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TRC 2008
The Effect of (Nonlinear) Pivot Stiffness on Tilting Pad
Bearing Dynamic Force Coefficients – Analysis
Jared GoldsmithResearch Assistant
Dr. Luis San AndresMast-Childs Professor
TRC Project 32513/1519 T3
2
XLTRC2
Project Goals
Enhance tilting pad bearing model by including nonlinear pivot flexibility for rocker, spherical, and flexure type pivots
3
Tilting Pad Journal Bearing Pivot Types
Tilting Pad Bearings
Y
X
PAD ROTATION
Y
X
PAD ROTATION
Y
X
PAD ROTATION
FLEXURE WEB
4
pivot
P
l
Y
X
e
journalpad
t
pivot
P
l
Y
X
e
journalpad
t
Film Thickness
Tilting pad journal bearing & coordinates
pivot
pad
pivot
pad
rotational stiffness
radial stiffness
PK
PK
)sin()()cos()()sin()cos( pkk
ppk
YXpk RreeCh
XoW
YoW
journal speed
film thickness
Film thickness:
Pad clearance ( ) and preload ( ) and journal eccentricity ( )pC pr YX ee ,
Pad angular rotation ( ), radial ( ) and transverse displacement ( ) fork k k padNk ,..1
5
Perturbation Analysis
Small amplitude journal motions about an equilibrium position
YoXo WW ,
YoXo ee ,
koP kohthk k
oko
ko ,,
tiXXoX eeete )( ti
YYoY eeete )(
tikko
k et )(
tikko
k et )(
tikko
k et )(
padNk ,..1
Applying an external static load with components ( ) to the journal determines its static equilibrium position ( ) with fluid
static pressure field , film thickness , and corresponding
equilibrium pad rotation and deflections ( )
Consider small amplitude journal center motions ( ) of
frequency about the static equilibrium point. Hence
YX ee ,
Consider small amplitude journal and pad motions about static equilibrium position (SEP)
and
for
6
Load and Pad EOMs
Bearing forces and Pad Equations of Motion
k
k
k
kP
kP
kP
k
k
k
kpad
F
F
M
F
F
M
M
padN
k
kX
tiXXoX FeWWW
1
padN
k
kY
tiYYoY FeWWW
1
The sum of the pad fluid film forces balance the external load applied on the journal, i.e.,
kkk
kkk
kkkkkP
kpad
mcm
mbm
cmbmI
M
0
0
Force and Moment EOMs for pad:thk
Matrix representing pad inertia and mass
Y
X
Journal Rotation
XoW
YoW
Bearing
Fluid film
7
Nonlinear Pad Pivot
Typical Nonlinear Pivot
Radial Force
PK
PF
oPK oPF
Po
PF
P
Radial Deflection
)( fFP
PoPoPP KFF
)( PooP fF
PoPK
The assumption of small amplitude motions about an equilibrium position allows the pivot reaction radial force to be expressed as
where
is the static load on the pivot and
is the force due to radial displacement
Consider a typical nonlinear force ( ) versus pivot radial deflection ( ) in a bearing pivot
PF
8
Pad Forces and Moment
Forces and moments acting on a pad
Pad Fluid Film Forces = integration of hydrodynamic pressure fields on pad
R
L
kkl
kl
L
L
kk
kY
kX dzRdPF
F
sin
cos
ti
k
k
kY
X
YX
YYYYXYY
XXXXYXX
ko
kYo
kXo
k
kY
kX
e
e
e
ZZZZZ
ZZZZZ
ZZZZZ
M
F
F
M
F
F
kpP
kXP
kY
k FRFFtRM ]sincos)[(
Moment on Pad:
Substitution of zeroth and first order pressure fields gives
