Abstract
Brake noise, a major source of warranty cost, is a complex problem involving a myriad of design and processing variables that include friction material properties, rotor design, caliper design, and vehicle suspension. Although noise is minimized through proper brake design and appropriate choice of the friction material formulation, the realization of noise-free brakes in commercial vehicles requires that critical friction material properties are maintained in production. Process variations can significantly alter friction material properties which can adversely influence noise performance.
In this paper, we explore ultrasonic methods to measure uniformity and consistency of brake pads non-destructively. These studies explore how ultrasonic measurements can be implemented as both part of a control scheme to improve the manufacture of friction materials and/or as a quality assurance method to ensure that noise-prone components do not enter the marketplace. Over 300 brake pads of 7 different configurations from 5 different manufacturers were non-destructively measured. Measurements on production pads demonstrated significant variations in both the average value and spatial uniformity of friction materials from various manufacturers. Process specific studies related measured ultrasonic characteristics to variations in manufacturing. To enable rapid, automated, testing with this method, a series of laboratory experiments was performed to identify optimal ultrasonic coupling methods, signal processing schemes and analysis methods. A mechanically scanned prototype system was assembled and used to test brake pads. Comparison of automated test results with those obtained using manual test methods established the feasibility of the automated testing scheme and the applicability of the ultrasonic method.
KEYWORDS: NVH, compressibility, quality assurance, friction material, ultrasound