Gear rattle is caused by engine torsional vibration (engine order response) imparted to the
transmission components, further causing the gears to oscillate within their functional
backlashes. These oscillations lead to the repetitive impact of gear teeth, which lead to
noisy responses, referred to as gear rattle. The lack of in-depth research into the effect of
lubricant on gear rattle has been identified as a deficiency in the previous research in
rattle. The aim ofthe current work is to address this shortcoming.
The thesis outlines a new approach in investigating the problem of idle gear rattle. The
approach is based on the assumption that under idling condition the teeth-pair impact
loads are sufficiently low and the gear speeds are sufficiently high to permit the
formation of a hydrodynamic lubricant film between the mating gear teeth. This film
acts as a non-linear spring-damper that couples the driver and the driven gears.
A torsional single-degree of freedom model is used in the development of the theory. The
model is then expanded into a seven-degree of freedom torsional model and finally into
an Il-degree of freedom model that also includes the lateral vibrations of the supporting
shafts. The Il-degree of freedom model is based on a real life transmission that is also
used in experimental studies to validate the model.
It is found that lubricant viscosity and bearing clearance (lubricant resistance in squeeze)
play important roles in determining the dynamics of the system and its propensity to
rattle. At low temperatures, the lateral vibrations of the shafts, carrying the gears interfere
with the gear teeth impact action. The severity of rattle is determined by the relationship
between the entraining and squeeze film actions of the hydrodynamic film. When the
latter dominates, the system can rattle more severely.
The numerical results are found to correlate well with the experimental findings obtained
from vehicle tests in a semi-anechoic chamber and also with those from a transmission
test rig in the powertrain laboratory.
Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.