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Aviation History
1917
1917 - 0402.PDF
experience shows that an excess of scrap results from suchattempts; the remedy would appear to be to reduce the specification figures. Such a reduction without definingthe composition would be disastrous. For example, a medium carbon steel can be made to conform to a specificationwhich is intended for a medium straight nickel steel, yet the properties, of these two steels in use are totally distinct. Much has been said with regard to the difficulty in deter-mining the yield, though it is easy where carbon steels are concerned. A curve of the nature shown in Fig. 13 is obtained.With an alloy steel giving a, high ultimate, the curve obtained is as shown in Fig. 14. These diagrams will probably be subjected to criticism,but they are inserted to emphasise the different behaviour of the two classes of steel under load. It is argued that as a difficulty exists in determining theyield point, it should be omitted from the specification. This is an erroneous idea, because in the majority of cases the steelis used in a state where the yield point can be determined and the curve shows a kick sufficient to be intelligible. It isbut seldom that a steel is used in the state giving the second curve. Even if this were not so, the yield point, as distinctfrom the elastic limit, can be determined by a " non-diluted " inspector within reasonable limits. It will be of interest to consider steel specification andproduction with jegard to certain essential parts. With 16.—Enlarged view of cracks. 100 magnifications. reference to connecting rods, the designers call for Jugherultimate strength and yield point than is demanded for a crankshaft. Here it is thought that they have not givensufficient consideration to the engine revolutions which govern the stress. In many cases where the engine drivesthe airscrew direct, the engine revolutions must necessarily be in the neighbourhood of 1,300 per minute. When anengine is designed for a gear reduction the engine revolutions must be expected to be 2,000 per minute or over; consider-able differences in inertia stress have therefore to be dealt with. The cases are somewhat different, and thereforemore consideration should be given to the choice of steel for what are obviously two different conditions. Another point which has been neglected by the designeris the danger arising from sharp corners. An example occurred in a connecting rod where the grooves for the whitemetal had this defect, and it was found that the rods repeatedly cracked from the corner of these grooves to the bolt hole. Onexamining these cracks under the microscope it was found that they extended into the white metal, which would indicatethat the cracks came into existence during running. This is clearly shown in Fig. 15 and Fig. 16. An interesting problem presents itself in a specification forstreamline wire. It is possible to obtain the same tensile strength per square inch in the butt end of a streamline wireas that which can be obtained in the swaged down portion; cold work on the latter can of course to a certain extent bealleviated by a suitable heat-treatment, but such heat-treat- ment must also necessarily alter the condition of the materialin the butt end, having the effect of lowering the tensile strength and increasing the elongation. The matter -then APRIL 26, 1917. becomes interesting, because the gross breaking stress ofthe butt end should not be less than the gross breaking stress of the swaged portion. The butt end is of the diameterfrom which the streamline portion is swaged, and must have a less tensile strength per square inch than the swaged downportion, because the cold work in the latter increases its tensile strength. Cold work beyond certain limits produces in the steel anundesirable structure that cannot be completely restore i by subsequent treatment. Excessive cold work lowers theresistance to stress reversal. In this connection the following tables of figures obtained from actual tests are of interest:— TESTS ON STREAMLINE WIRES. Vn-annealed. BLADES. BUTTS. v $ SBis •6 3 »r I-I ad fl g! g SOo! | .g ad . ,3 Max . ns . Equi v Load . u &d 0.302 0.285 0.292 0.298 0.298 0.292 0.412 0.405 0.413 0.397 0.406 0.076 0.078 0.074 0.073 0.077 0.077 0.098 0.099 0.098 0.096 0.094 0.096 i-39 1-35 1.30 i-35 1.40 1-39 2.40 2.42 2.39 2.26 2.29 2.36 77.0 77-5 76.6 76-577.2 78.5 75-4 76.875-2 75-5 76.3 75-7 8 8 7 8 8 7 9 9 8 10 10 6 0.208. 0.207' 0.207 0.209 0.208 9.207 0.293 0.293 0.293 0.290 0.291 0.292 2.09 2.03 2.05 2.14 2.16 2.14 3-92 4-°33-95 3-92 3-92 3.98 I.04 1.02 1.03 1.06 1.08 I.07 1.84 1.89 1.85 1.88 1.881.88 61.4 60.3 60.8 62 63 63 53 59 58 59 59-4 59-4 Annealed. BLADES. 0.301 0.297 0.293 0.294 0.299 0.294 0.414 0.404 0.413 0.406 0.408 0.412 I* a a§8 BUTTS. !-S 0.075 0.073 0.075 0.073 0.075 0.075 0.098 0.095 0.097 0.096 0.096 0.092 I 1.19 1.12 1.17 1.08 1.24 1.21 2.18 2.11 2.22 2.05 2.06 2.13 67.2 65.8 67.8 64.2 70.4 69 68.6 70.2 70.6 67.2 66.9 72.5 14 10 14 16 13 13 *7 13 8 6 12 9 0.207 I 2-°5 0.208 i 2.02 0.207 0.208 0.208 0.209 0.293 0.293 0.294 0.293 0.293 0.293 2.00 1.83 2.16 2.03 3-85 3-9O 3-9O 3-94 3-97 3-98 1.03 1.02 I.OI 0.91 1.08 1.00 1.81 1.83 1.82 1.85 1.86 1.87 pe r n. 60. 59. 59. 53-863.6 57:0 57-85Z'4 58.4 58.9 59-° The " Equivalent Max. Load " is the estimated load on an area equal to that of the core of the screw thread. Considerable difficulty has existed with regard to case-hardened parts where weight is of greater importance than in ordinary engineering practice; in aircraft, case-hardenedparts often cannot be left with a solid ore. It follows that the usual tensile test of the core is of doubtful value as a representa-tion of the condition .of the material under the cemented skin. For example, the gudgeon pin of an aero engine may be 25mm. in diameter, and bored with an i8-mm. hole. To case- harden such a part, the ends of the hollow portion may beclay plugged, but undoubtedly a certain amount of the cementing gases passes through the plugs; in any case itis quite certain that the inner portion of this shell has a greater carbon content than that of the core in the solid bar,and its condition is different. (To be concluded.) . % The Army Council and Military Aeronautics. IN an Order in Council, published in the London Gazette of April 24th, setting forth the individual responsibilities of the members of the Army Council, the following appears:— " (f) The Director-General of Military Aeronautics shall be responsible to the Secretary of State for so much of the business relating to the administration of the Army Air Service as is not subject to the control of either the Air Board or the Ministry of Munitions, and as may be assigned to him from time to time by the Secretary of State." 402
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