Machine Design
A flanged pipe joint will be a leak-proof joint, if the circumferential pitch of the bolts is (where d = Diameter of bolt hole)

Greater than 30d
Between 20d and 30d
Equal to inside diameter of pipe
Less than 20d

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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

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

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