Geartogether withrack (and pinion)There are also prerequisites for constant overlap of the meshing index circle and the pitch circle
If the gear has a 20-degree pressure angle, and we use a 20-degree pressure angle rack, the pitch circle of the gear will coincide with the indexing circle. However, if the gear's indexing circle pressure angle is 20 degrees, but we use a rack with a 15-degree pressure angle to match, then the gear's pitch circle is not coincident with the indexing circle. At present, the general user can find the rack design information is required to rack pressure angle is equal to the gear indexing circle pressure angle, under this premise your proposition is valid. However, there are many occasions when we use a rack that does not have a pressure angle equal to that of the indexing circle of the gear. In this case, the pitch circle of the gear is the diameter of the gear circle when the gear pressure angle is the same as the rack pressure angle.