# Study on Mechanical Properties and Application of a New Flexible Bolt
## 7 Conclusions  
Through innovation to the tail structure of flexible bolts, the installation of flexible bolts is free from the limitation of excavation height. The locking casing and the spring are extruded onto the rod body of flexible bolts at first. The extruded locking casing is then processed by a thread rolling machine. In this way, the flexible bolts can be installed in the same way as the rigid bolts, thus realizing the fast installation and improving the bolt support speed.  
The static tensile test results show that the diameter and length of locking casing exerts great impact on the mechanical property of flexible bolts. When the diameter of locking casing is fixed, the peak resistance and its corresponding displacement increase as the length of locking casing increases, but the increased amplitude tends to reduce gradually. When the length of locking casing is fixed, too large a diameter would make the mechanical property unstable. Through laboratory tests, the locking casing specifications to be applied in the field test are determined as follows: length: 120 mm; diameter: 30 mm; peak resistance: over 330 kN; corresponding displacement: over 25 mm. The flexible bolt can control the roof deformation in time by increasing the resistance rapidly, but at the same time it leaves some margin for minor deformation. The mechanical property of the flexible bolts is rather stable: no locking casing is pulled out to invalidate the bolt support. The failure mode is that the steel wires break in turn.  
As shown in the cyclic dynamic loading test compared with that of static tensile test, the peak resistance force of flexible bolts witnesses a slight rise to 347 kN on average after cyclic loading. This indicates that the mechanical property of flexible bolts remains stable after cyclic loading. In other words, flexible bolts can adapt to the stress fluctuation caused by the roadway excavation.  
The field test results show that the 4080 mm-long flexible bolts can penetrate the fracture circles of the roadway roof and anchor directly on the elastic circle. In this case, the thickness of the anchoring layer increases noticeably, which in turn strengthens the flexural rigidity of the roof and guarantees the roadway safety. The ball nut can be screwed up to the rod body by bolt drilled directly, saving the installation time by up to 50% and achieving timely support.  
In addition, the single support by flexible bolts can avoid the construction mess, which used to be a common occurrence when bolts of various specifications are used. Meanwhile, the number of flexible bolts in each row is reduced to four. This can increase the speed of bolt support, which in turn improves driving efficiency.  
Data curation, Z.X.; formal analysis, Z.X., J.W., and M.S.; funding acquisition, N.Z.; investigation, Z.X., N.Z., and Q.W.; writing--original draft, Z.X.; writing--review and editing, Z.X., N.Z., Q.W., and J.W. All authors have read and agreed to the published version of the manuscript.  
This study is financially supported by the Fundamental Research Funds for the Central Universities (2020 ZDPYZD02).  
The authors declare no conflict of interest.