The ISFD design is manufactured through electrical discharge machining (EDM). Integral “S” shape springs connect an outer and inner ring, and a squeeze film damper land extends between each set of springs. Bearing pads are housed in the inner ring. The unique design allows for high-precision control of concentricity, stiffness and rotor positioning, and it produces superior damping effectiveness by separating stiffness from damping.
While a conventional squeeze film damper (SFD) experiences a dynamic stiffness from the damper film that is dependent on amplitude and frequency, in the ISFD design, the stiffness is defined only by the springs. This allows for good predictability and precise placement of critical speeds and rotor modes, regardless of vibration amplitudes and frequencies.
ISFD damping is controlled by flow resistance at the oil supply nozzle and end seals. Whereas damping in a conventional SFD is generated by squeezing in the damper film and governed by circumferential film flow, the segmented ISFD design prevents circumferential flow and absorbs energy through the piston/dashpot effect.
Both the stiffness and the damping of the ISFD design are optimized for the application through a rigorous rotordynamic analysis. For the steam turbine, since steam whirl was one of the root causes of the subsynchronous vibrations, the analysis of the ISFD solution paid careful attention to modeling destabilizing seal forces and stage forces. A damped eigenvalue analysis without those forces showed a better stability margin by a factor of 12 with the ISFD design compared to the original bearings. With the destabilizing forces, the ISFD solution maintained a high stability margin (see Figure 2). The combination of low stiffness and optimum damping at the bearing support is the key in transforming bending modes to more rigid body modes and improving the overall stability and damping ratio of the rotor/bearing system.