Documentos encontrados: 29
A paper presented by E. Downham and R. Woods at the American Society of Mechanical Engineers Vibrations Conference in Toronto, Canada, September 8-10, 1971.
The paper presents a factual discourse on the vibration monitoring of continuously operating process machinery with particular reference to operational experience and case studies from Shell Chemicals U.K. Ltd.
The crankshaft of an internal combustion engine possesses severe design problems. Its geometric form is irregular, and it is subjected to a variety of cyclic forces, moments, and torsional influences. Subsequent development work is essential to ensure that manufacturing tolerances, crankshaft loading, and support stiffness are adequate so that transmission of these influences does not affect adversely the reliability or useability of the complete engine in service. The trend for increasing specific power output combined with reduced weight has fostered the introduction of successive generations of engines of high rotational speed yet lighter construction. Thus balance quality, which has mechanical, material, and fabrication implications, as well as environmental, noise, and vibration effects, is an important consideration in modern engine design.
A paper presented by M J . Neale and B.J. Woodley at the Symposium
of the Society of Environmental Engineers held at Imperial College,
London, September, 1975.
It is based on the much fuller report prepared for the Department of Industry in 1975, which has now been revised to include the latest
1978 data and is to be published by HMSO during 1 978 for public purchase.
Many people are needlessly apprehensive of performing their own dynamic balancing procedure. To help overcome these fears, this Application Note starts by showing how very simple and straightforward such a process can be when using B & K equipment.
The use of thyristors to control the power of electric locomotives and multiple unit passenger trains (EMU's) on electrified railways is an example of a novel technology being injected into an established engineering practice. Many railway operators with busy electric lines have been attracted to the potential advantages of power electronics, only to find that thyristors create a lot of problems which cannot be measured, analysed or solved by conventional methods. The most notorious of these problems is the interference inflicted by thyristor-controlled trains on signalling and telecommunications circuits, vital for the safety of the railway.
Compression techniques are common practice in measuring systems where the excitation signal is permanently applied. A control signal proportional to some quantity (voltage, sound pressure, vibration acceleration, etc.) is fed to the compressor of the generator which regulates the output signal so that the control signal is kept constant.
In many measurement situations, however, the pulse is much shorter (typically a few milliseconds). This is for example, the case in electroacoustic measurements and hydrophone measurements.
A method allowing to apply compression techniques when using shot tone-bursts is described in the following. Practical applications to microphone measurements are also described.
For a particular skimming operation being carried out on a lathe in our own Production Shop, the unbalance of the rotating assembly limited the operating speed of the machine to 630 r.p.m. if its vibration were to be kept to an acceptable level. Any increase in running speed affected the quality of the surface finish, reduced the life of the cutting tool, and caused local disturbance due to the resulting vibration of the machine itself and of a nearby wall partition. By balancing the machine in its normal operating mode a 50% increase in work rate was achieved.
The purpose of this article is to demonstrate how a modern FFT analyzer (B & K Type 2031) connected to a desktop calculator can be used to make automatic spectrum comparisons and thus detect significant changes in machine condition, while at the same time being insensitive to nonsignificant changes due to small speed variations etc. The cassette recorder included in the calculator can be used for mass storage of reference spectra and other data for a large number of measurements points.
Difficulties and delay are often experienced when commissioning hydro-electric plant because the initial unbalance of the rotating parts of the generating set proves tricky to eliminate by trial and error methods. The sheer size and mass of the generator rotor, together with the effects of the coupled turbine, make it important that field balancing be carried out — i.e., the complete set must be balanced in situ. Using suitable portable balancing instruments, such as the B & K Field Balancing Set Type 9500 (Fig. 1), a task which might take weeks using conventional methods can be completed in a day or two. This was performed recently at one of the NEBB power stations in Norway.
The introduction of the Luminance Contrast Standard Type 1104 and the Luminance Contrast Meter Type 1100 has practically opened up a new field in the study of lighting.
Why does contrast depend upon the lighting?
Can the contrast be predicted when the lighting system is still at the drawing-board stage? Can the contrast rendering factor be specified in advance?
This article deals with the nature of luminance reflectance and its relation to the contrast. The Contrast Standard Type 1104 is used throughout for examples, but the theory is general.
The reflectance characteristic is explained in physical terms, and it is shown how the reflection values and the contrast may be calculated.
The subjective listening impression in a room is a function of many acoustic qualities. The more parameters that can be viewed at the same time, the easier it is to evaluate the objective qualities of the room.
