Session: 07-08-01: Multibody Dynamic Systems and Applications

Paper Number: 94422

94422 - Multibody Representation on the Coupling Between Wave Generator and Flexspline in Strain Wave Gears 

Strain wave gears are widely employed in aerospace and industrial applications due to their capability to obtain a high reduction ratio with low backlash through light and compact design. In particular, they play a key role in robot joints and servo-actuators used in flight control systems for urban air mobility aircraft. These drives can be functionally described through their three main components: a ball or roller elliptical bearing (wave generator), a flexible external gear (flexspline), and a rigid internal gear (circular spline). The insertion of the wave generator inside the flexspline elastically deforms it, allowing its teeth to properly mesh with the ones of the circular spline, which is connected to the output shaft in the most common configurations. Since this component is mostly used in applications requiring the execution of repeatable and high precision movements, its health assessment is fundamental in assuring the required performance is reached and in avoiding unexpected failures. Nevertheless, due to the intrinsic complexity of its design, experimental measurements on the inside of the gearbox are difficult to perform. To overcome this issue, several mathematical models have been formulated in the literature to better understand the internal behavior of the mechanism and to replicate the working principle of the strain wave gear. However, the dynamic models so far available are based on simplified representation through average parameters, such as the global teeth meshing stiffness, making them not suitable for detailed analyses such as those required to develop diagnostics and prognostics routines. The usage of high-fidelity mathematical models in prognostics and health management is widely adopted in the literature. Such an approach aims to compensate for the lack of experimental data of faulty components which are essential to properly train data-driven algorithms for health features selection. A suitably detailed model of the gear, able to replicate its behavior in both nominal and degraded operating conditions, would allow simulating the presence of faults of different types and severity and to study how they affect the performance of the machine on which the faulty component is mounted. For this purpose, a high-fidelity multibody model of a strain wave gear, in which simulated faults including, but not limited to, wear, backlash, and tooth crack can be inserted, is under development by the authors. In this framework, the present paper describes how the initial conditions of the high-fidelity gear model can be obtained by simulating the insertion of the wave generator inside the flexspline. Through the proposed 2D model, it is possible to determine the positions of the flexible gear teeth after its deformation caused by the elliptical bearing and the forces exchanged between the flexspline teeth. The force distribution between the flexspline and the wave generator outer race can also be evaluated together with its dependence on the flexspline flexural stiffness. To do this, each tooth has been described as a single body with its own dynamics, connected to the surrounding teeth by equivalent visco-elastic models, which leads to an accurate definition of the dynamic properties of the system. The final deformed configuration is not imposed a priori, as commonly present in the literature, but it is directly obtained by the presence of the unidirectional contact with the wave generator outer race. As a first approach, ideal revolute joints have been considered as constraints for all the three main elements of the gear. However, in the continuation of this work this hypothesis can be removed considering elastic constraints and paving the way to vibrational analyses. The proposed model has been validated by comparing the simulation outcomes with literature data and it represents the first step in the development of a fully-realized digital twin of the component capable of replicating the system behavior in both nominal and faulty conditions.

Presenting Author: Andrea Raviola Politecnico di Torino

Presenting Author Biography: Andrea Raviola is currently a Ph.D. student at Politecnico di Torino (Italy), where he obtained his master’s degree in mechanical engineering in 2018. His main research interests are in the area of advanced diagnostic and prognostic of industrial robots and human-robot interaction. He is a member of the Mechatronics and Servosystems research group at the Department of Mechanical and Aerospace Engineering of Politecnico di Torino.

Authors:

Andrea Raviola Politecnico di Torino
Antonio Carlo Bertolino Politecnico di Torino
Andrea De Martin Politecnico di Torino
Roberto Guida Politecnico di Torino
Stefano Mauro Politecnico di Torino
Massimo Sorli Politecnico di Torino