Theory of Machine
The type of pair formed by two elements which are so connected that one is constrained to turn or revolve about a fixed axis of another element is known as

Sliding pair
Rolling pair
Spherical pair
Turning pair

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Theory of Machine
Open pairs are those which have

Point or line contact between the two elements when in motion
Elements of pairs not held together mechanically
Two elements that permit relative motion
Surface contact between the two elements when in motion

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