Machinery's Handbook, 31st Edition
172
Friction Friction
Properties of Friction.— Friction is the resistance to motion that takes place when one body is moved upon another, and is generally defined as “that force which acts between two bodies at their surface of contact, so as to resist their sliding on each other.” According to the conditions under which sliding occurs, the force of friction, F , bears a certain rela- tion to the force between the two bodies called the normal force N . The relation between force of friction and normal force is given by the coefficient of friction , generally denoted by the Greek letter μ . Thus: F N and # µ µ = = Example: A body weighing 28 pounds rests on a horizontal surface. The force required to keep it in motion along the surface is 7 pounds. Find the coefficient of friction. . 28 7 025 µ = = = If a body is placed on an inclined plane, the friction between the body and the plane will prevent it from sliding down the inclined surface, provided the angle of the plane with the horizontal is not too great. There is a certain angle, however, at which the body will just barely be able to remain stationary, the frictional resistance being very nearly overcome by the tendency of the body to slide down. This angle is termed the angle of repose, fre quently denoted by the Greek letter θ, and the tangent of this angle is the coefficient of friction. Thus, μ = tan θ . N F N F A greater force is required to start a body moving from a state of rest than to merely keep it in motion, because static (resting) friction is greater than sliding (motion) friction. Laws of Friction.— Unlubricated or Dry Surfaces: 1) At low pressures (low normal force per unit area) friction is directly proportional to the normal force between the two surfaces. As the pressure increases, the friction does not rise proportionally; but when pressure becomes abnormally high, friction increases at a rapid rate until seizing takes place. 2) Friction, both in its total amount and its coefficient, is independent of the area in contact, so long as the normal force remains the same. This is true for moderate pressures only. For high pressures, this law is modified in the same way as in the first case. 3) At very low velocities friction is independent of the velocity of rubbing. As velocity increases, friction decreases. Lubricated Surfaces: For well-lubricated surfaces, the laws of friction are considerably different from those governing dry or poorly lubricated surfaces. 1) Frictional resistance is almost independent of pressure (normal force per unit area) if the surfaces are flooded with oil. 2) Friction varies directly with speed at low pressures; but at high pressures the friction is very great at low velocities, approaching a minimum at about 2 ft/s (0.61 m/s), linear velocity, and afterwards increasing approximately as the square root of the speed. 3) For well-lubricated surfaces frictional resistance depends, to a very great extent, on temperature, partly because of a change in the viscosity of the lubricant and partly because, for a journal bearing, bearing diameter increases with temperature rise more rapidly than does shaft diameter, thus relieving the bearing of side pressure. 4) If bearing surfaces are flooded with oil, friction is almost independent of the nature of the material of the surfaces in contact. As the lubrication becomes less ample, the coefficient of friction becomes more dependent upon the material of the surfaces. Influence of Friction on Efficiency of Small Machine Elements.— Friction between ma- chine parts lowers the efficiency of a machine. Average values of efficiency, in percent, of
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online