Theory of Machine
In a disc clutch, if there are n₁ number of discs on the driving shaft and n₂ number of discs on the driven shaft, then the number of pairs of contact surfaces will be

n₁ + n₂ - 2
n₁ + n₂ + 1
n₁ + n₂
n₁ + n₂ - 1

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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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