Theory of Machine
In gears, interference takes place when

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

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Theory of Machine
Higher pairs are those which have

Point or line contact between the two elements when in motion
Surface contact between the two elements when in motion
Elements of pairs not held together mechanically
Two elements that permit relative motion

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