Failure Analysis of Bucket Elevator Conveyor Chain Links

Abstract

Although designers and manufacturers continue to strengthen the links between design, manufacturing and performance, failures still occur and will continue to occur for one reason or another. In view of this, the cause or reason for failure is paramount for future designs. Chain bucket elevator drives are among the primary systems used in the cement industry to convey powdered cement vertically. Conveyor chain components that suffer premature failure need to be replaced on a frequent basis, negatively impacting on productivity and increasing the cost of the operation. The main objective of this research is to determine the cause of failure of the chain links of a bucket elevator by carrying out failure analysis on both failed and un-failed chain link samples. The specific objectives of this research are to determine the point of initiation of the fracture, analyse the mechanism of failure and design new component to minimize future failure and test its performance through modeling and simulation. Failure analysis was performed on failed and un-failed chain link samples obtained from East African Portland Cement. The methodology adopted included preliminary examination, metallurgical analysis and chemical analysis. Preliminary examination done on ten failed samples of chain links using stereo microscope revealed a brittle fracture and chevron marks showed that the fracture began either from the core or boundary of the fractured surface and progressed xvi through the material until eventual fracture occurred. The point of fracture initiation was dependent on where the inclusion was located either at the boundary or the core. Metallurgical analysis done on both failed and un-failed chain link samples revealed that the micro-structure for both was that of a tempered martensite. The un-failed chain link sample has a lot of blow holes and inclusions which are as a result of a manufacturing defect within it but showed no crack within it. The failed chain link samples observed under an optical microscope revealed a lot of cracks on the fractured surface which propagated during loading. Chemical analysis revealed that the carbon content for sample 1, 2, 3, 4 and 5 were 0.131%, 0.133 %, 0.135 %, 0.202 % and 0.129 % respectively, which was below the required range of 0.27-0.34 % according to European standard EN 10293. Carbon increases the hardness of steels. The reduced carbon content improved the ductility of the steel. The cause of failure was deduced to be as a result of inclusions from which the cracks had initiated from. The existing chain link design was improved by re-designing to eliminate the neck. The chain link was tested through modeling and simulated using analysis system simulation software. The results obtained from the modeling and simulation show that the new design is an improvement on the existing design as it had better fatigue life, deformation, safety factor and von Mises stress than the existing design. The fatigue life increased from 8:25 1010 cycles to 1:08 1011 cycles which was 23.61% improvement on the existing design whilst the equivalent stress in the existing design reduced from 142 MPa to 133 MPa. The safety factor also increased from 3.77 to 4.26 where as the deformation xvii of 0.05644 mm in the new design was less compared to the existing design i.e. 0.06101 mm. These results obtained are beneficial to the manufacturer of the bucket elevator conveyor chains in that during the manufacturing process of the conveyor chain links, due diligence will be accorded not only to the carbon content but also all other constituent elements so that they meet the required standards. As a result of improved manufacturing process and design of the conveyor chain links, the user (East African Portland Cement) will be supplied with improved conveyor chain links. The improved conveyor chain link will minimize down-time thereby increasing productivity.