Table of Contents
Advertisement
Introduction
Mechanical
vibrations
in
a construction
occur
whenever a change
in the
forces
applied
to it takes place. This
change
in the
applied
forces
can
have
a
periodic or non-periodic
character.
If the
change
in forces, the rate of
change,
or the number of
force cycles applied are great enough,
the con-
struction
might be damaged. The purpose
of vibration
testing is to determine
the damaging
effects
that vibrations have
on a structure,
allowing it to be
re-designed to prevent
such
damage
from
taking place
under normal
operating conditions.
The importance of
vibration
testing depends upon
the severity
of damage
caused
by
a
breakdown,
and
has been fully re-
cognized
in recent
years.
As long
as only simple structures are
to be
considered,
it is possible to
calculate
the
effects
of
vibrations.
However,
as
the
structure
becomes more
and
more
complicated and
the
operating conditions
more and more
complex
and severe,
the
amount of calculation
involved is large. In
such circum-
stances, vibration
testing may be the
only
practical
solution
to the problem
of
determining
vibrational effects.
This is
especially
true of the
aircraft and missile field,
where not only the
periodic changes
in force
caused
by the
engines,
but
also
the
forces
due
to
air
flow turbulance play
an
important role.
Also gas
turbine parts
and
parts
or other fast running machinery
are
now normally
subjected
to
vibration testing before the final design of the part is
approved.
The
vibration
test may
be carried out
in different ways,
of
which the three
most important
are:-
The
shock
test
,
The random motion test,
and
The frequency
sweep
test.
For the design
engineer
the frequency
sweep
test is, in the
vast
majority
of cases,
the most preferable method
,
as
the natural
frequencies
of the
objects
under test
as
well
as
their Q-values
are
directly measured.
The frequency sweep
test is
carried out by
feeding
a vibration exciter
with
a certain amount of
power
at a slowly changing
frequency. Due to re-
sonances in
the test
object
and the
vibration exciter system,
the power
necessary to
vibrate
the test
object
at
a certain constant
level is
not,
how-
ever,
independent
of
the frequency. To keep the
vibration
level
constant,
the output from
a vibration
pick-up,
built
into the
shaker
or mounted on
it, is used to
control
the input power to the
vibration exciter.
Normally, the
control
of the
vibration
level is made in such
a
way,
that when the vibration
3
Advertisement
Table of Contents
