Mixed Friction Regime Research Articles

Experimental study on the feasibility of alternative materials for tilting pad thrust bearings operating in transition to mixed friction

In hydrodynamic bearings traditional bearing alloys: Babbitts and bronzes are most frequently utilized. Polymer sliding layers are sometimes applied as a valuable alternative. Hard diamond-like carbon (DLC) coatings, which are also considered for certain applications may show some advantages, as well. Although material selection is of secondary importance in a full film lubrication regime it becomes important in mixed friction conditions, which is crucial for bearings with frequent starts and stops. Experimental research aimed at studying the performance of fluid film bearings in the specific operating regime, including the transition to mixed friction, is described in the paper. The tests were carried out on four tilting pad bearings of different material compositions: Steel/bronze, DLC/steel, steel/polyether ether ketone (PEEK), and steel/Babbitt. The tests comprised stopping under load and reproduction of the Stribeck curve by decreasing rotational speed to very low values, and observing the changes of friction force during the transition to mixed friction regime. Analysis of the transition conditions and other results showed clear differences between the tested bearings, illustrating the feasibility of less popular material compositions for bearings operating in specific conditions. More specifically, the DLC/steel bearing was demonstrating superior performance, i.e. lower friction, transition to mixed friction occurring at higher load, and more stable performance at start-stop regime over the other tested bearings.

Multiscale Wear Simulation in Textured, Lubricated Contacts

Specific surface textures may reduce the friction and increase the lifting forces in lubricated contacts. For the detrimental operating condition of mixed friction, wear is induced by the solid contact. In this study, a methodology for wear calculation in textured, lubricated contacts is presented that considers the wear-induced surface topography evolution. Based on the Reynolds differential equation, the mass-conserving cavitation model according to Jakobsson, Floberg, and Olsson (JFO), a wear-dependent asperity contact pressure curve and the wear equation according to Archard, wear in a wedge-shaped, textured lubrication gap was calculated. The results show the wear behavior of textured lubrication gaps. Based on the wear simulations, the tribological behavior of the textured surfaces compared to smooth surfaces is discussed. It is evident that textures, which improve the tribological performance in the hydrodynamic lubrication regime, are not necessarily associated with low wear values in a lubrication condition in the mixed friction regime. The analysis of the wear-dependent parameters initially showed a ‘recovery’ of the tribological system with increasing wear until the performance decreased again after a specific reversal point. This behavior is attributed to the relative position of the surface textures in the lubrication gap.

Fretting friction and wear behaviors of spiral wound gasket (SWG) sealing surface

Abstract The vibration of the pipeline act on the gasket sealing surface in the form of fretting wear, reducing the sealing performance. In this paper, an experimental study was described on the fretting wear and frictional mechanism of interface between spiral wound gasket (SWG) and flange. The results showed that the gasket winding structure affects the variation of the friction coefficient fluctuation. Moreover, only the fretting behavior of mixed friction regime (MFR) and slip regime (SR) can be expressed stably. Additionally, fretting wear is dominated by abrasive and adhesive wear, and plastic deformation occurs under high load. The intense wear behavior mainly presented in the circumferential direction of small displacement and the radial friction process of large displacement.

Surface texturing of rolling elements by hard ball-end milling and burnishing

Constant velocity joint shafts are machine elements within the driveshaft of cars. The tribological conditions within the raceways are characterized by mixed friction. To improve the lifetime of these machine elements, surface topography and surface integrity have to be adjusted. The positive effect of subsurface compressive residual stresses on the rolling fatigue strength for roller bearings is well known. Additionally, the positive effects of surface textures on tribological conditions in the mixed friction regime are known from cylinder liners. Within this paper, a novel approach to generate tribologically optimized surfaces by hard ball-end milling and an additional mechanical surface treatment by roller burnishing is tested and applied. First, the effects of both processes on the resulting surface topography are analyzed separately. In addition, the interactions between ball-end milling and burnishing are investigated. Concluding, this paper provides parameters to generate a plateau surface by the combination of ball-end milling and roller burnishing.

Relative statistic fluctuation of sliding friction force

The relative fluctuation of sliding friction force in a stationary friction regime has been estimated by the statistic theory of the fluctuation of adaptive physical variables. General mathematical equations and corresponding estimations of relative fluctuations have been given for various friction regimes, such as the regime of mixed and boundary friction, including friction with cladding metal nanoparticles in lubricant and friction with selective transfer. It has been found that relative fluctuations of friction forces are fairly high compared with the fluctuation of physical variables related to the volume of thermodynamic systems. Relative fluctuations for a mixed friction regime are particularly high and can reach 1–10% of the average friction force.

Influence of run-in procedures and thermal surface treatment on the anti-wear performance of additive-free lubricant oils in rolling bearings

When rolling bearings are operated in the boundary and mixed friction regime, there is a risk of wear which can be minimized by the application of suitable lubricants with anti-wear additives. These lubricants form surface-bound additive-based tribolayers under tribological strain. The trend towards a reduction of additive concentration requires new methods of effective wear protection. In this study, two additive-free lubricants were investigated using highly wear-critical cylinder roller thrust bearings. The results show that both oils exhibit insufficient wear protection. This protection can be increased for one of the oils by an iron oxide-based tribolayer which is built up during a suitable run-in procedure. Furthermore, it is shown how the run-in tribolayer can be reproduced by a thermal treatment of the bearings. The increase in wear protection can be explained by a better lubricant–surface interaction due to the tribolayers, which leads to a better separation of the steel surfaces. The tribolayers are characterized using EPMA, FIB–TEM, and nano-indentation analyses.

