Theory of Machine
In a multiple V-belt drive, when a single belt is damaged, it is preferable to change the complete set to

Ensure uniform loading
Reduce slip
Reduce vibration
Ensure proper alignment

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Theory of Machine
Lower pairs are those which have

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

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