Method for manufacturing fixing members adapted for railway sleepers by cold forming

TECHNICAL FIELD The present invention relates to a method for manufacturing fixing members adapted for fixing railway tracks to corresponding sleepers by cold forming.

PRIOR ART

There are known fixing members or eye bolts that are adapted to fix railway tracks to corresponding sleepers, said sleepers being wooden, metallic or made of concrete. The sleepers may comprise embedded sheaths upon which the corresponding fixing member is threaded.

These types of fixing members, preferably made of steel, comprise a body formed by a threaded stud and a round head with a diameter greater than the stud and a bracket that is rectangular, square or of another shape, and which must comply with specific tensile strength requirements and other mechanical properties such as an absences of cracks etc, defined in accordance with the ISO 898-1 standard, in addition to bending strength.

The fixing members are manufactured by means of a hot- or cold-forming process using a steel workpiece of a certain size and which has been suitably straightened. Generally speaking, the head is hot-formed after the corresponding steel piece has been heated at a temperature in excess of 800°C and once the workpiece has been deformed in a plastic manner. The formed workpiece is then abrasive blasted to remove the scale formed during the hot-forming process and the diameter of the stud is reduced. When the forming process is complete, a cold-threading operation is carried out.

In some processes, the workpiece is subjected to a heat treatment that provides it with certain mechanical properties.

Document JP5318018A describes a method for manufacturing a threaded fixing member, which comprises a cold-forming stage that follows a prior stage involving cooling and quench hardening beneath the head and beneath the stud to be threaded, with the thread then being formed, with the result that delayed fracture is prevented by controlling the generation of internal tensions. Document US 3,877,281 discloses a method for manufacturing high-strength fixing members that have mechanical properties preventing the appearance of the phenomena of delayed fracture without plastic deformation due to the brittleness of the steel. To achieve this it describes a method that begins with the cold-forming of a starting piece, with the cross-section being reduced by at least 10%. It is then heated quickly up to a temperature ranging from 45O ⁰ C to the temperature corresponding to the change point A1. It is then hot-formed to provide the shape of the head, and said head is then cooled by means of air.

Additionally, document US 4,682,381 describes a hot-forming manufacturing method that begins with a cylindrical workpiece being subjected to a first operation in which a first square part adapted to receive the tool fitting the fixing member to the corresponding sleeper and a stud is formed. In a second stage, the diameter of the stud is reduced, with the shape of the first part being kept substantially the same. In a third stage the top part of the stud is compressed to form a collar between the first square part and the bottom part of the stud, and finally, in a fourth phase the bottom part of the stud is threaded.

These types of processes require a heat treatment stage as the minimum mechanical requirements laid down for their use in railway tracks are not fulfilled, which makes the manufacturing process longer and more complicated due to the addition of a heat treatment stage that requires specific controls.

DISCLOSURE OF THE INVENTION

The object of the invention is to provide a manufacturing method of a fixing member adapted for fixing railway sleepers, as defined in the claims.

The manufacturing method is a cold-forming process, in which, starting with a starting piece with a certain external diameter, a fixing member with the required mechanical properties is obtained without the need for the heat treatment of the fixing member, subsequent to the cold-forming process. In addition, the scale that forms during known hot-forming processes is avoided. The process is thus simplified and optimised, with the number of stages being reduced and with it the manufacturing time of said fixing member.

To achieve this, the manufacturing method comprises a stage in which the area of a first part of the starting piece is reduced between approximately 45% and approximately 65%, and a main forging stage for a second part of the starting piece, the forging ratio ranging from approximately 1 ,25 to approximately 1 ,50. These and other advantages and characteristics of the invention will be made evident in the light of the drawings and the detailed description thereof.

DESCRIPTION OF THE DRAWINGS

Figures 1 to 5 show a starting piece in the different phases of a manufacturing method for the cold-forging of a fixing member according to the invention.

Figure 6 shows a threading process to which the starting piece is subjected, subsequent to the cold-forming process shown in Figures 1 to 5, in order to create a first embodiment of the fixing member.

Figure 7 shows an additional stage in the cold-forming process shown in Figures 1 to 5.

Figure 8 shows a threading process to which the starting piece is subjected, subsequent to the additional stage shown in Figure 7, in order to create a second embodiment of the fixing member.

Figure 9 is a view in detail of the direction of the fibres of the fixing member shown in Figures 6 and 8.

DETAILED DISCLOSURE OF THE INVENTION

The fixing member 1 , shown in Figures 6 and 8, obtained according to the method of the invention, is adapted for fixing a railway track to a corresponding sleeper, neither the track nor the sleeper being shown in the figures.

