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

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

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Machine Design
Stress concentration is caused due to

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

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