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 tanα
T = W.r tanφ
T = W.r tan(φ + α)

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Theory of Machine
A system of masses rotating in different parallel planes is in dynamic balance if the

Resultant force and resultant couple are both equal to zero
Resultant couple is equal to zero
Resultant force is equal to zero
Resultant force is numerically equal to the resultant couple, but neither of them need necessarily be zero

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