| 1. |
- Skåre, Thomas, et al.
(författare)
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Static and Dynamic Friction Processes Under the Influence of external Vibrations
- 1992
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Ingår i: Wear. - : Elsevier BV. - 0043-1648. ; 154:1, s. 177-192
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Tidskriftsartikel (refereegranskat)abstract
- The influence of external vibrations on the frictional force has been studied, using different frequencies, amplitudes, loads and materials. The results show that it is possible to affect the frictional force between two surfaces in contact through vibration. The frictional force can be weakened chiefly through a separation of the surfaces or strengthened by welding phenomena in the contact surfaces. Experiments have shown that either an increase or a decrease in the frictional forces can be obtained when one of the parameters, surface pressure, frequency or amplitude, is changed. Surface roughness, vibration direction, relative speed and materials may also play a determining role. With vibration assistance, the tendency to stick-slip is decreased or eliminated.
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| 2. |
- Höglund, Erik
(författare)
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EHL and the use of image analysis
- 1994
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Ingår i: Wear. - : Elsevier BV. - 0043-1648 .- 1873-2577. ; 179:1-2, s. 49-56
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Tidskriftsartikel (refereegranskat)abstract
- This paper describes three cases where image analysis has been used to evaluate transient phenomena in elastohydrodynamic lubrication (EHL). In the first case, a CCD camera and image analysis are used to evaluate the ball trajectory after impact in the "jumping ball apparatus" for subsequent calculation of the limiting shear stress-pressure coefficient. The error of the method is < 1%. In the second case, the motion of the rollers in a roller bearing is traced with a video camera and image analysis. The rolling and sliding motion in the loaded and unloaded parts of the bearing was clearly visible. Although the shaft speed was only 23.6 rpm, the method can easily be used up to 1500 rpm using a conventional high speed video and up to 27 000 rpm with a 6000 pictures per second video camera. In the third case, the method was applied to interferograms of an EHL point contact to investigate lubricant film thickness. The method uses hue, saturation and intensity values from digitized colour interferometric images together with calibration vales. It can be used to evaluate transient as well as quasistatic phenomena in the contact. The method makes it possible to determine absolute values of film thickness without prior knowledge about the fringe order in the interferogram. The method has been shown to work well in the range 140-700 nm with white light but preliminary tests for considerably thicker films are very promising
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| 3. |
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| 4. |
- Larsson, Per-Olof, et al.
(författare)
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Oil drops leaving an EHD contact
- 1994
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Ingår i: Wear. - : Elsevier BV. - 0043-1648 .- 1873-2577. ; 179:1-2, s. 23-28
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Tidskriftsartikel (refereegranskat)abstract
- Many machine elements are lubricated with oil or grease. The flow of lubricant depends on different parameters such as surface roughness, surface energy, temperature, etc. A known problem with rolling bearings is that lubricants have to be refilled more frequently if a gas stream passes through the bearing. It is therefore important to know the behaviour of the lubricant in order to predict, e.g. bearing fatigue life due to a gas stream. By studying the droplets thrown out into the air after passing an elastohydrodynamic contact, the most important parameters for oil drop formation have been determined. The analysis is based on picture processing and statistical methods. The experiments show that an increase in temperature, viscosity or surface velocity will increase the total drop volume. Decreased contact pressure and pure rolling also increase the total drop volume. The volume flow, passing the Hertzian contact area, varied between 18 and 30% of the air born oil-drop flow and it is reasonable to assume that increased flow of air-borne droplets will decrease the film thickness due to insufficient replenishment.
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| 5. |
- Larsson, Roland, et al.
(författare)
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Elastohydrodynamic lubrication at pure squeeze motion
- 1994
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Ingår i: Wear. - 0043-1648 .- 1873-2577. ; 179:1-2, s. 39-43
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Tidskriftsartikel (refereegranskat)abstract
- Experimental and theoretical studies of elastohydrodynamically lubricated contacts normally assume static or quasi-static conditions. Non-steady conditions are, however, common. In this paper the case of a ball impacting a flat lubricated surface is investigated theoretically. This case implies transient conditions and the lubricating effect is due to pure squeeze action. Pressure and film thickness distributions are computed during impact and rebound. The results of the analysis show the effects of ball mass, initial impact velocity, lubricant properties and the thickness of the applied lubricant layer on the minimum film thickness. Increased impact velocity increases the minimum value of film thickness achieved during the bounce. The damping capacity of the lubricating film is very high at low impact velocity and small ball mass. In fact, the damping is so high that no rebound occurs if the velocity or the ball mass are smaller than certain critical values. The thickness of the lubricant layer has no influence on the results if it is greater than a certain value. If the pressure-viscosity coefficient is increased, the film becomes thicker
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| 6. |
- Larsson, Roland, et al.
