WO1998045537A1

WO1998045537A1 - Rail pads

Info

Publication number: WO1998045537A1
Application number: PCT/GB1998/001060
Authority: WO
Inventors: David Rhodes; Mark Young
Original assignee: Pandrol Limited
Priority date: 1997-04-09
Filing date: 1998-04-09
Publication date: 1998-10-15
Also published as: GB9707184D0; AU7057998A

Abstract

A cushioning rail pad (30, 30'), for use between a rail foot and a support in a railway rail fastening assembly, is made of elastically-deformable material and formed so as to have substantially smooth upper and lower major surfaces (33, 31). Between the upper and lower major surfaces (33, 31) there is located a plurality of cavities (35, 37) partially defined by a web (32) of material supported within the pad (30, 30') so as to deform under load in bending mode. The pad material has a flexural modulus greater than 40 MPa.

Description

RAIL PADS

The present invention relates to rail pads for use in railway rail fastening assemblies.

A rail pad is used in a railway rail fastening assembly to provide electrical and mechanical insulation and a conforming surface between the foot of a rail and the rail support, such as a concrete sleeper.

A basic type of rail pad comprises a sheet of rubber or plastics material having desired thickness and stiffness characteristics. Another previously- known type of rail pad comprises a rubber web which has studs projecting from each side, the studs on one side coinciding with the studs on the other side. When loaded, the studs deform, thereby reducing the stiffness of the pad as compared to a solid section of the same thickness and providing mechanical insulation between the rail and the sleeper. Such a pad typically has a stiffness of the order of 50 kN/mm, but has an overall height of around 10 mm.

Such relatively thick rail pads mean that other components of the assembly, particularly fastening shoulders, have to be relatively large in order to accommodate the height of the pad. This leads to higher material use and thus high cost.

Another type of studded pad, typically made of a plastics material called HYTREL™ (itself much stiffer than rubber) , has studs arranged such that the studs on one side of the pad do not coincide with those on the other. This allows the web, rather than the studs, of the pad to deform when loaded, so that such a pad can have a stiffness of around 100 kN/m and an overall height of only 6.5 mm. However, a drawback of this design is that the studs themselves must be small compared to their spacing, and so are subjected to high loads. This leads to high pad wear and, more importantly, wear to the contacting surface of the concrete sleeper. A thin shim can be used to reduce such wear, but this increases the overall height of, and increases the number of components in, the assembly.

There are many sleepers now in the world which were originally designed to accommodate a pad of thickness of around 5mm, and having a moderate or high stiffness. Now many railways are recognizing that track performance would be improved if a pad with a much lower stiffness could be introduced. However, the new pad must have the same or only slightly greater thickness than the original so that all of the other fastening components continue to fit together and function properly. Many elastic materials, such as natural rubber, are almost incompressible and are very stiff when they are constrained. Pads made from a plain surface slab of such materials tend to have very high stiffness. Hence there is a need for the pad design to provide some space for the material to deflect into.

According to an embodiment of the present invention there is provided a cushioning rail pad for use in a railway rail fastening assembly between a foot of the rail and a support to which a rail fastening device, for holding down the rail foot, is secured, the pad being made of elastically-deformable material formed so as to have substantially smooth upper and lower major surfaces, between which there is located a plurality of cavities, characterised in that the said cavities are partially defined by a web of material supported within the pad so as to deform in bending mode and in that the pad material has a flexural modulus greater than 40MPa (measured at 23°C using the ISO 178 method) .

It should be noted that all values for the flexural modulus mentioned in the specification are the values measured at 23°C using the ISO 178 method.

