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

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

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Machine Design
A bolt

Has both the ends threaded
Is provided with pointed threads
Has head at one end and other end fits into a tapped hole in the other part to be joined
Has a head on one end and a nut fitted to the other

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