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At least 289 records · Page 16

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NASA NTRS1992

Optimum design of a gearbox for low vibration

A computer program was developed for designing a low vibration gearbox. The code is based on a finite element shell analysis, a modal analysis, and a structural optimization method. In the finite element analysis, a triangular shell element with 18 degrees-of-freedom is used. In the optimization method, the overall vibration energy of the gearbox is used as the objective function and is minimized at the exciting frequency by varying the finite element thickness. Modal analysis is used to derive the sensitivity of the vibration energy with respect to the design variable. The sensitivity is representative of both eigenvalues and eigenvectors. The optimum value is computed by the gradient projection method and a unidimensional search procedure under the constraint condition of constant weight. The computer code is applied to a design problem derived from an experimental gearbox in use at the NASA Lewis Research Center. The top plate and two side plates of the gearbox are redesigned and the contribution of each surface to the total vibration is determined. Results show that optimization of the top plate alone is effective in reducing total gearbox vibration.

Inoue, Katsumi↗

NASA NTRS1992

Modal simulation of gearbox vibration with experimental correlation

A newly developed global dynamic model was used to simulate the dynamics of a gear noise rig at NASA Lewis Research Center. Experimental results from the test rig were used to verify the analytical model. In this global dynamic model, the number of degrees of freedom of the system are reduced by transforming the system equations of motion into modal coordinates. The vibration of the individual gear-shaft system are coupled through the gear mesh forces. A three-dimensional, axial-lateral coupled, bearing model was used to couple the casing structural vibration to the gear-rotor dynamics. The coupled system of modal equations is solved to predict the resulting vibration at several locations on the test rig. Experimental vibration data was compared to the predictions of the global dynamic model. There is excellent agreement between the vibration results from analysis and experiment.

Choy, Fred K.↗

NASA NTRS1991

Directional solidification of Pb-Sn eutectic with vibration

Pb-Sn eutectic alloy was directionally solidified at 1.4 to 3.2 cm/h with forced convection induced by axial vibration of the growth ampoule with a frequency of 10 to 40 Hz and an amplitude of 0.5 to 1.0 mm. To determine the exact growth rate, an interface demarcation technique was applied. The lamellar spacing was increased 10 to 40 percent in ingots solidified with vibration, compared to those solidified without vibration. The number of grain boundaries was increased by vibration. The average intensity of convection in the melt under axial vibration of the ampoule was estimated by comparing the experimental results with a theoretical model.

Caram, Rubens↗

NASA NTRS1990

A study of coupled rotor-fuselage vibration with higher harmonic control using a symbolic computing facility

A fundamental study of vibration prediction and vibration reduction in helicopters using active controls was performed. The nonlinear equations of motion for a coupled rotor/flexible fuselage system have been derived using computer algebra on a special purpose symbolic computing facility. The details of the derivation using the MACSYMA program are described. The trim state and vibratory response of the helicopter are obtained in a single pass by applying the harmonic balance technique and simultaneously satisfying the trim and the vibratory response of the helicopter for all rotor and fuselage degrees of freedom. The influence of the fuselage flexibility on the vibratory response is studied. It is shown that the conventional single frequency higher harmonic control (HHC) capable of reducing either the hub loads or only the fuselage vibrations but not both simultaneously. It is demonstrated that for simultaneous reduction of hub shears and fuselage vibrations a new scheme called multiple higher harmonic control (MHHC) is required. The fundamental aspects of this scheme and its uniqueness are described in detail, providing new insight on vibration reduction in helicopters using HHC.

Papavassiliou, I.↗

NASA NTRS1992

Modal simulation of gearbox vibration with experimental correlation

A newly developed global dynamic model was used to simulate the dynamics of a gear noise rig at NASA Lewis Research Center. Experimental results from the test rig were used to verify the analytical model. In this global dynamic model, the number of degrees of freedom of the system are reduced by transforming the system equations of motion into modal coordinates. The vibration of the individual gear-shaft system are coupled through the gear mesh forces. A three-dimensional, axial-lateral coupled, bearing model was used to couple the casing structural vibration to the gear-rotor dynamics. The coupled system of modal equations is solved to predict the resulting vibration at several locations on the test rig. Experimental vibration data was compared to the predicitions of the global dynamic model. There is excellent agreement between the vibration results from analysis and experiment.

