Question

when two spur gears having involute profiles on their teeth engage the line of action is tangential to the

Answer 1

The answer is pitch circle.

When two spur gears having involute profiles on their teeth engage, the line of action is tangential to the pitch circle. This circle is an imaginary circle that is concentric to a toothed wheel. The pitch of the teeth is measured along this wheel. The diameter of a pitch circle is calculated as the division of the number of teeth by the diametral pitch.