Does step-cooling make the trunnion-hub-girder assembly procedure fail-safe?

To assemble the fulcrum of a bascule bridge, a trunnion is shrink-fitted into a hub, followed by the trunnion-hub (TH) assembly being shrink-fitted into the girder of the bridge.  To shrink the TH assembly, it is cooled in a medium such as liquid nitrogen, and during this process in some cases, the hub cracked.  To reduce the possibility of such failures, a formal design of experiments study is conducted to find the influence of geometrical parameters such as

(1) hub outer diameter and

(2) the radial interference (at the TH interface), and

different cooling methods such as immersion in

(1) liquid nitrogen

(2) dry-ice/alcohol bath followed by liquid nitrogen

(3) refrigerated air chamber followed by in liquid nitrogen,

(4) refrigerated air chamber followed by in dry-ice/alcohol bath and then by in liquid nitrogen

on design parameters such as critical crack lengths and stress ratios. 

The cooling method contributes the most to increasing critical crack length (between 58% and 79%) and increasing the stress ratio (between 48% and 84%) when considering TH assembly procedures in representative fulcrum geometries.  Moreover, the second cooling method, which is immersing the TH assembly into dry-ice/alcohol bath followed by immersing into liquid nitrogen, give larger critical crack lengths (between 262% and 406%) and larger stress ratios (between 17% and 87%) compared to the current cooling method that uses only liquid nitrogen.

Related Papers

N. Collier, A.K. Kaw, G.H. Besterfield, M. Rahman, "Benefits of Staged Cooling of Composite Cylinders in Shrink Fitting", The Journal of Strain Analysis for Engineering Design, pp. 349-361, Vol. 41 (5), 2006.