Friction Stribeck curve

Fig. 3. Stribeck curve relating friction coefficient to absolute viscosity ]1, speed N in rpm, and unit loadp.

The process of transition from hydrodynamic to boundary lubrication can be described qualitatively by plotting the measured friction coefficients against film thickness, which depends on the operational conditions, such as load, sliding velocity and lubricant viscosity. A typical diagram known as the "Stribeck Curve is schematically shown in Fig. 27, in which the friction coefficients are given as a function of, ... 

The Stribeck curve gives a general description for the transition of lubrication regime, but the quantitative information, such as the variations of real contact areas, the percentage of the load carried by contact, and changes in friction behavior, are not available due to lack of numerical tools for prediction. The deterministic ML model provides an opportunity to explore the entire process of transition from full-film EHL to boundary lubrication, as demonstrated by the examples presented in this section. 

Dependent upon load, speed, viscosity, temperature and the nature of the lubricated surfaces in relative motion, differing degrees of separation of these surfaces occur. When significant oil film penetration occurs, frictional heating begins to cause wear, and then both friction and wear can be controlled by chemical/physical surface reactions. Within an engine a range of the above conditions operate. The influence of these conditions on friction can be seen from the Stribeck curve. Fig. 3.6. 

This dimensionless term is known as the hydrodynamic factor , where Fig. 8.2 represents the Stribeck curve in the log scale and shows that a compromise must be made with respect to lubricant viscosity, between the friction losses in the region of hydrodynamic (HD) lubrication, and bearing wear when passing through the regime of mixed friction. Both hydrodynamic and hydrostatic bearings operate with infinite service life below some critical value of load and above a critical value of speed assuming other modes of wear such as corrosion and erosion are not experienced. 

Stribeck Curve (1992) see l.M. Hutchings Tribology - Friction and Wear of Engineering Materials. Arnold (Butterworth-Heinemann), London. 

Fig. 19 The "Stribeck curve" variation of the friction coefficient with the ratio of speed and load.

It was shown that the friction force applied on the wafer was directly proportional to the downforce. In a dry pad condition without any slurry, the friction on the wafer was relatively constant with the wafer velocity. However, in the presence of polishing slurry on the pad, the friction force decreased with the wafer velocity (Figure 1.3). This phenomenon can be well explained by the Stribeck curve from tribology (Figure 1.4). 

The Stribeck curve explains the relationship between the coefficient of friction and the lubrication thickness with a certain constant called Hersey number. Hersey number is the multiplication of the viscosity of the lubricant and the velocity of the moving object divided by the pressure applied on the object. 

The Stribeck curve shows that the friction force applied on the moving surface decreases with the relative velocity of the moving object in the presence of lubrication. This is because the thickness of the lubrication film between two objects increases with the relative velocity. In the presence of the abrasive slurry, the friction between the wafer and the pad decreased with the wafer velocity. It is believed that this is caused by increased slurry thickness between the wafer and the pad from higher wafer velocity. 

Figure 11.21 Friction coefficient plotted as a function of fluid viscosity and shear velocity divided by load (Stribeck curve) with corresponding lubrication film thickness.

The effect of polymer brushes in different lubrication regimes is seen in Fig. 4a and b, in which the Stribeck curves obtained from pin-on-disk and MTM measurements are plotted, respectively. The coefficient of friction is plotted against speed multiplied by viscosity for all HEPES-glycerol mixtures both in the presence and the absence of the polymer. As expected, the effect of polymer in reducing the friction is predominantly seen in the boundary-lubrication regime. The effect of polymer on the friction is also extended to the mixed-lubrication regime... 

Lubrication. Friction in the presence of a lubricant, which is commonly used to mitigate high fiiction contacts, is a velocity dependent phenomenon that can be illustrated with the so-called. Stribeck curve (7, 60, 61). Tbee regimes of lubricated fiiction are identified (1) boundary, (2) nuxed, and (3) hydrodynamic lubrication. Frictional problems, such as wear or firiction-force fluctuations, so-called stick-slip motion, are usually associated with the mixed and boundary lubrication regime. Either velocity of the moving counterfaces is too low, pressure on the fiiction contact is too... 

