 The
onset of TEI is determined by a perturbation technique whereby one
imposes a disturbance in the system and examines its behavior. If the
disturbance dies away, then the system will tolerate the perturbation.
On the other hand, if the disturbance grows, it will lead to TEI. Our
research program focuses on the understanding of the nature and the
prediction of TEI in wet clutches used in automotive transmissions.
Therefore, TEI theory should take into account characteristics that may
play an important role in wet clutches, such as the surface roughness,
fluid film between the gap, permeability of the friction material,
geometry of discs, and varying operating speed. One of the primary
objectives of TEI theory is to develop a realistic model for a wet
clutch and, therefore, to determine the critical speed which, if
exceeded, could result in the formation of hot spots. Its main goal is
to provide the designer with a useful, predictive tool; one that
identifies the key parameters that control the process so that such
parameters could be appropriately chosen at the design stage. Choosing
the proper materials or alteration of the operating conditions would
reduce the chance of the formation of hot spots.
The theory of thermoelastic instability has recently gained
considerable attention in the literature. However, the great majority
of the applications of the TEI theory thus far have been limited to the
dry contact of smooth surfaces. To overcome these limitations,
analytical models are developed to take into account characteristics of
wet clutches. The derivations are very general and thus useful for
investigating a variety of systems and the solutions provide a complete
map for thermoelastic instability. The summary of developed models is:
1.
A transient 3-D thermohydrodynamic model was developed to investigate
the thermal aspects of the engagement process in a wet clutch. The
objective of this analysis is to predict the variation of temperature,
pressure and torque as a function of time during the engagement.
Results can be used to determine the important variables for developing
TEI models. The analysis includes surface roughness, permeability and
waviness of friction material, radial grooves, velocity slip condition,
and centrifugal force of ATF [1].
2. A simple insulator-conductor model was developed to examine the
role of surface roughness on the initiation of TEI. It is shown that
with consideration of surface roughness the severity of applied load on
the system naturally emerges in the formulation of the problem leading
to a more realistic TEI model. Note that traditional theories predict a
single critical speed regardless of the magnitude of its applied load
[2].
3. An idealized model consisting of a conductor separated by a film
of liquid lubricant from an insulator was developed for treating TEI in
wet clutches. Two governing equations are derived to include the
hydrodynamic pressure and viscous dissipation heat. Furthermore,
several new dimensionless parameters are introduced that allow one to
characterize the behavior of lubricated systems and their
susceptibility to TEI. TEI formulation reveals that there are two
distinct categories of solutions: the moving wave solution and the
stationary wave solution. The moving wave solution completely describes
the thermoelastic instability. The stationary wave solution
corresponding the lower limit of the moving wave solution is derived
when the wave speed is zero [3].
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