The fixing member 1 of the invention is a threaded member that meets the requirements set out in the ISO 898-1 standard, preferably, for quality classes 3.6, 4.6, 4.8, 5.6, 5.8, 6.8. Said fixing member 1 has been formed from a steel workpiece or starting material 2 that, depending on the properties required, may require annealing prior to forming.

The workpiece 2 has a chemical composition, in weight percentage, of C ≤ 0,40% and Mn ≤ 1 ,60%, the preferred composition being C<0.20%, and Mn≤1 %. Furthermore, the workpiece 2 comprises, in weight percentage, Si ≤ 0,55%, P ≤ 0,035%, and S ≤ 0,035%, the equivalent percentage weight of carbon being ≤ 0,45%, calculated according to the ASTM A-706 formula, in which it is established that the equivalent carbon is:

%C + %Mn/6 + %Cu/40 + %Ni/20 + %Cr/10 - %Mo/50 - %V/10 The fixing member 1 obtained according to the manufacturing method of the invention, comprises a first part or stud 2b that may be threaded along its entire length, as shown in Figure 6, or partially threaded, as shown in Figure 8, and a second part or head 2c adapted for the fixing of the sleeper.

The fixing member 1 is obtained from the workpiece 2 by means of a cold-forming process, shown in Figures 1 to 5 and Figure 7, followed by a threading process shown in Figures 6 and 8, with the heat treatment stages subsequent to said forming process not being required.

In a first stage, shown in Figure 1 , the workpiece 2 that is obtained from a wire that is straightened and then cut to the required size is provided. The workpiece 2, which is substantially cylindrical, has the aforementioned chemical composition and an initial diameter D1.

In a second stage, shown in Figure 2, the first part 2b of the workpiece 2 is subjected to a reduction in the cross-section area ranging between approximately 45% to approximately 65% of the initial area, the reduced diameter D2 being the final external diameter following the reduction, the second part or head 2c remaining substantially without modifications. In a preferred embodiment, the reduction in area of the first part 2b will range between approximately 50% and approximately 55%.

In a third stage, shown in Figure 3, the workpiece 2 is subjected to a secondary forging operation both of the first part 2b, the diameter D3 being the external diameter of the first part 2b following the secondary forging operation, and the second part 2c, the diameter D4 being the external diameter of the second part 2c following the secondary forging operation.

In a fourth stage, shown in Figure 4, a reduction of part 3 of the head 2c is performed, with a preferably rectangular shape being formed, although it may be square or of other possible shapes, said part 3 being adapted to receive a certain assembly tool.

In a fifth stage, shown in Figure 5, the head 2c is subjected to a main forging operation, the main forging ratio being the ratio between the final diameter D5 of the head 2c following the main forging and the initial diameter D1 of the starting workpiece 2. The main forging ratio ranges between approximately 1.25 to approximately 1.5, the preferred main forging ratio ranging between approximately 1.25 and approximately 1.35.

In addition, the stud 2b is also subjected to an auxiliary forging operation, the diameter D6 being the final diameter of the stud 2b following the auxiliary forging stage, and a tip 4 is formed on the free end of the stud 2b, in the event that this is necessary-

In the event that the fixing member 1 comprises a completely threaded stud 2b, the cold-forming process ends at this fifth stage, the workpiece 2 being subjected to a final threading phase, shown in Figure 6, giving rise to a first embodiment of the fixing member 1.

If, however, the fixing member 1 comprises the partially threaded stud 2b, the cold- forming process comprises a sixth stage, shown in Figure 7, in which a segment 2d of the stud 2b is subjected to an external diameter reduction, the final diameter D6 of the stud 2b being reduced after the auxiliary forging stage to a previous thread diameter D8. A threading operation is then performed on the segment 2d of the stud 2b, as shown in Figure 8, giving rise to a second embodiment of the fixing member 1.

In both cases, the threading is performed by means of a cold- or hot-forging operation, using methods known in the prior art, the suitable thread being formed by means of forging.

The fixing member 1 obtained according to this manufacturing method has a minimum tensile strength according to ISO 898-1 , both in tensile tests without wedges and tensile tests with wedges, with the fixing member always breaking in the threaded area and not at the connection of the head 2c with the stud 2b. In addition, the fixing member 1 shown in Figure 6 and that shown in Figure 8 respectively support a first bending of up to 15° of the segment 2b and a first bending of up to 15° of the threaded segment 2d of the stud 2b in relation to said stud 2b, without fractures, cracks or fissures appearing, and a second bending of up to 30° without fractures appearing. In addition, the direction of the fibres is optimum, as shown in Figure 9. As a result, the fixing member 1 meets the requirements set out the 8981 standard for the 3.6, 4.6, 4.8, 5.6, 5.8 and 6.8 quality classes.