Es una parte del sistema de virado que hace girar el rotor de la turbina durante las operaciones de arranque o parada, a efectos de homogenizar la temperatura del rotor y evitar pandeos o deformaciones. Es un anillo dentado o corona, montado en el eje del rotor, con el cual engrana un piñón impulsado por un motor externo.
De la referencia: "Barring gear

Barring gear (or "turning gear") is the mechanism provided to rotate the turbine generator shaft at a very low speed after unit stoppages. Once the unit is "tripped" (i.e., the steam inlet valve is closed), the turbine coasts down towards standstill. When it stops completely, there is a tendency for the turbine shaft to deflect or bend if allowed to remain in one position too long. This is because the heat inside the turbine casing tends to concentrate in the top half of the casing, making the top half portion of the shaft hotter than the bottom half. The shaft therefore could warp or bend by millionths of inches.

This small shaft deflection, only detectable by eccentricity meters, would be enough to cause damaging vibrations to the entire steam turbine generator unit when it is restarted. The shaft is therefore automatically turned at low speed (about one percent rated speed) by the barring gear until it has cooled sufficiently to permit a complete stop."

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Note added at 2 days8 hrs (2010-07-16 23:29:08 GMT) Post-grading
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Creo que has elegido una respuesta inadecuada. En tu siguiente pregunta (http://www.proz.com/kudoz/english_to_spanish/mechanics_mech_... menciona*On turn operation, the swivel pinion is pressed by tooth forces against the barring ring (5), which is fitted on the shaft.* Para que exista *toth force* debe haber dos juegos de dientes engranados, (más una posición adecuada de los ejes de giro) y esto no ocurre si una de las piezas en toma es un "anillo de cojinete"