FRICTION, in mechanics, is the adhesion or degree of adhesion of contacting surfaces; the resistance which opposes the slipping or sliding of two bodies in contact. In an active sense, it is the rubbing together of things; in a passive sense, it is the tendency to adhere or stick, presented by bodies in contact, whether moving or at rest. Gritty substances and sticky substances between surfaces tend to in crease friction; oils, fluids and gases inter posed tend to reduce friction. The degree of friction between surfaces is determined by: (1) the weight or pressure with which they contact; (2) the extent or surface of area in contact; (3) the degree of roughness or smoothness of the contacting surfaces; (4) the speed at which the surfaces move; (5) the extent or kind of lubrication employed. The angle of friction is the maximum slope at which one body will rest on another without the upper one sliding off by gravity. Efforts have been made to tabularize the angle of fric tion of different contacting surfaces, but so many conditions are involved that in practice such tables are worthless. It is generally true, however, that at 45 degrees almost all straight surfaces will slip and smooth lubricated surfaces can be depended upon to slip at half that angle. A block placed on a flat, smooth sur face can be moved by a force equaling two fifths of its weight.
When friction is considered with reference to two bodies at rest, it is called static friction. Instances of static friction in machinery are noticeable in friction couplings or friction discs, consisting of two discs placed close together, just out of contact, one of which rotates while the other is stationary, but when both discs are tightly pressed together by the action of a lever, they rotate together, as if they were one. Friction between bodies in motion is called kinetic friction. This exists in the journal or bearing of an axle, on the working surface of i cams, etc., machinery; and machine designers as well as operators of machines should keep down such friction all they can, as it uses power to no good end.
Static friction is measured by the force that is required to just cause one body to move upon the other, when the two are pressed together by a certain definite pressure; and the ratio of this force to the pressure with which the bodies are held in contact is called the "co efficient of statical Kinetic friction is measured by the force that is required to maintain one of the bodies that are in contact, in a state of uniform motion with respect to the other one; and the ratio of this force to the force with which the bodies are pressed to gether is called the "coefficient of kinetic fric tion." The coefficient of static friction between
two given substances may be determined by causing a weight composed of one of the sub stances to rest upon a smooth plane composed of the other substance. If the plane is nearly horizontal, the weight will not slip upon it; but by increasing the angle of inclination, a position will be found for the plane, such that the weight is just on the point of sliding. The angle that the plane then makes with the hori zontal is called the ((angle of repose' of the pair of substances of which the plane and the weight are composed; and it may be shown by the elementary principles of mechanics that the coefficient of statical friction for these sub stances is numerically equal to the natural tan gent of the angle of repose.
Rolling friction presents different char acteristics from rubbing or static friction. It arises from the contact between a stationary surface and a rolling wheel, sphere or cylinder, or between two rotating surfaces. The tractive effort required to draw a cart on a smooth level road is about one-thirtieth of the weight of cart and load. A carriage wheel running on the ground is an instance of rolling friction, while the friction in the axle box is rubbing friction if the bearing be a plain one, or rolling friction if ball-bearings or roller-bearings are em ployed. Some have thought there was no fric tion between the steel balls and the raceway of a ball-bearing, but it is obvious that the weight or pressure produces static friction and that the imperfections of the surfaces, even though very minute, must always exist and there is al most sure to be sonic dust or grit in such bear ings at some time, hence rolling friction has to be figured on by machine designers, even in the best bearings, though it may be trifling, as in the ball-bearings of a typewriter. Sliding fric tion is a term used as opposed to rolling fric tion and is identical with kinetic or rubbing friction, except that it suggests surfaces de signed to slide easily.