First, differences in definitions:
1. Pitch circle: the movement path of the node relative to the gear and the circle of pure rolling throughout the transmission procedure
2. Indexing circle: a particular circle with the modulus and pressure angle as the standard value, adopted for simplicity of computation.
Second, the difference in the pressure angle:
1. The pitch circle's pressure angle changes in tandem with its pitch circle diameter.
2. The indexing circle α=20° has a conventional value of the pressure angle.
Third, differences in nature:
1. A pitch circle cannot exist unless two gears are meshed.
2. A gear has an indexing circle.
Fourth, the difference in location:
1. The two circles are tangential constantly.
2. During non-standard installation the indexing circles either intersect or separate.
Fifth, differences in size:
1. The center distance affects the pitch circle.
2. Fixed is the indexing circle, d=mz.
Furthermore, whilst the radius of the pitch circle fluctuates and the indexing circle and the standard center distance are separated, a non-standard installation-that is, when the installation center distance is not equal to the standard center distance-has constant radius of the indexing circle. The meshing line moves in position, and the angle of engagement differs from the pressure angle on the indexing circle.