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Rotordynamics & Squeeze Film DampersFunded by National Science Foundation (1994-97) and TAMU Turbomachinery Research Consortium (1992-to

date)

Squeeze film dampers(SFDs) provide viscous damping to rotating structures, allowing for

reduction in vibration amplitudes and providing safe isolation from or of other structuralcomponents. SFDsare customarily used in aircraft jet engines, where rolling element bearings

provide little damping to the rotor-bearing system, and in high performance compressors as retrofit

elements in series with tilting pad bearings to soften bearing supports, reduce critical speeds, and

allow for an extra margin of system stability. Most aircraft gas turbine engines employ at least one

squirrel cage supported damper.

Squeeze film dampers derive their behavior from a lubricant being squeezed in the annular space

between a non-rotating journal and a bearing housing. The journal, typically mounted on the outer

race of rolling element bearings, whirls due to the forces exerted on the rotating shaft. The

squeeze film action generates hydrodynamic pressures and damping forces at the film locationswhere the instantaneous gap (f ilm thickness) is decreasing.

Squirrel cage supported dampers are the most commonly employed SFDdesign. Most large aircraft

gas turbine engines use at least one, and in many instances, two or three dampers in one engine.

The most distinctive feature of this damper configuration is the relatively large axial space required

in comparison to the bearing hydrodynamic length.

OBJECTIVES: Funds allowed construction of a fully instrumented test rig for measurement of the

imbalance response of a three disk rotor supported on SFDs(see figures below). The objectives of

the research are:

(a) to provide reliable imbalance response measurements in a rotor-SFDconfiguration similar to

that of an aircraft engine,

(b) to develop an empirical model to predict the forced dynamic performance of SFDs operating

with air entrainment leading to a bubbly air/oil mixture, and

(c) to develop a non-linear SFD-structural model should the test results from (a) evidence deviations

from linear behavior.

The first objective, fully completed, comprised the construction of the test apparatus and

measurements of the test rotor in squirrel-cage supported SFDsand integral SFDs. The rotor-

bearing system shows rigid body cylindrical and conical critical speeds below a top operating speed

of 10 krpm. More than two hundred test measurements have shown the experimental rotor-SFD

response to be linear even for large imbalance levels and off-centered damper journal operation.

The experimental results allow the identification (and analytical validation) of the damping

capability of integral squeeze film dampers and aid to determine the applicability of this novel

technology to aircraft jet engines. These results have made the third objective irrelevant.

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Other experiments conducted in a controlled orbit SFDrig have shown the effects of air ingestion on

the performance of SFDs. An analytical model for performance prediction of SFDswith bubbly

mixtures has also been completed.

ROTORDYNAMICS TEST RIG ROTOR & SFDs

The Squeeze Film Damper (SFD) rig consistst of a three disk massive rotor (92 lb) supported on high

precision angular contact ball bearings. The outer races of these bearings are supported on squeeze

film dampers. The rotor is driven by a 10 HP DC motor and power supply. The supports are

mounted on an isolated base attached to a table containing the motor and a protective cover. The

rig is instrumented with six (X&Y) shaft displacement sensors, two support accelerometers, and one

optical tachometer and keyphasor. A turbine-type flow meter, pressure gauges, and several

thermocouples indicate the lubricant flow rate, pressure, and oil temperature in/out from the

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dampers.

Additional instrumentation includes four oscilloscopes, a FFT analyzer, and digital displays indicating

rotor speeds and lubricant temperatures. The facility includes a 40 gallon oil tank, three gear pumps

(one main oil supply pump and 2 return pumps), and 2 forced air convection coolers (for the

lubricant and the drive motor). A Bentley Nevada ADRE for Windows DAIUcollects and processes

the test rig vibration measurements. The data processing software includes real time slow-roll

subtraction, order-tracking and synchronous response filtering. An instrumentation console containssignal conditioners and digital displays of the operating rotor speed, flow rate, supply pressures and

inlet/exit damper temperatures. The console includes the controls for operation of the lubrication

pumps and the oil cooling and heating elements. Three oscilloscopes display the rotor orbits at the

measurement locations. A fourth oscilloscope shows the bearing support housing accelerations, and

a frequency analyzer depicts the FFT of selected vibration signals.

