Fatigue of Counterweight Sheave Trunnions - Heavy Movable ...

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The u.ork began at the west tower. The first phase of the work was machining the fillet to remove fatigue-damaged material along with any cracks. A milling machine with a ball end mill was employed for the niachining. The XI'' radius of the ball end niill is larger than the existing %" fillet radius, which acts to reduce the stress concentration factor. Cracks were discovered in both west tower trunnions (north fillets only) during machining. The crack in the NW outboard fillet was the longest. This crack was continuous for 31 0' around the trunnion circumference. The crack in the SW inboard fillet had broken into nine shorter length cracks (-2"- 4" segments) at the specified 5/64" machining depth. Fatigue Life Extension of Main Countetweight Sheave Trunnions The first step in dealing with the unforeseen cracks was to determine their maximum depth. If the cracks were too deep, then repairs would not be practical. The cracks in the SW inboard transition fillet were "chased" locally using a dye- grinder until they were removed. The maximum crack depth was W' from the original fillet surface. The total trunnion diameter would be reduced by %" based on the orientation of the mill cutter. It was determined that the 30-year fatigue life was obtainable, partially due to the reduced stress concentration from the enlarged fillet. The fillet machining was resumed until the desired depth was achieved. Magnetic-particle and dye-penetrant tests were performed to ensure that the cracks were completely removed. A similar procedure was followed on the NW outboard transition fillet. The maximum crack depth was determined to be 318". The total trunnion diameter would be reduced by 1'' based on the orientation of the mill cutter. It was decided to increase the fillet radius to 1" to hrther reduce the stress concentration and ensure removal of the crack tip(s). Milling work progressed at a rate 0.0 13.' of material removed per hoiir. 'file milling machine was in operation around the clock for several days to remove these cracks. After ~nachining, the transition fillets were shot pecncd by the Metal Improvement Co. 'lhe cost for design and construction of this repair was $600,000. HEAVY MOVABLE STRUCTURES, INC. 91h ~iennial Movable Bridge Symposium
Acknowledgements Fatigue Life Extension of Main Countetweight Sheave Trunnions The Port Authority of NY & NJ and the Maine Department of Transportation provided valuable overall leadership that lead to the first field applications of machining and shot peening sheave trunnions for fatigue life extension. The Metal Improvement Company, headquartered in New Jersey, developed and engineered the equipment and techniques to shot peen trunnions on-site. They were consulted throughout the investigation and design phases for their knowledge and expertise in the field. Daskell Corporation, headquartered in New York, and Cianbro Constructors, headquartered in Maine, worked to high standards as general contractors charged with completing this challenging work. HEAVY MOVABLE STRUCTURES, INC 91h Biennial Movable Bridge Symposium
Page 2 and 3: Introduction Fatigue Life Extension
Page 4 and 5: It C.L. SHEAVE Fatigue Life Extensi
Page 6 and 7: Fatigue Life Extension of Main Coun
Page 10 and 11: Trunnion Repair and Strengthening O
Page 12 and 13: . . . 111. iv. v. vi. vii. viii. ix

Fatigue of Counterweight Sheave Trunnions - Heavy Movable ...

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