PSI - Issue 14

Sudheer Patri et al. / Procedia Structural Integrity 14 (2019) 688–695

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Sudheer patri et. al./ Structural Integrity Procedia 00 (2018) 000–000

Thus, the maximum axial load applied by the drive motor is more than the strength of the given joint. 5. Discussion and recommendations It is clear from the investigations carried out that the screw material does not meet the chemical composition and strength requirements of the specified standard (IS: 1367 (Part-3)-1979). Based on the hardness values, it is assumed that the estimated strength of the screws would be much lower than the specified values as per standard. This implies that the joint has failed at load, which is much lower than the designed value. Among the various possible loading cases resulting in failure, packing errors and operational malfunctions can cause the failure. Since no data is available on the axial misalignment of mobile assembly during packing, it can’t be concluded if packing errors caused the failure of the joint. However, they have potential to cause failure. In case of operation malfunction, there is a chance of over travel during the final stages of testing at manufacturer’s site, which got unnoticed.  Introduce a torque limiter in the drive line so as to limit the over load due to erroneous motor operation  Modify the control logic to switch off the motor power supply upon actuation of torque limiter in addition to existing interlocks  The packing scheme has to be modified to ensue zero axial misalignment of mobile assembly A new M5 screw, procured as per specifications, was tested and found meeting all the requirements specified in the standard. 6. Conclusion The failure analysis shows that inferior material coupled with over-load either due to operational malfunction or due to packing errors caused the failure. Recommendations on stricter quality control and design improvements to limit the over load on mobile assembly during erroneous operation are made. It is also suggested that the operation philosophy of motor be modified so as to sense over load and switch off the motor supply during operation. Care shall be taken to prevent undue over loads during packing and transportation. References IS: 1367 (Part-3)-1979: Technical supply conditions for threaded steel fasteners, Part-III mechanical properties and test methods for bolts, screws and studs with full loadability R. Lacalle, S. Cicero, D. Ferreno, J. A. Alvarez, 2008 “Failure analysis of a bolt in a scaffolding system”, Engineering Failure Analysis, 15, 237 246 Richard G. Budynas, J. Keith Nisbett, 2008, “Shigley’s Mehanical engineering design”, 8 th edition, Tata McGraw-Hill publishing Company V. Y. Yu, J. G. Kohl, R. A. Crapanzano, M. W. Davies, A. G. Elam, M. K. Veach, 2005, “Failure analysis of the M16 rifle bolt”, Engineering Failure Analysis, 12, 146-154 The following recommendations are made for preventing such occurrences in future:  Strict quality control to ensure all the components meet their design specifications