Theory of Machine
In gears, interference takes place when

Gear teeth are undercut
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
Gears do not move smoothly in the absence of lubrication

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Theory of Machine
The relation l = (2/3).(j + 2) apply only to kinematic chains in which lower pairs are used. This may be used to kinematic chains in which higher pairs are used, but each higher pair may be taken as equivalent to

Two lower pairs and two additional links
One lower pair and two additional links
Any one of these
Two lower pairs and one additional link

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