Lubricant ageing effects on wet clutch friction
characteristics

Abstract: Somewhere in the transmission of vehicles today, a wet clutch can often be
found. Characteristic of this type of clutch is that they operate under
lubricated working conditions. In earlier research, friction
characteristics and performance of wet clutches have been well investigated
by several authors. Studies have also been made in order to understand the
ageing of wet clutches. However, most lifetime studies have been made for
systems with paperbased friction discs and systems involving sintered
bronze friction discs remain unexplored.

Friction discs of sintered bronze is used in the Haldex limited slip
coupling (LSC), an all-wheel drive system used in cars from many different
manufacturers. In order to get a better understanding of how this system
can change over time, the study in this master thesis is focused on how
frictional performance is affected by oxidation of lubricant, testrig
ageing and additive content. This work has been conducted in cooperation
with Haldex Traction in Landskrona and Statoil Lubricants in Nynäshamn.

The oxidation effects on friction performance was examined using a modified
dry-TOST (Waterless Turbine Oil Oxidation Stability Test) on a fully
formulated lubricant. The oxidation time period was divided into five steps
from 48 hours to 408h and for each level of oxidation, a friction
performance test was run using a pin on disc machine.

Also an oil aged in a clutch disc testrig was tested for friction
performance. The test is constructed in order to verify that an oil-
friction disc combination will last the lifetime of the specific
application.

Since lubricant additives are vital to the performance of wet clutches the
effect of reducing the additive concentration in the oil was also studied,
in the range 10 to 100% of the standard additive formulation.

Results showed that a general friction increase can be seen for oxidation,
additive reduction and testrig ageing. Lubricant aged in testrig shows
significantly different friction characteristics with temperature than
lubricants aged by dry-TOST implying that dry-TOST alone is not a
sufficient method to evaluate lubricant ageing.

Further research has to be made in order to understand the ageing of wet
clutches. A better understanding of which mechanisms that are responsible
for the decomposition of a lubricant in a wet clutch system such as the
Haldex LSC is needed. This thesis has focused on lubricant ageing but no
attention has been paid to wear and ageing of friction discs. To
investigate and relate ageing of lubricant and friction discs is another
important task for future research.