MIL-STD-1227B
by the contracting officer. The mounting brackets shall be furnished by the
end item manufacturer and shall be installed in such a manner as to permit
assembly without deflection or deformation of brackets or engine mounting
pads.
5.4.1 Vibration isolators. Suitable vibration isolators which stress an
elastomer in shear or compression or both shear and compression shall be
used on all installations when shock mounting is specified in the end-item
specification. The method of calculation for determining the maximum allow-
able spring force of four vibration isolators, equally spaced about the
center of gravity of the end item for beam mounting (see figure 5) shall be
as specified in 5.4.2.1. When vibration isolators are not equally spaced
about the center of gravity, a complete analysis shall be conducted to
determine the maximum permissible spring force for each isolator.
5.4.2 Design requirements for direct mounting of end item. The maximum
resultant moment that the equipment mounting pads can absorb without defor-
mation is 650 pound-foot. This limiting moment shall not be exceeded.
Cantilever mounting of end items to the engine is prohibited. Cantilever
mounting of the engine to the end item is permissible provided the appli-
cable end-item specification does not require a free fall test.
5.4.2.1 Method for calculating maximum allowable spring force. The
final spring force shall not apply to the weight of the end item a G-
magnification factor that will cause a moment exceeding 650 pound-foot, when
calculated as shown in figures 6 through 9.
5.4.2.2 Procedure. The following procedure shall be followed in select-
ing. four vibration mounts to withstand the 18-inch, free-fall, shock load
-
requirements. The following example applies only when isolators are equally
spaced about the center of gravity of the entire end item.
(a)
Determine weight of equipment;
W eq = 80 pounds
Select a vibration isolator, e.g., k = 3,000 pounds/inch (obtain
(b)
spring rate from manufacturer).
(c )
Calculate the static deflection by the following equation:
= static deflection in inches
Where,
= spring constant in pound/inch
5
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