Introduction to Friction
When an object, such as a block of wood, is placed on a floor and sufficient force is applied to the block, the force being parallel to the floor, the block slides across the floor. When the force is removed, motion of the block stops; thus there is a force which resists sliding. This force is called dynamic or sliding friction. A force may be applied to the block, which is insufficient to move it. In this case, the force resisting motion is called the static friction or striction. Thus there are two categories into which a frictional force may be split:
(i) dynamic or sliding friction force which occurs when motion is taking place, and
(ii) static friction force which occurs before motion takes place
There are three factors that affect the size and direction of frictional forces:
(i) The size of the frictional force depends on the type of surface (a block of wood slides more easily on a polished metal surface than on a rough concrete surface).
(ii) The size of the frictional force depends on the size of the force acting at right angles to the surfaces in contact, called the normal force; thus, if the weight of a block of wood is doubled, the frictional force is doubled when it is sliding on the same surface.
(iii) The direction of the frictional force is always opposite to the direction of motion. Thus the frictional force opposes motion, as shown in Figure 16.1.
The coefficient of friction, µ, is a measure of the amount of friction existing between two surfaces. A low value of coefficient of friction indicates that the force required for sliding to occur is less than the force required when the coefficient of friction is high. The value of the coefficient of friction is
The direction of the forces given in this equation is as shown in Figure 16.2 The coefficient of friction is the ratio of a force to a force, and hence has no units. Typical values for the coefficient of friction when sliding is occurring, i.e. the dynamic coefficient of friction, are:
For example, the material of a brake is being tested and it is found that the dynamic coefficient of friction between the material and steel is 0.91. The normal force, when the frictional force is 0.728 kN, is calculated as follows:
Applications of Friction
In some applications, a low coefficient of friction is desirable, for example, in bearings, pistons moving within cylinders, on ski runs, and so on. However, for such applications as force being transmitted by belt drives and braking systems, a high value of coefficient is necessary.
Instances where frictional forces are an advantage include:
(i) Almost all fastening devices rely on frictional forces to keep them in place once secured, examples being screws, nails, nuts, clips and clamps.
(ii) Satisfactory operation of brakes and clutches rely on frictional forces
being present.
(iii) In the absence of frictional forces, most accelerations along a horizontal surface are impossible; for example, a person’s shoes just slip when
walking is attempted and the tyres of a car just rotate with no forward
motion of the car being experienced.
Disadvantages of frictional forces include:
(i) Energy is wasted in the bearings associated with shafts, axles and gears due to heat being generated.
(ii) Wear is caused by friction, for example, in shoes, brake lining materials
and bearings.
(iii) Energy is wasted when motion through air occurs (it is much easier to cycle with the wind rather than against it).
Two examples of design implications, which arise due to frictional forces and how lubrication may or may not help, are:
(i) Bearings are made of an alloy called white metal, which has a relatively low melting point. When the rotating shaft rubs on the white metal bearing, heat is generated by friction, often in one spot and the white metal may melt in this area, rendering the bearing useless. Adequate lubrication (oil or grease) separates the shaft from the white metal, keeps the coefficient of friction small and prevents damage to the bearing. For very large bearings, oil is pumped under pressure into the bearing and the oil is used to remove the heat generated, often passing through oil coolers before being re- circulated. Designers should ensure that the heat generated by friction can be dissipated.
(ii) Wheels driving belts, to transmit force from one place to another, are used in many workshops. The coefficient of friction between the wheel and the belt must be high, and dressing the belt with a tar-like substance may increase it. Since frictional force is proportional to the normal force, a slipping belt is made more efficient by tightening it, thus increasing the normal and hence the frictional force. Designers should incorporate some belt tension mechanism into the design of such a system.
