Hydroelectric generators are used to generate power in many parts of the United States. One of their interesting details is that despite how massive they are—a typical 5–20 MW generator can measure 8–12 ft in diameter and weigh about 50 tons—they are built to extremely demanding tolerances.

My company, BMI, was contracted to install new keybars in a hydroelectric generator stator frame. The stator frame houses the stator core that sits inside the generator. The keybars act as structural components to ensure the massive iron core is kept within the roundness tolerance.

Keybar manufacturing tolerances are tight, at this particular installation about 0.005 in. radius over the 24 in. keybar length. During installation, a series of fixtures are needed to hold the keybars in place and allow adjustment in and out, side to side, and to control twist during assembly so the stator’s inside diameter is held to a tolerance of +/– 0.01 in. over the 8–12 ft diameter.

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