ROSTA
3.5
Anti-vibration Mounts
Technology Anti-vibration Mounts
The vibration technology basically differentiates between
two principal types of oscillation appearances (fig.
1
). Si-
nusoidal oscillations of working equipments are usually
amortised in an
overcritical
installation manner, shocks
and impacts in a
subcritical
mounting manner.
Manufacturers and suppliers of anti-vibration mounts usually offer different types of machine mount with varying natural fre-
quencies to meet the required
detuning
between the excitation frequency of the machine and the natural frequency of the
anti-vibration mount.
Oscillations
Shocks
Frequency Proportion
l
(fig.
2
)
l
>
√
2
:
Overcritical
efficient vibration isolation, clearly definable ef-
fectiveness, also efficient solid-borne noise ab-
sorption
l
= 1:
Resonance field
uncontrolled swing-up, in the long term destruc-
tive for machine and mounts
l
< 1:
Subcritical
vibration isolation not definable, isolation results
have to be measured out (before and after mount
installation).
1
Overcritical installations (
l
>
√
2
)
On overcritical installations the natural frequency of the
mounts should show at least a detuning factor of 1:1,414 in
regard to the excitation frequency of the machine. Usually,
very efficient anti-vibration mounts feature a deep deflection
capability offering a low natural frequency. Most of the gen-
erators, compressors, blowers and chargers are, therefore, in
overcritical
manner installed on relatively “soft” mounts.
The resulting
detuning
proportion
provides information
about the expected
isolation-effectiveness
in % of the
machine suspension. The adjacent chart (fig.
3
) and the cal-
culation formula (fig.
4
) inform about the resulting vibration
isolation in %.
2
3
1. Isolation of Oscillations and Shocks
0.2
1
2
3
4
5
√
2
0.4
0.6
0.8
1
2
(20%)
(40%)
(60%)
(80%)
unterkritisches
D = 1.0
D = 0.25
D = 0
Transmissibility V =
transmitted power
excited power
Frequency proportion
l
=
Excitation frequency (machine)
Natural frequency (damper)
Frequency proportion
l
Isolation range
Damping
function
function
overcritical
subcritical
Resonance
Vibration isolation W[%]