A round baler having a main frame, a tailgate pivotally connected to the main frame and an apron extending around a plurality of guide rolls disposed in the main frame and the tailgate. A pair of take up arms, rotatably mounted on the main frame, carry at least one additional guide roll for the apron. A bale forming chamber, including the apron, varies in size from a bale starting position to a full bale position. A pair of levers, connected to rotate with the take up arms, have attached thereto a tensioning mechanism to urge the take up arms to the bale starting position of the apron. The machine is provided with apparatus for wrapping the formed bale with twine and a unique system that operates in conjunction with the levers to automatically trip the wrapping apparatus when the bale formed in the chamber has attained a desired size.