Theory of Machine
The velocity of sliding of meshing gear teeth is(Where ω₁ and ω₂ are angular velocities of meshing gears and ‘y’ is distance between point of contact and the pitch point)

(ω₁/ω₂) y
(ω₁ + ω₂)/y
(ω₁ + ω₂) y
(ω₁ × ω₂) y

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Theory of Machine
Open pairs are those which have

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

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