Harbor Freight 12" disc sander vibrates (only) during startup

[Bert McMahan]

I recently picked up one of the older Central Machinery 12" disc sanders from an estates sale for cheap. It has a weird, intense vibration for about a half second during startup. Once it's running, it runs nice and smooth and works well. When I turn it off it coasts for forever, but doesn't seem to have a vibration during slowdown.

Is that just some centrifugal switch engaging/disengaging? I'm wondering if a bearing is on its way out. Unfortunately my Googling is only bringing up threads of people who have vibration while it's running, which isn't my case. Obviously this isn't a really nice sander, so if that's just how it's built I can live with it.

[Robert Hayward]

I have a Powermatic DS20 that is similar to what you describe also on start up only. I think it is a current inrush that causes it. With mine it is closer to three to five seconds though. Once the disc get up to a certain RPM the noise goes away, has done it since new. The disc coast time after power off is in the 18 minute range with my DS20. Glad it has a hand brake.

[Richard Coers]

When a bearing is "on its way out", it starts making noise. Usually through the entire rpm range and often most noisy at full rpm.

[Ken Fitzgerald]

As devices spin up to speed, it's not unusual for them to vibrate as they pass through speeds that are not their final run speed and to do similar things as they shut down for the same reason. It has todo with harmonic balance and mechanical resonant frequency.

Vibration at its normal operating frequency is an issue.

[Bert McMahan]

I just realized I forgot to follow up on this- it turned out the screw holding the wheel on the arbor had loosened, and it was mainly attached with just the adhesive on the sandpaper disk! When I removed the old disk, it was clear that the whole thing was loose! I tightened it up and the vibration is gone.

I hadn't noticed it because it appeared to "seat itself" after spinning up, which is why the vibration stopped, so the whole thing didn't really flex when I sanded anything with it and put some light pressure on it. I'd also only used it a couple of times before replacing the abrasive and noticing the loose screw.

[glenn bradley]

It be a QA issue. I went shopping for one at HF with my dad and we had to return three before we got one that ran smooth. It has continued to run, smooth for years and years.