Spindle drive vibration

When test running the spindle thru various rpm's I noticed I get get some vibration at certain speeds.
I don't notice any vibration at all in high range at any RPM, but in low range I see the idler jumping around and the motor starts shaking. Especially around 800-1000 rpm.

I looked at the belts pretty close any they look fine. I wonder, since the machine did not run for a long while, if the low drive belt is deformed from tension being applied to the same spot this entire time? I ran it about 5 minutes the other day and it seemed to calm down a little. I guess if I were energetic I could swap the belts around, or just replace them, but it's more time and just another hundred bucks!

Richard

The nice thing about the belt drive is they are not under tension until it is used. It could be a large amount of goop stuck in the belt and sheave. I recently rebuilt my entire Z axis head and you wouldn't believe how much crud gets up there. I am sure you made sure all the bolts holding the motor are secured and nothing is missing. Where is it vibrating? Is the actual cut vibrating or is the machine vibrating.

One thing that happens/ed on one of my machines is there is a harmonic at a certain RPM that made the machine sound like it was going to explode. The harmonic was on the Z axis cover and once I found it I just bolted a stiffener to the sheet metal and that took care of it.

I checked a little closer today and the motor, and motor mount along with the idlers shake pretty bad from 600-900 RPM.
It looked like the mounting bolts were loose but after checking they were all tight. Kind of crowded in there so I can't get a good look.
I take it this could be from screwed up rubber isolated mounts?
If so, is it worth it just just make some solid mounts here at the shop?

Richard