Sliding and friction

The authors thank the Deutsche Forschungsgemeinschaft (DFG) for financial support within the frame of the Collaborative Research Centre 483 High performance sliding and friction systems based on advanced ceramics". 

Within the frame work of the Centre of Excellence in Research CER 483 High performance sliding and friction systems based on advanced ceramics one approach is to apply advanced ceramics as friction material for e.g. a dry running motor vehicle clutch using the ceramic specific benefits as wear and temperature resistance combined with lightweight design to fulfil today s demands as e.g. functionality, life time and reliability. 

A number of friction studies have been carried out on organic polymers in recent years. Coefficients of friction are for the most part in the normal range, with values about as expected from Eq. XII-5. The detailed results show some serious complications, however. First, n is very dependent on load, as illustrated in Fig. XlI-5, for a copolymer of hexafluoroethylene and hexafluoropropylene [31], and evidently the area of contact is determined more by elastic than by plastic deformation. The difference between static and kinetic coefficients of friction was attributed to transfer of an oriented film of polymer to the steel rider during sliding and to low adhesion between this film and the polymer surface. Tetrafluoroethylene (Telfon) has a low coefficient of friction, around 0.1, and in a detailed study, this lower coefficient and other differences were attributed to the rather smooth molecular profile of the Teflon molecule [32]. 

For many centuries the application of materials for low friction and wear in sliding and rolling contacts primarily involved wood, stone, leather, iron, and copper. Almost all engineering materials have since been employed at one time or another in the continuing search for the best bearing material. Final selection is commonly a judgment based on the most essential material properties, ease of appHcation, and cost. 

Because of the steep pitch required, slides are limited in application. They are most commonly used to bridge the gap between roller-conveyor systems on two floors, because the roller conveyor can take the container off the slide rapidly and ehminate or reduce the chance for collisions. Slides may also be used when containers can be chuted from an upper floor to a manually loaded carrier. The use of several rollers at the feed point is recommended for easy dehveiy to the sloping section. If the drop is short and containers light, a roller cleanout will prevent backup of containers on the slide. The slope of gravity slides is a function of container weight, size, and friction characteristics and should be selected with care to be sure that containers do not move either too swiftly or not at all. Slides usually use flat steel sheet. 

Boundary lubrication is perhaps best defined as the lubrication of surfaces by fluid films so thin that the friction coefficient is affected by both the type of lubricant and the nature of the surface, and is largely independent of viscosity. A fluid lubricant introduced between two surfaces may spread to a microscopically thin film that reduces the sliding friction between the surfaces. The peaks of the high spots may touch, but interlocking occurs only to a limited extent and frictional resistance will be relatively low. 

Fig. 21—Comparisons of friction forces from simulations of commensurate and incommensurate SAMs in relative sliding (a) friction exhibits a linear dependence on applied normal load, (b) friction increases logarithmically with the sliding velocity.

The chapter concentrates on the aspect of force transmission during limited sliding. The friction contribution to traction and the resulting wear, both in laboratory experiments and in tire traction and... 

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.)... 

The model can also be used when both slip components act simultaneously. The limiting force is still determined by the friction coefficient and pressure. The total traction at a point x in the contact area is the vector sum of the two components. This determines the point of onsetting sliding and hence the circumferential contribution is the total less the side force contribution. This is shown diagrammatically in Figure 26.25 [33]. 

Piggott, M. R. (1987). Debonding and friction at fiber-polymer interface. I Criteria for failure and sliding. Composites Sci. Technol. 30, 295-306. 

Sigl, L.S. and Evans, A.G. (1989). Effects of residual stress and frictional sliding on cracking and pull-out in brittle matrix composites. Mech. Mater. 8, 1-12. 

Fuzes. A lubricant is expected to perform the jobs of minimizing friction, wear, and galling between sliding or rolling parts. It must do these jobs under two types of conditions (1) those which are inherent in the component element itself - such as load, speed, geometry, and frictional heat - and (2) those which are imposed from external sources -such as temp and compn of the surrounding atm, nuclear radiation, inactive storage, vibration, and mechanical shock. The imposed conditions are usually the more restrictive ones for lubricant selection... 

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