Abstract: Fatigue testing machine is essential for evaluating material performance, and the innovative design of their actuating mechanisms remains a key challenge. A novel actuating configuration, termed the variable-track crank, is proposed by integrating a dual eccentric sleeve mechanism, a crank-slider mechanism, and a planetary gear mechanism. Its working principle is described, and a kinematic model is analytically established and validated through numerical simulation and prototype experiments. A combined theoretical and data-driven method is used to investigate the influence of component clearance on slider motion waveform error under different operating frequencies and input amplitudes. A sinusoidal summation model is introduced to explain and quantify the error. Results show that the proposed configuration enables real-time continuous amplitude adjustment within 0-40 mm, with a maximum adjustment (40 mm) completed in 0.2 s. It eliminates the need for slip rings and features compact structure, wide dynamic range, and high responsiveness. The waveform error mainly arises from linkage clearance, showing weak dependence on frequency but a strong nonlinear relationship with input amplitude: waveform distortion increases significantly at small amplitudes and gradually stabilizes at larger amplitudes. The findings provide guidance for the design of fatigue testing machines.

Key words: variable-track crank, dynamic amplitude adjustment, kinematic model, linkage clearance