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

3) For thick objects, the inertias are computed about two axes:

Ø About the axis of rotation.
Ø About the axis of each section parallel to the axis of rotation.

4) For a general object, about the 3 perpendicular axes of symmetry, mass moment inertia
exists in such a way that, one of them is maximum and the others are intermediate and
minimum. These are called the principal mass moment of inertias.

The most commonly used MMOI’s for engineering calculations are as tabulated:

Component Description Mass moment of inertia
Cuboid: Since the cuboid is symmetric about

each axis, it has 3 principal inertias.
W

CG

H

L

Hollow shaft: The mass moment of inertia about the
axis of the cylinder can be got by

subtracting the inertia of solid cylinder
with radius with that of the

L cylinder with radius For the CG

axis it is treated as a thick rod.

Component Description Mass moment of inertia
Thin rod: The mass moment of inertia is sought
about an axis through the CG of the rod.
The inertia about the axis of the rod is
CG
not considered as the entire mass is
almost concentrated along the axis of
the rod.
Thick Cylinder:

CG
For a thin disc:
x

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