Multiple Drive Shaft Fractures in Lift Oil Pumps

Several high-pressure lift oil pump failures were reported from the field. These assemblies are used to inject lubrication oil into the sliding bearings of heavy rotating turbomachinery equipment, such as steam and gas turbine rotors, often weighing in excess of a hundred tons. This ensures that the rotor shaft “floats” on a lube oil film even at low rotational speeds, when hydro-dynamic lubrication conditions have not yet been fully established. A fractured spline shaft, which was the driven shaft of one of the failed pumps, was received from the client for the determination of the metallurgical root cause of failure. The subject spline shaft failed due to torsional overload. This cracked the hardened case and initiated high cycle fatigue (HCF) cracking as secondary damage. The main fatigue cracks were nucleated at overload fractures in the hardened case, in the fillet radii at the base of the teeth of the spline shaft. No evidence of any material defects that could have contributed to the failure or could even have been causative for it was found.