Barring gear is the term normally used for the complete mechanism provided for the rotation of the turbine generator shaft at a very low speed after unit stoppage for any reason. The requirement for this is very important for all capacities of turbine generator units. Once the unit is "tripped" (i.e. the turbine stop valve is closed cutting off steam to the turbine) the unit starts slowing down (also known as "coasting down"). When it reaches a dead stop, if it is allowed to be in one position for some time there is a tendency for the turbine shaft to deflect or bend.
This deflection is because of the heat inside the turbine casing tends to get concentrated in the top half of the casing, thus making the top half portion of the shaft hotter than the bottom half. The shaft therefore warps or bends by millionths of inches, only detectable by monitoring meters such as the eccentricity meter on the operators' console.
But this small amount of shaft deflection would be enough to cause vibrations and wreck the whole turbine generator unit when the huge mass starts spinning. Even the whole concrete building vibrating has been observed in some cases where barring was not used. Therefore the shaft is not allowed to come to a dead stop, but a mechanism known as turning gear or barring gear automatically takes over to rotate the set at a particular set low speed.