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Measurements of Drag Torque and Lift Off Speed and Identification of Stiffness and Damping in a Metal Mesh Foil BearingChirathadam, Thomas A. 2009 December 1900 (has links)
Metal mesh foil bearings (MMFBs) are a promising low cost gas bearing technology
for support of high speed oil-free microturbomachinery. Elimination of complex oil
lubrication and sealing system by installing MMFBs in oil free rotating machinery offer
distinctive advantages such as reduced system overall weight, enhanced reliability at
high rotational speeds and extreme temperatures, and extended maintenance intervals
compared to conventional turbo machines. MMFBs for oil-free turbomachinery must
demonstrate adequate load capacity, reliable rotordynamic performance, and low
frictional losses in a high temperature environment.
The thesis presents the measurements of MMFB break-away torque, rotor lift off and
touchdown speeds, temperature at increasing static load conditions, and identified
stiffness and equivalent viscous damping coefficients. The experiments, conducted in a
test rig driven by an automotive turbocharger turbine, demonstrate the airborne operation
(hydrodynamic gas film) of the floating test MMFB with little frictional loses at
increasing loads. The measured drag torque peaks when the rotor starts and stops, and
drops significantly once the bearing is airborne. The estimated rotor speed for lift-off
increases linearly with increasing applied loads. During continuous operation, the
MMFB temperature measured at one end of the back surface of the top foil increases
both with rotor speed and static load. Nonetheless, the temperature rise is only nominal
ensuring reliable bearing performance. Application of a sacrificial layer of solid
lubricant on the top foil surface aids to reduce the rotor break-away torque. The
measurements give confidence on this simple bearing technology for ready application
into oil-free turbomachinery.
Impact loads delivered (with a soft tip) to the test bearing, while resting on the
(stationary) drive shaft, evidence a system with large damping and a structural stiffness
that increases with frequency (max. 200 Hz). The system equivalent viscous damping
ratio decreases from ~ 0.7 to 0.2 as the frequency increases. In general, the viscous
damping in a metal mesh structure is of structural type and inversely proportional to the
frequency and amplitude of bearing motion relative to the shaft. Impact load tests,
conducted while the shaft rotates at 50 krpm, show that the bearing direct stiffness is
lower (~25% at 200 Hz) than the bearing structural stiffness identified from impact load
tests without shaft rotation. However, the identified equivalent viscous damping
coefficients from tests with and without shaft rotation are nearly identical.
The orbits of bearing motion relative to the rotating shaft show subsynchronous
motion amplitudes and also backward synchronous whirl. The subsynchronous vibration
amplitudes are locked at a frequency, nearly identical to a rotor natural frequency. A
backward synchronous whirl occurs while the rotor speed is between any two natural
frequencies, arising due to bearing stiffness asymmetry.
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Metal Mesh Foil Bearings: Prediction and Measurement for Static and Dynamic Performance CharacteristicsChirathadam, Thomas 14 March 2013 (has links)
Gas bearings in oil-free micro-turbomachinery for process gas applications and for power generation (< 400 kW) must offer adequate load capacity and thermal stability, reliable rotordynamic performance at high speeds and temperatures, low power losses and minimal maintenance costs. The metal mesh foil bearing (MMFB) is a promising foil bearing technology offering inexpensive manufacturing cost, large inherent material energy dissipation mechanism, and custom-tailored stiffness and damping properties. This dissertation presents predictions and measurements of the dynamic forced performance of various high speed and high temperature MMFBs.
MMFB forced performance depends mainly on its elastic support structure, consisting of arcuate metal mesh pads and a smooth top foil. The analysis models the top foil as a 2D finite element (FE) shell supported uniformly by a metal mesh under-layer. The solution of the structural FE model coupled with a gas film model, governed by the Reynolds equation, delivers the pressure distribution over the top foil and thus the load reaction. A perturbation analysis further renders the dynamic stiffness and damping coefficients for the bearing. The static and dynamic performance predictions are validated against limited published experimental data.
A one-to-one comparison of the static and dynamic forced performance characteristics of a MMFB against a Generation I bump foil bearing (BFB) of similar size, with a slenderness ratio L/D=1.04, showcases the comparative performance of MMFB against a commercially available gas foil bearing design. The measurements of rotor lift-off speed and drag friction at start-up and airborne conditions are conducted for rotor speeds up to 70 krpm and under identical specific loads (W/LD =0.06 to 0.26 bar). The dynamic force coefficients of the bearings are estimated, in a ‘floating bearing’ type test rig, while floating atop a journal spinning to speeds as high as 50 krpm and with controlled static loads (22 N) applied in the vertical direction. The parameter identification is conducted in the frequency range of 200-400 Hz first, and then up to 600 Hz using higher load capacity shakers.
A finite element rotordynamic program (XLTRC2) models a hollow rotor and two MMFBs supporting it and predict the synchronous rotor response for known imbalances. The predictions agree well with the ambient temperature rotor response measurements. Extensive rotor response measurements and rotor and bearing temperature measurements, with a coil heater warming up to 200 ºC and placed inside the hollow rotor, reveal the importance of adequate thermal management.
The database of high speed high temperature performance measurements and the development of a predictive tool will aid in the design and deployment of MMFBs in commercial high-speed turbomachinery. The work presented in the dissertation is a cornerstone for future analytical developments and further testing of practical MMFBs.
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