Theory of Machine
The example of successfully constrained motion is a

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

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Theory of Machine
In gears, interference takes place when

Gear teeth are undercut
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

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