Theory of Machine
The Whitworth quick return motion mechanism is formed in a slider crank chain when the

Slider is a fixed link
Smallest link is a fixed link
Coupler link is fixed
Longest link is a fixed link

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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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