Tamping tine and ballast: teaming up
When comparing measurements of new and worn ballast, the following behaviour can be observed: when the tamping tine comes into contact with the ballast, and it has to overcome the resistance that results in further compaction, the tamping force noticeably increases. Figures illustrating the behaviour of worn ballast demonstrate this. The tamping force reaches its maximum immediately before the start of the unloading phase, before deflection begins. In this phase, the tine subsequently loses contact with the ballast and withdraws from the ballast prior to the start of the next cycle.
In contrast, ballast in good condition behaves differently: the tamping tine hits ballast much earlier, and the tamping force reaches its maximum after a moderate increase. The tamping force decreases if the tamping tine's speed decreases; the tamping tine continues moving towards the sleeper that needs to be tamped. A deflection movement cannot be observed. Unlike worn ballast, the consequence of the unloading response is negative values. In a subsequent cycle, the tamping tine tends to come into contact with the ballast earlier. This indicates a plastic deformation of the new ballast resulting from mechanical stress.
Thanks to a brand new measurement and analysis method, it is now possible for the first time to observe the movement behaviour of ballast throughout the entire tamping process. In addition, the method provides an unprecedented level of detailed insight into the ballast condition. The tamping tine is particularly important here: the new method allows it to function as both a working and measurement tool.
To determine the ballast bed condition, researchers used the findings from the TAMP research project in combination with the installed measurement technology and the algorithm developed for evaluation. This makes it possible to identify the perfect timing for ballast bed cleaning or renewal. Additionally, the working parameters are to be adapted to the respective ballast bed condition in the future, thus achieving a further increase in tamping quality. Finally, this system considerably increases the quality of infrastructure systems and reduces life-cycle costs.
Basic principle: phases of the tamping process
Tamping tines and ballast form an interaction system during tamping. The tamping process is divided into three phases: ballast penetration (using the tamping tines), the squeezing movement and lifting of the tines, and relocating the tamping unit to the next sleeper. By dividing the tamping process into separate phases, it is possible to evaluate the energy required for each phase. The squeezing movement is of particular importance. The squeezing process begins a few moments after the final tamping depth has been reached and represents the main ballast compaction process.
A mechanical model of the interaction between tamping tine and ballast has been developed in addition to prior experiments.