This final article completes the tour of the world of torque by delving into the subject of ultrasonic bolt measurement

In previous articles Norbar has examined the role of torque wrenches in the rail industry and their specific applications which range from fish-plate bolts through to bolts used on rolling stock. The articles have covered the different types of torque wrench available, from insulated wrenches through to electronic torque wrenches, like NorTronic, and their suitability for different rail applications.

Also covered was the use of torque multipliers, including hand torque multipliers and electric torque tools, such as EvoTorque, that are increasingly being used for bolted steelwork applications for rail infrastructure, along with locomotive and rolling stock manufacture and maintenance.

Most recently examined was measurement and calibration, specifically the importance of accurately applying torque in rail applications.

Finally ultrasonic bolt measurement is put in focus. Ultrasonic measurement provides a very precise method of determining the elongation of a bolt during the tightening process. Normally, there is enough tolerance for torque alone to be used as the means of assembly. In cases involving more critical applications consideration should be given to bolting friction and its variables.

Friction, both under the head, and in the threaded portion, absorbs the vast majority of the applied force. So, a small change in the friction causes a large change in the applied load and large changes in the load can lead to joint failure, which in real applications such as trackside bolting, can cause critical failure and the potential for fatal accidents.

Ultrasonic measurement is not effected by friction. When the elongation of a fastener is measured, the stress in the bolt is being calculated, which is directly proportional to load. However, there are many difficulties inherent in good ultrasonic technique, in order to achieve an accurate reading of the applied load.

How does ultrasonic measurement work?

Using material constants, ultrasonic measurement equipment, such as the Norbar USM-3, converts this TOF into an ‘ultrasonic length’ of the fastener, providing a baseline from which future measurements will be made.

Ultrasonic measurement of bolt preload is made possible by introducing a sonic pulse at one end of the fastener and accurately measuring the time of flight (TOF) required for the echo to return from the opposite end.


When the fastener is tightened, the TOF increases and the USM-3 will again utilise material constants to eliminate the effects of stress and temperature variations on sound velocity, providing an accurate elongation or load measurement.

The USM-3 uses state of the art hardware and software to achieve these measurements with maximum automation, and minimises the need for operator interpretation. Offering digital recording and transmission of data, in addition to analogue signal output the USM-3 provides a complete system for measurement, recording, and control of fastener tension in the most demanding applications.

Bolt preparation

In order to have a good ultrasonic measurement the right preparations must be made. The type of bolt can greatly change the scope of the job.

It is very important to have your bolt prepared as non-parallel or rough surfaces can cause the signal noise to rise. The transducer contact area determines the amount of signal coupled from the transducer to the bolt.

Surface preparation is very important. The optimum situation is to have an area prepared on the end of the bolt large enough for the transducer to sit perfectly flat. It is extremely important that the transducer does not rock or tilt during the measurement process.

Grade markings are a specific problem.

When grade marks are located on the outside perimeter, there is often room in the centre for the transducer. However, they may have to be ground off to allow transducer connectivity. Saw cut bolts and bolts that are sanded flat with a hand grinder typically have a surface not compatible with the transducers and are likely to provide poor results.

Couplant, the adhesive between the transducer and the bolt is also important. Norbar can offer advice regarding appropriate bolt preparation.


In any piece of measurement equipment, proper calibration is essential. Calibration of ultrasonic bolt load measurement equipment is a very complex subject, but it is important to understand. For many applications, the default factors are completely adequate, and no further calibration work is required.

For the highest precision, however, individual bolts must be calibrated, along with calibration of the transducer, velocity, stress factor and temperature factors. This requires the use of suitable tensile testing equipment.

Measuring the unloaded length

In order to measure the elongation of a bolt, it is necessary to measure the unloaded length. At first, it might seem that this step is unnecessary. Why not machine all the bolts to a known length and then only one ultrasonic measurement (of the stretched bolt) would be required?       Unfortunately, the variation in sound velocity from bolt to bolt, and in fact, from place to place on a single bolt, makes this unworkable. Instead, it is necessary to measure an unloaded length for each bolt, store that length, and then recall it as the baseline reference after the bolts are tightened.

Measuring the elongation

With the Norbar USM-3 a number of values other than the length of the bolt are stored. These include the gain and threshold settings, the amplitude of the peaks, and the frequency of the echo. All these settings are retained for each bolt, and when measuring elongation, the stored values are recalled and compared to the current bolt with reference to the stored baseline value.

A reading of the elongation can be taken placing the transducer back on the bolt, in the same location used to measure the unloaded length. Once it has been verified that the bolt group and temperature is correct and that the bolt number is the same, the measurement process can begin.

The USM-3 can produce reports by sending data via its USB port. These reports can be sent to a printer directly, to a computer using a communications port, or to a computer using the Sonic Bolt software. The report format is designed to be imported into an Excel spreadsheet software for creating graphs and data analysis.


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