Patent Publication Number: US-2013228634-A1

Title: Screw dowel for fastening rails

Description:
FIELD OF THE INVENTION 
     The invention relates to a screw anchor for the concrete attachment in a railroad tie or non-ballasted track according to the preamble of claim  1 . Moreover, the invention relates to a railroad tie or non-ballasted track with at least one screw anchor according to the invention. 
     PRIOR ART AND PROBLEM 
     Screw anchors for the concrete attachment in a railroad tie are used in railway engineering for connecting rail to tie. In this arrangement, the rail attachment is connected to the tie by means of rail screws. The screw anchors are often made of plastic and can be cast into a concrete tie or a non-ballasted track during manufacture. One reason that the anchors are often made of plastic is because the rail screw must be electrically insulated from the tie and because plastic is a cheap flexible material which is simple to process and use. 
     A disadvantage resulting from this is that the screw anchors made of plastic often have different mechanical and physical properties from the surrounding material. In the case of the coefficient of thermal expansion, many plastics, and also the materials conventionally used as anchor material, have an approximately 10-fold greater coefficient of thermal expansion than concrete. This can lead to disadvantages, particularly in concrete tie manufacture but also in later operation, as an anchor initially in form-fit contact with the concrete expands more than the surrounding concrete when the temperature rises so that pressure is exerted on the concrete. Cracks and fissures can occur around the anchor or radially if there are high thermal stresses. Depending on the rail attachment system and the specific clamp system used (e.g. tension clamp), forces are introduced into the concrete tie resulting in lateral tensile stresses, which can cause a widening of the crack and can encourage subsequent crack growth. Water and/or dirt can thereby penetrate into the concrete and, as a result, further damage to the tie can occur due to cracks getting bigger, which has a negative effect on the service life of the tie. 
     DESCRIPTION OF THE INVENTION 
     The object of the invention is to develop an anchor for the concrete attachment in a railroad tie or non-ballasted track, said anchor being characterized by a reduced build-up of stress in the surrounding concrete material. 
     This object is achieved on the one hand by the screw anchor described in claim  1 . The object is further achieved by the railroad tie or non-ballasted track described in claim  11 , which has at least one anchor according to the invention. 
     The screw anchor according to the invention for the concrete attachment in a railroad tie or non-ballasted track is characterized in that it comprises a threaded section with external threads disposed on the exterior of the screw anchor and internal threads disposed on the interior of the screw anchor and further an upper partial section without external threads which is disposed in the installed state close to the anchor opening. It is further characterized in that it comprises two components made of different materials, namely a base body made of plastic and a cylindrical ring made of a steel material, which is attached to the base body in a form and/or force fit manner. 
     The screw anchor according to the invention has the advantage that, by virtue of the cylindrical ring made of a steel material, stresses arising from the expansion of the plastic are absorbed by this ring and thus cannot radiate into the concrete material. It is thereby achieved that fewer and smaller cracks occur in the concrete material, and the concrete tie thus achieves a longer service life. As a result, the advantageous properties of two materials are implemented in a hard/soft combination. 
     Such an anchor can be thereby produced that the cylindrical ring as an insertion part is inserted into an injection molding tool, which can be carried out in an automated manner, whereby plastic material can be injected in such a way that the ring externally surrounds the anchor. 
     A further preferred embodiment consists in the cylindrical ring extending with the base body in the installed position upwards into the region of a feed opening for a rail screw, but which is preferably covered by a plastic collar as component of the screw anchor. The plastic collar is intended to cover the cross-section of the cylindrical ring and have a thickness of 3 to 5 mm. This ensures that adequate electrical insulation exists between the tie screw and the cylindrical ring and, in addition, corrosion of the steel is counteracted. This has the advantage that the greatest temperature fluctuations, which occur at the surface of the railroad tie or non-ballasted track, are best absorbed by a ring positioned in such a way. 
     Furthermore, it is preferred that the ring is attached in a form and/or force-fit manner to the base body of the anchor by an appropriate adjustment of the internal diameter. As a result, gluing, locking and other attachments are avoided, which leads to a simplified production method and reduced costs. 
     A further preferred embodiment consists in that the ring has a wall thickness between 0.5 mm and 5 mm. This has the advantage that the ring is sufficiently thick to contain the expansion of the plastic material while being simultaneously sufficiently thin to be cost- and material-efficient. 
     In addition, it is preferred that the ring in the installed position extends downwards as far as the upper start of the external threads. This offers the advantage that the ring can surround the maximum amount of material and thus prevents the expansion of as much plastic material as possible, thereby also preventing the formation of cracks, while steel is simultaneously saved as the ring is only present in the (upper) region in which the greatest temperature differences occur. The lower plastic region with the external threads is used to secure the screw anchor in the concrete. 
     A further preferred embodiment consists in the coefficient of thermal expansion of the steel material of the cylindrical ring being at least 10 times less than that of the plastic material of the base body, which it surrounds. This has the advantage that when there are temperature fluctuations the steel ring expands only a little relative to the plastic, and the ring can thus fulfill its function to a particular degree, namely to prevent the expansion of the plastic material even at high temperatures; the linear coefficient of thermal expansion of the material of the cylindrical ring is preferably less than 3·10 −5  l/K. 
     A further preferred embodiment consists in the coefficient of thermal expansion of the steel material being approximately equivalent to that of the concrete of the tie. In this arrangement, it has an advantageous effect that concrete and steel are then very similar in terms of their coefficient of thermal expansion and thus no stresses occur between these two materials when there are temperature fluctuations. 
     Furthermore, a screw anchor is preferred in which the steel material is characterized by a modulus of elasticity high enough and a hardness great enough that the cylindrical ring attached to the anchor is not expanded or only negligibly expanded by the expanding plastic of the base body when heated within the temperature range which occurs during normal use. Due to this high strength of the steel, pressure stresses between anchor and surrounding concrete are thus prevented. The modulus of elasticity of the material of the cylindrical ring is maximum 150 GPa. 
     Advantageously, the screw anchor is characterized in that its external thread is a round thread. Such a thread is advantageously distinguished by easy manufacturability and a high strength in the connection thus achieved with the concrete. Moreover, a round thread has proved to be very beneficial in terms of the force-fit in radial stresses between screw anchor and concrete material. 
     Furthermore, it is preferred that the screw anchor is characterized in that the cylindrical ring is thinly coated with plastic. In this arrangement, the steel material of the cylindrical ring is protected from external influences (e.g. rain) and thus corrosion, which has an advantageous effect on the long-term service life of the ring. In addition, it is not visible from outside. Furthermore, this ensures in addition that the anchor and the screw possibly contained therein are electrically insulated from the surrounding concrete, which is advantageous for ties which are to be used on electrified lines. The aforesaid enveloping can be achieved in a simple manner in the injection molding method by over-molding. 
     A further preferred embodiment of the invention consists in a railroad tie or non-ballasted track being provided with at least one screw anchor according to any of the previous embodiments. This has the advantage that the railroad tie is characterized by an increased service life by virtue of the advantages already mentioned above. 
    
