Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates to isolators used to
fasten two components of a structure together in
such a way as to reduce the transmission of vibra-
tion and noise between the two, and more partic-
ularly to such isolators which are in the form of
resilient elastomeric bushings or grommets intended
to come between one of the components and a astener
passing through it securing it to the other com-
ponent.
Vibration isolators in the form of resilient
bushings or grommets, fabricated of an elastomer
and designed to be inserted in an aperture in a
component and through which a fastener may be
passed for the purpose of securing the component to
another, are well known. Generally, such isolators
take the form of a pair of thick-walled hollow
cylindrical tubes, each having a radially-disposed
flange at one end, or are a combination of such a
flanged tube and a resilient washer. The tubes are
dimensioned to tightly fit into an aperture passing
through the component to be secured, and, in turn,
to be tightly fit by a fastener, such as a bolt,
passing through the tubes' central bores. The
flanges (or the flange and the washer) are designed
to engage the surfaces of the componen-t adjacent
and surrounding the aperture, and to be held
captive to the component by the fastener when the
fastener is secured to the other component.
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~ 5 iS well known, such isolators function by
elastically joining the two components together to
form a mechanical oscillator whose natural frequency,
by design, is less than any vibrational frequency
to be attenuated by at least some 29%, the amount
of attenuation depending on the ratio of the two
frequencies and becoming greater the greater the
- ratio of the frequency to be attenuated to the
natural frequency becomes. The natural frequency
of translational oscillation of a component of a
given mass supported through a resilient isolator
is directly proportional to the square root of the
spring stiffness of the isolator, and this in turn
depends on the type of stress the isolator is
subjected to, the isolator's composition, and its
geometry and dimensions.
For grommet-type isolators, loads are seen by
the grommet as compressive stresses, and the spring
stiffness of such an isolator is the product of the
compression modulus of the material and the cross-
sectional area of the isolator normal to and
bearing the load divided by the thickness of the
isolator in the direction of the applied force.
For resilient elastomers, the modulus itself is
dependent upon shape, as such materials are highly
incompressible and achieve their resiliency through
elastic flow, the free surfaces of the elastomer
bulging outward as the load is applied. For a
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given loading, the modulus increases as the cross-
sectional area bearing the load increases and as
the free surface area (bulge area) decreases. The
dependence of modulus on these two areas is usually
expressed as a dependence on a single parameter,
the shape factor, which is defined as the ratio of
the load bearing area to the bulge area. The
larger the shape factor, the larger the modulus.
It will be appreciated that the stiffness of,
and therefore the attenuation afforded by, an
isolator may be different in different directions.
A fundamental problem encountered in designing an
isolator is in providing the requisite stiffness in
different directions while keeping the isolator
within an allowable space envelope. In the case of
a grommet-type isolator, the isolator is generally
symmetric about an axis, and only axial and rac1ial
stiffnesses need be considered. Axial loads are
taken in compression by the flanged head portions
of the isolator, while radial loads are seen by the
smaller diameter middle portion.
Obtalning low frequency attenuation is difficult,
particularly with regard to axial loads. As has
been mentioned, the na-tural frequency varies as the
square root of the spring stiffness; thus, a ten-
fold decrease in sprin~ stiffness results in only a
three-fold decrease in spring stiffness results in
only a three-fold decrease in natural frequency.
In prior art isolators, the spring stiffness in the
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axial direction typically was decreased through
decreasing the load bearing area by decreasing the
radius of the flanged head, or through increasing
the thickness of the flanged head, or both. It
should be noted that in varying the spring stiff-
ness of the isolator bv varying the radius of the
flange while maintaining a constant flange thick-
ness, while the load bearing area varies as the
square of the radius, the shape factor only vaxies
as the radius, inasmuch as the bulge area varies
directly as the radius. To achieve a higher-
powered dependence of shape factor on radius, in
order to more strongly al.ter the spring stiffness,
one must simultaneously increase the thickness of
the flange as its radius is decreased. To obtain a
desired low axial spring stiffness b~ such des.ign
approaches often results in impractical designs, in
that the required flange diameter becomes too small
to properly secure the isolator to the component or
the flange becomes too thick, thereby exceeding the
allowable space envelope as well as providing a
less stable configuration.
