Rail Pad Elastic Modulus Classification and Track Vertical Stiffness Control Technology
What are the core control methods for the elastic modulus grading of under-rail pads?
The core control methods for the elastic modulus grading of under-rail pads are concentrated in three aspects: material formula adjustment, foaming process optimization and filler addition ratio control. The synergistic effect of the three can realize the precise gradient control of modulus from low to high. Material formula adjustment is the basic method. Low-modulus pads increase the proportion of natural rubber to more than 70% to improve elasticity; medium-modulus pads use natural rubber and styrene-butadiene rubber in a ratio of 6:4, taking into account elasticity and stiffness; high-modulus pads increase the proportion of styrene-butadiene rubber and cis-butadiene rubber to enhance structural stiffness. Foaming process optimization is for foamed pads. Low-modulus pads adopt a high foaming rate process with a foaming rate of 60%-70%. More internal pores result in lower modulus; high-modulus pads adopt a low foaming rate process with a foaming rate of 30%-40%. Fewer pores result in higher stiffness. Filler addition ratio control is an auxiliary method. Add carbon black, calcium carbonate and other fillers to the rubber base material. The filler proportion increases from 10%-15% of low modulus to 20%-30% of medium and high modulus. Fillers can improve the compressive strength of the pad, thereby increasing the elastic modulus. The precise matching of the three methods can control the elastic modulus of the pad within the design range of the corresponding grade with a deviation of no more than ±5MPa.
Why are under-rail pads with low elastic modulus preferred for high-speed lines?
High-speed lines have extremely high requirements for track smoothness, vibration and noise reduction. Low elastic modulus pads are preferred because of their excellent elastic buffering performance, which can accurately match the working condition requirements of high-speed lines. Low-modulus pads have strong elastic deformation ability. When trains pass at high speed, the pads can absorb the vertical impact energy of wheel and rail through their own large deformation, reduce the vertical vibration amplitude of the track, and improve the smoothness and comfort of train operation. Low-modulus pads can effectively attenuate the transmission of wheel-rail vibration to sleepers and track beds, reduce vibration noise, meet the noise control standards along high-speed lines, and reduce the impact on the surrounding environment. The wheel-rail contact stress of high-speed trains is more concentrated. The flexible contact of low-modulus pads can disperse the contact stress, reduce the contact fatigue damage of the rail head, and extend the service life of the rail. In addition, low-modulus pads can adapt to the seamless track structure of high-speed lines, provide a small vertical deformation space for the thermal expansion and contraction of the rail due to temperature, release part of the temperature stress, and avoid rail cracks caused by excessive stress. The vertical stiffness of low-modulus pads is low, which can make the overall vertical stiffness of the track softer and match the low stiffness requirement of high-speed trains for the track.
Why does the design of under-rail pads for heavy-haul lines need to take into account both high modulus and high compression resistance?
Heavy-haul lines bear the load characteristics of large axle weight and repeated rolling. The pad design takes into account both high modulus and high compression resistance to provide sufficient vertical support while avoiding plastic deformation of the pad and ensuring track stability. High-modulus pads have high vertical stiffness, which can provide stable vertical support for heavy-haul rails, resist the vertical pressure brought by large axle weight, prevent vertical subsidence of rails, ensure the smoothness of the rail surface, and avoid the impact of rail surface deviation on train operation. High compression resistance enables the pad to maintain structural integrity under the action of long-term repeated large loads without diseases such as compressive deformation and depression, ensuring the long-term stability of the elastic performance of the pad and avoiding sudden changes in the vertical stiffness of the track due to deformation. If the pad only has high modulus but insufficient compression resistance, it will undergo plastic deformation under long-term compression, the modulus will gradually decrease, and the vertical stiffness of the track will also decrease accordingly; if it only has high compression resistance but too low modulus, it cannot provide sufficient vertical support, and the rail is prone to subsidence. The pad design that takes both into account can keep the vertical stiffness of the track of heavy-haul lines stable for a long time, adapt to the harsh working conditions of large load and strong vibration, and reduce the replacement frequency of the pad.
How do the thickness and elastic modulus of under-rail pads coordinately control the vertical stiffness of the track?
The thickness and elastic modulus of under-rail pads are two core factors for controlling the vertical stiffness of the track. The two act synergistically through the way of "modulus setting the foundation and thickness fine-tuning" to realize the precise control of vertical stiffness. First, determine the elastic modulus grade of the pad according to the line working conditions: low modulus for ordinary-speed and high-speed lines, high modulus for heavy-haul lines. The modulus grade directly determines the basic value of the overall vertical stiffness of the track, with high modulus corresponding to high basic stiffness and low modulus corresponding to low basic stiffness. Then, under the same modulus grade, realize the fine-tuning of vertical stiffness by adjusting the pad thickness. For every 1mm increase in pad thickness, the vertical stiffness of the track will decrease by 3%-5%, and vice versa, the stiffness will increase when the thickness decreases. For example, when high-modulus pads are used in heavy-haul lines, if the basic stiffness is too high, the pad thickness can be appropriately increased to 12-15mm to reduce the local vertical stiffness and make the track stiffness distribution more uniform; when low-modulus pads are used in high-speed lines, if the basic stiffness is too low, the thickness can be appropriately reduced to 8-10mm to increase a small amount of vertical stiffness and ensure the supporting capacity. The coordinated control of the two must follow the principle of "modulus as the main and thickness as the auxiliary", and at the same time ensure that the pad thickness is within the design range to avoid pad instability due to excessive thickness and insufficient support due to excessive thinness.
What are the adaptation requirements of different track structures (ballasted/ballastless) for the elastic modulus of under-rail pads?
The structural characteristics of ballasted and ballastless tracks are significantly different, and the adaptation requirements for the elastic modulus of the pad are also completely different. The core is to match the pad modulus with the stiffness of the track structure to realize the balance of the overall stiffness. The track bed of ballasted track is composed of ballast, which has a certain elasticity and deformation capacity, and the overall stiffness is relatively low. Therefore, it is adapted to medium and low elastic modulus pads with a modulus controlled at 20-60MPa. The medium and low modulus pads can form coordination with the elasticity of the ballasted track bed, make the overall vertical stiffness of the track more uniform and avoid sudden stiffness changes. The track bed of ballastless track is an integral concrete structure with high rigidity and poor deformation capacity, and the overall stiffness is much higher than that of ballasted track, which needs to be adapted to the modulus according to working conditions: low-modulus pads (≤20MPa) for high-speed ballastless tracks, which make up for the rigidity of ballastless track bed through the elastic buffering of low-modulus pads; high-modulus pads (80-150MPa) for heavy-haul ballastless tracks, which can provide stable support, match with the high stiffness of ballastless track bed and ensure vertical supporting capacity. In addition, the pad modulus of ballasted track can be adjusted flexibly, while the ballastless track has higher precision requirements for the pad modulus, and the deviation must be controlled within ±3MPa to ensure the stability of track stiffness.