T.O. 33B-1-1
3-36
Finally, any cables and cable leads used with and for cable-wrapped coils must have good quality electrical connections.
Poor connections result in overheating and reduced coil amperage.
3.3.12.4
Electromagnet Technique.
Parts can be magnetized longitudinally by placing them between the pole pieces of a pair of electromagnets with the
fields of the two electromagnets being directed in the same direction through the part.
3.3.12.5
Yoke Technique.
Still another method is the magnetizing of parts between the feet of yoke or probe.
3.3.12.6
Selection of Current Level.
3.3.12.6.1
General.
A number of factors must be considered when determining current levels for longitudinal magnetization of parts. Some
of the more important factors are:
a. The coil diameter and the number of turns.
b. Cross-sectional area of the part and the coil.
c. The length to diameter (L/D) ratio of the part.
d. The size, shape, and composition of the part.
e. The orientation of the part within the coil.
f. The kind of discontinuities being sought and their ease of detection.
3.3.12.6.1.1
The magnetizing field strength in the center of the magnetizing coil increases with the current passing through the coil
and is proportional to the number of turns. The field strength decreases if the coil radius is made larger.
3.3.12.6.1.2
Rule-of-thumb formulas have been developed to help determine the amount of amperage required to induce an adequate
longitudinal magnetic field in a part. These formulas apply particularly well to cylindrically shaped parts and are
explained with examples shown in the following paragraphs. However, as discussed previously, blind adherence to
these “rules of thumb” can result in overmagnetization with a subsequent loss of inspection sensitivity.
3.3.12.6.2
Cross Sectional Area.
It is critical to determine the relationship between the cross-sectional area of the part and the cross-sectional area of the
coil(s). This relationship/ratio will determine whether the part can be inspected within a coil of a given diameter by
lying the part in the bottom or next to the side of the coil wall, or by centering the part in the coil, and which formula
will be used for estimating the amperage required. The cross-sectional area for the part and coil are determined as
follows:
A = pr2
Where: A = Cross-sectional Area
p = 3.1416
r = radius (1/2 of the diameter). The diameter of the part shall be taken as the largest distance
between any two points on the outside circumference of the part.
Example:
A 12-inch diameter coil is to be used to inspect a part having a 2-inch diameter.
Area of Coil (12" diameter)
Area of Part (2" diameter)
A =pr2
A=pr2
