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BACKGROUND OF THE INVENTION 
     The present invention relates to construction of railway grade crossings where railroads intersect vehicular roads, and in particular to such grade crossings where a portion of the vehicular roadway includes concrete panels that are supported atop the railroad ties of the railroad track. 
     At railway grade crossings cast concrete “filler” panels or slabs are used to fill the spaces between the rails and along the outer side of each rail to provide a roadway surface. Such concrete panels rest on top of the railroad ties, with each panel covering several ties and having its top surface aligned with the roadway surface to establish a smooth crossing for vehicles. Despite having been engineered to withstand the weight of vehicular traffic, these panels are subject to wear and can fail prematurely. 
     The concrete filler panels used in grade crossings are typically not loaded other than by their own weight. When a heavy truck passes over the crossing, the panels are subjected to bending stresses, tending to deflect downward where the tires of vehicles pass over areas of the panels that are not directly supported by the ties. If the tops of the ties are not even with each other, a panel might bridge the distance between several ties without actually contacting the tops of intermediate ties. If a panel is flexible enough, under a heavy road-traffic load it might deflect so that the undersurface of the panel is brought into contact with the tops of low-standing intermediate ties. Once the panel touches the top of a low-standing tie, it is then supported by that tie and does not deflect further. In some cases, it is not the bending stress sustained by the entire panel that causes the panel to fail. Rather, it is the fact that the undersurface of the panel is in tension as it repeatedly strikes against the upper surface of the tie so that tiny chips are broken away from the bottom surface of the panel, leading to eventual surface cracks and propagation of the cracks. Premature failure of a panel in such railway crossings is most likely to occur when the ties are unusually uneven. Although the tops of all the ties should be at the same height at the rail-attachment point, the top surfaces of the ties are often not at exactly the same heights except at the rail-attachment points. Also, some ties have manufacturers&#39;logos or other writing in raised relief on their top surfaces. Concrete panels and concrete ties both have metal reinforcing bars included within the concrete, and these reinforcing bars can cause slight distortion of the surfaces of the concrete components. Further, due to the relatively large size of the panels, the underside surfaces of the panels may not be completely flat. 
     Variation in ties and concrete filler panels is taken into account when the panels are designed, and the amount of bending stress the panel might experience should not ordinarily cause the panel to fail. However, the panels still do fail, and in order to counter premature failure of the concrete panels, pads of rubber or rubberlike materials have been used atop the ties to distribute the loads of motor vehicle traffic more evenly. The presence of rubber tie pads between the ties and the panels distributes the forces caused by projecting irregularities on the tops of the ties, helps compensate for uneven ties, reduces the pressure applied to the bottom surfaces of the panel when it is in tension and protects the panel from repeated impact on the ties. Such a pad is disclosed in published Canadian patent application No. 2,281,110, and an article in the May 2000 issue of  Mechanical Engineering.    
     While pads may improve the longevity of the concrete panels, vibration caused by a train passing along the tracks at a grade crossing can cause the pads to migrate from their optimal position between the ties and the concrete filler panels, walking themselves out of position. 
     The pad disclosed in the Canadian application identified above includes end flaps to discourage movement of the tie pads. The pad disclosed in the  Mechanical Engineering  article uses a hollow cell to address this problem. Applicant believes that there are disadvantages to both these designs and has invented an improved tie pad. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The present invention provides an improved tie pad that resists movement with respect to its supporting tie. A tie pad according to the present invention has flanges that extend downwardly from the side margins of a main panel of the pad and upwardly projecting shoulders near the interconnection of the main panel with the flanges. The upwardly projecting shoulders provide frictional contact against the bottom surfaces of the concrete filler panels, and when pressed downward by the concrete filler panel, the shoulders push the flanges against the edges or sides of the tie, causing the pad to engage and grip the tie and preventing the pad from migrating from its proper position atop the tie. 
     The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1 is a diagrammatic view showing the use of concrete filler panels at a railway grade crossing, and showing the placement of elastomeric tie pads according to the present intention between the ties and the concrete filler panels. 
