METHOD OF MAKING A THREADED CONNECTION FOR REINFORCING BARS

METHOD OF MAKING A THREADED CONNECTION FOR REINFORCING BARS.

The present invention relates to a procedure for making a screw thread on a corrugated bar according to the introduc- tory part of claim 1.

The corrugated bar used as raw material in the procedure of the invention for making a screw thread is produced from round section by a special forming method which produces the ribs of the corrugaged bar and simultaneously increases the strength of the steel during the manufacture. This manufac¬ turing method increases the strength of the corrugated bar, and in addition the ribs produced become hardened, being of a considerably harder material.

Normal screw threads for a nut on a corrugated bar are made using known techniques either by rolling or by cutting. In these cases, the cross-section of the bar is reduced in the threaded portion and the tensile capacity of the bar is com- pletely determined by the cross-section of the thread. The reduction in tensile capacity of the cross-section of the thread as compared to a solid bar is of the order of 20 - 30 %. Thus, the tensile capacity of a threaded bar is exclu¬ sively determined by the cross-section of the thread, leav- ing the capacity of the rest of the bar unused, which means uneconomic use of steel. An economic target is to produce a thread whose tensional area is larger than or as large as the nominal area of the corrugated bar.

For the manufacture of a screw thread having the full ten¬ sile capacity of the corrugated bar, several methods have been patented. These are based on expanding the end of the corrugated bar by the cold battering method in room tempera¬ ture. For example, patent application GB 2 227 802 presents a bar joint for use in the reinforcement of concrete, in which the cross-section of the bar ends to be joined is enlarged by cold battering and the ends are provided with a conical thread. Fl-application 890509 presents a procedure for making mechanical joints between round reinforcement bars, in which the bars are joined together by means of a threaded sleeve placed at the juncture. According to this application, one or both ends of the bars to be joined are cold battered before threading. The battering is performed over the length of the part to be threaded and is so imple¬ mented that the root diameter of the threaded portion is at least equal to or larger than the normal diameter of the bars to be joined.

Cold battering causes no changes in the material or strength properties of corrugated bars. After the cold battering, the thread is produced on the battered area by cutting. This method preserves the strength properties of the steel bar unchanged, but it also removes material from the surface of the bar. By the cold battering method, the end of the corru¬ gated bar can only be enlarged over a short length because the material structure of steel does not withstand cold bat¬ tering well enough to allow a corrugated bar to be provided with a thread longer than that required for a nut. For joints requiring a long thread, the cold battering method is inadequate.

The object of the present invention is to eliminate the drawbacks of previously known techniques and to achieve a procedure for making a thread on a corrugated bar which pre¬ serves the increased strength of steel achieved during the manufacturing of the corrugated bar as well as the hardness of the steel surface and the toughness of the interior parts of the bar even during the threading process, allowing a thread with a full tensile capacity to be made on the corru¬ gated bar.

In the procedure of the invention, the end of the corrugated bar is machined by removing the corrugation ribs and flank fillets of the bar. Next, the bar end is heated and then hot battered, thereby increasing its cross-sectional area. After the hot battering, the battered end of the corrugated bar is cooled. The bar end is threaded by rolling. The details of the features characteristic of the procedure of the inven¬ tion are presented in the attached claims.

This procedure allows to produce a thread with a tensional cross-section as large as or larger than the net cross-sec¬ tional area of a solid corrugated bar, which is decisive in respect of the bolt ratings. Moreover, regardless of the diameter of the corrugated bar, the threaded portion can be of a desired length depending on the use it is designed for. This means that all of the tensile capacity of the corru¬ gated bar can be utilized, including the threaded portion, and the procedure makes it possible to produce a thread of any length as required. Thus, a threaded corrugated bar can be used in applications requiring a thread longer than that required by the nut length, in other words, the thread can be long enough to allow adjustment as required. Such appli¬ cations include e.g. the anchor bolt joints of pillars.

In the following, the invention is described in detail by the aid of an example by referring to the attached drawing, in which

Figure 1a presents a corrugated bar and figure 1b a corru- gated bar with a machined end.

