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

sin (θ + φ) + 1/ 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

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Theory of Machine
Oldham's coupling is the

Second inversion of single slider crank chain
Second inversion of double slider crank chain
Third inversion of double slider crank chain
Third inversion of slider crank chain

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