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Document Title
Fundamentals of Stress and Vibration Chapter Title
[A Practical guide for aspiring Designers / Analysts] 2. Engineering Mechanics

Therefore, friction force is given by:

Friction force = μ ∗ N

For a static situation Friction force = μ static ∗ N

It is important to note that, the coefficient of friction is operative only to ensure static equilibrium.
In situations where the motion is impending, the static coefficient of friction is fully operative. For
dynamic situations, that is, when the motion is sustained, we have:

Friction force = μ dynamic ∗ N

However, from the discussions so far, we could conclude that the dynamic coefficient of friction is
lesser that the static coefficient of friction.

If the difference between static and dynamic friction is large, then, a phenomenon called “stick-slip”
friction is observed.

Example, the chattering motion of chalk against the board, viper blades against the windshield,
violin string against the bow, etc.

Friction is an important design parameter, be it, joints or surfaces in contact, say, piston and
cylinder or gear meshing or bearing friction. In all the aforementioned cases, friction must be
designed for optimum functionality.

Example, a typical rear wheel of an automobile is attached to the hub with bolts whose pretension
provides the normal force and the available friction coefficient ensures that the wheel does not slip
when accelerated (traction developed against the road surface).

Therefore, mathematically, number of bolts used to assemble the rear wheel to the hub is given by:

maximum traction
Number of bolts n =
preload ∗ μ static

Facts of Static and Dynamic Friction:

1) Friction force does not depend on the area of contact but depends only on the normal
contact force.
2) Friction force slightly reduces at high velocities. However, it remains constant for
moderate velocities.
3) Static friction is fully operative (maximum) only in situations where the motion is
impending.
4) For metals, friction increases with increase in temperature.
5) Friction is reduced via lubrication and high quality surface finish (low height of
irregularity).

Page 62 QP No. SSC/Q4401, Version 1.0, NSQF Level 7, Compliant with Aero and Auto Industries,