Dry friction

In this section we discuss dry friction, also called solid or Coulomb friction. Dry friction occurs when two solid surfaces are in direct contact without any other components such as lubricants or adsorbed surface layers involved. One might object that in practice no such surface exists under ambient conditions and that all surfaces are somehow contaminated. However, the basic principles of dry friction may apply even in this situation. 

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Brakes and clutches operate both dry and wet. In dry friction couples, the heat is removed by conduction to the surrounding air and stmctural members. Wet friction couples operate within a fluid, usually an oil, which absorbs the heat and maintains the couple at relatively low (below 200°C) temperatures. The fluid also traps the wear debris. 

Dry-friction whiri. This type of whip is experienced when the surface of a rotating shaft comes into contact with an unlubricated stationary guide. The effect takes place because of an unlubricated journal, contact in radial clearance of labyrinth seals, and loss of clearance in hydrodynamic bearings. 

Dry-friction whirl Any speed Nf = —nN tip clearance effects, balance pistons Shaft in contact with... 

When two bodies are in contact and there is a tendency for them to slide with respect to each other, a tangential friction force is developed that opposes the motion. For dry surfaces this is called dry friction or coulomb friction. For lubricated surfaces the friction force is called fluid friction, and it is treated in the study of fluid mechanics. Consider a block of weight W resting on a flat surface as shown in Figure 2-5. The weight of the block is balanced by a normal force N that is equal and opposite to the body force. Now, if some sufficiently small sidewise force P is applied (Figure 2-5b) it will be opposed by a friction force F that is equal and opposite to P and the block will remain fixed. If P is increased, F will simultaneously increase at the same rate until... 

The mechanisms of static friction and stick-slip motion, as discussed in the last section, are supposed to be a good description of dry friction. Another case, perhaps more general in engineering practices, to be addressed in this section is lubricated sliding where liquid lubricant, consisting of a few molecule layers, is confined between two solid walls. Both experimental and theoretical studies indicate, as we have discussed in Chapter 5, that there are substantial changes in rheology of the confined lubricant, and the liquid may transit practically to a solid-like state when film thickness becomes molecularly thin [32,33]. 

Weiss, M. and Elmer, F. J., Dry Friction in the Frenkel-Kontorova-Tomlinson Model Dynamical Properties," Zeitschrift fur Physik B—Condensed Matter, Vol. 104, No. 1,1997, pp. 55-69. 

FIGURE 26.24 Braking force as function of slip using the brush model with (dry) a constant friction coefficient and (wet) a friction coefficient which depends on the slip speed because of wet lubrication. Braking stiffness 90,000 N, dry friction coefficient 1.2, load 4500 N, speed at the onset of braking 40 km/h, critical speed 210 km/h. 

W = wt of dried friction compn, determined as described under opn b)... 

In the light of the above, contact abrasion methods can be classified into ordinary, used for measurements at room temperature, and high temperature. Classed among abrasion resistance test methods at room temperature are the following Bohme s, Mindt s, Harvey s, McGee s, Blondel s, dry friction with balls, dry friction without balls, and Siever s method. 

Blondel s method involves abrading a rotating sample in a rotating porcelain mortar filled with porcelain pellets. A variation on this method is dry friction of a plane surface with steel balls rolling under pressure. 

Mackensen-Zeiss blower method, 39,101 Martens sclerometer, 48, 49 McGee s method, 92 Mean Mohs-Mackensen hardness, 106 Method, Blondel s 92 Bohme s disc, 90 Brinell, 36, 58 Davidenkov scratch point, 50 Davidenkov s, 50 dry friction without balls, 92 Harvey s 92... 

O 13.21% sulfur-yel ndls(from benz+petr eth), mp 77-8°(dec) readily sol in ale, eth, benz or chlf diffc sol in petr eth. It can be prepd by shaking benzaldehyde-p-tolylhydrazone in ligroin, or better in benz soln, with a si excess of hydrogen peroxide. This compd expl spontaneously at RT when dry. Friction or impact does not cause it to explode(Refs 1 2)... 

At low sliding velocities and high loads, the lubricating film is squeezed out of the gap. This leads to so-called boundary lubrication. Friction coefficients under these conditions are typically 100 times higher than under hydrodynamic lubrication conditions, but still substantially smaller than for dry friction under UHV conditions. This is due to the fact that the surfaces are still wetted by molecular layers of the lubricant, even under conditions where the local stress is high enough to deform the surface asperities. Under these conditions friction depends more on the chemical constitution of the lubrication layer than on its viscosity. 

It should be noted, that most clean surfaces (unless prepared and kept in UHV) are covered by a thin layer of contamination. As a consequence, in experiments performed under ambient conditions, we have to consider that boundary lubrication rather than dry friction might be present. 

Amontons law of macroscopic, dry friction states that the friction force is proportional to the load and does not depend on the apparent contact area ... 

The force that arises in a dry friction environment can be computed using Coulomb s... 

Friction is the tangential resistance offered to the sliding of one solid over another, due to dry friction. Friction is an apparently simple phenomenon with very complex mechanisms that take place on a variety of length scales, from atomic to nano and up. The study of friction is part of the engineering-scientific discipline of tribology,3 which is the scientific study of friction, wear, and lubrication (6). It was Leonardo da Vinci (1452-1519) who discovered the first two laws of friction, namely, that the area of contact has no effect on friction and that friction is proportional to the load. These two laws were rediscovered later by Guillaume Amontons (1663-1705), and later Charles-Augustin Coulomb (1736-1806), added the third law ... 

Consequently, Bowden and Tabor (7) specify two factors that are responsible for dry friction the first, and usually the more important factor, is the adhesion that occurs at the... 

In view of these complexities, it is remarkable that Eq. 4.1-4 represents numerous metal-metal, dry frictional data rather well, for both the static and sliding cases. Polymers, on the other hand, exhibit an even more complex frictional behavior on metal. This is, perhaps, not surprising, since the physical situation involves a relatively soft, viscoelastic, and temperature-dependent material in contact with a hard, elastic, and much less temperature- and rate-dependent material. Empirical evidence of these complexities is the nonlinear relationship between the frictional force and the normal load... 

F. Heslot, T. Baumberger, B. Caroli, and C Caroli, Creep, Stick-slip and Dry Friction Dynamics Experiments and a Heuristic Model, Phys. Rev., E49, 4973 (1994). 

The silicon carbide-based ceramic layer is obtained by CVD activated by a microwave plasma (2.45 GHz) on the tantalum previously cleaned by an argon plasma (56 min, 133 Pa, 250 W) at a temperature identical to the deposition temperature (T = 570°C), lower than the substrate annealing temperature. After readjustment of the total pressure and microwave power by introduction of a precursor (TMS) in the argon flow, the SiC coating is produced under selected conditions (66 Pa, TMS/Ar = 0.2/5.5 l.h-, T = 570°C, 350 W) derived from a previous parametric study. Coatings obtained then presented low dry friction coefficients. The mechanical properties, which are essential in order to be able to appreciate the mechanical stability, were acquired by several methods ... 

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