Optimization of New Plastic Bracket NVH Characteristics using CAE 2012-36-0195

NVH requirements are critical in new driveline developments. Failure modes due to resonances must be carefully analyzed and potential root causes must have adequate countermeasures. One of the most common root causes is the modal alignment. This work shows the steps to design and optimize a new plastic bracket for an automotive half shaft bearing. This bracket replaces a very stiff bracket, made of cast iron. The initial design of plastic bracket was not stiff enough to bring natural frequency of the system above engine second order excitation at maximum speed. The complete power pack was modeled and NVH CAE analysis was performed. The CAE outputs included Driving Point Response, Frequency Response Function and Modal analysis. The boundary conditions were discussed deep in detail to make sure the models represented actual system. After some iteration, weaker areas were identified and the design was changed, increasing stiffness and shifting some low frequency modes to higher frequencies. The remaining mode below engine second order could not be changed adequately, so a different strategy needed to be taken. An elastomeric isolator was added between bearing and bracket, in order to dampen the vibrations. The material chosen was EPDM, due to its damping coefficient and high temperature resistance. The model was submitted to a new analysis, when the stiffness of the isolator could be determined in order to match the resonant frequency. This isolator reduced the transmissibility of the vibration through bracket and the amplitude of the vibration was decreased to an acceptable level with this strategy.