Elasis uses Detailed CAE Model of Gearbox to Reduce Design Time

A new dynamic and vibroacoustic prediction method based on a pure CAE model demonstrated the potential to reduce design time and engineering costs by accurately predicting gear noise in the design phase. Elasis and LMS worked together to simulate gearbox noise using a detailed CAE model that simulates noise generation mechanisms at the source, under varying operating conditions. Experimental testing was used at every phase of the modeling process in order to validate the CAE model. This approach provides more information on the root causes of noise problems than prototype testing and makes it possible to develop solutions earlier, faster and at lower cost.

As automobile manufacturers continue to substantially reduce engine and road noise, the transmission is becoming much more important as it is less and less masked by other sources of noise. Transmission designers must address noise problems occurring in many different conditions, such as clutch clunk noise, gear whine, gear rattle and driveline boom. Two major gearbox noise classes encountered in practice include gear whine and gear rattle.

Gear whine is a tonal noise that occurs at orders of the rotational engine speed that correspond to the number of teeth in the different gear pairs. The number of teeth in contact can vary through each revolution, with two teeth being in contact at one point in time and three at another, for example. In addition, the point of contact on a given gear pair continually moves along the face of the teeth. One can view each tooth as a beam that is clamped at its root. The bending stiffness of these beams changes as the contact point moves along this beam. For one given tooth, it rises as the contact point moves towards the root diameter and falls as the contact point moves towards the outer diameter. This phenomenon accounts for additional stiffness variation as a function of time.

Gear rattle results from the fact that non-power transmitting gears are free to move within boundary limits defined by the tangential clearance between the teeth of different gears and the radial and axial clearance between gear and shaft. These gears impinge upon other gears and shafts, resulting in metal-to-metal impacts. These impact forces generate a wideband vibration frequency content. The impacts are driven by engine speed fluctuations due to combustion forces. Gear whine and rattle sources are transferred to the shaft bearings, which finally excite the transmission housing. The housing vibrates and acts as a noise radiator.

Today, transmission manufacturers typically address these problems by spending considerable time and energy during the prototype testing phase. They evaluate transmission noises that occur when gears are being shifted, as the engine is accelerated in various gears and while idling. The problem with this approach is that it is very expensive and time-consuming to build prototypes, perform physical testing and then modify the prototypes in an effort to solve the problem. Another problem is that the amount of information provided by physical testing is limited by the number and accuracy of sensors that can be used to instrument the transmission.

For these reasons, Elasis started to evaluate and understand gearbox noise in the design stage, before prototypes become available. The aim is to be able to assess the noise and vibration performance of the transmission in the early design stages, to optimize the design while reducing or eliminating the need for troubleshooting during the prototype phase. An approach Elasis and LMS successfully introduced is the analysis of the transmission housing, analyzing the way the transmission housing vibrates and radiates noise under known forces at the bearings, and how this noise is transmitted to the vehicle interior. The transmission housing was modeled using Finite-Element (FE) and Boundary-Element (BE) models, and the forces at the bearings were identified experimentally, for example by means of Transfer Path Analysis (TPA). The combination of test and simulation is also used to identify weaknesses in the housing and suggest effective corrections, such as adding ribs and changing wall thickness.

The input forces for this method can be identified on the basis of the current gearbox design, if available for testing, or with realistic forces defined from experience with similar gearbox types. This approach is appropriate when the noise problem is related to gearbox housing resonances, which amplify the internal forces generated at the bearings. However, since this problem is mainly caused by high excitation forces, structural modifications made to the housing may be insufficient and another approach must be used.

Elasis is a Fiat-owned engineering company, which works mainly for Fiat Auto and on the Fiat-GM Powertrain joint venture (FGP), carrying out research and development on vehicles and powertrains. In the current project, LMS and Elasis set out to go beyond this method by analyzing the sources of the noise in a new transmission in detail. The challenge was modeling the full complexity of the internal operation of the transmission to the level of detail required to accurately predict internal dynamic forces in the wide frequency range that is associated to gear rattle.

LMS engineering specialists developed an approach that combines multibody simulation and BE analysis to capture the root causes of noise generation within the transmission. The first step was using LMS multibody software to create a dynamic model of the internal transmission mechanism that incorporates gear stiffness, gear backlash, bearing characteristics, and hub and sleeve connection properties. The flexibility of components, such as shaft and housing, was represented by using modal parameters obtained from FE models. In the project, a constant stiffness at the contact of each pair of teeth was considered, as the focus was mainly on gear rattle and the stiffness fluctuations due to the moving contact between teeth were neglected. When the focus is on gear whine, it is possible to extend the procedure. One can, for instance, use an analytical relationship that provides the stiffness of the tooth contact as a function of the parameters related to the contact location. The next step is building a BE model of the housing using LMS acoustic simulation software, in order to predict the noise radiated under the simulated conditions.

The NVH department of Elasis has built a state-of-the-art test rig to evaluate manual gearboxes. It is a Virtual Engine Simulator (VES) that offers the ability to reproduce speed and torque fluctuations up to 500 Hz, which are similar to those generated by a physical internal combustion engine. This eliminates the need to have a real engine and ensures that the gearbox is the sole source of the recorded noise, enabling an in-depth study of gear rattle noise and eliminating the mask effects of engine noise. One important insight gained during the project is that velocity fluctuations at the engine shaft primarily increase gear rattle at low rotational speed.

The input to the multibody simulation were the combustion pulsations represented by the engine speed fluctuations, and the resistance torque as seen by the transmission, which represents the vehicle load. The multibody simulation was carried out in fifth gear, by inputting the rotational speed on the flywheel while applying a resistance torque at the output shaft. The gearbox housing acted as the noise radiator. As it was simulated as a flexible component characterized by the modes of an FE model, the modal participation factors as a function of time were computed during the multibody simulation. They were transferred to the frequency domain and applied to the vibroacoustic model of the gearbox, which yielded the noise radiation prediction.

This method will enable Elasis to make better design decisions in the early phases of the development when prototype results are not available. It will also become possible to obtain far more detailed information on the performance of proposed designs and evaluate alternate decisions faster and less expensive, when compared to the build and test method. As these new simulation methods are further refined and used more widely within Elasis, they yield substantial improvements of the transmission design process.