(författare)
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Simplified solution to the combined squeeze-sliding lubrication problem
- 1994
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Ingår i: Wear. - : Elsevier BV. - 0043-1648 .- 1873-2577. ; 173:1-2, s. 85-94
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Tidskriftsartikel (refereegranskat)abstract
- The unfavourable elastohydrodynamic lubrication situation in combined squeeze and sliding motion has been analysed both theoretically and experimentally. In experiments a rotating roller impacted and rebounded on a lubricated surface. It was found that oil film breakdown always occurs at the end of the impact time, when the contact force is low. It has also been found that there exists an upper limit for the sliding velocity. Below this limiting velocity no oil film breakdown occurs. This paper is an initial attempt to explain theoretically why oil film breakdown takes place towards the end of the impact, and why an increasing sliding velocity reduces the capability of the oil film to separate the lubricated surfaces. If the oil film's elastic and damping behaviour are taken into consideration it can be shown that a considerable phase shift between maximum contact force and oil film breakdown will arise. It has been found that the squeeze action dominates the pressure formation in the contact and thus the hydrodynamic effect of sliding motion is moderate. Furthermore, several effects, such as non-Newtonian behaviour, surface roughness, temperature rise, starvation and deformations, which are not included in the theoretical model, may decrease the oil film thickness if the sliding velocity increases
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| 7. |
- Lundberg, Jan, et al.
(författare)
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High-speed video photographs of lubrication breakdown in squeeze-sliding contact
- 1992
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Ingår i: Wear. - 0043-1648 .- 1873-2577. ; 157:2, s. 427-433
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Tidskriftsartikel (refereegranskat)abstract
- Using photographs from a high-speed video camera it was confirmed that the main part of breakdown of a lubricating film will appear at the end of the contact time for a contact simultaneously subjected to squeeze and sliding motion. This corresponds with earlier findings using totally different equipment for electric detection of the asperity contact. The present investigation used glass and steel as the lubricated surfaces, instead of steel and steel as was the case in the earlier investigation. In combination with far less stiff equipment, the new materials gave longer contact time and larger elastic deformations of the contact bodies. It was also verified that increased surface roughness, increased sliding velocity and decreased viscosity increase the risk of oil film breakdown
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| 8. |
- Lundberg, Jan
(författare)
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Lubrication of machine elements during combined squeeze and sliding motion
- 1993
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Ingår i: Wear. - 0043-1648 .- 1873-2577. ; 169:2, s. 161-166
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Tidskriftsartikel (refereegranskat)abstract
- The motion when two parts in a machine come into contact can be a normal, sliding or rolling approach, or a combination of the three. The case of combined normal and sliding motion can be very unfavourable from the point of view of lubrication. Nevertheless, this situation does occur, for example in a gear mesh and in heavily loaded rolling-element bearings.The following factors in the case of lubrication of machine elements during combined normal and sliding motion were studied experimentally: oil viscosity, surface texture, shear strength of oil and maximum pressure. The pressure also involves the parameters normal force, Young's modulus, Poisson's ratio and surface curvature.Based on the experimental results, an equation has been deduced which describes how the above-mentioned factors influence the permissible limiting sliding velocity Vsl without oil film breakdown: Vsl=0.127×10-6(v0.1-1.575)(ψ-13.1-1.707)(3840-pmax)This equation agrees well with results from experiments carried out by other authors, and is valid if combined sliding and impact between the machine elements, resulting in a limited contact time, are present.
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| 9. |
- Lundberg, Jan
(författare)
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Viscosity dependence of squeeze/sliding lubrication
- 1992
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Ingår i: Wear. - 0043-1648 .- 1873-2577. ; 155:1, s. 31-36
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Tidskriftsartikel (refereegranskat)abstract
- It has been found experimentally that increased lubricant viscosity will increase the permissible sliding speed without leading to breakdown of the lubricant film while the normal velocity is held at a constant level. The dependence approaches a square root function. It is also apparent that with a squeeze velocity present, increased sliding velocity will decrease the oil-film thickness. An initial attempt at an explanation is given in terms of a combination of Reynolds' equation and further experimental results.
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| 10. |
- Wikström, Victoria, et al.
(författare)
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Wear of bearing liners at low speed rotation of shafts with contaminated oil
- 1993
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Ingår i: Wear. - : Elsevier BV. - 0043-1648 .- 1873-2577. ; 162-164:2, s. 996-1001
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Tidskriftsartikel (refereegranskat)abstract
- An experimental and theoretical investigation was carried out to investigate the relationship between wear of bearing liners and low shaft speed, contamination, oil temperature, bearing load and time. Experimentally, it was found that oil with no external debris added produced only slight polishing of the liners. When the oil was contaminated with 0.02 wt.% iron or quartz particles of a known distribution (less than 32 μm diameter), increased wear was detected. Also, comparison of the results of the clean and iron-contaminated tests with those for quartz-contaminated oil show that the character of the worn liner area has changed from evenly rubbed zones to equally divided polished stripes in the circumferential direction. During the tests, the bearing friction was measured. In no test did the friction rise drastically, as would have been the case if severe wear and scoring had appeared. In actual applications, though, this sometimes occurs. This may be explained by severe contamination in industrial lubricating systems, with larger particles and higher debris concentration. A clean lubricating system is thus crucial in order to avoid heavy wear at low speeds. The experimental results were then compared with those indicated theoretically for film thickness at low speed. The theoretical minimum film thickness at low speed was much smaller than the size of the debris. The results indicate that if So-1 <=340, B/D >=0.73 and C>=0.18 mm, no severe wear will occur.
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