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

Gas bearings Micro Turbomachinery (< 0.5 MW) ADVANTAGES ASME Paper No. GT2002-30404 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.htm Oil-Free bearing High rotating speed (DN value>4M) Simple configuration Lower friction and power losses Compact size Gas bearings AIAA-2004-5720-984 Gas Foil Bearing GT 2004-53621 Flexure pivot Bearing

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

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

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

2007: Control of bearing stiffness / critical speed 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

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.

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

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

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

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

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

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)

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!

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

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.

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.

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

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

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

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

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