Theory of Machine
Whirling speed of the shaft is the speed at which

Torsional vibrations occur
Shaft tends to vibrate in longitudinal direction
Combination of transverse and longitudinal vibration occurs
Shaft tends to vibrate vigorously in transverse direction

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Theory of Machine
Two pulleys of radii r₁ and r₂ and at distance x apart are connected by means of a cross 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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