Choy, Fred K.↗

NASA NTRS1993

Evaluation of a vibration diagnostic system for the detection of spur gear pitting failures

A vibration diagnostic system was used to detect spur gear surface pitting fatigue in a closed-loop spur gear fatigue test rig. The diagnostic system, comprising a personal computer with an analog-to-digital conversion board, a diagnostic system unit, and software, uses time-synchronous averaging of the vibration signal to produce a vibration image of each tooth on any gear in a transmission. Several parameters were analyzed including gear pair stress wave and raw baseband vibration, kurtosis, peak ratios, and others. The system provides limits for the various parameters and gives a warning when the limits are exceeded. Several spur gear tests were conducted with this system and vibration data analyzed at 5-min. intervals. The results presented herein show that the system is fairly effective at detecting spur gear tooth surface fatigue pitting failures.

Townsend, Dennis P.↗

NASA NTRS1993

Vibration and noise analysis of a gear transmission system

This paper presents a comprehensive procedure to predict both the vibration and noise generated by a gear transmission system under normal operating conditions. The gearbox vibrations were obtained from both numerical simulation and experimental studies using a gear noise test rig. In addition, the noise generated by the gearbox vibrations was recorded during the experimental testing. A numerical method was used to develop linear relationships between the gearbox vibration and the generated noise. The hypercoherence function is introduced to correlate the nonlinear relationship between the fundamental noise frequency and its harmonics. A numerical procedure was developed using both the linear and nonlinear relationships generated from the experimental data to predict noise resulting from the gearbox vibrations. The application of this methodology is demonstrated by comparing the numerical and experimental results from the gear noise test rig.

Choy, F. K.↗

NASA NTRS1993

Raman scattering measurements of vibrational and rotational distributions in expanding nitrogen

Vibrational and rotational population distributions in expanding nitrogen in the supersonic nozzle of a reflected shock tunnel were investigated using spontaneous Raman scattering spectroscopy. The experimental data are compared with two multitemperature nonequilibrium calculations, one based on the Landau-Teller model for vibrational relaxation, the other on the solution of the vibrational master equations. Good agreement was found between both the Landau-Teller and the master equation solutions for the vibrational energy mode, but the experimentally inferred rotational temperatures were found to be systematically low, due to collisional narrowing. The dramatic increase in vibrational relaxation rates for a cooling flowfield reported in most previous studies was not reproduced by the present experiments.

Gillespie, Walter D.↗

NASA NTRS1991

The NASA/industry design analysis methods for vibrations (DAMVIBS) program - Accomplishments and contributions

A NASA Langley-sponsored rotorcraft structural dynamics program, known as Design Analysis Methods for VIBrationS (DAMVIBS), has been under development since 1984. The objective of this program was to establish the technology base needed by the industry to develop an advanced finite-element-based dynamics design analysis capability for vibrations. Under the program, teams from the four major helicopter manufacturers have formed finite-element models, conducted ground vibration tests, made test/analysis comparisons of both metal and composite airframes, performed 'difficult components' studies on airframes to identify components which need more complete finite-element representation for improved correlation, and evaluated industry codes for computing coupled rotor-airframe vibrations. Studies aimed at establishing the role that structural optimization can play in airframe vibrations design work have also been initiated. Five government/industry meetings were held in connection with these activities during the course of the program. Because the DAMVIBS Program is coming to an end, the fifth meeting included a brief assessment of the program and its benefits to the industry.

Kvaternik, Raymond G.↗

NASA NTRS1992

Two-mode elliptical-core weighted fiber sensors for vibration analysis

Two-mode, elliptical-core optical fibers are demonstrated in weighted, distributed and selective vibration-mode-filtering applications. We show how appropriate placement of optical fibers on a vibrating structure can lead to vibration mode filtering. Selective vibration-mode suppression on the order of 10 dB has been obtained using tapered two-mode, circular-core fibers with tapering functions that match the second derivatives of the modes of vibration to be enhanced. We also demonstrate the use of chirped, two-mode gratings in fibers as spatial modal sensors that are equivalents of shaped piezoelectric sensors.

Vengsarkar, Ashish M.↗

NASA NTRS1993

Vibration and noise analysis of a gear transmission system

Choy, F. K.↗

NASA NTRS1993

Evaluation of a vibration diagnostic system for the detection of spur gear pitting failures

Townsend, Dennis P.↗

NASA NTRS1992

A global ab initio potential for HCN/HNC, exact vibrational energies, and comparison to experiment

An ab initio (i.e., from first principles) calculation of vibrational energies of HCN and HNC is reported. The vibrational calculations were done with a new potential derived from a fit to 1124 ab initio electronic energies which were calculated using the highly accurate CCSD(T) coupled-cluster method in conjunction with a large atomic natural orbital basis set. The properties of this potential are presented, and the vibrational calculations are compared to experiment for 54 vibrational transitions, 39 of which are for zero total angular momentum, J = 0, and 15 of which are for J = 1. The level of agreement with experiment is unprecedented for a triatomic with two nonhydrogen atoms, and demonstrates the capability of the latest computational methods to give reliable predictions on a strongly bound triatomic molecule at very high levels of vibrational excitation.

