An industrial high speed Siemens 12 MW steam turbine operating at 10,700 rpm drives a Flender gearbox pinion,which is connected to a low speed gear shaft,with gear ratio at 17.1,driving a Siemens generator at 1,500 rpm(50Hz).A high amplitude sub-synchronous vibration,at 0.89X,occurred in the high speed pinion at low motor electrical loads.The Zollern gearbox pinion bearings consisted of offset half two pad bearings which were found to be unstable,exciting one of the pinion-steam turbine shaft modes at the 0.89X frequency.A contributing factor to the problem was an oil leak into the high speed coupling.Tilt pad bearings could not be used to fix the problem due to the limited available bearing space in the gearbox pinion.New pressure dam bearings with three pads and two pressure dams were designed to fix the problem.The bearings were implemented and the 0.89X high vibration problem was eliminated by the pressure dam bearing pinion bearing design.This paper describes the system rotor dynamic analysis and pressure dam bearing design process.