STATE OF THE ART TECHNOLOGY:INTEGRALSQUEEZE FILM DAMPERS

(ISFDs)

Modern technological advances in metal

working allow the development of integral

squeeze film dampers (ISFDs). This ingenious

design is made possible by a wire Electrical

Discharge Machining (EDM) process.

ISFDsare compact mechanical elements with

a length no larger than the bearing itself,

and comprised of arcuate pads attached to a

bearing housing via thin wire-EDMwebs.ISFDscan also be machined as split segments

allowing rapid retrofit. Replacement of

squirrel cage supported SFDs by integral dampers brings the following benefits to an aircraft gas

turbine engine:

1. Reduced overall weight and length of the entire aircraft engine structure

2. Elimination of squirrel cage components ISFDs compact and with reduced number of parts

3. Ability to support axial thrust loads without locking the damper lateral motion

4. Accurate positioning (centering) by precise design and construction of the support web stiffness

and pad film clearances5. Split configuration which allows easier assembly and inspection than with any other damper

design

A comprehensive study of the forced performance of Integral Squeeze Film Dampersis one of the

main objectives of research and further development.

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RESEARCH COMPLETED

IN 1996/1997:Measurements of the imbalance

response of the test rotor

supported on open ends, integral

squeeze film dampers (ISFDs) have

been completed. The dampers are

compact with integral radial

stiffness procured by wire EDMthin

webs. The ISFDshave length and

diameter equal to 3.8 inches

(96.52 mm) and 0.91 inches (23.0

mm), with a clearance equal to 9

mils (0.229 mm). The tests are conducted with an ISO VG 10 oil at room temperature (73 F). The

measurements include shaft speed, vibration displacements at six shaft locations, and two

accelerations at the support housings. Other measurements include oil temperatures, feed pressures

and flow rate.

Tests identifying the structural stiffness of each ISFDverify the design value (20 klb/in).

Measurements of the synchronous rotor response with increasing imbalance masses are performed

from coast-down tests. The measured vibration peak response at the rotor first critical speed is

used to extract the system damping force coefficients and subsequent identification of the ISFD

damping coefficients. The experiments show the open ended ISFDsto damp well the rotor response

for the cylindrical modes of vibration, with peak vibration amplitudes proportional to the magnitude

of the imbalances. Large rotor motions up to 80% of the nominal ISFDclearance are measured, and

without shifts in the first critical speed denoting an absence of damper stiffness hardening. The

test system damping coefficients increase slightly with the amplitude of rotor motion through the

first critical speed. From these, the damping coefficients for the ISFDs are extracted and agree wel

with predictions from a full-film open ends, integral damper FEM model. This model is based on the

solution of the classical Reynolds equation without fluid inertia effects for incompressible, isoviscous

fluids flowing through the thin film land between the flexural pads and the damper housing. Given a

specified damper journal position and instantaneous velocity, the program calculates the damper

reaction forces and damping force coefficients in the (X,Y) directions.

RESEARCH COMPLETED IN 1997/98: SEALED INTEGRAL SQUEEZE FILMDAMPERSAdditional work on the experimental facility includes measurements of the test rotor- ISFD

responses to couple mass imbalances and for ISFDswith end seals. The goal is to determine the

effect of controlled end gap seals on the integral damper viscous force coefficients and their

influence on the imbalance response of the test rotor. The measurements also include damper flow-

rates and maximum temperature rise of the lubricant.

Measurements of the rotor synchronous response to couple imbalances exciting the conical mode of

vibration further demonstrate the effectiveness of the integral SFDsto reduce rotor vibrations at

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this mode. Additional imbalance

response measurements show the

effect of controlled end gap seals on

increasing the ISFDsdamping

coefficients while still allowing for

cooling lubricant flow through the

dampers.

