Theory of Machine
In gears, interference takes place when

Pitch of the gears is not same
Gears do not move smoothly in the absence of lubrication
The tip of a tooth of a mating gear digs into the portion between base and root circles
Gear teeth are undercut

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Theory of Machine
The contact ratio is given by

(Length of the path of approach)/(Circular pitch)
(Length of the arc of approach)/cosφ
(Length of path of recess)/(Circular pitch)
(Length of the arc of contact)/(Circular pitch)

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