
In the picture:
RB — radius of base circle;
RF — radius of tooth root circle;
Bending stiffness Kb:

Shear stiffness KS:

The axial compression stiffness Ka is as follows

Where:
N — number of teeth of gear tooth;
α 0 — pressure angle;
α 2 — upper half tooth angle of base circle,
For the external meshing pair, the expression of α 1 on the driving and driven gears is different, as follows:
Driving gear:

Driven gear:

Where:
θ 1 — the rotation angle of driving gear. If the rotation starts from the reference position, θ 1 = 0 can be defined;
N1 — number of teeth of driving gear;
N2 — number of teeth of driven gear.