Quote Originally Posted by GITRDUN View Post
The encoder is actually easy to get to on this one. Ill pull the belt and pulley off and inspect or replace. When i say its vibrating while oriented i mean its violently vibrating. It sounds like a hammer drill. I know enough not to grab a firm hold on the spindle nose just in case, i wouldnt recommend any one doing that either but thought i would share it. The harder you push on a servo motor the harder it pushes back.
That's a better description of your issue and definitely not a good thing.

For what you are describing now, I would doubt that the belt will be the root cause as I have never witnessed a belt do something like that. A worn belt/pulley will not help things granted, but it's quite possible that the encoder is bad as you suspect.

When the machine is at a commanded rpm, does it hold the commanded speed well or does it make big jumps; a few jumps in rpm is fairly normal? Any issues with orientation changing?