Load bearing structure and method of manufacture thereof

Abstract

A load bearing structure configured to bear a load, the structure comprising a multiple cells. The load bearing structure has a length, a center, and a cell density, which varies at least along the length of the load bearing structure, which weighs at least 6 kg.

Description

BACKGROUND

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.

BRIEF DESCRIPTION

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.

DRAWINGS

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:

FIG. 1 is a front elevation perspective view of a load bearing structure in accordance with an embodiment of the invention.

FIG. 2 is a top perspective view of the load bearing structure of FIG. 1.

FIG. 3 is a bottom perspective view of the load bearing structure of FIG. 1.

FIG. 4 is a plot illustrating a load on the load bearing structure and cell density versus the length of the load bearing structure of FIG. 1.

FIG. 5 is front view of a load bearing structure with consolidated parts in accordance with another embodiment of the invention.

FIG. 6 illustrates a method for manufacturing a load bearing structure in accordance with an embodiment of the invention.

DETAILED DESCRIPTION

As noted, the present invention provides a load bearing structure for bearing a load. FIGS. 1-3 illustrate such a load bearing structure, for example, a railway sleeper 10 in accordance with one embodiment of the invention. The railway sleeper 10 holds and supports the rails, bearing a load P at localized zones in the railway sleeper, illustrated in FIG. 1 as equally distributed (P/2 for each rail). The load P typically represents the load of railway vehicles imparted to the sleeper 10, typically through two rails (not shown). The railway sleeper 10 may be supported by girders 12 as shown, or other alternate supports such as those employed in ballast tracks, or other alternate supports such as those employed in ballastless tracks may be used. Critical parameters for railway sleeper performance include load bearing capacity or bearing stiffness, and dimensional stability or gauge maintenance. Importantly, vibration absorption characteristics of the railroad sleeper 10 translate into passenger comfort through reduction in noise and vibrations. Other important considerations include the weight of the railway sleeper, its cost, and the cost and complexity of additional fixtures required for facilitating load bearing, such as rail fasteners. An aspect of the invention resides in providing a railway sleeper or a load bearing structure that offers improvement in these parameters.

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 (FIG. 3) having walls, within the railway sleeper 10 body, the cells 14 configured to bear loads according to the spatial loading requirements of the railway sleeper 10. The cells 14 may proceed from top of the sleeper 10 to the bottom (along the height) or from left of the sleeper 10 to the right (along the width). More specifically, it was observed that the key loading for railway sleepers comes from the rail loading, as illustrated by loads P/2 shown in FIG. 1. Accordingly, the cells 14 are distributed based on the expected loading characteristics of the load bearing structure 10. For example, the invention advantageously provides a high density of cells 16 immediately below the zone of load transfer from the rail to the sleeper. A reduced cell density 18 may be employed at locations distant from the primary load transfer zones. By spatially positioning the cells 14, the load bearing capacity of the railway sleeper is concentrated in the zones where the loading is high, and this concept is employed for both transverse and longitudinal load bearing capacity of the railway sleeper. It is noted here that the term “spatially varying” includes variations along the length, breadth and height of the load bearing structure. By concentrating material where it is required, the present invention performs the intended load bearing function at about a third of the material weight in some cases. Thus, the load bearing structure provided by the present invention offers significant weight and cost reductions over conventional load bearing structures.

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 FIG. 4, wherein the cell density 20 is at a maximum at the locations of concentrated loading, illustrated by curves 30, for the embodiment of FIGS. 1-3. In another embodiment, the cell density varies symmetrically from the center of the load bearing structure, and in another embodiment the cell density varies un-symmetrically from the center. Also, in certain embodiments, the cells 14 may have a closed configuration with walls enclosing a volume from all sides, as opposed to an open configuration of illustrated embodiment of FIGS. 1-3, in which the cells have at least one open side without a wall. It is appreciated that the load bearing structures being discussed have a load bearing capacity above about 10 kN, for example support beams, and railway sleepers 10 for which typical loads are about 200-500 kN.

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.

FIG. 6 illustrates a method 100 for manufacturing a load bearing structure in accordance with an embodiment of the invention. In step 110, a mold having cavities configured to form the load bearing structure, the structure comprising cells is provided. In step 120, a suitable polymeric material is injected into the mold. The polymeric material may be a single polymeric material, a mixture of polymeric materials, or a composite material. In step 130, the load bearing structure is recovered from the mold. It is appreciated here that various injection molding techniques, or other techniques such as high pressure plastic injection molding, high or low pressure structural foam molding, gas assist injection molding, extrusion, thermoset molding, injection-compression molding, water assist molding, multi shot molding are generally known in the art, and any of such obvious techniques may be used without deviating from the scope and spirit of the invention. In multi-shot injection molding, the railway sleeper is molded in two or more injection shots of the polymeric material. For example, a left half of the part along length is first molded, and then a right half is molded to complete the sleeper. In another embodiment, the sleeper may be manufactured in different parts. These different parts are molded separately, and then joined together using mechanical type of joints for example, Dovetail joint or other fastening techniques known in the art, for example thermoplastic welding, bolt and screws, among others. In one embodiment, the railway sleeper 10 comprises an open cell configuration (or “rib cell”) as illustrated in FIG. 3 that meets the engineering requirements of a load bearing structure such as a railway sleeper and may be manufactured by an injection molding process and provides for faster cycle time and more cost effective fabrication relative to the manufacture of structures lacking the open cell configuration. In an embodiment, functional parts, such as those not required for the purpose of bearing the load, for example rail fasteners, rail supports, bolt holders, among others, may be advantageously co-molded into the load bearing structure 10. For example, FIG. 5 illustrates a railway sleeper 10 having consolidated rail fasteners 22.

