Theory of Machine
Which of the following is a turning pair?

Shaft with collars at both ends fitted into a circular hole
Lead screw of a lathe with nut
Ball and a socket joint
Piston and cylinder of a reciprocating steam engine

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Theory of Machine
Open pairs are those which have

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

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