Determining Physical Properties for Rotating Components Using a Free-Free Torsional FRF Technique

This paper presents a test methodology to determine the physical properties of stiffness and damping for powertrain rotating components using a free-free torsional frequency response measurement. The test methodology utilizes free-free boundary conditions and traditional modal test techniques applied to symmetric rotating components with substantially large bounding masses of known inertia. A modal test on the rotating component is executed by mounting accelerometers on opposing tangential bosses in the same direction on each of the inertial masses and impacting one of the bosses with a modal hammer to acquire frequency response functions (FRF's). Physical properties are then extracted from the FRF's using fundamental vibration relationships for an assumed two degree of freedom system. Stiffness and damping values for a variety of hollow tube carbon fiber drive shafts and a comparable steel-aluminum shaft are reported using the methodology presented. The technique is also shown to be useful in estimating fluid inertia for rotating machinery with complex blade and fluid passage geometry, such as torque converters. Measurement of fluid inertia for a range of torque converter turbine diameters was found to be within 7% of estimates obtained from computed aided design models.