Dynamic friction coefficient

Classical, macroscopic devices to measure friction forces under well-defined loads are called tribometers. To determine the dynamic friction coefficient, the most direct experiment is to slide one surface over the other using a defined load and measure the required drag force. Static friction coefficients can be measured by inclined plane tribometers, where the inclination angle of a plane is increased until a block on top of it starts to slide. There are numerous types of tribometers. One of the most common configurations is the pin-on-disk tribometer (Fig. 11.6). In the pin-on-disk tribometer, friction is measured between a pin and a rotating disk. The end of the pin can be flat or spherical. The load on the pin is controlled. The pin is mounted on a stiff lever and the friction force is determined by measuring the deflection of the lever. Wear coefficients can be calculated from the volume of material lost from the pin during the experiment. 

The friction coefficient can be measured in two ways the static friction coefficient Qus) and the dynamic or kinetic friction coefficient (fikX The static friction coefficient is defined as the ratio of the force required to initiate relative movement and the normal force between the surfaces the dynamic or kinetic friction coefficient is defined as the ratio of the friction force to the normal force when the two surfaces are moving relative to each other. For simplicity, much of the research has focused on the dynamic friction coefficients wherein the two surfaces move at a relative constant velocity. Most of the friction studies on skin have dealt with the dynamic friction coefficient and the subscript k is usually dropped. This overview references the dynamic coefficient of friction unless otherwise noted. 

Comparative Studies of the Changes in Dynamic Friction Coefficient (/n) with Increasing Hydration (Hydration) and Decreasing Hydration (Dryness)... 

Table 16.7 Guide to Dynamic Friction Coefficients for Different Forms of Molybdenum Disulphide...

Dynamic friction coefficient Kinematic viscosity, m /s Mass fraction of species i Mass fraction of species i in phase k Density, kg/m ... 

The basic mechanism inherent in the Nose-Hoover equations of motion is that the variable p acts as a dynamic friction coefficient that controls the... 

Fig. 6.5 Effect of different additives on static and dynamic friction coefficients...

At this point, a distinction must be made between static and dynamic friction coefficients. The force required to overcome the resistance to starting a movement is not the same as the one necessary to maintain an existing movement. The former corresponds to the static friction coefficient, the latter to the dynamic friction coefficient [52], For instance, silk exhibits high static and low... 

Softeners also reduce the dynamic friction coefficient between the fabrics and the iron, thereby making gliding easier [27]. The resistance to an iron is easily quantified by measuring the height of an inclined plane at which the iron starts to move it decreases as the number of rinses with a fabric softener increases the opposite is observed in the absence of softener [4],... 

Where Q = the heat generated in the friction process F = applied normal force at the contact C = the dynamic frictional coefficient at the contact point and L = sliding distance prior to the frictional ignition. 

Surface properties The molecular configuration of PTEE imparts a high degree of antiadhesiveness to its surfaces, and for the same reason these surfaces are hardly wettable. PTFE possesses the lowest friction coefficients of all solid materials, between 0.05 and 0.09. The static and dynamic friction coefficients are almost equal, so that there is no seizure or stick-slip action. Wear depends upon the condition and type of the other sliding surface and obviously depends upon the speed and loads. 

Silicon grease and fluorocarbon mould release agent lubricants appear to convert the stick-slip motion into nearly constant slip behaviour, and further reduce the dynamic friction coefficient. This loss of stick-slip motion and reduction in the friction coefficient when using lubricants is considered to be a useful indication for engineers who deal with the process equipment and mechanics [56]. 

After the sliding motion of the solid is initiated, the force F, that must be exerted to maintain its motion at a fixed velocity is also proportional to the normal force, but the proportional coefficient is lower than the static friction coefficient, and it is called the sliding friction coefficient or dynamic friction coefficient... 

Macroscopic friction tests on the NTS monolayer and bilayer in air were performed by sliding a stainless steel ball along a distance of 20 mm at a sliding velocity of 90 mm/min under a load of 0.2 N. Both thin films showed similar dynamic friction coefficients of 0.080-0.083, while a large difference was observed in wear resistance. The friction coefficient of the NTS monolayer increased after the 150 friction cycles, as shown in fig. 5.8a. On the other hand, the bilayer demonstrated a stable friction coefficient until 2200 friction cycles (fig. 5.8b). A wear track was clearly observed on the surface of the monolayer after 300 friction cycles (fig. 5.8c), while only a slight wear track was present on the surface of the bilayer, even after 1000 slidings, as shown in fig. 5.8d. Generally, a wear track should be formed by direct contact of the stainless steel probe with the silicon substrate at the point where... 

Figure 5.11 shows the dynamic friction coefficient of PMMA brushes of various thicknesses in air, cyclohexane, and toluene. Compared with the friction coefficient of the polymer brush in the dry state, the value decreased in both organic solvents... 

The degree of wear/friction improvement depends on the particular plastic to which the PTFE is being added [41], As examples, the above referenced study showed that the addition of 15% low MW PTFE to a 6/6 polyamide, a polyacetal resin, and a polycarbonate reduced the specific wear rates by 97%, 46%, and 97%, respectively. The dynamic friction coefficients were reduced by 76%, 48% and 97%, respectively. Although these are reasonable comparisons, wear and friction performance can depend on the exact contact conditions such as velocity, pressure, and countersurface composition. Commercial compounds of thermoplastics with PIFE will vary depending on the application requirements but an addition level of 15-20% is usually considered optimum for the best wear performance. 

Figure 13. Variation of the dynamic friction coefficient with the velocity.

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