As　the　transmission　gear　box　on　the　top　of　the　blast　furnace　works　permanently　under　the　heavy　load,　high　temperature,　dust　and　blast　furnace　gas,　its　working　condition　is　very　severe.　In　order　to　avoid　its　sudden　malfunction,　its　vibration　and　the　grease　during　the　working　period　were　detected.　Furthermore,　the　data　were　analyzed　using　the　frequency　spectrum　and　action　spectrum　methods.　Finally　the　test　on　the　material　of　the　failure　parts　was　made　after　the　gear　box　was　disassembled.　The　above　research　manifested　that　the　running　stop　of　most　transmission　gear　box　on　the　top　of　the　blast　furnace　was　caused　by　the　lock　of　the　cross-rolling　bearing,　and　one　of　the　reasons　of　the　lock　was　the　damage　of　the　cage.　The　rolling　elements　in　the　bearing　disturbed　each　other　after　the　damage　of　the　cage,　and　the　moving　condition　of　some　rolling　elements　changed　from　rolling　to　sliding.　As　a　result,　the　rolling　elements　pressed　each　other　and　were　damaged,　finally　the　rotation　of　gear　ring　was　impeded　and　locked.