Theory of Machine
In considering friction of a V-thread, the virtual coefficient of friction (μ₁) is given by

μ₁ = μ/cos β
μ₁ = μ sin β
μ₁ = μ/sin β
μ₁ = μ cos β

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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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