Metal working – Method of mechanical manufacture – Assembling or joining
Reexamination Certificate
2000-03-29
2002-01-08
Hughes, S. Thomas (Department: 3726)
Metal working
Method of mechanical manufacture
Assembling or joining
C238S029000, C238S083000, C264S149000, C264S171110
Reexamination Certificate
active
06336265
ABSTRACT:
SCOPE OF THE INVENTION
The present invention relates to an improved method of manufacturing a composite railroad cross tie, and more particularly a railway cross tie characterized by a core which is encased in plastic or resin.
BACKGROUND OF THE INVENTION
Conventionally, railway or railroad cross ties or “sleepers” have been formed from hardwood logs. The logs are cut into an elongated rectangular shape and typically have a width of between about 8 and 10 inches, a height of 6 to 8 inches, and a length of between about 7 and 9 feet. In use, the cross ties are positioned resting on a concrete rail bed or partially submerged within ballast such as crushed gravel or rock. Between about 20 and 40 cross ties are used to support each rail section of railroad track by driving spikes into the cross ties so that the spike heads engage a lower flange on each rail. Conventional hardwood railroad cross ties present disadvantages in that given the scarcity of hardwoods they are expensive to produce and susceptible to decay.
To prolong the life-span of hardwood railroad cross ties, it is known to coat or paint the sides and ends of hardwood cross ties with preserving chemicals, such as coal tar creosote or the like in an effort to delay their deterioration. The use of creosote as a preservative, suffers the disadvantage that it is a toxic substance and a suspected carcinogenic. Creosote coated cross ties therefore result in potential environmental hazards both in the initial coating of the cross ties, and through the possibility of the creosote leaching into the surrounding soil or water table.
When chemically treated with preservatives, hardwood cross ties will have a typical life span ranging from a maximum of about fifty years where optimum conditions and drainage occur, to as low as two years in high humidity environments. Even when cross ties are treated with decay inhibiting chemicals, the chemical preservatives will typically only penetrate between 2 and 10 mm into the ends and exterior surfaces of the cross tie. As the hardwood dries, it differentially shrinks with age. As a result of wood shrinkage, it is common for deep cracks or checks to form in the sides of cross ties and which may extend up to two inches into the cross tie surfaces. These cracks or checks in turn permit water and insects to reach the untreated interior portion of the wood, speeding the cross tie decay.
When repairing a rail section to replace cross ties which have deteriorated, it is often the practice to replace all of the cross ties along the entire rail section, regardless of whether or not even the majority of the hardwood cross ties may have deteriorated to such an extent as to be in need of replacement. Chemically treated hardwood railroad cross ties suffer a further disadvantage in that the toxic chemicals present a disposal difficulty for discarded cross ties, given environmental concerns over the hazardous chemical preservatives. As a result, it is frequently necessary to not only pay for new railroad cross ties, but also to pay a surcharge for the disposal of each railroad cross tie which is replaced.
In an attempt to overcome the disadvantages associated with conventional hardwood railroad cross ties, various individuals have proposed concrete, composite and manufactured cross tie constructions for use in place of hardwood logs. Concrete cross ties are very heavy, weighing as much as three times that of a hardwood cross tie, and are expensive to install. As well, concrete cross ties have a tendency to crack, and also take a heavy toll on the moving rail cars and cargo due to their lack of energy absorbing characteristics.
U.S. Pat. No. 4,150,790 to Potter, which issued Jun. 20, 1995 discloses a steel beam reinforced lignocellulosic cross tie. U.S. Pat. No. 4,083,491 to Hill, which issued Aug. 18, 1975 discloses a cross tie formed from two end blocks which are joined by a pair of metal sides. The manufactured sleepers or cross ties of Hill and Potter have not yet achieved commercial success as they are expensive to manufacture, and further they do not address the difficulties associated with the disposal of the millions of existing creosote impregnated hardwood cross ties which are currently in use.
U.S. Pat. No. 3,416,727 to Collins, which issued Dec. 17, 1968 discloses a laminated railroad tie formed from a shredded hardwood filler and synthetic resin made from waste wood. Collins suffers a disadvantage in that in addition to the added expense of manufacture, the use of shredded wood fiber may in fact increase the degradation of the cross tie. Plastic cross ties are very expensive, with the result that their use is restricted to areas which are difficult to access, such as tunnels, which are one of the most expensive areas for replacing cross ties and which offsets the high initial cost of plastic cross ties. Plastic cross ties are also usually made from polyolefine compounds which tend to stretch or elongate and creep under the heavy loads, particularly at elevated temperatures, which restricts their use. Like Hill and Potter, plastic based cross ties such as those proposed by Collins also do not address the problem of disposal of existing hardwood cross ties.
It has been proposed to provide a composite cross tie which consists of an inner core material of natural or engineered wood which is completely encased in an outer plastic shell. The inventors have appreciated, however, potential difficulties in the manufacture of coated core members. To ensure consistent finished cross tie dimensions, suggested methods of manufacture would involve injection molding the coating about each core. Injection molding is cost intensive from an equipment and tooling point of view. This process also shows relatively long cycle times in manufacturing due to the relatively thick layer of plastic needed to encapsulate the core.
SUMMARY OF THE INVENTION
The present invention overcomes at least some of the disadvantages of prior art railroad cross ties by providing a composite cross tie having an inner core member encased in plastic or resin. The coating layer is applied to the inner core by passing the core member through an enlarged die head which is configured to form an extrusion coating about the core in substantially the desired finished dimension.
Another object of the invention is to provide an improved method of forming a composite railroad cross tie to permit its manufacture quickly and economically in a continuous extrusion process.
The ends of the ties can be coated by resin provided in the extrusion process or by separate end caps either molded prior to the extrusion process or after the extrusion process.
The present invention also seeks to overcome the disadvantages associated with the prior art by providing an improved railroad cross tie or “sleeper”, which has a water impermeable outer coating or shell to provide enhanced resistance to decay.
Another object of the invention is to provide a composite railroad cross tie which is characterized by an inner core member of natural or engineered wood, and extruded outer plastic coating layer or a pair of end covers or caps, which together with the coating layer substantially isolate the core from moisture and/or insects which may otherwise speed its decay. These end caps can be added to the core before or after the extrusion process.
Another object of the invention is to provide a railway or railroad composite cross tie which may be easily and economically manufactured, and which has a core formed from new or recycled hardwood, engineered woods, concrete, plastic composites or other such structurally suitable materials.
Another object of the invention is to provide a composite railroad cross tie which has substantially the same dimensions as a conventional creosote treated hardwood cross tie, so as to facilitate the replacement of worn hardwood cross ties partially submerged within rail bed ballast.
A further object of the invention is to provide an improved method of manufacturing a composite railroad cross tie having substantially the same stability, weight
Hughes S. Thomas
Omgba Essama
Riches McKenzie & Herbert LLP
LandOfFree
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