| 1. |
- Almqvist, Torbjörn, et al.
(författare)
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A comparison between computational fluid dynamic and Reynolds approaches for simulating transient EHL line contacts
- 2004
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Ingår i: Tribology International. - 0301-679X .- 1879-2464. ; 37:1, s. 61-69
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Tidskriftsartikel (refereegranskat)abstract
- When simulating elastohydrodynamic lubrication (EHL), the Reynolds equation is the predominating partial differential equation for prediction of the fluid flow. Also very few attempts have been carried out using the full momentum and continuity equations separately. The aim of this investigation is to compare two different approaches for simulation of EHL line contacts where a single ridge travels through an EHL conjunction. One of the approaches is based on the Reynolds equation, addressing the coupling between the pressure and the film thickness. The solver uses the advantages of multilevel techniques to speed up the convergence rate. The other approach is based on commercial CFD software. The software uses the momentum and continuity equations in their basic form, enabling numerical simulations outside the contact regions, as well as in the thin film region to be carried out. The numerical experiments show that, under the running conditions chosen, only small deviations between the two approaches can be observed. The results are encouraging from several viewpoints: validation of the codes, the possibilities of further developments of the CFD approach and the justification of using a Reynolds approach under the running conditions chosen
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| 2. |
- Almqvist, Torbjörn, et al.
(författare)
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The Navier-Stokes approach for thermal EHL line contact solutions
- 2002
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Ingår i: Tribology International. - 0301-679X .- 1879-2464. ; 35:3, s. 163-170
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Tidskriftsartikel (refereegranskat)abstract
- The complicated nature of the EHL-problem has so far forced researchers to develop their own computer codes. These codes are ultimately based on the Reynolds equation, and if thermal EHL-simulations are required, a simultaneous solution of the equation of energy also has to be performed. To date only a few attempts to solve the full equations of momentum and continuity as well as equations of energy have been performed. However, such an approach will give extended possibilities of simulating EHL-contacts; i.e. the computational domain can be expanded and it will be possible to simulate the flow, not only in the contact but also around the contact. Another possibility is to investigate how the altering length scales of the surface roughness influence the behaviour of the flow in the contact. However, the aim of the work presented in this paper is to investigate the possibilities of using a commercial CFD-code (computational fluid dynamics code) based on the above-mentioned equations for simulating thermal EHL. The rheology is assumed to be Newtonian and the equations of momentum and continuity are then commonly referred to as the Navier-Stokes equations (N-S equations). The geometry chosen for the simulations is a smooth line contact geometry, for which the results from the simulations show that it is possible to use the N-S equations for thermal EHL for contact pressures up to approximately 0.7 GPa. The code used in this work is the commercial CFD software (CFX 4.3 user guide). There is a limitation in the N-S approach due to a singularity that can occur in the equation of momentum when the principal shear stresses in the film become too high. However, a thermal approach makes it possible to simulate EHL-contacts at higher loads compared with an isothermal approach, due to the reduction of the viscosity in the former approach. The singularity is not present in the Reynolds approach.
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| 3. |
- Björklund, Stefan
(författare)
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The influence of surface roughness in elliptical contacts
- 2001
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Ingår i: Tribology International. - 0301-679X .- 1879-2464. ; 34:12, s. 841-845
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Tidskriftsartikel (refereegranskat)abstract
- The influence of surface roughness on contact behaviour is of great importance in many tribological situations. In the last decade several methods to calculate the pressure distribution and the real contact area in contacts between rough surfaces have been described. A problem arising for slender elliptical contacts, such as between gear teeth, roller and raceway, cam and follower, etc., is that the size of the contact is much greater than the size of the asperities. Accordingly the number of contact nodes necessary for an accurate solution to the problem becomes excessively large. This paper describes a method to calculate the influence of three-dimensional surface roughness in contacts that are very long in one direction. The method is based on restricting the calculations to a subpart of the real contact area, while the rest of the contact is taken into account by mirroring techniques. The results show that the real contact area is very sensitive to the amplitude of the roughness, while the waviness is less important. An equation is suggested from which the real area can be calculated if the smooth case contact parameters and two roughness parameters are known.
