Thomas Abraham Chirathadam

Measurement of Structural Stiffness and Damping Coefficients in a Metal Mesh Foil Bearing

Engineered Metal Mesh Foil Bearings (MMFB) are a promising low cost bearing technology for oil-free microturbomachinery. In a MMFB, a ring shaped metal mesh (MM) provides a soft elastic support to a smooth arcuate foil wrapped around a rotating shaft. The paper details the construction of a MMFB and the static and dynamic load tests conducted on the bearing for estimation of its structural stiffness and equivalent viscous damping. The 28.00 mm diameter, 28.05 mm long bearing, with a metal mesh ring made of 0.3 mm Copper wire and compactness of 20%, is installed on a test shaft with a slight preload. Static load versus bearing deflection measurements display a cubic nonlinearity with large hysteresis. The bearing deflection varies linearly during loading, but nonlinearly during the unloading process. An electromagnetic shaker applies on the test bearing loads of controlled amplitude over a frequency range. In the frequency domain, the ratio of applied force to bearing deflection gives the bearing mechanical impedance, whose real part and imaginary part give the structural stiffness and damping coefficients, respectively. As with prior art published in the literature, the bearing stiffness decreases significantly with the amplitude of motion and shows a gradual increasing trend with frequency. The bearing equivalent viscous damping is inversely proportional to the excitation frequency and motion amplitude. Hence, it is best to describe the mechanical energy dissipation characteristics of the MMFB with a structural loss factor (material damping). The experimental results show a loss factor as high as 0.7 though dependent on the amplitude of motion. Empirically based formulas, originally developed for metal mesh rings, predict bearing structural stiffness and damping coefficients agreeing well with the experimentally estimated parameters. Note, however, that the metal mesh ring, after continuous operation and various dismantling and reassembly processes, showed significant creep or sag that resulted in a gradual decrease of its structural force coefficients.Copyright

Measurements of Drag Torque, Lift-Off Journal Speed, and Temperature in a Metal Mesh Foil Bearing

Metal mesh foil bearings (MMFBs) are a promising low cost gas bearing technology for high performance oil-free microturbomachinery. Elimination of complex oil lubrication and sealing system by deploying MMFBs in rotorcraft gas turbine engines offers distinctive advantages such as reduced system weight, enhanced reliability at high rotational speeds and extreme temperatures, and extended maintenance intervals compared with mineral oil lubricated bearings. MMFBs for oil-free rotorcraft engines must demonstrate adequate load capacity, reliable rotordynamic performance, and low frictional losses in a high temperature environment. The paper presents the measurements of MMFB break-away torque, rotor lift-off and touchdown speeds, and temperature at increasing static load conditions. The tests, which were 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 lifts off. The estimated rotor speed for lift-off increases linearly with the applied static load. During continuous operation, the MMFB temperature measured at the back surface of the top foil increases both with rotor speed and static load. Nonetheless, the temperature rise is mild, demonstrating reliable performance. Application of a sacrificial layer of solid lubricant on the top foil surface reduces the rotor break-away torque. The measurements give confidence on this simple bearing technology for ready application into oil-free turbomachinery.

Metal Mesh Foil Bearing: Effect of Motion Amplitude, Rotor Speed, Static Load, and Excitation Frequency on Force Coefficients

Metal mesh foil bearings (MMFBs), simple to construct and inexpensive, are a promising bearing technology for oil-free microturbomachinery operating at high speed and high temperature. Prior research demonstrated the near friction-free operation of a MMFB operating to 60 krpm and showing substantial mechanical energy dissipation characteristics. This paper details further experimental work and reports MMFB rotordynamic force coefficients. The test rig comprises a turbocharger driven shaft and overhung journal onto which a MMFB is installed. A soft elastic support structure akin to a squirrel cage holds the bearing, aiding to its accurate positioning relative to the journal. Two orthogonally positioned shakers excite the test element via stingers. The test bearing comprises a cartridge holding a Copper wire mesh ring, 2.7 mm thick, and a top arcuate foil. The bearing length and inner diameter are 38 mm and 36.5 mm, respectively. Experiments were conducted with no rotation and with journal spinning at 40–50 krpm, with static loads of 22 N and 36 N acting on the bearing. Dynamic load tests spanning frequencies from 150 to 450 Hz were conducted while keeping the amplitude of bearing displacements at 20 µm, 25 µm, and 30 µm. With no journal spinning, the force coefficients represent the bearing elastic structure alone because the journal and bearing are in contact. The direct stiffnesses gradually increase with frequency while the direct damping coefficients drop quickly at low frequencies (< 200 Hz) and level off above this frequency. The damping combines both viscous and material types from the gas film and mesh structure. Journal rotation induces airborne operation with a hydrodynamic gas film separating the rotor from its bearing. Hence, cross-coupled stiffness coefficients appear although with magnitudes lower than those of the direct stiffnesses. The direct stiffnesses, 0.4 to 0.6 MN/m within 200–400 Hz, are slightly lower in magnitude as those obtained without journal rotation, suggesting the air film stiffness is quite high. Bearing direct stiffnesses are inversely proportional to the bearing motion amplitudes, whereas the direct equivalent viscous damping coefficients do not show any noticeable variation. All measurements evidence a test bearing system with material loss factor (γ) ∼ 1.0, indicating significant mechanical energy dissipation ability.

