Positioning The Test Object; Externai Supports - BRUEL & KJAER System V Instruction Manual

3.6.2 . Positioni ng the test o bject
3.6.
If possible, the effective ce ntre of gravity of the test object should be pla ced 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 centre of gravity is not on the axis of the moving ele m ent, a moment is applied to
the table , of mag n itude equ al to the product of t he force n eeded to accelerate the test ob­
ject and the d istance between the movi ng eleme nt axis and the centre of gravity . This
moment may deflect the moving element to the side and, if deflection is excessive, the
side overload pads on the m ovi ng element will rub on the centre post. Th is probl em can
be m i nimized by providing 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 t h e location of the centre of
mass . If the problem is serious , a nd 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 h elpfu!.
3.7.
3 .6.3. Exte rnai Supports
The flexur e systems of B & K Exciter Heads are designed 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 nce it can lead to problems caused by additional mass ,
cross motion, distortion and/ or alignment.
Th e probl e m of excessive side loads is most com mon when vibrating large obj ects in t he
horizonta l directi on. The prob lem is most severe when th e centre of gravity of the object
is remote from the m ount ing s urface .
M any techniques have been devised to redu ce the side force s on the moving element . A
few of these te chniques are:
a) Provide one or more wires or elastic cords from
the gravitationa l f orce.
an overh ead support to counteract
b) Place the object on a slip table to counteract the gravitat ional force . Most of these
slip tables are esse ntially a flat plate supported by a thin f ilm of oil on a flat granite
block .
c) Drive the object through a flexible link that is stiffax ially b ut bends re latively easily.
A thin rad , t hi n ned down regions on a larger diameter rad , or a wire in tension may
be a suitable solution for a particular problem .
d) Support the load with linear be ar ings. If systems usi ng ball or roller bearings are
used , sam e means shou ld be pr ovided to pr e- Ioad every ball or roller, or rattl ing and
excessive d istortion will res ult. A ir bea rin gs are smooth , and typically free of distor­
tion , but the low stiffn ess may cause ratatianal resonance probl ems . High pressure
linear oil bearings wo rk weil if carefully made and 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!.
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