Coefficient of Friction
The coefficient of friction is a measure of the resistance to motion between two surfaces in contact. It is a dimensionless quantity that represents the ratio of the force of friction between the surfaces to the force pressing them together. A higher coefficient of friction indicates greater resistance to motion, while a lower coefficient indicates less resistance.
7 Key excerpts on "Coefficient of Friction"
eBook - ePubExtreme Tribology
Fundamentals and Challenges
- Ahmed Abdelbary(Author)
- 2020(Publication Date)
- CRC Press(Publisher)
...Commonly, the friction coefficient or its equivalent, as a dimensionless quantity, is used to represent the magnitude of the resistance to relative motion between bodies (Blau, 1992). Measurement techniques involve either the direct measurement of forces that resist relative motion between two or more bodies of matter or the indirect measurement of the effects of those forces. Some tribometers are designed to measure the force required to initiate relative motion (static friction force), and some are intended to measure the resisting force on objects that are already in motion (kinetic friction force). In lubrication conditions, tribometers are designed to focus on metering the effects of the interfacial fluid or grease. Many friction measurement systems are designed to simulate specific applications, however, other measurement systems are designed to isolate and study frictional effects under highly controlled laboratory conditions that do not simulate any particular engineering application. As a result, there are literally hundreds, if not thousands of devices designed to quantify frictional resistance. Basic friction measurement methods are described here. 1. The inclined plane method, ASTM D4521. This simple method to measure friction force (F f) is based on a static balance of forces that requires only the angle (θ) of tilt to be measured at the instant when relative motion begins. According to Fig. 3.18, the static friction coefficient (μ s) is given by: μ s = tan θ (3.16) 2. The horizontal plane method, ASTM D4521. This method is directly utilized to measure the force required to begin motion and to continue relative motion, i.e., to measure static and kinetic friction forces between a specimen and counterface, as shown in Fig. 3.19. A typically 1000 gm dead weight is placed on the inclined slider. The inclined slider is raised until sliding begins. The Coefficient of Friction is equal to the tangent of the angle at which sliding begins...
eBook - ePubBasic Engineering Mechanics Explained, Volume 1
Principles and Static Forces
- Gregory Pastoll, Gregory Pastoll(Authors)
- 2019(Publication Date)
- Gregory Pastoll(Publisher)
...Below each diagram is a free-body diagram of the block, from which the equation for the value of N is obtained. The weight of the block is W in each case. In the third and fourth diagrams, additional external forces have been introduced. The Coefficient of Friction, μ This coefficient, given the symbol μ (the Greek letter ‘mu’), is a number, ranging in value from zero to approximately 1, that indicates the difficulty of sliding one material relative to the other. The nature of the two materials in contact dictates what the Coefficient of Friction will be. A Coefficient of Friction of zero indicates perfect ‘smoothness’, namely no frictional resistance whatsoever. A Coefficient of Friction of 1 indicates a large amount of frictional resistance. This value is sometimes regarded as the maximum possible value for μ. However, instances have been recorded in which μ > 1. (See the table that follows.) The friction coefficient has meaning only when both materials are specified. For example, ‘the Coefficient of Friction for rubber on glass’ can be specified. One cannot speak of the Coefficient of Friction for only one material, for example, for steel. If both surfaces are of the same material, for example: steel, then one must specify ‘the Coefficient of Friction for steel on steel’. It is important to realise that the terms ‘smoothness’ and ‘roughness’ that are sometimes used to indicate the extent of a frictional resistance, are not referring directly to the physical smoothness or roughness of the surfaces. Some very approximate values for the Coefficient of Friction for various combinations are listed in the table below...
eBook - ePub- Alan Hendrickson(Author)
- 2012(Publication Date)
- Routledge(Publisher)
...The coefficient defines the relationship between the force pushing the two materials together, the normal force, F n, and the maximum resistance to movement the setup will offer, the force due to static friction, F fs. The coefficients for a wide variety of pairs of materials in contact are listed in tables in many physics reference books, but these values are nearly useless for theatre because they are for materials we will never use, glass on nickel, or zinc on cast iron for instance. Table 4-1 does give a very short list of potentially useful coefficients, and these can be used to estimate the coefficients for similar materials and situations, but in general the most accurate coefficients for our use come from measurements using a mock-up of the actual materials to be used. This will be discussed in the Measuring Coefficients of Friction section below. The formula for the maximum force due to static friction does not include any term for the area of contact. This is because within wide limits the force due to friction is independent of the area of contact. For example, if a number of stage weights is used to hold open a door, it should not matter whether the weights are piled one atop the other, or placed side by side. Only when the area becomes small enough that the forces involved cause the one material to dig into the other will this area independence vanish. A material’s surface finish or texture has a major impact on its Coefficient of Friction. A smooth steel block will slide more easily across another material than will an identically loaded steel file. The file’s teeth dig into the other surface, forcing a shearing action on that surface rather than a sliding action. Wood grain can have a similar interlocking interaction. Two wood blocks with their grains parallel to each other but perpendicular to the direction of movement will generally have higher coefficients than if their grains are perpendicular...
eBook - ePub- N. (Nehemiah) Hawkins(Author)
- 2018(Publication Date)
- Perlego(Publisher)
...FRICTION AND VISCOSITY OF FLUIDS. Frictional Resistance. —The resistance with which bodies oppose the movement of one surface on another is termed friction. It depends on the nature, and the roughness of the surfaces in contact; at the commencement of the sliding, it is greater than when the motion is continued. Friction is in effect an equivalent force exerted in a direction opposite to that in which the sliding occurs. Its whole amount is the product of two factors: the first of these, which sums up the effect of the nature and condition of the surfaces, is called the Coefficient of Friction ; the second, which is the sum of all pressures, as weight strain, and the adhesion due to magnetism (when employed), which act to urge the two bodies together, i.e., perpendicularly to the surface of contact, is called the normal pressure. But this law holds only where, with dry surfaces, the pressure is not enough to indent or abrade either; or, with wet surfaces, not enough to force out the unguent...
