Summary

This is a pulley and weight hanger assembly suitable for use in a physics classroom. Designed for use with SimpleSumo robots: https://www.thingiverse.com/thing:2574286

On this setup I recommend using pennies for weights because they are standard small equal units of weight, easily available, and cheap.

The pulley uses one 608 bearing. This link is for a fidget spinner bearing which will spin with the least resistance, although you will have to protect it from dirt: https://www.amazon.com/LONMAX-Bearing-Tri-Spinner-Fidget-Spinner/dp/B06XTZJDCC/ref=as_li_ss_tl?rps=1&ie=UTF8&qid=1507876654&sr=8-6&keywords=608+bearing&refinements=p_85:2470955011,p_72:2661618011&linkCode=sl1&tag=engin09-20&linkId=63649e3a9e2da6a033db7e5c64d36aaf

Uses one 5/16" bolt to hold the assembly together.

Video of experiment involving this pulley setup shown at 6min 18 sec:
https://youtu.be/JAGPEJZuJWw