Failure Analysis of Two Screws Used in the Fixing System of Ball Mills Internal Covering
Francisco Bahia Pena1; Geraldo Lúcio de Faria1
1 Universidade Federal de Ouro Preto
doi:10.20906/CPS/COB-2015-0894
Resumo
This work presents a study case about the failure analysis of two steel screws, with different geometries, used in the fixing system of industrial ball mills internal covering. This kind of failure has been often occurred in some Ball mils installed in Brazil. Two distinct screws, failed in service, were sampled and submitted to standardized procedures of failure analysis. Studies about the screws work conditions, as well about the material specifications for this application, standardized by international associations, were done. Chemical, macro and microstructural analysis by optical and scanning electronic microscopes, microfractographic analysis and hardness test were executed in samples of the both studied screws, aiming to determine the failure main causes. Stress concentration factors were calculated in screw threads applying mathematical models proposed by different researchers. The stress concentration factors for static and dynamics loading were compared for the both different geometries. The obtained results showed that screw thread roots are the main stress concentration points in screw structures. This fact was aggravated due to the occurrence of corrosion pitting in these areas caused by its use in an aggressive environment. It was possible to conclude that fatigue cracks have nucleated in these areas of high stress concentration and have growth until a critical size, spreading unsteadily and causing the screws failure.
Palavras-chave: Fixing Screws; Ball Mills; Failure Analysis; Fatigue