Theory of Machine
The two elements of a pair are said to form a higher pair, when they

Permit relative motion
Are kept in contact by the action of external forces, when in motion
Have a surface contact when in motion
Have a line or point contact when in motion

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Theory of Machine
Two pulleys of radii r₁ and r₂ and at distance x apart are connected by means of an open belt drive. The length of the belt is

π (r₁ - r₂) + (r₁ + r₂)²/x + 2x
π (r₁ + r₂) + (r₁ - r₂)²/x + 2x
π (r₁ + r₂) + (r₁ + r₂)²/x + 2x
π (r₁ - r₂) + (r₁ - r₂)²/x + 2x

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