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.
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
2012-36-0195
Optimization of new plastic bracket NVH characteristics using CAE
With radial and axial Stiffness shown curves it was possible to
design a ring to achieve the design targets. The figure 12
shows a comparison of the Driving Point Responses of the
first proposal (baseline), the 1st iteration and the final design.
It shows the differences in terms of amplitude of vibration and
frequency for the three designs after bracket changes and
tuning the EPDM isolator. The proposed design with the presence of EPDM isolator shifts the response peaks between
250 and 300 Hz to a frequency above 300 Hz. It also reduces
the amplitude of the responses around 120 Hz.
Figure 12 – DPR curves for initial, 1st. iteration and final
bracket proposals
The figure 13 shows the modal analysis of the initial
(baseline) proposal and final one. The reduction of amplitudes
can be seen in the images. The modes in 103.5 and 137.5Hz
had smaller amplitudes and have not shown big influence in
the system response.
Figure 13 - Modal Analysis Baseline (a) vs. New Proposed
Design (b)
SUMMARY/CONCLUSIONS
This study has shown a way to design a new plastic part
replacing an existing one made of cast iron. The optimization
process was described in order to provide a better
understanding of how the available tools can be used to
achieve useful results for new applications. As expected, the
plastic part design was not able to achieve natural frequency
targets due its smaller stiffness. The package limitations also contributed to limit the improvement of the component.
According to the simulation results, the strategy of
compensate noise factor, tuning the EPDM isolator to the
frequencies below the target was efficient to reduce vibration
amplitudes. The next step of this development are building
prototypes of the plastic bracket and running a DOE (Design
of Experiments) to confirm EPDM ring tuning. This DOE
would consist in a series of physical modal analysis
measurements with rings with different stiffness. This can be
used to determine statistically the optimum value for EPDM
ring stiffness to dampen the resonant frequencies.
REFERENCES
1. CHEAH, Lynette et al. Factor of Two: Halving the Fuel Consumption of New U.S. Automobiles by 2035 Cambridge: Laboratory for Energy and Environment
Massachusetts Institute of Technology, 2007.
2. CHEAH, Lynette et al. Meeting U.S. passenger vehicle fuel economy standards in 2016 and beyond. Burlington:
Elsevier, 2010
3. HEYWOOD, John B. Assessing the Fuel Consumption and GHG of Future In-Use Vehicles PEA-AIT International Conference on Energy and Sustainable Development: Issues and Strategies (ESD 2010) The
Empress Hotel, Chiang Mai, Thailand. 2-4 June 2010.
4. MARK, Herman F. Encyclopedia of Polymer Science & Technology. 3rd ed. Hoboken: John Wiley & Sons, Inc.,
2004.
5. RHODIA, Relatório Técnico FS: 2010-120, São Bernardo
do Campo: Rhodia, 2010.
6. RHODIA, TECHNYL A 118L V50 - A FT 051 - FICHA TÉCNICA - VERSÃO 01, São Bernardo do Campo:
Rhodia, 2000.
7. RHODIA, TECHNYL A 218 V40 Product Datasheet – A FT 110- 2010, São Bernardo do Campo: Rhodia, 2010.