MEASUREMENTS OF ROTOR LIFT-OFF AND
BREAK UP TORQUE IN A METAL MESH FOIL BEARING
FOR USE IN AUTOMOTIVE TURBOCHARGERS
Keun Ryu
Thomas Chirathadam
Graduate Research Assistants
Brian Rice
UG Research Assistant
UNIVERSITY OF VIRGINIA,
AEROSPACE ENGINEERING
Charlottesville, VA 22903
TL
Luis San Andrés
Mast-Childs Professor
TEXAS A&M UNIVERSITY, MECHANICAL ENGINEERING
COLLEGE STATION, TX 77843
Abstract
Gas bearings enable the commercial success of high speed microturbomachinery operating at high temperatures and virtually friction free. Metal mesh gas foil
bearings are a low cost alternative to replace oil-lubricated bearings in passenger vehicle turbochargers. However, during rotor start up and shut down, the rotor
operates in contact with the foil bearings thus demanding of a large break-up torque to overcome the dry friction. Early rotor lift-off in the bearings enables nearly
friction free operation. Measurements of break-up torque on a metal mesh bearing as a function of shaft speed and static load are obtained in an existing
turbocharger driven test rig. Bearing performance characteristics such as power loss and ultimate load capacity are experimentally determined. The bearing
experiences the highest torque at low shaft speeds, dropping significantly once the rotor lifts off. Increases in static load lead to an increase in bearing break-up
torque and delay rotor lift off to a higher speed.
Experimental Facility
Terminology
Figure 2. Schematic view of
test rig [2]
n
ow
S p e e d [R P M ]
t-d
40000
Start-up
30000
20000
10000
Gas film operation region
0
0
10
20
30
40
Time [s]
Torque [N-mm]
Bearing torque
vs. time
50
Touch-down
40
Lift-off
30
20
10
0
10
20
30
40
http://www.grc.nasa.gov/WWW/Oilfree/turbocharger.htm
P o w e r L o s s [W ]
Power loss
vs. time
Time [s]
Turbochargers
100
90
80
70
60
50
40
30
20
10
0
De
cre
as
0
www.microturbine.com
e in
Po
we
r lo
10
20
Air supply pressure
into turbine is
manually increased
until rotor overcomes
the dry friction and
begins to rotate freely.
Rotor runs up to 40
krpm and then coasts
down.
Drag torque is large
and peaks at rotor
speed of ~2 krpm. A
sharp drop in torque
indicates rotor lift-off,
while sharp increase
evidences touchdown.
Maximum power loss
due to friction occurs
at start-up and
gradually decreases
as lifted rotor speeds
up.
ss
30
40
Time [s]
Turbo compressor
Figure 3. Rotor start-up/shut-down cycle over a
40 second interval, 13 N static load (vertical).
www.turbomagazine.com/features/0110tur_
1994_toyota_supra/photo_09.html
http://www.miti.cc/newsletters/150hpcom
pressozr.pdf
- Lift-off speed and torque during start-up and shut-down
- Drag torque for increasing rotor speed and static loads
- Ultimate load capacity
- Rotor temperature for increasing static load
Acknowledgment
This study is supported by National Science Foundation under REU#0552885 program.
The support of NASA (agreement NNX07P98A )and Honeywell Turbo Technologies are
also acknowledged.
Torque [N-mm]
Rotordynamic measurements of a metal-mesh gas
foil bearing mounted on a turbocharger rotor
Bearing torque
vs. speed
60
Research Objective
10
8
5
10
15
20
25
55 N-mm
50
40
Run up
10 N-mm
30
Run-up
Coast Down
20
10
Coast down
0
0
10000
20000
30000
40000
50000
Speed [rpm]
Figure 4. Bearing torque versus
speed under a 13 N static load.
Once rotor lifts-off, operation with a gas
film reduces friction 82 % (~ 6 times)
Drag torque is
maximum at low rotor
speed, prior to rotor
lift-off. The bearing
torque at top speeds,
> 40 krpm, drops to
18% of the torque at
start-up.
Friction coeff.
