Now consider the system of Fig. 4-19. The lead of the screw, L, is defined as the linear distance that the mass travels per revolution of the screw. In principle, the two systems in Fig. 4-20 and Fig. 4-21 are equivalent. In Fig. 4-20, the distance traveled by the mass per revolution of the pinion is 27rr. By using Eq. (4-47) as the equivalent inertia for the system of Fig. 4-19, we have