Sliding speed

The coefficient of friction between two unlubricated solids is generally in the range of 0.5-1.0, and it has therefore been a matter of considerable interest that very low values, around 0.03, pertain to objects sliding on ice or snow. The first explanation, proposed by Reynolds in 1901, was that the local pressure caused melting, so that a thin film of water was present. Qualitatively, this explanation is supported by the observation that the coefficient of friction rises rapidly as the remperarure falls, especially below about -10°C, if the sliding speed is small. Moreover, there is little doubt that formation of a water film is actually involved [3,4]. 

While pressure melting may be important for snow and ice near 0°C, it is possible that even here an alternative explanation will prove important. Ice is a substance of unusual structural complexity, and it has been speculated that a liquidlike surface layer is present near the melting point [17,18] if this is correct, the low /t values observed at low sliding speeds near 0°C may be due to a peculiarity of the surface nature of ice rather than to pressure melting. 

The lubricating properties of tears are an important feature in normal blinking. Kalachandra and Shah measured the coefficient of friction of ophthalmic solutions (artificial tears) on polymer surfaces and found no correlation with viscosity, surface tension or contact angle [58]. The coefficient of friction appears to depend on the structure of the polymer surfaces and decreases with increasing load and sliding speed. 

It is known that even condensed films must have surface diffusional mobility Rideal and Tadayon [64] found that stearic acid films transferred from one surface to another by a process that seemed to involve surface diffusion to the occasional points of contact between the solids. Such transfer, of course, is observed in actual friction experiments in that an uncoated rider quickly acquires a layer of boundary lubricant from the surface over which it is passed [46]. However, there is little quantitative information available about actual surface diffusion coefficients. One value that may be relevant is that of Ross and Good [65] for butane on Spheron 6, which, for a monolayer, was about 5 x 10 cm /sec. If the average junction is about 10 cm in size, this would also be about the average distance that a film molecule would have to migrate, and the time required would be about 10 sec. This rate of Junctions passing each other corresponds to a sliding speed of 100 cm/sec so that the usual speeds of 0.01 cm/sec should not be too fast for pressurized film formation. See Ref. 62 for a study of another mechanism for surface mobility, that of evaporative hopping. 

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. 

Discuss why stick-slip friction is favored if fi decreases with sliding speed. 

The wear, W, of friction materials can best be described by the wear equation (32,33) W = KP° P where K is the wear coefficient, P the normal load, D the sliding speed, t the sliding time, and a,b, and c ate a set of parameters for a given friction matenal—rotor pair at a given temperature. 

Fig. 38—Friction coefficient at different sliding speed [60]. Load 2 N, Concentration of UDP 0.3 %, Base oil PEG.

Figure 40 shows the relationship between the friction coefficient and the sliding speed for PEGs with different concentrations of UDP. It is clear that the friction coefficient of all lubricants is about 0.22 at the speed of about 0.1 mm/s. With increasing sliding speed, it drops sharply in the speed range from 0.1 mm/s to 15 mm/s, and then maintains about 0.03 when the speed is more than 15 mm/s. 

Fig. 24—Area percentage in different inverse Knudsen number ranges, D is inverse Knudsen number A, is the area satisfied with the inequalities in the horizontal abscissa, (a) sliding speed v = 9.57557 m/s (b) sliding speed v=40 m/s.

Fig. 29—Effect of van der Waals force on loading capacity of a O type slider under different pitch angles. Input parameter was set as minimum film thickness ho=6 nm, roll angle =0, sliding speed u=25 m/s, length of the slider L=. 2S mm, width of the slider B = 1.1 mm, mass of the slider M=1.6 mg.

Surface Geometry Amplitude (/am) Wavelength (xa) Load (N) Sliding speed (mm/s)... 

FIGURE 26.12 Friction coefficient of a natural rubber (NR) gum compound as function of the ice temperature at three different speeds (left) and friction coefficient of four different gum compounds having different glass transition temperatures as function of the ice track temperature at a constant sliding speed of 0.005 m/s. (From Heinz, M. and Grosch, K.A., ACS Spring Meeting, St Antonio, 2005.)... 

In order to calculate the operative log a-iv value a relation is required between the temperature rise in the contact area and the sliding speed. According to Carslaw and Jaeger [30] the maximum temperature rise, occurring for rubber near the end of the contact area, is given by the following relation ... 

FIGURE 26.20 The log a v speed function of the previous chart is combined with the friction master curves for a natural rubber (NR) and a styrene-butadiene rubber (SBR) gum compound on glass showing the limited range of friction values (and their position on the log a-iv axis for different testing conditions) which are obtained when the sliding speed is increased. 

FIGURE 26.52 Sliding abrasion of three different tread compounds as function of temperature at a sliding speed of 0.01 m/s (a) styrene-butadiene rubber (SBR), (b) ANR, (c) NR,-tread compound, —gum compound. 

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