C.H. Bovington, S. Korcek, J. Sorab "The Importance of the Stribeck Curve in the Minimisation of Engine Friction. , Leeds-Lyon Tribology Symposium, Sep 1998... 

Much has been done [3,4,5,6] to reveal the mode of lubrication in such Joints through the measurement of friction in Joint simulators of increasing sophistication. Each simulator was capable of measuring the friction to high accuracy. The recorded values of friction were generally similar to those associated with mixed or boundary lubrication. When plotted in the form of a Stribeck curve, the friction factors revealed variations consistent with the expected characteristics for mixed lubrication, with suggestions of either boundary or fluid-film lubrication when relatively low or high viscosity lubricants were used respectively. 

The velocity-dependent data strongly suggest that the PLL-g-PEG is functioning as a very effective boundary lubricant in this aqueous environment, since the expected increase in friction at low velocities—frequently observed in the Stribeck curve—was not observed in the presence of PLL-g-PEG. 

Figure 6. Engine Stribeck Curve. Upper lines are least-squares fits through the data on oils with no friction modifier (FM) at viscosities above and below 4cP. Lower lines are lower bounds for oils with friction modifier.

Bearing friction is measured as a function of speed at a range of temperatures (up to 150 °C). Stribeck curves generated using this rig show the variation of friction with speed as the lubrication regime changes from full film hydrodynamic to mixed hydrodynamic. 

This simple surface feature enables insight to some important effects within mixed lubrication regime. After these results three different real surface topographies are compared with each other. Finally the friction calculation will be explained and two Stribeck curves are presented. 

The next step is to calculate Stribeck curves from boundary to hydrodynamic lubrication. With the help of the aforementioned assumptions as well as material- and fluid parameters, it is possible to calculate friction coefficients over sliding velocity. Fig. 24 shows the results for the lapped and fig. 25 for the polished surface topography. 

The last section describes the friction calculation with fluid and solid friction. The new model enables the calculation of Stribeck curves over several orders of magnitude. But it has proven that the assumptions concerning solid friction calculation are not sufficient yet. Since it could be shown that solid contact spots cannot be calculated by Reynolds equation but rather must be identified by a... 

The authors applied this technique successfully on the same engine under motored conditions. The results were used to validate an engine valve-train friction model [5]. The experiments were carried out at different engine speeds and lubricant temperatures, studying the behaviour of 0W20 lubricant with and without an organic friction modifier. Both mean and instantaneous friction was measured and found to correlate very well with the Stribeck curve. 

Friction in Lead Screw Drives Fig. 1.1 Stribeck curve [11]... 

Liquid lubrication mechanism. There are four defined regimes of liquid lubrication hydrodynamic (thickness of lubricant film (h), h > 0.25 pm), elastohydrodynamic (h 0.025 to 2.5 pm), boundary (h 0.0025 pm), and mixed. These regimes are dependent on oil viscosity (Z) and relative velocity (V) and are inversely proportional to the load (L), (ZV/L). Fig. 5.1, known as the Stribeck-Hersey curve, depicts these regimes in terms of friction coefficient versus viscosity, velocity, and load (ZV/L) (Fusaro, 1995). 

Fig. 5.1. Stribeck-Hersey curve coefficient of friction (f or p) as function of viscosity (Z) velocity (V) / load parameter (L) h = thickness of lubricant film (Fusaro, 1995)...

Lubrication mechanism How do you expect each of the following regimes to change in going from thickness of lubricating film h to a coefficient of friction, p in the Stribeck-Hersey curve Fig.5.1 There are three defined regimes of liquid lubrication (a) hydrodynamic, h 25 pm, (b) elastohydrodynamic, h = 0.025 to... 

Although not present in the usual Stribeck diagrams, a reduction of f, probably related to thermal effects, can also occur by increasing the speed after reaching the fiiU fluid regime for nonconformal contacts. However, shape, dimensions and temperatime can alter the trends of the friction curves, as shown in Fig. 18, and in particular the hmiting A values for which the transitions between the different regimes of lubrication occur. 

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