DAKOTA ULTRASONICS MAX II Operation Manual page 49

Dakota Ultrasonics
Figure 1 Sound path in a good bolt
Smooth, even surfaced bolt ends that seat the entire active surface of the transducer
with minimum gap are required for accurate signal transmission. Bolt ends may need
to be cleaned, ground, etc. to achieve the required surface.
Avoid:
 Rough or irregular surface's which prevent adequate contact with the transducer.
Irregular or rough surfaces can be filled with couplant, but energy transmission
will still be reduced and dispersed causing mode conversions to occur from
reflections off the side walls of the bolt, as shown at left of Figure 2.
 Bolt ends not perpendicular to the axis of the bolt, as shown at right of Figure 2.
Energy will be transmitted toward the side wall and reflect along the bolt, yielding
poor signal quality and possible measurement errors. Avoid alignment errors
exceeding 2 degrees.
Figure 2 Rough and angled transducer contact
 Rusted, dirty, or thick paint-covered bolt ends. These coatings prevent sonic
energy from traveling between the transducer and the bolt. Very thin coating or
plating is acceptable.
 Bolt ends with recessed grademarks, as shown at left of Figure 3. Couplant can
be used to fill recessed grademarks. Small indentations cause some loss of signal
strength, but normal measurement is still possible. Large or numerous
indentations cause the signal to be too weak for a reliable measurement.
 Bolt ends with raised grade marks, or indentations with a raised edge, which
cause the transducer to be seated at an angle to the axis of the bolt, thus
preventing adequate contact, as shown at right of Figure 3.
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