The lateral stability of a nonlinear rail vehicle wheelset subject to random track irregularities is analyzed by the random plus sinusoid describing function method. The nonlinearities considered are wheel/rail profile geometry and friction force saturation. A state space, dual-input, equivalent linearization procedure is developed and applied to the wheelset model. It is shown that on perfectly smooth track creep force saturation increases the critical speed and limits the maximum lateral amplitude. The nonlinear geometry creates several stability boundaries (stable and unstable) and limits the maximum wheelset excursion to lower values than the creep force saturation does. This method predicts the experimentally observed phenomenon of intermittent hunting, i.e., a two-mode response randomly shifting between a limit cycling and a nonlimit cycling behavior, when running on rough track near the critical speed. The predicted response is compared to the results from digital simulations and is shown to be quite accurate.

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