T.O. 33B-1-1
3-14
3.1.9.4
Portable Equipment.
CAUTION
Contact prods shall not be used on aerospace components or parts, and SHALL be
labeled: NOT FOR AIRCRAFT USE.
Portable MT units are manufactured in a variety of sizes, shapes, voltages, and current outputs. Portable equipment
operates on the same principle as stationary equipment. However, the compactness allows areas to be inspected where
larger equipment may prohibit access. Portable equipment is usually operated on 110 or 220 volt AC and is rated
between 200 and 1000 amperes. Portable equipment can be either AC, or a combination of AC and halfwave DC.
Portable equipment is suitable for examining small areas in large components where suspected cracks may be found.
For example, critical engine mount fittings and landing gear assemblies, which are difficult to inspect in stationary
units, can be examined quickly with minimum disturbance and with attention concentrated on points most subject to
cracking. Portable equipment can be moved to large items in need of magnetic particle testing and inspections can
often be performed without disassembly.
3.1.9.5
Contact Prods.
CAUTION
When parts are being magnetized by the use of spring loaded contact clamps to
generate circular magnetization, the contact clamps shall not conduct more than
800 amperes.
When a part is too large to fit into a stationary unit, or if only mobile or portable equipment is available, then the part,
or areas of the part, can be magnetized using cables and two hand-held prods. The current passing between the two
contact prods creates a circular field. Great care must be used to prevent local overheating, arcing, or burning the
surface being inspected, particularly on high-carbon or alloy materials where hard spots or cracks could be produced.
3.1.9.6
Contact Clamps.
Contact clamps can be used with cables instead of contact prods, particularly when the parts are relatively small in
diameter. Parts, like engine mounts and tubular structures, are inspected by positioning the clamps so that current
passes through the area under inspection. Care must be used to avoid burning of the part under the contact clamps.
Burning and heating may be caused by dirty contacts, insufficient contact clamp pressure, or excessive currents.
Cracks may be produced as a result of the transient heating.
3.1.9.7
Hand probe Or Yoke.
For occasional inspections of small parts and localized inspection of large parts, magnetic hand probes or yokes are
often adequate and easy to use. Probes or yokes are able to put a strong magnetic field into that portion of the part that
lies between the poles of the probe or yoke. Hand probes or yokes can be excited by either AC or DC electrical
currents, or they can be a permanent magnet.
3.1.9.8
Electromagnetic Probe or Yoke.
Electromagnetic probes or yokes are U-shaped cores of soft iron with a coil wound around the base of the U (see Figure
3-18). When alternating current or rectified alternating current is passing through the coil, the core is magnetized.
This produces an electromagnetic probe or yoke that functions similarly to a permanent horseshoe magnet. A probe or
yoke may be used to induce only a longitudinal field in a part. No electrical current passes through the part.
