Machine Design
In replacing the V-belts, a complete set of new belts is used instead of replacing a single damaged belt because

New and old belts will cause vibrations
Belts are available in sets
The new belt will carry more than its share and result in short life
Only one belt cannot be fitted with other used belts

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Machine Design
In an oil lubricated journal bearing, the coefficient of friction between the journal and the bearing

Remains constant at all speeds
Becomes minimum at an optimum speed and then increases with further increase in speed
Is maximum at zero speed and decreases monotonically with increase in speed
Is minimum at zero speed and increases monotonically with increase in speed

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