Theory of Machine
Lower 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
Two elements that permit relative motion
Elements of pairs not held together mechanically

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Theory of Machine
In gears, interference takes place when

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

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