Restraint keys are a cost effective method of providing axial restraint for clutches and holdbacks . The restraint key is longer than the clutch through bore length or clutch width . The single block key will prevent axial movement in only one direction and must be used with some other means to prevent movement in the other direction such as a set collar or a step in the shaft . The double block key will prevent axial movement of the clutch in either direction . All restraint keys must be secured in the shaft keyseat . This can usually be accomplished by cutting the keyseat with an end mill without breaking out at the end of the shaft .