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
Higher pairs are those which have

Elements of pairs not held together mechanically
Point or line contact between the two elements when in motion
Surface contact between the two elements when in motion
Two elements that permit relative motion

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