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BRUEL & KJAER System V - Positioning the Test Object; Externai Supports

BRUEL & KJAER System V
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3.6.2
.
Positioni
ng
the
te
st o
bject
3.6.
If
possible,
the
effective
ce
ntr
e
of
gravity
of
the
test
object
should
be placed on
the
axis
of
the
moving
element.
close
to
the
table
surface,
and
should
remain
fixed
in
position
throughout
the
frequency
range
. For
same
specimens,
this
is
not
possible.
If
the
centr
e of
gravit
y is
not
on
the
axis
of
the
m
oving
ele
m
ent,
a
mo
ment
is ap
plied
to
the
table
,
of
mag
n
it
ude e
qu
al
to
the
produc
t of the
for
ce needed
to
accelerate
the
test
ob
-
je
ct
and
the
di
stance
between
the
movi
ng
eleme
nt
axis
a
nd
the
centre
of
gr
a
vity
.
This
moment
may
deflect
the
moving
element
to
the
side
and,
if
deflection
is
excessive,
the
sid
e
overload
pads
on
the
moving
element
will
rub
on
the
centre
post.
This
probl
em
c
an
be mi
nimize
d by
providin
g a
counterweight
so
that
the
centre
of
gravity
of
the
combina-
tia
n is
on
the
axis.
The
test
object
is
likely
to
have
resonances
of
its
own
. These
resonances
may
apply
mo-
ments
to
the
moving
element
or
cause
an
effective
shift
in the
location
of
the
centre
of
mass
. If
the
problem
is
serious
, and
if
only
part
of
the
force
capability
of
the
exciter
is
needed
to
vibrate
the
test
object
to
the
desired
leve!, a
judicious
addition
of
mass
to
the
moving
system
may
be help
fu!.
3.7.
3.
6.3.
Exte
rnai
Supports
The
flexur
e
systems
of
B & K
Exciter
Heads
ar
e de
signed
to
operate
with
side
loads
larger
than
commonly
are
encountered
. If,
however,
it is
necessary
to
vibrate
objects
that
are
likely
to
apply
unusually
large
side
forces
to
the
moving
element.
it is
advisable
to
use
same
form
of
externai
support
to
reduce
these
loads.
An
externa
I
support
should
not
be
used
uniess
it
is
necessary,
si
nc
e it can lead
to
pr
ob
l
ems
caused
by
additio
nal
mass
,
cross
motion,
distortion
and
/
or
ali
gnment.
The pr
obl
em
of
e
xcessive
side
loads
is
most
co
m
mon
wh
en vi
br
at
ing
large
ob
je
cts
in
the
hor
iz
onta
l
dir
ecti
on.
The
p
ro
bl
em
is
most
seve
re
whe
n
th
e c
entre
of
gravit
y
of
the
object
is
remote
fr
om
the
mo
un
t
ing
s
urfac
e .
M
any
techniques
have
been
devised
to
reduce
the
side
force
s
on
the
moving
element
. A
few
of
these
tec
hniques
are:
a)
Provide
one
or
more
wires
or
elastic
cords
from
the
gravitationa
l force.
an
overh
ead
support
to
counteract
b)
Place
the
object
on
a
slip
table
to
counteract
the
gr
av
itat
ional
force
.
Most
of
these
slip
tables
are esse
ntially
a
flat
plate
supported
by a
thin
fi
lm
of
oil
on
a
flat
granite
block
.
c)
Drive
the
object
through
a
flexible
link
that
is
stif
fax
ially
b
ut
bends
rela
tively
easily.
A
thin
rad , t
hi
n
ned
do
wn
regions
on
a
larger
diameter
rad ,
or
a
wire
in
tension
may
be a
suitable
solution
for
a
particular
problem
.
d) S
upport
the
load
with
linear
bearings.
If
syste
ms
using ball
or
roller
bearings
are
used
, same
means
shou
ld be pro
vid
ed
to
pre-Ioad
every
ball or r
oller,
or
ra
ttl
ing
and
excessiv
e d
istortion
will
res
ult.
Air bea
rin
gs are
smooth
, and
typically
free
of
distor-
tion
,
but
the
low
stiffn
ess
may
cause
ratatianal
resonance
pro
bl
ems
.
High
pressure
linear
oil be
arings
wo
rk
weil
if
carefully
made
an
d w eil
maintained
.
However,
they
tend
to
be
heavy
and
expensive
and
ca n be
ru
ined
by a li
ttle
dirt
in
the
oi!.
18

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