Correlation of a negative friction–velocity slope with squeal generation under reciprocating sliding conditions

Under reciprocating sliding, it is found that the friction strongly depends on both displacement and frequency. This characteristic may be used to verify the negative friction–velocity slope mechanism of squeal generation. Test parameters such as the frequency and amplitude of reciprocating sliding are found to have a significant influence on the friction. In the present test, extensive test parameters are included. The frequency is ranged from 0.1 to 5 Hz, amplitude from 0.5 to 5 mm. In order to assure the accuracy of friction measurement, the friction force is measured with two different types of load cells: stress-strain and piezoelectric. Two different types of test apparatus are used to explore the dependence of the friction on both displacement and frequency. One of the test apparatus has a rotational degree of freedom between the frame fastening a specimen and load cell, which can almost keep a constant nominal normal force between two matched specimens regardless of the misalignment and wear of the specimens. The other does not have the degree of freedom. The reciprocating sliding is imposed in both sine and triangle waves. The test result shows that a negative friction–speed slope is not always responsible for vibration and associated squeal. Many squeals are also found to occur in the areas with positive friction–speed slopes.