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 inversion of a mechanism is

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

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