A combine harvester comprises a straw chopper (10) which is driven by a belt (18) via a first drive pulley (14) that is rotatable about a fixed shaft (17). An output pulley (16) is also rotatable about the fixed shaft and drives the chopper. A coupling sleeve (19) is slidably mounted on the drive pulley or output pulley, the sleeve being axially slideable between the two pulleys between two positions. In a first position the sleeve connects the pulleys co-rotation about the fixed shaft to drive the chopper. In a second position the sleeve is positioned so that the two pulleys are disconnected so as to disconnect drive to the chopper.
