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

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

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Theory of Machine
Open pairs are those which have

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

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