Fatigue Cracking of Welded Structures Under Dynamic Loading

Two examples concerning fabricated mild steel rotor spiders which failed due to lack of torsional rigidity, probably supplemented by the presence of high internal stress, are described. The machine concerned in the first case was a 3,000 hp three-phase slip-ring motor. In the second case the machine was a 200 kW alternator, direct-driven by a diesel engine running at 750 rpm. Both the foregoing failures reveal the same basic weakness, i.e., insufficient rigidity when subjected to variations or reversals of torque. In the first case, the bars welded to the arms were inadequately supported in a lateral direction, so that excessive stresses of a fluctuating nature were set up in the welds as a result of the frequent load changes that arose in service. This weakness was eliminated when designing the replacement spider. In the second example, failure also arose as a result of deficient torsional rigidity with the consequent development of excessive stresses in the welds at the junctions of the bars with the sleeve, the torque being of a fluctuating character due to the impulses imparted by the engine.