Involute Gear Mechanism

Involute gear tooth, the contact point starts closer to one gear, and as the gear spins, the contact point moves away from that gear and towards the other. The pitch diameter is the effective contact diameter. As the gear turns, the contact point slides up onto the thicker part of the top gear tooth. Thus, pushes the top gear ahead. As the teeth continues to rotate, the contact point moves further away going outside the pitch diameter. Then the contact point starts to slide onto the skinny part of the bottom tooth, subtracting a little bit of velocity from the top gear to compensate for the increased diameter of contact. Thus, the involute gear tooth produces a constant ratio of rotational speed.