Theory of Machine
The factional torque for square thread at the mean radius r while raising load W is given by

T = W.r tan(φ - α)
T = W.r tanφ
T = W.r tanα
T = W.r tan(φ + α)

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Theory of Machine
In gears, interference takes place when

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
Gear teeth are undercut
Pitch of the gears is not same

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