Installation; Mechanical Installation Of Exciters Using Type 4801 Body; Direct Mounting; Mounting Upon A Seismic Block - BRUEL & KJAER System V Instruction Manual

2. INSTALLATION
2.1. MECH ANIC AL INSTALLATION OF EXCITERS USIN G TYPE 4801 BOD Y
The technique used to install the Exciter Body 4801 depends upon the usage. For most
applications , the Base of the exciter may be placed directly upon the floor. For more criti­
cal applications , other mounting techniques may be preferred.
2. 1.1 . Direct Mounting
The easiest way to mount the Exciter Body 4801 is to place the Base directly on the
floor. This technique is suitable whenever the forces transmitted from the exciter to the
floor do not produce objectionable results and when the motions coupled to the exciter
from the floor do not affect the usefulness of the exciter. Motion transmitted to the exci­
ter through its body is likely to be insignificant for operation frequencies over 30 Hz and
when an appreciable fraction of the available force is used.
The forces transmitted to the building are seldom bothersome when the exciter is placed
on a substantiai masonry floor. If the exciter is placed on a thin wooden floor , the forces
transmitted from the exciter to the building are likely to cause annoyance , in the form of
vibration and noise . For some situations, an adequate reduction in the irritation leveiover
that obtained with direct mounting can be achieved by placing an inch or two of soft re­
silient material under each corner of the Base .
2. 1.2 . Mounting upon a seismic block
Vibration isolation better than that provided by the trunnion springs of the exciter usually
is obtained only by use of a seismic block.
A seismic block consists of a large mass, typically reinforced concrete supported by a rel­
atively soft spring system . The mass is typically ten times as large as the mass of the ex­
citer itself. Hence the block characteristics dominate and the entire complex of exciter
and seismic block may be treated as a simple mass on a spring, i . e., a single degree of
freedom system . Such a system has a resonant frequency corresponding to
f=_l_~
k
2TT m
where k equals the overall spring stiffness of the system and m is the total mass.
Since the mass of the V System Exciter is about 85 kg (187 Ib), depending on which head
is chosen, its seismic block should have a mass of about 850 kg (1870 Ib) This corre­
sponds to 0,35 m
3
(12,5 ft 3
)
of reinforced concrete . If the Exciter should work down to
1 Hz , then the spring stiffness of the suspension system should be
k = (2
TT) 2
m
=
33,4 N/ mm (190 Ib / in)
7