Purpose This study aims to simulate the actual forces applied to drill rods during the borehole drilling for gas drainage from underground coal mines to satisfy the requirements of drill rods' fatigue tests.
Methods This study designed a dynamic loading system for full-size drill rods in coal mines. This system consists of a drill rod loading platform, a hydraulic pump unit with power recovery, and a measurement and control system, allowing for the composite loading of dynamic torque, axial force, and radial displacement on drill rods of different specifications. In this system, torque is loaded to drill rods using the hydraulic power recovery method. Specifically, two identical hydraulic motors, acting as loads mutually, are mechanically connected to the tested drill rod. The active motor drives the drill rod and the loading motor to rotate, while the loading motor returns the output flow to the active motor, thus achieving torque loading and power recovery. The energy loss of the closed-loop system is compensated by two hydraulic compensating pumps. The loading of axial force and radial displacement is controlled based on the electro-hydraulic proportion. Based on various variables collected by sensors, the programmable logic controller (PLC) performs closed-loop adjustment of the force and displacement of the loading cylinder to achieve precise control. During tests, various operating parameters and power recovery efficiency are acquired using the measurement and control system and then displayed in the upper computer in real time. The loading capacity of the dynamic loading system was verified using a round drill rod measuring φ 127×3000 mm.
Results and Conclusion The results indicate that the torque and rotational speed of the dynamic loading system can be adjusted by changing the flow or pressure of hydraulic compensating pumps, the displacement relationship between the active and loading motors, and the gearbox gear, thereby meeting various operation conditions of the tests. In the case of the same displacement between the active and loading motors, the power recovery efficiency can reach the theoretical maximum (77%). Under the operating condition of a low rotational speed combined with a high torque, the measured torque loaded to the drill rod reached the maximum (35080 N∙m), corresponding to a rotational speed of 28 r/min, motor power of 50.7 kW, and power recovery efficiency of 66.2%. This suggests an effective reduction of the installed power of the dynamic loading system. Overall, the proposed dynamic loading system for drill rods in coal mines overcomes the limitations of existing devices for fatigue tests, such as insufficient loading capacity and high energy consumption, providing strong equipment support for drill rod tests and being of great application value.
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