The challenges of ultrasonic tension measurement

Neill Brodey, Managing Director of Norbar, explains some of the challenges that have to be overcome in using ultrasonic tension measurement.

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 and Norbar Torque Tools Ltd continues to offer the widest range of torque tools available with new models being added regularly.

In some cases, however, variations in friction or joint geometry mean that torque measurement may not offer the required degree of accuracy.

In others, the clamping force may have to be monitored over the joint's lifetime.

Both these situations can benefit from the use of ultrasonic tension measurement.

Neill Brodey, Managing Director of Norbar, explains some of the challenges that have to be overcome in using the technology and the benefits that it can offer.

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The ultrasonic length is determined by introducing 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.

The change in bolt length as it stretches increases the echo time and the increasing stress level in the bolt reduces the sound velocity, which further increases 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 physics governing this process are clearly understood and have been employed for many years in the fields of active sonar, or radar.

Send a pulse of energy toward an object and then measure the time between the initial pulse and the returning echo.

While the concept is comparatively simple and ultrasonic measurement can produce astoundingly accurate results, the selection of the optimum bolt and transducer and their coupling can be difficult in tension measurement.

Low frequency versus high frequency Ultrasonic measurement requires the transmission of a suitable quantity of ultrasonic energy through the length of the bolt.

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.

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So, a low frequency transducer is able to achieve an echo in a longer bolt, or in a bolt made of metal with higher resistance to sound transmission (attenuation).

While the lower frequency has more penetration power, it also produces more unwanted noise and its energy tends to spread, much like an unfocussed beam of light.

When low frequency energy is introduced at the end of a bolt, a significant portion is bounced from side to side within the cylindrical shape, producing a noisy and distorted echo.

Higher frequency pulses tend to travel more directly down and back the centreline of a bolt, with less noise and distortion.

Effect of transducer diameter The best balance between maximum frequency and noise suppression requires the selection of the best transducer for bolt measurement.

The diameter of the transducer (which is generally specified by the diameter of the actual piezoelectric crystal) directly effects energy transmission.

Larger diameter crystals have greater ability to send and receive energy, and less of the energy spreads laterally.

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Bolt end conditions affect measurement Dirty, rusty, heavily painted, or uneven bolt ends create uneven echoes and interfere with precise ultrasonic measurement.

The echo ricocheting back through the bolt travels further than the echo returning directly along the bolt axis, reaching the transducer at a slightly different time.

Bending of the bolt under load and non perpendicular bolt ends create similar problems.

Bending of the bolt is a fairly common occurrence, normally due to nonparallel faces of materials being clamped, or by bending these faces during tightening.

Pipe flanges or joints with partial gaskets are prone to this problem.

When the bolt is bent, both ends of the bolt are forced out of perpendicular to the path of the ultrasonic pulse.

A new solution - the Norbar USM-1 Instruments with simple user interfaces are a Norbar trademark.

The USM- 1employs Digital Signal Processing and sophisticated software built on thirty years of experience in this field.

The result is a quantum leap in ultrasonic tension measurement that overcomes many of the traditional difficulties.

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In particular: - · The selectable tone burst pulse system can send the maximum amount of energy to the ultrasonic transducer, allowing the broadest possible range of transducers for a given application.

· The low noise and automatic gain features of the receiver system allow signal detection and measurement in the most difficult applications.

· Automatic digital signal analysis optimises the measurement process and warns of potential problems.

· Adjustment of the tone burst pulser system optimises the amplitude of the ultrasonic signal.

This is accomplished by adjusting the number of synchronised impulses sent to the transducer.

It is important to select the correct transducer for an application, ensure that it seats correctly and that reflecting ends are perpendicular.

With reasonable care, though, we can safely say that ultrasonic tension measurement is a viable option in the factory today.

Modern electronics and software have dramatically reduced both operator training costs and measurement set-up time.

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Traditional torque measurement continues to be an excellent solution for many applications, but where greater accuracy is required or repeated measurements have to be taken, ultrasonics offers an ideal solution.

Norbar is committed to being at the forefront of developments in both these areas.

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