High speed epicyclic gearboxes used in gas turbogenerators are typically designed with either carburized and hardened or
nitrided gears. Each of these types of gear has its own merits and limitations. While carburized and hardened gears offer
optimum design, nitrided gears are sometimes preferred because of their low heat-treat distortion. In terms of load
capacity, carburized and hardened gears perform satisfactorily not only under steady load but also heavy shock load.
Nitrided gears, on the other hand, are known to operate successfully only under steady state load. The fatigue life of these
gears reduces significantly under heavy shock load. That there is also aconsiderable reduction ofnitrided gears life under
rated load with wide fluctuations is unknown. A recent failure of an epicyclic gearbox with nitrided gears that was
operating under such loading confirms this fact. In this case, the gearbox was used to reduce the speed of a gas turbine
rated 10.4 roW. During the engine wash cycle, the load at the generator end varied from 0 to 10 mW at 1.5 cycles per
second for about 25 seconds duration, with the peak occasionally reaching to 12 mW. No such failure is reported in
similar gearboxes that are operating at various installations that did not experience this type of load fluctuations.