Machine Design
The maximum efficiency of a square threaded screw is

(1 - sinφ)/ (1 + sinφ)
(1 - sinφ)/ (1 + cosφ)
(1 + sinφ)/ (1 - sinφ)
(1 + cosφ)/ (1 - sinφ)

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Machine Design
Spring index is

Its capability of storing energy
Ratio of coil diameter to wire diameter
Load required to produce unit deflection
Indication of quality of spring

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