μb = 0.31
μg = 0.055
10000
9000
8000
7000
6000
5000
4000
3000
2000
1000
0
15000
17000
19000
21000
23000
25000
27000
Speed [rpm]
Figure 6. Bearing ultimate load
capacity versus rotor speed
Temperature
vs. static load
u
Sh
Rotor speed
vs. time
50000
0
Micro gas turbine
12
Bearing drag torque
increases linearly with
applied load (includes
the bearing weight) at
a speed of 40 krpm
(rotor lifted off).
Figure 5. Bearing torque versus
applied load at 40 krpm
Experimental Results
60
Future applications
14
0
Load Capacity [Pa]
Table 1. Main parameters of MMFB bearing and test rig
B
WBEARING=2.73 N
Mass [g]
278.2
e
Cartridge length [mm]
35.9
a Cartridge outer diam. [mm]
54.2
r
Cartridge inner diam. [mm]
42.2
i
Mesh length [mm]
30.5
n
Mesh thickness [mm]
8
g
Top foil diam. [mm]
28.1
Top foil thickness [mm]
0.127
T R
Journal diam. [mm]
28.05
e i
Speed range [rpm]
0 - 70,000
s g
Pressure range [psi]
0 - 125
t
Torque Arm Length [mm]
119.2
Gas Bearings
• readily available at low cost
• material compactness provides control of stiffness
• enable high temperature operation
Torque [N-mm]
Bearing torque
vs. static load
Figure 1. Turbocharger test rig
[2]
Metal Mesh Gas Bearings
Metal Mesh Foil Gas Bearings
16
Total Load [N]
W
• eliminate oil and remove pumping and sealing systems
• reduce drag power and heat generation
• allow weight reduction
• improve overall system efficiency & reliability
• enable higher and lower temperature capability
18
6
Temperatuer [°C]
2
20
Load capacity
vs. speed
Gas bearing – compliant, self-acting film bearing uses
air as the working lubricant.[1]
Break-up torque – Applied shaft torque to overcome
dry-friction (contact) and allow shaft rotation with gas film.
Lift-off speed – Rotor speed at which thin gas film
evolves to support load acting on bearing and without
rubbing.
Load (W) capacity – The maximum load that the
bearing can withstand at a particular speed until sliding
contact occurs with sudden rise in drag torque [1].
Power loss – The mechanical loss of energy caused by
the sliding friction between the top foil and test rotor.
T orque
μb– Break-up friction coefficient
μ=
μg– Gas film friction coefficient (idem)
D
Experimental Results (cont’
(cont’d)
Rotor runs up to 40
krpm and is statically
loaded. Rotor
decelerates by closing
air supply into turbine.
Ultimate load
capacity determined
from sudden increase
in torque at a rotor
speed that is
proportional to applied
load.
65
60
Rotor temperature is
proportional to applied
load in gas film
operating region.
55
50
45
40
35
30
25
20
0
5
10
15
20
25
Load [N]
Figure 7. Temperature of rotor free end versus applied
load. Operation at 40 krpm
Conclusions
1) During a rotor start-up/shut-down cycle, bearing torque drops
significantly at rotor lift-off and raises sharply at touch-down.
Break-up torque during start-up is ~ 34% larger than that at shutdown.
2) Once rotor lifts, torque decreases as rotor speed increases
demonstrating operation in cushion of gas film. Within 3040krpm, torque is a minimum.
3) Friction coefficient reduced by ~ six times once rotor lifts as
opposed to operation with rubbing contact.
4) Gas film operating torque and rotor temperature increase linearly
with respect to applied static load.
5) Ultimate load capacity increases proportionally as rotor speed
increases.
Metal mesh foil bearings perform best at high speeds where the
gas film can support higher loads and no dry friction occurs. At
low speeds metal mesh foil bearings show high friction. The
challenge is to design a bearing that reduces both break-up
torque and drag torque during gas film operation. Further
research on solid lubricant coatings could achieve the goal.
References
[1] DellaCorte, C., 1997, “A New Foil Air Bearing Test Rig for Use to 700 °C
and 70,000 rpm,” NASA TM-107405.
[2] San Andrés, L., and Kim, T.H., 2008, “Measurements of Structural
Stiffness and Damping in a Metal Mesh Bearing and Development of a Test
Rig for Foil Gas Bearings,” TRC-B&C-5-08.