Loctite 648 that I use is also for cylindrical parts mounted on shafts. Seems very similar to what Keith recommended but his is more for bearings. 648 is for gears, sprockets and pulleys, like for a CNC machine. Both may be the right tool for the job.
Here's the details from Loctite:
"When you’re working with holding gears and sprockets on gearbox shafts, or rotors on electric motor shafts, LOCTITE® 648 is ideal for preventing loosening and leakage from shock and vibration. This green, low viscosity, high strength, anaerobic-curing retaining compound provides reliable bonds of cylindrical assemblies, even when exposed to high temperature environments. It cures on active metals (e.g. mild steel) as well as passive substrates like stainless steel and plated surfaces, and it even tolerates minor surface contaminations from oils such as cutting, lubrication, anti-corrosion and protection fluids without the use of a primer or cleaner."
Thanks. I really appreciate that! This servo is being used as a spindle motor, so there is probably less acceleration/deceleration associated with this use and therefore less torque. In either case, I will certainly use the Loctite.
For what it is worth, the PDF (From Loctite) shows the difference between the 648 and 620. Essentially 620 is thicker, initial cure is a bit slower and it is rated for higher temperature. Seems like both would do the job.
I fired up the spindle for the first time today to see how the pulleys/belts align (see below). When I reversed direction, the belt rose up on the rear pulley slightly (Less than 1/8"). Just curios if the belts track differently as they wear in? Are you able to make minor adjustments while the loctite is curing?
My small router has belt reduction on the X and Y axis with 1" wide Gates poly belts and there is slight belt creep when the motor changes direction. It's been like this for almost 2 years with no issues.
The 648 that I use allows me plenty of time for adjustment. Seems like it's still slightly tacky the next day but after that it hardens up and hold great. If you even need to remove it just apply a little heat to the pulley and it slips right off. I'm sure it's the same procedure for the 620.
Both may be the right tool for the job.