Bentley, Joseph A.↗

NASA NTRS1994

Vibration reduction in helicopter rotors using an actively controlled partial span trailing edge flap located on the blade

This report describes an analytical study of vibration reduction in a four-bladed helicopter rotor using an actively controlled, partial span, trailing edge flap located on the blade. The vibration reduction produced by the actively controlled flap (ACF) is compared with that obtained using individual blade control (IBC), in which the entire blade is oscillated in pitch. For both cases a deterministic feedback controller is implemented to reduce the 4/rev hub loads. For all cases considered, the ACF produced vibration reduction comparable with that obtained using IBC, but consumed only 10-30% of the power required to implement IBC. A careful parametric study is conducted to determine the influence of blade torsional stiffness, spanwise location of the control flap, and hinge moment correction on the vibration reduction characteristics of the ACF. The results clearly demonstrate the feasibility of this new approach to vibration reduction. It should be emphasized than the ACF, used together with a conventional swashplate, is completely decoupled from the primary flight control system and thus it has no influence on the airworthiness of the helicopter. This attribute is potentially a significant advantage when compared to IBC.

Millott, T. A.↗

NASA NTRS1995

Ball bearing vibrations amplitude modeling and test comparisons

Bearings generate disturbances that, when combined with structural gains of a momentum wheel, contribute to induced vibration in the wheel. The frequencies generated by a ball bearing are defined by the bearing's geometry and defects. The amplitudes at these frequencies are dependent upon the actual geometry variations from perfection; therefore, a geometrically perfect bearing will produce no amplitudes at the kinematic frequencies that the design generates. Because perfect geometry can only be approached, emitted vibrations do occur. The most significant vibration is at the spin frequency and can be balanced out in the build process. Other frequencies' amplitudes, however, cannot be balanced out. Momentum wheels are usually the single largest source of vibrations in a spacecraft and can contribute to pointing inaccuracies if emitted vibrations ring the structure or are in the high-gain bandwidth of a sensitive pointing control loop. It is therefore important to be able to provide an a priori knowledge of possible amplitudes that are singular in source or are a result of interacting defects that do not reveal themselves in normal frequency prediction equations. This paper will describe the computer model that provides for the incorporation of bearing geometry errors and then develops an estimation of actual amplitudes and frequencies. Test results were correlated with the model. A momentum wheel was producing an unacceptable 74 Hz amplitude. The model was used to simulate geometry errors and proved successful in identifying a cause that was verified when the parts were inspected.

Hightower, Richard A., III↗

NASA NTRS1995

Active Suppression Of Vibrations On Aircraft Structures

Method of active suppression of nonlinear and nonstationary vibrations developed to reduce sonic fatigue and interior noise in high-speed aircraft. Structure of aircraft exhibits periodic, chaotic, and random vibrations when forced by high-intensity sound from jet engines, shock waves, turbulence, and separated flows. Method of suppressing vibrations involves feedback control: Strain gauges or other sensors mounted in paths of propagation of vibrations on structure sense vibrations; outputs of sensors processed into control signal applied to actuator mounted on structure, inducing compensatory forces.

Maestrello, Lucio↗

NASA NTRS1996

Holographic Vibration Testing With Video/Computer Imaging

In improved system for holographic vibration testing, holographic interferograms indicating shapes of vibrational modes recorded by video camera under computer control. Results available almost immediately, and images distributed simultaneously to multiple computer terminals connected to vibration-testing computer via local-area network. Designed to replace prior photography-based system for identifying natural vibrational modes of propfan blades; adaptable to similar vibration testing of other objects.

Miller, Christopher J.↗

NASA NTRS1996

Improved Estimation Of Vibrations In Engines And Pumps

Method of estimating vibrational loads upon, and vibrational power dissipated in, rocket engines, pumps, turbomachines, and similar flow-through-type machinery derived from older vibration-scaling concept of estimating vibrations of components of rocket engine in design stage, without complicated mathematical modeling and calculations. Methods involve scaling vibrations via global variables like mass-flow rates and exit velocities of engine exhausts. Present method called "Improved Composite Loads Spectra-Component Scaling Criteria".

Chow, David C.↗