Abstract
Research and /or Engineering Questions/Objective:
Several mechanisms are assumed to cause brake squeal. The dμ/dv<0 theory is one of them. dμ/dv represents the slope of friction coefficient for sliding velocity. If dμ/dv is negative, disc vibrations may increase due to negative damping. It has been said for a long time that negative dμ/dv affects brake squeal. Many researchers measured dμ/dv to clarify squealing factors. In those results, the slope of friction coefficient at a steady sliding speed was used for dμ/dv. However, the pad undergoes minute vibrations during brake squealing. So, we thought that dμ/dv should be measured under vibrations.
Methodology:
We developed a test rig that can measure the friction coefficient at minute vibrating velocity in addition to the conventional method. A piezo-actuator applies minute vibrations to a pad specimen in the tangential direction. The frictional force between the pad specimen and disc rotor is measured by a dynamic load sensor installed under the pad specimen. A slide block restricts the vibrations to the tangential direction, and reduces frictional loss due to the tangential vibrations under the thrust load. The vibrating acceleration of the pad specimen is measured by an accelerometer set on the pad specimen. To obtain vibrating frictional force, the measured force is separated from the inertia of mass using phase plane description. The inertia of mass has the same phase of acceleration, and the target signal is a component having the same phase of velocity. Then, dμ/dv is obtained by FFT treatment of the measured signals.
Results:
dμ/dv at the minute vibrating velocity was compared with that at the steady sliding speed. The pad specimen was cut from a full sized brake pad of NAO material into cubes of 10 mm3 including the backing plate. The measurement results showed that the slope of friction coefficient for the steady sliding velocity was negative at almost all of the measured velocities, and changed slightly to a positive value as the sliding speed of the disc increased. The average value was -0.014 sec/m. On the other hand, the measurement results of dμ/dv at the minute vibrating velocity were positive, and the average value for 6 measurements was 0.019 sec/m. From these results, the dμ/dv value was increased by applying minute vibrations. This tendency was also confirmed for a brake pad of L/S material.
Limitations of this study:
To obtain the correlation of dμ/dv with brake squeal occurrence, the frequency dependency of dμ/dv should be measured at the squealing frequency. Especially for in-plane brake squeal, dμ/dv measurements at higher frequencies such as 6 kHz, 13 kHz, and more are needed. On the other hand, it is important for this measurement to avoid resonance vibration in the actuating frequency. The most dominant part on the resonance frequency is the jig supporting the piezo-actuator. So, the frequency range of measurement is limited to 4 kHz on the high end by the size of the jig.
What does the paper offer that is new in the field in comparison to other works of the author:
The measurement results showed that dμ/dv at minute vibrating velocity was a large value compared with that at steady sliding speed for the NAO material. In this paper, additional results for an L/S material will be shown. The dμ/dv of the L/S material was 0.030 at steady sliding speed, and that at the minute vibrating velocity was 0.093. Although the dμ/dv at the steady sliding speed was a positive value, the same tendency was confirmed for both NAO and L/S materials.
Conclusion:
A dμ/dv measuring device was developed. The device can measure dμ/dv at minute vibrating velocities in addition to conventional dμ/dv at the steady sliding speed of a disc. The dμ/dv at minute vibrating velocity has a different value compared with that at the steady sliding speed of the disc for both NAO and L/S materials.