Machine Design
The contact ratio is the ratio of

Length of arc of approach to the circular pitch
Length of path of contact to the circular pitch
Length of arc of recess to the circular pitch
Length of arc of contact to the circular pitch

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Machine Design
A hollow saddle key is

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
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

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