Theory of Machine
The maximum efficiency of spiral gears is (where θ = Shaft angle, and φ = Friction angle)

cos (θ - φ) + 1/ cos (θ + φ) + 1
sin (θ + φ) + 1/ cos (θ - φ) + 1
cos (θ + φ) + 1/ cos (θ - φ) + 1
cos (θ - φ) + 1/ sin (θ + φ) + 1

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Theory of Machine
In a steam engine, the earlier cut-off with a simple slide valve may be obtained by increasing the steam lap and the angle of advance of the eccentric but keeping constant the travel and lead of the valve, this method will

Reduce maximum opening of port to steam
All of the listed here
Cause withdrawing or throttling of steam
Reduce length of effective stroke of piston

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