TM 5-805-4/AFJMAN 32-1090
d. Ribbed neoprene pads. Neoprene pads with
is pumped up to the necessary pressure to carry its
ribbed or waffle-pattern surfaces are effective as
load. Since the chamber is subject to very slow
high frequency isolators in series with steel
leakage, a system of air mounts usually includes a
springs. In stacks of 2 to 4 thicknesses, they are
pressure sensing monitor and an air supply (either
also used for vibration isolation of flow power
a pump or a pressurized air tank). A group of air
rotary equipment. The pads are usually about 1/4
mounts can be arranged to maintain very precise
to 3/8 inches thick, and they compress by about 20
leveling of a base by automatic adjustment of the
percent of their height when loaded at about 30 to
pressure in the various mounts. If air mounts are
50 lb/in2. Higher durometer pads may be loaded
used in a design, an active air supply is required.
up to about 100 lb/in2. The pads are effective as
Operational data should be obtained from the
isolators because the ribs provide some shearing
manufacturer,
action, and the spaces between the ribs allow
8-3. Mounting Assembly Types.
lateral expansion as an axial load is applied. The
manufacturer's literature should be used for
In this paragraph, five basic mounting systems are
proper selection of the material (load-deflection
described for the vibration isolation of equipment.
curves, durometer, surface area, height, etc.).
These mounting systems are applied to specific
e. Felt pads. Felt strips or pads are effective for
types of equipment in paragraph 8-6. Certain
reducing structureborne sound transmission in the
general conditions relating to all the systems are
mounting of piping and vibrating conduit. One or
first mentioned.
more layers of 1/8 or 1/4 inch thick strips should
a. General conditions.
be wrapped around the pipe under the pipe clamps
(1) Building uses. Isolation recommendations
that attach the piping to building structures. Felt
are given for three general equipment locations:
pads will compress under long time and high load
on grade slabs, on upper floors above noncritical
application and should not be used alone to vibra-
areas, and on upper floors above critical areas. It
tion isolate heavy equipment.
is assumed that the building under consideration
f. Cork pads. Cork pads, strips, or blocks may be
is an occupied building involving many spaces that
used to isolate high frequency structureborne
would require or deserve the low noise and vibra-
noise, but they are not recommended for high load
tion environments of such buildings as hotels,
bearing applications because cork gradually com-
hospitals, office buildings, and the like, as charac-
presses under load and loses its resilience. High
terized by categories 1 through 4 of table 2-1.
density construction cork is sometimes used to
Hence, the recommendations are aimed at provid-
support one wall of a double wall. In this applica-
ing low vibration levels throughout the building. If
tion, the cork will compress slightly with time,
a building is intended to serve entirely such uses
and it will continue to serve as a high frequency
as those of categories 5 and 6 of table 2-1, the
isolator (say, for structureborne noise above about
recommendations given here are too severe and
100 to 200 Hz), but it will not provide good low
frequency isolation at equipment driving fre-
can be simplified at the user's discretion. An
quencies of about 10 to 60 Hz. Years ago, before
on-grade slab usually represents a more rigid base
other resilient materials came into widespread
than is provided by a framed upper floor, so the
use, cork was often misused under heavy vibrating
vibration isolation recommendations can be re-
equipment mounts: full area cork pads were fre-
laxed for on-grade installations. Of course, vibra-
quently loaded at rates of 1 to 5 lb/in2. This is
tion isolation treatments must be the very best
such a low loading rate that the cork appears stiff
when a high-quality occupied area is located im-
and does not provide the desired resilience. If cork
mediately under the MER, as compared with the
case where a "buffer zone" or noncritical area is
is to be used for vibration isolation, a load deflec-
located between the MER and the critical area.
tion curve should be obtained from the supplier,
(2) Structural ties, rigid connections. Each
and the cork should be used in the central linear
piece of isolated equipment must be free of any
region of the curve (possibly loaded at about 10 to
structural ties or rigid connections that can short-
20 lb/in2). With this loading, the compressed mate-
rial will have an initial deflection of about 5% and
circuit the isolation joint.
(a) Electrical conduit should be long and
will continue to compress gradually with age.
"floppy" so that it does not offer any resistance or
g. Air springs. Air springs are the only practical
constraint to the free movement of the equipment.
vibration isolators for very low frequencies, down
Piping should be resiliently supported. Limit stops,
to about 1 Hz or even lower for special problems.
shipping bolts, and leveling bolts on spring isola-
An air mount consists of pressurized air enclosed
tors should be set and inspected to ensure that
in a resilient reinforced neoprene chamber. The air
8-3
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