This paper will therefore try to expand the traditional “1-dimensional” acoustic reverberation time measurement – first, to a “2-dimensional frequency response measurement” of the various acoustic surfaces using a gated sine tone burst, and then to a “3-dimensional” plot of how the various frequency responses change as function of time when the reflections arrive. The 3D plots are obtained using a gated pink noise pulse, Real-Time Analyzer, a calculator and a digital plotter.
However, it requires, of course, some experience to interpret the 3D results just as is requires more experience to interpret a frequency response than a single number on a voltmeter.
The paper will show the influence of various time resolutions and frequency resolutions as well as rectangular and Gaussian time weightings.
In recent years there has been a rapidly developing interest in the field of mechanical dynamics for a variety of reasons. The overall result is that the dynamic behaviour of a machine or structure is now an important factor in design and development along with the analysis of static stresses and deflections, and is normally studied in its own right, rather than just being allowed for in an excessive "safety factor", or treated as an afterthought when problems have been encountered.
Forced vibration techniques provide very powerful tools for the investigation of structures, structural elements, and the materials from which the structural elements were formed.
One of the main advantages of using the impedance or mobility theory is to avoid the solution of a classical set of differential equations. This is often achieved by application of electrical circuit theory to an electrical analogy of the system in question.
Microphones are often used to measure high dynamic sound pressures produced for example by sonic booms, explosions, jet engines, etc., which also involve a significant low-frequency content. For these applications, it may often be necessary to know precisely the behaviour of the measuring system under those special conditions. This may include sensitivity, linearity, distortion, low-frequency response, etc.
The B & K High Pressure Microphone Calibrator Type 4221 has been developed to meet these needs. It allows measurements at levels up to 164 dB SPL in continuous operation and up to 172 dB with tone-burst excitation, and covers a frequency range from below 0,01 Hz to 1000 Hz.
This paper describes the 4221 and reviews its main operating characteristics. Typical applications and the corresponding measuring setups are also discussed.
Investigations of a New Insert Earphone Coupler, Impedance Measurements of Volumes, Applications of Small Artificial Ears
By the use of gating techniques, many types of acoustic measurement normally confined to anechoic rooms, can now be made in ordinary reflective rooms. Of primary importance are the measurements of loudspeaker frequency response and directional characteristics.
However, the technique is also applicable to other areas such as measurement of distortion, early reflections, absorption, and phase response. These measurements are made possible by means of a Gating System which provides the necessary tone burst test signal and measures the peak amplitude of the received signal, which is gated by an adjustable time window to eliminate the influence of reflections and loudspeaker transient distortion.
The use of a compressor loop is common practice in measuring systems where the excitation signal is continuously applied. The High-Pressure Microphone Calibrator Type 4221 allows measurements to be made up to 1000 Hz (instead of 300 Hz without compressor) at levels up to 162dB re. 20/iPa.
Tone-burst excitation allows higher sound pressure levels (max. 172dB) to be reached. In order to eliminate problems due to the coupler resonance, compression techniques have been applied to tone-burst operation. This Application Note describes the set-up used and discusses the measurement results.
In general, amplitude non-linearities of a hearing aid may be measured using three different distortion measurements: harmonic, difference-frequency, and intermodulation distortion. Up to the present time, harmonic distortion measurements have been the easiest to make, and hence the most commonly used. However, with the advent of the new instrumentation discussed in this Application Note, the intermodulation and difference-frequency distortion measurements have become significantly easier and thus merit consideration.
The "sound" of a Hi-Fi set is to a great extent room dependent. Very often, the final result is determined by the room rather than by the actual equipment. Fortunately, these influences may readily be measured.
The objective test method, which seems to correspond best with subjective judgment, is the "1/3 octave, pink-weighted random noise method" carried out in the listening room. From measurements made according to this method, it is possible to say which Hi-Fi equipment will give the best result in the actual listening room. In practice, the method can be carried out in many ways.
The simplest and least expensive method uses a pink noise test record and a precision sound level meter. The most sophisticated and accurate technique (also the most expensive), uses a pink noise generator and a real-time, third octave analyzer. The results of such measurements show the frequency response characteristics of the loudspeaker/listening room combination.
This application note will describe the measurement possibilities, and it will show some results of the methods and compare them with results obtained by listening tests. Five loudspeakers in three different rooms and a test team consisting of five critical listeners were used.