EP-A- 0581202 (WEGU) discloses a rubber rail pad through part of which cavities of circular cross- section extend. The aim of these cavities is to allow space for the material of the pad to deflect when the pad is under load, while retaining a flat surface (non-flat surfaces may trap water, leading to corrosion of the rail or abrasion of the sleeper) . To ensure that such a pad is not too stiff for its purpose, therefore, the pad must be made of relatively soft material, having a flexural modulus of around 5-10MPa. It is not possible within such a pad, if only 5 to 6mm thick, to provide sufficient shaping and acceptable membrane thickness to produce a really low stiffness. Thus, the rail pad of EP-A-0581202 can be said to have a "stiff" structure and so requires a soft material to achieve the desired pad stiffness. In contrast, a rail pad embodying the present invention can be said to have a "soft" structure and requires a stiff material to obtain the necessary pad stiffness. In particular, since the cavities of a pad embodying the present invention are partially defined by a web of material supported within the pad so as to deform in bending mode, the structure of the pad is much less stiff then the prior art pads, which are loaded primarily in compression.

For example, a pad embodying the present invention may have a static stiffness measured between 18 and 90kN of between lOOkN/mm and 150kN/mm, whereas conventional pads would typically have a stiffness in excess of 300 kN/mm.

A rail pad embodying the present invention may be made of material having a flexural modulus greater than 55MPa, such as the plastics material HYTREL™, which is available in a number of grades having flexural moduli in the range from 55MPa to 570MPa. Material having a flexural modulus greater than 200MPa may be most appropriate .

A pad embodying the present invention may comprise: a first layer forming the said lower major surface of the pad; a second layer, spaced apart from the first layer; a third layer, spaced apart from the second layer and forming the upper major surface of the pad; a first plurality of projections which extends from the second layer to the first layer and which are spaced apart from one another; and a second plurality of projections which extend from the second layer to the third layer, and which are spaced apart from one another, the spacing of the first and second pluralities of projections being such that the projections of the first plurality are offset from those of the second plurality, except possibly around the periphery of the pad, and the said cavities of the pad being defined by the said projections of the first and second pluralities and the said first, second and third layers .

Reference will now be made, by way of example, to the accompanying drawings, in which:

Figure 1 shows a schematic end view of a railway rail fastening assembly;

Figure 2 shows a pictorial perspective view of one type of previously-considered rail pad; Figures 3, 4, 5 and 6 show cross-sectional views of different respective embodiments of the present invention;

Figure 7 shows a partial cross-sectional view of another pad embodying the present invention; Figure 8 shows a perspective view of the pad of

Figure 7 ; Figures 9a and 9b show further rail pads embodying the present invention;

Figures 10a, 10b and 10c also show further rail pads, of which the pads of Figures 10b and 10c embody the present invention and the pad of Figure 10a does not;

Figure 11 shows a plan view illustrating part of a pad manufacturing process;

Figure 12 shows a schematic cross-sectional end view of another railway rail fastening assembly; and

Figure 13 shows a partial perspective view of another rail pad embodying the present invention.

Figure 1 is a cross-sectional view of a railway rail fastening assembly 1 comprising a rail 10 which is held in place on an upper surface 110 of a concrete sleeper 11 by means of fastening clips 12 inserted into shoulders 13. The shoulders 13 may be cast into the concrete sleeper 11, or may be secured in some other way. A rail pad 20 is provided between the lower surface of a bottom flange 101 of the rail 10 and the upper surface 110 of the sleeper 11.

Figure 2 shows one of the known rail pads described above. The rail pad 20 comprises a planar web 22, from which a plurality of studs 24 project.

The studs 24 are distributed across both the upper and lower surfaces of the web 22, and are arranged so that studs projecting from one side do not coincide with studs projecting from the other side. This arrangement allows the short section of web between the adjacent studs to act as a resilient beam, so that when the pad is loaded the web itself deflects.

A pad 30 embodying the present invention, which may be used instead of the pad 20 in Figure 1, is shown in cross-sectional side view in Figure 3 and comprises lower and upper layers 31 and 33, and an internal web 32, which layers and web are spaced apart from, and extend substantially parallel to, one another, horizontally when in use. A first set of elongate connecting ribs 34 connect the lower layer 31 and the web 32. The connecting ribs 34 are spaced apart from one another and extend along the length of the pad 30. Spaces 35 are defined between the ribs 34.