Identification of Rotordynamic Force Coefficients of a Metal Mesh Foil Bearing Using Impact Load Excitations

Metal mesh foil bearings (MMFBs) are inexpensive compliant gas bearing type that aim to enable high speed, high temperature operation of small turbomachinery. A MMFB with an inner diameter of 28.00 mm and length of 28.05 mm is constructed with low cost and common materials. The bearing incorporates a copper mesh ring, 20% in compactness, and offering large material damping beneath a 0.127 mm thick preformed top foil. Prior experimentations (published papers) provide the bearing structure force coefficients and the break away torque for bearing lift off. Presently, the MMFB replaces a compressor in a small turbocharger driven test rig. Impact load tests aid to identify the direct and cross-coupled rotor dynamic force coefficients of the floating MMFB while operating at a speed of 50 krpm. Tests conducted with and without shaft rotation show the MMFB direct stiffness is less than its structural (static) stiffness, ∼25% lower at an excitation frequency of 200 Hz. The thin air film acting in series with the metal mesh support and separating the rotating shaft and the bearing inner surface while airborne reduces the bearing stiffness. The equivalent viscous damping is nearly identical with and without shaft rotation. The identified loss factor, best representing the hysteretic type damping from the metal mesh, is high at ∼0.50 in the frequency range 0–200 Hz. This magnitude reveals large mechanical energy dissipation ability from the MMFB. The measurements also show appreciable cross directional motions from the unidirectional impact loads, thus generating appreciable cross-coupled force coefficients. Rotor speed coast down measurements reveal pronounced subsynchronous whirl motion amplitudes locked at distinct frequencies. The MMFB stiffness hardening nonlinearity produces the rich frequency forced response. The synchronous as well as subsynchronous motions peak while the shaft traverses its critical speeds. The measurements establish reliable operation of the test MMFB while airborne.

Performance Characteristics of Metal Mesh Foil Bearings: Predictions Versus Measurements

Proven low-cost gas bearing technologies are sought to enable more compact rotating machinery products with extended maintenance intervals. The paper presents an analysis for predicting the static and dynamic forced performance characteristics of metal mesh foil bearings (MMFBs) which comprise of a top foil supported on a layer of metal mesh of certain compactness. The analysis couples a finite element model of the top foil and underspring support with the gas film Reynolds equation. Comparison of predictions against laboratory measurements with two bearings aims to validate the analysis. The predicted drag friction factor in one bearing (L = D = 28.00 mm) during full film operation is just f ∼ 0.03 at ∼ 50 krpm, agreeing well with measurements at increasing applied loads. The predictions further elucidate the effect of the applied load and rotor speed on the bearing minimum film thickness, journal eccentricity and attitude angle. For a second bearing (L = 38.0 mm, D = 36.5 mm), predicted bearing force coefficients show magnitudes comparable with the measurements, with less than 20% difference, in the 250–350 Hz excitation frequency range. While the predicted direct stiffness coefficients are rather constant, the experimental force coefficients increase with frequency (max. 400 Hz), due mainly to the increasing amplitudes of dynamic force applied to excite the bearing with a set amplitude of motion. The analysis under predicts the direct damping coefficients at high frequencies (>300 Hz). The cross-coupled stiffness and damping coefficients are typically lower (< 40%) than the direct ones. The bearings operated stable at all speeds without any sub synchronous whirl. The reasonable agreement of the predictions with the available test data promote the better design and further development of MMFB supported rotating machinery.Copyright

Gas Bearing Technology for Oil-Free Microturbomachinery: Research Experience for Undergraduate (REU) Program at Texas A&M University

