The first two were mounted on the tractor with a too long PTO shaft. Lift up the arms and bang. The last one was a blockage and a locked up slip clutch. The driver thought if he kept re starting the PTO with the engine flat out it would clear it. We lock the PTO slip clutch as the drivers never notice its slipping and it gets red hot in minutes, not good in the middle of a large poultry shed.design fault or dumb farmers.....?