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.