Learning from Failure Investigation of Worm Wheel Gearbox in a Wire Rod Mill

Abstract

Failure of gearbox has always incurred huge loss to the industries due to interruption in operation and consequent repair or replacement cost. This study presents the investigation of a gearbox failure in wire rod mill of a steel plant. Visual and macroscopic examination of the fracture surfaces was carried out to understand the characteristics of failure. Keyway on the shaft and the key were found to be damaged non-uniformly suggesting the movement of key during service before final failure of the shaft. Ratchet marks with smooth fracture surface of shaft indicated fatigue failure. Chemical analysis confirmed the material of shaft, key and hub as EN 24, 45C8 and gray cast iron, respectively. Microstructural observation of the shaft showed inclusions in banded tempered martensitic structure. No inclusions were found in pearlitic matrix of the key, and hub revealed graphite flakes in pearlite matrix. Hardness of all three components and mechanical properties of the shaft were found to be satisfactory. Fractography of the shaft using scanning electron microscopy revealed striations, thus confirming the mode of failure as fatigue. Misfit of the key caused fracture of the shaft in keyway area during service and subsequent failure of the gearbox.