Analysis of the Reasons for the Difficulty in Disassembling the Tie Rod Pin Shaft of Rotary Drilling RIGS and Countermeasures

Key words: Disassembly of the drill mast of rotary drilling rig, structural improvement design of the mating hinge point

Abstract: During the construction process of rotary drilling RIGS, the drill masts placed in sections must be connected one by one to form a whole before the installation of drill pipes and drill bits can be completed. When the lower section of the drill mast is connected to the main section, the luffing cylinder of the drill mast and the tie rod hinged to the lower section of the drill mast are used to jointly complete the reconnection of the drill mast. Now, in view of the problems that occurred during the assembly process, the author puts forward suggestions for improving the hinge point structure.

Rotary drilling RIGS are one of the rapidly developing construction machinery for pile foundation construction in building engineering in recent years. Whether it is the construction site of the main venues for the 2008 Beijing Olympics, which is known as the "Green Olympics and People's Olympics", or the construction line of the Qinghai-Xizang Railway in the high-cold western regions of the motherland with special environmental protection requirements, rotary drilling RIGS can be seen everywhere. Due to the fact that this equipment integrates environmental protection, energy conservation, high efficiency and intelligent control, world-renowned companies such as Bauma from Germany, BSP from the UK, Mette, CMV, NCB and Casagranti from Italy have all had their own specialized products enter the sales market of drilling RIGS in China, which currently has a relatively small capacity. Of course, powerful domestic enterprises are not willing to lag behind and are all engaged in the development and manufacturing of such products. With the construction of the Beijing-Shanghai high-speed railway about to kick off, it is bound to witness another peak in the sales of rotary drilling RIGS.

Looking at the operation preparation process of large and medium-sized rotary drilling RIGS (when moving over long distances or transporting, the drill pipe must be removed from the drill mast and placed separately), the work that must be done before construction is to connect the drill masts placed in sections into one piece, and then complete the connection of the pins between the cylinder rod of the pressurized oil cylinder and the pressurized support ear of the power head in sequence, as well as the installation of the drill pipe and the drill bit. During the process of connecting the main section of the drill mast (the middle part is hinged to the drill mast luffing cylinder) with the lower section, it is necessary to use the drill mast luffing cylinder and the tie rod hinged between the tripod and the lower section of the drill mast to complete the connection. After investigation, it was found that whether it is the connection or disassembly of the drill mast, there will be encounters with the lower hinge point of the tie rod, the connection or removal of the lower section of the drill mast and its connecting pin shaft. In the actual operation process, due to various reasons, it is very easy to cause difficulties in the installation and removal of this pin shaft. Almost every operation requires a sledgehammer to strike, which is labor-intensive and time-consuming, causing great headaches for the workers.

After observation, it was found that most rotary drilling RIGS are composed of a parallelogram mechanism consisting of a tripod, a boom, a luffing pull rod and a turntable. Within the luffing range of the tripod, regardless of how the length of the luffing cylinder below it changes, all components installed on the tripod, along with the tripod, are always in planar motion. During long-distance relocation (transportation), to ensure the smooth passage of the carrying vehicles through Bridges, culverts and other structures, the drill pipes and drill bits of the drilling rig are unloaded and transported separately. The positions of the power head, luffing mechanism (including boom, luffing tie rod and tripod) and drill mast are shown in Figure 1. For the sake of description, let the hinge point connecting the tripod and the luffing tie rod be a, the hinge point connecting the boom be b, the hinge point connecting the boom and the turntable be c, the hinge point connecting the turntable and the luffing tie rod be d, the hinge point connecting the tripod and the main section of the drill mast be e, the hinge point connecting the main section of the drill mast and the lower section of the drill mast be f, the hinge point connecting the lower section of the drill mast and the tie rod be g, and the other hinge point of the tie rod coincides with point a.

During the process of joining the main section and the lower section of the drill mast into one piece, the luffing cylinder of the drill mast gradually elongates first. At this point, the main section of the drill mast is rotating around an axis passing through point e and perpendicular to the longitudinal plane. Point f is moving in a circular motion around point E. Point g, on the one hand, needs to maintain a constant distance from point f, and on the other hand, it also needs to move in a circular motion around point A. In fact, the trajectory of point g's movement is the result of a compound motion. As the length of the luffing cylinder of the drill mast keeps increasing, the angles between the sides of the quadrilateral formed by points a, e, f, and g in Figure 1 are constantly changing. Once the corresponding planes of the main section of the drill mast and the guide rail of the lower section of the drill mast coincide, the two sections of the drill mast can be connected as one. Then, the connecting pins between the pull rods and the lower section of the drill mast can be removed. After installing the drill rods and drill bits, Then you can enter the construction operation procedure. When the main section and the lower section of the drill mast are connected as one, points a, e and g form a stable triangle (see Figure 2). Theoretically speaking, at this point, the length of the luffing cylinder of the drill mast is just right. Apart from the weight of the components themselves, there are no additional forces at the three hinge points of the triangle. It is very easy to remove the connecting pin between the tie rod and the lower section of the drill mast. However, this is not the case in actual use.

Whether the two sections of the drill masts are connected by bolts or pins, there is always a certain gap between the holes and the bolts (pins). Theoretically, only when the axes of the connection holes of the two sections of the drill masts are completely aligned can the connecting parts (bolts or pins) be installed. During the actual operation by the workers, as long as the connecting parts can be inserted through the connection holes of the two sections of the drill mast, the length of the luffing cylinder of the drill mast will stop extending. When the bolts are tightened or the pin shafts are fixed, additional forces will be generated between the main section connection hinge point of the drill mast and the three hinge points e, g, and a of this cylinder. If the two sections of the drill mast are joined by bolts, once the connecting pin between the lower section of the drill mast and the tie rod is removed, the bolt still needs to be tightened again. After the axes of the connection holes of the two drill masts are completely aligned, if the large cavity of the cylinder continues to supply oil, there will still be additional forces remaining at the above four hinge points after the drill masts are connected. Once the additional force is generated, due to the imperfect micro-control of the cylinder length by the hydraulic system, it is currently very difficult to completely eliminate the additional force through hydraulic operation. Moreover, due to the fact that the operator of the drilling rig is not very skilled, the use of a sledgehammer to strike when loading and unloading the pin shaft is an inevitable result of the current structure.

In conclusion, to avoid retightening the bolts during the connection of the two sections of the drill mast, a method can be adopted where the length of the drill mast luffing cylinder is "slightly excess" than the ideal size, subjecting the pull rod to a certain pressure. When the bolt is tightened to the specified torque, the cylinder rod of the drill mast luffing cylinder can be retracted to solve this problem. To achieve this goal, the connection hole between the lower section of the drill mast and the tie rod must be changed from a circular structure to a long circular one. The actual distributable area of the long round holes is shown in Figure 3. The width (denoted as B) is the same as the diameter of the existing circular hole, and its length (denoted as L) is determined by the ratio of the minimum controllable stroke of the luffing cylinder of the drill mast (determined by the adjustment accuracy of the hydraulic system control components), the distance from the hinge point of the main section of the drill mast to point e and from point g to point e. As long as the length L is selected reasonably, the current stubborn problem of difficult installation and removal of the connecting pin shaft between the tie rod and the lower section of the drill mast can be completely overcome.

If the length of the drill mast luffing cylinder is slightly shorter than the ideal size, the problem can also be solved. However, the distribution area of the long round holes is slightly different. For the bolt connection structure, it must be tightened a second time.