will produce low levels of inherent vibration. As the gear teeth wear, however, backlash between meshing teeth increases and this is reflected in an increase in the vibration energy. As the vibration energy is dissipated through the gear box, it excites resonances and exerts extra dynamic loads on gear teeth. It is of great interest to the maintenance engineers to predict the occurrence of backlash between gear teeth. The fact that vibration signals carry much information relating to running conditions of gear boxes can be used to solve this problem. In this study, a mathematical model was formulated to simulate the effect of backlash between gear teeth on the vibration spectrum of gear boxes. A single stage helical gear box was used to demonstrate this model. In this simulation, the transmission shafts were treated as lumped parameter systems. The elasticity of shafts, bearings, and gear teeth were also included. The grounded-chair representation was used to obtain the equations of motion that were solved using the Runge-Kutta method. The vibration spectra of the gear box system were obtained using fast Fourier transform (FFT). These spectra can be used to construct a vibration chart which, in turn, can be used to determine the proper times for maintenance of gear boxes to avoid failure of such units.

No abstract

Active sonar systems that transmit large time-bandwidth (TW)-product linear frequency modulated (LFM) waveforms and receive echoes from targets of unknown range and speed can suffer considerable correlation losses that cannot be predicted from conventional (narrow-band) ambiguity function theory. As is well known, the theory can be modified to include the effects of Doppler distortion on large TW-product signals by correlating the received signal against a reference that is a time-compressed version of the transmitted signal. In this article, the effects of multipath (or target highlight structure) and Doppler on the correlation process for rectangular-weighted large TW-product LFM waveforms are examined. Gaussian-weighted waveforms are also considered to examine sidelobe behavior. It is shown that in a multipath environment, the correlator output peak does not generally occur at the correct Doppler reference channel. This is due to the constructive/destructive interference of the summation of complex delay-Doppler autocorrelation functions associated with each return. A summation technique that identifies the appropriate Doppler reference channel is proposed; this technique allows the target parameters to be estimated if the signal-to-noise ratio is sufficiently high.

No abstract

Acoustic bottom backscattering measurements and the corresponding geoacoustic properties of the ocean bottom are presented for an experiment conducted in the shallow waters of the North Atlantic. The bottom scattering strength data, which were obtained with a high resolution (narrow beamwidth) parametric sonar, were measured as a function of frequency (5–20 kHz), grazing angle (4°–10°), azimuthal angle (± 55°), and pulse length (0.4–10 ms). The supporting environmental measurements included box cores for determining the acoustic properties of the sediment and stereo photography for calculating the two-dimensional roughness spectrum of the sea floor. [Work supported by NAVSEA 63R.]