Damping in rotating machine bearings

Damping helps remove energy from a system through resistance to motion. For rotating equipment, damping is necessary to prevent vibrations from damaging the rotor, bearing or other components. Unlike rolling element bearings, fluid film bearings have a significant amount of damping. But even the damping provided by fluid film bearings is not sufficient for all rotating machinery. The stiffness of the fluid film bearing can counter the damping capability. Therefore, an additional means of increasing “effective” damping to the system is needed.

To make the damping effective, it may be necessary to allow for additional motion by softening the bearing support. Thus, rotating machinery designers are using dampers between bearings and ground. Using a damper can help increase stability, reduce rotor response, increase separation margin between operating speed and critical speeds, reduce transmitted forces from rotor to ground (reduced pedestal vibrations and reduced bearing wear), and decrease sensitivity to changes in the rotor, such as buildup on a rotor component.

ISFD vs. SFD

an outer and inner ring, and a squeeze film damper land extends between each set of springs. In a conventional squeeze film damper (SFD), damping is generated by squeezing oil in the damper film and is governed by circumferential film flow, which makes it difficult to control oil flow resistance. In contrast, the segmented ISFD design prevents circumferential flow, and damping is controlled by flow resistance at the oil supply nozzle and the end seals. The ISFD design absorbs energy through the piston/dashpot effect (like a shock absorber).

While a conventional SFD experiences a dynamic stiffness from the damper film that is dependent on vibration amplitude and frequency, in the ISFD, the stiffness is defined by the springs. This allows for better predictability and precise placement of critical speeds and rotor modes.

Both the stiffness and the damping of the ISFD design are optimized for the application through a rotordynamic analysis. Thus, they can help maximize the ratio of energy transmitted to the bearing locations and improve the stability of the system. The ISFD design can also 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.

(The author is Chief Engineer at Waukesha Bearings, headquartered in Pewaukee, Wis., USA. Blair has responsibilities for fluid film bearing research & development activities, including new products and the refinement of bearing design tools and methods.)