PSI - Issue 14

Sudheer Patri et al. / Procedia Structural Integrity 14 (2019) 688–695 Sudheer patri et. al.,/ Structural Integrity Procedia 00 (2018) 000–000

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4.2.1 Packing errors Before transportation of SNDHM from the manufacturer’s site, it is meticulously packed to ensure that the neutron detectors are not subjected to undue vibrations or impact loads during transportation. Detailed packing scheme is beyond the scope of the paper. However, it involves positioning the mobile assembly in its top positions and locking it from both axial and lateral movements by an Aluminium block. In case if the mobile assembly is positioned with an axial misalignment with respect to its top position, then tensile loads get induced in the mobile assembly and transmitted to the screwed joint under study. Such induced tensile loading, if higher than the strength of the joint under study, may result in fracture of all the screws. 4.2.2 Loading during transportation The SNDHM is fixed to a transportation structure and transported in horizontal condition on a multi axle trailer. During acceleration or braking action, the MA is subjected to inertia loads. The acceleration / deceleration needed for the failure of the joint can be calculated as follows: The mass of mobile assembly = 450 kg The total inertia load = the breaking load = 62280 N (as calculated above) Therefore, ƒ…‡Ž‡”ƒ–‹‘ ൌ ୫ ୪୭ ୟ ୟ ୱ ୢ ୱ ൌ ଺ ସ ଶ ହ ଶ ଴ ଼଴ = 138 m/s 2 > 14 g Acceleration / deceleration greater than 14 g is not practically possible. Hence, we can conclude the failure would not have occurred during transportation. 4.2.3 Operational malfunction The mechanism is operated by a motor-reducer assembly. Redundant reed switches, with 2/3 logic, are provided to stop the motor at mobile assembly top and bottom positions before reaching the mechanical stops. Protection of mechanism from over load was taken care in the design by providing torque limiter. However, during testing, it was found that the torque limiter was not getting engaged after slipping. Hence the torque limiter was replaced with a coupling and additional reed switches were provided for protection of mechanism. However, any malfunction of the reed switches or control logic may result in over travel of mobile assembly pressing against mechanical stop. Then, motor delivers maximum torque, which induces axial loads in mobile assembly and the same are transmitted to the screwed joint under study. Such induced tensile loading, if higher than the strength of the joint under study, may result in fracture of all the screws. The maximum axial load generated due to motor operation is calculated as follows: The mechanism is operated by a 0.5 HP squirrel cage induction motor- reducer unit with 48 rpm output speed. Thus the maximum torque developed by the motor is calculated as follows. ݉ܽ ݔ ݅݉ ݑ ݉ ݐ ݋ ݑݍݎ ݁ ൌ ெ௢௧௢௥ ௉௢௪௘௥ ௔௡௚௨௟௔௥ ௩௘௟௢௖௜௧௬ ൌ ଴Ǥହ כ ଶ ଻ସ଺ כ గ כ ర ల ఴ బ ൌ ͹Ͷ ܰ݉ Efficiency of ball screws = 0.9 From energy balance, axial force * lead = efficiency* torque * 2* π ݄ܶ݁ ݎ ݂݁݋ ݎ ݁ǡ ܽš‹ƒŽ ˆ‘”…‡ †‡˜‡Ž‘’‡† ൌ ݂݂݁݅ܿ݅݁݊ܿ כ ݕ ்௢௥௤௨௘ כ ଶ כ ௅௘௔ௗ గ ൌ ͲǤͻ כ ଻ସ כ ଶ כ ଴Ǥ଴଴଺ గ = 67858 N