The education of undergraduate mechanical engineering students (UGME) in state of the art technology development for microturbomachinery (MTM) is of importance to ensure the availability of qualified labor satisfying MTM manufacturer and end used needs, as well as to encourage the students towards pursing advanced degrees in science and engineering. National Science Foundation (NSF) funds a three-year summer Research Experience for Undergraduates (REU) Program (#0552885) to conduct hands-on training and research in mechanical, manufacturing, industrial, or materials engineering topics related to technological advances in microturbomachinery. The paper details the progress in research achieved by four UG students during 10 weeks in the summer of 2008. The students, assisted by seasoned graduate students and expert faculty, conducted work in aspects of gas bearing technology from manufacturing bearing components, to conducting rotordynamic performance tests, and to predicting rotordynamics performance. During the program, the students attended to a number of technical seminars including vocational and counseling presentations and preparation for admission to graduate school. The paper showcases the students’ technical posters produced upon completion of their 10 week research program: (1) Precision Tooling for Manufacturing an Underspring of a Generation II Foil Bearing, (2) Measurements of Imbalance Response of a Rotor Supported on Gas Foil Bearings, (3) Predictions of Nonlinear Rotordynamics of Rotor-Foil Bearing Systems, and (4) Measurements of Rotor Lift-Off and Break Up Torque in a Metal Mesh Foil Bearing. NASA GRC and Honeywell Turbocharging Technologies also provided support that enabled the success of the NSF REU program. URL http://reumicro.tamu.edu provides full descriptions on the program, topics of study, faculty involved and participating students.Copyright

Performance Characteristics of Metal Mesh Foil Bearings: Predictions vs. Measurements

Proven low-cost gas bearing technologies are sought to enable more compact rotating machinery products with extended maintenance intervals. The paper presents an analysis for predicting the static and dynamic forced performance characteristics of metal mesh foil bearings (MMFBs) which comprise of a top foil supported on a layer of metal mesh of certain compactness. The analysis couples a finite element model of the top foil and underspring support with the gas film Reynolds equation. Comparison of predictions against laboratory measurements with two bearings aims to validate the analysis. The predicted drag friction factor in one bearing (L = D = 28.00 mm) during full film operation is just f ∼ 0.03 at ∼ 50 krpm, agreeing well with measurements at increasing applied loads. The predictions further elucidate the effect of the applied load and rotor speed on the bearing minimum film thickness, journal eccentricity and attitude angle. For a second bearing (L = 38.0 mm, D = 36.5 mm), predicted bearing force coefficients show magnitudes comparable with the measurements, with less than 20% difference, in the 250–350 Hz excitation frequency range. While the predicted direct stiffness coefficients are rather constant, the experimental force coefficients increase with frequency (max. 400 Hz), due mainly to the increasing amplitudes of dynamic force applied to excite the bearing with a set amplitude of motion. The analysis under predicts the direct damping coefficients at high frequencies (>300 Hz). The cross-coupled stiffness and damping coefficients are typically lower (< 40%) than the direct ones. The bearings operated stable at all speeds without any sub synchronous whirl. The reasonable agreement of the predictions with the available test data promote the better design and further development of MMFB supported rotating machinery.Copyright

Measurements of Rotordynamic Response and Temperatures in a Rotor Supported on Metal Mesh Foil Bearings

Gas bearings in power generation microturbomachinery (MTM) and for automotive turbocharger applications must demonstrate adequate thermal management without performance degradation while operating in a harsh environment. The paper presents rotor surface temperatures and rotordynamic measurements of a rigid rotor supported on a pair of metal mesh foil bearings (MMFBs) (L = 38.1 mm, D = 36.6 mm). In the tests, to a maximum rotor speed of 50 krpm, an electric cartridge heats the hollow rotor over several hours while a steady inlet air flow rate at ∼ 160 L/min cools the bearings. In the tests with the heater set to a high temperature (max. 200 °C), the rotor and bearing OD temperatures increase by 70°C and 25°C, respectively. Most rotor dynamic responses do not show a marked difference for operation under cold (ambient temperature) or hot rotor conditions. A linear rotordynamics structural model with predicted MMFB force coefficients delivers rotor response amplitudes in agreement with the measured ones for operation with the rotor at ambient temperature. There are marked differences in the peak amplitudes when the rotor crosses its (rigid body) critical speeds. The test bearings provide lesser damping than predictions otherwise indicate. Waterfalls of rotor motion show no sub synchronous whirl frequency motions; the rotor-bearing system being stable for all operating conditions. The measurements demonstrate that MMFBs can survive operation with severe thermal gradients, radial and axial, and with little rotordynamic performance changes when the rotor is either cold or hot. The experimental results, accompanied by acceptable predictions of the bearings dynamic forced performance, promote further MMFBs as an inexpensive reliable technology for MTM.© 2013 ASME