N86-3

WHIRL/WHIP DEMONSTRATION

Robert Grissom Bently Nevada Corporation

M i nden , Nevada 89423

Fluid flow in bearings and seals, set in motion by shaft rotation (Figure l), gener- ates dynamic forces which may result in a well-recognized instability known as whirl and whip. vibrations in which the amplitude may vary from very small to nearly the limit of the bearing or seal clearances. Oil whirl i n ;ubi-icated bearings, in particular, typically occurs at somewhat less than half rotative speed. As the rotative speed increases, the frequency relationship remains constant until the whirl frequency approaches the first balance resonance. at a nearly constant frequency asymptotically approaching first balance resonance, i ndependent of i ncreasi ng rotative speed. can permit, prevent, or eliminate this instability.

These are lateral, forward precessional, self-excited, subsynchronous

Now the whirl is smoothly replaced by whip

Changes in beari ng/seal radi a1 1 oadi ng

F'LuD3 IN MOTION DUE TO SHAFT ROTATION

SOURCE OF INSTABILITY! Figure 1. - Bearing/seal fluid-generated instability.

06 J ECTIVE

The oil whirl/whip rig demonstrates the effects of fluid dynamic forces generated by the rotating shaft. At low rotative speeds, this produces changes of the jour- nal static equilibrium position within the bearing. lationship between any load direction and the average journal oqlii 1 ibrium position (attitude angle) (Figure 2). At higher rotative speeds, the instability threshold is observed as a function of unidirectional radial load, unbalance, and rotor configuration.

The demonstrator shows the re-

https://ntrs.nasa.gov/search.jsp?R=19860020723 2020-03-16T01:58:01+00:00Z

Figure 2. - Attitude angle between load direction and average journal equilibrium position.

ORBITS:

WHIRL WHIP

Figure 3. - Oscilloscope presentation of oil whirl and oil whip orbital motion (vibration precession) of journal, cdined with once-per-turn reference (keyphasor) signal. may move forward or backward relative to time depending on ratio of vibration frequency to rotative speed.

Keyphasor marks

speea.

The f i xed relat ionship between o i l whi r l frequency and ro ta t i ve speed, as well as the t rans i t ion t o o i l whip a t a frequency s l i g h t l y below the f i r s t balance resonance frequency, may be observed on the oscilloscope (Figure 3) and spectrum analyzer. Note the d i s t i nc t i ve oscil loscope o r b i t pat tern o f the two Keyphasor marks s l o w l y ro ta t ing counter t o time d i rec t ion (stroboscopic ef fect ) during o i l whi r l and the mul t ip le Keyphasor marks during o i l whip (variable frequency rat ios).

ROTOR R I G

The r o t o r r i g consists o f a s ingle disk r o t o r supported i n an O i l i t e ( o i l impreg- nated, sintered bronze) bearing a t the inboard end and an o i l lubr icated Lucite journal bearing a t the outboard end. The journal i s 0.980" i n diameter w i th 0.006" t o 0.013" diametrical clearance. O i l i s grav i ty fed through an ax ia l groove (Figure 4).

The disk may be posit ioned anywhere along the shaft t o modify the st i f fness. loading o f the journal bearing may be control led w i th horizontal and ver t i ca l springs o r a nylon s t i c k (Figure 5).

The ro to r i s driven by a variable speed (0-12,000 rpm) e l e c t r i c motor through a f 1 exi b l e coup1 i ng.

Radial

41 6

1. Variabla s p n d a leca ic mmr 8. 011 supply menolr 2. flexibla coupling 9. V l r t i c a l pmx1oity prob. 3. 8earinq supmrt 10. Luclta har tmi

5. lTx.37P shaft 12. 011 p9 6. 1 U4 & disk 13. O i l d n l n ms.)’nlr

4. Ol l f ta -ring 11. HortoPnt.1 p m x i ~ l t Y PmM

7. u r d 14- h m P1.m

Figure 4. - Rotor rig for demonstrating oil uhirl/uhip. Figure 5. - Application of preload uith nylon stick.

INSTRUMENTATION

X-Y proximity probes located at the journal bearing provide position and vibration information for the orbit and time base display on the oscilloscope. Vector Filter (DVF 2), an automatic or manually tuned bandpass filter, displays rotative speed, amplitude, and phase. frequency domai n di spl ay.

The Digital

An FFT spectrum analyzer generates a

MEASUREMENT PARAMETERS

The parameters o f interest are:

Rotor equilibrium position and vibration measured at the journal. * Average oil swirling ratio, A. * Oil whip frequency. * Direction of vibration precession.

Change in threshold of stability with changes in disk position and un- balance (two additional stability thresholds). Relative ‘radial load changes (magnitude and direction) to stabilize or destabi 1 ize system.

RESULTS

Figures 6 through 15 illustrate the transition from whirl to whip and the e f f e c t o f unbalance force on the whirl threshold. The vibration mode shape for whirl and whip and variations in stability threshold with rotor configuration are also shown.

41 7

A m GI

2 X - 3: P

J Y

E c 0

STABI!

e

RUN: 8

- _- - in= I = - - -

41, 5: m-

FIRST BALANCE

RESONANCE

Figure 6. - Cascade spectrun of vertical vibration response during startup with well-balanced

. . . . . + 1

f! 0 FIRST * Y

BALANCE RESONANCF

Figure 7. - Cascade spectrun of vertical vibration response during startup with moderately t

rotor. Note stable region for synchronous vibrations associated with increased unbalance vi bration.

rotor.

la 1 anced

41 8

OF POOR QUALITY

N FIRST * P

BALANCE RESONANCE

Figure 8. - Cascade spectra of vertical vibration response during startup with more severely unbalanced rotor. Note expanded stable region of synchronous vibrations.

(a) Whirl mode - disk and journal motion in phase. (bl Whip mode - journal motion 90° ahead of disk motion.

. .. . Figure 9. - Rotor modes revealed by perturbation testing.

41 9

X

0

!!!

O6l I .I . . Figure 10. - Cascade spectrum of main rotor runup vertical vibration response and o i l whirl inception measured at oil bearing position. Disk located next to motor.

K W

a 5f

Figure 11. - Cascade spectrun of main rotor runup vertical vibration response measured at disk position. Disk located next to motor.

420

e R

3"

X

E STABILITY - a

THRESHOLD s.

O . U X

RUN: 116

Figure 12. - Cascade spectrun of main rotor runup vertical vibration response measured a t o i l bearing position. Disk located a t shaft midspan.

X

STABILIV THRESHOU)

&

8 AI- S-\-

x P

~ J u o . I c T <C.IEHM(IN x 1060)

figure 13. - Cascade spectrum of main rotor runup vertical vibration response measured a t disk position. Disk located a t shaft midspan.

421

"F RUN: 128

FREGuprcI CCWdTSAXN x

Figure 14. - Cascade spectrun of main rotor runup vertical vibration response measured at bearing position. Disk located next to oil bearing.

a '0 X

B s STABILITY$

THRESHOLD - P

Figure 15. - posi ti on. of figures

- .- CRhJuOcY <-IN x IBBB)

Cascade spectrun of main rotor runup vertical vibration response measured at disk Disk located next to oil bearing. Canpare stability threshold versus disk position 10, 12, and 14.

422