Friction behaviour in strip draw test of pre-stretched high strength automotive aluminium alloys

Aluminium alloys are being increasingly used in the automotive industry as a means of light-weighting the vehicle body to reduce emissions. The forming characteristics of aluminium alloys, however, differ from those of conventional deep-drawing quality materials, and the drive to reduce vehicle development times has led to reliance on simulation of forming operations to establish feasibility in the early design stages. It is known that friction has a major influence on the forming characteristics of a material and changes as the material plastically deforms and surface roughness increases, causing a transition from hydrodynamic to a mixed friction regime where metal to metal contact is increased. In the simulation environment however, friction is usually assumed to be constant. The development of higher strength alloys requiring higher forming forces suggest that a constant friction value may not always be a valid assumption. In this paper the friction behaviour of three commercially available automotive aluminium grades are compared, at different levels of strain, using a strip draw test. The results show that for a material with a standard mill finish friction is anisotropic, remaining so even at relatively high strains and the anisotropy is independent of the direction of strain. The friction coefficient increases with the level of strain but the use of solid wax lubricants helps to overcome this maintaining a uniform friction at strains up to 10%. For aluminium with an EDT finish the roughening effects due to plastic strain are completely overcome and again a constant friction value is valid. In some alloys the formation of stretcher strains also help to reduce the metal to metal contact promoting hydrodynamic friction and a constant friction coefficient at lower strains.

Ice friction: the effect of thermal conductivity

AbstractThe effect of thermal conductivity on ice friction is studied systematically for different metallic slider materials over a wide range of temperatures, and sliding velocities. By thermally insulating the slider with fiberglass, the isolated effect of thermal conductivity on ice friction is investigated. A decrease of the friction coefficient in the boundary friction regime and an earlier onset of the mixed friction regime in terms of sliding velocity are found. Furthermore, the dependence of the ice friction coefficient on sliding velocity is compared for different sliding materials. It is found that the influence and importance of thermal conductivity decreases with increasing sliding velocity.

Parallel Sliding Load Support in the Mixed Friction Regime. Part 2—Evaluation of the Mechanisms

When two parallel surfaces slide parallel to each other in the presence of a liquid, classical lubrication theory shows that no load carrying capacity should result. In Part 1 of this work the experimental data showing that such load support does occur are reviewed. In this paper several of the proposed mechanisms for parallel sliding load support are evaluated by combining the mechanism with a mixed friction model and comparing predictions to experimental results. Both water and oil type bearings are considered. While there are exceptions, it has been found that in general the thermal wedge, microasperity lubrication, microasperity collisions, and squeeze effects cannot account for the large apparent load support in the experimental cases studied. However, small deviations from parallel geometry have a strong effect and can easily account for the observed load support, but several questions remain. It is concluded that future investigations should be directed to developing a better understanding of how favorable macrogeometries might be developed and searching for other sources of pressure generation in parallel sliding.

Parallel Sliding Load Support in the Mixed Friction Regime. Part 1—The Experimental Data

When two parallel surfaces slide parallel to each other in the presence of a liquid, classical lubrication theory shows that no load carrying ability of the lubricant should result. In experiment after experiment it has been clearly demonstrated that a large and useful load carrying capability often does develop in such situations. Since the successful operation of a significant fraction of sliding bearings and face seals may depend upon this phenomenon, a better understanding would be very useful for bearing and seal design. In this paper much of the known data on parallel sliding experiments are reviewed and compared. Seals as well as bearings are included in this data base. A wide range of conditions and viscosities are included. Some recent work on parallel sliding in water is examined. A comparison of parallel sliding to tilted sliding is also made. It is shown that a strong load support mechanism is present in all of the experimental results. The experiments clearly show that as speed is increased the bearing surfaces are lifted up such that asperity contact and friction are reduced. While explanations are often given for individual sets of experiments, the pervasiveness of this behavior suggests that perhaps there is some mechanism common to all parallel sliding which is not well understood. A mixed friction model is developed and used to explain some of the results. Conclusions are reached concerning several characteristics of parallel sliding. The paper concludes that given the importance of this phenomenon, careful evaluation of the various possible load support mechanisms should be made so that an effective direction for further research can be established.

Thermomechanical aspects of mixed friction in concentrated contacts

Important aspects of partial elastohydrodynamic lubrication are discussed. For heavily loaded counterformed machine parts such as cams and followers, gear teeth, and ball- and roller-bearing elements working in the mixed friction regime, thermomechanical effects on the micro scale have been investigated. The temperature distribution resulting from friction has been determined, assuming that an axially symmetrical source of heat occurs on the peak of the contacting asperity. The thermal deformations and stresses in interacting asperities are obtained with the finite element method. A statistical analysis of the thermomechanical quantities has been carried out. Conclusions about the temperatures and the thermal stresses in contacting asperities have been formulated. A thermoelastic coupling of the interacting rough surfaces has been proposed as a mechanism of the scuffing mode of failure in concentrated contacts.