28th Turbomachinery Consortium Meeting

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28th Turbomachinery Consortium Meeting
Dynamic Forced Response of a Rotor-Hybrid Gas
Bearing System due to Intermittent Shocks
Luis San Andrés Mast-Childs Professor Principal
Investigator
Keun Ryu Research Assistant
TRC-BC-1-08
2008 TRC Project GAS BEARINGS FOR OIL-FREE
TURBOMACHINERY
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Micro Turbomachinery (lt 0.5 MW)
ADVANTAGES

• High energy density
• Compact and fewer parts
• Portable and easily sized
• Lower pollutant emissions
• Low operation cost

http//www.grc.nasa.gov/WWW/Oilfree/turbocharger.h
tm
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Gas Bearings for MTM
 Gas bearings for micro turbomachinery (lt 0.5 MW
) must be
Simple low cost, small geometry, low part
count, constructed from common materials,
manufactured with elementary methods.  
Load Tolerant capable of handling both normal
and extreme bearing loads without compromising
the integrity of the rotor system.
High Rotor Speeds no specific speed limit (such
as DN) restricting shaft sizes. Small Power
losses.
Good Dynamic Properties predictable and
repeatable stiffness and damping over a wide
temperature range.
Reliable capable of operation without
significant wear or required maintenance, able to
tolerate extended storage and handling without
performance degradation.
Modeling/Analysis (anchored to test data)
readily available
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Gas Bearings for MTM
Thrust in TRC program Investigate conventional
bearings of low cost, easy to manufacture (common
materials) and easy to install align. Combine
hybrid (hydrostatic/hydrodynamic) bearings with
low cost coating to allow for rub-free operation
at start up and shut down

Major issues Little damping, Wear at start
stop, Instability (whirl hammer),
reliability under shock operation
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Gas bearing test rig
Max. operating speed 100 kpm 3.5 kW (5 Hp) AC
integral motor Rotor length 190 mm, 28.6 mm
diameter, weight0.826 kg
Components of high-speed gas bearing test rig
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Gas Bearings for MTM
GT 2008-50393
2007 Control of bearing stiffness / critical
speed
Displacements at RB(H)
5.08 bar
2.36 bar
5.08 bar
Blue line Coast down
2.36 bar
Red line Set speed
Controller activated system
Peak motion at critical speed eliminated by
controlling supply pressure into bearings
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2007-2008 Objectives
Demonstrate the rotordynamic performance,
reliability, and durability of hybrid gas bearings

• Rotor motion measurements for increasing gas feed
  pressures and speed range to 60 krpm.
• Install electromagnetic pusher to deliver impact
  loads into test rig.
• Perform shock loads (e-pusher lift-drop) tests
  to assess reliability of gas bearings to
  withstand intermittent shocks without damage.

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TEST gas bearings
TEST gas Bearings
Flexure Pivot Hybrid Bearings Promote stability,
eliminate pivot wear, engineered product with
many commercial applications
worn pads surfaces
Clearances Cp 38 45 mm, Preload 7 5 mm
(20) Web rotational stiffness20 Nm/rad
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2008 Gas Bearing test rig layout
E-pusher Push type solenoid 240 N at 1 inch
stroke
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Electromagnetic pusher tests
Multiple impact
Impact duration 20 ms E-force 400 N (pk-pk)
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Manual lift drop tests
Multiple impact
Lift off to 515 cm (1030
rotation)
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Coast down E-pusher tests
Ps5.08 bar (ab)
Displacements at LB(H)
Intermittent shocks Impact force 100400 N
46 krpm
Shock 15 g Transient rotor response 40 µm
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Coast down manual lift drop tests
Shock induced acceleration At base 520 g At
housing 510 g
Ps3.72 bar (ab)
Beyond critical speed Synchronous frequency is
isolated from shocks Below 20 krpm Large
fluctuation of synchronous response
Displacements at LB(H)
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Waterfall manual lift drop tests
Displacements at LB(H)
Ps2.36 bar (ab)
Excitation of rotor natural frequency. NOT a
rotordynamic instability!
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Rotor response manual lift drop tests
Ps2.36 bar (ab)
Shock loads applied
Shock loads applied
Overall rotor amplitude increases largely.
Subsynchronous amplitudes larger than synchronous
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Rotor response manual lift drop tests
Ps2.36 bar (ab)
Natural frequency of rotor-bearing system
(150190 Hz)
Natural frequency of test rig (40 Hz)
Rotor-bearing natural frequency increases with
rotor speed. Natural frequency of test rig also
excited.
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Rotor response manual lift drop tests
Ps2.36 bar (ab)
15 krpm
Drop induced shocks 30 g Transient response Full
recovery within 0.1 sec.
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Rotor speed vs time (No shocks)
Dry friction (contact)
With feed pressure long time to coast down
demonstrates very low viscous drag!
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Rotor speed vs time (Manual lift-drop tests)
Overall coast down time reduces with shock loads
( 20 sec)
No shocks
Exponential decay (No rubs) even under severe
external shocks
No shocks
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Conclusions

• Under shock loads ( up to 30 g), natural
  frequency of rotor-bearing system (150-200 Hz)
  and test rig base ( 40 Hz) excited. However,
  rotor transient motions quickly die!
• For all feed pressures (2-5 bar), rotor transient
  responses from shocks restore to their before
  impact amplitude within 0.1 second. Peak instant
  amplitudes (do not exceed 50 µm)
• Even under shock impacts, viscous drag effects
  are dominant, i.e., no contact between the rotor
  and bearing.
• Hybrid bearings demonstrate reliable dynamic
  performance even with WORN PAD SURFACES

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TRC Proposal Gas Bearings for Oil-Free
Turbo-machinery Identification of Bearing Force
Coefficients from Base-Induced Excitations
TASKS

• Set up an electromagnetic shaker to deliver
  excitations (periodic loads of varying frequency)
  to the test rig.
• Measure the rotor response due to base induced
  excitations.
• Identify frequency dependent bearing stiffness
  and damping coefficients from measured rotor
  transient responses at increasing rotor speeds.
• Compare the identified bearing force coefficients
  to predictions from XLTRC2 computational models.

B
TRC
2008
/200
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UDGET FROM
FOR



Support for graduate student (20h/week) x 1,600
x 12 months,
Fringe benefits (2.5) and medical insurance
(194/month)



22,008


Tuition fees three semesters (3,996x3)
Supplies for test rig



17,992

Total Cost

40,000










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Electromagnetic shaker

• Shaker force peak amplitude (sine) 98 N (22 lbf)
• Useful frequency range 5 9000 Hz

LDS V406/8 PA 100E
Operating rotor speed range 170 Hz 1 kHz
10 krpm
60 krpm
Y
X
Low frequency excitations simulate road surface
effect on MTM
Z
Identify frequency dependent bearing force
coefficients at increasing rotor speeds