In a certain steel making plant, one casting crane was mended to increase its main lifting capacity from 160t to 180t. To support 180t lifting load, main trolley was rebuilt and two main girders were mended. The casting crane could work normally, before its main lifting capacity was increased. After it was mended, connecting bolts to fix main trolley rails were often loosing. If the pretightening force was increased, girth welds to fix bolts were often ruptured. In order to solve these problems, the root causation must be found. In this paper, finite element analysis is applied to aiding this research. Connecting bolts to fix main trolley rails and girth welds to fix bolts are selected as main researching objects of FEA. They are studied within girders system, and special load conditions of this crane are considered carefully. By FEA, an inversion is used to determine pretightening force of bolts in girders system before and after the increasing of main lifting capacity of this crane. After FEA results of bolts and its girth welds are carefully analyzed, root causations of loosing of bolts and rupture of girth welds are found.Finally, a method to solve these problems is offered to ensure this crane working safely.
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