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Aviation History
1955
1955 - 0062.PDF
62 FLIGHT, 14 January 1955 VIBRATION TESTING —of Aircraft Gas Turbines: Some Useful Adaptations of Standard Instruments THERE is a tendency—in which airline publicity agentshave doubtless played an innocent part—to assume thatthe turbojet and turboprop are, by comparison with the piston engine, virtually vibrationless. The assumption is one that demands analysis. Very briefly, it can be said that, whereas the reciprocating action of piston engines leads to fairly high amplitudes of vibration, the comparatively low rotational speeds mean that the forces generated are not usually very large; the rotary motion of jet engines, on the other hand, results in a much lower amplitude of vibration but, due to the high rotational speeds, the associated forces are usually of a much higher order. The more usual effects of vibration are well understood; andin aircraft practice provision is always made against the commonest of all, which is the tendency of nuts and screws andother threaded parts to work loose. Against some other effects— e.g., the tendency of metal to crack—there is little or no defenceunless an impracticable weight of material is used. In any machinery, vibration tends to set up a vicious circle, the vibra-tion causing wear and the wear in turn increasing the vibration. In gas-turbine manufacture, vibration testing can be used toadvantage in providing a rapid check on correct manufacture of individual components and on accurate assembly. An engineincorporating parts that are faulty in respect of dimensions, weight or fitting will seldom run smoothly, so that vibration testreadings can be considered as a nominal criterion of quality of manufacture. The actual range over which readings may beexpected to extend will, of course, be determined by the design of the engine. The point at which a particular engine falls withinthis range will, however, provide an excellent objective estimate of its general quality and of its performance in service.To take such vibration readings, any one of several standard vibration meters now available may be used. The electronicslaboratory which forms part of the instrument department of the Bistol Aeroplane Company's engine division has, however,suitably modified a large number of simple valve voltmeters. These are understood to be most effective for the limited dutiesfor which they are intended. Valve yoltmeters are used at Filton for two main purposes.The first is the conventional one of setting up other testing equip- ment in the laboratory. The second is the rather unusual applica-tion of making vibration measurements on aircraft engines by using the voltmeters to measure the signals obtained from seismicvibration pick-ups. This latter application again falls into two groups: first, research on new and development engines and, secondly, routine testing of established engines as they come away from the produc- tion line. These three fields of operation require somewhat differentcharacteristics from the instruments employed. The valve volt- meters used are, in fact, adaptations of the well-known DaweType 613 valve voltmeter but each is suitably modified for the purpose for which it is to be used. For laboratory measurements involving other instruments,the obvious requirement is extreme sensitivity coupled, of course, with stability. This sensitivity is obtained by increasing thegain of the three-stage video amplifier employed in the Type 613 to give nine voltage ranges with full-scale sensitivities of 3mv,lOmv, 30mv, O.lv, 0.3v, l.Ov, 3.0v, lOv and 30v. The resulting instrument, known as the Type 614 sensitive valve voltmeter, isdirectly calibrated down to a value of 100 microvolts on the lowest range. A cathode follower is used before the amplifier for measure-ments up to the one-volt range so that the multiplier may operate at the low impedance level of the cathode circuit. Above onevolt, a frequency-compensated divider direct on the high- impedance input is employed. The large amount of negative feedback provided on the*amplifier, together with the stabilized high-tension supply^, renders the reading virtually independent of mains variation and:of ageing of the valves. The miniature diode rectifier operates on a linear part of thecharacteristic so that the scale divisions on the meter are practi- cally linearly spaced. This is an important feature when similarinstruments are being used for routine production testing, as will be seen later. Another useful feature for such use is that the circuit is arrangedto "limit" on large inputs. This means that the meter will not be damaged by occasional overloads of several hundred timesthe magnitude of normal inputs for the particular range. This sensitivity renders the Type 614 very useful for calibratingand checking the frequency response of other instruments, valve voltmeters used on test beds, and varied equipment—speciallyproduced by the instrument department for engine testing— such as filters and frequency-response instruments for vibrationtesting. Slight modifications of the Type 614 voltmeter have also ren-dered it suitable for measuring vibration on research and develop- ment engines. These modifications are usually external to the A J47 turbojet, from a Sabre, on test with Southern Instruments seismic vibration pick-ups clamped to compressor and turbine housings. (Right) This enlarged view shows how a pick-up can be readily attached to any part of an engine by adjustable-jaw self-locking pliers.
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