A turbomachine has a housing, a rotor shaft extending along and rotatable about an axis in the housing, and a pair of axially spaced axial-thrust bearings in the housing carrying the shaft and normally only permitting a predetermined small axial displacement of the shaft in the housing. An impeller is rotationally fixed on the shaft. An axially fixed transmission gear is rotationally coupled to the shaft and has an axially directed gear face. A collar fixed axially on the shaft has an axially directed collar face confronting the gear face and normally spaced from the gear face by a clearance equal to more than the predetermined axial displacement. Thus on failure of one of the bearings and axial overtravel of the shaft the gear face and collar face engage each other and limit axial movement of the shaft.