Gas Bearings for OilFree Turbomachinery 28 th Turbomachinery

Gas Bearings for Oil-Free Turbomachinery 28 th Turbomachinery Consortium Meeting Dynamic Forced Response of a Rotor-Hybrid Gas Bearing System due to Intermittent Shocks Keun Ryu Research Assistant Luis San Andrés Mast-Childs Professor Principal Investigator TRC-B&C-1 -08 2008 TRC Project GAS BEARINGS FOR OIL-FREE TURBOMACHINERY

Gas Bearings for Oil-Free Turbomachinery Micro Turbomachinery (< 0. 5 MW) ADVANTAGES • High energy density • Compact and fewer parts • Portable and easily sized • Lower pollutant emissions • Low operation cost ASME Paper No. GT 2002 -30404 http: //www. grc. nasa. gov/WWW /Oilfree/turbocharger. htm Gas bearings Gas Foil Bearing • Oil-Free bearing • High rotating speed (DN value>4 M) • Simple configuration • Lower friction and power losses • Compact size Flexure pivot Bearing AIAA-2004 -5720 -984 GT 2004 -53621

Gas Bearings for Oil-Free Turbomachinery 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 Oil-Free Turbomachinery 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 rubfree operation at start up and shut down Major issues: Little damping, Wear at start & stop, Instability (whirl & hammer), & reliability under shock operation

Gas Bearings for Oil-Free Turbomachinery Gas bearing test rig Max. operating speed: 100 kpm 3. 5 k. W (5 Hp) AC integral motor Rotor: length 190 mm, 28. 6 mm diameter, weight=0. 826 kg Rig housing Bearing shell and Load cells Bearing cover Gas bearing Shaft and DC motor Components of high-speed gas bearing test rig

Gas Bearings for Oil-Free Turbomachinery. GT 2008 -50393 Gas Bearings for MTM 2007: Control of bearing stiffness / critical speed 5. 08 bar 2. 36 bar Displacements at RB(H) 2. 36 bar Blue line: Coast down Red line: Set speed Controller activated system Peak motion at “critical speed” eliminated by controlling supply pressure into bearings

2007 -2008 Objectives Gas Bearings for Oil-Free Turbomachinery 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 Gas Bearings for Oil-Free Turbomachinery 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 Gas Bearings for Oil-Free Turbomachinery E-pusher : Push type solenoid 240 N at 1 inch stroke

Electromagnetic pusher tests Gas Bearings for Oil-Free Turbomachinery Multiple impact Impact duration ~20 ms E-force ~400 N (pk-pk)

Manual lift & drop tests Gas Bearings for Oil-Free Turbomachinery Multiple impact Lift off to 5~15 cm (10~30° rotation)

Coast down: E-pusher tests Gas Bearings for Oil-Free Turbomachinery 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

Gas Bearings for Oil-Free Turbomachinery 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)

Gas Bearings for Oil-Free Turbomachinery Waterfall: manual lift & drop tests Ps=2. 36 bar (ab) Displacements at LB(H) Rotor speed decreases Excitation of rotor natural frequency. NOT a rotordynamic instability!

Rotor response: manual lift & drop tests Gas Bearings for Oil-Free Turbomachinery Ps=2. 36 bar (ab) Shock loads applied Overall rotor amplitude increases largely. Subsynchronous amplitudes larger than synchronous

Rotor response: manual lift & drop tests Gas Bearings for Oil-Free Turbomachinery 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 Gas Bearings for Oil-Free Turbomachinery 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) Gas Bearings for Oil-Free Turbomachinery Dry friction (contact) With feed pressure: long time to coast down demonstrates very low viscous drag!

Gas Bearings for Oil-Free Turbomachinery 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 Gas Bearings for Oil-Free Turbomachinery • Under shock loads ( up to ~30 g), natural frequency of rotorbearing 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

Gas Bearings for Oil-Free Turbomachinery TRC Proposal: Gas Bearings for Oil-Free Turbo- machinery – Identification of Bearing Force TASKS Coefficients from Base-Induced Excitations • 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 XLTRC 2 computational models. BUDGET FROM TRC FOR 2008/2009: Support for graduate student (20 h/week) x $ 1, 600 x 12 months, Fringe benefits (2. 5%) and medical insurance ($194/month) Tuition & fees three semesters ($3, 996 x 3) + Supplies for test rig Total Cost: $ 22, 008 $ 17, 992 $ 40, 000

Gas Bearings for Oil-Free Turbomachinery Electromagnetic shaker LDS V 406/8 – PA 100 E §Shaker force peak amplitude (sine): 98 N (22 lbf) §Useful frequency range: 5 ~ 9000 Hz Operating rotor speed range: 170 Hz ~ 1 k. Hz 10 krpm ~ 60 krpm Y X Z Low frequency excitations: simulate road surface effect on MTM Identify frequency dependent bearing force coefficients at increasing rotor speeds