Effects of vibrations on friction and wear of automobile disc brake pads and electric motor carbon brushes

This dissertation explores the effects of vibrations on friction and wear of carbon brushes in electric motors and brake pads in automobile disc brakes. Micro-vibrations generated by a piezoelectric actuator, when applied to a carbon brush such that the brush vibrated in a rocking mode, reduced brush wear, compared to the case without vibrations applied. When the applied vibrations were near the system natural frequency, much more wear reduction was achieved. The wear reduction mechanism may involve the rocking vibrations transferring the real contact spots between the brush and the rotor to new locations within the apparent are of contact, before thermal mounds evolved; it also may have opened gaps between the brush and the rotor so that abrasive wear particles could escape, consequently avoiding thermal mounds and abrasive wear. Passive vibrations induced by surface waviness inscribed on a wavy disc rotor in an automobile disc brake system increased friction force, compared to the case with a smooth rotor with very little surface waviness; while the friction force increased, pad wear was maintained at the same level. The temperature of the wavy rotor was lower than that of the smooth rotor. These effects may be due to rocking vibrations of the pad preventing thermal mounds, and increased heat transfer from the rotor to the air. In addition, it is believed that brake fade, a phenomena wherein brake forces in friction brakes “fade” or reduce due to high temperature, occurred to the smooth rotor system. Measured wear results differ considerably from laboratory to laboratory, and within a given laboratory, from test to test, even with the same material pairs and the same experimental conditions. Vibration characteristics appear to be one of the main causes of variability of the wear results. This is shown through analysis of brush wear data with different experimental conditions.