The bending fatigue limits is generally given from a chart when designing the strength of the gear

which has brought great inconvenience to driving the design of CAD with many charts.Considering the impact factors of the bending fatigue limit（gear materials

hardness of heat treatment

materials and heat treatment quality） and the nonlinear features between these impact factors and gear bending fatigue limit

A method of using four-storey BP neural network model to determine the gear bending fatigue limit is put forward to.An analytical expression is established based on two special points of bending strength limit

using its value as BP network training samples.The trained BP network model can be directly used for establishing the limits of gear bending fatigue strength in engineering design

some cases confirmed the error of no more than 5 percent.