Reducing both vibration emission and immission have become important
objectives in operating machinery and other equipment. The continuous
gains in machine performance achieved during the past several years have
generally provided for increases in rotation speed and cutting speed as
well as in the impact force available for non-cutting shaping. For this
reason, the amount of vibration generated and emitted to the environment
has increased, requiring of manufacturers that they intensify their
vibration-isolation measures in the context of environmental protection.
Principle of Vibration Isolation
The objective in installing a machine on vibration-isolating mounts
is to reduce its impulse and sinusoidal vibration. In particular, it is
the amplitude of the elastically-mounted machine's movement that is to
be held within certain constraints. In choosing a vibration insulator it
is therefore absolutely necessary to provide for sufficient damping
capacity!
Vibration Calibration
Isolation of periodically-actuated vibrations
The effectiveness of vibration isolation depends to a great extent
on the relationship between the rotational speed of the machine and
the natural frequency of the insulator (damping ratio). In general it
is true that the effectiveness of vibration isolation rises as the
natural frequency of the insulator drops, that is, as the ratio
between the frequency of the vibration (rotational speed of the
machine) and the natural frequency of the insulator rises. The curve
below shows that isolation only starts to occur when this ratio
exceeds 2. If the ratio is less than 2, the vibration may easily be
amplified or strengthened. Typically, the objective is to achieve a
ratio of between 3 and 4. A ratio of 3 is considered to be the lowest
effective value, a ratio of 4 to be an economic limit. A ratio of
greater than 4 cannot be justified on economic grounds, as the
incremental material cost is proportionally much greater than the
incremental isolation effectiveness thereby achieved.
Impact Isolation
The critical characteristics of an impact are its duration,
distribution and intensity. Isolating an impact converts it from a
short-term high-amplitude impulse into one which lasts longer but
only carries a small fraction of the original force. In contrast to
the periodically-actuated vibrations, the isolated system vibrates
in the resonant frequency of the insulated machine and not according
to its rotational speed. The remaining impulses transmitted by the
insulators drop in intensity as the resonant impulse duration rises
and the resonant frequency of the insulated machine drops.
Types of Vibration Isolation
Vibration-damping technology is divided into active and passive
types. If the vibrations created by a machine are to be prevented
from entering the environment, we speak of "active
isolation". If a shock-sensitive machine used for fine
manufacturing or finishing operations is to be isolated from
vibrations emanating from the environment, we speak of
"passive isolation".
Important Terminology
Damping = is the physical characteristic of an
insulator that causes it to limit resonant vibrations. Mechanical
energy is thus converted into heat.
Isolation = means damping or shielding impulse
energies otherwise transmitted to the floor or foundation or to
the machine.
