Idle shake is an important NVH attribute. Vehicles with good NVH characteristics are designed to perform excellent in IDLE and SHAKE conditions. Typically, tactile vibrations at idle are measured at the driver seat and steering wheel. Vibrations caused by engine excitation at idle are passed through several paths to the body structure. The dominant paths being the engine mounts and the half-shafts, either one of them or both can be a major factor influencing the perceived idle vibration in a vehicle. In the past, modeling the half-shafts accurately has been a challenge and often time has been ignored because of modeling complexity. This has led to idle CAE predictions not correlating with test data.
The aim of this paper is to describe a finite element modeling method of half-shaft to predict idle vibrations levels. The proposed model includes all the major components of a physical half-shaft: the main shaft and the two constant velocity (CV) joints, modeling of two distinctly different types of CV joints is presented. The first CV joint type is Rzeppa CV joint and the second is Tripod CV joint. The connections between the main shaft and the two joints are represented by linearized elastic springs, the stiffness values were calculated using contact theory and infinitesimal elasticity.
The stiffness calculations include the effect of half-shaft angle, applied nominal torque and joints orientation relative to the shaft. The CV joints stiffness variation with applied nominal toque are highly non-linear functions. Furthermore, the half-shaft angle has a nonlinear relationship with the CV joint stiffness; these relations are captured in the proposed method. Since the vehicle idle is at a particular mean toque and half-shaft angle, the proposed method demonstrates a linearization technique to develop a linear model that can be used in linear full vehicle idle model. The vehicle level results show good correlation. For further verification, at the subassembly level, a test fixture was developed and standalone half-shaft assembly was tested. The measured vibrations were compared with those obtained from the finite element model under the same boundary conditions and excitation and a good correlation was observed.