This paper presents the investigation of bearing failure in a compressor test rig which has been satisfactorily in operation. Two pair of single row angular contact ball bearings, each pair in tandem, and the pairs in turn in back to back support the rotor shaft to meet compressor thrust and to provide moderate stiffness for smooth operation. Since being experimental rig, it is reassembled several times involving bearing replacements on each occasion. Recently one bearing on one side pair is found severely damaged and the other bearing of the pair with minor damage. The bearing with minor damage supported the rotor and this evaded the initial diagnostics until the time of severe damage. Initial investigation revealed that the primary cause of failure is washed off by the secondary failure. A thorough, systematic investigation with the help of fishbone approach is carried in order to find out the root cause of the failure and also to decide on new rig. This investigation revealed lack of preload as the major cause and bearing seat clearance as minor problem. New procedure is adopted to ensure proper preload and the rig is back in operation.

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