CA1138904A - Vibration isolator - Google Patents

Abstract

ABSTRACT

A two-piece grommet-type vibration isolator comprised of a pair of resilient disks provided with concentric internal circular depressions and centered circular apertures, 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 disks constant and varying the diameter of the depression.

Description

~ 3~

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.

~13~04 ~ 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 ~*~ 4 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 3~3~

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 113~

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 1~3~

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.

1~3~

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 ~38~

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.

_ g _ 1gl ;18~09L

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 ~31~

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 ~3~

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 ~3~t~3(:~

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 ~L3~

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 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.

Claims (13)

The embodiments of the invention to which an exclusive property or privilege is claimed are defined as follows:

1. A bushing for use in fastening two components together so as to reduce the trans-mission of mechanical vibration and noise between said components, said bushing comprising in com-bination:
a sleeve of a resilient material substantially formed into a thick-walled right circular cylin-drical tube terminating in a first end and a second end and having an interior axial bore;
a flange of a resilient material substantially in the form of a disk affixed externally to and substantially concentric with said sleeve at said first end thereof; and a lip of a resilient material substantially in the form of an annulus affixed peripherally to said flange and directed away from said second end.

2. A bushing according to claim 1 further comprising a rigid cylindrical tube tightly fitted in said interior axial bore.

3. A bushing according to claim 2 wherein said rigid cylindrical tube is so dimensioned and disposed as to extend from said bore at at least one of said first and second ends and not by a distance greater than the extent of said lip beyond said flange at said first end.

4. A bushing according to claim 1 and further comprising a rigid circular washer concen-tric with said sleeve affixed to said lip.

5. A bushing according to claim 1 or 2 wherein said sleeve and flange are integral with one another and are made of the same material.

6. A washer for use in fastening two com-ponents together so as to reduce the transmission of mechanical vibration and noise between said components, said washer comprising in combination a circular disk of a resilient material having a central circular aperture penetrating there-through from a first planar side to a second planar side of said disk and a lip of a resilient material substantially in the form of an annulus affixed peripherally to said first planar side of said disk.

7. A washer according to claim 6 further comprising a rigid cylindrical tube tightly fitted into said central circular aperture.

8. A washer according to claim 7 wherein said rigid cylindrical tube is so dimensioned and disposed as to extend from said circular aperture at said first planar side of said disk by a dis-tance not greater than the extent of said lip beyond said disk.

9. A washer according to claim 6 and further comprising a rigid circular washer concentric with said disk affixed to said lip.

10. A washer according to claim 6 wherein said disk and said lip are integral with one another and are made of the same material.

11. A vibration isolator for fastening two components together, said isolator comprising two parts each including in combination:
a disk of a resilient material having a central circular aperture penetrating therethrough from a first planar side to a second planar side of said disk;
a lip of a resilient material substantially in the form of an annulus affixed peripherally to said first planar side of said disk; and wherein further at least one of said parts includes a sleeve of a resilient material sub-stantially in the form of a thick-walled right circular cylindrical tube having an interior axial bore concentric with said disk, the said disk of said one part being affixed to one end of said sleeve so as to form an external flange.

12. A vibration isolator according to claim 11 wherein said two parts are disposed so that the said second planar sides of said disks are closer to one another than the said first planar sides of said disks.

13. A vibration isolator according to claim 12 wherein said disks and sleeves are made of the same material.