Machine Design
A bolt of M 24 × 2 means that

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

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Machine Design
A hollow saddle key is

A taper key which fits half in the key way of hub and half in the key way of shaft
A taper key which fits in a key way of the hub and the bottom of the key is shaped to fit the curved surface of the shaft
A taper key which fits in a key way of the hub and is flat on the shaft
Provided in pairs at right angles and each key is to withstand torsion in one direction only

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