Question

Which of the following key fits in the keyway of the hub only (i.e., there is no keyway on the shaft)?

• A. Sunk key
• B. Saddle key
• C. Feather key
• D. Woodruff key

Hint:

Think of a Saddle. Just as a saddle sits on top of a horse without going into the horse, a Saddle key sits on top of the shaft without a deep groove (keyway) cut into it.

Answer 1

The Correct Option is B

Keys are machine elements used to connect a rotating machine element (like a gear or pulley) to a shaft to prevent relative rotation and transmit torque. Keys are categorized based on how they sit in the shaft and the hub. 1. Characteristics of a Saddle Key: A

Saddle key is a unique type of key that

does not require a keyway in the shaft.

• Hub Mounting: A keyway is cut only in the hub of the mounting part (the gear or pulley).

• Shaft Interface: The key sits in the hub's keyway, and its bottom surface is shaped to grip the shaft purely through friction.

2. Types of Saddle Keys:

• Hollow Saddle Key: The bottom of the key is curved to match the circular profile of the shaft.

• Flat Saddle Key: The bottom is flat, requiring a small flat surface to be machined onto the shaft, but still no deep "groove" or keyway.

3. Comparison with Other Keys:

• Sunk Key: These are "sunk" into both the shaft and the hub, meaning a keyway must be cut in both. This is the most common type for heavy power transmission.

• Woodruff Key: An easily adjustable semicircular key that sits in a deep, curved keyway in the shaft and a standard keyway in the hub.

• Feather Key: A type of sunk key that allows the hub to slide axially along the shaft while transmitting torque.

4. Conclusion: Because the saddle key relies on friction and does not physically "lock" into a groove in the shaft, it is the only option that fits exclusively in the hub's keyway. It is typically used for light-duty applications where preserving the shaft's structural integrity is more important than transmitting high torque.