An alternative design approach has been to
incorporate radial grooves in the flange, thereby
altering the load bearing area and the shape factor
of a flange filling a given space envelope. It
should be noted that, for a gi~en number of groo~es
of a given depth, the shape factor again does not
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vary as rapidly as the load bearing area, ~ince any
change in the load bearing area results in a change
in the same sense in the bulge area. It will be
appreciated that the design analysis of such a
flange is more laborious than that for a simple
flange, and further, that the tooling necessary to
produce such a part is more complex and expensive.
Accordingly, an object of the present inven-
tion is to provide a grommet-type isolator which
can be configured to have a low axial spring
stiffness without either the flange diameter
becoming too small to properly secure the isolator
to the component or the flange becoming too thick,
thereby exceeding the allowable space envelope.
~nother object of the present invention is to
provide a family of such isolators wherein the
flanged heads o the isolators all occupy a similar
spatial envelope while providing differing axial
spring stiffnesses.
~ further object is to provide a design for
such isolators which allows a simple design analysis
and tooling.
Yet another object of the present invention is
to provide an isolator which has maximum stability
for a given load bearing area.
These and other objects are met in the two-
piece isolator of the present invention in which
each piece is a resilient disk provided with a
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concentric internal circular depression having a
centered circular aperture, at least one of the
disks being affixed flange-like externally to one
end of a resilient cylindrical tube. The axial
spring stiffness of the isolator may be varied by
keeping the overall dimensions of the resilient
disk constant and varying the diameter of the
depression. By varying the spring stiffness in
this manner, not only may the natural frequency of
an isolator be reduced while maintaining the
outside diameter of the flange, but also a minimal
shape factor may be obtained for a given load
bearing area and flange thickness, as the bulge
area actually increases for a decrease in load
bearing area.
Bushings made in accordance with this inven-
tion may therefore be made to have various axial
spring stiffnesses while nevertheless occupying
similar spatial envelopes. In particular, this
type of bushing may be made to have a minimal axial
spring stiffness for a given flange outside diameter
and axial thic~ness.
Inasmuch as the bushing has the form of a
series of concentric surfaces of re~olution cut by
their normal planes, the tooling necessary to cast
the elastomer is relatively simple, easily fabricated,
and low in cost, particularly so in comparison to
radially-grooved bushings~ This shape also lends
itself to simpler mathematical analysis, for design
purposes, than does that of the radially-grooved
bushing.
Further, since axial loads are supported
peripherally bv a flange of maximum allowable
diameter and minimal thickness, rather than being
distributed over a number of radially directed pads
or over a smaller diameter or axially thicker
flange, the bushing of the present invention is
relatively more stable than bushings alread~ known
or obvious to persons skilled in the art.
The invention is illustrated by way of example
in the accompanying drawings wherein:
Fig. 1 is an isometric view looking toward one
end of a bushing made in accordance with the
principles oE the present invention;
Fig. 2 is an isometric view looking toward the
opposite end of the bushing shown in Fig. l;
Fig. 3 is a sectional view, taken along the
axis, of the bushing of Fig. l;
Eig. 4 is a sectional view, taken along the
axis of a pair of bushings of the type shown in
Fig. 1 assembled to form an isolator between a pair
of structural members; and
Fig. 5 is a sectional view, taken along the
axis, of an alternative embodiment of the present
invention.
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Referring to Figs. 1 through 3, there may be
seen bushing 20 made in accordance with the prin-
ciples of the present invention. ~ushing 20 is
comprised of sleeve 22 having an external flange 24
disposed about one end. Sleeve 22 and flange 24
are preferably formed as a unitary element, and can
be made out of any of a wide variety of synthetic
or natural elastomers, such as neoprene or poly-
urethane rubber. A preferred material is flouro-
silicone rubber.
Sleeve 22 is substantially in the form of athick-walled right circular cylindrical tube having
an outer surface 26 and concentric inner surface 28,
as may be seen by reference to Fig. 3. The end of
sleeve 22 distal from flange 24 is finished off in
a substantially flat end face 30. Secured in the
bore bounded by inner surface 28 is tube 32.
