Mechanical or hydraulic devices that regulate machine speed are known as:

The term for mechanical or hydraulic devices that regulate machine speed is "governors." Governors work by automatically adjusting the input to maintain a desired speed in machinery, ensuring stable operation under varying load conditions. This is especially important in engines or turbines where maintaining constant rotational speed is crucial for efficiency and performance.

In contrast, carriages refer to the components that support or move something, typically in a linear direction, and do not involve speed regulation. Keyways are grooves used in mechanical engineering to secure parts together, allowing for rotational movement, but they do not control speed. Lifts, on the other hand, are mechanisms designed to raise or lower loads vertically and are not specifically focused on speed regulation. Thus, governors are the appropriate term for devices that manage and regulate the speed of machines.