Damping for increased stability

15th May 2015

Fig. 1. Conventional squeeze film damper    Fig. 1. Conventional squeeze film damper
Fig. 2. ISFD Design Fig. 2. ISFD Design
Fig. 3. Flexure Pivot Tilt Pad Journal Bearing with ISFD Technology Fig. 3. Flexure Pivot Tilt Pad Journal Bearing with ISFD Technology
Fig. 4. ISFD Technology for a Rolling Element Bearing Fig. 4. ISFD Technology for a Rolling Element Bearing
Fig. 5: Finite Element Analysis of ISFD ’S’ Spring Fig. 5: Finite Element Analysis of ISFD ’S’ Spring

Damping decreases vibrations by removing energy through resistance to motion. For rotating equipment, damping is necessary to control vibrations and prevent them from damaging the rotor, bearings or other components in the machine.

Rolling element bearings provide little damping, while fluid film bearings provide a significant amount. With either, the stiffness of the bearing can counter the effectiveness of the bearing damping by limiting motion.

For many machine designs (pipeline centrifugal compressors or aircraft engines, for example), the bearing damping may not be sufficient to suppress all damaging vibrations. Therefore, a means of increasing ‘effective’ damping to the system is needed.

To increase damping, rotating machinery designers are using fluid films or compliant materials between bearings and ground. To make the damping ‘effective’, it may be necessary to allow for additional motion by softening the bearing support.

The result of using an engineered damper can be increased stability, reduced rotor response, increased separation margin between operating speed and critical speeds, a reduction in forces transmitted from rotor to ground, reduced pedestal vibrations, reduced bearing wear, and decreased sensitivity to changes to the rotor, such as material buildup on a rotating component.

ISFD versus SFD

In a conventional squeeze film damper (SFD), damping is generated by squeezing oil in the damper film and is governed by circumferential film flow as shown in Fig. 1, which makes it difficult to control oil flow resistance.

In contrast, the segmented Integral Squeeze Film Damper (ISFD) design prevents circumferential flow, and damping is precisely controlled by flow resistance at the oil supply nozzle as well as the end seals, as shown in Fig. 2. The ISFD design absorbs energy through the piston/dashpot effect, like a shock absorber.

With ISFD technology, shown in Fig. 3, the damper is integral to the bearing. Integral ’S’ shape springs connect an outer and inner ring, and a damper land extends between each set of springs. ISFD technology can be used to provide additional damping to not only fluid film bearings but also rolling element bearings, as shown in Fig. 4.

Stiffness of the ISFD design is defined by the springs, as shown in Fig. 5. This allows for better predictability and precise placement of critical speeds and rotor modes compared to a conventional SFD, which may experience non-linear damping and a stiffness effect from damper film cavitation.

Both the stiffness and the damping of the ISFD design are optimised for each application through a rigorous rotordynamic analysis. Thus, ISFD technology can maximise the ratio of energy transmitted to the bearing locations and significantly improve the stability and dynamic response of the system.

As an added benefit, the ISFD can be designed and manufactured to center the rotor under static (rotor weight) load. Most conventional SFDs rely on a separate means to counter the static load, such as O-rings in eccentrically turned grooves and/or helper springs.

Barry Blair is Chief Engineer, Waukesha Bearings.


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