DB Minden is testing optical strain gages from HBM
Many tests and inspections are mandatory for approval of new railway vehicles. These include test runs with wheelsets enabling the forces between wheel and rail to be measured. At present, DB Systemtechnik in Minden, Germany, an independent testing authority, explores the possibilities resulting from the use of optical strain gages, for example K-OL from HBM.
Both the vertical and the lateral force component between rail and wheels needs to be determined when taking wheel-rail contact force measurements. The forces effective between rail and wheel cause a deformation of the wheels and the axle that is measured with strain gages and converted online into force values using complex software. Conventional strain gages (SG) are currently being used for this purpose.
High installation effort
At the testing authority in Minden, strain gages are installed on a wheelset that comprises two wheels and the axle. This setup requires a total of 48 strain gages per wheel (Fig. 1). Signal lines are led into the hollow axle through a hole. The complete electronics including amplifiers and signal transmission is located at the end of the axle.
Installation of strain gages and signal lines is complex and takes several weeks per wheelset. The strain gages need to be individually installed and connected. Since the wheelset is subject to relatively high mechanical stress during the test runs, the signal lines, too, need to be completely bonded.
Test stand setup
When the strain gages, the signal lines and the electronics have been installed, the measuring wheelset needs to be calibrated. For this purpose, the complete wheelset is built into a specially developed test stand (Fig. 2). The cable entry holes from the areas of SG installation to the amplifier constitute a relevant stability influence of the measuring wheelset. The required effort to consistently ensure crack-free parts through ultrasonic tests is thus relatively high.
Another drawback of the test setup is that due to the place of installation the measurement electronics - with its rotating part installed in and its stationary part installed on the axle - is subject to the extremely high stress in the area of the wheelset.
Improvement through use of optical strain gages
DB Systemtechnik in Minden consistently works on optimizing and further developing the method to eliminate the above-described drawbacks of the measuring wheelsets. One approach that is currently being followed is to use optical strain gages like K-OL from HBM (Fig. 3).
Reduced installation effort
A major benefit when using optical strain gages is the significantly reduced installation effort. "Using optical strain gages would enable us to shorten installation times from four to five weeks at present to few days," explains Andreas Brodtka, head of production of measuring wheelsets at DB Systemtechnik. The reduced installation effort and less maintenance during the test runs would, of course, result in reduced costs. As the complex compensation for interference would not be needed anymore, 20 strain gages per wheel would be sufficient. "The fact that up to 20 optical strain gages are installed in a single fiber is of special importance", states the test engineer.
Installation of the signal lines without drilling
In addition, the thin glass fiber could be led to the exterior without any hole having to be drilled in the axle. A solution enabling laying of the optical fiber without any hole is currently being developed. In the area of transition from the rotating to the stationary part on the wheelset that will still be necessary, the signals transmitted by the optical fiber could be sent via a so-called optical slip-ring assembly. The interrogator evaluating the signals of the optical strain gages could then be located well-protected inside the vehicle and receive signals via optical fibers. The interference-prone electronics in the axle would no longer be needed.
At present, optical strain gages are being used at DB Systemtechnik in Minden only within the scope of a research project, but the test results show the potential of the optical strain gages.