Aging and replacement standards for rail pads
- What are the main manifestations of under-rail pad aging?
One of the main manifestations of aging of the pad under the rail is the decrease in elasticity. The pad, which originally had good elasticity, has a reduced elastic modulus under long-term load and environmental effects, and cannot effectively absorb train vibration, resulting in increased track vibration. It is common for cracks to appear on the surface. In the early stage, they may be small cracks. As aging intensifies, the cracks will gradually expand, increase, and even break, affecting the integrity and bearing capacity of the pad. The pad will show obvious hardening or softening. Rubber pads may become hard and brittle after aging, while some polymer pads may become soft and sticky due to reasons such as the volatilization of plasticizers. Both situations will cause their performance to deviate from the design requirements. Changes in size, such as shrinkage or expansion, will increase the matching gap between the pad and the sleeper and rail, making it impossible to fit tightly, affecting the buffering effect and track stability. In addition, aging may also cause the color of the pad to change, such as fading and darkening, and may be accompanied by surface powdering and peeling.
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What factors accelerate the aging of under-rail pads?
High temperature is an important factor. In hot areas or under long-term sunlight exposure, the temperature of the under-rail pad rises, accelerating the oxidation reaction of the material, leading to the breakdown of molecular chains and reducing the service life. Ultraviolet radiation in sunlight can also cause photochemical reactions in the pad material, destroying the chemical structure of the material and causing aging such as cracking and hardening. In a humid environment, especially in rainy areas or areas with high groundwater levels, the under-rail pad is in a humid state for a long time, which can easily cause hydrolysis of the material. If the environment is corrosive (such as containing salt, acid, alkali, etc.), it will further accelerate the corrosion and aging of the pad. The long-term alternating load of the train causes fatigue damage to the under-rail pad. The repeated deformation and recovery process makes the internal structure of the material fatigue, and gradually loses its original performance, which is more obvious in heavy-haul railways and high-speed railways with large loads.
- How to formulate a reasonable replacement standard for under-rail pads?
The replacement standard should be formulated based on the performance parameters of the under-rail pad. When the elastic modulus of the pad deviates from the design value by more than a certain range (such as 20%), and the vibration absorption capacity is significantly reduced, it should be replaced. The degree of surface damage is an important basis. If the area of cracks on the pad surface exceeds 10% of the total area, or there are obvious fractures, it must be replaced. The change in size is also a consideration. When the shrinkage or expansion of the pad causes the fitting gap with the rail or sleeper to exceed 3mm, affecting the normal function, replacement is required. Combining the service life of the pad, according to the material type and use environment, the reference service life is formulated. For example, the service life of rubber pads in general environments is about 8-10 years, and they should be inspected in advance and replaced if necessary. The replacement standard should also consider the actual operation of the railway. If the track vibration, noise, etc. exceed the specified limit due to the aging of the pad, it should be replaced in time regardless of other indicator
- What are the key points of the under-rail pad replacement process?
Before replacement, the track status should be surveyed to determine the model, specification, and quantity of the under-rail pads that need to be replaced, and prepare the corresponding replacement materials and tools. When removing the old pad, the rail should be properly supported to avoid excessive displacement of the rail, which may affect the track geometry. The old pad should be completely removed to avoid residual debris affecting the installation accuracy and performance of the new pad. When installing the new pad, ensure that the pad is in the correct position, closely fits the sleeper and rail, and there is no displacement or warping. Positioning tools can be used to assist installation to ensure that the center line of the pad is consistent with the center line of the rail. During the installation process, avoid scratches and pollution on the surface of the pad to prevent affecting its performance. After replacement, the track geometric parameters such as gauge and level need to be re-measured and adjusted to meet the standard requirements. Finally, a trial run is conducted to observe the vibration and noise when the train passes, and confirm that the performance of the replaced pad meets the standard.
- How to detect the aging degree of under-rail pads?
A rebound tester is used to detect the elasticity of the under-rail pad. By measuring the rebound value of the pad after being hit and comparing it with the rebound value of the new pad, the more the rebound value decreases, the more obvious the decrease in elasticity and the more serious the aging degree. Ultrasonic testing equipment is used to emit ultrasonic waves to the pad. According to the propagation speed and reflection of the ultrasonic waves, judge whether the internal structure of the pad has changed, such as whether there are internal cracks and looseness caused by aging. Measure the size of the pad and compare it with the original size to calculate the shrinkage or expansion rate. When the shrinkage or expansion rate exceeds 5%, it indicates that the aging degree has affected its normal use. Observe the surface state of the pad, record the number, length, area of cracks as well as hardening, softening, chalking and other conditions, and grade according to the established aging grade standards to determine the aging degree. In addition, load tests can be conducted to simulate the train load, measure the deformation of the pad under a certain load, and excessive deformation indicates that the pad is seriously aged and its bearing capacity is reduced.