Abstract: The invention provides a crop harvesting machine comprising crop processing means, feeder means including feed rolls for feeding crop material to the processing means, drive means operable to drive the crop processing means, further drive means operable to drive the feeder means and detector means operable to detect foreign objects in the crop material in use handled by the machine before they reach the crop processing means, the further drive means comprising a drive reversing mechanism and a belt drive transmission with a belt drive engaging mechanism which is movable between an inoperative position and a drive-engaging position, and the detector means being operable upon detection of an undesirable object to initiate instantaneous arrest of the feed rolls and to move the belt drive engaging mechanism from its drive-engaging position to its inoperative position.