Theory of Machine
In a mechanism, the fixed instantaneous centers are those centers which

None of the listed here
Moves as the mechanism moves, but joints are of permanent nature
Remain in the same place for all configurations of the mechanism
Vary with the configuration of the mechanism

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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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