Failure of Derricking Shaft of Dragline Excavator
Severe damage to the jib of a dragline excavator resulted from failure of the shaft which carried the derricking sheaves at the apex of the "A" frame. Failure occurred within the hub of the center sheave of the group of three at the right-hand end of the shaft. The shaft was manufactured from a 0.5% carbon, 1% chromium steel heat treated to give a hardness value of 300 VDP. The material was in the hardened and tempered condition and showed no abnormalities which would predispose to early failure. The content of non-metallic matter was only of nominal amount. Failure of the shaft resulted from fatigue due to the cumulative action of the repeated stresses which it had been subjected to during service. The shaft had been subjected to repeated stress applications sufficient to result in the initiation and development of a fatigue crack at the radial hole. To prevent a repetition of the failure it was recommended that the stress-raising effect of the holes be reduced by chamfering or preferably rounding-off the edges. Furthermore, rotation of the shaft should be prevented so that the radial holes were positioned on the opposite side of the shaft.