Sensitivity analysis of cooling methods and geometric parameters in the assembly procedure of bascule bridge fulcra

Abstract

To assemble the fulcrum of a bascule bridge, a trunnion is shrink fitted into a hub, followed by shrink fitting of the trunnion-hub (TH) assembly into the girder of the bridge. To shrink the TH assembly, it is cooled in a medium such as liquid nitrogen; however, during this process in some cases, the hub cracks. To reduce the possibility of such failures, a formal design-of-experiments study is conducted to find the influence of geometrical parameters such as the hub outer diameter and the radial interference (at the TH interface), and different cooling methods on design parameters such as the critical crack lengths and stress ratios. Cooling methods include immersion in, first, liquid nitrogen; second, a dry-ice-alcohol bath followed by liquid nitrogen; third, a refrigerated air chamber followed by liquid nitrogen; and, fourth, a refrigerated air chamber followed by a dry-ice-alcohol bath and then liquid nitrogen. The cooling method contributes the most to increasing the critical crack length (between 58 and 79 per cent) and increasing the stress ratio (between 48 and 84 per cent) when considering the TH assembly procedures in representative fulcrum geometries. Moreover, the second cooling method, which is immersion of the TH assembly in a dry-ice-alcohol bath followed by immersion in liquid nitrogen, gives larger critical crack lengths (between 262 and 406 per cent) and larger stress ratios (between 17 and 87 per cent) compared with the current cooling method that uses only liquid nitrogen.