A bearing cage retainer (11) for a rolling element rotor bearing in a vacuum pump. The bearing cage retainer being configured to have an operational arrangement in which, at the maximum longitudinal axial displacement limit of the outer race (5) in the direction of the retainer, the bearing cage retainer is disengaged from the bearing cage (7), and a failure configuration where the dislocation of the bearing cage by a longitudinal axial displacement of the bearing cage relative to the outer race in the direction of the retainer and in which the bearing cage retainer engages said bearing cage and the bearing cage maintains the separation of the rolling elements within the bearing.