means for outputting a message when the wear position deviates by more than {HEAD ~ a permissible degree} of {TAIL ~ wear} 
of
means for outputting a message when the wear position deviates by more than { HEAD ~ a permissible degree } of { TAIL ~ wear }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL