ON THE STATIC LOAD PERFORMANCE OF A LARGE
SIZE, HEAVILY LOADED SPRING SUPPORTED
THRUST BEARING
Rasool Koosha
Graduate Research Assistant
Luis San Andrés
Mast-Childs Chair Professor
and Fellow STLE
1
STLE 74th Annual Meeting & Exhibition,
Nashville, Tennessee,
May 19-23, 2019
J. Mike Walker ’66 Department of Mechanical Engineering,
Texas A&M University
Introduction: Tilting Pad Thrust Bearings (TPTBs)
Control rotor axial placement in rotating machinery.
Offer advantages such as low power loss, simple
installation, and low-cost maintenance.
• As lubricant is sheared
• Film and pad temperatures increase.
• Load capacity of bearing depends on
lubricant viscosity,
• a function of temperature.
• Pad thermally and mechanically
induced deformations
• shape the operating fluid film
thickness and determine the bearing
forced performance.
2
Thrust Collar
Speed: Ω
Bearing Housing Orifice
Pivot
Fluid Film Pad Tilting
Static Load: W
Introduction: Spring-Supported Thrust Bearings (SSTBs)
For large size high power density applications, SSTBs are
preferred over pivoted ones. SSTBs produce
lesser elastic deformations,
self-adjustment against thrust collar misalignment,
better heat dissipation.
3
The performance of
SSTBs during machine
start-up and shut-down
processes is critical to
ensure safety and
reliability.
Prior work on SSTBs
4
Elastic deformations produce a convex pad top surface, and rotating
collar rubs on a pad to wipe out its Babbitt layer.
Bearing: SSTB in a turbine
generator unit, OD = 2.7 m
Chambers and Mikula Failure analysis
Operating Condition:
load/Pad = 2.1 MPa
Speed = 257 rpm (𝑅𝑂Ω = 37 m/s).
Time for thermal balance is proportional to pad thickness2
~ 9 h for a 0.25 m thick Babbitt & steel pad.
Bearing: SSTB OD = 2.0 m
Ettles et al. Transient response analysis
Operating Condition:
load/Pad = 3.0 MPa
Speed = 500 rpm (𝑹𝑶Ω = 53 m/s).
1987, STLE Trans., 31.
2003, J. Tribol, 125.
More work on SSTBs
5
Optimization process with objective functions to increase minimum
fluid film thickness while minimizing pad temperature raise. The
optimum design has:
Thickness for a bearing pad equal to 12% of its circumferential
arc length.
Spring bed arrangement extending edge-to-edge in the radial
direction.
Yet only supporting 55% of the pad circumferential length.
Bearing: SSTB
Distinct bearing configurations
Ettles et al. Bearing Design optimization
Operating Condition:
load/Pad = 4 MPa
Speed = 300 rpm.
2016, J. Tribol, 138.
TPTB current computational analysis
2D hydrodynamic pressure on pad surface.
Cross-film viscosity variation.
Accounts for turbulent flow effects.
3D temperature distribution in fluid film.
Heat conduction to the pads.
Accounts for turbulent flow effects.
3D temperature distribution in pad and liner.
Heat transfer boundary condition all side
of a pad.
The acting pressure & temperature gradient in a pad produce elastic deformation.
6
Elastic deformations in a pad
Advantage:
High
Accuracy.
7
Pad and Liner deformation
field from in-house FE model
Babbitt
Pivot
Peak
deformation
In-House Finite Element
model couples pad and
spring-bed stiffness.
Pressure induced
deformation
Temperature induced
deformation
3D deformation field for a
pad
Pad rigid body motion
due to spring bed
flexibility
Both pad deformation and
rigid body motion change the
film thickness bearing
performance
9
A SSTB holding a hydraulic turbine
Max surface speed ΩRo 23.6 m/s
Specific load per pad W/(Ap Np) 4.0 MPa
Outer/inner radius, Ro/Ri 1.43
Radial/circumferential length, L/B 1
Pad thickness/outer radius, H/L 0.2
Pad material Steel
Lubricant Thick Oil
Large size 16-pad SSTB:
Spring-beds extend
• Edge-to-edge radial length of a pad
• Partially in circumferential direction.
Pads include
• a hydrostatic pocket for hydraulic lift
• an internal multiple-pipe cooling system
Issue: SSTB shown repeated failures after accumulating a
number of startup /shutdown processes ranging from 3k to 5k.
10
Objective of Predictive Analysis
Three cases of operation:
Case 1: Nominal operating condition (4.0 MPa specific load/pad and
23.6 m/s OD surface speed) without cooling system.
To evaluate hydrodynamic load performance of bearing.
Case 2: Nominal operating condition with an active cooling system.
Bearing performance compared to Case 1 evaluates the
effectiveness of the cooling system.
Case 3: Operation with idle collar (zero speed):
To evaluate hydrostatic load performance.
Bearing performance for (a) a hot pad (keeps temperature from
nominal condition) to simulate a fast shut down, and (b) with a
cold pad that assumes a slow process of thermal equilibrium as
shaft speed decreases.
11
Case 1: Bearing operating at
nominal condition
with inactive cooling system.
OD speed = 23.6 m/s,
Specific load/pad=4 MPa
12
C1: Fluid Film Thickness
200
180
160
140
120
100
80
220
240
[µm]
Minimum
Film
Thickness
At mean diameter Fluid Film Thickness field
OD surface speed = 23.6 m/s,
Specific load/pad=4 MPa
Nominal Operating condition.
Cooling system OFF
Minimum film thickness = 74 µm
Occurs within the pad surface (non at
the trailing edge)
Maximum film thickness = 246 µm at
pad OD - trailing edge.
Tra
ilin
g E
dg
e
Le
ad
ing
Ed
ge
Pressure field on pad
13
Case 1: Pressure Field
At mean diameter4.0
3.0
2.0
1.0
0.0
0.5
1.5
2.5
3.5
Hydrostatic
pressure
OD surface speed = 23.6 m/s,
Specific load/pad=4 MPa
Low pressure zone at the pad trailing
edge inner and outer diameter
Nominal Operating condition.
Cooling system OFF
Pressure field is extended fully along
the pad radial length but not the
circumferential length
14
C1: Film & Pad Temperatures
4035
2520
510
0
15
30
Pad Temperature riseFluid film Temperature rise
Peak Temperature
Collar Side
Pad SideTop
Side
Bottom
Side
45
50
Pocket Pocket
OD surface speed = 23.6 m/s,
Specific load/pad=4 MPa
Both pad and fluid film show a significant temperature rise extending well into the
pad.
[⁰C]
Nominal Operating condition.
Cooling system OFF
15
C1: Pad Elastic Deformations
50
0
-50
-75
-100
-150
-25
25
-125
Top
Side
Total deformation
Bottom
Side
Thermal deformations are larger than
mechanical deformations.
Thermal deformation warp the pad to
make a convex surface.
Nominal Operating condition.
Cooling system OFF
[µm]
OD surface speed = 23.6 m/s,
Specific load/pad=4 MPa
16
Case 2: Bearing operating at
nominal condition
with active cooling system.
OD speed = 23.6 m/s,
Specific load/pad=4 MPa
17
Case 2: Fluid Film Thickness
200
180
160
140
120
100
80
[µm]
Minimum Film Thickness
At mean diameter Film Thickness on Pad
OD surface speed = 23.6 m/s,
Specific load/pad=4 MPa
Nominal Operating condition &
active cooling system
Minimum film thickness of 81 µm at pad
trailing edge.
Maximum film thickness of 196 µm at
the pad OD & trailing edge.
Compared to case 1:
• Larger min film and smaller max film.
• Location of minimum film thickness moves
toward inside of pad.
Pressure Field (normalized with
respect to pad mean pressure)
18
C2: Fluid Film Pressure
4.0
3.0
2.0
1.0
0.0
0.5
1.5
2.5
3.5
At mean diameter
OD surface speed = 23.6 m/s,
Specific load/pad=4 MPa
Hydrostatic
pressure
Nominal Operating condition &
active cooling system
Pressure field covers whole pad
surface.
Compared to case 1:
Pressure field shows a slower drop
from the center to the edges along the
circumferential direction.
19
C2: Film & Pad Temperatures
40
35
25
20
10
5
0
15
30
Pad Temperature riseFilm Temperature rise
Collar Side
Pad SideTop
Side
Bottom
Side
OD surface speed = 23.6 m/s,
Specific load/pad=4 MPa
Maximum Temperature
[⁰C]
Nominal Operating condition &
active cooling system
PocketPocket
Temperature rise is moderate (max 10C) except at pad trailing
edge (40C) where cooling lines do not reach.
Compared to case 1: Pad temperature rise is 10⁰C higher.
20
C2: Pad Elastic Deformations
Max surface speed = 23.6 m/s,
Specific load/pad=4 MPa
40
20
0
-10
-30
-40
-50
-20
10
30Top
Side
Bottom Side
Total deformation
Mechanical (pressure induced) deformations larger
than thermal deformations. Pad warps at ID and OD.
