Turning Gear Failure

Dear Fellows,

We faced a failure of turning gear in our 50MW aero derivative gas turbine. The turning gear failed from its casted hub. The Turning gear is motorized having jaw coupling. The motor tripped on overload and rotor was not moving even on manual torque.

The Gas turbine side de-coupled, and manual torque could not rotate the generator side. Then Generator jacking oil & Thrust bearing oil pressures were checked. Jacking oil pressures were ok, but thrust bearing oil pressures (responsible to maintain float both active / inactive side of generator during barring) were found 15 Bar & 80 bar.

We think uneven pressure might have pushed the generator rotor with zero clearance on active side causing rotor movement hard and Jaw coupling being non-flexible kept forcing resulting in gear hub cracking.

We replaced thrust oil pressure hoses and replaced gearbox and rotated machine on manual torque. It was moving free. The thrust pressures were 22 and 70 bar after hose replacement (still not equal but machine was free to move on barring).

What is the possible reason of failure if it is not thrust oil pressure difference? How we can avoid such failures in future?