Preparation of high friction brake shoe material and its tribological behaviors during emergency braking in ultra-deep coal mine hoist

Abstract

The ultra-deep coal mine exhibits the larger speed (>18 m/s) and terminal load (≥240 t/cycle) during hoisting as compared to the ordinary deep coal mine. The larger coefficient of friction and brake specific pressure between brake shoe and brake disc are required to provide the larger friction force and brake torque in order to realize the safe and reliable braking. Therefore, the preparation of high friction brake shoe material and its tribological behaviors during emergency braking in ultra-deep coal mine hoist were investigated in this study. The tribo-brake test rig of brake shoe material with the maximum speed 20 m/s and maximum specific pressure of 2.0 MPa was developed based on the similarity principle. Mechanical properties and conventional tribological behaviors (coefficient of friction, wear rate, temperature rise rate, surface morphology and composition) of resin based brake shoe materials of different formulas were investigated to obtain the optimal formula of high friction brake shoe material employing the uniform experimental design method. Conventional tribological properties of self-made and commonly used WSM-3 brake shoe materials were compared. Effects of initial braking speed and brake specific pressure on tribological behaviors of high friction brake shoe material during emergency braking with the large speed and brake specific pressure were explored. The results show that the optimal formula of brake shoe material consists of the optimal basic formula (phenolic resin 9.8%, NBR 3.8%, vermiculite powder 4.9%, conductive carbon black 2.6%, ceramic fiber 10.6%, graphite 3.8%, sepiolite 8.7%, wollastonite 19.2%, barium sulfate 10.6%, diatomite 7.9%, stearic acid 1.5%, zircon 4.5%, mica 3.8%, kaolin 3%, antimony trioxide 3%, and lead oxide 2.3%), graphene 4%, carbon fiber 2% and glass fiber 4%. As compared to WSM-3 brake shoe materials, the high friction brake shoe material presents the higher coefficient of friction and the medium volume wear rate. During emergency braking, the average coefficient of friction at the stabilized stage shows an increase at first and then a decrease with increasing initial braking speed, and a decrease with increasing brake specific pressure. Increases of initial braking speed and brake specific pressure cause increases in the temperature and temperature rise amplitude of friction disc surface.