Vibration insulating systems for high-speed railways using polyurethane elastomers.

Vibration insulating systems for high-speed railways using polyurethane elastomers

Following the success of the development of a vibration insulator for the Danchoku track (ref. 1), the development of a resilient tie for ballasted track was demanded with the object of saving the maintenance of the track, and reducing vibration and noise. This article presents the process of the development and application of the vibration insulator railways, namely a resilient tie for ballasted track, by covering the bottom and sides of the concrete tie with resilient material.

Over ten thousand of the resilient ties we have developed have already been laid on the Tokaido and Sanyo Shinkasen and conventional railways.

Directions of development

The resilient tie was proposed (ref. 2) as shown in figure 1, to satisfy the following requirements:

* cost saving by decreasing the amount of resilient material used;

* increasing manufacturing efficiency and adhesion strength by applying the RIM system (ref. 1);

* preventing peeling of the vibration insulating material from the concrete tie;

* preventing wear of ballast.

Development of the vibration insulator

Shape design of a vibration insulator In order to save cost, the amount of resilient material used was decreased as much as possible by limiting its use in the bottom area, which was entirely covered with resilient material, up to a minimum height on the sides and edges (65-110 mm from the bottom) that were covered, as shown in figure 1.

Optimum thicknesses of the vibration insulator were studied to obtained vibration insulation and durability simulatenously, as well as cost savings, and they were determined to be 15mm for the bottom and 10mm for the sides. With these thicknesses, the amount of resilient material used for the vibration insulator was greatly reduced to one-third of the Danchoku type.

Spring constant The objective value of practical spring constant was determined to be under 40 MN/m (on ballast, static measurement) corresponding to a value of under 10 MN/m measured for the test piece (100x100x15 mm)

Vibration insulator Development of a vibration insulator proceeded according to the following four approaches:

* Filler-rich casting system - A system in which low cost fillers such as rubber powder reclaimed from waste tires are added in high concentrations to a polyurethane casting elastomer.

* R-RIM system - Glass fiber and rock fiber are used as reinforcing materials.

* RIM system with reinforcing net - RIM system in which reinforcing fiber net is set on the surface layer of the insulator bottom.

* Preform system - A system separately mass-producing vibration insulator by the RIM system and then combining it with the concrete body by means of adhesion.

These four approaches were examined simulatenously. During the development, design of material, trial manufacture of prototypes and evaluations were repeated while creating and applying new evaluating methods such as the "high temperature corner indentation test" for more accurate evaluation of mechanical durability against stress concentration caused by sharp edges on ballast.

With total evaluation of vibration insulating effect, durability, mass-productivity and cost performance, low-foaming polyurethane elastomer UF-OT (MDI, polyether polyol) using polyester net as a reinforcing material has been developed.

Molecular design of polyurethane elastomer

Low-foaming polyurethane elastomer UF-OT was developed with the object of high strength and high durability with a static spring constant of under 10 MN/m on the test piece. Taking mass production into consideration, expensive raw materials such as NDI were avoided. Static spring constant was adjusted by controlling the molecular weight of polyether polyol, the content of chain extenders and the density of the product. Table 1 shows the basic formulation.

Table : Table 1 - basic formulation

Polyether triol      MW 400-5000
Chain extender       (EG or 1.4-BDO)

Amine catalysts Water Silicone surfactant

MDI prepolymer NCO content 5-20%

This study indicated that the larger molecular weight of polyether polyol resulted in a lower static spring constant but in lower durability. On the other hand, its smaller molecular weight increased durability but unnecessarily increased spring constant. Figure 2 shows the tendencies that the higher the content of 1,4-BDO, a main chain extender used for the study, the higher the static spring constant; the lower the content, the lower the spring constant; and higher content of 1,4-BDO which means lower content of polyether polyol, increased strength but brought a worse result for compression set, probably because the concentration of the crosslinking decreased. The relationship between density of the product and static spring constant is shown in figure 3.

Based on the above study, UF-OT has been developed as the material with composition satisfying required property such as strength, durability and static spring constant. Polyester net was used as a reinforcing material inserted into the surface layer of the insulator to prevent cracks caused by ballast contact.

