that gaffe a long time ago and learnt from the mistake.

[Coder]

And yes - I made sure that the flat and set screw were tightened against each other properly.

The only reason I included that was that I committed that gaffe a long time ago and learnt from the mistake.

No problem - always good to hear a suggestion!

[crfriend]

No problem - always good to hear a suggestion!

One of the joys of the human condition is that not only can we learn from our own mistakes, but also from the mistakes of others (if they're considerate enough to be honest and open about 'em).