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| 4. |
- Boschkova, K., et al.
(författare)
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Study of thin surfactant films under shear using the tribological surface force apparatus
- 2001
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Ingår i: Tribology International. - 0301-679X .- 1879-2464. ; 34:12, s. 815-822
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Tidskriftsartikel (refereegranskat)abstract
- Static and dynamic behaviour of thin surfactant films in aqueous solution of hexadecyltrimethylammonium salicylate (C16TASal) were investigated using the tribological surface force apparatus. Normal force measurements show that 0.15 mM C16TASal builds up an innermost film of approximately 8-11 Angstrom thickness at each mica surface, indicating that the surfactant adsorbs in a flat conformation. Furthermore, the height of the force barrier at approximately 60 Angstrom is low (ca 2 mN/m) indicating that the second adsorbed layer is easily pushed out. Addition of salicylate salt to 0.15 mM C16TASal give rise to a more close packed structure, with a total thickness of 62-65 Angstrom. indicative of a micellar or bilayer arrangement at the surfaces. Furthermore, the frequency dependence of the shear modulus was investigated both at close separation at the innermost force barrier and at larger separations (up to 300-400 Angstrom). The visco-elastic measurements show that the elasticity modulus, G ', dominates over the loss modulus, G , for all studied cases, indicative of a more solid-like than liquid-like film. Finally, it is shown that shear at high contact pressures induces new aggregate structures at the surface.
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| 5. |
- Glavatskih, Sergei
(författare)
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A method of temperature monitoring in fluid film bearings
- 2004
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Ingår i: Tribology International. - 0301-679X .- 1879-2464. ; 37:2, s. 143-148
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Tidskriftsartikel (refereegranskat)abstract
- In the present paper a method to improve temperature monitoring of fluid film bearings is described. The method is tested in an industrial tilting pad thrust bearing. Test results for steady state and transient operating conditions are reported. Temperatures monitored by a thermocouple through the utilisation of the proposed method are compared to those measured by thermocouples in the pad backing and in the collar. The method is also tested on a PTFE-faced bearing. Test results show that the proposed method improves sensitivity to thermal transients in conventional babbitted bearings and provides adequate means of temperature monitoring in the PTFE-faced bearings.
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| 6. |
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| 7. |
- Kassman Rudolphi, Åsa, et al.
(författare)
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Stationary loading, fretting and sliding of silver coated copper contacts : Influence of corrosion films and corrosive atmosphere
- 1997
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Ingår i: Tribology International. - 0301-679X .- 1879-2464. ; 30:3, s. 165-175
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Tidskriftsartikel (refereegranskat)abstract
- The degradation mechanisms of silver coated copper contacts were studied under stationary loading, reciprocated sliding and fretting conditions. The model contact consisted of two crossed cylinders (Ø = 10 mm) mounted in a small test chamber. Prior to performance testing corrosion films were grown in a large chamber of Battelle class III atmosphere. The tests were performed in four atmospheres, specified with respect to temperature, relative humidity and pollutant gases. The normal force was 20 or 60 N and the contacts were powered with a direct current of 10 A. The study shows that the presence of a thin pre-grown corrosion film on top of the silver coating strongly influences the contact resistance, the friction and the degradation mechanisms of the contact. Generally, the corrosion films reduced the tendencies to adhesion between the contact surfaces, which subsequently reduced the deformation. Moreover, the corrosion films had very different effects in different contact situations. While the impairing effect on the contact resistance was almost neutralized during fretting it was substantial in some sliding cases. In contrast to the pre-grown corrosion film, the test atmosphere was not found to influence the contact performance.
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| 8. |
- Larsson-Kråik, Per-Olof, et al.
