Removal of Longitudinal and Circular Fields

T.O. 33B-1-1 3-84 3.6.8.3 Ring Shaped Parts. For the demagnetization of ring-shaped parts an effective method is to pass a central conductor through the ring. The central conductor is energized with AC, and the current caused to decay to zero by means of either a step-down switch or a stepless current control. The latter method of decay can be much more rapid (down to a few seconds) than the step-down switch. This method can also be used with reversing, decaying or step-down DC as well. 3.6.8.4 Long Parts. Long parts, such as rods, bars, and tubes may retain an objectionable amount of residual magnetism from the earth's magnetic field. As the earth's field extends from the north to the south pole, it is desirable to demagnetize these types of parts by withdrawing from an AC coil in an east-and-west direction. This will minimize the effect of the earth's field on the residual magnetism in the parts. 3.6.8.5 Large Structures. Frequently, large structures such as engine mounts may require demagnetization and demagnetizing coils of suitable size may not be available. In such case each individual extension from the structure, such as the legs of a mount, should be placed within the coil as close to the wall as possible and withdrawn. The structure should then be reversed. The other end is then brought close to the face of the coil and rotated, so that all parts of the structure are passed across the open face of the coil. The entire structure is finally withdrawn four to five feet from the coil before it is shut off. In handling such tubular structures, it is important that they be moved to and from the coil in an east-and-west direction. 3.6.8.6 Removal of Longitudinal and Circular Fields. In considering the problem of demagnetization, it is important to remember that a part may retain a strong residual field after having been circularly magnetized, and yet exhibit little or no external evidence of such a condition. Such a field is difficult to remove, and there is no easy way to check the success of demagnetization. There may be local poles on a circularly magnetized piece at projecting irregularities or changes or sections, and these can be checked with a field indicator. However, to demagnetize a circularly magnetized part, it is often better to first convert the circular field to a longitudinal field. The longitudinal field does possess external poles, is more easily removed, and the extent of removal can be easily checked with a field indicator. 3.6.9 Measuring Residual Leakage Field Intensities. Leakage field intensities can be measured by quantitative or comparative methods. Quantitative measurements usually involve the use of instruments in conjunction with search coils, probes, or Hall-effect cells. Such instruments are classified as laboratory equipment and are not generally found in field locations. For purposes of determining the effectiveness of demagnetization efforts, residual field intensities are measured by comparative methods. 3.6.9.1 Field Indicator. The field indicator, a pocket instrument, is used to determine the comparative intensity of leakage fields emanating from a part. A typical field indicator is shown in Figure 3-38. The theory of operation is quite simple. Figure 3-38. Typical Field Indicators