The baling chamber of one type of round baler is defined by belts which are trained over rolls connected to a drive. Stones can enter the nip formed between the belts and some of the rolls, e.g., a lower front belt driving roll, and thereby cause poor tracking of the belts. To reduce the problem, a shield device is snap-fit mounted on the roll. The device includes a nip bridging plate an upright portion of which abuts the belt run advancing to the nip and a downwardly sloping lower portion of which bridges the nip. Thus the device covers the nip preventing access to it by the stones, and stones which fall upon it slide down the sloping portion of the plate eventually to find their way harmlessly to the ground.