Machine Design
A hollow saddle key is

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

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Machine Design
A bolt of M 24 × 2 means that

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

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