| 1. |
- Björling, Marcus
(författare)
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Friction in elasto hydrodynamically lubricated contacts : the influence of speed and slide to roll ratio
- 2011
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Reducing losses in transmissions has become a high priority in the automotive market during recent years, mainly due to environmental concerns leading to regulations placed on the automotive industry to drive the development of vehicles with lower fuel consumption and CO2 emissions. Rising fuel prices and increasing environmental concerns have also made customers more prone to purchase more fuel efficient vehicles. In addition to the fuel savings that could be achieved by increased efficiency of transmissions there are other benefits as well. A more efficient transmission will in general generate less heat, and experience less wear. This will lead to fewer failures, longer service life of components, and possibly longer service intervals. Furthermore this implies a possibility to reduce coolant components, thus reducing the total weight of the system, leading to a further decrease in consumption and a lower impact on the environment due to a reduction of material usage. A low weight design is also beneficial for vehicle dynamics and handling. In addition to the automotive market, gears are extensively used in many other fields, such as wind power and industry. In some cases a substantial part of the losses in a gear transmission is attributed to gear contact friction due to sliding and rolling between the gear teeth. To better understand the contact friction phenomena in gears an experimental apparatus capable of running under similar conditions to gears is chosen. By using a ball on disc test device the contact friction can be measured in a broad range of speeds and slide to roll ratios (SRR). The results are presented as a 3D friction map which can be divided into four different regions; Linear, Non-linear, mixed and thermal. In each of these regions different mechanisms are influencing the coefficient of friction. Several tests have been conducted with different lubricants, EP- additive packages, operating temperatures, surface roughness and coatings. The method gives a good overview, a system fingerprint, of the frictional behaviour for a specific system in a broad operating range. By observing results for different systems, it is possible to identify how different changes will influence the coefficient of friction in different regimes, and therefore optimize the system depending on operating conditions. Among other things the tests have shown that reducing base oil viscosity increases contact friction in most operating conditions, introducing an earlier transition from full film to mixed lubrication, and increasing full film friction in many cases with high sliding speeds. An increase in operating temperature could both increase, and decrease the coefficient of friction depending on running conditions. Introducing smoother surfaces reduces the coefficient of friction at lower entrainment speeds since thinner lubricant films are required to avoid asperity collitions. By applying a DLC coating on one or both surfaces in a EHL contact, the friction coefficient is shown to decrease, even in the full film regime.
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| 2. |
- Sahlin, Fredrik
(författare)
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Hydrodynamic lubrication of rough surfaces
- 2005
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Interacting surfaces are frequently found in mechanical systems and components. A lubricant is often added between the surfaces to separate them from mechanical contact in order to increase life and performance of the contacting surfaces. In this work various aspects of hydrodynamic lubrication are investigated theoretically. This is where interacting surfaces are completely separated by a fluid film which is often the desired operating condition of machine components when wear and friction is to be reduced. Different flow regimes can be identified within the scope of hydrodynamic lubrication. If the surfaces are separated by a thick fluid film the influence from surface asperities is small and the surfaces can be treated as smooth. If the rate of change in film thickness with respect to the spatial directions is significantly large and if the flow velocity or Reynolds number is large, the ordinary fluid mechanical approach treating viscous flow with Computational Fluid Dynamics (CFD) has to be used. CFD is used to investigate influence from the use of an artificial microscopic surface pattern on one of the two interacting surfaces. The influence from the pattern is isolated from any other pressure generating effects by keeping the interacting surfaces parallel. Results are shown for different shapes of the micro-pattern. If the Reynolds number decreases, the system enters a regime called Stokes flow where the inertia effects are neglected. The full CFD approach is compared with the Stokes for various physical and geometrical cases. If the change in film thickness is small in the spatial directions, the thin film approximation is applicable and the full momentum equations describing fluid flow together with the mass continuity equation can be reduced to the Reynolds equation. Depending on boundary conditions, low pressures can occur at location of expanding fluid gap leading to tensile stress applied to the lubricant. However, a real liquid lubricant can only resist small tensile stresses until it cavitates into a mixture of gas and liquid. This often happens close to atmospheric pressure due to contamination and dissolved air into the liquid and occurs at higher pressures than the actual vaporization. To avoid pressures reaching too low levels, a general cavitation algorithm applied to the Reynolds equation is presented that accommodates for an arbitrary density-pressure relation. It is now possible to model the compressibility of the lubricant in such a way that the density-pressure relation is realistic through out the contact. The algorithm preserves mass continuity which is of importance when inter-asperity cavitation of rough surfaces is considered. For small film thicknesses the surface roughness becomes important in the performance of the lubricated contact. Even the smoothest of real surfaces is rough at a microscopic level and will influence the contact condition. The Reynolds equation still applies since the heights of the surface asperities are small compared to the spatial elongation. Treatment of the roughness of a real surface in a deterministic fashion is however beyond the scope of today's computers. Therefore other approaches need to be employed in order to take the surface roughness into account. In this work a homogenization method is used where the governing equation of the flow condition is formulated with a two-scale expansion, the global geometry and the roughness. Solutions are achieved for the limit of the roughness wavelength approaching zero and the method renders a possibility to treat the two scales separately. A method to generate dimensionless flow factors compensating for the surface roughness is developed. The flow factors, once solved for a particular surface, can be used to compensate for the surface roughness in any smooth global problem for any film thickness.
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