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Note that the following section, all though primarily related to setup and use of the
optional DC Static Centering Unit Type 1056, contains important practical informa-
tion when setting up Modal Exciters Types 4825 and 4826 for use with a tension wire
stinger. This information applies to any setup that uses a tension wire stinger.
DC Static Centering
Pretensioning of a tension wire stinger can also be achieved using the (optional) DC
Static Centering Unit Type 1056. This allows for use of tension wire stinger technolo-
gy without a spring-based mechanical pretensioning found on dedicated horizontal
exciter stands such as Lateral Modal Exciter Stands UA 1607 and UA 1608
DC Static Centering Unit Type 1056 becomes especially useful when limited space or
skewed angle mounting is necessary. In both cases, the size and form of Lateral
Modal Exciter Stands UA 1607 and UA 1608 may prevent it from being successfully
installed. Using DC Static Centering Unit Type 1056 means that Modal Exciters Types
4825 and 4826 – mounted in trunnion – can be placed at any position and angle, and
a tension wire can still be used.
In a practical setup, first connect the input transducer to be used in the test to the
chosen input location on the structure. Then screw the 10–32 UNF to tension wire
adaptor into the input transducer, insert one end of the piano wire and secure its
position into the adaptor by fastening the Allen screw in the adaptor. The piano wire
is fed into the modal exciter, and is fastened by turning the chuck nut clockwise
with a spanner.
For the excitation setup to function properly, DC Static Centering Unit Type 1056
must now be adjusted to provide the maximum possible pre-tensioning for the par-
ticular setup. When applying the necessary pre-tensioning force, the following
should be observed:
1) The structure under test must not move when preloaded. If the structure does
move, the boundary conditions for the structure under test should be changed
to accommodate for the necessary pre-tensioning force or, often a better solu-
tion, a setup using push/pull stingers should be used instead of tension wire .
2) Maximum excitation force and useful frequency range will depend upon pre-ten-
sioning applied, length of the tension wire, boundary conditions of modal exciter
stand, compliance of boundary conditions of structure under test. In most appli-
cations, a frequency range up to a maximum of approximately 500 Hz can be
expected.
1 In a vertical excitation setup, a third possibilty for pretensioning exist: A free "hanging" mass may
simply be attached to the end of the tension wire coming through the modal exciter, providing a
pretensioning force equal to the product of the mass and gravitational acceleration. This solution is
especially useful when using an additional trunnion, mounted in a back-to-back configuration with
the trunnion into which the modal exciter is allready mounted. This back-to-back configuration
provides convinient and ample space for the free "hanging" mass.
CHAPTER 2
9
Operation
1
.
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