Machine Design
A bolt of M 24 × 2 means that

The nominal diameter of bolt is 24 mm and pitch is 2 mm
Cross-sectional area of the threads is 24 mm²
The pitch of the thread is 24 mm and depth is 2 mm
The effective diameter of bolt is 24 mm and there are 2 threads per cm

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

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