Stability analysis of running-in system based on Nyquist curve of friction vibration

This paper examines the stability of the running-in process using a Nyquist curve analysis of the vibration due to friction. The Nyquist stability criterion uses the frequency characteristic to assess the stability of systems; so when the (−1, j0) point is enclosed by Nyquist curve, the closed-loop system is unstable. Running-in and wear tests of a piston ring against a diesel engine cylinder were conducted on a pin-on-disc tester. The analysis showed when the running-in is confined to the surface layers, the point (−1, j0) is not enclosed by the Nyquist plot and the process is stable; When the running-in is in the severe wear stage, the point (−1, j0) is enclosed by the Nyquist plot, the system is unstable; When running-in is in the stable wear stage, the point (−1, j0) is not enclosed by the Nyquist plot and the process is stable. This method could be used to predict the progress of the running-in process and identify the run-in state.