Machine Design
The dedendum circle diameter is equal to (where φ = Pressure angle)

Addendum circle diameter × cosφ
Pitch circle diameter × sinφ
Pitch circle diameter × cosφ
Clearance circle diameter × cosφ

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Machine Design
A bolt of M 24 × 2 means that

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

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Machine Design
Jam nut is a locking device in which

Through slots are made at top and a cotter pin is passed through these and a hole in the bolt, and cotter spitted and bent in reverse direction at other end
A slot is cut partly in middle of nut and then slot reduced by tightening a screw
One smaller nut is tightened over main nut and main nut tightened against smaller one by loosening, creating friction jamming
A hard fibre or nylon cotter is recessed in the nut and becomes threaded as the nut is screwed on the bolt causing a tight grip

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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
New and old belts will cause vibrations
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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