GT Flexure Pivot Hybrid Gas Bearings Keun Ryu Research Assistant ASME TURBO EXPO 2009, Orlando, Fla Dr. Luis San Andrés Mast-Childs Professor Fellow ASME TURBOMACHINERY LABORATORY TEXAS A&M UNIVERSITY Supported by TAMU Turbomachinery Research Consortium ASME paper GT Dynamic Forced Response of a Rotor-Hybrid Gas Bearing System due to Intermittent Shocks

GT Flexure Pivot Hybrid Gas Bearings Micro Turbomachinery (< 0.5 MW) High energy density Compact and fewer parts Portable and easily sized Lower pollutant emissions Low operation cost ADVANTAGES 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 ASME Paper No. GT /Oilfree/turbocharger.htm

GT Flexure Pivot Hybrid Gas Bearings Ideal 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

GT Flexure Pivot Hybrid Gas Bearings Thrust of research 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

GT Flexure Pivot Hybrid Gas Bearings 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 Components of high-speed gas bearing test rig Rig housing Bearing shell and Load cells Gas bearing Bearing cover Shaft and DC motor Test rig

GT Flexure Pivot Hybrid Gas Bearings Test rig Positioning Bolt LOP Rotor/motorBearingSensorsLoad cell Air supply Thrust pin

GT Flexure Pivot Hybrid Gas Bearings Promote stability: no cross-coupled stiffnesses Eliminate pivot wear, contact stresses, pad flutter Minimize manufacturing and assembly tolerances’ stack-up Flexure pivot tilting pad hybrid bearing Clearances Cp =38 & 45  m, Preload =7 & 5  m (~20%) Web rotational stiffness=20 Nm/rad worn pads surfaces

GT Flexure Pivot Hybrid Gas Bearings Zhu & San Andres (2004) GT Gas bearing for oil-free applications. Good comparisons with: TAMU work on flexure pivot tilting bearings Delgado & San Andres (2004) Computational model for hydrodynamic operation, with application to hybrid brush seals San Andres (2006) Computational model for hybrid operation validated by Zhu (2004) measurements. Code used by 20+ companies Stable to 99 krpm 60 KRPM GT GT Journal of Tribology, 129 San Andres & Ryu (2007) Operation with worn clearances and LOP/LBP configuration J. Eng. Gas Turbines and Power, 2008, 130

GT Flexure Pivot Hybrid Gas Bearings 2008: Control of bearing stiffness / critical speed Peak motion at “critical speed” eliminated by controlling supply pressure into bearings Controller activated system Displacements at RB(H) 5.08 bar 2.36 bar Blue line: Coast down Red line: Set speed 2.36 bar 5.08 bar J. Eng. Gas Turbines and Power, 2008, v. 130

GT Flexure Pivot Hybrid Gas Bearings 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.

GT Flexure Pivot Hybrid Gas Bearings E-pusher : Push type solenoid 240 N at 1 inch stroke 2008 Gas bearing test rig layout

GT Flexure Pivot Hybrid Gas Bearings Electromagnetic pusher tests Impact duration ~20 ms E-force ~400 N (pk-pk) Multiple impact

GT Flexure Pivot Hybrid Gas Bearings Manual lift & drop tests Multiple impact Lift off to 5~15 cm (10~30° rotation)

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

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

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

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

GT Flexure Pivot Hybrid Gas Bearings 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)

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

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

GT Flexure Pivot Hybrid Gas Bearings Overall coast down time reduces with shock loads (~ 20 sec) Exponential decay (No rubs) even under severe external shocks Rotor speed vs time ( manual lift-drop tests ) No shocks

GT Flexure Pivot Hybrid Gas Bearings 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 Conclusions:

GT Flexure Pivot Hybrid Gas Bearings Dominant challenges in gas bearing technology Current research focuses on coatings (materials), rotordynamics (stability) & high temperature (thermal management ) –Bearing design & manufacturing process better known. Load capacity needs minute clearances since gas viscosity is low. –Damping & rotor stability are crucial –Inexpensive coatings to reduce drag and wear at low speeds and transient rubs at high speeds –Engineered thermal management to extend operating envelope to high temperatures Need Low Cost & Long Life Solution!

GT Flexure Pivot Hybrid Gas Bearings 2009 Gas bearing test rig layout connecting rod pushes base plate!

GT Flexure Pivot Hybrid Gas Bearings Rotor speed coast down tests Ps = 2.36 bar (ab) Subsynchronous response: 1)24 Hz (Harmonic of 12 Hz) 2)Natural frequency 193 Hz Shaker input frequency: 12Hz Synchronous Dominant! excitation of system natural frequency is NOT an instability!