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FLAWS HOW THEY ARE ,

19th March 1929, Page 73
19th March 1929
Page 73
Page 73, 19th March 1929 — FLAWS HOW THEY ARE ,
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Which of the following most accurately describes the problem?

SCIENTIFICALLY DETECTED.

I T is rare that metal parts fail

suddenly, for usually a crack develops slowly, and it is not until the breaking point is reached that the user is aware of anything 711383. In commercialmotor operation the failure itself may be the cause of comparatively small expenditure, but the consequent loss of use of a chicle or disruption of scrvice may be very damaging, both directly and indirectly.

To aid the maintenance engineer and others in their search for latent defects the magnetic detection system handled by the Equipment and Engineering Co., Ltd., 2 and 3, Norfolk Street, Strand, London, W.C.2, is of undoubted value. It is, we are told, employed by many of the foremost engineering undertakings. Many methods exist for the detection of flaws, such as that of heating the part to cause oil exudation and inspection under magnifying devices. These and others are open to the objection that human fallibility is ever-present.

The principle used in the system here described is highly to magnetize the sus pected part and then to pour over it a solution containing iron in a finely graduated form. The lines of force caused by magnetizing flow throughout the length of the part. A crack breaks the lines of force, causing them to emerge on the surface of the steel, forming magnetic poles north and south. The ferrous material in suspension in the liquid gathers in the area where the lines of force emerge, bridges the poles and thus forms an easily visible and definite line locating the crack.

Various types of magnetizer can be obtained, a special one for the motor and aeroplane industries being probably the most interesting to readers of this journaL It is equipped with detachable pole tips of blank form for machining according to the user's requirements. As it is important that the correct amount of magnetism be imparted to the various items to be tested and to allow for the differing permeabilities, of various steels, a potentiometer control is generally provided. The design of this permits reversal of the power to remove from the part tested any permanent magnetism which might be harmful, leaving it entirely neutral.

Another advantage of the potentiometer is that it safeguards the highly inductive magnetizing coils frOm injury should the current be switched off inadvertently during a test.

Other designs include magnetizers for gear pinions, small parts of high carbon content and objects of unusual shape.

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