| 1. |
- Jacobson, Bo
(författare)
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The Stribeck memorial lecture
- 2003
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Ingår i: Tribology International. - 0301-679X. ; 36:11, s. 781-789
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Tidskriftsartikel (refereegranskat)abstract
- Stribeck's publications Wugellager fur beliebige Belastungen' and 'Die wesentlichen Eigenschaften der Gleit- und Rollenlager' were remarkable for their time. Many of the findings were revolutionary and are still considered quite modem. The method used by Stribeck to calculate load distribution between the balls within a ball bearing is still used today, otherwise, a full finite element analysis would be required. His measurements and analysis of bearing friction is also astonishingly accurate. Most of the ball-bearing types studied by Stribeck are not in use any more, but the deep-groove ball bearings he investigated were similar enough to be compared with modern deep-groove ball bearings. Stribeck measured the friction coefficient 0.0015 and SKF gives the friction coefficient 0.0015 for deep-groove ball bearings in the catalogue from 1989. For the first time, in the SKF catalogue from 1989, the life calculation method took into account contamination also. Stribeck had stated already 100 years ago that cleanliness was very important for bearing life and function. Stribeck's investigation of journal bearing friction, as a function of load and speed, was also extremely important, as he showed the possibility of finding a point of minimum friction for lubricated applications. He also showed that the friction for sliding bearings started at high friction at low speeds, decreased to a minimum friction when metal to metal contact stopped, and then increased again at higher speeds, which is the well known 'Stribeck curve'. (C) 2003 Elsevier Ltd. All rights reserved.
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| 2. |
- Ståhl, Jonas, et al.
(författare)
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A non-Newtonian model based on limiting shear stress and slip planes - parametric studies
- 2003
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Ingår i: Tribology International. - 0301-679X. ; 36:11, s. 801-806
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Tidskriftsartikel (refereegranskat)abstract
- Parametric studies and corresponding results are presented using a theological model based on the limiting shear stress and possible occurrence of slip planes. The model is applied to elastohydrodynamically lubricated line contacts with smooth surfaces and isothermal conditions. A few investigations are carried out where different parameters are varied. The first study investigates the influence on the film thickness distribution due to a variation of the maximum Hertzian pressure when the slide-to-roll ratio is constant. The second study investigates how far the non-Newtonian region propagates at low slide-to-roll ratios for a few different values of the Hertzian pressure. The results show that it is a remarkably small slide-to-roll ratio necessary to cause slip planes in a large part of the Hertzian contact zone. A third study regards the influence of the entrainment velocity on the film thickness generation at different slide-to-roll ratios. Finally some theological parameters are varied. First, only the limiting shear stress at atmospheric pressure (tau(0)) is varied, and second, a few different lubricants are studied, each with their own set of rheological parameters. (C) 2003 Elsevier Ltd. All rights reserved.
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| 3. |
- 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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| 4. |
- 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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| 5. |
- 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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| 6. |
- 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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| 7. |
- 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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| 8. |
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| 9. |
- 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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| 10. |
- 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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