The exciting force f sin ωt and the restoring force of spring stiffness ky2 are applied to the worm screw. From Eq. (5), the force FSy that is applied to the y axis of the worm screw by the vibration, except for the force that is applied from the worm wheel via the gear tooth, can be calculated as follows.This FSy becomes an applied force to the contact point by the worm screw. We now consider the forces generated between the teeth of the worm wheel and the worm screw. The tooth surface is inclined by the lead angle and the pressure angle. To simplify the analysis, we consider only the lead angle γ , which has a significant effect on the friction of the teeth. Figure 3b shows the worm gear observed along the x axis. Another figure that removes the worm screw so that the contact point of the teeth can be seen is also shown. If torque is applied to the worm wheel shaft, a pushing force F is generated on the gear teeth via the worm wheel. This F becomes an applied force to the contact point by the worm wheel. Therefore, the contact point of the gear tooth is applied both forces of FSy and F. Moreover, the worm screw can rotate around its axis, thus, if the amplitude of the vibration is small, the motion of the contact point can be considered as the motion on the yz plane. If we fix the coordinate system to the worm wheel, F relatively behaves as a force that is applied to the worm screw in the opposite direction. Therefore, the worm screw can be treated as an object on which a force of F acts while it rests against a surface inclined at γ , as shown in Fig. 3c. Of course, F includes both forces generated by spring ky1 and the torque applied to the worm wheel. However, if the amplitude of the vibration is small, F is dominated by the force generated by torque T rather than the force generated by spring ky1 . Therefore, we assume F as constant to simplify the equation. (5)( MW + MS)y′′ + (ky1 + ky2)y = f sin ωt(6)F Sy =− ky2y + f sin ωtBy this simplification, F can be treated similar to a force generated by the gravitational acceleration, as shown in Fig. 3c. Because the pressing force caused by the vibration applied via the worm screw is FSy , the normal force N of the gear tooth surface is obtained as follows: