Bucket: Steam turbine blade, not water holder

To most people, a bucket is a simple open container: cylindrical or tapered, with a handle, used to carry water, sand, or refuse. It is one of humanity's oldest tools, and the word itself reaches back to Old French and the shape of a wooden vessel.

In mechanical engineering, a bucket is something entirely different. It is a single curved blade or vane attached to the rotor of a steam turbine or gas turbine. Hot steam or compressed hot gas flows past these blades in sequence, pushing on the curved surface and causing the rotor to spin. The bucket's aerodynamic profile, its pitch angle, and its material (usually steel or nickel-based alloy to withstand thermal stress) are critical to the turbine's efficiency and power output. A large steam turbine may have hundreds of buckets arranged in rows across multiple stages, each row stepping down the pressure and temperature of the working fluid.

The metaphor is not accidental. The bucket catches the flow of steam much as a water bucket catches rain, then deflects it to do work. Early turbine designers borrowed the word from the everyday object because the principle is similar: a curved container receives a moving fluid and guides it in a useful direction. In a fire bucket (a safety device holding water or sand for fighting small fires), the container's sole job is to hold the extinguishing agent. In a bucket brigade used in industrial electronics, the metaphor shifts again: capacitors act like tiny buckets passing a signal from one to the next, one step per clock pulse. Each is a discrete delay, but the image of liquid motion, or charge motion, being handed along is the same.

The turbine bucket is named for its shape and function, not for any confusion with a water bucket. But the name has stuck for over a century, and engineers speak of bucket erosion (damage from steam droplets), bucket vibration, and bucket failure as matter-of-factly as any mill worker speaks of carrying water.