FRICTION is the resistance offered to the motion of one portion of matter upon another. It is due partly to the molecular attraction between the surfaces and partly to the nature and condition of those surfaces. It is in the nature of a force which acts between the surfaces in contact and resists their sliding or rolling upon each other. This force, termed the force of friction (F), is independent, in a solid body, of the area of the surfaces in contact and the speed of the motion (unless it be very small) of the two bodies. It is proportional, however, to the force with which the surfaces are pressed together, that is, it is that force times some factor which is always less than unity. This factor is called the coefficient of friction and is designated by or f. Thus if a ioo lb. iron block (pressing the surfaces together with a force P) slides along a horizontally placed iron plate when a force of 20 lb. (F) is exerted against it, the ratio between the weight of the block and the force required to move it is the coefficient of friction (A) If the block of iron is placed upon the iron plate, one end of which is raised until the weight of the block will just overcome its tendency to slide, then the angle of the plate to the horizontal is called the angle of repose, which is generally denoted by 0. In the case of the iron block on the iron plate (0) will be between 14° and 8° depending upon the roughness of the surfaces. The force which is required to overcome the friction of starting a body so that it will slide along another surface is called the friction of rest or static friction. This always is greater than the force necessary to keep it sliding in uniform motion, which is called the friction of motion or kinetic friction.
All surfaces are more or less rough, even those which are well machined and polished show, under the microscope, small pro jections and depressions. When any surfaces have been at rest for some time static friction is always increased. This is be cause two dry surfaces left under pressure embed themselves, the one in the other, more and more thoroughly as time passes, until with the exclusion of air, in some instances adhesion occurs. With lubricated surfaces that have been at rest for some time, and when the lubricant has been pressed out, a somewhat similar action occurs. As a result the starting effort is always much greater than the running effort ; starting effort may even approxi mate the values for solid friction, which is the term used to denote friction when there has been no lubrication. After the surfaces have been forced into motion. the lubricant is drawn between them and the force necessary to drive the part rapidly decreases as the lubricant begins to produce a film. When a perfect film is maintained between the surfaces they are then held apart and are without contact. The condition then follows the laws of fluid friction covering the movement of particles of liquids in contact with each other or with other bodies. This can be likened to the effect of a boat going through the water, giving motion to various strata or films of water. (See ViseosITv. ) The film in contact with the boat travels at about the speed of the boat, but every succeeding film or layer has less motion, the outside layer being then in contact with still water. The frictional resistance between the films is very great as the speed of the boat increases.
The laws of fluid friction as given by Thurston are: Frictional resistance is, (I) independent of the pressure between the masses in contact; (2) directly proportional to the area of rubbing surface; (3) proportional to the square of the relative velocity at moderate and high speeds; and to the velocity, nearly, at low speeds; (4) independent of the nature of the surfaces of the solid against which the stream may flow, but dependent to an extent upon the degree of roughness; (5) proportional to the density of the fluid and related in some way to its viscosity.
The following table (Thomsen) of approximate values for the coefficient of friction shows coefficient of friction with various conditions:— In the latest studies of the behaviour of bearings and lubricants it has been pointed out by Bernard (Industrial and Engineering Chemistry, April 1924) that the main variables influencing the lubrication of a given bearing of normal design and construction are the ratio z where z is the viscosity of the lubricant, p n is the speed of rotation of the journal, and p is the pressure on the bearing. When speeds are high and loads are light viscosity is the only property of the lubricant which influences friction. With low speeds and heavy loads friction is largely effected by the ability of the lubricant to adhere to the bearing surfaces and resist rupture. (See also BEARINGS; LUBRICATION AND LUBRI CANTS.) (W. F. P.)