As shown in the figure, the forming process of the tooth profile surface of the involute straight tooth spur helical gear is demonstrated. S is the generating surface, and there is no sliding friction between the base cylinder and the generating surface. Obviously, the non sliding friction of the straight line KK ‘on the base circle generatrix NN’ makes the motion process of the straight line become a surface, that is, the surface of the straight tooth involute cylindrical gear. When two spur helical gears are kneaded for transmission, the contact line of the tooth profile surface is an oblique straight line, as shown in figure (b).

An involute is a line formed by the intersection point between the end face and the tooth profile surface without sliding friction. In any case, the end face maintains a vertical relationship with the central axis of the spur helical gear, and its position relationship can be seen during the movement process.