A second set of elongate connecting ribs 36 extend between the web 32 and the upper layer 33, and extend along the length of the rail pad 30. The second ribs 36 are again spaced apart across the width of the pad 30, so as to define spaces 37 therebetween. The second set of ribs 36 are spaced such that they do not coincide with the first ribs 34 in a direction perpendicular to the planes of the layers 31 and 33; that is no vertical line passing through the upper and lower layers 33 and 31 intersects more than one rib 34, 36.

When in use, the pad is positioned between the rail foot and the sleeper, and so load is applied to the lower and uppers layers 31 and 33 in a direction substantially perpendicular to the layers. The relative positions of the first and second ribs 34 and 36 allow the web to deflect, as if the sections of material between the ribs were short beams.

The pad shown in Figure 3 has relatively low initial stiffness. When the web 32 comes into contact with the lower layer 31 and the upper layer 33 comes into contact with the web 32, the stiffness of the pad increases markedly. When this happens, no more deflection of the layers is possible. Only a very small deformation of the ribs themselves is then possible, but this effect is small compared with the layer deflection, so the pad becomes much stiffer. This "bi-linear" stiffness characteristic is advantageous, since it combines an attractive low stiffness at low and moderate load levels with a much higher stiffness at higher loads which serves to limit deflections when excessive loads are applied.

Since the upper and lower surfaces of the pad are smooth, unlike studded pads, particulate material, such as gravel, sand or grit, and water cannot easily penetrate between the rail and pad, or between the pad and sleeper.

In a preferred embodiment of the invention, the open ends of the pad would be sealed to prevent ingress of such material, for example by using a hot blade to cut out the shape of the pad.

The pad shown in Figure 3 is preferably produced by extruding a hard plastics material such as HYTREL™ in an appropriate process. Such an extrusion process enables the pad shown in Figure 3 to be produced more cheaply than conventional pads, which must be individually moulded.

In addition, the smooth external surfaces of the pad not only helps to prevent ingress of erosion agents between the pad and the sleeper or rail, but also increase the friction coefficient between the rail and the pad, compared with the conventional pads, thereby improving rail creep resistance. The use of smooth surface rail pads may also be advantageous in the construction of slab track, in which the rail and rail pads are suspended in their working positions and concrete is poured, to form the underlying slab track, up to the required level under the pad. If a studded pad is to be used in the assembly during such "top-down" construction of the slab track, a "dummy" pad of appropriate size and shape must be used, since otherwise the spaces between the studs on the underside of the pad would be filled with concrete. This "dummy" pad is then substituted, once the concrete has set, by a studded pad, but this extra process obviously involves additional cost. In contrast, a smooth surface pad may be used immediately, avoiding the need for any substitution.

Alternative cross-sections of rail pads embodying the present invention are shown in Figures 4 , 5 and 6. In Figure 4, the web 32 is provided with protrusions 41 and 43 which extend into the spaces 35 and 37 respectively. The protrusions 41 and 43 allow the stiffness characteristics of the pad to be modified. In particular, the deflection at which the pad stiffness increases can be reduced, since the protrusions come into contact with the upper and lower layers 31, 33 when the loading reaches a predetermined level . Similarly, the embodiment shown in Figure 5 has protrusions 45 and 47 which extend from the upper and lower layers 31 and 33 respectively into the spaces 35 and 37, in order to vary the point at which the increase in stiffness occurs. Figure 6 shows a fourth embodiment of the invention in which the internal web 40 of the pad varies in height above the lower layer, such that parts of the web 40 come into contact with the lower layer 33, preventing further deflection, at a predetermined loading level.

One additional possible use for this change in stiffness characteristic would be to use different cross-sections under different parts of the rail. For example, if the pad is arranged such that the ribs run parallel to the axis of the rail, it would be possible to lower the deflection at which the stiffness increases on one side of the pad relative to the other side. Such a pad could be used to reduce the effects of uneven loading on the rail, since uneven loading can cause the rail to roll. In particular, it is often the case that loads act mainly on the outer ("field") side of the pad, causing gauge widening rail roll. Such gauge widening could be reduced by stiffening the "field" side of the pad.

