Martin Trapp

Squeak and Rattle Behavior of Elastomers and Plastics: Effect of Normal Load, Sliding Velocity, and Environment

The use of plastics and elastomers, for interior and exterior automotive components, presents a risk of frictionally incompatible materials contacting each other, resulting in squeaks, ticks, chirps... Fords NVH S&R Department, and MB Dynamics have developed a tester (Figures 1 and 2) that can measure friction, and sound, as a function of sliding velocity, normal load, surface roughness, and environmental factors that allows us to provide up-stream engineering information to Forward Model Design Engineers. When material pairs undergo sliding contact, friction forces can cause elastic deformation adjacent to the contacting surfaces. The elastic deformation is a mechanism for storing energy and sound is produced when the energy is released. The sound that we hear may be a squeak or squeal (multiple stick-slip) or a tick (single stick-slip). However if the sliding material composition (e.g. coatings, low friction additives...) and the structure (surface roughness or stiffness/compliance) of the sliding components is properly selected, we can avoid or minimize noise by minimizing elastic deformation and therefore the release of stored elastic energy. The release of the stored elastic energy can occur when the kinetic friction is less than the static friction. This difference can be duplicated during single excursion events or when undergoing sine or random vibration. In the case of thermoplastics, cold temperatures can have a major influence on acoustic properties by reducing chain mobility (Tg) at the sliding contact surface leading to an increase in the surface contact stiffness and therefore changing the elastic deformation properties.

Buzz, Squeak and Rattle Detection for Modules, Subsystems and Components

This chapter describes the best ways of performing a physical shaking test for engineering development or for production quality audits. BSR detection is not just another vibration test. There are many challenges which can trip up the unsuspecting practitioner, such as the uncommonly low vibration levels which typically have acceleration levels in the range of 1 to 5 m/sec 2 , or the relatively uncommon frequency range of BSR detection. The necessity for quietness when conducting BSR tests is emphasized, with techniques for achieving low sound levels in the test area. The important issue of how to evaluate noise levels using sones rather than dBA measurement is discussed. The effect that the quality of test fixtures can have on BSR testing is illustrated with examples. Examples of ways of measuring multiple directions of vibration and the use of control accelerometers are shown. Different types of vibration generators, the characteristics of power spectral density profiles, the objective laboratory analysis of noise and psychoacoustics are discussed. An application example (including test methods and lessons learned) of BSR testing of instrument panels is given in detail.

Friction Sliding and Rattle Impact Analysis

The post-processing method described in this study characterizes and detects friction sounds from the recorded acoustic signals by correlating the results from their objective and spectral analyses with the measured time traces for contact forces. The reported results indicate that surface roughness may well be responsible for friction sounds observed during transitions from static to sliding friction. The favorable correlation of the results from the method with the results from the listening session indicates that the present work may be viewed as a useful step toward the development of a standard for material friction pair testing to reduce automotive squeaks. Further improvements of the method are needed to account for the other properties of material interfaces. For nylon and polypropylene (PP) thermoplastics, the psychoacoustic annoyance (PA) of rattle increased as the hardness increased and the Izod impact strength decreased, and as the shear and flex modulus increased. Plate modes could be a factor; therefore, future analysis will investigate what effects different boundary conditions, C-C-C-C (C = constrained) or C-C-C-F (F = free), have on the acoustic rattle impact output. PA as a function of specific modulus and specific flexural rigidity for metals showed significant differences between Al, Mg, steel, and brass. The Al and Mg samples had high PA values, similar to the highly filled nylon 66 samples, and the steel and brass samples had low PA values, similar to the PP samples. The 95th percentile of the loudness (N5 loudness in sones) was the single best correlated objective metric to a subjective jury evaluation.