Figure 1: Stress and fatigue analysis gearbox and drivetrain components for a light-weight electric utility vehicle.  The FEMAP model was analyzed using NX Nastran SOL 601 (ADINA).

This was a classic stress analysis of a gearbox housing. It is something we done many times at Predictive Engineering for off-shore equipment, motorcycles, automotive, aerospace and now for a small electric utility vehicle. The client was great to work with and supplied us with the load path breakdown from gear-to-gear-to-gear. Although this is something we have done in the past, teamwork is always great since it helps prevents cross-communication. The objectives of this work was to determine stress levels in the gearbox housing to assess durability and if, there would be sealing issues between the main casing and its cover. Oil leakage between seals can be determined via contact pressure plots. The trick on such work is to ensure sufficient mesh density between the contacting surfaces that one sees true pressure and not a mesh artifact. Prior work with Norton Motorcycles helped to refine this seal analysis. Final results showed that the design was adequate for service and would be “leak free”. 

Figure 3: Stress and vibration analysis of 3,000 HP, Dual-Shaft, 8-speed transmission for the efficient pumping of heavy fluids

This FEA consulting project started from initial concept and was carried forward to a field tested 3,000 HP transmission. It was the first of its kind using a compact dual-shaft arrangement to provide 8-speeds off of a 12 cylinder diesel engine running at 2,300 RPM. The bearing forces were calculated based on gearing arrangement (1^st gear to 8^th gear) and input torque. Contact was used to simulating the shaft loading into bearings and then into the cast ductile iron transmission housing. The sectional housing (four main sections) was bolted together with contact enforced across the bolt preloaded interface. The dominate load case for the transmission housing was, of course, 1^st gear. The analysis work studied bolt loading for long-term life based on the end-clients own internal transmission bolt specifications along with stress limits based on the long-term fatigue life of their transmission and bolt materials. The model shown in the graphic had 3+ million nodes and captured nonlinear bolt pre-load with contact. Additionally, a normal modes (eigenvalue) analysis was performed to determine the first significant vibration modes. The objective was to ensure that the first mode of the transmission assembly was higher than the diesel engine. The model was validated during operational tests from stress to vibration to “oil-tight sealing” at all assembly joints.

Figure 5: Two examples of hydraulic variable speed transmissions were studied. Stresses in the transmission cases were driven by hydraulic pressures and fluid operating temperatures (thermal-stress). Bolt load failure analysis was pivotal in both transmissions to ensure their in-the-field durability

Hydraulic variable-speed transmissions have been used for decades. In these two FEA consulting projects for different clients, we investigated the stress and thermal-stress response of the transmission housings. In the first project, the client was interested in improving the seal performance of the top head gasket. Besides the usual stress analysis under operating pressure we did a complete bolt analysis (spreadsheet based from FEA bolt axial and shear loads) to a gasket analysis. Our recommendation was quite simple – use a higher bolt preload. In the second FEA project, the client was having problems with the system once it had been in operation for a few hours. Due to thermal growth of the components (aluminum impeller matched up with a ductile iron housing), some binding was occurring between the components causing bolt breakage. The FEA model was able to accurate predict this behavior and the manufacturing tolerances were modified to allow for additional clearance along with thinning out a few regions to allow for greater flexibility during thermal expansion.