I think one size of small sprocket in the KOP has the flanges on it, but the wider belts don't fit on it! Adding the flanges is probably the quick and easy way to keep the belts on.
We ended up using chains for all our roller drives, mainly because of the availability of different sprockets and the fact that you can make chain be whatever length it needs to be pretty easily, no need to go shopping for just the right length belt. And it stays on the sprockets by design
