What is the term for the open spot on a shaft where a key is affixed?

The term for the open spot on a shaft where a key is affixed is known as a keyseat. A keyseat is a machined groove or slot that allows a key to fit into the shaft, enabling the transmission of torque between the shaft and a connected component, such as a gear or pulley. This design ensures that the key maintains a precise alignment while preventing relative motion, thus facilitating efficient power transfer.

The other terms do not accurately describe this feature. A collar refers to a circular component used to secure or hold an assembly in place but does not specifically relate to the machined area for a key. A coupling is a device used to connect two shafts for motion transmission but is separate from the keyseat. The term runout shoulder refers to a specific part of the shaft where dimensional tolerance is checked and is not related to the placement of a key.