I have a Servo Products CNC mill with about a 1/8" thick round drive belt. I picked up a couple that are supposed to be for the machine. I went to measure speeds for various settings for future reference and was fine on the low range. On high range about halfway up the speed range the belt started coming off the pulley due to speed. It was still running in the grooves but getting much larger. A little higher speed and it was barely making contact with the spindle pulley and slipping terribly even with no load on the spindle. I ran through the speeds some time ago and don't recall this issue.

Should I be taking the belt off the pulleys when not in use so it doesn't get stretched out? I see McMaster Carr has lots of selection of these type belts in both neoprene and urethane. I am not sure the material of the current belt but it is black. Which would be better? The smallest pulley is about 1" diameter and goes about 20k RPM if the manufacturer claims are accurate. How do I measure the belt needed? Do I measure the relaxed belt length or the belt length when it is on the pulleys?