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.