My welch pump siezed up after a few months of makign the occasioanl screeching sound. I decided it was time to tear it down and rebuild it. It turned out a small screw had fallen into the main vacuum chamber. I had to replace the springs whihc eject the metal fins whihc form the vacume tight seal in the main pumping chambers. I also went ahead and replaced the rotary shaft seal. Many thanks to Cody Moynihan for a lot of consulatation when tackling this.
The screw!
It's been rebuilt and pumped down to 1 mTorr!