Tube 32 is in the form of a solid right circular
cylinder having an axial bore 34. The axial length
of tube 32 is chosen to be slightly greater than
that of sleeve 22 (the length of sleeve 22 is
considered to be the distance between its end
face 30 and the annular surface 40 of flange 24),
and tube 32 is so disposed in sleeve 22 so as to
protrude from the sleeve a sli~ht distance beyond
end face 30, as will be described in detail herein-
after. Tube 32 is provided with substantially
smooth planar end,surfaces 36 and 33, respectively
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adjacent to and remote from end face 30. Tube 32
is fabricated from a substantially rigid material,
and in a preferred embodiment is of aluminum,
although it will be understood that other materials
can be used provided they possess the requisite
rigidity. While in a preferred method of manu-
facture sleeve 22 is molded around tube 32 so as to
form an integral unit, it will be obvious to one
skilled in the art that the sleeve could be pre-
formed with a hollow bore defined by inner sur-
face 28 and tube 32 later inserted and secured by a
suitable cement.
Flange 24 extends outward from the end of
sleeve 22 remote from end face 30. The flange is
in the form of a disk-like radial extension to the
sleeve and is substantially concentric therewith.
Flange 24 is provided with a pair of substantially
smooth planar surfaces 40 and 42 respectively
adjacent to and remote from outer surface 26 of
sleeve 22. The flange is dimensioned so as to
situate surface 42 at the same distance, or further
from, the plane of end face 30, as is surface 38 of
tube 32. The flange has an outer cylindrical
surface 44. Surface 42, rather than being con-
tinuous radially across the extent of flange 24, is
in the form of an annular lip, being centrally
relieved by a depression 45 which is concentric
with outer cylindrical surface 44 and defined by
inner cylindrical surface 46 and bottom surface 48.
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The axial extent of inner cylindrical surface 46 is
chosen to be less than that of outer cylindrical
surface 44~ Furthermore in accordance with this
invention, the axial extent (length~ and the
diameter of inner cylindrical surface 46 are chosen
from considerations of the desired axial spring
stiffness, as will be described hereinafter. The
central region of depression ~5 is typically
occupied by tube 32. Fillet 49, not essential to
the present invention, may be formed between bottom
surface 48 and tube 32, if desired.
Pairs of bushings 20 fabricated in accordance
with the present invention are used to form an
isolator. Referring to Fig. 4, there may be seen,
in fragmentary sectional view, a pair of components 50
and 52 connected together through an isolator
formed by such a pair of bushings 20a and 20b used
in conjunction with washer 54 and bolt 56. In a
preferred embodiment, bushings 20a and 20b are
identical to one another in dimensions and com-
position, although it will be appreciated that in
principal they could differ from one another.
Thus, for instance, the sleeve 22 and tube 32 of
one of the bushings could be reduced in axial
length to the point where one of the bushings
forming the isolator is essentially nothing more
than a resilient washer haviny a rigid hollow core,
the sleeve portion of the isolator being provided
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entirely by the other bushing. Bolt 56, passing
through washer 54 and tubes 32 and threaded into a
tapped hole in component 52, holds the pair of
bushings 20a and 20b captively engaged in a hole,
sized to accept sleeves 22, in component 50.
Flange 24a of bushing 20a is held by bolt 56, in
compression between washer 5~ and component 50,
while flange 24b of bushing 20b is held similarly
in compression between components 50 and 52. The
amount of compxessive force which may be applied by
the bolt is limited, in a preferred embodiment, by
the axial lengths of tubes 32, which have been
dimensioned to have a combined length less than the
combined thickness of component 50 and unstressed
flanges 24a and 24b by the amount of the desired
compression. It will be appreciated that a similar
control of the amount of compression of the flanges
can be achieved through other means, as ~y the use
of a torque wrench. As an aid in assembly, washer 54
may be affixed, with epoxy or other adhesive, to
surface 42 of flange 24a. Otherwise, flange 24a is
similar to flange 24b, and the structure and
function of all like-numbered parts of bushings 20a
and 20b are the same as for bushing 20.
As indicated hereinbefore, an isolator may
also be formed by combining a single bushing 20
with a resilient washer. A resilient washer 20c
suitable for such use is shown in Fig. 5. As may
be seen, washer 20c comprises flange 24c and
tube 32c. ~ube 32c is dimensioned so as to be
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slightly shorter in axial extent than the axial
extent of flange 24c, and is provided with a planar
end surface 36c disposed to be coplanar with
annular surface 40c of the flange. Annular sur-
face 40c extends radially from tube 32c to outer
surface 44. In all other respects, flange 24c is
similar to flange 24 and tube 32c, to tube 32.
