Abstract: The causes of early failure and short service life of the cold upsetting die for short and coarse spherical rollers in practical use were analyzed, and the structure of the spherical roller die was improved. The experimental results show that the improved die life has been greatly improved, while the roller forming quality has also been greatly improved.

Key words: spherical roller; Cold heading; Mold; Combined type

Existing Problems and Cause Analysis

The feeding methods of spherical rollers include turning and cold heading. For small spherical rollers with a diameter less than 22mm, Z31-25 single station cold heading machine is widely used for feeding in China. There are uncertain factors such as large dimensional changes and unstable stamping quality in single station cold heading forming. Especially for short and coarse small spherical rollers (with a diameter of 15~22mm and a length of 12~20mm), in the cold heading process, on the one hand, due to the length dimension of the roller being smaller than the diameter dimension, the material section is prone to deformation and large deformation during cutting; On the other hand, due to the large upsetting deformation, in order to ensure that the outer diameter of the roller is full and the end surface is flat, the cold upsetting force borne by the mold during stamping is the largest relative to the conical and cylindrical rollers. Therefore, there have been problems such as early failure and short service life of cold upsetting dies in the actual cold upsetting process.

Due to the limitations of its own length and size, the design of the concave die structure of short and coarse small spherical rollers has undergone a process of change from integral (Figure 1) to group (Figure 2). According to the stress analysis of the concave die during cold heading, the friction between the bar and the concave die mainly occurs in the axial region of the small end chamfer of the roller during processing, and this region is also subjected to an axial tensile stress. Therefore, the integral concave die is most likely to fail at an early stage, with the main forms of failure being chamfer damage, falling blocks, and so on. For small spherical rollers with a roller diameter of no more than 14 mm, the service life of the concave mold can reach 5000 grains; However, for small spherical rollers with a roller diameter size greater than 14 mm, the service life of the concave mold can only reach 2000 grains, and after failure, the entire concave mold is scrapped, resulting in significant waste. In addition, the overall concave mold is processed using GCr15, which is a one-piece mold. During heat treatment, the local quenching method is used. Due to the low compressive strength of GCr15 and the shallow hardening layer during heat treatment, the mold has a short service life.

              Figure 1 Integral                Figure 2 Group Composition

The small combined concave mold (Figure 2) adopts a pre-stressed combined structure, using a pre-stressed ring (transition sleeve). This structure is characterized by separating the forming part of the mold, without parting the upper and lower molds of the work sleeve, and using high-performance mold materials to process the working cavity. However, during the cold heading process, the roller chamfer of the female die is subjected to the combined action of axial tensile stress, radial compressive stress, and friction. This is still the weakest link of the female die, and early fatigue and cracking often occur at the chamfer of the female die in use.

2. Improvement measures

According to the problems existing in the structure of the original spherical roller concave mold, some enterprises have adopted a 7-body concave mold, with the structure shown in Figure 3. However, when using the concave die with this structure to press short and coarse spherical rollers, the upper die of the double layer sleeve is relatively short, and under the impact force of the cold heading cycle, the core of the upper die of the working sleeve is easy to run. In order to further optimize the die structure and improve the die life, the improvement measures taken are as follows:

Figure 3 Seven-body Female Die

The prestressed combined concave mold is used and the mold cavity is divided. Using a prestressed composite die can achieve the required pre compression stress on the die layer, reduce the outward displacement of the die, and thereby reduce the tensile stress generated during cold heading; Parting the mold cavity can prevent stress concentration at the chamfer and reduce friction. After parting, the thickness of the double layer cover is relatively thin, which is prone to outward movement during use. This can be solved by increasing the interference amount or extending the thickness of the double layer cover. However, due to the versatility of the cover, there are situations of repeated use. If the interference amount is increased, the versatility of the cover will be damaged. Therefore, it is decided to lengthen the thickness of the double layer sleeve and lengthen the positioning surface to prevent the outer movement of the double layer sleeve. Considering the overall thickness of the mold, the thickness of the double layer sleeve is extended by 8-15mm.

Assembly dimensions. After the double layer sleeve is thickened, the total assembly height and cavity depth change. Therefore, a step hole is machined from the large end of the inner cone for the working sleeve in the double layer sleeve, and a step is machined from the small end face of the base, as shown in Figures 4 and 5. During assembly, install the small steps on the base into the inner hole behind the work sleeve, with a gap fit between the two, which can ensure both the mold cavity depth and the overall assembly height. In addition, during the assembly process, the shrinkage of the upper and lower molds is inconsistent, so a gap of more than 0.15 mm should be ensured between the parting dimensions of the upper and lower molds.

Figure 4 Improved upper mold      Figure 5 Improved lower mold

According to the improvement measures, the structure of the improved cold upsetting die for short and coarse small spherical rollers is shown in Figure 6. The improved concave mold structure has the following characteristics: the mold adopts a combined structure, and after the cavity part fails, common parts (such as jackets and screw pads) can also be reused; The lengthening of the positioning surface of the double layer sleeve prevents the double layer sleeve from moving outward during operation; The early failure at the chamfer is avoided after the parting of the working cavity.

Using the improved die structure, experiments were conducted on cold heading dies for several types of rollers such as 361023024. The test data are shown in Table 1. From the table, it can be seen that the improved service life of the concave die has been greatly improved, and the forming quality of the processed rollers has also been greatly improved.

Table 1 Comparison of mold life and roller processing quality before and after improvement

3. Conclusion

Statistical data from tracking tests indicate that the improvement of the structure of the concave die for short and coarse small spherical rollers has achieved good results, and early failure modes such as chamfering, chipping, and cracking have been eliminated. The concave die with this structure can be used in large quantities during the cold heading and feeding process of small spherical rollers.

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