Fluid film impedances: },{ kkk CiKZ ,,,,, YX
9
Reduced Force Coefficients
Frequency reduced force coefficients for tilting pad bearing
pad
RR
RR
N
k
kb
kfP
ka
kXYRR
YYYX
XYXXR ZZZZCiK
ZZ
ZZZ
1
1 ][]][[][
where
and
Assuming the pads move with the same excitation frequency ω as the journal whirl frequency, the frequency reduced coefficients are
k
YYYX
XYXXkXY ZZ
ZZZ
k
YYY
XXXka ZZZ
ZZZZ
k
YX
YX
YX
b
ZZ
ZZ
ZZ
Z
k
kc
ZZZ
ZZZ
ZZZ
Z
][][][][][ 2 k
masskc
kpivot
kpivot
kfP MZCiKZ
Matrices representing pivot stiffness and damping coefficients
k
PP
P
PPkpivot
KK
K
KK
K
0
00
0
][
k
PP
P
PPkpivot
CC
C
CC
C
0
00
0
][
10
Progress
Tilting pad bearing pivot
Modified tilting pad bearing model now accounts for spherical and rocker nonlinear pivot stiffness
0
100
200
300
400
500
0 2 4 6 8 10
Load (kN)
Piv
ot S
tiffn
ess
(MN
)
Spherical pivot stiffness versus load
Pivot
Pivot housing
PF Assumptions:
•Spherical pivot – point contact
•Rocker pivot – line contact
11
Test Bearing
Test bearing description
Y
X
PAD ROTATION
Carter and Childs* five pad, rocker pivot, tilting pad bearing (LBP) and (LOP)
2m
Bearing Parameters Values
Rotor diameter 101.59 mm
Pad axial length 60.33 mm
Pivot offset 60%
Pad number (arc length) 5 (57.87)
Radial pad clearance .1105 mm
Pad inertia 2.49E-4 kg-
Preload 0.282
Radial bearing clearance .0792 mm
Mobile DTE ISO 32
31 cSt
5.5 cSt
Viscosity @ 40° C
Viscosity @ 100° C
Density @ 15°C
Specific heat
850 kg/m3
1951 J/(kg-K)
Fluid Properties
* ASME Paper No. GT2008-5069
12
(LBP) Rotordynamic Force Coefficients
Experimental and predicted direct stiffness and damping coefficients
Carter and Childs measured and predicted nonsynchronous direct stiffness
Rotor speed = 4000 RPM Bearing loaded in –Y direction (LBP)
0
100
200
300
400
500
600
700
800
0 5 10 15
Load [kN]
Sti
ffn
es
s [
MN
/m]
Kxx
Kxx Th
Kyy
Kyy Th
The TPB model (rigid pivot) generally over predicts stiffness and damping coefficients
13
Static Results
Direct static stiffnesses versus load
Original XLTRC2 (rigid pivot) and modified XLTRC2 (flexible pivot) predicted direct static stiffnesses versus static load
0
50
100
150
200
250
300
0 2 4 6 8
Load [kN]
Dir
ect
Sti
ffn
ess [
MN
/m]
Kxx - Flexible Pivot
Kxx - Rigid Pivot
Kyy - Flexible Pivot
Kyy - Rigid Pivot
Flexible pivot
Rigid pivot
Rotor speed = 4000 RPM Bearing loaded in –Y direction (LBP)
14
-0.35
-0.30
-0.25
-0.20
-0.15
-0.10
-0.05
0.00
0 1 2 3 4 5 6 7
Load [kN]
Ey/
Cp
Flexible Pivot
Rigid Pivot
Static Results
Journal eccentricity versus load
Original XLTRC2 (rigid pivot) and modified XLTRC2 (flexible pivot) predicted journal eccentricity versus static load
Flexible pivot
Rigid pivot
Rotor speed = 4000 RPM Bearing loaded in –Y direction (LBP)
15
Future work
Future work
•Perform extensive comparisons between predictions and Childs et al. experimental TPB stiffness and damping coefficients
•Include pivot friction for spherical pivots