Stationary Equipment

Table 3-4. Relative Permeabilities for Some Ferromagnetic Materials

Figure 3-27. Stationary Wet Magnetic Particle Inspection Unit

TM-1-1500-335-23 Nondestructive Inspection Methods Manual
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T.O. 33B-1-1 3-43 3.3.12.7.3 Inherent with longitudinal magnetization when using a coil is the difficulty in producing good indications near the ends of the part. This difficulty is caused by the leakage field that emanates from the magnetic poles generated at the part ends. Longitudinal magnetization of a cylindrical part in a coil will produce free magnetic poles at the end of the part. The direction of the magnetic field within the part will be in the same direction as the magnetization force generated by the coil. However, since the flux lines are continuous, the flux lines that traverse from one pole to the other within the part must return outside the part, and in doing so travel in a direction opposite to the applied magnetizing force. This results in a reduction in field strength at the surface of the part and is called free pole demagnetization. The inspection of areas near the ends of such parts is improved when the quick break in the magnetizing current is used. The resulting rapid decay of the field generates a pulse of induced current in the same direction as the original magnetizing current, which in turn produces a strong surface residual field over most of the length of a part. Parts must be moderately retentive for this type of residual inspection, and their shape must be generally cylindrical and have no long slots or cuts that would interrupt an induced current path around in the part near its outer surface. It must be mentioned that the use of yokes or field flow magnetization will also assure an adequate inspection of the ends of generally cylindrical objects. 3.3.13 Equipment. A variety of equipment is available which can be used for either circular or longitudinal magnetization. The equipment ranges in size from small, general-purpose portable units capable of being carried by hand to large, custom-built stationary units with separate power supplies. 3.3.13.1 Stationary Equipment. A typical stationary horizontal wet magnetic particle inspection unit of intermediate size is shown in Figure 3-27. The unit has two contact heads for either direct contact or central conductor, circular magnetization using a copper rod between the heads, or a cable connected to a contact block between the heads. Many of the units contain a coil used for longitudinal magnetization. The coil and one contact head are movable on rails. The other contact head is fixed; the contact plate on it, being air cylinder operated, provides a means for clamping the part. The unit has a self-contained power supply with all the necessary electrical controls. Magnetizing currents are usually three phase full-wave DC or AC depending upon usage requirements. The units are made in several different sizes to accommodate different length parts and with various maximum output currents. A full-length tank with pump, agitation and circulation system for wet inspection media is located beneath the head and coil mounting rails. A hand hose with nozzle is provided for applying the bath. On special units automatic bath application facilities are provided.

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