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Aviation History
1962
1962 - 1959.PDF
FLIGHT International, 6 September 19< testing cars for noise and vibration The importance of vibration Vibration is in itself most unpleasant if it has to be endured for any length of time. Even more tiring is the accom panying noise. A driver who is tired reacts more slowly to an emergency, while an irritable driver can constitute a menace on the roads. From the point of view of safety on the roads, it is therefore important to minimize vibration and, with it, noise. Fatigue is not restricted to the occupants of cars: components subjected to continuous vibration also tend to become 'fatigued' and eventually fail. The importance of metal fatigue was underlined by the investigation into the 'Comet' disasters. These were due to fatigue failure of parts of the fuselage, but fatigue testing has been carried out for many years now on such components as springs, bolts and test samples of various alloys to establish that they will stand up to the vibration likely to be encountered in normal service. The lower the vibration actually encountered the greater the margin of safety against failure. Another reason for combating vibration is that it tends to loosen fasteners such as nuts, rivets and pins. If a component in an assembly starts rattling as a result of a loose nut, this will result in increased vibration, which in turn will cause further nuts to work loose. Unless this vicious spiral is stopped at an early stage, the unit will literally 'shake itself to bits" at an increasingly rapid rate. Everyone knows how quickly a car can fall into a state of serious disrepair if small defects are not attended to promptly. Fig. 1 Type 390A Goodmans vibration generator with special supporting spider for suspension from the bracket mounted on the front bumper bar. Every body has a natural frequency of vibration, depending on its size, shape and structure. It will vibrate much more readily at this natural frequency than at any other and is said to be in 'resonance' with the source of vibration at this frequency. Although the problem of vibration in cars has ntil recently been tackled primarily from the driver's r tint of view, to provide greater riding comfort, the mechi ;a] implications of vibration reduction are now rece: ing increasing attention. Testing for noise and vibration has been introduced as a standard routine by several motor car manufacturers and it may be of interest to describe the latest method by which such tests are carried out. A new method of vibration testing At first, vibration tests were carried out by simply running prototype models or production cars on roads of difficult kinds and by observing the behaviour of the test cars under varying conditions. To obtain comparative data was practically impossible, even when objective instru ments such as sound-level meters were used to measure the noise, instead of relying on the purely subjective Fig. 2 Two vibrators were mounted, one at either side, to simulate road conditions at the front end of the car. The car is not jacked up: the vibrators can be fitted in a matter of minutes. judgment of the tester. The difficulty lay in driving different cars over the same road, at the same speed, in the same manner, under the same ambient conditions - in practice an impossible task, even on a test track. The latest method is to use vibration generators which, as their name implies, generate the required vibration and impart them to the equipment under test. In this way the response of the vehicle to the range of vibrations likely to be met with in service can be studied without inter ference from road noise or engine noise. Moreover, the effect of the application of each frequency in turn can be measured and the exact point at which each resonant condition appears can be noted for subsequent correction. Test equipment and procedure The equipment used in the test described here simple. It consists of a pair of Type 390A vi; generators made by Goodmans Industries Ltd ( Works, Wembley, Middlesex) and of a power os made by the same company. The vibration gen are a development of the moving-coil loudspeak which Goodmans are so well known. The vibrat made in a number of sizes, capable of exerting fc a few ounces and up to 25, 50, 250 and even 18. The model used on this occasion, Type 390A, c . ven ration uiom ;ilator rators rs for rs are •es of )0 lb. ers a
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