How the normalpitch ($$P_N$$) and axial pitch ($$P_c$$) related to a helical gear with helix angle $$\alpha$$?

A

$$P_c = P_N - \cos \alpha$$

B

$$P_c = \frac{P_N}{\cos \alpha}$$

C

$$P_N = P_c. \cos \alpha$$

D

$$P_N = \frac{P_c}{\cos \alpha}$$