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Aviation History
1938
1938 - 0671.PDF
MARCH IO, 1938. FLIGHT. 245 THE INDUSTRY (CONTINUED) The first stage (left) is to pass a direct current through the component to be tested. After switching off the direct current, the part is immersed in the tank of iron-impregnated liquid (right) and an alternating current is passed through it for a few seconds. FAULT DETECTION A New Method of Inspecting Aero Engine Components A NEW and improved apparatus for'inspecting steelaero engine components, using the electro-mag-netic flux principle, has been developed jointly by the engine Department of the Bristol Aeroplane Co., Ltd., and the Metropolitan-Vickers Electrical Co., Ltd. For several years the Bristol Company has extensively employed the well-known Magnaflux and Electroflux tests. It will be recalled that both of these tests are based upon the principle of subjecting the component to an intense electro-magnetic field and coating it with a liquid containing finely divided particles of iron. Any cracks that may exist in the component are then revealed by the accumulation of iron particles around their edges, due to local disturbance of the magnetic lines of force. In the Magnaflux method an electro magnet is so arranged that its principal lines of force pass longitudinally across a gap formed by two pole pieces between which the component is placed. In the Electroflux test, on the other hand, a heavy electric current is passed through the component itself, and generates within the latter a magnetic field whose lines of force form a series of rings in planes normal to the path of the current. Due to the respective arrangements of the magnetic lines • of force, these two tests are each directional, to a certain extent, in their sensitivity to the presence of cracks. Relative to the horizontal axis of the component as placed in position for test, the Electroflux method is more vigorous in disclosing longitudinal cracks, whilst the Mag- naflux more readily detects transverse cracks. Whilst in actual practice both methods are sufficiently adaptable to render very good service in the inspection of a wide variety of components, it has been found that there are certain cases where either method by itself is not absolutely infallible. The new apparatus now introduced, however, combines ^e advantages of both the Magnaflux and Electroflux instruments. As will be seen from the accompanying photographs, this combined instrument is very compact,a iu simple to operate. The inspection is carried out inwo stages. First, the component is gripped between the two hinged pole pieces of the electro magnet as shown on the left. The control switch is then moved so as to pass direct current through the solenoid coil surrounding the magnet core, which is contained within the cabinet. The component is thus fully magnetised. Next, the direct current is switched off and the pole pieces moved downwards by a handle until the component is completely immersed in the tank of iron-impregnated liquid, as shown on the right. Thereupon, the switch is moved over to pass a heavy alternating current through the component for a few seconds, thus temporarily super- imposing a transverse magnetic field upon the longitudinal one still retained by the component. The latter is then removed for visual examination of any cracks revealed by the deposition of iron particles. Components which have satisfactorily passed the test are afterwards demagnetised completely by passing them through the field of a separate electro magnet energised by alternating current. The new instrument has already proved its great value for the closer inspection of certain components, where the occasional presence of minute cracks has hitherto been suspected, but not revealed by either the Magnaflux or the Electroflux tests. A further link has thus been added to the chain of vigilant inspection operations which maintain the very high quality of Bristol engines. A 24-ft. Airscrew!T HE casual visitor to the Airscrew Company's factory at Weybridge might well be astounded on encountering a two- bladed wooden airscrew of twenty four feet diameter. On regaining his powers of speech and enquiring as to the aeroplane for which it was intended, he would learn that it was not an aircraft component at all, but a fan, as used in industrial cooling towers. Properly designed airscrew-type ventilating and forced- draught fans have, it is claimed, a number of advantages over the orthodox centrifugal type. The Airscrew Company are obviously in a position to design them properly, and a cata- logue sets forth details of all types downwards from the in- dustrial giants mentioned. Wind-tunnel fans are included.
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