Dissertation Defense: Assessment of Nonlinear Damping Elements for Vibro-Impacts in Automotive Torsional Systems with Discontinuous Nonlinearities

Committee

Prof. Rajendra Singh, Chair (ME)

Prof. Brian Harper (ME)

Prof. Donald Houser (ME)

Prof. Jason Dreyer

Abstract

The topics of this dissertation are driven by a demand for more accurate drivetrain torsional system models in ground vehicle industry as they may be used to troubleshoot transient dynamic, noise and vibration problems. Such driveline systems contain many discontinuous nonlinear elements that have been extensively studied in the literature. However, damping mechanisms involved in the transient events remain yet to be adequately defined and quantified. This forms the chief goal of this research and thus two recent laboratory experiments (as published) are reexamined and analyzed to investigate nonlinear damping characteristics that were not fully addressed before. Overall, dominant damping elements associated with vibro-impact phenomena in a typical vehicle drivetrain system includes four sources: contact damping and hysteresis associated with gear mesh stiffness, oil squeeze damping in the gear mesh clearance, friction in the tapered rolling element bearings, and dissipative elements in a multi-staged clutch. First, the gear mesh dissipation contribution is considered to be proportional to the load carried by gear teeth. Dynamic hysteresis is investigated using a combined Hertzian contact stiffness expression (for the point contact) along with alternate impact damping formulations based on dry contacts. Further, the squeeze damping phenomenon is successfully formulated using the Reynolds equation. A lubricated contact damping expression is presented to calculate the minimum film thickness calculation during contact losses between gears. The results of using a combined dry contact hysteresis and oil squeeze damping elements between the gear teeth show that predictions match much better with experimental results. Next, global damping is represented by a combination of viscous and dry friction elements. Friction torques in tapered bearings, which, are geometrically coupled with the hypoid gear mesh force, are modeled by using a smoothened Coulomb friction model. It was found that inclusion of bearing friction in the driveline model has a significant influence on the impacts in term of number of impacts as well as on the shapes of impulsive contact forces. Finally, the damping associated with transient dynamics of a clutch-damper subsystem has been identified using computational, signal processing, and experimental methods. A novel method that combines nonlinear hysteretic and viscous damping from transient vibration acceleration measurements is proposed and validated. Two wavelet logarithmic decrement type formula are developed in the framework of a general continuous wavelet analysis for both dry friction and viscous damping parameters. This method simultaneously yields both viscous damping ratio and dry friction damping estimates directly from acceleration measurements without a need to perform any integration process. Major contributions of this research are as follows. First, gear mesh damping during the impact process due to material and lubricant squeeze between mating teeth contributions are identified, modeled, and validated. Second, bearing friction torques is formulated and its dynamic interactions with gear mesh forces are investigated. Third, refined damping models for multi-staged clutch suitable for impulsive type loadings are proposed and justified. Also, a novel identification damping approach based on wavelet transform analysis of measured accelerations during dynamic tests is presented. Overall, the refined models and parameters formula can be utilized to troubleshoot the transient vibrations of a nonlinear torsional system, while providing scientific and physical insights.