A round baler includes a bale-forming chamber with a belt assembly for rolling a bale, the assembly includes a plurality of parallel side-by-side belts trained about rollers transverse to the belts, wherein the rollers have a shaft with a plurality of belt-driving sleeves mounted thereon. The belt-driving sleeves have a circumferential groove and the belts have a longitudinal protrusion to track the belt in the groove of the belt-driving sleeve.