Patent Application
20060131437
This invention relates generally to load bearing structures, and specifically, to efficient load bearing structures and methods for manufacturing the same.
Load bearing structures such as railway ties or railway sleepers serve to transfer the rail loading from the wheel load, which is around 500 kN, to support the structure of the train and the railroad base, to facilitate in gauge maintenance, and to absorb vibrations imparted to the railway tracks, among other functions. Popular conventional materials for railway sleepers include concrete, steel and wood. Concrete is typically a very rigid material and therefore has poor shock absorption characteristics, while steel also suffers from poor vibration absorption characteristics, while use of wood is increasingly being discouraged because it results in depletion of natural resources. In fact, in many countries, policies discontinuing the use of wooden sleepers have been affected. Accordingly, there is a need in the industry for an alternative material for railway sleeper due to problems with conventional sleepers. For instance, polymeric railroad sleepers have emerged as a probable alternative. Some contemplated polymeric railway sleepers include recycled, reinforced plastic constituents, sandwich and hybrid concepts. These conceptualizations, however, suffer from a number of disadvantages, such as high weight, leading to an increase in material costs, and low strength to weight ratios, among others.
Further, the weight of the conventional railway sleepers ranges from 100 kg to 200 kg, and in general there exists a need for railway sleepers with reduced weight. There is also a need to improve the load bearing capacity, gauge maintenance and vibration characteristics of the alternate material railroad sleepers, and a need for efficiently manufacturing such railway sleepers.
Briefly, in accordance with one embodiment of the invention, there is provided a load bearing structure. The load bearing structure is configured to bear a load, the load bearing structure includes a number of cells and the load bearing structure has a length, a center, and a cell density. The cell density of the load bearing structure varies along the length. The load bearing structure has a mass of 6 kg or more.
In accordance with another embodiment of the invention, there is provided a method for manufacturing a load bearing structure. The method includes providing a mold configured for making the load bearing structure, injection molding a suitable composite material into the mold, and recovering the injection-molded load bearing structure.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
As noted, the present invention provides a load bearing structure for bearing a load.
According to an inventive aspect of the invention, the load bearing structure is non-uniformly composed to efficiently support the load. Specifically, the load bearing structure comprises multiple cells 14 (
In an embodiment, the cell density is configured to vary according to the loading of the load bearing structure 10. Such a configuration is illustrated by the plot of
In addition to performing the critical functions such as load bearing, gauge maintenance, weight and cost reductions, the material constituents for the railway sleeper may also be tailored for desired features. In an embodiment, rail fixtures such as sloped top surface, transverse rail supports, rail fasteners, bolt holders among others, may be integrated in the sleeper, thereby beneficially providing a consolidated parts feature. Parts consolidated at the time of manufacture advantageously eliminate the need to attach or fix those parts when the load bearing structure is put in service, reducing the need for labor and equipment at that time.
In an embodiment the load bearing structure, such as the railway sleeper 10, comprises a polymeric material. Polymeric materials include thermoplastics and thermosets, and combinations thereof. More specifically polymeric materials suitable for use in the load bearing structures of the present invention may be selected from materials such as polycarbonates, polyamides, olefin polymers, polyesters, polyestercarbonates, epoxides, polysulfones, polyethers, polyetherimides, polyimides, silicone polymers, phenol formaldehyde resins, mixtures of the foregoing polymers, copolymers of the foregoing polymers, and mixtures thereof.
In another embodiment, the load bearing structure comprises a composite material. The composite material typically includes an organic polymeric matrix with a filler material dispersed in the organic polymer matrix. Suitable materials for use as the organic polymeric matrix include thermoplastics, thermosets and combinations thereof. The organic polymeric matrix and the filler material are chosen to impart desired properties to the load bearing structure, such as decreased thermal dimensional variation (thermal expansion or contraction), high bearing strength, rigidity, and vibration damping characteristics, among others. Suitable filler materials include glass fibers, carbon fibers, polymeric fibers, natural fibers, and zeolites, among others. The load bearing structure may further be configured to comprise functional surfaces such as surfaces comprising a weatherable coating layer, chemical resistance coating, surfaces comprising an anti algae coating, surfaces comprising an anti slip coating, and combinations thereof. In other embodiments, the load bearing structure's polymeric material or filler material may impart the above functionalities.
