In view of the present contact ratio of pin gear is mostly calculated by the graphology
thereby causing the fault with a large error in the result of calculation.Therefore the analytical method is used
from the setting-up of tooth surface equation of pin gear
the method for directly calculating the contact ratio of pin gear is deduced and the more accurate theoretical range of addendum circle radius of pin gear is deduced.And also a quantification of the mutual relationship of contact ratio and addendum circle thickness is built