When discussing the benefits of simulation, time and money savings are usually the biggest headlines, as they should be. But these also promote better design. To avoid multiple iterations and meet design specifications, engineers tend to design conservatively, as was the case at TürkTraktör (Turkey’s largest producer of tractors). The problem with this approach is that it tends to result in designs that are heavy, expensive, and thus less than optimal. While under-designed parts often reveal themselves in physical tests, overdesigned parts are difficult, even impossible to identify.
To create more optimal designs prior to prototyping, Bias Engineering was contracted to develop an accurate dynamic model of the powertrain system. They selected Adams, which makes it easy to evaluate the performance of alternative designs, and has the ability to utilize finite element models from external programs to take the flexibility of a mechanism into account.
Hunkar Yurt, Simulation Engineer at Bias Engineering, modeled the spur and helical gear pairs using the Adams/Machinery gear module. He modeled these gears by specifying the gear type, selecting simple or detailed modeling method, selecting the contact modeling method, defining the location and geometric parameters, choosing the gear material, etc. He modeled the bevel, hypoid and planetary gears by starting with their solid model geometry and adding contact pairs with the impact formulations. The hypoid gear pair and one of the planetary gear pairs were modeled as flexible bodies in MSC Nastran and the modal neutral files were imported into Adams. This made it possible to calculate the stress distribution on these gears. After the simulation was completed, fatigue life of the pinion gears was calculated by exporting Adams REQ files into nCode DesignLife for calculation of fatigue life based on Dang Van theory. The flexibility of shafts was included in the calculations through the discrete flexible links approach in Adams. A discrete flexible link consists of two or more rigid bodies connected by beam force elements. The use of flexible shafts makes it possible to more accurately calculate bearing loads and investigate the eccentric behavior caused by shaft deflection. Most of the bearings were modeled as kinematic constraints. During initial simulations, engineers recognized that a pinion gear bearing was exposed to high loads. At this point the decision was made to take advantage of the Kisssoft integration in the Adams/Machinery module to model the nonlinear stiffness of rolling bearings within the Adams/Machinery environment. The bearing was selected from the Adams/Machinery Bearing library by referencing the product code and selecting the detailed modeling method which includes flexibility. Adams/Machinery also calculated the service life of the bearing. The initial model was created for validation purposes and was based on a tractor which was already in production. Yurt ran the model under the same conditions as tests that had been performed at TürkTraktör. The simulation results matched up very closely to physical testing for values such as torque and power.
With the simulation model validated, TürkTraktör engineers will begin using it as part of the design process to reduce costs and test alternative designs.
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