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Micro-Hydrodynamic Lubrication - Implementation of a Local Scale Model

Mårtensson, Henrik LU (2016) MME820 20161
Machine Elements
Abstract
This master’s thesis has been carried out in collaboration with the dynamic, acoustic and tribology simulation group in the engine development department at Scania CV in Södertälje, Sweden.
When simulating tribological conjunctions, such as a journal bearing, it is often necessary to take the surface topography of the two bearing surfaces into account. The surface topography can influence the hydrodynamic pressure in a bearing and some topographies are known to provide better lubrication than others. In addition, in the mixed lubrication regime there is also asperity contact between the surfaces. However, since the roughness is a short wavelength feature, accounting for the surface topography directly when solving the hydrodynamic and the... (More)
This master’s thesis has been carried out in collaboration with the dynamic, acoustic and tribology simulation group in the engine development department at Scania CV in Södertälje, Sweden.
When simulating tribological conjunctions, such as a journal bearing, it is often necessary to take the surface topography of the two bearing surfaces into account. The surface topography can influence the hydrodynamic pressure in a bearing and some topographies are known to provide better lubrication than others. In addition, in the mixed lubrication regime there is also asperity contact between the surfaces. However, since the roughness is a short wavelength feature, accounting for the surface topography directly when solving the hydrodynamic and the contact mechanics problem would require a mesh which is too fine to be of any practical use in an industrial development project, as the computational time would be immense. A common approach to overcome this problem is the multi-scale method. The problem is dived into a global scale problem and a local scale problem. On the local scale, which deals only with the surface topography, correction factors for the surface roughness or “flow factors” are calculated which is then used when the global scale problem, the bearing with smooth surfaces, is solved.
In this master’s thesis, a local scale model has been implemented. It uses an asperity contact mechanics code based on the elastic half-space theory, allowing the flow factors to be calculated using the deformed surface topography in the mixed lubrication regime. Using the implemented local scale model, flow factors for the stochastic, averaged and homogenised Reynolds equation are calculated and compared for both artificially generated and real measured surfaces. Different boundary conditions for the micro-bearing problems in the averaged flow model are investigated as well. Furthermore, a hydrodynamic model of a slider bearing is modelled and simulated in Matlab using a mass-conserving cavitation algorithm together with an elasto-hydrodynamic model of a journal bearing in the multibody simulation software AVL Excite Power Unit.

Simulation results from both global scale models suggest that a surface with longitudinal roughness, i.e. roughness striations along the sliding directions would result in better tribological performance in terms of higher minimum oil film thickness and lower hydrodynamic and asperity friction power loss than a surface with transversal roughness. However, the roughness amplitude seems to be most dominant influential factor in the performance of a tribological contact and that the actual surface topography is of minor importance. Also, it is concluded that the influence of the surface roughness and flow factors is only apparent for small separations in or close to the mixed lubrication regime, i.e. for high loads or low sliding speeds. (Less)
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author
Mårtensson, Henrik LU
supervisor
organization
course
MME820 20161
year
type
H2 - Master's Degree (Two Years)
subject
language
English
id
8889221
date added to LUP
2016-08-25 16:19:04
date last changed
2016-08-25 16:19:04
@misc{8889221,
  abstract     = {This master’s thesis has been carried out in collaboration with the dynamic, acoustic and tribology simulation group in the engine development department at Scania CV in Södertälje, Sweden.
When simulating tribological conjunctions, such as a journal bearing, it is often necessary to take the surface topography of the two bearing surfaces into account. The surface topography can influence the hydrodynamic pressure in a bearing and some topographies are known to provide better lubrication than others. In addition, in the mixed lubrication regime there is also asperity contact between the surfaces. However, since the roughness is a short wavelength feature, accounting for the surface topography directly when solving the hydrodynamic and the contact mechanics problem would require a mesh which is too fine to be of any practical use in an industrial development project, as the computational time would be immense. A common approach to overcome this problem is the multi-scale method. The problem is dived into a global scale problem and a local scale problem. On the local scale, which deals only with the surface topography, correction factors for the surface roughness or “flow factors” are calculated which is then used when the global scale problem, the bearing with smooth surfaces, is solved.
In this master’s thesis, a local scale model has been implemented. It uses an asperity contact mechanics code based on the elastic half-space theory, allowing the flow factors to be calculated using the deformed surface topography in the mixed lubrication regime. Using the implemented local scale model, flow factors for the stochastic, averaged and homogenised Reynolds equation are calculated and compared for both artificially generated and real measured surfaces. Different boundary conditions for the micro-bearing problems in the averaged flow model are investigated as well. Furthermore, a hydrodynamic model of a slider bearing is modelled and simulated in Matlab using a mass-conserving cavitation algorithm together with an elasto-hydrodynamic model of a journal bearing in the multibody simulation software AVL Excite Power Unit.

Simulation results from both global scale models suggest that a surface with longitudinal roughness, i.e. roughness striations along the sliding directions would result in better tribological performance in terms of higher minimum oil film thickness and lower hydrodynamic and asperity friction power loss than a surface with transversal roughness. However, the roughness amplitude seems to be most dominant influential factor in the performance of a tribological contact and that the actual surface topography is of minor importance. Also, it is concluded that the influence of the surface roughness and flow factors is only apparent for small separations in or close to the mixed lubrication regime, i.e. for high loads or low sliding speeds.},
  author       = {Mårtensson, Henrik},
  language     = {eng},
  note         = {Student Paper},
  title        = {Micro-Hydrodynamic Lubrication - Implementation of a Local Scale Model},
  year         = {2016},
}