Coefficient static friction

Typical static friction coefficients are given in Table 1. These data demonstrate that the absolute traction values for synthetic surfaces are satisfactory in comparison with natural turf, provided that shoes with the appropriate surfaces are employed. Synthetic surfaces by virtue of their constmction are to a degree directional, a characteristic which, when substantial, can significantly affect both player performance and ball roU. This effect is evident in a measurement of shoe traction in various directions with respect to the turf—pile angle. Some traction characteristics are directiy affected by the materials. 

The net area of this intimate contact is called the real area of contact Areai. It is assumed that plastic flow occurs at most microscopic points of contact, so that the normal, local pressures correspond to the hardness aj, of the softer of the two materials that are in contact. The (maximum) shear pressure is given by the yield strength cry of the same material. The net load L and the net shear force Fs follow by integrating aj, and cry over the real area of contact Areai. That is, L = cs, Arca and Fs = ayAreai. Hence, the plastic deformation scenario results in the following (static) 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. 

FIGURE 33.1 Variation of the friction coefficient versus time during the recording of an experiment on the forearm. The two curves correspond to two different loads applied on the sliding pad. The maximum of the curves corresponds to the static friction coefficient and the asymptotic value to the kinetic coefficient.3... 

Under static conditions, 0 rises linearly with time and the static friction coefficient rises as In t. The steady-state value of 0 during sliding can be found by setting the time derivatives in Eq. (6) to zero, yielding O,. = D /v. Inserting this in Eq. (5) gives p = Po + hi(w/r)o). Thus the steady-state kinetic... 

Assuming constant normal pressure in the contacts, which is equivalent to a linearity between L and A, the static friction coefficient obeys the following general scaling laws for rigid objects where roughness exponent H = 0 applies ... 

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... 

Contacts mechanics in the MHz range is much different from its low-frequency counterpart. For instance, static friction coefficients probed with MHz excitation are often much above 1. Contact mechanics at short time scales should be of substantial practical relevance. 

TABLE 8.4. Static Friction Coefficients for Different Material Contacts... 

The relationship between the normal force and the friction force is used to define the coefficient of static friction. Coefficient of fiiction is the ratio of the force that is required to start the fiiction motion of one surface against another to the force acting perpendicular to the two surfaces in contact. Friction coefficients will vary for a particular plastic from the value just as motion starts to the value it attains in motion. The coefficient depends on the surface of the material, whether rough or smooth. These variations and others make it necessary to do careful testing for an application that relies on the fiiction characteristics of plastics. Once the fiiction characteristics are defined, however, they are stable for a particular material fabricated in a prescribed method. 

Carrier fluid viscosity. Pa s Pj, Solids-wall friction coefficient during the downswing motion, -Solids-wall static friction coefficient, -... 

The frictional behaviour of polymers differs somewhat from that of perhaps more familiar materials. The frictional force tends to be proportional not to load (as in the classical case) but to speed. The coefficient of friction is very dependent on the nature of the two surfaces in contact, but is generally low, when suitable pairs are selected. This means that plastics gears can usually be run without external lubricants. Often the static friction coefficient is lower than the dynamic, which helps to explain the absence of slip-stick phenomenon exhibited by some plastics systems in motion this is especially marked with PTFE, which has an exceptionally low coefficient (around 0.02). The non-classical response of plastics materials results from their much lower modulus. Their frictional response is characterized by adhesion and deformation. 

Data using various contacting surfaces are presented in Table 10.9. The static friction coefficients for the flocked surfaces against the solid metal surfaces, brass... 

Table 10.9 Sliding angle and static friction coefficient between various surfaces involving flocked waste surfaces conditioned at 60% RH...

Static-frictional force is proportional to the normal force, F, applied to the solid, and the dimensionless proportional coefficient is called the static friction coefficient of the material and is usually denoted... 

The static friction coefficient of a solid material depends not only on the material but also on the nature of the substrate, its surface finish (e.g., roughness), its surface conditions (e.g., oxidized, etched), and the type of atmosphere (e.g., air, gas, vacuum). For most materials the static coefficient of friction is drastically increased when contact surfaces are put under vacuum because the lubricating action of molecules disappears. This explains why moving parts and bearing in a vacuum or under reduced pressure are technological challenges. On the other hand, the lubricating action of liquids is obvious except if they react chemically ... 

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... 

Table 1.5. Static friction coefficients of selected solids on themselves in air and in a vacuum ...

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