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.