In use, resilient washer 20c may be combined
with a bushing 20 to form an isolator (not illus-
trated) in essentially the same manner as two
bushings 20 may be combined. For example, re
silient washer 20c may be used in place of one of
the bushings 20 in the installation shown in
Fig. 4, the washer being placed with surface 40c
contacting component 50 and with tube 32c con-
centric with the aperture through the component.
The assembly is then secured in the same way as are
bushings 20a and 20b. It will be recognized that,
since resilient washer 20c is not provided with a
sleeve, any radial loads imposed on such an isolator
will be carried primarily by the sleeve 22 of the
bushing 20 used in conjunction with the resilient
washer. In all other respects, the performance of
the isolato~ formed by combining a bushing 20 and a
resilient washer 20c is similar to that of an
isolator formed from two bushings, like parts of
re~ilient washer and bushing performing like
functions. Thus, axial loads would be carried by
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both flange 24 of the bushing and flange 24c of the
resilient washer.
Inasmuch as an isolator formed from a palr of
bushings 20 or a bushing 20 and a washer 20c do not
differ significantly from one another in the
details of construction, assembly, and operation
not hereinbefore described, the remaining detailed
description will be of an isolator formed from a
pair of bushings.
With reference to Fig. 4, motions of com-
ponents 50 and 52 relative to each other are seen
by bushings 20a and 20b as dynamic compressive
loads. Motions of the components toward or away
from each other are seen as axial compressive
forces by flange 24b and flange 24a respectively,
while motions in the directions normal to this are
seen as radial compressive forces by sleeves 22.
For a given spatial envelope (i.e., for given
outside diameters of flanges 24 and 24b and sleeves 22
and for a given combined axial length of tubes 32
and thickness of component 50) the spring stiffness
of the isolator depends, as will be understood by
those skilled in the art, on the material of con-
struction and the remaining dimensional parameters.
Thus, the radial spring stiffness depends, among
other thingsl on the axial extent of sleeves 22
~which, to~ether with the already established
outside diameter of the sleeves, determines the
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load bearing area~ and the inside diameter of the
sleeves - i.e., the outside diameter of tubes 32 -
(which, with the sleeves' outside diameter, estab-
lishes the radial thickness of the sleeves and
their bulge areas). In the case of flanges 24a
and 24b, the axial and radial dimensions of outer
cylindrical surfaces 44 are predetermined by the
desired spatial envelope, and the axial and radial
dimensions of inner cylindrical surface 46 may be
changed to vary the area of surface 42 (the load
bearing area for axial loads) and inner cylindrical
surface 46 (the bulge area in part~. Bushings made
in accordance with this invention therefore may be
made to have various axial and radial spring
stiffnesses while nevertheless occupying similar
spatial envelopes. In particular, this type of
bushing may be made to have a minirnal axial spring
stiffness for a givan flange outside diameter and
axial thickness.
Inasmuch as the shape of the bushing is a
series of concentric surfaces of revolution cut by
their normal planes, the tooling necessary to cast
the elastomer is relatively ~imple, and may be
easily fabricated at relatively low cost, partic-
ularly so in comparison to a radially-grooved
bushing. In this connection it is important to
appreciate that substantially the same mold can be
used to make bushings ~ith identical outside
dimensions but different spring stiffnesses, since
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the dimensions of the cavity 45 can be varied by
employing different core pieces in the mold. As a
consequence a variety of isolators differing in
spring stiffnesses can be produced at only a
nominally greater equipment cost than is required
to make isolators of identical spring stiffnesses.
The shape of the bushin~ of the present
invention also lends itself to simpler mathematical
analysis for design purposes than does that of the
radially-grooved bushing. Further, since axial
loads are supported peripherally by a flange of
maximum allowable diameter, and minimal axial
thickness, rather than distributed over a n~lmber of
radially directed pads, or over a smaller diameter
or axially thicker flange, the bushing is relatively
more sta~le than such bushings.
Since changes may be made in the illustrated
embodiment without departing from the scope of the
invention, this description of the preferred
embodiment is for purposes of illustration only,
and should not be interpreted in a limiting sense.
