Theory of Machine
The two links OA and OB are connected by a pin joint at O. If the link OA turns with angular velocity ω₁ rad/s in the clockwise direction and the link OB turns with angular velocity ω₂ rad/s in the anticlockwise direction, then the rubbing velocity at the pin joint O is (where r = Radius of the pin at O)

ω₁.ω₂.r
(ω₁ - ω₂) 2r
(ω₁ + ω₂) r
(ω₁ - ω₂) r

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Theory of Machine
Higher pairs are those which have

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

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