Abstract: The supporting structure design and shafting dimension chain arrangement will directly affect the safety and reliability of mechanical systems. In this paper, the structural principle and working condition of an oil mist separator were analyzed, and it has been revealed that suitable clearance between one end of the outer ring and the shell hole is necessary to fit the thermal elongation of shaft in the mechanical system with fixed support at both ends. As an example, a failed bearing was analyzed by surface morphology observation and metallurgical structure examination. It is found that the bearing balls rode over the ring land under excessive axial loading, resulting in high-temperature failure during short period. In order to verify the relationship between excessive axial loading and shafting dimension chain arrangement error, assembly dimension chain has been calculated and it has been confirmed that the clearance between one end of the outer ring and the shell hole may have disappeared and the failure is related to shafting dimension chain arrangement error. Accordingly, suggestions have been put forward, including components matching before assembly, components dimension adjusting and clearance measuring.