Line of Action
Line of action is the common tangent line of the base circles of a pair of involute gears in contact.
When a pair of involute gears is meshed, the contact occurs on the line of action and the contact point moves along the line of action.
In short, the line of action is the trace of the contact point of a pair of mating involute gears.
Also, on the line of action, the length from the start of the tooth contact to the end of contact is called the length of action.
As shown in the figure below, the common tangent of the pair of base circles, A - B, is the line of action, and the contact point moves along this A - B.
