overspeed

To overspeed is to run a rotating machine, typically an engine or turbine, above its maximum continuous rated speed specified by the manufacturer. This is distinct from brief transient excursions; overspeeding means sustained operation in the danger zone. The consequence is accelerated wear, bearing failure, blade distortion, and potentially catastrophic mechanical failure.

Overspeeding occurs most often in turbomachinery, centrifugal compressors, and reciprocating engines. In a turbine, overspeed reduces the safety margin for blade resonance and increases centrifugal stress on rotating elements exponentially, since stress scales roughly with the square of rotational speed. In a positive-displacement pump or compressor, excessive speed generates internal pressures beyond the component's design envelope, damaging seals and impellers. A diesel engine overspeeded will experience detonation, piston scuffing, and valve float.

Most machines are protected by mechanical or electronic governors that sense speed and throttle back fuel, air, or flow to maintain setpoint. A failed governor is a common cause of inadvertent overspeeding. Tachometers and electronic speed sensors provide operator feedback. On critical equipment, alarm setpoints trigger shutdown well before the absolute speed limit.

Undersized or worn governors, fuel control unit failures, and loss of load (in turbines, a sudden loss of generator load) can cause runaway speeds. The term originates simply from the verb speed, meaning to go fast, and the prefix over, meaning beyond the permitted range. It is both a technical fault condition and an operational error.