In the control of noise we basically
considered three areas: the source, the path, and the receiver. Vibration control may
involve one or a combination of the following techniques.
(a) Source alteration
In the control of vibration it is
important to first check and see if the noise or vibration level can be reduced by
altering the source. This may be accomplished by making the source more rigid from a
structural standpoint, changing certain parts, balancing, or improving dimensional
tolerances. The system mass and stiffness may be adjusted in such a way so that resonant
frequencies of the system do not coincide with the forcing frequency. This process is
called detuning. Sometimes it is also possible to reduce the number of
coupled resonators that exist between the vibration source and the receiver of interest.
This technique is called decoupling. Although these techniques can be
applied during design or construction, they are perhaps more often used as a correction
scheme. However, it is also important to ensure that the application of these schemes does
not produce other problems elsewhere.
In general, vibration isolators
can be broken down into three categories: (i) metal springs, (ii) elastomeric mounts,
and (iii) resilient pads. Before examining each of these areas, a few general comments
can be made which are pertinent to all categories. We must always remember that we are
assuming a single-degree-of-freedom system, and therefore our analysis will not be exact
in every case. However, practical systems are normally reduced to this model because it is
the only one that we understand thoroughly.
When building or correcting a design,
always ensure that the machine under investigation and the element that drives it both
rest on a common base. Always design the isolators to protect against the lowest frequency
that can be generated by the machine. Design the system so that its natural frequency will
be less than one-third of the lowest forcing frequency present. The isolation device
should also reduce the transmissibility at every frequency contained in the Fourier
spectrum of the forcing function.
(i) Metal springs
Metal springs are widely used in
industry for vibration isolation. Their use spans the spectrum from light, delicate
instruments to very heavy industrial machinery. The advantages of metal springs are: (a)
they are resistant to environmental factors such as temperature, corrosion, solvents, and
the like; (b) they do not drift or creep; (c) they permit maximum deflection; and (d) they
are good for low-frequency isolation. The disadvantages of springs are (a) they possess
almost no damping and hence the transmissibility at resonance can be very high; (b)
springs act like a short circuit for high-frequency vibration; and (c) care must be taken
to ensure that a rocking motion doe not exist.
Careful engineering design will
minimize the effect of some of these disadvantages. For example, the damping lacked by
springs can be obtained by placing dampers in parallel with the springs. Rocking motions
can be minimized by selecting springs in such a way that each spring used will deflect the
same amount. In addition, the use of an inertia block that weighs from one to two times
the amount of the supported machinery minimizes rocking lowers the center of gravity of
the system, and helps to uniformly distribute the load. High-frequency transmission
through springs caused by the low damping ratio can be blocked by using rubber pads in
series with the springs. A typical damping ratio for steel springs is 0.005.
The design procedure for
selecting springs for vibration isolation is outlined below:
A machine set operating at 2400
rpm is mounted on an inertia block. The total system weighs 907 N. The weight is
essentially evenly distributed. We want to select four steel springs upon which to mount
the machine. The isolation required is 90%.
(ii) Elastomeric mounts
Elastomeric mounts consist primarily
of natural rubber and synthetic rubber materials such as neoprene. In general, elastomeric
mounts are used to isolate small electrical and mechanical devices from relatively high
forcing frequencies. They are also useful in the protection of delicate electronic
equipment. In a controlled environment, natural rubber is perhaps the best and most
economical isolator. Natural rubber contains inherent damping, which is very useful if the
machine operates near resonance or passes through resonance during "startup" or
"shutdown." Synthetic rubber is more desirable when the environment is somewhat
Rubber can be used in either tension,
compression or shear; however, it is normally used in compression or shear and rarely used
in tension. In compression it possesses the capacity for high-energy storage; however, its
useful life is longer when used in shear. Rubber is classified by a durometer number.
Rubber employed in isolation mounts normally ranges from 30-durometer rubber, which is
soft, to 80-durometer rubber, which is hard. The typical damping ratio for natural rubber
and neoprene is z = 0.05.
One word of caution when dealing with
rubber: it possesses different characteristics depending upon whether the material is used
in strips or bulk, and whether it is used under static or dynamic conditions. The steps
for selecting an elastomeric mount are essentially those enumerated in the previous
section on metal springs. The following examples will illustrate the procedure.
A drum weighing 120 N and
operating at 3600 rpm induces vibration in adjacent equipment. Four vertical mounting
points support the drum. Choose one of the isolators shown in Figure 6 so as to achieve
90°/ vibration isolation.