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| 2. |
- Almqvist, Andreas, et al.
(författare)
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Development of a lubrication simulation model
- 2009
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Ingår i: Svenska mekanikdagarna. - Stockholm : Svenska nationalkommittén för mekanik. ; , s. 74-
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 5. |
- 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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| 8. |
- Matsuoka, Atsuko, et al.
(författare)
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Correlation of sister chromatid exchange formation through homologous recombination with ribonucleotide reductase inhibition.
- 2004
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Ingår i: Mutat Res. - 0027-5107. ; 547:1-2, s. 101-7
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- We conducted the recombination and sister chromatid exchange (SCE) assays with five chemicals (hydroxyurea (HU), resveratrol, 4-hydroxy-trans-stilbene, 3-hydroxy-trans-stilbene, and mitomycin C) in Chinese hamster cell line SPD8/V79 to confirm directly that SCE is a result of homologous recombination (HR). SPD8 has a partial duplication in exon 7 of the endogenous hprt gene and can revert to wild type by homologous recombination. All chemicals were positive in both assays except for 3-hydroxy-trans-stilbene, which was negative in both. HU, resveratrol, and 4-hydroxy-trans-stilbene were scavengers of the tyrosyl free radical of the R2 subunit of mammalian ribonucleotide reductase. Tyrosyl free radical scavengers disturb normal DNA replication, causing replication fork arrest. Mitomycin C is a DNA cross-linking agent that also causes replication fork arrest. The present study suggests that replication fork arrest, which is similar to the early phases of HR, leads to a high frequency of recombination, resulting in SCEs. The findings show that SCE may be mediated by HR.
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| 9. |
- 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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| 10. |
- Sahlin, Fredrik
(författare)
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Lubrication, contact mechanics and leakage between rough surfaces
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Mechanical components with interacting surfaces are central in everyday life. Interactions between surfaces, such as the sliding, rolling and bouncing, on one another may be found everywhere. These surfaces are often required to withstand severe conditions resulting in wear, which may ultimately lead to failure. To reduce the risk of failure, a lubricant is often added between the surfaces that partly or completely separates the surfaces from direct mechanical contact. The physical conditions existing between two interacting surfaces are complex, with parameters such as colliding surface asperities, mechanical deformations, lubricant flow transport and chemical reactions. A combination of all these parameters will affect the output parameters of interest for the consumer, e.g., friction, wear rate and leakage a.k.a. energy efficiency, service life and environmental impact in other terminologies. To design components with improved performance, more knowledge of the tribological (interacting surfaces) interface is needed both from experimental and theoretical viewpoint. In this work, various aspects of the lubrication between surfaces were theoretically simulated, to gain a greater understanding of tribological interfaces and to develop tribological design tools. When the interacting surfaces are separated by a thick fluid film, the influence from surface asperities is small and the surfaces can be treated as if they were smooth. For this type of lubrication condition, Computational Fluid Dynamics (CFD) is used to investigate the influence from a surface pattern applied onto one of the two interacting surfaces. It is shown that parallel surfaces generate a pressure increase originating from fluid inertia between the surfaces as a result of introducing the micro-pattern. In some lubricant films low pressures may occur at region of an expanding gap between the interacting surfaces. A liquid lubricant can only resist small tensile stresses until it cavitates into a mixture of gas and liquid. Hence, a cavitation model is presented that accommodates for an arbitrary lubricant compressibility. It was found that the geometry and lubricant starvation at the inlet of the tribological interface, as well as the compressibility model, are significant factors for load carrying capacity of the lubricant when cavitation is considered. For thin lubricant films, surface roughness becomes important in the performance of the tribological interface. Direct numerical simulations of the interface with measured surface roughness requires too many degrees of freedoms to be accounted for in computations. Therefore, a homogenization method is used, where the gap between the surfaces in the tribological interface may be modeled by two scales; a global geometry scale and a local surface roughness scale, where the method enables the two scales to be treated separately. A method to generate dimensionless flow factors to compensate 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. It is shown that the cumulative distribution of heights of the surface roughness (bearing ratio) completely determines the lubricating conditions for two-dimensional roughness and that the effects of the roughness increase as the film thickness decreases. By further decreasing the film thickness, into the mixed lubrication regime, the surface asperities will start to collide and take over some of the load carried by the fluid. The surface roughness has a crucial influence on the performance in this regime. A model to simulate the linear elastic perfectly plastic deformation of rough surfaces is developed. The model is based on FFT to improve the computational efficiency. Thus, the model is suitable to accept periodic input, which is a demand for the homogenization method previously mentioned. To consider both the asperity collisions and the hydrodynamic effects, a mixed lubrication model is developed capable of using three-dimensional measured surface roughness as input. The model is based on computing flow factors that carry the effects of a specific surface roughness in all regimes from completely separated surfaces to dry contact and full asperity deformation. An efficient simulation procedure is described, from importing the roughness measurement data to the simulation of a complete application. Linear elastic perfectly plastic displacement is considered and a homogenization method for fluid transport is used. The mixed lubrication model is validated through experiments with good correlation. It is shown that real deterministic surface roughness measurements may be efficiently imported and used in the model. Also, any global geometry may be simulated in any regime once the rough surface flow factors have been calculated. An important property of a tribological interface, especially in seals, is the flow or leakage through the contact. In many applications, leakage is crucial in terms of environmental impact. By using the mixed lubrication model previously developed, leakage through a range of measured elastomer and seal surfaces is investigated (see cover figure). It is found that the mixed lubrication model permits an efficient analysis of the leakage, even though real measured surface roughness was used as input. Moreover, the valley roughness parameters are shown to be important in characterizing the leakage and the peak roughness parameters are important for the percolation threshold.
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