A pair of concentrically mounted drive shafts (820, 840) are rotatable with respect to one another. Each drive shaft engages and drives at least one endless chain upon which lugs are mountable. A driven power shaft (860) directly drives one (820) of the drive shafts. The other drive shaft has a coaxially-mounted gear wheel (850) that intermeshes with a pinion wheel (870) that is rotatable and is interconnected with the driven power shaft such that when the pinion wheel is inhibited rotating power is delivered from the driven power shaft through the locked pinion gear to the gear wheel and drive shaft (840) affixed thereto.