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The power of 3

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How can rolling stock wheel diameter and profile be assessed quickly and accurately without removal of the wheelset? Tools have been available for some time for mapping the profile, but it’s a skilled job that takes time. Measuring the diameter has long been a problem, but now a new device is being used in UK depots that allows both of these important tasks to be accomplished in seconds – and it can do a lot more besides.

‘The power of 3,’ says the publicity strapline. ‘Behind the three laser lines lies the secret of Calipri.’ So what is Calipri and how does it work? The name didn’t give much away to your writer, and what’s this about three lasers? Well, lasers can be used to measure, so maybe it’s measuring in threes?

Of course – three dimensions! Measure the curvature of a circle and you can calculate its diameter. Design a device that can make that measurement from any angle and the result would be a rather useful tool. In developing Calipri, Austrian company NextSense GmbH has done precisely that.

Hot measurement

Based in Graz, NextSense produces a wide range of laser-based profile measurement devices for the automotive and steel industries. Being able to measure hot metal by using non-contact techniques is clearly advantageous! The development of those techniques for problem solving within the rail industry has been a logical step.

It was logical, too, that Mechan Limited should become the UK distributor for Calipri. Known worldwide for its range of vehicle lifting jacks, wheel and bogie drops and bogie testing machines, Mechan is keen to take on new products to complement its existing business.

Calipri is a truly portable laser scanner. For most applications, the sensor device can be hand held with the collected data being stored on a tablet PC that’s carried on a comfortable body strap. The gathered information can be downloaded at the end of the testing plan and software packages can be used to analyse and store the data.

Quick profiles

Wheel profile measurement is a key part of the wheelset inspection process and, by using Calipri, it can be accomplished within a few seconds. A testing plan that identifies the vehicle, the wheel profile type and wheelset identity is set up on the tablet PC and the sensor is then pointed at the wheel rim. The three central laser lines projected by the scanner can be seen on the wheel and the scanner is positioned so that these lines also intersect the centre of the wheel. Guiding the sensor in an arc around the wheel profile allows the required information to be gathered. Audible prompts and a visual display on the sensor head make this guiding process straightforward.

Measuring each wheel takes around 10 seconds, which is fast when compared to contact type measuring techniques. The small wheels associated with low floor tram vehicles are notoriously difficult to access, so a smaller version of the Calipri sensor head is available for this purpose.

Accuracy of measurement is not reliant upon operator skill. The user moves the sensor by hand over the object to be measured and the image processing system combines segments obtained from various viewing directions. In this way the measurements are free of operator influences. The results are shown on the sensor display, as well as on the tablet PC and comparisons can be made with the standard wheel profile.

Although the system cannot by itself locate wheel defects such as wheel tread flats or surface spalls, it can be used to assess the severity of such damage.

This measurement module can also be used in the wheel shop to determine the correct depth of cut required for re-profiling.

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Worried about wear?

It doesn’t stop at wheel profiles. Calipri is a multi-functional profile measurement device. By means of a modular concept it can also be used to measure wheel diameter, back- to-backs and brake disc wear.

In each of these cases the module package includes the necessary software and supplementary gauges. When measuring brake disc wear, for instance, a small magnetic gauge can be attached to the edge of the disc. The laser scans the gauge and brake disc simultaneously, with the gauge providing a reference of known width. This allows the software included with the module to calculate the disc thickness and degree of hollowness of the disc faces.

The diameter of a rail vehicle wheel is a critical wear parameter and the Calipri wheel diameter module can be used to measure it very accurately. A carbon fibre gauge is attached to the wheel. The size of the gauge has to be selected according to the nominal diameter of the wheel to be measured – three gauge sizes are available.

Two placement cylinders have to touch the rolling circle anywhere on its circumference and a magnet is switched on to retain the gauge in position. The gauge has two measurement cylinders that act as reference points and these are scanned by the three laser lines at the same time as the wheel tread. If the wheel profile has been recorded, the software can then calculate the diameter of the rolling circle.

The ovality and eccentricity of rail vehicle wheels (radial and axial runout) can also be assessed. Again, these are key maintenance tests that can be undertaken quickly using the appropriate Calipri module. A magnetic wedge is attached to the inside of the wheel flange and the laser scanner is supported on a tripod stand. As the wheel is rotated the trigger wedge provides the start and end reference points for the scan. Once the wheel rotation test is completed, graphs can be displayed to show the degree of axial and radial runout.

Back-to-back (wheel clearance) measurements are also very simple using Calipri. A two-part telescopic gauge is placed on the rails and then extended so that contact points on the gauge meet the back of each wheel flange. The sensor is then used to scan two reference cylinders on the gauge and the software can then calculate the wheelset back-to- back measurement.

Infrastructure too

As well as wheelset measurements, Calipri can be used in the analysis of rail and switch profiles. The procedure is very similar to the technique used for measuring wheel profile. Comparison with standard rail profiles contained within the software gives a very accurate indication of rail wear.

When particularly accurate measurements are required, a geometry module can be clamped to the rail, guaranteeing the measurement of an exactly defined cross section, including conic objects such as crossings and switch rails.

Two software packages are available – ‘Analyzer’ for extended analysis of the recorded profile curve and ‘Explorer’ for more detailed processing of downloaded data. Analyzer allows a profile to be visualised, measured and compared with the relevant standard reference profile. Explorer takes things further by providing structured management, storage and analysis of measurement data.

Wear limits, tolerances and historical runs are documented. This allows the remaining life of an asset to be predicted and maintenance intervals to be tailored accordingly. NextSense can also provide powerful ‘Wheel-Rail analysis’ software that can be used to examine in great detail the wheel/rail kinematics and evaluate wear parameters.

Calipri has been exported by NextSense to many rail operators throughout the world. It has been formally approved by DB in Germany and by OBB in Austria, where it will be used at all depots. SNCF too will use Calipri throughout France.

In the UK, Calipri is now in use with South Eastern Trains, Siemens and Bombardier. Network Rail also has one system for assessing its on-track plant and the RAIB also has two systems, together with a suite of Vampire modelling software.