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Thermal power plants mostly employ bowl mills to crush coal which in turn is used in the boiler for generating steam. These bowl mills encounter failures with respect to grinding components and/or other accessories. In the recent times, fretting failure is assuming more significance with respect to failure of the bowl mill components. Fretting, generally, occurs when there is a relative movement, of about a few microns, between two mating surfaces that are under a high contact stress. In the present case study, involving gear box of a bowl mill, input shaft has failed due to fretting and investigations revealed concluding evidences. Various investigation techniques like, visual, stereo microscopic examinations and Scanning Electron Microscopic studies, etc. were carried out to ascertain the root cause for failure. These techniques revealed that the shaft failed due to fretting and results are discussed in the paper.

Gasturbines are provided with cutouts for minimizing vibrations. Round cutouts are usually favored but there are various designs which offer more advantages over the circular shape. The objective of this work was to compare stress reduction during the induced vibration in the hollow gas turbine shafts by introducing different cutouts. Round, square and elliptical cutouts under different orientations were compared. It is observed that a square cutout at 45˚ orientation has the least stress concentration. This is due to the effective orientation of plastic strains along the principal axis.