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No description available.

Digitized by Kansas Correctional Industries

This thesis presents the results of an investigation of bearing failure at a loaded hole in carbon fibre composite laminates. The investigation included finite element analysis and a program of experimental testing for quasi-isotropic laminates, uni-directional laminates and laminates with fibres steered along predefined trajectories. The investigation formed part of a research program in the Cooperative Research Centre for Advanced Composite Structures Ltd (CRC-ACS) support by the Office of Naval Research from the United States. Work in progress in the CRC includes research defined in this thesis including the addition of nano-particles to the resin to improve the bearing strength. The literature survey undertaken in this thesis identified that the micro-buckling theory proposed by Professor C.T. Sun from Purdue University, Indiana USA, is the best analysis procedure for initial bearing failure in pin-loaded laminates. This failure theory has therefore been implemented in a Fortran program using the results from a finite element analysis for the nonlinear contact problem of a pin bearing on a hole in a composite laminate. The finite element analysis is executed using the commercial finite element system MSC.MARC. The numerical predictions have been validated by a test of progressive failure with the aid of an acoustic emission monitoring system. The acoustic emission parametric and frequency analysis is performed using the acoustic emission software Vallen Systeme. Fast fourier transformation of the waveform have been achieved to distinguish and identify microfailure mode. Failure mechanisms have been verified by post-mortem microstructural examination. There are 3 failure mechanisms associated with bearing failure in carbon fibre reinforced epoxy composite, the first is fibre matrix interfacial failure, followed by fibre fracture, and subsequently matrix splitting. The application of AE technique has been applied successfully to characterise the development of failure.

Micro-turbomachinery (MTM) implements gas bearings in compact units of enhanced mechanical reliability. Gas bearings, however, have little damping and are prone to wear during frequent rotor start-up and shut down conditions. Externally pressurized gas bearings provide a simple solution to overcome excessive drag and allowing rub-free operation during transient response events. Some commercial MTM currently implements gas foil bearings, a costly proprietary technology with few, if any, proven reliable predictive design models. The thrust of this research is to investigate conventional bearings of low cost, easy to manufacture (common materials) and easy to install and align. Flexure pivot tilting pad bearings offer little or no cross-coupled stiffnesses with enhanced rotordynamic stability. These bearings, modified for hydrostatic pressurization, demonstrated superior rotordynamic performance over other simple gas bearing configurations. The test rig comprises of a rigid rotor, 0.825 kg and 28.6 mm in diameter, supported on two hybrid flexure pivot hybrid gas bearings, each with four pads and 60% pivot offset and 0.6 mm feeding holes. Experimental results show that external pressurization stiffens the gas bearings, increasing the system critical speed while reducing the modal damping. Most importantly, the tests demonstrate that external pressurization is not needed for super critical speed operation. In practice, the supply pressure could be shut off at high speeds with substantial savings in operational efficiency. In addition, controlling the feed pressure while the rotor passes through its critical speeds can eliminate high amplitude motions because of the bearings’ inherent little damping. In 2007, the test rig integrates an inexpensive automatic air pressure regulator to control the supply pressure into the gas bearings. The measured system dynamic response determines the regulator control scheme with a programmed schedule over a rotor speed region enclosing the system critical speeds. Rotor speed coast-down tests with controlled supply pressure into the bearings demonstrate the effective elimination of large rotor motion amplitudes while crossing the system critical speeds. The simple on-off supply pressure control, i.e. a sudden increase in pressure while approaching a critical speed, is the best since it changes abruptly the bearing stiffness coefficients and moves the system critical speed to a higher speed. A rotordynamic analysis, integrating bearing force coefficients predicted by an existing TRC computational model, forwards critical speeds in agreement with the test results. Predicted rotor synchronous responses for the cases with controlled supply show an excellent correlation with the measured responses. The experiments validate the predictive tools and demonstrate the controllable rotordynamic characteristics of the flexure pivot hybrid gas bearings.

