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Aviation History
1939
1939 - 1029.PDF
356 FLIGHT. APRIL 6, 1939 lisation and porosity in castings and the direction of lines of flow in forgings and stampings. The machine tool room is, of course, essential for the preparation of test-pieces for tensometer and impact tests, for sectioning castings for macro and micro examination, and for the construction of intricate pieces of apparatus for investigating specialised problems. Also essential is the dark-room, which is of very modern type with a light-lock so that doors are eliminated. The dark-room is itself divided into two sections, one for radio graphic processing and the other for micro-photography work. As in the other rooms, air conditioning maintains the temperature at an even 65 deg. F. On the first floor there is the second chemical analysis laboratory previously referred to, and in this the subject of corrosion is also investigated. The main corrosion rigs are nine in number, and specimens of the materials under test are hung from rotating discs, which are inclined at an angle of 45 deg. to the vertical. The discs are driven by electric motors through suitable reduction gearing, and as they revolve the specimens are immersed in the corroding liquid once every ten seconds. The corroding effect of various liquids is investigated ; for example, the ordinary water supply, salt solution, ethylene glycol, and so on. Spray and impingement corrosion tests are also carried out. In another room on the first floor the work of a physical laboratory is carried out, such properties as thermal con ductivity, the coefficient of expansion, specific gravity, Young's modulus and specific heat being determined. Fatigue and Creep Fatigue testing is of particular importance, as metals often fail due to the fact that they literally become fatigued or tired under stresses which are lower than the accepted figures for strength. There are seven fatigue- testing machines, and they are equipped with furnaces and temperature control. The load is applied to a transverse test-piece by means of a knife edge operated by a spring which is compressed by a cam rotating at 1,000 r.p.m. A battery of '' creep '' testing machines has also been installed, for under certain conditions metals actually flow under very low stresses. There are six creep test machines, and these also are equipped with furnaces and control equipment, so that the metals can be tested at the temperatures actually experienced in internal-com bustion engines. A test-piece in the form of a small beam about 11 in. long has the load applied transversely by means of a knife edge, lever and weights. Creep is measured by a differential method and the use of an-optical lever. For the determination of expansion, a bar of material 15 in. long is heated uniformly by means of an oil bath. The expansion is measured by means of a micrometer read ing to 0.0001 in. For the investigation of thermal conductivity heat is put in at one end of a bar and the quantity travelling along (Above) Inspection of illuminated radiographs. (Left) Radiographic examination of a crankcase casting by the Philips equipment. The complete plant is lead-screened for safety. measured by a constant-flow water calorimeter. Thermo couples are used to determine the temperature gradient along the bar, and external loss of heat is prevented by a guard ring which has a temperature gradient corresponding to that of the bar. The results are correlated with electri cal conductivity as determined by a Kelvin double bridge. In order to investigate the damping capacity of various metals a damping capacity machine of the torsional type designed by the Cambridge Instrument Company is em ployed. In this the test bar is fixed at each end, while the middle is clamped to a heavy metal dumbell. The bar is made to oscillate and a stylus carried by the dumbell records the amplitude of the vibrations on a celluloid disc. On examination of the celluloid disc under a microscope the loss of amplitude is measured and the damping capacity can then be calculated. Friction is investigated by two friction test machines. In these an electric motor operates through a suitable drive a rotor in which are held four cylindrical test-pieces in such a manner that the end of each test-piece rubs against a stationary disc. The materials of test-pieces and station ary disc can be chosen and changed as required. The rubbing surfaces are immersed in an oil bath which can be heated so as to obtain the desired temperature of operation, and the desired load can be applied by means ol a system of levers and a spring. On one of the two machines the load can be partly relieved for alternate revolutions. The speed at which the testing rig is driven is measured by a revolution counter and the power absorbed in driving the rig by an ammeter. The oil in which the rubbing surfaces are immersed can be examined chemically for wear products, and the surfaces of the test specimens and of the rubbing disc can also be viewed. In a bearing test machine a straight shaft is driven by a variable-speed electric motor through a suitable gear box and coupling. The shaft is arranged to receive three bearings. The centre bearing reproduces the conditions of a main bearing, there being no load on it except that introduced by the shaft. The other two bearings can be given a direct loading by means of levers and weights, and also, at the same time, an alternating load is produced by springs which are cam operated. Oil is circulated into the main bearing and along the shaft to the two end bearings, in which, obviously, the cycles of a big-end bearing can be reproduced. The temperature of the bear ings is measured by thermometers in the housings, and the power absorbed in driving the machine by a Watt meter. There is also a torsional fatigue tester, in which a test bar—under heat, if necessary—can be set into torsional vibration by means of a motor-driven cam and lever operated by an electric motor. Another small machine reproduces the conditions per taining to a valve stem and its guide and an ingeniously simple rig has been devisod for testing hot scaling.
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