What is a contact point ?
A contact point is where a pair of gears touch each other. In the diagram below P1, P2 and P3 are the contact points.
As shown in the diagram below, when involute gears mesh, the contact points of both gears move along the line of action which is the common tangent of the two base circles.
At that moment, the trace that is created by the contact point is called the trajectory of the contact point.