Air foil bearings offer several advantages over oil-lubricated bearings in high speed micro-turbomachinery. With no contact between the rotor and bearings, the air foil bearings have higher service life and consequently lesser standstills between operations. However, the foil bearings have reliability issues that come from dry rubbing during start-up/shutdown and limited heat dissipation capability. Regardless of lubricating media, the hydrodynamic pressure generated provides only load support but no dissipation of parasitic energy generated by viscous drag and the heat conducted from other parts of the machine through the rotor. The present study is a continuation of the work on hybrid air foil bearings (HAFB) developed by Kim and Park, where they present a new concept of air foil bearing combining hydrodynamic air foil bearing with hydrostatic lift. Their experimental studies show that HAFB has superior performance compared to its hydrodynamic counterpart in load capacity and cooling performance. In this article, the bearing stiffness and damping coefficients of HAFB are calculated using a linear perturbation method developed for HAFB. The study focuses on circular HAFB with a single continuous top foil supported by bump foil. The research also includes a parametric study which outlines the dependence of the stiffness and damping coefficients on various design parameters like supply pressure ( P s ), feed parameter ( Г s ), excitation frequency (v), and bearing number (Λ). Furthermore the present research also includes experimental investigation of HAFB with bump foil as compliant structure. In the first phase of the experimental research a high speed test facility was designed and fabricated. The facility has the capability of running up to 90,000 RPM and has an electric motor drive. This article gives detailed description of this test rig and also includes data acquired during the commissioning phase of the test rig. The test rig was then used to measure the load capacity of HAFB.

Hydrodynamic flexure pivot tilting pad gas bearings (FPTPGBs) can enable successful operation of oil-free microturbomachinery. This work presents the experimental and analytical study of such bearings with different damper configurations. A test rig was constructed that could safely operate a ~28.6 mm, 0.8 kg rotor beyond 120 krpm. A time domain orbit simulation, which integrates nonlinear equations of motion for the rotor-bearing elements, was implemented as the primary analysis tool to predict rotor-bearing responses to imbalance, the presence and location of critical speeds, etc. Complementary analyses were also performed with a model that uses linear bearing impedance coefficients to predict system natural frequencies. Imbalance response testing verified that the rotor-bearing system behaved linearly in the region above the critical speed, and orbit simulations predicted the response to a calibrated imbalance with notable agreement. Viscoelastic dampers added behind the FPTPGB pads delayed the onset of subsynchronous vibrations (from 43 krpm without damper to above 50 krpm with damper) of the system with bearing clearance increased by shims. Midrange subsynchronous vibrations initiated at ~20 krpm were eventually suppressed by ~25 krpm due to the stabilizing effect of rotor centrifugal growth. The viscoelastic dampers had a negligible effect on suppressing these midrange subsynchronous vibrations in experiments, but this was not demonstrated in simulations, presumably due to much lower stiffness contribution of the damper at lower frequencies. The ideal, perfectly aligned models in the simulations were able to tolerate shims up to only 10% of nominal clearance, but the test rig exhibited surprising stability with shims as much as 200% of nominal clearance; this increase may be caused by imposed eccentricities due to misalignments in the test rig. FPTPGBs supported by compliant bump foils can have the ability to tolerate rotor misalignments and shock loading like foil gas bearings. Simulation studies on imbalance response characteristics for several bearing shell mass and support stiffness configurations present initial design guidelines for the application. Namely, results showed that FPTPGBs favored large bearing shell mass and large support stiffness, while FPTPGBs with radial compliance favored small bearing shell mass with large support stiffness.

