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
Gear teeth are undercut
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
The inversion of a mechanism is

Obtained by reversing the input and output motion
Changing of a higher pair to a lower pair
Turning its upside down
Obtained by fixing different links in a kinematic chain

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