Theory of Machine
The example of successfully constrained motion is a

All of these
Piston reciprocating inside an engine cylinder
Motion of the shaft between a footstep bearing
Motion of an I.C. engine valve

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Theory of Machine
In gears, interference takes place when

Pitch of the gears is not same
Gear teeth are undercut
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

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