What is the primary purpose of a shaft key in machine design?

The primary purpose of a shaft key in machine design is to transfer torque between the shaft and the attached component. A shaft key is a small, often rectangular piece of metal fitted into a groove in a shaft and a corresponding slot in the hub of the component such as a gear, pulley, or wheel. This design allows for a solid mechanical connection that prevents slipping while enabling efficient transfer of mechanical power or torque from the shaft to the component it drives.

In scenarios where torque needs to be efficiently transmitted, the key ensures that the rotational motion of the shaft translates effectively into the rotational motion of the attached component, which is vital for the proper operation of machinery. While it might provide some level of alignment between the components, its primary role revolves around supporting the shear forces generated during operation to ensure reliability and performance.

In contrast, connecting two shafts without any movement implies a fixed relationship that does not adequately represent the function of a key. Measuring torque is typically done with separate devices, such as torque wrenches or load cells, and controlling speed is a function often managed by variable frequency drives or gear ratios, rather than through the key itself.