(författare)
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Pressure fluctuations as grease soaps pass through an EHL contact
- 2000
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Ingår i: Tribology International. - 0301-679X .- 1879-2464. ; 33:3-4, s. 211-216
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Tidskriftsartikel (refereegranskat)abstract
- Soap lumps entering a grease-lubricated elastohydrodynamic lubrication (EHL) contact were traced as they passed through the contact by using optical interferometry in a standard Ball & Disc apparatus and a high-speed video camera with light enhancer. One specific soap lump could be traced when it was passing through the contact and sharp images of the passing lump could be captured. This paper presents a combined experimental and numerical approach to determine what is happening when such lumps pass through the contact. From the fringe pattern, obtained by optical interferometry, a film thickness map is created by using image processing. This is done for every time step (1 ms) as the lump is passing through the contact. These maps serve as input to a numerical computation of the pressure by assuming elastic deformation theory. Consequently, no assumptions about the rheological behaviour of the grease have to be made. Two greases were studied, based on the same synthetic poly-α-olefin but thickened with Li-12-OH and Li-complex respectively. It is seen that the soap thickened lumps can cause deep elastic indentations accompanied by large pressure fluctuations. The pressure level will in some cases be more than doubled due to lumps entering the contact region. The effects on noise level and operational life are also discussed.
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| 9. |
- Lundberg, Jan, et al.
(författare)
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A new method for determining the mechanical stability of lubricating greases
- 2000
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Ingår i: Tribology International. - 0301-679X .- 1879-2464. ; 33:3-4, s. 217-223
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Tidskriftsartikel (refereegranskat)abstract
- Mechanical stability is of central importance when dealing with the long-term service-length of grease-lubricated roller bearings. Poor stability will lead to consistency degradation of the grease, because of mechanical forces between the rolling parts of the bearing. The result can be leakage of grease through seals, or at worst a total failure of the bearing. The present investigation was initiated because present-day methods for prediction of mechanical stability show weak correlation with real service-length. The aim of the project was to develop a useful alternative. In order to fulfil this, both field tests and laboratory tests were carried out. In the field tests, nine different commercial greases were examined in the wheel bearings of five ore waggons, used for transporting ore by railroad from the Kiruna Mine in northern Sweden to Narvik in northern Norway for shipping to foreign markets. The test ore waggons travelled a distance of about 300,000 km during a period of 3 years. Small samples of greases were taken, on eight different occasions, for consistency testing. After the end of the test period, the damage on the bearings was also studied. In the laboratory tests, new undestroyed greases of the same brand as in the field tests were examined using conventional methods, such as the V2F, the Roll Stability Test and the Grease Worker. Comparisons between the field tests and these laboratory tests indicate poor correlation. In addition to these conventional methods, the relevance of the shear strength of the greases to the prediction of the mechanical stability was also tested. The shear stress τL depends on the applied pressure p, thus τL=τO+γ·p where τO is the shear stress at atmospheric pressure, γ is a property of the lubricant in the same way as viscosity or density. It was found that γ correlates well with the mechanical stability in service. Increased γ values lead to a decrease in the mechanical stability. One reasonable explanation is that high γ values correspond to high shear stresses in the grease, and thus severe conditions for the thickener.
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| 10. |
- Lundberg, Jan
(författare)
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Influence of surface roughness on normal-sliding lubrication
- 1995
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Ingår i: Tribology International. - 0301-679X .- 1879-2464. ; 28:5, s. 317-322
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Tidskriftsartikel (refereegranskat)abstract
- Contact between machine components can involve normal, sliding and rolling motion, either singly or in combination. Combined normal and sliding motion, which occur for example in the meshing of gear teeth and in heavily-loaded rolling elements, can present problems for lubrication. The purpose of the present experimental study was to investigate how surface roughness affects the lubricant film characteristics under conditions of combined normal and sliding motion. The experimental arrangement consisted of a rotating roller which impacted a stationary ball in the presence of a lubricant. Under the same conditions of normal surface approach, increasing the surface roughness significantly decreased the level of roller sliding that could occur without breakdown of the lubricating film. This behaviour was similar to a step function. Of the several surface roughness parameters investigated, only those which involved the maximum peak-to-valley height correlated well with experimental results. In general, surface roughness had a greater effect on oil film breakdown than did either viscosity or load.
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