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DISSERTATIONS
Author: |
Bakolas Vasileios |
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Title: |
Mixed friction and the development of micropitting on the flanks of
spur gears. |
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Abstract: |
The goal of
this work was the experimental and theoretical investigation of the
mixed lubrication regime, with special interest for the conditions
relevant to micropitting as well as the investigation of the
existence of a critical parameter that could be used for the
prediction of this particular gear failure.
The search for this critical parameter was concentrated on the
stress field beneath the surface of the active gear flanks, since
the nature of micropitting suggests that the flank fails
mechanically.
For the theoretical investigation of the mixed lubrication contact a
model was developed that is based on the superposition principle.
The model assumes that the contact of two rough surfaces in the
presence of a lubricant can be sufficiently approximated be the
superposition of a lubricated contact of smooth surfaces and a dry
contact between rough surfaces. The major innovation of this model
is that for the first time a mixed lubrication model can predict the
real pressure and shear stress distribution on the surface of the
contacting bodeis, thus allowing the exact calculation of the
subsurface stress field.
For the practical application of this model, knowledge is necessary
regarding various parameters that it is know that the influence the
behavior of a linear contact under mixed lubricating conditions,
such as:
· the bulk temperature field of the active flanks
· the roughness of the mating surfaces
· the physical and rheological properties of the lubricant
· the dry friction coefficient.
At the same time a number of experiments were carried out in order
to collect data that were necessary for the execution and the
verification of the calculations. Due to the fact that the
experiments described in the literature are time-consuming and don't
lead to safe conclusions regarding the existence of a critical
parameter that is connected to the operating conditions, a new
experimental procedure was developed which had the advantage of more
rapid execution and the fact that it provides a clear link between
operating conditions of a gear pair and the occurrence of
micropitting.
The application of the model for the conditions of the experiments
and the subsequent calculation of the subsurface stress field along
the mesh line allowed the identification of a critical shear stress,
the mean maximum shear stress Γ25 , whose value is directly linked
to the appearance and intensity of micropitting.
Thus the experimental procedure in connection with the mixed
lubrication model are capable to determine a critical stress value
for a gear pair system which is described among others by the
material of the flanks, their roughness, the lubricant and the
additives used. This stress can be compared to the stress resulting
of the operation of the gear pair and can be determined with the use
of the mixed lubrication model in order to establish the potential
micropitting risk. |
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