Asperity pressure

Some preliminary asperity pressures calculation with the found parameters, suggest that the usual film parameter on the Stribeck curve can lead to wrong conclusions about the tribological severity. Some discussion about the expected asperity pressures of new and worn ICE cylinder bores are also presented. 

The micro-contact pressures, that arise when a lubricated piston ring slides against the cylinder bore, were calculated with the computer program described in [15]. The asperity pressures were calculated according the G-W model. The hydrodynamic pressures are calculated by a simplified Reynolds equation where the calculation of the squeeze effect was turned off. Increase of oil viscosity by pressure was not considered, neither shear thinning effects. The following condition was investigated ... 

Pc Pf = contact (asperity) pressure and fluid (lubricant) pressure... 

Most of the disagreement with the more exact solution appears in the low speed and heavy asp ty contact regions where asperity interaction is predominately the mechanism supporting the load. Compared to the universal treatment of the hydrodynamic pressure and asperity contact in the full-scale micro EHL model [4-5], the macro-micro approach superimposes the asperity contact pressure obtained fiom an off-line contact simulation to the hydrodynamic pressure from the average flow analysis, resulting in an under-estimation of asperity deformation but an over-estimation of the average asperity pressure. The next step in the macro-micro analysis partly compensates for this effect. 

The mechanism of boundary lubrication may then be pictured as follows. At the unusually prominent asperities, the local pressure exceeds the yield pressure... 

The coefficient of friction for copper on copper is about 0.9. Assuming that asperities or junctions can be represented by cones of base and height each about 5 x 10" cm, and taking the yield pressure of copper to be 30 kg/mm, calculate the local temperature that should be produced. Suppose the frictional heat to be confined to the asperity, and take the sliding speed to be 10 cm/sec and the load to be 20 kg. 

Dry Sliding. When two surfaces mb, the real area of contact involves only sufficient asperities of the softer material so that their yield pressure balances the total load (3). As the initial load W increases, the real contact area illustrated in Figure 1 increases proportionately according to the relation... 

Yield pressurep of the asperities is about three times the tensile yield strength for many materials. The real area of contact is frequently a minute fraction of the total area. With a typical bearing contact stress of 3 MPa and a bron2e bearing asperity yield pressure of 500 MPa, for instance, less than 1.0% of the nominal area would involve asperity contact. 

As we said in the introduction, friction absorbs a lot of work in machinery and as well as wasting power, this work is mainly converted to heat at the sliding surfaces, which can damage and even melt the bearing. In order to minimise frictional forces we need to make it as easy as possible for surfaces to slide over one another. The obvious way to try to do this is to contaminate the asperity tips with something that (a) can stand the pressure at the bearing surface and so prevent atom-to-atom contact between asperities (b) can itself shear easily. 

The existence of asperity contacts in mixed lubrication causes great many local events and significant consequences. For example, the parameters describing lubrication and contact conditions, such as film thickness, pressure, subsurface stress, and surface temperature, fluctuate violently and frequently over time and space domain. It is expected that these local events would have significant effects on the service life of machine elements, but experimental measurements are difficult because of the highly random and time-dependent nature of the signals. Only a few successes were reported so far in experimental studies of mixed lubrication, mostly limited to the artificially manufactured... 

The average pressure, Pc, due to asperity contact can be obtained from Eq (5) by dehningp =FM, which gives... 

As a result of asperity contact, the nominal contact zone is split into a number of discrete areas that can be cataloged either to the lubrication region or asperity contact area (Fig. 2). The mean hydrodynamic pressure in the lubrication regions, pi, can be calculated by the average flow model, while contact pressure is estimated via Eq (7). Consequently, the film thickness is determined through numerical iterations to... 

The model has been applied successfully to predicting the performances of bearings, gears, seals, and engines [10-12]. A fundamental limitation of the statistic models is their inability to provide detailed information about local pressure distribution, film thickness fluctuation, and asperity deformation, which are crucial for understanding the mechanisms of lubrication, friction, and surface failure. As an alternative, researchers paid a great interest to the deterministic ML model. 

The word deterministic" means that the model employs a specific surface geometry or prescribed roughness data as an input of the numerical procedure for solving the governing equations. The method was originally adopted in micro-EHL to predict local film thickness and pressure distributions over individual asperities, and it can be used to solve the mixed lubrication problems when properly combined with the solutions of asperity contacts. 

Instead of dividing the computation domain into lubrication regions and asperity contact areas, the mixed lubrication model proposed by the present authors assumes that the pressure distribution over the entire domain follows the Reynolds equation ... 

In this model, there will be no asperity contacts in the traditional sense, but as the film thickness between the interacting asperities decreases below a certain level, the right-hand terms in Eq (9), which represent the lubricant flow caused by pressure gradient, become so insigniheant that the pressure can be predicted by a reduced Reynolds equation [16,17] ... 

As described previously, two different strategies have been developed in the DML model to solve the pressure distributions for hy drod5mamic lubrication and asperity contacts, simultaneously. The advantage and disadvantage of the two methods deserve a further discussion. 

A major shortage of the method is that the border positions and boundary pressure distributions between the hydrodynamic and contact regions have to be calculated at every step of computation. It is a difficult and laborious procedure because the asperity contacts may produce many contact regions with irregular and time-dependent contours, which complicates the algorithm implementation, increases the computational work, and perhaps spoils the convergence of the solutions. 

Due to technical difficulties in measurement, experimental results for the local film thickness and pressure distributions caused by asperities are quite limited that only two experiment cases recently have been reported which are employed here as the comparison reference. 

In summary, the height distribution of surface roughness, characterized by the skewness and kurtosis, may present a significant influence on certain performances of mixed lubrication, such as the real contact area, the load carried by asperities, and pressure distribution, while the average film thickness and surface temperature are relatively unaffected. 

It has been generally recognized that asperities are flattened by contact or hydrodynamic pressure, and the extent of the deformation depends on the load, speed, and slide-to-... 

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