Gas Turbine - Ratcheting and Barring, Turbine Engine Startup

This is a very important device and young engineers find it difficult to understand. Ratcheting of gas turbine refers to slow rotation of the turbine shaft. The rotation is carried out so as to avoid the sag of the rotor. By ratcheting, an unequal temperature profile along the rotor is prevented. Ratcheting is administered by means of a ratchet motor, a jaw-clutch and the associated assembly comprising of limit switches. For a GE machine, ratcheting is done every three minutes. The turbine shaft is rotated by 45° in each cycle. The basic idea is to prevent sagging of rotor and prevention of a bow formation that could otherwise pose a high vibration upon a subsequent start.