An auxiliary drive mounted to a reel mower cutting unit includes an auxiliary drive housing with an input shaft rotated by the reel mower cutting unit, an output shaft for rotating an attachment, and a belt transmitting rotational power from the input shaft to the output shaft. An eccentric bearing housing is positioned in an opening in the auxiliary drive housing. The output shaft is inserted through a bore in the eccentric bearing housing that is offset from the center of the eccentric bearing housing. The eccentric bearing housing may be rotated within the opening to tighten the belt by shifting the position of the output shaft with respect to the input shaft.