Bolt tension, rather than torque, is the most reliable method of
ensuring the security of mechanical joints, particularly in critical
applications. Neill Brodey describes the method, the problems and their
solution.
The standard way to ensure that a bolted or clamped joint is correctly
tensioned is to use torque measurement. For many applications it remains
the most reliable, cost-effective solution but in some cases, variations
in friction or joint geometry will make that torque measurement
inaccurate, while in others, the clamping force may need to be monitored
over the joint's lifetime. Both these situations can benefit from the use
of ultrasonic tension measurement but the technology does present a
number of challenges.
The ultrasonic length is determined by transmitting a sonic pulse at one
end of the bolt and accurately measuring the time required for the echo
to return. As the fastener is tightened, two things happen that can be
measured: as the bolt stretches, so the echo time lengthens; and with
increasing stress, the velocity of sound in the bolt reduces, having a
further influence on the echo time. The change in ultrasonic length is
displayed as elongation or tension, and this data can be used to
calculate induced stress if required. The ultrasonic method can produce
very accurate results but the selection of the optimum bolt and
transducer - and their coupling - can be difficult in tension measurement.
Firstly, the relationship of the energy pulse frequency to its
penetration is important. Lower frequencies produce longer wavelengths
that will travel further through a given substance, while higher
frequencies produce shorter wavelengths. Thus a low frequency transducer
is able to achieve an echo in a longer bolt, or in a bolt whose material
offers a high acoustic attenuation. Lower frequency transmission,
however, has less directional quality, resulting in scattered echoes from
the bolt sides and hence a higher level of unwanted noise and signal
distortion.
The best balance between maximum frequency and noised suppression is
achieved by selecting the right type of transducer. Its diameter (which
is generally dictated by the diameter of the piezoelectric crystal) has a
direct effect on energy transmission; larger diameter crystals have
greater ability to send and receive energy, and there is less lateral
spread. But the problems don't stop there. Dirty, rusty, thickly coated
or uneven bolt ends create uneven echoes that affect measurement
accuracy. Bending of the bolt under load - such as may occur in pipe
flanges with partial gaskets - and non-perpendicular bolt ends create
similar problems.
The author's company has spent many years developing the ultrasonic
sounding technique for bolt tension measurement, and advances in DSP and
software are now at a stage where many of the problems described above
can be overcome. One of the company's latest instruments, the USM-1,
employs a selectable tone burst pulse system, which sends the maximum
amount of energy to the transducer. This allows the broadest possible
range of transducers for a given application. The receiver's low noise
and automatic gain features, moreover, enable signals to be detected even
in the most adverse conditions, while automatic signal analysis optimises
the measurement process and warns of potential problems.
Neill Brodey is managing director of Norbar Torque Tools
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