Learning from Failure Investigation of Worm Wheel Gearbox in a Wire Rod Mill
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CASE HISTORY—PEER-REVIEWED
Learning from Failure Investigation of Worm Wheel Gearbox in a Wire Rod Mill Sachin Pawar . Goutam Mukhopadhyay
Submitted: 8 June 2020 / Accepted: 14 July 2020 ASM International 2020
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. Keywords Fatigue
Gearbox Shaft EN 24 Misfit Striations
S. Pawar (&) G. Mukhopadhyay R&D and Scientific Services, Tata Steel, Jamshedpur 831001, India e-mail: [email protected]
Introduction Wire rod mill produces bar, rod and wire by rolling of billets. Rolling involves use of heavy equipment and machines. One of the vital part of these machine is gearbox whose primary function is to transmit the power. Gearbox failure causes major breakdowns and consequent losses to the industries. In a study [1] on failure of gearbox, it was concluded that 70% failures were due to bearings, 26% were due to gears and 4% due to other components. Aim of this study is to conduct a failure analysis of a gearbox in wire rod mill. As explained in Fig. 1, failure analysis starts with identifying the failure mode.
Experimental Procedure Visual Observations Just after the failure, site visit was done and the following components of gearbox were visually observed. Shaft: Heavy scoring marks were observed in the keyway region. Width of keyway was found to be increased due to cracking and removal of pieces from both sides. Three of such cracked pieces of shaft from keyway region were found inside the gearbox during dismantling. Multiple ratchet marks were observed along the length of the keyway as shown in Fig. 3. This indicates cracks initiation was from multiple points. Worm wheel hub: Heavy scoring marks were observed in the keyway region. Also, a piece of it was found to be cracked from keyway. A broken piece of hub was found inside the gearbox during dismant
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