Research on friction characteristics of the support mechanism of the drilling robot under axial-torsional load

Because the coiled tubing drilling robot (CTDR) needs to pull the coiled tubing (CT) and provide the weight on bit (WOB), the traction force is an essential parameter for evaluating the performance of the CTDR. The friction performance between the support mechanism of the CTDR and the borehole wall will directly affect the traction force. At present, there is not a research paper about the mechanical behavior of the interaction between the support mechanism and the borehole wall. On this basis, a model of contact mechanics of the friction block of the support mechanism and the borehole wall was established. The model of contact mechanics integrates the combined factors of the torsional forces and axial forces generated by the drill bit and the CTDR. The friction performance between the friction block of the support mechanism and the borehole wall under different pa-rameters and conditions was investigated. It is found that the size of the rake angle of the tooth has little effect on the damage of the borehole wall only under normal force. And, the rake angle of the tooth is negatively correlated with the equivalent friction coefficient. The embedded depth is positively correlated with the equivalent friction coefficient. When the rake angle of the tooth is greater than 35 degrees, the influence of the embedded depth on the equivalent friction coefficient is weakened. When the embedding depth of the friction block reaches 0.3 mm, the equivalent friction coefficient is greater than 1. When the bevel angle of the tooth is 20 degrees, the friction block performs best. The optimized structural parameters of the friction block are as follows: rake angle of the tooth of 35 degrees, back angle of 60 degrees, spacing of 4 mm, and bevel angle of the tooth of 20 degrees. Further contact experiments were conducted between a single tooth of the friction block and a rock sample. The dif-ferences between the experimental results and the numerical simulation results were analyzed. The correctness of the theoretical model is verified by experiments. This paper reveals the relationship between structural pa-rameters and equivalent friction coefficient. The results can provide a theoretical basis for the structural design of the friction block for high-safety and high-traction CTDR.