Theory of Machine
Oldham's coupling is the

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

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Theory of Machine
The contact ratio is given by

(Length of the path of approach)/(Circular pitch)
(Length of path of recess)/(Circular pitch)
(Length of the arc of contact)/(Circular pitch)
(Length of the arc of approach)/cosφ

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