Nominal Operating condition &
active cooling system
[µm]
Mechanical deformations are larger than thermal
deformations.
Compared to Case 1: Pad deformations are smaller.
21
Case 2: Lessons Learnt
The bearing produces a sufficiently thick fluid film thickness.
Even with an active cooling system, the predicted pad temperature
rise and ensuing thermal deformations are significant at the trailing
edge where cooling lines do not reach.
The current arrangement of springs under a pad, with free leading
and trailing edges, improves hydrodynamic performance as it opens
the film at the leading edge to let flow in. [1] Ettles et al. 2016, J. Tribol, 138(4)
22
Inactive vs Active Cooling Systems: Lessons Learnt
Active cooling system effectively removes heat from bearing pad.
Bearing with an active cooling system produces
Larger fluid film thickness.
Reduced low pressure area.
Lower pad temperature and fluid film temperature rises.
Smaller pad thermal deformations.
Not the
same
23
Case 3: Bearing operating w/o
shaft speed
and with active cooling system.
Speed = 0
Specific load/pad=4 MPa
HOT pad: keeps same temperature as that
when operating at nominal speed.
COLD pad: reaches temperature field as if
operating at (very) low speed (steady state).
Simulate bearing performance after a
fast shut down in shat speed
24
C3: Fluid Film Hydrostatic Pressure Fields
Hydrostatic Lift
Cold Pad4.0
3.5
2.5
1.0
0.5
0
3.0
2.0
1.5
Hot Pad
Surface speed = 0 m/s
Specific load/pad=4 MPa
Hydrostatic Lift
Hot pad shows slightly higher peak pressure.
Compared to case 2:
Pressure drops quickly from pocket towards pad edges, leading and trailing.
Larger pocket pressure needed to carry load.
Hydraulic Pressure for Lift ACTIVE
25
C3: Fluid Film Thickness Fields
[µm]
Minimum Film Thickness
Cold Pad
45
40
30
15
10
5
0
35
25
20
Hot Pad
Max surface speed = 0 m/s
Specific load/pad=4 MPa
Minimum film: 6 µm for cold pad & 1 µm for hot pad
Compared to Case 2:
Minimum film is significantly smaller and occurs at the pad
ID and OD (as it warps).
1 mm
6 mm
26
C3: Pad Deformation (thermal+pressure)
Peak
Deformations
Top
Side
Bottom
Side
10
0
-20
-30
-50
-40
-10
20
30Cold PadHot Pad
Top
Side
Bottom
Side
surface speed = 0 m/s
Specific load/pad=4 MPa
Hot pad shows significantly larger pad deformations.
Unlike the cold pad, the deformations of the hot pad at the trailing edge are positive
The film thickness reduces at trailing edge
[µm]
27
Case 3 vs. Case 2: Lessons Learnt
The current bearing design, pad dimensions and spring-
bed arrangement under the pads, is not capable of
producing a sufficiently large fluid film thickness for a
hydrostatic operating condition.
Operating with a hot pad during a shutdown process
affects bearing performance since at a low speed 0
speed, the minimum fluid film thickness is critically small
(order of surface roughness) and which increases the risk
of contact.
Shutdown Process: Cold Pad vs Hot Pad
Pad minimum film thickness
Min
. F
ilm
Th
ickn
ess
Speed
Shutdown
(hot pad)
Very slow shutdown
(cold pad)0.9
0.7
0.6
0.5
0.5
0.3
0.2
0.1
0.0
1
0.8
10.90.80.70.60.50.40.30.20.10
A hot pad
produces a
smaller film
thickness.
Difference
increases at
very low
speeds.
Specific load/pad=4 MPa
nominal
speed
Closure
Pad pressure induced deformations are significant to
warp a pad (convex curvature) at both its leading & trailing edges to
open (enlarge) the film thickness; and
the pad OD and ID warp toward the thrust collar (concave curvature)
and then reduce the film thickness.
The pad internal cooling system effectively limits a pad temperature
rise, hence thermal elastic deformations are moderate, except at the
pad trailing edge where the cooling lines do not reach.
During a slow shut-down process with a hot pad, the model predicts a
smaller film thickness than that if the bearing pads are cooled at a
steady rate.
A fast shutdown may lead to sudden seizure of one or more pads in
the bearing.
ON THE STATIC LOAD PERFORMANCE OF A LARGE SIZE, HEAVILY LOADED
SPRING SUPPORTED THRUST BEARING
Future Work
To better design both the pad and spring bed
geometry and disposition to ensure reliable
operation
during a nominal operating condition, and
during quick start-up and shut-down processes.