The generating line is a straight line. When the generating line rolls along the base circle, the locus formed by any point A on the generating line is an involute. In the figure, the arc AK is the locus formed by point A, and the arc AK is the involute.
It can be seen from Figure 1 that the characteristics of the involute are as follows: ① The vertical line at any point on the curve is always tangent to the base circle; ② There is no involute in the base circle; ③ The length of arc AB is equal to the length of line KB.
In Figure 2, there are two meshing gears. The upper gear is the driving gear, which rotates clockwise; The lower gear is a driven gear, which rotates counterclockwise. The internal common tangent N1N2 of the two gear base circles is called the theoretical meshing line. The N1 point and N2 point are the two limit points in the meshing process, and the actual meshing point can only be within the segment N1N2. Point B1 is the intersection of the addendum circle of the driven gear and the theoretical meshing line N1N2, and is the starting point of the actual meshing of the two gears. With the rotation of the gear, the meshing point of the gear moves along the theoretical meshing line N1N2. The meshing point on the driving gear tooth profile moves from point B1 to the top of the tooth, while the meshing point on the driven gear tooth profile moves from point B1 to the root of the tooth.
When the meshing point moves to point B2 (point B2 is the intersection of the driving gear addendum circle and the theoretical meshing line N1N2), the two gears are disengaged, so point B2 is the end point of the actual meshing of the two gears, and section B1B2 is called the actual meshing line. It can be seen from the meshing process that the actual meshing line B1B2 is one section of the theoretical meshing line N1N2. Because it is the meshing of full gear, the first pair of gear teeth must have entered the meshing state when they are disengaged at point B2, otherwise the transmission will be discontinuous.