Figures 7 and 8 show partial and perspective views respectively of a rail pad of generally similar construction to that shown in Figure 3. As can be seen from Figures 7 and 8, the central web 32' is shaped so as to produce spaces 35' and 37', between connecting ribs 34' and 36', which have curved profiles. The curvature of the web 32' reduces the stresses imparted on the web and the ribs 34' and 36' when the pad is loaded in use. Thus, the pad of Figures 7 and 8 can be considered to be an improved version of the Figure 3 pad. The embodiments shown in Figures 3 , 7 and 8 are probably the most useful, since they have the lowest stiffness for a given thickness of rail pad over the greatest range of loads and deflections.

Figures 9a and 9b show partial cross-sectional views of two further rail pads embodying aspects of the present invention. Figure 9a shows a pad having lower, central and upper layers 131, 132 and 133. The layers are spaced apart from one another and are joined by connecting ribs 134a, 134b, 136a and 136b. In a manner similar to the previously-described designs, the centre web 132 bends under loading, causing the pad to deflect . The Figure 9a pad can provide controlled deflection, which is relatively large, and is a "squash flat" design, i.e. the pad deflects until the layers come into contact with one another.

Figure 9b shows a honeycomb-type section in which the centre web 232 is effectively provided by a series of discrete parts, each of which is connected to the upper and lower layers 233 and 231, by means of connecting ribs 234, 236. Spaces 235, 235a and 237 are defined between the ribs. When loaded, this cross- section tends to deform by bending in the ribs 234 and 236.

The rail pad of Figure 10a, which does not embody the present invention, has a pair of layers 331 and 332 with elongate holes 335 extending through the pad defined by connecting ribs 334 which extend between the layers 331 and 332. Such a pad is capable of damping vertical compression only and is comparatively stiff. Only a relatively small vertical deflection is required before the ribs 334 buckle.

In contrast, Figures 10b and 10c show further rail pads embodying the present invention. The rail pad of Figure 10b has two layers, 431 and 433. The layers 431 and 433 are connected to one another by inclined elongate ribs 434, which define spaces 435 therebetween. The ribs are curved such that the spaces have curved upper or lower surfaces . Such a pad can have a varying stiffness characteristic across its width, and is relatively straightforward to manufacture by extrusion. Large deflections are possible with such a pad.

The rail pad of Figure 10c has two outer layers, 531 and 533 connected by protrusions 534 and 536. As is readily apparent from the drawing, the protrusions 534 and 536 define a series of spaces 535 and 537. This pad provides a small deflection before the stiffness of the pad increases. However, such a pad uses a relatively large amount of material and the smaller holes 537 can be difficult to produce. With reference to Figure 11, which illustrates a pad cropping process, it can be seen that continuous extrusion of the section shown in Figures 3 to 10 allows different sized pads to be stamped from one sheet of extruded material. This allows pads of varying width to be produced cheaply and easily.

In the example shown in Figure 11, the pads are produced so that the elongate ribs run laterally with respect to the rail, which provides an essentially uniform stiffness characteristic across the rail.

Alternatively, portions 53 and 63, which are cut out to accommodate the rail fastening shoulders, could be cut from the sides of the extrusion, thereby providing a pad having connecting ribs running substantially parallel to the rail. Such a pad can be of varying cross-section across the rail width, in order to provide varying stiffness characteristics across the rail.

In some applications the rail 10 is supported on a continuous concrete plinth 11' on a continuous rail pad 20', as shown in Figure 12. The rail 10 is fastened down with shoulders 13 and clips 12 at discrete intervals, commonly 0.6m - 0.7m. Continuous pads 20' for such applications are often extruded, but in the past it has been difficult to achieve low stiffness levels in such extruded continuous pads. Low stiffness is desirable with a view to reducing vibrations transmitted from the rail 10 into the plinth 11' , which is often an important consideration on this type of track construction, commonly used on urban metro tracks. Thus, because of their relatively low stiffness, the pads of Figures 3 to 10 are very suitable for this application. An example of a pad 30' embodying the present invention, which is suitable for this purpose, is shown in Figure 13.