The synchronous horizontal rotor (p-

p) response for increasing levels of

rotor imbalance is shown here for

dampers with end seal gaps equal to 3

mils. The experiments show the sealed ends ISFDsto damp well the rotor response for the cylindrica

mode of vibration and with peak vibration amplitudes proportional to the magnitude of the disk

imbalances. Note that the rotor peak amplitude for the largest imbalance is nearly 90% of the

damper radial clearance (0.230 mm). Damping coefficients extracted from the peak amplitudes are

also shown below as a function of the peak rotor amplitude for various end gap seal clearances (3, 4

and 5 mils). Damping coefficients for the open ended dampers are also included. The dampingvalues at zero rotor eccentricity correspond to the test results from impact response experiments

without rotor spinning.

The paramount effect of the end

seal gap clearance is clearly

demonstrated from the

experiments. Tighter end gap seals

offer more damping, up to two

times the magnitude obtained with

the open ended dampers. However

the most notable finding is that the

damper flow rate is not reduced as

the end seal clearance decreases,

thus allowing for the integral

dampers to perform their function

satisfactorily without lubricant

overheating, as would be the case

of a conventional damper with tight end seals.

RESEARCH COMPLETED IN 998-1999: SERIES TILTING PAD BEARING-

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AND INTEGRAL SF-DAMPER

High performance, high speed turbomachinery

demands appropriate means to ensure structural

isolation of components and stringent rotor

vibration limits with tolerance to sudden

imbalance loads due to blade loss events, shock,

and maneuver actions. Squeeze film dampers arean effective mean to reduce vibrations and to

suppress instabilities in high performance aero-

engine systems. Integral squeeze film dampers

(ISFDs) offer distinct advantages such as reduced

overall weight and length of the damper

structure with less number of parts, accuracy of

positioning (centering), and a split segment construction allowing easier assembly, inspection and

retrofit than with any other type of damper. Flexure pivot tilting pad bearings offer similar

construction features as the ISFDs while minimizing assembly stack up tolerances and avoiding pivot

wear and fretting. The series combination of a tilting pad bearing and a squeeze film damper has

been implemented in numerous process compressors in the petrochemical industry to introduce

flexibility and damping to the bearing supports. The proper design of these two mechanical

elements allows for the optimum damping coefficient at the bearing support and accurate

relocation of the (rigid mode) rotor bearing system critical speeds away from the operating speed

range.

Measurements of imbalance responses of a test rotor

supported on SFDshave been conducted since 1996. These

experiments address to rotor-SFDconfigurations typical ofaircraft gas turbines where safety and stability dictate the

use of ball bearings instead of fluid film hydrodynamic

bearings. In 1999 we are conducting measurements of the

synchronous imbalance response of the test rotor

supported on flexure pivot, tilting pad bearings and

integral SFDs. The major objectives of the experiments

are to determine the combined effect of the hydrodynamic

bearings and SFDs on the location of critical speeds and

effective logarithmic decrement, and to demonstrate the

effectiveness of this bearing pair combination on reducing

amplitudes of rotor vibration. The experimental results will allow benchmarking of predictive

computational tools for estimation of force coefficients in both tilting pad bearings and squeeze

film dampers.

THE EXPERIMENTS

Coast down rotor response measurements will be recorded for dry and wet dampers, i.e. without

and with lubricant, for increasing mass imbalances located at the rotor middle disk. The programs

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SFDFLEXSand HYDROTRCMwill be used to predict the force coefficients of the integral dampers

and flexure pivot bearings over a frequency range. The XLTRCprogram will allow the prediction of

the rotor synchronous response using equivalent impedance for the series SFD-tilting pad bearing at

the bearing supports. The predictions will be correlated to the test measurements to evidence the

effectiveness of the SFDsin suppressing rotor vibrations. A continuation project included the contro

of the damper stiffness based on the regulation of the feed pressure.

To learn more, order/read ourPublicationsACKNOWLEDGMENTS

The support from National Science Foundation (NSF) and the Turbomachinery Research Consortium

(TRC) is gratefully acknowledged. Thanks to Dr. F. Zeidan, KMC Bearings, Inc., for his assistance

and support.