Introduction

The coal mining activities carried out in England in the 17th century gave rise to the railroads, where the first transports used animal traction in rail-oriented vehicles. In addition, the invention of the steam engine led to significant developments in the railway infrastructure, creating the first prototype locomotives and enabling mechanical traction in freight wagons.

The first railroads had their tracks supported on rigid blocks and, in some cases, directly on the ground. However, this construction model presented several problems in terms of track durability. In order to remedy these problems, the engineers of the time realized that more resilient supports were needed, capable of absorbing the impacts of operation.

In order to improve the resilience of the infrastructure elements of the permanent way, the need for a sleeper and ballast set was defined. For a material to be used as ballast, it must have characteristics such as:

• Resilience, adequate rigidity and the ability to distribute operating stresses;
• Ability to withstand and provide stability for lateral and longitudinal stresses and to maintain the geometry of the road;
• Capacity to drain rainwater.

The importance of these elements in the railway infrastructure is therefore evident, and care must be taken both with their correct dimensioning and with their maintenance during operation.

The passage of wagons and passenger cars generates longitudinal, lateral and vertical stresses due to wheel-rail contact, thus transferring loads to the sleepers and ballast of the permanent way. These loads cause the ballast to deteriorate, generating thinner elements and changing the track modulus. Knowing the loads helps define maintenance parameters.

Development

The definition of the vertical loads acting on the permanent way is calculated statically, using the weight of the wagon and its components plus its material load. However, there are the dynamic movements of the wagon - yaw, pitch and roll. Each of these movements is characterized by rotations around the 3 axes of the wagon and occur due to the transfer of mass from the damping system and the railway dynamics itself, decreasing or increasing the loads on the wheels as shown in Figure 1 and Figure 2.

Figure 1: Yaw, pitch and roll rotation axes.

Figure 2: Reduction/increase in wheel load due to dynamic effects of the wagon.

This change in load, also known as the dynamic amplification factor (_Fd), is defined as the ratio between the dynamic load (_Cd) and the static load (_Ce), as shown in the following equation.

These load values can vary along the railroad due to the degradation of the existing ballast, geotechnical factors in the region and changes in the system's rigidity, such as the entry and exit of railroad bridges, defects in the track and rolling stock. In order to obtain the real values of the amplification coefficients, it is possible to carry out an experimental study to collect data on site along the railroad. Check out one of Kot's projects below!

Case de sucesso: Determinação do fator de amplificação dinâmico de uma ferrovia

In order to collect data on the dynamic amplification factors during the passage of railway trains over the track, strain gauges were installed on the rail, as shown in Figure 3.

Figure 3: Instrumentation points.

The data acquisition made it possible to verify the deformation of the rail caused by the vertical loads due to the passage of each wheel of the train, as shown in Figure 4. With this data, a finite element analysis can be carried out and the loads applied to verify the sleepers and ballast in the region analyzed.

Figure 4: Typical measured deformation profile of the 4 wheels of a wagon.

To define the static loads, a loaded train passed the instrumented site at a speed of 5 km/h. From this data, the load values on the rails of 20 wheels were collected to obtain the average static stresses in order to remove possible effects of load unbalance, wheel defects, among others.

After defining the static loads, the train was measured at the operating speeds on the stretch to collect data on the dynamic factor of the track in question.

Using statistical calculations, the upper control limits and the average of the data were defined in order to construct the graph in Figure 5, shown below.

Figure 5: Dynamic factor values obtained during the study.

Conclusion

Data collection made it possible to verify the real loads acting on the permanent track components. The highest load amplification factor found on the section evaluated was 1.3, which is in line with the literature (see Figure 6) for speeds of up to 60 km/h, as adopted by Brazilian railroads.

Figure 6: Dynamic amplification factors x speed according to international standards.

Dynamic factors lower than 1 are expected and normal, and do not necessarily correspond to unrealistic values. The wagons have various rigid body modes, such as pitch and roll movements, in addition to the impact loads acting on the couplings, which cause excitations, resulting in variations in the loads acting on the bogies.

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