1. Mast-Childs Professor Principal Investigator
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-B&C-1-08
2008 TRC Project
GAS BEARINGS FOR OIL-FREE TURBOMACHINERY

2. Gas bearings Micro Turbomachinery (< 0.5 MW) ADVANTAGES
ASME Paper No. GT
ADVANTAGES
High energy density
Compact and fewer parts
Portable and easily sized
Lower pollutant emissions
Low operation cost
Oil-Free bearing
High rotating speed (DN value>4M)
Simple configuration
Lower friction and power losses
Compact size
Gas bearings
AIAA
Gas Foil Bearing GT
Flexure pivot Bearing

3. Gas Bearings for MTM
 Gas bearings for micro turbomachinery (< 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

4. 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

5. 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, weight=0.826 kg
Rig housing
Bearing shell and
Load cells
Gas bearing
Bearing cover
Shaft and DC motor
Components of high-speed gas bearing test rig

6. 2007: Control of bearing stiffness / critical speed
Gas Bearings for MTM GT
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

7. 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.

8. TEST gas bearings TEST gas Bearings worn pads surfaces
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 stiffness=20 Nm/rad

9. 2008 Gas Bearing test rig layout
E-pusher
: Push type solenoid
240 N
at 1 inch stroke

10. Electromagnetic pusher tests
Multiple impact
Impact duration ~20 ms
E-force ~400 N (pk-pk)

11. Manual lift & drop tests
Multiple impact
Lift off to 5~15 cm (10~30° rotation)

12. Coast down: E-pusher tests
Ps=5.08 bar (ab)
Displacements at LB(H)
Intermittent shocks
Impact force 100~400 N
46 krpm
Shock ~15 g
Transient rotor response ~ 40 µm

13. Coast down: manual lift & drop tests
Shock induced acceleration
At base 5~20 g
At housing 5~10 g
Ps=3.72 bar (ab)
Beyond critical speed: Synchronous frequency is isolated from shocks
Below 20 krpm:
Large fluctuation of synchronous response
Displacements at LB(H)

14. Waterfall: manual lift & drop tests
Displacements at LB(H)
Ps=2.36 bar (ab)
Rotor speed
decreases
Excitation of rotor natural frequency. NOT a rotordynamic instability!

15. Rotor response: manual lift & drop tests
Ps=2.36 bar (ab)
Shock loads applied
Shock loads applied
Overall rotor amplitude increases largely. Subsynchronous amplitudes larger than synchronous

16. Rotor response: manual lift & drop tests
Ps=2.36 bar (ab)
Natural frequency of rotor-bearing system (150~190 Hz)
Natural frequency of test rig (~40 Hz)
Rotor-bearing natural frequency increases with rotor speed. Natural frequency of test rig also excited.

17. Rotor response: manual lift & drop tests
Ps=2.36 bar (ab)
15 krpm
Drop induced shocks ~30 g
Transient response
Full recovery within ~ 0.1 sec.

18. Rotor speed vs time (No shocks)
Dry friction
(contact)
With feed pressure: long time to coast down demonstrates very low viscous drag!

19. 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

20. Conclusions
Under shock loads ( up to ~30 g), natural frequency of rotor-bearing system ( 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

21. 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
UDGET FROM
TRC
FOR
2008
/200 9 :
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

22. 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