| 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. |
- Brewe, David E., et al.
(författare)
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The effect of vibration amplitude on vapour cavitation in journal bearings
- 1987
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Ingår i: Wear. - : Elsevier BV. - 0043-1648 .- 1873-2577. ; 115:1-2, s. 63-73
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Tidskriftsartikel (refereegranskat)abstract
- Computational movies were used to analyse the formation and collapse of vapour cavitation bubbles in a submerged journal bearing. The effect of vibration amplitude on vapour cavitation was studied for a journal undergoing circular whirl. The boundary conditions were implemented using Elrod's algorithm which conserves mass flow through the cavitation bubble as well as the oil film region of the bearing. In the calculations, 0.1 ε εmax, where ε is the instantaneous eccentricity and 0.4 εmax 0.9 for the different cases studied. For the case 0.1 ε 0.4, no vapour cavitation occurred. For the case in which 0.1 ε < 0.9, vapour cavitation was present for 76% of the total time
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| 3. |
- Broitman, E., et al.
(författare)
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Mechanical and tribological properties of CNx films deposited by reactive magnetron sputtering
- 2001
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Ingår i: Wear. - 0043-1648 .- 1873-2577. ; 248:1-2, s. 55-64
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Tidskriftsartikel (refereegranskat)abstract
- The hardness, elasticity, wear rate and friction coefficient of carbon nitride (CNx) films of defined microstructure and composition are presented. CNx films were deposited by dc reactive magnetron sputtering from a C target in N2/Ar plasma. Films were grown on Si (001), Ni, and HSS substrates to thickness of ~0.5 µm at a total pressure of 3 mTorr with the N2 fraction varied from 0 to 1, and the substrate temperature Ts, varied from ambient to 350°C. The mechanical and tribological properties of the coatings were evaluated by nanoindentation and dry ball-on-disk test. For CNx (0 = x = 0.35) films deposited below 200°C (amorphous structure), the elastic recovery and hardness do not change significantly with increasing N concentration, however, the friction coefficient increases from 0.19 to 0.45, while the coating wear rate is low. For CNx (0 = x = 0.15) films grown at Ts = 350°C, where a transition from a graphite-like to a "fullerene-like" phase occurs, a dramatic increase in hardness and elasticity is observed. Furthermore, the rms surface roughness decreases from 15.0 to 0.4 nm. For 0.15 = x = 0.20, CNx films deposited at Ts = 350°C (fullerene-like phase) exhibit a smooth surface, high hardness and elasticity (~90% recovery), and a coefficient of friction against hard steel of ~0.25. For all substrates, film friction coefficient tends to increase as the nitrogen content in the film is increased. Results also indicate the formation of a transfer layer which improves the tribological properties of the films. © 2001 Elsevier Science B.V. All rights reserved.
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| 4. |
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| 5. |
- Del Din, Massimo, et al.
(författare)
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Wear characteristics with mixed lubrication conditions in a full scale journal bearing
- 1999
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Ingår i: Wear. - 0043-1648 .- 1873-2577. ; 232:2, s. 192-198
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Tidskriftsartikel (refereegranskat)abstract
- Increased awareness of environmental problems has stressed the importance of switching from traditional lubricants to more environmentally friendly alternatives. Different investigations with standard test methods indicate that such a switch is possible without loss of lubricating power. The question arises if the statement above is true for industrial lubricating conditions, without laboratory cleanliness. This paper presents a study of friction and wear in a two grooved journal bearing at different shaft speeds, oil temperatures and contamination levels. A number of tests have been conducted combined with a theoretical analysis of film thickness and lubricating regime. The aim was to investigate whether an environmentally adapted rape seed-synthetic ester oil could replace a traditional mineral oil in a full scale application. The results show that the rape seed-synthetic ester oil gives significantly lower values of wear regardless of the operating conditions and there is also a tendency of lower values of frictional torque compared with the mineral oil. Mainly operating in the mixed lubrication regime, no signs of impending bearing failure have been registered, even though a large amount of silica particles added to the oil gave higher wear values than with uncontaminated oil.
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| 6. |
- Eriksson, Mikael, et al.
(författare)
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Wear and contact conditions of brake pads: dynamical in situ studies of pad on glass
- 2001
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Ingår i: Wear. - 0043-1648 .- 1873-2577. ; 249:3-4, s. 272-278
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Tidskriftsartikel (refereegranskat)abstract
- A pin-on-disc machine has been used to make in situ observations of the dynamics of the contact between brake pad materials and a glass disc, while simultaneously monitoring the friction force. The machine, that is normally used to study elastohydrodynamical film formation, has been slightly modified and fitted with a brake pad material sample instead of a rolling ball. The glass disc facilitates direct microscopy and video recording of the dynamic contact situation using a microscope equipped with a CCD-camera. The most obvious features of the topography of a brake pad are the contact plateaus, small flat islands rising typically a few microns above the rest of the surface. These plateaus are based on the more wear resistant constituents of the pad such as fibres and abrasive particles, but also include softer ingredients. The study shows that the softer and more porous areas surrounding the contact plateaus are worn mainly through three-body abrasion. When the disc is sliding against the plateaus, a large number of small particles (wear debris) are transported through the narrow labyrinth between the surfaces, milling down the weaker constituents. The wear debris can also form very small particles or a continous friction film that may become compacted in front of the initial contact plateaus based on, e.g. a metal fibre. In this way larger but softer plateaus form. The investigation elucidates the rapid changes of the contact situation on a microscale.
