623ZZ bearing with Nylon Pulley of 21.5mm diameter
This bearing is fitted inside a nylon pulley which may have many advantages. First, the nylon is hard so it does not get distorded when it got in contact with an hard surface. It is a perfect tool to create convey systems, translatable tray holder, any king of translation setup on a hard surface, etc.
As the nylon is hard, it does not waste mechanical energy as it does not get distorded (which reduce consume some energy).
As the Nylon is hard, it is not the best to be used on rough or unevent surface.
If you do have a lathe, you can also machine the nylon surface to give him the desired V, U, I shape depending on specific needs.