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