TM 1-1520-264-23
1.4.8.1.1 Magnetic Yokes and Probes. Portable induced field inspection equipment is generally referred to as either a
probe or a yoke. These terms are synonymous and differ due to manufacturer's nomenclature. They are small, portable,
easy to use, and can be used on or off the helicopter. They induce a strong magnetic field into that portion of a part that
lies between the poles or legs. This limits the magnetization to longitudinal; however, by turning the probe 90 on the
part for the second position, circular and longitudinal magnetization to the axis of the part has been produced. Some
yokes and probes have both AC and DC capabilities while others have AC only. All procedures in this manual use AC.
AC provides a very desirable and useful field. The vibratory action of AC adds significantly to the magnetic particle
mobility enhancing the formation and build-up of larger and sharper indications at discontinuities. An AC magnetic field
is also used when it is necessary to reveal only surface cracks, common to in-service parts due to fatigue and stress
cracking. Yokes and probes utilizing AC for magnetization also have the additional advantage that they can be used for
demagnetization.
1.4.8.1.2 Hand-held Coil. For longitudinal magnetization of bolts, shafts, spindles, axles, and similar small parts, the
hand-held coil offers a simple, convenient method of inspecting for transverse cracks. It allows for equipment
maintenance inspections wherever a coil can be applied around the part. Parts are magnetized and demagnetized with
the same coil.
1.4.8.2 Safety Precautions During Magnetic Particle Inspections. Follow safety precautions and instructions contained in
this manual and the Nondestructive Inspection Methods manual listed in Table 1-1.
•
Black lights generate considerable heat during use. Extreme care must be exercised to prevent contacting
the housing with any part of the body.
•
To prevent injury to eyes, do not look directly into black light.
•
Prolonged direct exposure of hands to the filtered black lights main beam may be harmful. Suitable gloves
should be worn when exposing hands to the main beam.
a.
Follow manufacturer's instructions when using black lights and filter.
b.
Do not wear sunglasses or glasses with light-sensitive lenses during fluorescent magnetic particle inspections.
They can contribute to improper interpretation of defects.
CAUTION
Do not operate magnetic particle equipment within 36 inches of aircraft instruments.
1.4.9 Demagnetization of Inspection Parts. Following magnetic particle inspection of a part, the residual magnetic field
in the part shall be reduced to the lowest possible level. This must be done prior to returning the part to service or
rejecting it as a defective part. Unless this is done properly, the residual magnetism may cause adverse influence on
instruments, unnecessary wear on parts, or attract ferrous metal chips and dust into bearing surfaces. After
demagnetization a magnetic field strength meter shall be used to measure residual fields. Readings in excess of three
units are not acceptable.
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