Axle Flange Sources of Driveline Imbalance: Calculation Methods from Runout and Location Error, with Validation

The axle, or differential, flange is understood to be a large source of vehicle driveline imbalance, or unbalance, through defining the center of rotation of a driveshaft. The tolerances and methods of manufacturing and assembly are therefore very important. The aim of the current investigation, is to understand and quantify the imbalance contributions from flange radial and axial runout, along with location error between the driveshaft and axle flange. An overview of the measured radial and axial runouts from a population of 100 axle assemblies is presented, including correlation of the imbalance amplitude distributions to some standard probability density functions. It was found from the investigation, that it is important to understand the nature of any source of runout, relative to any subassembly/component-level balancing, in modeling the transfer function from runout to imbalance loading. Methods for calculating the imbalance of an assembled driveline are presented, which include the mass levels of important driveline components in their calculation. Two methods of quantifying the weight contribution are given—the “distributed weight” method and the “hung mass” method. The latter is known to be used in some parts of the industry. The “distributed weight” method led to an improved estimate of the contribution from the driveshaft tube mass on the imbalance force at the axle flange. Two methods of dealing with the radial and axial runout of an axle flange are discussed—the industry-standard “CFRO” method (combined flange run out) and the “IIRA” (individual imbalance forces from radial and axial runout) method. The CFRO method tended to lead to overestimation of the level of runout imbalance. The contribution of imbalance due to location error between the driveshaft and axle flange is identified and quantified with results, for the 100 axle assemblies of measured data, given. It is hoped and intended to be a useful guide to the NVH engineer doing the calculation of imbalance loading from axle runout.