SCIENCE AND TECHNOLOGY ENGLIAH FOR MECHANICAL ENGINEERING BD/AC=nA/nB 6.17 but BD/AC= BP/AP 6.18 and na/nR is the speed ratio. Therefore, AP and BP must be pitch radii and point P must be the pitch point Path Of Contact And Contact Ratio The path of contact is the line described by the point of contact between two mating teeth during rotation. In involute gearing, the path of contact coincides with the line of action. It begins where the addendum circle of the driven sear intersects the line of action, and ends where the addendum circle of the driving gear intersects the line of action. This definition ignores possible inter ference conditions with pinions of small tooth numbers, which are outside the scope of this textSCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING BD/AC = nA / nB 6.17 but BD/AC = BP/AP 6.18 and nA /nB is the speed ratio. Therefore, AP and BP must be pitch radii and point P must be the pitch point. Path Of Contact And Contact Ratio The path of contact is the line described by the point of contact between two mating teeth during rotation. In involute gearing, the path of contact coincides with the line of action. It begins where the addendum circle of the driven sear intersects the line of action, and ends where the addendum circle of the driving gear intersects the line of action. This definition ignores possible interference conditions with pinions of small tooth numbers, which are outside the scope of this text