    
     
       SHORT DESCRIPTION OF THE DRAWINGS 
       Purely by way of example, further features of the invention will emerge from the following description of embodiments which are shown in the accompanying drawings, in which: 
         FIG. 1  shows a cross-section of a first embodiment of a screw anchor according to the invention; and 
         FIG. 2  is a second embodiment of a screw anchor according to the invention. 
     
    
    
     WAYS OF EXECUTING THE INVENTION 
     The screw anchor  10  represented in  FIG. 1  consists of the base body  22  made of plastic material and is formed of an upper partial section  16  without external thread, a middle partial section  14  with external threads  18  disposed on the exterior of the screw anchor  10  and internal threads  28  disposed on the interior of the screw anchor and a lower partial section  12 . The internal thread can also be executed as a round thread but can differ from the external thread  18  in terms of thread flank and thread pitch. 
     The upper partial section  16  is executed essentially in a cylindrical shape and constructed integrally with the thread section  14  as well as the lower partial section  12 . Form-fit elements in the shape of ribs extending longitudinally to the screw anchor can be provided on the external circumference of the lower partial section  12 , said elements preventing or at least impeding the screw anchor from radially distorting relative to the surrounding concrete material after the concrete mass of the railroad tie or non-ballasted track has been cast around the screw anchor. 
     The screw anchor represented in  FIG. 1  is provided with a cylindrical ring  20  made of a steel material, said screw anchor extending along the upper partial section  16  and terminating flush with the upper opening  26 . It consists of steel, although it can also consist of a different material characterized by a high modulus of elasticity and a coefficient of thermal expansion which is approximately equivalent to that of concrete. The cylindrical ring  20  is connected to the base body  22  of the screw anchor through the upper partial section  16  of the base body  22  being injected into the cylindrical ring  20 . The screw anchor is thus fabricated by the cylindrical ring being inserted into the injection mold and then the plastic material of the base body  22  being injected according to the mold represented in  FIG. 1 . The cylindrical ring thereby possesses a wall thickness between  0 . 5  mm and  5  mm so that, with low material input, sufficient strength is guaranteed to contain the expansion of the plastic material. 
     As is visible from  FIG. 1 , the cylindrical ring  20  in the installed position represented in the figure with a substantially vertical orientation of the longitudinal axis of the screw anchor terminates at the top flush with the upper partial section  16  of the base body. 
     The cylindrical ring  20  with the base body  22  is generally intended to reach into the region of the feed opening but be covered there by an insulating plastic collar  24  as a component of the anchor, The plastic collar has a thickness of 3 to 5 mm. The function of a plastic collar over the cylindrical ring consists in creating adequate electric insulation between a rail screw screwed in the anchor and the cylindrical ring. Furthermore, the plastic collar counteracts the corrosion in the ring. 
     The embodiment shown in  FIG. 2  corresponds essentially to that according to  FIG. 1 , which is why the reference numbers of this figure are adopted here, but differs by the addition of a coating of the steel ring in the form of the plastic collar  24 . This coating is executed on the outer circumference of the entire steel ring and prevents the steel ring from coming into direct contact with the surroundings and thus corroding or creating an undesirable electrical contact between a screw screwed into the anchor and the railroad tie or non-ballasted track.