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

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

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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
A taper key which fits half in the key way of hub and half in the key way of 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

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