Failure analysis for a reduction gear and bolts failed by fatigue was carried out by means of fractography, and the values of stress amplitude during service were estimated using a relation between striation spacing and stress intensity factor range, ΔK. The failed reduction gear had been used for a hoist with a rated load of 5 tonf attached to a travelling crane, and fatigue cracks initiated from the root of teeth. The bolts had been used for connection of slewing circle and counter weight of a truck crane. Metallurgical examination, fatigue test and calculation of applied stress were also performed to clarify the failure causes. The stress distributions of 32 high strength bolts of 20mm diameter on a slewing circle were measured by strain gauges mounted on the bolts, and the measurement was carried out using the same type of track crane as the failed one.
The value of stress amplitude evaluated from the striation spacing was found to be almost equal in stress to that of the result of calculation or actual measurement of stress. Therefore the combined fractographic-fracture mechanics method was found very valuable in the quantitative analysis of fatigue failure of machine parts. The failure cause of the gear was related to the lack of hardness of tooth root. The fatigue limit of the bolts was 4.8kgf/mm2. The couse of failure for the bolts was due to overload during operation. Moreover, a high stress concentration for few bolts in the slewing circle may also be related to the failure cause.
