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.
