Theory of Machine
The efficiency of a screw jack is maximum, when (where α = Helix angle, and φ = Angle of friction)

α = 90° - φ
α = 90° + φ
α = 45° + φ/2
α = 45° - φ/2

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Theory of Machine
Two pulleys of radii r₁ and r₂ and at distance x apart are connected by means of a cross belt drive. The length of the belt is

π (r₁ - r₂) + [(r₁ - r₂)²/x] + 2x
π (r₁ + r₂) + [(r₁ - r₂)²/x] + 2x
π (r₁ + r₂) + [(r₁ + r₂)²/x] + 2x
π (r₁ - r₂) + [(r₁ + r₂)²/x] + 2x

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