Arc of Approach and Arc of Recess:

Arc of approach is the locus of a point on the pitch circle from the starting of engagement to the pitch point. The arc of recess is the locus of a point through pitch point up to the ending of engagement of two mating gears.

Arc of Contact

This is the locus of a point on the pitch circle from the starting of engagement to the ending of engagement of two mating gears.

                                    Arc of Contact = Arc of Approach + Arc of Recess