From what i understand, they are a sealed bearing commonly used on small agricultural machinery.
For whatever reason, the 5/8 keyed shaft is made from soft metal and the hardened inner bearing shell will cut a grove in the surface of the jackshaft if this area is not regularly lubricated.
When the wear becomes significant (1mm into the surface on both sides of the jackshaft), it changes the angle of the jackshaft relative to the chain as the small 10 tooth (or optional 9 and 11 tooth) sprocket meshes with the chain side plates.
With 8 speed chain, it doesn't seem to be much of a problem but when using (narrower) 9 speed chain, the wear shows up by having the sprocket teeth smash side plates in half.
Typically (and in my situation which includes a lot of off-road riding) i've found that it takes around 6,000 kilometers before the jackshaft shows wear that makes it unusable; needing replacement.
Having said, regular lubrication with a graphite based motorcycle chain lube, significantly reduces wear at the point where the inner bearing shell meets the 5/8 keyed shaft.
As things currently stand, i've upgraded the 5/8 keyed shaft from mild steel to stainless steel, but have yet to see how it stands the test of time.