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| 7. |
- Gåhlin, Rickard, et al.
(författare)
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Wear volume and wear distribution of hydraulic motor cam rollers studied by a novel atomic force microscope technique
- 1998
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Ingår i: Wear. - 0043-1648 .- 1873-2577. ; 220:1, s. 1-8
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Tidskriftsartikel (refereegranskat)abstract
- The wear volume and wear distribution of chromium steel cam rollers in a high-torque hydraulic motor has been investigated. The cam roller is a part of a novel silicon nitride/chromium steel journal bearing system. An atomic force microscope (AFM) was used to obtain topographical images of the cam roller surfaces before and after use in a full-scale test of the hydraulic motor. The surfaces were marked with small Vickers indentations to make it possible to reposition the AFM to the same locations. To measure the microscopical wear and produce high resolution maps of the local distribution of wear, a recently developed method was utilised. The method is based on two techniques to treat digital topographical images. To map the distribution of wear, the image of the unworn surface is substracted by the image of the worn surface. To measure the wear volume, the bearing histogram is used to calculate a volume relative to a fixed depth. The calculated volume of the unworn surface is then subtracted by the volume of the worn surface. The ceramic/metal system displayed an extremely low wear rate. corresponding to a typical total mean wear depth of about 30 nm. The surface topography showed very limited changes with the minute wear mainly localised to the uppermost part of the surface ridges. The adopted method thus allowed a unique high resolution mapping and volumetric measurement of the initial stages of wear (1 mg lost out of 600 g) on a real machine element. This high resolution analysis is promising for improving tribological testing of real machine elements with long expected wear lives, by reducing the need for excessively accelerated tests or extremely long and costly test durations.
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| 8. |
- Holgerson, Mikael
(författare)
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Apparatus for measurement of engagement characteristics of a wet clutch
- 1997
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Ingår i: Wear. - 0043-1648 .- 1873-2577. ; 213:1-2, s. 140-147
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Tidskriftsartikel (refereegranskat)abstract
- The problem of obtaining smooth gear changing whilst maintaining long life of wet clutches in automatic transmissions demands more knowledge about their behaviour. Experimental and theoretical studies on the engagement of a wet clutch have been carried out. A wet clutch test rig which can apply a drive torque during engagement was developed. The apparatus could also vary the sliding velocity, inertia, force rate, and lubrication. Measured output data included normal force, brake torque, sliding velocity and temperature over time. The input parameters and output characteristics obtained were similar to those in automatic transmissions used in cars. The friction characteristics as well as power and temperature were investigated. A simple model was developed to estimate the engagement performance which gave a good approximation of the performance measured in the tests. The friction is high in the beginning and end of the engagement cycle and lower inbetween. There is a torque peak just before the clutch stops owing to friction characteristics. The maximum developed power occurs at about half of the engagement time, while the maximum temperature is just before clutch stop
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| 9. |
- Höglund, Erik, et al.
(författare)
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Effect of pressurised gases on the viscosity of polyalkylene glycols
- 1997
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Ingår i: Wear. - 0043-1648 .- 1873-2577. ; 207:1-2, s. 24-28
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Tidskriftsartikel (refereegranskat)abstract
- Experiments have been conducted into the effect of pressurised hydrocarbons on the viscosity of a polyalkylene glycol. This lubricant is intended to be used in a sub-sea compressor/pump unit and the risk of diluting the lubricant by the surrounding natural gas, thus reducing the lubricant's viscosity, was to be investigated. Dilution can cause insufficient lubrication of the compressor's moving parts. A pressurised Höppler type viscometer was used for the tests which were conducted at 50 °C with a methane based hydrocarbon mixture and with pure nitrogen. The viscosity of the lubricant was reduced to 24% after 24 h exposure at 100 bar with the hydrocarbon mixture. A gas analysis was performed which indicates that the heavier hydrocarbons used are relatively soluble in the lubricant even at relatively low pressures. Nitrogen was found to have a very limited effect on the viscosity even if some was dissolved in the oil. A long duration test of 11 days was also made with nitrogen. This test did not show any significant reduction of the viscosity.
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| 10. |
- 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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