Figure 2 illustrates the hot battering procdure.

Figure 3a presents a hot battered bar end and figure 3b a corrugated bar provided with a screw thread according to the invention.

In the procedure for making a full-capacity screw thread, the end of the corrugated bar is first machined by turning it so as to remove the corrugation ribs 1 and the flank fil¬ lets 2 (figure 1a) from the bar area 3 to be threaded (fig¬ ure 1b). In this way, the hardest parts of the corrugated bar are removed. In the manufacturing process of corrugated bars, the rib material undergoes the greatest changes. In the procedure of the invention, the parts of the hardest material, which constitute an impediment to hot battering as employed in the thread-making procedure, are removed from the corrugated bar.

The machined end 3 of the corrugated bar (figure 1b) is heated in a controlled manner so that a smooth temperature difference is created in the machined area 3 between the bar end 4 and the beginning 5 of the ribbed portion, the tem¬ perature being highest at the end 4 of the corrugated bar and falling smoothly towards the other end 5 of the machined portion. The temperature of the unmachined portion 6 of the corrugated bar is not raised except by heat transfer from the heated portion 5.

The heated corrugated bar 7 (figure 2) is locked in place by means of a hydraulic press 8 so that it cannot move. With another hydraulic press 9, a closed cylindrical mould 10 is pressed against the bar end 11 so that the end 11 of the corrugated bar begins to be hot-battered and its cross-sec¬ tional area increases and becomes equal to the internal dia¬ meter of the cylindrical mould 10 in the press.

The end 7 of the corrugated bar is expanded so much that the cross-sectional area of the thread 15 to be formed will be at least equal to the cross-section of the rest of the bar 7, so that the tensile capacity of the bar is fully pre¬ served even in the threaded portion.

The pressing force is applied from the end 11 of the bar towards the locking part 8 and is large enough to batter the bar and increase its cross-sectional area to the size of the mould. The purpose of the changing distribution of tempera- ture in the machined portion of the bar is to ensure that the hot battering effect will start from the end 11 of the bar and, as the pressing force is increased, advance towards the other end 12 of the machined portion. With the smoothly changing temperature, the advance of the battering of the bar can be controlled all the time, and it also ensures that the battering will not start at the middle of the machined portion. Moreover, the temperature rising towards the end 11 of the bar ensures that the portion to be battered will not buckle before the battering effect sets in at the hottest point 11. The moulding is only stopped after the whole machined portion 13 has expanded and fills the mould 10.

After the hot battering, the battered end 14 of the bar

(figure 3a) is cooled in a controlled manner so that the original strength characteristics of the corrugated bar can be preserved during the cooling process.

To make a full-capacity screw thread, the rolling method as known in prior art is used, whereby the cylindrical portion 14 formed on the bar via hot battering is worked with rollers to form a screw thread on the battered end of the bar without removing any material from it.

Through the rolling process, a thread is formed on the sur¬ face of the bar, and the rolling also has a strengthening effect on the material as the steel material 17 under rol¬ ling is cold formed, thereby increasing its strength and hardness. The cold strengthening effect of the rolling does not reach the interior part 18 of the bar, so the material inside the bar remains tough and the toughness characteris¬ tics of the whole threaded portion of the bar are preserved.

The rolling for the forming of the thread is only started after the end of the corrugated bar has been cooled to room temperature. The thread is made on the whole battered por¬ tion 14 of the corrugated bar. After this, no more turning is done on the bar.

By using the rolling method, the original hardness of the material in the threaded portion, which was lost during heating, is restored. In addition, the rolling also causes the bar material to be cold-strengthened in the threaded portion, enabling the original hardness of the surface of the corrugated bar to be restored in this part of the bar. The cold strengthening effect of the rolling does not reach the interior parts of the bar, so the good toughness proper¬ ties of the corrugated bar can be preserved even in the threaded portion.

It is obvious to a person skilled in the art that different embodiments of the invention are not restricted to the ex¬ ample described above, but that they may instead be varied within the scope of the following claims.