Static and Dynamic Coefficients of Friction

FIGURE 13.3 Dwell-time dependence of the static coefficient of friction. 

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Dry Lubricant. The static and dynamic coefficients of friction for the parylenes are low and virtually the same. This feature is an advantage in the use of a parylene coating as a dry lubricant on the bearing surfaces of miniature stepping motors. Coating a threaded ferrite core significantly reduces the abrasion to coil forms (82). 

A broad choice of lubricating and reinforcing additives leads to a good range of coefficients of friction, from 0.1 up to 0.3 for the static coefficients, and low wear from excellent to good. For example, static and dynamic coefficients of friction are, respectively ... 

Polymer emulsions (150 to 250 nm) deposit on the external surface of fabrics and on fibers. They improve the softness and the ease of ironing since they reduce both static and dynamic coefficients of friction. 

There are no data available on the effect of WPC density on the slip coefficient. However, it is known that polyethylene of lower density has a better traction than that of a higher density. In other words, HDPE is characterized by a low coefficients of friction, and the higher the density (specific gravity), the lower the static (and dynamic) coefficient of friction. For polyethylene density of 0.915 g/cm, coefficient of friction equals to 0.50 for 0.932 g/cm, it is equal to 0.30, and for 0.965 g/cm, it is equal to 0.10 [3]. 

Coefficients of fiiction of light conveyor belts are determined according to European standards.A metallic test panel under load, which generates together with the panel a normal force of 50 N, is pulled on the surface of a test piece. Forces required to initiate movement and maintaining it are recorded and results used for calculation of static and dynamic coefficients of friction. ISO, and the identical British standard, contain method of determination of frictional proper-... 

The simple concept of constant static and dynamic coefficients of friction does not assume anything on how the transition from stick to slip occurs. The transition from sticking to sliding between two transparent PMMA blocks was studied experimentally in detail by Rubinstein et al. [928] who found that the onset of friction is connected to the propagation of crack wave fronts, which lead to a rupture of the interface. 

For plastic film and sheeting the most common test is ASTM D1894-08 Standard Test Method for Static and Kinetic Coefficients of Friction of Plastic Film and Sheeting that is capable of measuring both the static and dynamic COFs. 

The frictional properties of PTFE arc unique Its unusually low static coefficient of friction decreases with increasing load and is lower than the dynamic coefficient of friction. This precludes stick-slip behavior. The low surface energy also prevents wetting by liquids other than low-surface-tension fluids like fluorocarbons. 

Assume isothermal operation and that the inlet pressure to the solids conveying zone equals the base pressure of a fully loaded hopper. Bulk density of the feed is 595 kg/m3, the static coefficient of friction in the hopper is 0.3, and the dynamic coefficients of friction on barrel and screw are 0.45 and 0.25, respectively. 

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. 

The detergent in a transmission or farm tractor hydraulic fluid formulation is a key component for setting the frictional properties of the fluid, including the dynamic coefficient of friction, the static coefficient of friction and the slope of a friction versus sliding speed curve with a particular set of friction elements. Even more importantly, a detergent retains these properhes for extended periods of time [27-29]. 

From a quantitative study of the effect of lubricating oils in static generation by yams, the role of dynamic coefficient of friction was compared with that of electrical conductivity (85). In order to avoid static difficulties in yam processing, the yams must have a low coefficient of friction and, more importantly, have the highest possible conductivity. Whatever the coefficient of friction may be, the voltage approaches zero if the conductivity becomes high enough (85). 

The static coefficient of friction implies the force required to first move one substrate relative to the other, and the dynamic coefficient of friction refers to the force required to continue the movement. Methods of determining the coefficient of friction of some plastic materials are given in ASTM D1894 and ASTM D3028. 

PTFE has excellent properties, including chemical inertness, heat resistance (at both high and low temperatures), electrical insulation, and low coefficient of friction (static 0.08 and dynamic 0.01), and it is nonstick over a wide temperature range (-260°C to... 

Fluoroplastics are, generally, slippery materials with smooth surfaces due to their low coefficients of friction. The coefficient of friction decreases with an increase in fluorine content. Numerous mechanical applications depend on low friction of these plastics with little or no lubrication. In the case of perfluoroplastics, such as PFA, FEP, and PTFE, dynamic coefficient of friction is larger than static coefficient of friction. It... 

In the sections below a brief overview of static solvent influences is given in A3.6.2, while in A3.6.3 the focus is on the effect of transport phenomena on reaction rates, i.e. diflfiision control and the influence of friction on intramolecular motion. In A3.6.4 some special topics are addressed that involve the superposition of static and transport contributions as well as some aspects of dynamic solvent effects that seem relevant to understanding the solvent influence on reaction rate coefficients observed in homologous solvent series and compressed solution. More comprehensive accounts of dynamics of condensed-phase reactions can be found in chapter A3.8. chapter A3.13. chapter B3.3. chapter C3.1. chapter C3.2 and chapter C3.5. 

Static friction decreases with an increase in load, and the static coefficient of friction is lower than the dynamic coefficient. The tendency to creep must be considered carefliUy in FEP products designed for service under continuous stresses. Creep can be minimized by suitable fillers. Fillets are also used to improve wear resistance and stiffness. Compositions such as 30% bronze-fiUed FEP, 20% graphite-filled FEP, and 10% glass-fiber-filled FEP offer high PV values ( 400(kPa-m)/s) and are suitable for beatings. 

Low coefficient of friction (with equal dynamic and static coefficients). 

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