A picture of the part is below. It is the wheel — don't know what it would be called specifically — in it. It's about 4-5" in diameter and about 1-2" wide. In this case it is actually driven by a motor.
The element in the picture showing the cross-sectional area is part of a large (roughly 7 foot diameter) loop made out of a metal pipe approximately 1-1.5" in diameter. That loop sits on top of the wheel. So what you're looking at below is the perspective if you were to be looking down at the wheel from above. The wheel (item I'm asking about) when driven, results in the loop spinning at the same rate as the wheel. The only thing that causes the loop to spin with the wheel is friction between the two surfaces.
There's about a 200 lb downwards force on the ring that increases the friction between it and the wheel — inhibiting slippage. Also, the wheel spins relatively slowly — maybe 5° per second.
The problem: I'm trying to source this part, but am having trouble finding something that will serve the purpose. It's really a roller, but every time I search for "roller" I get something more along the lines of this:
Which fundamentally is very different. Even if I search "rubber roller" it's just the same thing but with a rubber coating. The main difference is that what I'm looking for has a dip that sort of hugs the aforementioned loop. All of these "rollers" I'm finding — even if I specify a much smaller width do not have that component; which makes me feel like I'm using the wrong terminology.
What would this part really be called and where can I find something that serves its purpose?