The transient response of a complex loudspeaker system can often be improved simply by repositioning the speakers. How much axial movement is required will be indicated by studying the phase response of the system. This Application Note will concentrate primarily on phase response, but will also consider subjects such as minimum and nonminimum phase behaviour and phase responses of conventional types of loudspeaker cabinets and microphones. It will interpret phase response curves and give practical examples.
Vibration signals measured at the external surfaces of a machine contain a great deal of information as to the internal processes and a number of monitoring and analysis techniques can be applied to extract information from these signals.
This paper discusses the area of applicability of the various techniques, which fall into three main categories:
(i) Field measurement with portable instruments.
(ii) Field recording followed by detailed analysis.
(iii) Permanent on-line monitoring.
Principles for the selection of suitable instrumentation and data processing systems are discussed, with respect to the types of information which can be extracted, the degree of complexity of the machine, and the amount of data to be handled.
The most important analysis technique is narrow band spectral analysis, and this can be carried out by normal swept-frequency techniques, high-speed swept frequency techniques or real-time analysis techniques. Where required, the spectrum can be obtained in digital form from analogue analyzers or by direct digital analysis. Other useful techniques include time domain averaging and cepstrum analysis.
Practical cases associated with machine health monitoring in a number of chemical plants are included as illustrations and a comprehensive bibliography is given in the paper.
After a discussion of some basic concepts and definitions, the present article discusses the various ways in which the cepstrum can be obtained, and finally gives some results of the application to gearbox fault diagnosis. Much of the information is of course generally applicable, e.g. to other applications of cepstrum analysis and to other cases where it is desired to detect sideband growth.
On the 12th and 13th November 1974 a special Telephonometry Symposium was held in Copenhagen. Hosts for the symposium were KTAS (Copenhagen Telephone Company) and JTAS (Jutland Telephone Company), and initiative for the meeting was mainly due to the efforts of Mr. P. V. Arlev and S. A. Jager of JTAS. and Sweden contributed to the success of the meeting.
The symposium consisted of lectures and discussions related to developments in the field of telephonometry, and participants from Engand, Denmark, Finland, Norway Of the papers presented, two summaries and six full texts were later printed in the B & K Technical Review No. 1, 1975, copies of which are available on demand from B&K. This Application Note prints a further five papers plus an extra paper which although not presented at the symposium received some discussion there.
The idea of the Fast Fourier Transform (FFT) first appeared in 1965. It provides an algorithm which gives a far faster computation of the Discrete Fourier Transform than was previously possible, and has created for it a complete new range of applications where its use was previously thought impracticable. This Application Note starts off with a brief look at the philosophy behind FFT, and then goes on to describe a practical application of it using a Computer Type 7504 and a Digital Event Recorder Type 7502 as a single channel FFT Analyzer.
The concept of the Power Spectral Density of a periodic time function is well known, as are the techniques involved in its measurement. An equivalent measurement, using similar techniques, may be obtained from a single impulse. However, it is then more commonly referred to as the Energy Spectral Density. This Application Note looks first at some of the theory behind Energy Spectral Density, and then goes on to look at the practical side of its measurement for short duration impulsive signals using a Digital Event Recorder Type 7502 and a Heterodyne Analyzer Type 2010.
With the "Weighting Network, -3 dB/Octave" Type WB 0078 PSD measurements in absolute PSD unit V2/Hz can be made by means of frequency analyzers with constant percentage bandwidth.
Time averaging together with some sort of weighting are the two most important processes in the data reduction which is necessary in almost any measurement of a signal. For signals involving only high frequencies the time averaging normally causes no problems, but for low frequency signals or single pulses it is more important to know the influence of the averaging process and of a finite averaging time. The main purpose of this paper is to compare two simple types of averaging and find out how to choose the parameters to get most equal results.
One of the problems associated with narrow band analysis of vibration (or acoustic) signals is the length of time required to obtain the complete spectrum, typically 10 to 30 minutes for three decades. Where this is to be done only occasionally, then the original signal could often be analysed in situ or at least recorded on a tape reel for the full 30 minutes or so. This paper focuses on alleviating this issue.
Fast pressure variations may seriously increase the fatigue loading of the tubes and other components of a piping system. Therefore, it is important to find a simple way to detect such variations and their distribution over the system. One promising method is to measure the extensional vibrations of the tube walls as the tube expands and contracts with the pressure variations. Naturally, care must be taken that other vibration modes are eliminated in the measurements.