Design and application of an electric servo-driven time-dependent shear testing machine for rock mass stability

The shear strength testing of rocks provides the fundamental parameters of cohesion and internal friction angle, which are critical for evaluating rock mass stability in mining, tunneling, slope engineering, and dam foundations. Direct shear testing is the most widely adopted standard method for obtaining these parameters. However, conventional electrohydraulic servo-driven machines for the direct testing of the shear strength of rocks suffer from inherent drawbacks that severely limit their applicability in modern university laboratories. Continuous constant-power hydraulic operation generates noise exceeding 70 dB(A), disrupting teaching activities; hydraulic oil temperatures exceeding 60 ℃ degrade seals and cause leakage, preventing continuous load holding beyond a few hours; and heavy cast iron shear molds require manual handling, posing a risk of mechanical injury to inexperienced student operators. To address these issues, this study develops a 300 kN electric servo-driven shear testing machine for the time-dependent behavior of rocks; this machine integrates automatic sample loading and unloading capabilities.

The mechanical system employs a rigid portal-type frame welded from Q345 high-strength steel plate. This frame was verified by finite element analysis to ensure structural integrity under 300 kN compound loading. Two high-precision ball screw servo-driven electric cylinders provide normal and shear loads, driven by servo motors with planetary reduction drives. A novel electric-push-rod automatic sample handling mechanism is designed: the electric push rod drives the lower shear mold along guide rails, whereas a reset guide plate automatically realigns the upper mold. This design eliminates manual lifting of 20 kg mold blocks and reduces sample changeover time to within 2 min. The fully closed-loop control system is built entirely from domestic motion control hardware and servo drivers, featuring proprietary data acquisition cards, 300 kN spoke-type load cells that interface directly without external signal conditioning, and 23-bit high-resolution incremental encoders. The host software implements pre-loading control that automatically switches to displacement holding as soon as the contact force exceeds a preset threshold. It also incorporates dual-target control that simultaneously monitors force and displacement to trigger timely shutdown in the event of unexpected specimen failure, thereby protecting the equipment and the laboratory.

Systematic calibration demonstrated post-correction force and displacement indication errors within ±1%, satisfying the design specifications. Control performance tests showed pre-loading overshoot below 0.236% and maximum steady-state error of 0.06%, markedly surpassing Grade 1 testing machine standards. Operating noise was 48.1 dB(A), approximately 25 dB lower than that of conventional hydraulic machines. Power consumption during bidirectional full-load holding was only 0.78 kW, achieving over 50% energy savings compared with hydraulic systems that typically consume over 2 kW continuously. Typical direct shear tests on iron ore specimens yielded a cohesion of 11.76 MPa and an internal friction angle of 67.18°. Under 50 kN normal and 30 kN shear loadings, a 75 h continuous time-dependent shear test exhibited force fluctuations below ±0.014%, verifying exceptional long-term stability for creep and time-dependent failure research.

This electric servo-driven time-dependent shear testing machine fundamentally resolves oil leakage, high noise, and limited load-holding durations that plagued hydraulic systems. Its automatic sample handling mechanism substantially enhances operational safety and convenience for student users. Meanwhile, the low-power, clean-energy design aligns with national dual-carbon strategies and green laboratory construction requirements. The machine provides a high-precision experimental platform suitable for undergraduate rock mechanics instruction and for advanced research on time-dependent shear behavior and creep failure.