Frictional melting

Based on the frictional melting layer assumption, we take this part of the skate blade drag to result from the power required to overcome the longitudinal viscous stress exerted on the blade by the lubricating fluid, considered to have a Couette flow. That is ... 

Figure 2 shows the thickness of the lubricating layer, / , as a function of distance along the dynamic contact zone. The upstream boundary value of h is the depth of the quasi-liquid layer based on the ice surface temperature." The quasi-liquid layer serves as the lubricant over the first 0.1% of the contact zone. The upper curve represents the frictional melting term (first term on the rhs of Eq (5)). The effects of adding squeeze flow and heat conduction into the ice are shown by the two intermediate curves. The lowest curve combines all factors, and shows... 

Under conditions typical of competitive speedskating, frictional melting, squeeze flow and heat conduction into the ice all play an important role in determining skate blade lubrication. Pressure-induced freezing point depression and the quasi-liquid layer are accounted for in the model, but they play only a minor role in determining the kinetic ice friction coefficient. 

Figure 8.8 Stages in the ultrasonic welding process. In Phase 1, the horn is placed in contact with the part, pressure is applied, and vibratory motion is started. Heat generation due to friction melts the energy director, and it flows into the joint interface. The weld displacement begins to increase as the distance between the parts decreases. In Phase 2, the melting rate increases, resulting in increased weld displacement, and the part surfaces meet. Steady-state melting occurs in Phase 3, as a constant melt layer thickness is maintained in the weld. In Phase 4, the holding phase, vibrations cease. Maximum di lacement is reached, and inter-molecular diffusion occurs as the weld cools and solidifies. ...

The combined effect of a high contact pressure of the order of 30 psi (2 kg cm ) with sliding speeds up to kO mph has also received attention and the results are discussed in terms of frictional melting of the ice at the i nterface. 

It is a common experience that the friction of rubbers and of most other materials on ice is extremely low. Under some circumstances the reasons for the low coefficient of friction may be ascribed to frictional melting or to pressure melting on the ice. The friction of rubber on smooth surfaces has been carefully studied j but until recently > little scientific work has been done to investigate the mechanism of the friction of rubber on ice. This paper describes all the recent work which we have carried out on the subject and shows that under some circumstances the friction of rubber on ice may be very high indeed with friction coefficients well in excess of unity. 

E. Southern We have tried to do this but because of the lack of precision in the results on the mastercurve it is not possible to draw useful conclusions. A further complication is the effect of frictional melting which will cause partial lubrication in the contact area. We have a simple theory based on Jaeger s solution of the problem of heat generated during sliding friction but we have not carried out any experiments yet to verify it. 

Key Words solids conveying, friction, melting, singlescrew extrusion. 

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]. 

Another indication of the probable incorrectness of the pressure melting explanation is that the variation of the coefficient of friction with temperature for ice is much the same for other solids, such as solid krypton and carbon dioxide [16] and benzophenone and nitrobenzene [4]. In these cases the density of the solid is greater than that of the liquid, so the drop in as the melting point is approached cannot be due to pressure melting. 

Substances in this category include Krypton, sodium chloride, and diamond, as examples, and it is not surprising that differences in detail as to frictional behavior do occur. The softer solids tend to obey Amontons law with /i values in the normal range of 0.5-1.0, provided they are not too near their melting points. Ionic crystals, such as sodium chloride, tend to show irreversible surface damage, in the form of cracks, owing to their brittleness, but still tend to obey Amontons law. This suggests that the area of contact is mainly determined by plastic flow rather than by elastic deformation. 

To the extent that the segmental friction factor f is independent of M, then Eq. (2.56) predicts a first-power dependence of viscosity on the molecular weight of the polymer in agreement with experiment. A more detailed analysis of f shows that segmental motion is easier in the neighborhood of a chain end because the wagging chain end tends to open up the structure of the melt and... 

As a tme thermoplastic, FEP copolymer can be melt-processed by extmsion and compression, injection, and blow molding. Films can be heat-bonded and sealed, vacuum-formed, and laminated to various substrates. Chemical inertness and corrosion resistance make FEP highly suitable for chemical services its dielectric and insulating properties favor it for electrical and electronic service and its low frictional properties, mechanical toughness, thermal stabiUty, and nonstick quaUty make it highly suitable for bearings and seals, high temperature components, and nonstick surfaces. 

Spin Welding. Spin welding is an efficient technique for joining circular surfaces of similar materials. The matching surfaces are rotated at high speed relative to each other and then brought into contact. Frictional heat melts the interface and, when motion is stopped, the weld is allowed to soHdify under pressure. 

Metalloid peroxides behave as covalent organic compounds and most ate insensitive to friction and impact but can decompose violentiy if heated rapidly. Most soHd metalloid peroxides have weU-defined melting points and the mote stable Hquid members can be distilled (Table 3). Some... 

Fibers. The principal type of phenoHc fiber is the novoloid fiber (98). The term novoloid designates a content of at least 85 wt % of a cross-linked novolak. Novoloid fibers are sold under the trademark Kynol, and Nippon Kynol and American Kynol are exclusive Hcensees. Novoloid fibers are made by acid-cataly2ed cross-linking of melt-spun novolak resin to form a fuUy cross-linked amorphous network. The fibers are infusible and insoluble, and possess physical and chemical properties that distinguish them from other fibers. AppHcations include a variety of flame- and chemical-resistant textiles and papers as weU as composites, gaskets, and friction materials. In addition, they are precursors for carbon fibers. 

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