Theory of Machine
When the nature of contact between the elements of a pair is such that one element can turn about the other by screw threads, the pair is known as a

Screw pair
Turning pair
Sliding pair
Spherical pair

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Theory of Machine
The contact ratio is given by

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

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