The gear transmission is realized by the sequential engagement of the pairs of gears. The ratio of the length of the actual meshing line to the pitch of the base circle is called the coincidence degree. In order to enable the gear to be continuously driven, it should be ensured that when the pair of gears are disengaged, the latter pair of gears will enter the mesh, so the degree of coincidence should be greater than 1.
In order to keep the two gears continuously and smoothly in the transmission, it must be ensured that at least the same time before the end of the engagement of the previous pair of tooth profiles, the latter pair of tooth profiles have entered the engagement, otherwise the transmission will be interrupted. As can be seen from the figure on the right, when a pair of teeth are disengaged at point B1, the latter pair of teeth just enter the mesh. If the pitch of the base is further increased to separate the meshing points, the pair of teeth will be off at point B1. When opening, the latter pair of teeth can not reach the B2 point meshing condition, so that the transmission is interrupted or unstable. If, on the contrary, if the base circle pitch is reduced and the meshing point is closer, the former pair of teeth will appear at point B1. When disengaged, the latter pair of teeth have already reached the B2 point engagement, thus ensuring the continuity and smoothness of the transmission. That is, the base pitch must be less than the actual mesh length.