Under ideal conditions, the normal line of the standard involute tooth surface follows the direction of the theoretical engagement line during engagement, in which no relative velocity occurs and therefore no impact occurs.

As shown in the diagram, O1 and O2 are the rotation centers of small and large wheels respectively, and Pb1 and Pb2 are the joints of small and large wheels respectively.

In actual conditions, the load causes deformation of the previous engagement pair (tooth pair 1), resulting in relative pitch difference, which results in early contact between the tip point of the large gear and the surface of the small gear when the engagement pair (tooth pair 2) is engaged, resulting in meshing interference and a meshing impact.