Abstract: The invention relates to crop harvesting machines fitted with detector means for detecting the presence of undesirable objects such as metallic objects. The invention seeks to solve the problem of known machines in which some form of slipping clutch is provided which operates when drive is arrested due to the presence of an undesirable object and which may overheat, and the problem of normal drive being re-established without first ensuring the removal of the offending object. According to the invention a harvesting machine comprises crop processing means, feeder means for feeding crop material to the processing means, a drive train for driving the feeder means, and detector means for detecting undesirable objects passing through the feeder means, the detector means being operable upon the detection of an undesirable object to initiate instantaneous arrest of the drive to the feeder means and to break the drive train for the feeder means.