eBook - ePubIndustrial Tribology
Tribosystems, Friction, Wear and Surface Engineering, Lubrication
- Theo Mang, Kirsten Bobzin, Thorsten Bartels(Authors)
- 2011(Publication Date)
- Wiley-VCH(Publisher)
...Chapter 3 Friction Friction is the mechanical force which resists movement (dynamic or kinetic friction) or hinders movement (static friction) between sliding or rolling surfaces. This force of friction is also called “external friction.” The work done by friction can translate into deformation, wear, and heat. Internal friction as part of external friction when lubricants are used results from the friction between lubricant molecules; this is described as viscosity (see Chapter 7) (The meaning of internal friction in material science is the force resisting motion between the elements making up a solid material while it undergoes deformation). The causes of external friction are, above all, the microscopic contact points between two sliding surfaces; these cause adhesion, material deformation, and grooving. Energy which is lost as friction can be measured as heat and/or mechanical vibration. Lubricants should reduce or avoid the microcontact which causes the greatest part of external friction. 3.1 Kinetic and Static Friction Different from static friction, kinetic friction occurs under conditions of relative motion. The American Society for Testing and Materials (ASTM) defines the kinetic Coefficient of Friction as the coefficient under conditions of macroscopic relative motion of two bodies. The kinetic Coefficient of Friction, which sometimes is called the “dynamic Coefficient of Friction,” is usually somewhat smaller than the static Coefficient of Friction. The static coefficient of ?friction is defined as the Coefficient of Friction corresponding to the maximum force that must be overcome to initiate macroscopic motion between two bodies (ASTM). The maximum value of static friction is sometimes referred to as “limiting friction.” 3.2 Coefficient of Friction Figure 3.1 defines the Coefficient of Friction as the dimensionless ratio of the friction force F and the normal force N...
eBook - ePub- A. D. Johnson(Author)
- 2017(Publication Date)
- CRC Press(Publisher)
...A. Coulomb, occurs between contacting surfaces in the absence of any lubricating fluid. An appreciation of the effects of friction may be gained by considering a block resting on a horizontal surface, Figure 6.2. The block applies a normal load N to the surface due to gravity acting on its mass. The force P is necessary to move the block along the surface and increases until the block is on the point of moving. At this point applied force = resisting force (friction) P = F r Fig. 6.2 Block resting on a horizontal surface Any addition to the applied force P will merely accelerate the block. There are four important concepts which relate to friction and these are considered below. The friction force F r is proportional to the normal load N. This concept is true only within the limits indicated in Figure 6.3, which shows the relationship between F r and N. The useful portion is limited to the straight line section of the curve. Since F r and the normal force Fig. 6.3 The limits of the linear relationship between the friction force and the normal load. N are proportional, their relationship is constant and can be written: F r N = μ (6.1) where μ is the Coefficient of Friction. This can be arranged to give the basic equation: F r = μ N (6.2) When the block is on the point of moving the applied force is P = F r = μ N (6.3) The friction force is independent of area of contact between the surfaces. In order to understand how the friction force can be independent of area, consideration must be given to the source of friction. It is generally understood that dry friction results from point contact between the microscopically rough surfaces, as shown in Figure 6.1. When on the point of moving, the applied force P is the same as the friction force F τ and it is this force which is needed to move the block over the contact points. Increasing the area creates more contact points and proportionally distributes the normal force N...
eBook - ePub- W. Bolton(Author)
- 2015(Publication Date)
- Routledge(Publisher)
...Increasing the applied force results in a force being reached when motion just starts. The value of the frictional force that has to be overcome before motion starts is called the limiting frictional force. When motion occurs there must be a resultant force acting on the object accelerating it (Newton’s second law) and thus the applied force must have become greater than the frictional force. The following are the basic laws of friction: Law 1 The frictional force is always in such a direction as to oppose relative motion and is always tangential to the surfaces in contact. Law 2 The frictional force is independent of the areas of the surfaces in contact. Law 3 The frictional force depends on the surfaces in contact and its limiting value is directly proportional to the normal reaction between the surfaces. The coefficient of static friction μ s is the ratio of the limiting frictional force F to the normal reaction N: The coefficient of kinetic friction μ k is the ratio of the kinetic frictional force F to the normal reaction N : Table 26.1 gives some typical values for these coefficients. Table 26.1 Coefficients of friction Surfaces Static coefficient Kinetic coefficient Steel on steel 0.7 0.6 Copper on steel 0.5 0.4 Glass on glass 0.9 0.4 Example A block of wood of mass 2 kg rests on a horizontal surface, the coefficient of static friction being 0.6. What horizontal force is needed to start the block in horizontal motion? Take g to be 9.8 m/s 2. The limiting frictional force, i.e. the force to start the object is motion, is F = μN = 0.6 x 2 x 9.8 = 11.76 N. Example What acceleration is produced when a horizontal force of 50 N acts on a block of mass 2.5 kg resting on a horizontal surface where the coefficient of kinetic friction is 0.5? Take g to be 9.8 m/s 2. The normal force N = 2.5 g and so the frictional force F is F = µN = 0.5 x 2.5 x 9.8 = 12 N...
