Rail-vehicle Buffers Absorber Test

Application: Transportation
Authors: Elsys AG, Schwab Verkehrstechnik AG
Date: May 2013
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Buffers on railway vehicles usually are seen as simple devices. Nowadays however, they should not only be absolutely reliable, but also must have optimal properties to absorb run-up energy from trailing wagons safely. Nowadays buffers on railway vehicles, as designed and utilized by Schwab Verkehrstechnik, AG, should be considered Hi-Tech components.

The absorption of the energy is achieved through hydraulics. It is clear that with sophisticated hydraulic absorption buffer systems, just static data collection will be insufficient to understand the operation of these modern bumpers.

The forces over the entire stroke (the distance the buffer can move) with hydraulic buffer systems, other than with spring-loaded buffers, under ideal circumstances will be constant. That way the smallest acceleration values will be realized. The force on the buffers will be proportional to the square of the impact speed. Ideally the buffers will decelerate vehicles entirely, before running into the end-stop at the end of the stroke.

The measurement set-up in Fig. 1 is able to record all dynamic parameters when wagons run up against each other. All eight signals required will be recorded simultaneously.

Railcar Absorber Test Block-Diagramm

Figure 1 - Block Diagram AbsorberTest

Two loaded freight wagons are pushed towards each other with speeds v1 and v2. The absorbed energy is easy to calculate with:


Kinetic Energy E = m1 * m2
–––––––
m1 + m2
* v2
––
2

where
m1 = Mass 1 (40'000 kg)
m2 = Mass 2 (40'000 kg)
v = Speed (v1 + v2)
The energy is distributed over both bumpers.

The freight wagons are moved with hydraulic actuators (not shown in the block diagram). The speeds v1 and v2 are more or less equal. With a Triangulation Laser System the sum of both speeds (v1 + v2) just before the impact are measured.

All measurement signals are acquired prior, during and after the impact and recorded as a function of time. Immediately after, or later on a separate PC, they are processed and archived with the TranAX analysis software. All calculations can be executed with TranAX. Graphs and measurement results can be directly transferred to test reports in EXCEL or WORD.

Since not only single measurement values are acquired, but signal curves with high temporal resolution, many interesting and detailed conclusions can be derived from analyzing the acquired data. This is of great importance for the further research and development of effective train bumpers.

A similar measurement set-up can also be deployed successfully in the manufacturing and quality control of rail car bumpers. This way norms and standard procedures can be easily adhered to and maintained.

Railcar Absorber Test - Recorder Pressure Signals

Figure 2 - Recorded Signals

In Fig. 2 all the important recorded signals are shown. Unwanted oscillations are clearly visible. These oscillations are present almost the entire time the hydraulic actuators are active. From that conclusions for improvements to the hydraulic energy absorption design can be drawn.

By zooming in (in the lower part of the screen) details can be observed more precisely. The pressure trajectory in the first buffers shows relative high peaks with short duration.

Through filtering (immediate performed by TranAX calculations) the oscillations on the signals can be reduced significantly. An X-Y diagram shows the units “Force” and “Displacement” plotted against each other for both bumpers. The energy generated by the controlled impact is represented by the plot area inside the curves. It is computed by TranAX by calculating the integral of the Force and Displacement signals.

Railcar Absorber Test - Measurement Results

Figure 3 - X-Y diagram shows the units “Force” and “Displacement” plotted against each other for both bumpers.

Figure 3 also shows that the acceleration in the buffers is relatively constant during approximately 160 msec and that no extreme values have occurred.

Irreversable absorption test at 30 km/h

Figure 4 - Irreversable absorption test at 30 km/h

Figure 4. shows another application of the measurement system. Hereby the impact effects on front couplings are dynamically tested. These front couplings from Schwab Verkehrstechnik AG function as reversible and irreversible energy absorbers. Run-up energy from small impacts up to 10 km/h is absorbed reversibly by hydraulics. Energy from faster run-up impacts is absorbed irreversibly by crash elements.

Simultaneous with recording the electronic measurement data a high speed video camera films the test impacts in real time. The with the TranAX analysis software equipped data acquisition system, is able to play back the measurement signals synchronously to the video recordings.

Position sensors used to measure displacement or distance, are too slow for fast impact events, therefore the video recordings are helpful to actually determine the velocity and displacement.

The principle working of hydraulic shock absorbers

Figure 5 - the principle working of hydraulic shock absorbers

Figure 5 shows the principle working of hydraulic shock absorbers. Part of the oil in the oil-container (1) when under pressure, moves through an orifice (4) and a multiplier valve (5) into oil-container (2) and pressurizes a gas-spring (3). The orifice and the pressure in the gas-spring determine the dynamic behavior of the shock absorber.

Bigger impacts (accidental collisions) up to 36 km/h activate a pull linkage facility that in turn gives the hydraulic shock absorber the maximum available stroke range. Then, when reaching the maximum stroke range consequently hitting the end-stop in the pull linkage system, the increased impact force activates a hydraulic predetermined breaking point. These break-points are necessary to fulfill the collision safety requirements and follow norms formulated for modern railway traffic.

Only short peak forces occur when activating these predetermined breaking points. With the TranAX analysis software, these short peak forces are reliably acquired and analyzed.