     FIG. 2 is a sectional view of the railway grade crossing shown in FIG. 1, taken in the direction of line  2 — 2  in FIG.  1 . 
     FIG. 3 is a sectional view of the railway grade crossing shown in FIG. 1, taken in the direction of line  3 — 3  in FIG.  2 . 
     FIG. 4 is a sectional detail view, taken in the direction of line  4 — 4  in FIG. 2, showing the profile of an extruded elastomeric tie pad such as those shown in FIG. 1, in position on top of a relatively low-standing tie in a railway grade crossing such as that shown in FIG.  1 . 
     FIG. 5 is a view similar to that of FIG. 4, except that the elastomeric tie pad is shown compressed by the concrete filler panel. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings which form a part of the disclosure herein, a railway grade crossing  10  shown in FIG. 1 includes a railroad track  12  having parallel rails  14 ,  16  supported on cross ties  18 , which are typically set into ballast (not shown). The ties  18  are preferably of concrete, as it permits economical manufacture with well-defined shapes and of relatively accurate dimensions but could be of wood or other material. A road  20  for vehicular traffic is shown crossing the railway track  12  at an angle  22  of approximately 90° but the road could also cross at a significantly different angle. Concrete filler panels  24  and  26  have respective upper surfaces  28  and  30  located at substantially the same height as the upper surface  32  of the road  20  on either side of the crossing  10 . In the particular grade crossing  10  shown in FIG. 1 the road  20  is sufficiently wide such that two wide panels  24  arranged end-to-end are needed between the rails  14  and  16 . Similarly, on each side of the track  12  two smaller side panels  26  have been placed end-to-end outside the rails  14  and  16  with their upper surfaces  30  aligned with the road surface  32  of the road  20 . The wide panels  24  are known as gauge panels, and the narrower panels  26  are known as field panels. In constructing the grade crossing  10  the concrete filler panels  24  and  26  are lowered into place with a suitable hoist, using hook eyes  34  which are provided in the panels for that purpose. Elastomeric rail boot or seal strips  36  and  38  are installed between the panels  24 ,  26 , and the rails  14 ,  16 , as shown also in FIG.  2 . 
     Tie pads  40 ,  42  similar to each other except for their lengths, are located directly on the tops of the ties  18  as may also be seen in FIGS. 2 and 3. The gauge panels  24  rest on top of the gauge pads  40  and the field panels  26  rest on top of the field pads  42 . 
     The tie pads  40  and  42  must be correctly located and kept in place on top of the ties  18  so it is important that the pads resist movement once they are installed. The panels  24  and  26  could be especially liable to premature failure should the pads  40 ,  42  be displaced from their proper positions between the ties  18  and the panels. While tie pads  40  and  42  may be secured to the ties  18 , or to the underside of the panels  24  and  26  by adhesive, preferably the tie pads are held in position on the ties by the relationships between the respective shapes of the tie pads and the ties. 
     As shown in FIG. 3, each tie pad  40 ,  42  is held in position on the width  44  of each tie  18  by flanges  46  that rest on a diagonal surface  48  of the chamfered upper longitudinal edges of the ties  18 . 
     The pads  40 ,  42  also need to be held in the proper positions along the length  50  of the ties  18 . This may be done in a number of ways. For example, in FIG. 2, the pad  40  is restrained from longitudinal movement along the length  50  of tie  18  by abutting against the rail attachment hardware  92 . Alternatively, shoulders on the tie  18 , attachment to the panels  24  and  26  or abutment against adjoining structure such as ballast or the roadway  20  may be used. 
     As shown in FIG. 3, the pads  40  and  42  support the filler panels  24  and  26  atop the ties  18 , preventing direct contact between the tops of the ties and the undersides of the panels. When the bottom surface of panel  24  or  26  is loaded in tension by the weight of a vehicle  54  on the upper surface  28  or  30  of one of the filler panels, surface irregularities such as bumps on the top surface of the ties  18  do not press directly against the bottom surface of the panels, and the forces resulting from such irregularities are spread over a larger area by the elastic deformation of the tie pads  40  and  42  at such points. 