Micro-turbomachinery (MTM) implements gas bearings in compact units of enhanced mechanical reliability. Gas bearings, however, have little damping and are prone to wear during frequent rotor start-up and shut down conditions. Externally pressurized gas bearings provide a simple solution to overcome excessive drag and allowing rub-free operation during transient response events. Some commercial MTM currently implements gas foil bearings, a costly proprietary technology with few, if any, proven reliable predictive design models. The thrust of this research is to investigate conventional bearings of low cost, easy to manufacture (common materials) and easy to install and align. Flexure pivot tilting pad bearings offer little or no cross-coupled stiffnesses with enhanced rotordynamic stability. These bearings, modified for hydrostatic pressurization, demonstrated superior rotordynamic performance over other simple gas bearing configurations. The test rig comprises of a rigid rotor, 0.825 kg and 28.6 mm in diameter, supported on two hybrid flexure pivot hybrid gas bearings, each with four pads and 60% pivot offset and 0.6 mm feeding holes. Experimental results show that external pressurization stiffens the gas bearings, increasing the system critical speed while reducing the modal damping. Most importantly, the tests demonstrate that external pressurization is not needed for super critical speed operation. In practice, the supply pressure could be shut off at high speeds with substantial savings in operational efficiency. In addition, controlling the feed pressure while the rotor passes through its critical speeds can eliminate high amplitude motions because of the bearings’ inherent little damping. In 2007, the test rig integrates an inexpensive automatic air pressure regulator to control the supply pressure into the gas bearings. The measured system dynamic response determines the regulator control scheme with a programmed schedule over a rotor speed region enclosing the system critical speeds. Rotor speed coast-down tests with controlled supply pressure into the bearings demonstrate the effective elimination of large rotor motion amplitudes while crossing the system critical speeds. The simple on-off supply pressure control, i.e. a sudden increase in pressure while approaching a critical speed, is the best since it changes abruptly the bearing stiffness coefficients and moves the system critical speed to a higher speed. A rotordynamic analysis, integrating bearing force coefficients predicted by an existing TRC computational model, forwards critical speeds in agreement with the test results. Predicted rotor synchronous responses for the cases with controlled supply show an excellent correlation with the measured responses. The experiments validate the predictive tools and demonstrate the controllable rotordynamic characteristics of the flexure pivot hybrid gas bearings.

The present work advances progress on the validation against measurements of linear and nonlinear rotordynamic models for predicting the dynamic shaft response of automotive turbochargers (TCs) supported on floating ring bearings. Waterfall spectra of measured shaft motions at the compressor and turbine ends of a test TC rotor evidences a complex response, showing synchronous (1X) and multiple subsynchronous frequencies along the entire operating speed range (maximum shaft speed ~ 65 krpm). Postprocessing of the raw test data by mathematical software allows filtering the synchronous and subsynchronous vibration components for later comparisons to predicted shaft motions. The static performance of the floating ring bearings is analyzed with in-house software (XLSFRBThermal®), which considers thermal expansion of the shaft and bearing components as well as static loading on the bearing due to lubricant feed pressure. In addition, the program calculates rotordynamic force coefficients for the inner and outer films of the floating ring bearing. The turbocharger Finite Element (FE) structural model for the linear and nonlinear analyses includes lumped masses for the compressor and turbine wheels and the thrust collar. The mass imbalance distribution on the TC rotor is estimated from the test data using a procedure derived from the two-plane balancing method with influence coefficients. The linear model yields predictions of rotor synchronous (1X) response to imbalance and damped eigenvalues. The analysis evidences that the rotor cylindrical-bending mode is unstable at all shaft speeds while the rotor conical model becomes more unstable as lubricant feed pressure decreases. The predicted synchronous (1X) motions agree well with the test data, showing a critical speed at approximately 20 krpm. The linear stability results indicate the existence of three critical speeds occurring at 4, 20 and 50 krpm. The second critical speed corresponds to the rotor cylindrical-bending mode, showing larger amplitudes of motion at the compressor nose than at the turbine end. The third critical speed associated to the rotor first bending modes is well damped. In the nonlinear transient analysis, the nonlinear equations of motion of the system (rotor-FRB) are integrated, and the bearing reaction forces are calculated at each time step in a numerical integration procedure. The model then yields predictions of total motion which is decomposed into synchronous (1X) and subsynchronous motions, amplitudes and frequencies. The nonlinear analysis predicts synchronous (1X) amplitudes that correlate well with the test data at high shaft speeds (> 30 krpm) but underpredicts the imbalance response at low shaft speeds (<30 krpm). The time transient simulations predict multiple frequency subsynchronous motions for shaft speeds ranging from 10 krpm to 55 krpm, with amplitudes and frequencies that are in good agreement with the measurements. Finally, the shaft motion measurements and predictions demonstrate that the turbocharger dynamic response does not depend greatly on the lubricant feed pressure and inlet temperature for the conditions tested.