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
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
In worm, gears, pitch lead angle is

The angle between the tangent to the pitch helix and the plane of rotation
The angle between the tangent to the pitch helix and an element of the cylinder
None of these
Half the angle between two inclined faces in axial plane

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