     In FIG. 4, the profile of one embodiment of a tie pad is shown in a relaxed condition, with a gauge pad  40  atop the central portion of a tie  18 . A gauge panel  24  is located above the tie pad  40  and the tie  18 . The tie shown in FIG. 4 is relatively low-standing in comparison with other ties (not shown) on either side of it. As a result, the panel  24  barely rests on tie pad  40 . 
     Referring to FIGS. 4 and 5, tie pad  40  includes a main panel  56  extending horizontally along the top of the tie  18 . A pair of flanges  46  located along the side margins  58  of the main panel  56 , extend diagonally downward along the surfaces  48  of the chamfered longitudinal edges of the tie  18 . An outer, or lower, margin portion  60  of each flange  46  extends downwardly along the vertical side  62  of the tie  18 . Raised shoulders  64  project upwardly above a plane  57  defined generally by the top surface of the main panel  56  and are located proximate the side margins  58  of the main panel and the inner, or upper margin  66  of the flange. In the exemplary embodiment the shoulders extend longitudinally along the entire length of the tie pad  40  or  42  but this need not be so. The shoulders  64  in the exemplary embodiment are located wholly over the flanges  46 , outboard of the side margins  58  of the main panel  56 , but shoulders that are not wholly over the flanges could achieve the intended results. 
     In the exemplary embodiment, a bottom cavity  68  is defined in the bottom face of the main panel  56  of the tie pad  40  to provide clearance for a logo or lettering  70  often found in raised relief on the top of a molded concrete tie  18 . The central portion  72  of the main panel  56  above the bottom cavity  68  thus has a thickness  74  less than the thickness  76  of the two lateral portions  78  of the main panel. 
     The embodiment shown in FIGS. 4 and 5 has a pair of grooves  80  in the top face of the main panel  56  laterally outward on each side from the location of the bottom cavity  68 , resulting in a strip of material whose thickness  82 , is also less than the thickness  76  of the lateral portions  78  of the main panel  56 . The top grooves may be used to receive adhesive, and also contribute to the flexibility of the tie pad  40 . The top grooves  80  and the bottom cavity  68  result in a jog portion  84 . Although the jog portion  84  is shown as perpendicular to the general plane of the main panel  56 , the jog portion may form other angles with the general plane of the main panel. 
     The tie pads  40  and  42  are preferably formed by extruding suitable thermoplastic elastomeric material from the same tool or die. A suitable material for the tie pads  40  and  42  is a rubber or rubberlike material with an ability to withstand weather conditions and to remain elastic throughout the expected range of temperatures in the environment of the grade crossing  10 . A suitable material would preferably have a hardness in the range of 25 to 80 Shore A Durometer. One acceptable material is an extrudable thermoplastic synthetic rubber material called SANTOPRENE™with a hardness of 65A, a combination of highly crosslinked rubber particles in a continuous matrix of thermoplastic material, available from Advanced Elastomer Systems, L. P., of Akron, Ohio, 
     In the exemplary embodiment, the thick lateral portions  78  of the main panel  56  preferably have a thickness  76  of about 0.250 inch, while the thicknesses  74  and  82  of the material in the central portion of the main panel  56  may be 0.125 inch. The thickness  86  of the diagonal portions of the flanges  46  is preferably about 0.020 inch, while the outer or lower margin portions  60  of the flanges  56  may have a thickness of 0.125. 
