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

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Machine Design
The effective stress in wire ropes during normal working is equal to the stress due to

Axial load plus stress due to bending
Axial load plus stress due to acceleration/retardation
Bending plus stress due to acceleration/retardation
Acceleration/retardation of masses plus stress due to bending

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