Mechanical Engineering Principles

172 MECHANICAL ENGINEERING PRINCIPLES

can be transmitted, and (b) maximum

force which can be transmitted when the

belt is running at a constant speed.

[(a) 675 N (b) 652.5 N]

15.3 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.

Problem 4. State three advantages, and

three disadvantages of frictional forces.

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 accel-
erations along a horizontal surface are impos-
sible; 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).

Problem 5. Discuss briefly two design

implications that arise due to frictional forces

and how lubrication may or may not help.

(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 lubri-

cation (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 cool-

ers 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 it may be increased by

dressing the belt with a tar-like substance. Since

frictional force is proportional to the normal force,

a slipping belt is made more efficient by tighten-

ing it, thus increasing the normal and hence the

frictional force. Designers should incorporate some

belt tension mechanism into the design of such a

system.

Problem 6. Explain what is meant by the

terms (a) the limiting or static coefficient of

friction, and (b) the sliding or dynamic

coefficient of friction.

(a) When an object is placed on a surface and

a force is applied to it in a direction parallel to

the surface, if no movement takes place, then the

applied force is balanced exactly by the frictional

force. As the size of the applied force is increased,

a value is reached such that the object is just on

the point of moving. The limiting or static coeffi-

cient of friction is given by the ratio of this applied

force to the normal force, where the normal force

is the force acting at right angles to the surfaces in

contact.

(b) Once the applied force is sufficient to over-

come the stiction its value can be reduced slightly

and the object moves across the surface. A partic-

ular value of the applied force is then sufficient to

keep the object moving at a constant velocity. The

sliding or dynamic coefficient of friction is the ratio

of the applied force, to maintain constant velocity,

to the normal force.