     The shoulders  64  are preferably located above the upper, or inner, margin portions  66  of the flanges  46 . In the exemplary embodiment, the shoulders  64  have a height  88 , above the main panel  56  of the tie pads, of 0.125 inch when uncompressed, for example, and preferably within the range of 0.100-0.150 inch. Each shoulder  64  has a relaxed width  90  of 0.250 inch, for example. The rounded upper surface of each shoulder  64  makes it progressively more difficult to compress the shoulder as clearance between the top of the tie  18  and the underside of the filler panel  24  or  26  is reduced. The weight of the panels  24  and  26 , on the shoulders  64 , causes the flange  46  to press downwardly against the diagonal surface  48  of the tie  18 . This tends to keep the pads  40 ,  42  in place since the flanges would have to move upwardly in order to permit the pads to move laterally. Further, since the shoulders  64  are located outwardly of the side margins  58  of the main panel  56 , the flange tends to pivot downwardly and inwardly at the side margin  58 . Thus, the flanges  46  on both margins of the tie pads  40  and  42 , engage both longitudinal edges of the tie  18 , preventing lateral movement of the tie pad with respect to the tie if in response to movement of the track  12  as vehicles move over the panels  24  and  26  as a train moves through the grade crossing  10 . Contact between the shoulders  64  and the underside surface of the panels  24  or  26  also frictionally resists movement of the pads  40  and  42 , longitudinally and laterally with respect to the tie  18 . 
     The top grooves  80  and the bottom cavity  68  leave the central portion  72  of the main panel  56  adapted to accommodate slight differences in width of the tie  18  by flexure of the material in the jog portions  84 . 
     As shown in FIG. 5, when a concrete filler panel  24  or  26  is supported by a tie pad  40  or  42  located on top of a higher-standing tie  18 , or when the weight of a vehicle on the top of the panel  24  or  26  deflects a panel  24  or  26  downwardly toward a tie pad  40  or  42  atop relatively low-standing tie  18 , the shoulders  64  are compressed by the panel  24  or  26  and are urged down and bulge outward along the diagonal surface  48  of the chamfered upper edges of the tie  18 , urging the flanges  46  downward with respect to the main panel  56  of the pad  40  or  42 . 
     The weight of the concrete filler panels  24  or  26  and vehicles carried atop the panels also tends to compress the thick lateral portions  78  of the main panel  56  of the tie pod, causing the material of the lateral portions  78  to bulge toward the shoulders  64  and the central portion  72  of the main panel, as shown in FIG.  5 . The top grooves  80  and the bottom cavity  68  provide space into which the squeezed elastic material can bulge. With the elastomeric tie pads  40  and  42  thus squeezed between the concrete panels  24  and  26  and the tie  18 , the resistance to further compression of the elastomeric pad material is great enough so that the weight of vehicles on the panels  24  and  26  is efficiently transferred to the ties  18 . 
     The elastomeric pads  40 ,  42  also act to protect the concrete filler panels  24 ,  26  by effectively enlarging the area of contact, between the top of a tie  18  and the underside of a concrete filler panel.  24  or  16  where a bump or other surface irregularity is present. This is particularly valuable when the bottom of a concrete panel  24  or  26  is in tension, as when a vehicle is pressing down on its upper side surface  28  or  30  between two relatively high ties  18 . The tie pads  40 ,  42  thus help to reduce the likelihood of fretting, development of surface cracks and propagation of cracks in the bottom surfaces of the concrete filler panels  24 ,  26 , making the panels less likely to fail prematurely. 
     So long as the pads cover the ties  18  sufficiently to achieve the results discussed above, the tie pads  40 ,  42  can be shorter than the ties. Although the exemplary tie pads  40  and  42  are shown for use with a flat tie with a chamfered edge, the invention may be used with ties with other shapes. While the flange  46  in the exemplary embodiment is shown extending along the entire length of the tie  18 , this arrangement may not be necessary to prevent lateral movement of the pads  40  and  42  with respect to the tie. 
     While the invention has been described with respect to a railroad crossing at a roadway, it would be applicable in any situation where a dynamic load bearing surface is supported by spaced apart ties or similar supporting structure. 
     The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow.

Summary:
Elastomeric pads positioned on top of railroad ties have flanges that extend along chamfered corners of railroad ties and shoulders that extend upward and are located along the tops of the flanges. Pre-cast concrete panels that are commonly provided at railway grade crossings between and alongside the rails rest on the pads. The shoulders are compressible by the weight of the concrete panels and help to keep the flanges in place on the chamfered edges of the ties and resist movement of the pads from their intended positions between the concrete panels and the ties. The pads may be extruded of thermoplastic synthetic rubber.