Mechanical properties and durability of UF-OT

The results of evaluation tests on mechanical properties and durability of UF-OT using specimens collected from actual resilient ties for the Shinkansen are shown in table 2. [Tabular Data Omitted]

The static spring constant of UF-OT almost met the objective value and was stable with only a slight fluctuation of around +1.5 even after the heat aging. The spring constant of resilient tie (real size) was measured on ballast and the value was 20-30 MN/m for each rail fastening. Figure 4 indicates the fluctuation of the dynamic spring constants at different frequencies; the measurements were preformed within similar load range as that for static spring constants. The measured value at 0 Hz shown in figure 4 is the static spring constant. The dynamic spring constants of UF-OT tend to increase with an increase in excitation frequency to less extent than normal vulcanized (solid) rubber because the loss factor tan [delta] of UF-OT is about 0.1, which is less than a half of that of normal vulcanized (solid) rubber. When vibration insulators are used in order to decrease rail supporting spring constants, it is quite important that the dependency of dynamic spring constants on frequency be minor, because it means less fluctuation of rail supporting spring constants caused by different train speeds.

As the resilient ties are laid on ballast and used under heavy train load, vibration insulators are required of high strength in the surface layer, enough to withstand the pressure of ballast sharp edges. To evaluate the strength of the surface layer, a corner of a cube of which edges and a vertex was processed to be 1R was put into the specimen for depth of 7 mm and the conditions of the surface were observed after 24 hours of aging at 70 [degrees] C. The result was that a depression about 1 mm in depth remained but cracks were prevented from progressing by the polyester net inserted into the surface as reinforcing material so the insulator maintained a high enough strength. In addition, a compression set test according to ASTM standards indicated satisfactory results that only a very slight decrease in thickness of about 1.3% was measured.

Thermal resistance, water resistance and alkali resistance of UF-OT were evaluated through tensile and elongation tests using dumbbell specimens. The results of the tests are shown in table 2 and figure 5. As UF-OT did not change in tensile strength nor elongation and slightly decreased in tear resistance after heat aging, UF-OT was found to clearly have enough heat resistance. Although it showed about a 10% decrease in tensile strength after immersion in water and alkaline solution, it was allowable for practical use as the surface skin layer would prevent liquid immersion to some extent. [Tabular Data Omitted]

Manufacturing process of resilient ties for ballasted track

For molding of the UF-OT used for resilient ties, the RIM system was employed to integratedly mold it to the concrete tie. Various primers such as synthetic rubber, epoxy resin, silane resin, acrylic resin and urethane resin were compared for the strongest adhesion between polyurethane and concrete, and among them a one-component polyurethane primer was selected. It was also found that satisfactory adhesion strength could be obtained by removing laitance on the concrete surface and controlling the water content of the surface.

Results of a field test on the Shinkansen track

The following results were found from the field test of resilient ties for ballasted track within a block of the ballasted track on a viaduct of the Shinkansen.

* With the resilient ties for ballasted track, rail supporting spring constants decreased to 10-20 MN/m for each rail fastening.

* Vibration acceleration level of the track induced by the Shinkansen decreased by about 7 dB, and noise level behind the viaduct decreased by 1-2 dB.

* Acceleration of sinking of ballast induced by the train vibration was retarded.

* As the progression of track misalignment was reduced by one-quarter, track maintenance work was also reduced.

* Vibration insulators were confirmed to have satisfactory durability of practical use because no cracks, peeling and changes in hardness were found even after five years of field tests on the Shinkansen.

Conclusions

In order to reduce structural vibration and noise induced by high speed trains, we have developed the new vibration insulator made of low-foaming polyurethane elastomer (UF-OT). The insulator must reduce the transmission of vibration between a running train and the under structure including the track, and to bear up under dynamic train loads and other conditions over long periods. Therefore, a key point of the development is how to decrease the spring constant while keeping the mechanical properties and durability.

As compared with vulcanized (solid) rubber one, this insulator, consisting of an outside skin and inner foam, has the following advantages:

* The spring constant is low enough to control the vibration under heavy compressive load.

* It has good durability against dynamic compression fatigue caused by ballast edge under degradative conditions.

* The dynamic modulus becomes nearly independent of frequency ( 30Hz). Therefore, the track modulus is not affected by train speed.

Results of the running tests on the Shinkansen track prove that the vibration insulator gives a 7 dB decrease in the vibration and a 1-2 dB decrease in the noise. It is clear that the vibration insulator saves track maintenance cost by the retardation of ballast sinking. It is quite effective to control the vibration and noise along high-speed railways.

References

1. Sato, Y., F. Ohishi, et al. 1982. Quarterly Report, 23(2): 58-63. 2. Miura, S. F. Ohishi, et al. 1990. RTRI Report, 4 (5): 9-17.

PHOTO : Figure 1 - resilient tie for ballasted track

PHOTO : Figure 2 - effect of content of 1,4-BDO on static spring constant

PHOTO : Figure 3 - effect of foam density on static spring constant

PHOTO : Figure 4 - frequency dependence of dynamic spring constant

PHOTO : Figure 5 - changes in tensile strength after the treatments