Strain gauge measurements on the check rail system of railway switches

By Roar Andreassen, Rune Nilsen. Norut Narvik

When a train runs on the diverting track of a switch, the wheels on the outer rail will pass the gap in the crossing plate (the frog). During this phase, the guiding of the axles will be done by the check rail (guard rail). The load from a train acts in a complicated way when the train passes a switch because the forces from jaw movements and the guiding from the check rail appear in combination with the gravitational forces. Scaling strain gauges by applying known lateral forces on the check rail when a real train is moving through the switch will be quite complicated.

Norut Narvik is developing a method to assess the nature of these forces by means of strain gauge measurements in combination with FE simulation.

In check rail project 1 (LKAB iron ore railway station) strain gauges was attached to two of the support brackets and to three positions of the check rail, see Figure 1. The trains passed the switch at low speed with some variations (5 – 15 km/hr).

Figure 1. Monitored location. S1 and S2 on the check rail. L2A, L2B, L3A and L3B on brackets. Bracket L3 rests on the same sleeper as the crossing (the frog).

Data were recorded using a Campbell Scientific CR1000 logger and a GSM link to a web server.

To obtain consistent strain readings, the FE model of the check rail components were used to find locations with smoothly distributed strains. The check rail itself behaves as a beam, but in a complicated manner because of its many and relatively stiff supports. The brackets however, seemed more promising, with steady compressive strain distribution along its back edges, see Figure 2 and Figure 3.

The logging was done at 20 Hz. It was also found that the frequency of 20 Hz was too low to capture the peak strains in the check rail, but sufficiently to reliably determinate the compressive peak strains on the back side of the brackets, as long as the train velocity was low (5 – 15 km/hr).

Figure 2. Choosing the location for the strain gauge. Right: strain results from FE model when 72 kN was applied to the bracket model.

Figure 3. Attaching the strain gauges

Calculation of the forces based on strain measurements

When there are only small deformations of the switch components, and when the strains are in the elastic range, the forces will be proportional with the measured strains. Hence, a gain factor for calculating force from measured strains can be derived from the FE model.

The estimated maximum force found on bracket L3 was estimated to 72 kN. The average estimated force on L3 was 46 kN with standard deviation 10 kN.

These results were used to calculate the expected fatigue life of the brackets.

The effect of train velocity

Among railway technicians there are various theories weather the forces on a check rail will depend on the train velocity in the slow velocity range or not.

The results from 10 train passages showed that the average peak forces (measured as strains) did not depend on the train velocity, see Figure 4.

Figure 4. The effect of train velocity in the range 5 – 15 km/hr.

Further projects

The check rail project 2 (Bane NOR, Norwegian national railway infrastructure) is now running. It is monitoring one of the first switches when the trains enter the Narvik railway station. The main objective is to assess the effect of increased axle load of the heavy haul iron ore trains. The FE model is refined to include the effect of sleepers and the elastic foundation.  Temporary results and method details will be presented on the IHHA Narvik 2019 conference.

This video is a FE simulation showing the details when a train wheel runs along a 3 meter long rail and imposes a force on the check rail. The force on the check rail appears after 300 mm from the start of the check rail and disappears 300 mm before the end of it. The weight of the train wheel is 150 kN and the lateral force is 60 kN. The deflection is about 1 mm, but is exaggerated in the video.

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