Coefficient of friction dynamic

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). 

Unfilled Teflon PFA has been tested in mechanical appHcations using Teflon FEP-100 as a control (24). Tests were mn on molded thmst bearings at 689.5 kPa (100 psi) against AISI 1080, Rc 20,16AA steel, and at ambient conditions in air without lubrication. A limiting PV value of 5000 was found. Wear factors and dynamic coefficients of friction are shown in Table 4. 

Friction is the resistive force that we experience when we try to slide one object over the surface of another. The coefficient of friction is the ratio of the lateral force required to slide the surfaces past one another relative to the force holding them in contact. Polymers exhibit two coefficients of friction the static coefficient of friction is a measure of the force required to initiate movement, the dynamic coefficient of friction is a measure of the force required to sustain movement at a constant rate. In general, the force required to initiate sliding is somewhat greater than that required to maintain a constant rate of movement. 

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 ... 

Because Darnell and Mol neglected the width of the flight and allowed the dynamic coefficient of frictions to be set equal for part of the derivation, the model is rarely used. The model adaptation of Tadmor and Klein is used instead. This model will be described next. 

Like the original Darnell-Mol model, the Tadmor-Kleln model was developed with only very limited rate data because solids conveying measuring devices were not available at that time. Moreover, dynamic coefficient of friction data were also unavailable during their model developments. Complete plastlcatlng extrusion simulations, however, were developed with the Tadmor-Kleln model using engineering friction coefficients to estimate the characteristics in the process. 

The dependence of rate on the discharge pressure was considerably less for HIPS resin as compared to HDPE resin. As shown in Fig 5.13, the solids conveying rate for HIPS resin decreased to a lesser extent with increasing discharge pressure, and the rate did not seem to be dependent on temperature in this temperature range. This result is consistent with the dynamic coefficient of friction for HIPS resin, as shown by Fig. 12.17. As shown in Fig. 12.17, the coefficient did not depend to a high level on temperature or velocity in this low temperature range. 

This section wiil compare the modified Campbeii-Dontula modei to the experimen-tai data for the LDPE resin. The physicai properties for this resin were provided in Section 5.3.2. The dynamic coefficients of friction are avaiiahie in Appendix A5 and are reported as stress at a sliding interface. The coefficient of friction is related to the stress at a sliding interface as follows ... 

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. 

Friction Between Two Surfaces Answer the following questions or discuss statements (a) Two clean, highly polished steel surfaces when brought into contact appear to stick as if having very high coefficient of friction, (b) Would the dynamic coefficient of friction between steel and a polymer increase or decrease with increasing surface... 

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. 

ETFE resins are very often compounded with varied ingredients, such as glass fibers and bronze powder, to attain certain mechanical properties. For example, glass fibers are added at 25 to 35 wt% levels to increase modulus and improve wear and friction characteristics. By adding 25% glass fibers, for example, the dynamic coefficient of friction is reduced from about 0.5 to about 0.3 69... 

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 32.2 Outline of the ranges in the dynamic coefficient of friction. These ranges reflect measurement of untreated normal skin friction in vivo. Reproduced from Sivamani et al.25... 

Table 5.17. Wear factor and dynamic coefficient of friction of different polymers containing PTFE Polymist. Courtesy of Ausimont USA, Inc.

Polymer PTFE, % Wear factor Dynamic coefficient of friction ... 

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]. 

Macroemulsions remain on the external surface of fabrics. They achieve an excellent lubrication through the decrease of the dynamic coefficient of friction (see below). An excellent softness results. 

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. 

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). 

A normal force P acting on an interface between two solids (Fig. 2.11) creates a tangential friction force pP, p being the (dynamic) coefficient of friction. Then, the friction power is... 

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]. 

Dynamic coefficient of friction is the coefficient of friction of a body in motion, once slipping has begun and the body continues sliding at a relatively constant... 

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