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.

TABLE 1
Parameter	Comparative example	Example 1
Part Volume	30700	cm3	16320	cm3
Part Weight at density of 1.5	46	kg	24	kg
gm/cm3
Maximum Von-Mises stress	56.48	N/mm2	36.16	N/mm2
Maximum deflection	0.65	mm	0.52	mm
Shot capacity of machine =	36840	cm3	19584	cm3
Minimum 1.2 × Part volume

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.

Claims

1. A load bearing structure configured to bear a load, said load bearing structure comprising a plurality of cells, said load bearing structure having a length, a center, and a cell density, wherein said cell density varies at least along said length, and said load bearing structure having a mass of at least 6 kg.

2. The load bearing structure of claim 1, wherein said plurality of cells comprises open cells.

3. The load bearing structure of claim 1, wherein said cell density varies symmetrically from the center of said load bearing structure.

4. The load bearing structure of claim 3, wherein said cell density is concentrated in a region bearing a substantial component of said load.

5. The load bearing structure of claim 1, wherein said cell density varies unsymmetrically from the center of said load bearing structure.

6. The load bearing structure of claim 1, wherein said plurality of cells comprises at least one closed cell.

7. The load bearing structure of claim 14, wherein said load is greater than about 10 kN.

8. The load bearing structure of claim 1, which is a support beam.

9. The load bearing structure of claim 1, which is a railway sleeper.

10. The load bearing structure of claim 9, further comprising a consolidated part.

11. The load bearing structure of claim 9, wherein the consolidated part is a rail support mechanism.

12. The load bearing structure of claim 11, wherein said rail support mechanism is a support structure attachment mechanism.

13. The load bearing structure of claim 1 comprising at least one polymeric material

14. The load bearing structure of claim 13, wherein said polymeric material is selected from the group consisting of themoplastics, thermosets and combinations thereof.

15. The load bearing structure of claim 14, wherein said polymeric material is selected from the group consisting of polycarbonates, polyamides, olefin polymers, polyesters, polyestercarbonates, epoxides, polysulfones, polyethers, polyetherimides, polyimides, silicone polymers, mixtures of the foregoing polymers, copolymers of the foregoing polymers, and mixtures thereof.

16. The load bearing structure of claim 1 comprising at least one composite material.

17. The load bearing structure of claim 16, wherein said composite material comprises an organic polymeric matrix and a filler material dispersed therein.

18. The load bearing structure of claim 17, wherein the filler material is a glass fiber.

19. The load bearing structure of claim 17, wherein the filler material is a carbon fiber.

20. The load bearing structure of claim 17, wherein the filler material is a polymeric fiber.

21. The load bearing structure of claim 17, wherein the filler material is a natural fiber.

22. The load bearing structure of claim 17, wherein the organic polymeric matrix is selected form the group consisting of thermoplastics, thermosets and combinations thereof.

23. The load bearing structure of claim 17, wherein the filler material is configured to decrease dimensional variation.

24. The load bearing structure of claim 23, wherein the filler material is selected from the group consisting of carbon fibres and zeolites.

25. The load bearing structure of claim 17, wherein the load bearing structure comprises functional surfaces.

26. The load bearing structure of claim 25, wherein said functional surfaces comprises a coating selected from the group consisting of weatherable coating, chemical resistance coating anti algae coating, anti slip coating and combinations thereof.

27. The load bearing structure of claim 9, wherein the railway sleeper is configured to be compatible with a girder bridge, a ballasted track or a ballast less track.

28. A method of manufacturing a load bearing structure, the method comprising:

providing a mold having cavities complimentary to the load bearing structure;

molding a polymeric material into the mold; and

recovering the load bearing structure from the mold,

wherein the load bearing structure configured to bear a load, the load bearing structure comprising a plurality of cells, the load bearing structure having a length, a center, and a cell density, wherein said cell density varies at least along said length, and the load bearing structure having a mass of at least 6 kg.

29. The method of claim 28, wherein said molding comprises one selected from the group consisting of high pressure plastic injection molding, high pressure structural foam molding, low pressure structural foam molding, gas assist injection molding, extrusion, thermoset molding, injection-compression molding, water assist molding and combinations thereof.

30. The method of claim 28, wherein the mold has cavities complimentary to a rib cell structure.

31. The method of claim 28, further comprising co-molding a functional part into the load bearing structure.

32. The method of claim 28, further comprising surface-treating the load bearing structure.

33. The method of claim 32, wherein said surface treating is selected from the group consisting of painting, plating, electrolytic coating, spraying and combinations thereof.