Theory of Machine
The frictional torque transmitted in a flat collar bearing, considering uniform wear, is (where r₁ and r₂ = External and internal radii of collar respectively)

(2/3) μ W [(r₁³ - r₂³)/(r₁² - r₂²)]
(2/3) μ W (r₁ + r₂)
(1/2) μ W (r₁ + r₂)
(1/2) μ W [(r₁³ - r₂³)/(r₁² - r₂²)]

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Theory of Machine
In gears, interference takes place when

The tip of a tooth of a mating gear digs into the portion between base and root circles
Gear teeth are undercut
Pitch of the gears is not same
Gears do not move smoothly in the absence of lubrication

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