Torsional Fatigue of Pinion Gear Shafts

Two pinion shafts, part of a centrifuge assembly, both failed within two weeks of installation. The centrifuge was originally used to process a slurry; however, prior to the failure the facility began using the centrifuge to process a dense rubber material that the centrifuge was not designed for. The expected service life of the pinion shaft is approximately 7 years.

Both shafts exhibited angled fracture surfaces at the keyway end of the shaft opposite the pinion gear. Fretting was observed on the shaft surface as well as the keyway slot. A secondary crack was observed originating at the corner of the keyway slot of both shafts that propagated in a spiral around the shaft below the surface. One shaft exhibited a complete fracture in this location while the other shaft only exhibited a partial crack. Features consistent with fatigue were observed on both fracture surfaces on the macro and micro level including beach marks and ratchet marks.

Metallurgical sections through the partially cracked shaft at both the primary angled fracture surface and secondary subsurface crack showed multiple secondary cracks in the angled primary fracture surface typical of multiple origin torsional fatigue. Cracking was only observed at one corner of the keyway consistent with unidirectional loading of the shaft.

The microstructure of the sectioned shaft consisted of martensite and bainite with an average hardness of 44 HRC. No indication of case hardening was observed. EDS of the shaft showed it was an alloy steel.

The failure of the shafts was from unanticipated vibrations of the shaft while in service due to excessive loading of the centrifuge beyond the design specifications. These vibrations combined with the slip fit of the shaft caused fretting of the shaft surface and keyway, leading to the formation of two separate instances of fatigue.