| Ford’s Detroit NVH division
is working with UK software firm Romax to reduce transmission whine. Romax
is also working with Japanese OEMs and transmissions suppliers to solve
noise issues in hybrids and dual clutch transmissions.
The more refined other aspects of vehicle NVH become, the more gearbox
whine stands out. It’s a pure tone that is more apparent to passengers
than background wind and tyre noise. It’s a headache for manufacturers.
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Gearbox whine stands out more as other NVH issues get
resolved |
Transmission error causes the whine. If the gears were all perfect and
there were no deflections in the shaft, power would transmit perfectly
from one gear to another. Instead the gears displace where the teeth mesh
and vibrations occur. When these reach the gearbox housing they become
noise.
Romax R&D manager Dr Jamie Pears said: “Even if you remove the
noise in prototypes, you can’t be sure of the effects of small variations
in manufacturing. Vibration is very sensitive to these.”
Researchers at Mercedes-Benz’ Ulm Research Centre in Germany have
been investigating similar subjects. Engineers there are working on ways
of managing tolerances in the production of engines, axles and transmissions.
The aim of the application-oriented research is to attain product maturity
at an earlier stage and so reduce costs. Besides trying to make shifting
gears smoother, the researchers are trying to reduce transmissions noise.
Product and production modelling researcher Christian Glöggler said:
“It would be great to be able to use tolerance management to be
able to do this, but we’re still a long way from realising this.”
Simulating the whine is complex. It involves not only predicting the original
transmission error, it also has to take into account the transmission
through the rest of the system’s gears, shafts and bearings. Any
false assumptions would cause large errors in the results.
Normally updating an existing gearbox for a new vehicle might require
three prototypes; a dual clutch transmission might take four. Romax says
its software can reduce this to just one or two.
The Romax NVH tool predicts the vibration on the housing and throughout
the gearbox. The information is displayed either as a moving animation
or the vibration at individual points on the model can be displayed as
a graph of amplitude against frequency. Campbell diagrams – a way
of representing the interaction between frequency, rpm and the vibration
response – are also possible.
“Because the model is created by defining parameters for the gears,
you can easily investigate the effects of the tolerances and manufacturing
variability,” said Pears. You then try to make your design insensitive
to the tolerances that you can’t get rid of.”
Being able to edit input parameters is important – it isn’t
possible in a conventional FEA model. To make a change you have to go
back and redo a large part of the model.
Ford is looking to implement the tool in its development process. Romax
is working with US and Japanese firms on a tool to simulate hybrid transmissions.
It is still identifying customer requirements but expects to have a viable
tool next year.
Pears said: “Hybrids have a lot of possible configurations, so the
system is much more complex. Simulation tools become more important because
you need to simulate before you fix the configuration – that’s
when your costs start to increase.”
Romax is also working on ways to introduce automatic design for Six Sigma
and Taguchi analysis into the tool. The idea is to be able to specify
all the tolerances within the model and then predict the variability automatically.
But this will take years to accomplish. Such functions run the model through
large numbers of cycles – Romax is working on ways to speed up its
core analysis.
© Automotive Engineer, 2007 |
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