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
Stress concentration is caused due to

All of these
Pitting at points or areas at which loads on a member are applied
Abrupt change of section
Variation in properties of material from point to point in a member

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