Knowledge of Rail Pad Aging Inspection and Replacement
How to detect under-rail base plate aging on-site with simple methods?
Elasticity can be judged by a pressing test: press the base plate surface hard with fingers. If the rebound is slow and obvious indentation remains after 3 seconds, it indicates that the elasticity has attenuated and aging has occurred. Visual inspection can check for cracks on the surface; cracks longer than 5mm or accounting for more than 10% of the area are moderate aging, and through cracks are severe aging. Measure the thickness with calipers; if the thickness decreases by more than 10% compared with the original value, it means severe wear and aging and needs to be replaced. Tap the base plate lightly to listen to the sound: aged base plates make a crisp "ding-ding" sound due to increased hardness, while new ones make a dull "dong-dong" sound. Combined with surface color change, rubber base plates changing from light yellow to dark brown can also initially judge aging.
What are the differences in hardness aging standards for under-rail base plates of different materials?
The standard hardness of rubber base plates is Shore A 60-70 degrees, and replacement is required if the hardness changes by more than ±15 degrees after aging (i.e., below 45 degrees or above 85 degrees). The standard hardness of HDPE base plates is Shore D 60-70 degrees, and aging is indicated when the hardness is above 80 degrees or below 50 degrees. The standard hardness of the surface rubber layer of composite base plates is Shore A 55-65 degrees, and the bottom HDPE layer is Shore D 65-75 degrees. Any layer with excessive hardness is judged as aging. The core reason for hardness change is the degradation of the internal structure of the material, such as the loss of plasticizer in rubber or the breakage of molecular chains in HDPE. During testing, 5 test points should be evenly selected on the base plate surface, and the average value should be used as the judgment basis to avoid local errors.
What are the specific steps for the elastic recovery rate test of under-rail base plates?
First, select a 100mm×100mm base plate sample to ensure there are no obvious cracks or damages. Apply 50% of the rated load (50KN for rubber base plates, 80KN for HDPE base plates) with a pressure testing machine and hold for 30 minutes. After unloading, let it stand for 1 hour and accurately measure the thickness change of the sample. The elastic recovery rate is calculated as (thickness after unloading - thickness after loading)/(original thickness - thickness after loading) × 100%. Rubber base plates with an elastic recovery rate ≥80% are qualified, HDPE base plates ≥75% are qualified, and composite base plates ≥78% are qualified. Those below the standard need to be replaced.
What are the differences in the replacement cycles of under-rail base plates under different service environments?
In dry inland areas with weak environmental corrosion, the replacement cycle of rubber base plates is 8-10 years, HDPE base plates 15-20 years, and composite base plates 12-15 years. In coastal humid areas with high humidity and salt content, the replacement cycle of rubber base plates is shortened to 4-6 years, HDPE base plates 10-15 years, and composite base plates 8-12 years. In high-altitude areas with strong UV radiation, EPDM rubber base plates have a replacement cycle of up to 15-20 years due to strong weather resistance, while ordinary rubber base plates need to be replaced every 6-8 years. Due to long-term high pressure, polyurethane base plates on heavy-haul railways have a replacement cycle of 5-8 years, which is about 30% shorter than that of ordinary lines. Urban rail transit has large passenger flow and frequent vibrations, so the base plate replacement cycle is usually 20% shorter than that of conventional railways.
After cracks appear on the under-rail base plate, how to judge whether it needs to be replaced?
If the crack length exceeds 6mm or the width exceeds 0.3mm, it cannot be repaired and must be replaced directly; otherwise, it will affect the shock absorption effect and bearing safety. Cracks with length ≤6mm and width ≤0.3mm can be repaired, and the repaired base plate can still maintain about 80% of the original service life. If the crack is located at the edge of the base plate or in a stress-concentrated area, it is recommended to replace it even if the size is small, as cracks in this area are prone to rapid expansion. Heavy-load or high-speed lines have higher requirements for the integrity of the base plate, and cracks longer than 3mm need to be replaced to avoid excessive track vibration. The repaired base plate needs to be re-inspected for compressive strength and elastic recovery rate to ensure it meets the usage standards.