The pads shown in the Figures are generally between 5 and 10mm in thickness, and preferably 6.5mm in thickness.

Although the rail pads described above have ribs running therethrough, it will be apparent that a pad embodying the present invention may instead of ribs have stud-like or similar projections arranged in an appropriate manner (the pad being made by a process other than extrusion in this case) .

Claims

1. A cushioning rail pad (30, 30') for use in a railway rail fastening assembly (1) between a foot (101) of the rail (10) and a support (11) to which a rail fastening device (13), for holding down the rail foot (101), is secured, the pad (30, 30') being made of elastically-deformable material formed so as to have substantially smooth upper and lower major surfaces (33, 31), between which there is located a plurality of cavities (35, 37) , characterised in that the said cavities (35, 37) are partially defined by a web (32) of material supported within the pad (30, 30') so as to deform in bending mode and in that the pad material has a flexural modulus greater than 40MPa (measured at 23┬░C using the ISO 178 method) .

2. A rail pad as claimed in claim 1, wherein the pad material has a flexural modulus greater than 55MPa

(measured at 23┬░C using the ISO 178 method) .

3. A rail pad as claimed in claim 2, wherein the pad material has a flexural modulus of more than

200MPa.

4. A rail pad as claimed in claim 1, 2 or 3 , wherein the pad (30) comprises: a first layer (31) forming the said lower major surface of the pad (30) ; a second layer (32) , spaced apart from the first layer (31) and forming the said web of the pad (30) ; a third layer (33) , spaced apart from the second layer (32) and forming the said upper major surface of the pad (30) ; a first plurality of projections (34) which extend from the second layer (32) to the first layer (31) and which are spaced apart from one another; and a second plurality of projections (36) which extend from the second layer (32) to the third layer

(33) , and which are spaced apart from one another, the spacing of the first and second pluralities of projections (34, 36) being such that the projections of the first plurality (34) are offset from those of the second plurality (36) , except possibly around the periphery of the pad, and the said cavities (35, 37) of the pad (30) being defined by the said projections of the first and second pluralities (34, 36) and the said first, second and third layers (31, 32, 33) .

5. A rail pad as claimed in claim 4, wherein the projections (34, 36) are in the form of elongate ribs which are spaced apart across the pad.

6. A rail pad as claimed in claim 4 or 5, wherein the first and third layers (31, 33) are substantially planar and are substantially parallel to one another.

7. A rail pad as claimed in claim 6, wherein the second layer (32) is substantially planar and is substantially parallel to the first and third layers (31, 33) .

8. A rail pad as claimed in claim 7, wherein the projections (34, 36) extend perpendicularly with respect to the second layer (32) such that no two projections (34, 36) coincide with a line substantially perpendicular to the second layer (32) .

9. A rail pad as claimed in claim 4, 5, 6 or 7, wherein the second layer (32) is of varying cross- section across the pad.

10. A rail pad as claimed in claim 9, wherein the second layer (32) is shaped such that the cavities (35, 37) between respective pairs of projections (34, 36) have curved bounding surfaces .

11. A rail pad as claimed in any one of claims 4 to 10, wherein the first layer (31) includes further projections (45) which extend therefrom into the cavities (35) between adjacent projections of the first plurality (34) .

12. A rail pad as claimed in any one of claims 4 to 11, wherein the third layer (33) includes further projections (47) which extend therefrom into the cavities (37) between adjacent projections of the second plurality (36) .

13. A rail pad as claimed in any one of claims 4 to 10, wherein the second layer (32) includes further projections (41) which extend therefrom into the cavities (35, 37) defined between adjacent projections of the first and second pluralities (34, 36) .

14. A rail pad as claimed in any preceding claim, formed as a continuous rail pad layer.

15. A rail pad as claimed in any preceding claim, wherein the pad material is HYTREL .