Embodiments of the present invention utilizing surfaces and cells for performance enhancement and spatially varying cell density configurations in load bearing structure 10 have been described. The invention however is useful in other alternative configurations as well. For example, in an embodiment of the invention, the spatially varying cell density configuration can be custom designed for girder bridge supports. In yet another embodiment of the invention, the spatially varying cell density configuration can be custom designed for use in combination with ballasted tracks. In yet another embodiment of the invention, the spatially varying cell density configuration can be custom designed for use in combination with ballastless tracks. In one embodiment of the present invention, a load bearing structure comprises at least one organic polymeric matrix material and at least two reinforcement filler materials wherein a first reinforcement filler material has a negative thermal expansion coefficient (for example carbon fibers) and a second reinforcement filler has a positive thermal expansion coefficient (for example glass fibers). The two reinforcement fillers are present in amounts such that the overall thermal expansion coefficient is zero. The use of fillers having offsetting thermal expansion characteristics is useful in controlling the dimensional integrity of a structure, for example gauge maintenance in railroad applications.
Numerical Evaluation Section
A comparison of performance parameters of a conventional polymer railway sleeper (Comparative Example), and a railway sleeper comprising spatially varying cell density structure (Example 1), indicates the advantages brought forth by various embodiments discussed above. The following data was generated by simulating various Railway sleepers by forming a test mesh using Hypermesh™ software, and testing it for strength (maximum Von-Mises stress and maximum deflection) using ABAQUS™ software, and for manufacturing (Injection mold, moldability, productivity and shot capacity of machine) using Moldflow™ software. The term “example” as used herein will be understood to refer to a numerical simulation outcome, and not an actual physical test.
As can be seen from Table 1, the part volume and part weight values required to meet the engineering requirements for the load bearing structure of the Comparative Example as compared to Example 1 indicates a substantially higher volume and mass of material is required for a polymeric railway sleeper having a conventional design relative to the sleeper design of Example 1, which represents polymeric railway sleepers possessing spatially varying cell density.
The railway sleeper of Example 1 comprising a rib cell structure advantageously provides for an injection molding process that is a simple manufacturing process, simple injection molds, low tooling cost, easy moldability, high productivity and lower shot capacity machine, as compared to that of the Comparative Example. The volume and weight reduction in Example 1 is substantial, and accordingly the reduction in material costs is also substantial.
As can be seen from Table 1, the high part volume, and accordingly high part weight values are required to meet the engineering requirements for the load bearing structure of the Comparative Example as compared to Example 1, which is polymeric railway sleepers possessing spatially varying cell density. The railway sleeper of Example 1 comprising a rib cell structure provides for an advantageous injection molding process, requiring a low shot capacity of injection molding machine for Example 1, as compared to that of the Comparative Example. The advantage of weight reduction exhibited by Example 1 is substantial, as is the associated cost reduction. The weight reduction of Example 1 in comparison to conventional sleepers, such as those of wood is especially advantageous. A low von-Mises stress and a low deflection of the Example 1 in comparison to the conventional polymeric sleeper indicates a higher stability and toughness of the Example 1, and are further advantages explored by the present design provided by the invention. While preserving desired strength, and other desirable parameters such as vibration damping, the railway sleeper of Example 1 weighs about 24 kilograms, whereas conventional sleepers may weigh between 100 and 200 kilograms, and current polymeric sleepers may weigh up to about 50 kilograms. Additionally, ease of manufacture, which is an important factor, is also an advantageous aspect of the embodiment of Example 1. This is also illustrated by nearly half the shot capacity required for manufacturing the conventional polymeric sleeper. Other advantages pertaining to manufacture include use of injection molding process, simple injection molds, low tooling costs, high moldability and productivity as compared to the conventional polymeric sleeper. Therefore, the configurations illustrated in Example 1, as projected and compared with the Comparative Example are a substantial improvement over the existing art.
The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood by those skilled in the art, that variations and modifications can be effected within the spirit and scope of the invention.
