The engagement state of the tooth surface can be determined by the formula at any time during the transmission process.From the analysis, it can be seen that the direction of engagement line of back engagement state is different from that of back engagement state, which results in the change of engagement position. As shown in the figure, the dotted line represents the engagement line of back engagement state.

From the nature of the involute, it can be concluded that the pressure angle between the engagement point P and G of the tooth surface and back meshing state alpha Pli and alpha Gli satisfy:

In the formula, alpha Pli is calculated by formula; alpha is dynamic engagement angle; Z1 is the number of teeth of gear 1.

The pressure angle at the engagement point of the gear in back engagement can be determined by the formula: