O-ring belts evolved from flat belts used in early power transmission systems, which wore unevenly and slipped.
Designed as a circular cross section belt, these specialized O-rings operate in a pulley with a groove in low-torque situations.
Especially useful in 3-D applications, round belts come in various forms:
with solid or hollow cores
in many standard sizes
non-standard lengths can be spliced to your size requirements