no longer a
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effective inspections and problem diagnoses can be made
by "listening"' to these high-frequency sounds.
||By JOHN FREEMAN
General Manager Cost Reduction Activities Oakland, California.
that truck components are growing in complexity would not surprise
anyone closely associated with the industry. This growing sophistication
is largely the result of the introduction of electronics, larger
vehicles and government regulations. To keep pace with the equipment,
a knowledge of what just a few years ago was considered to be
space-age technology is becoming required in many fleet maintenance
One example of this growing sophistication by fleets is the increased use
of ultrasonic test equipment as a diagnostic tool. Results have been encouraging
with downtime and related costs being reduced.
Prior to 1964, when the nation's space industry was almost the sole user
of ultrasonic detection equipment, no one in the trucking industry could
have realized that these units were later to become "tools of the
trade." Today the devices are being employed nationwide in maintenance
shops to check and inspect such items as: internal combustion engine valving
and piston blow-by; gaseous piping and ducting; air brake systems; bearings;
cam and tappet assemblies; gear trains; seals in refrigerated van bodies,
clean rooms and air ducts; various hydraulic components and many other
"Ultrasonics" is a word and an area of technology that some shop managers
might still consider to be rather foreign. This remoteness, however, is mostly
just imagined and certainly not something to be feared. Except for their frequencies,
ultrasonic waves are exactly the same as their audible counterparts. In fact,
many lower frequency, "sonic'' waves possess ultrasonic counterparts. The
hissing of leaking pressurized air, the noise of jet planes and the jangling
of keys are all examples of common sounds that are often heard but also contain
unsuspected ultrasonic frequencies which cannot be heard.
Many people use sounds to detect problems of one sort or another on a regular
basis. The watch repairman listens to the tick of a watch. The plant engineer
listens for the sound of escaping steam.
Because ultrasonic waves are not as penetrating as those of lower, audible
frequencies, it's generally easier to pinpoint the source of the sound.
Moreover, there's usually much less noise interference. In particular,
conversation and most background noises are normally absent. Extremely
small sounds can, therefore, be detected and analyzed.
generator (above) is used to help find leaks in air seals.
|This article will describe
some of the tests which can be performed with an ultrasonic analyzer,
but first a brief discussion of the instrumentation is in order.
The detector is basically a microphone and associated electronics,
which is sensitive only to ultrasonic energy in the frequency
range of 35,000 to 45,000 Hertz. These signals are amplified
by the self-contained circuitry and converted either to sounds
which can be heard by the mechanic or readings on a meter. The
most useful type of instrumentation is designed specifically
for the type of tests done on fleet vehicles. It includes a contact
probe which can be used in place of the microphone, a separate
ultrasonic generator that is used to check for faulty seals in
cabs or trailers and some sort of device which can help localize
the source of air-borne sound.