CA1088963A - Vibration damped structures and objects - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A vibration damped structure in which vibration energy is absorbed in a viscoelastic material is characterized by the viscoelestic material being formed as bores in a rigid part that is decoupled from at least a part of the structure.
The rigid part bears on the structure in such a manner that it participates in vibrational oscillations of the structure.
The viscoelastic material adheres to at least the limiting surfaces of the bores.

Description

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Reduc Acoustics AB
Sundbyber~
Sweden , Vibration damped structures ~

The present invention relates to vibration damped structures and objects, which normally emit substantial sound when set in vibra-tion.
Structures, as related to here, could be structures made of com-mon structural materials such as steel or other metals, concrete, in-cluding light concrete, plaster, wood, including structural parts manu-factured o~ wood fibres or wood chips, as well as synthetic materials.
Structures herein referred to can be buildings and parts thereof, such as frameworks, beams, slabs, walls and ~loors; constructional works, such as bridges; machines and engines.
Objects, as related to in this specification, could be e.g. con-tinuously or intermittently rotating, oscillating or translatory move-ing objects, which due to their movement, or through strokes, blows or vibrations transmitted to same, are set in sympathetic vibrations and thereby emit sound. Circular saw blades, ~or instance, are an example of such rotating objects, which, as is known, by sawing emit substantial sound. Intermittentl~ movable objects which due to blows emit sound, can be exemplified by rock drills for rock drilling ma-chines.
Other objects, herein referred to, could be such, which due to turbulence or cavitation, possibly in combination with some other cause of vibration~ when operating in fluids start oscillating, e.g.
turbine blades and pump rotor blades.
Further objects related to hexein, could be such which are im-mobile per se, but due to contact wlth a source of ~ibration can be . . . _ . . .
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- 2 ~ 9~3 ~~ set in sympathetic vibrations, e.g. engine foundations.
It is previously known -to increase -the inner damping in plates by applying a thin layer of viscoelastic material between two plates. Such a unit is usually called a sandwich plate. As a sandwich plate starts vibrating the two plates move relative each other, whereby shearing takes place in the viscoelastic layer adhering to the plates, which layer thereby absorbs a substantial part of the vibrating energy.
V.S. Patent Specification 3,828,504 disclosures corres-pondingly damped structuresof concrete and light concrete.
In the damped sandwich plates as well as the structuresaccording to the cited Patent Specification the plates and the structure parts respectively are totally separated by the viscoelastic layer.
The Swedish Patent Specification 7407174-7 discloses structures, wherein partial damping layers are applied. Also in certain embodiments of the invention described therein the structure parts are separated from each other by the viscoelastic damping layer in such a manner that in each section through the structure at least two structure parts and possibly a viscoelastic damping layer can be distinyuished.
For the damping of many structures and objects an application of the prior art as in practice unsuitable. Even if it is possible to achieve a satisfactory damping by employ-ing the prior art, several applications imply that an activepart of an object not without important disadvantages in its full extent can be divided into two or more parts with intermediate viscoelastic layers.
It is also known -to damp structures and objects by applying, on a surface thereof, one or more damping fields in the form of a metal sheet or foil for damping structure borne noise provided with a viscoelastic damping layer comprising a self-adhesive glue.
The object of the present invention is partly to remove certain inconveniences with the prior art and partly to offer a substi-tute for the prior art in certain applications, which results in an equally good damping performance. According to the present invention this has been achieved in that the - invention has been given the characterizing features set forth in the appended claims.

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- 2a -Thus the invention consists of a structure or object, wherein vibration energy is absorbed in a viscoelastic material, characterized in tha-t the viscoelastic material is applied in first bores in a rigid part which is decoupled from at least one part of the structure or object and may be a detail of the structure or object, said part bearing on the structure or object in such a manner that it participates in vibrational oscillations of -the structure or object, said viscoelastic material adhering to at least the limiting sur-faces of the bores.
The invention is described more in detail below, reference being made to the accompanying drawings, wherein fig. la-f and fig. 2a-f show sections through different fundamental embodiments of the inven-~ ,-~.., , .. . , ~ , . ~ . . ~ , . ..
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3 1~ 3 tion regarding the location of the viscoelastic dampin~ material in a damped structure or object, fig. 3 shows a section through an examp-le according to the invention o~ a damped structure of concrete or light concrete, fig. 4 shows a section through a damping field accor-ding to fig. 2b applied outside a structure or object, fig. 5 shows similarly a damping field according to fig. lb set in a structure or object, fig. 6 shows a damping field arranged according to fig. la, which is let into a structure or object and, furthermore, is provided with a covering layer, figs 7-11 show examples of different damped objects in accordance with the invention, where figs 7 and 8 show a part of and a section through a part of a saw blade, respectively, figs 9 and 10 show a rock drill and on an enlar~ed scale a section through same, respectively, fig. 11 sho~s a section through a cylin-drical shaft, and figs 12 and 13 show, respectively, a section throug~
two additional fundamental embodiments of the invention.
In figs la-f and 2a-f the reference numeral 1 refers to a part of a structure or object, 2 refers to a rigid structure part or a se-parate stiffening part arranged against part 1, and 3 refers to a vis-coelastic damping material.
In fig. la part 2 is provided with bores 4. The bores are filled with the viscoelastic material 3 adhering to the walls of the bores and to part 1. According to fig. lb the viscoelastic material 3, or another viscoelastic material, isapplied not only in the bores 4 but also as a conventional damping layer 5 between the parts 1 and 2.
In ~ig. lc part 1 has been provided with dead-end bores 6 cor-responding to the bores 4 in part 2 and the viscoelastic material 3 continuously fills up the respective bores 4 and 6 facing each other and adheres to the walls of the bores. According to fig. ld the visco-elastic material or another viscoelastic material, urthermore, has been arranged between the parts 1 and 2.
In fig. le the same device as in fig. la is shown, with the ex-ception that a layer 7 of a non-viscoelastic material is placed be-tween the parts 1 and 2. This layer 7 can, e.g. in certain applica-tions, be electrically isolating, but, in any case, for the coopera-tion of parts 1 and 2, it has to be connected to part 1, e.q. through glueing, and the viscoelastic material 3 has to adhere to the layer 7. Fig. lf shows the same arrangement of the viscoelastic material as fig. lc except in that a layer 7, corresponding to the layer of ~ig.
le, has been placed between the parts 1 and 2. In this latter case, .: ~

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_ ~Oi~9~i3 the layer 7 doesnot need to and, in certain cases, shouldnot adhere to any of the parts 1 and 2, but these are united by the viscoelastic material in the bores 4 and 6.
Fig. 2a~f correspond to a respective one o~ fig. la-f except for the bores 4' being dead-end bores, extending from the surface o~
part 2 facing part 1.
The embodiments according to fiy. la-f have the advantage that the bores may be made after the parts 1 and 2 have been applied against each other; this is especially favourable in the embodiments according to figs lc, d and f, where the purpose is to achieve co-axial bores

4 and 6. Further advantages with the embodiments according to fig. la-1~ are, ~irstly, that the viscoelastic material 3 can be filled into the bores, especially in the embodiments according to figs la, c, e and f, a~ter the parts have been placed against each other and, second-ly, that if the viscoelastic material first is applied to part 1, as is suitably done in the embodiments according to figs lb and d, and thereafter part 2, provided with bores, is placed onto part 1, the bores 4 will contribute to avoid air entrapments between the parts 1 and 2 and thereby toachieve a better adhesion between the parts.
Furthermore, gases, possibly formed when the viscoelastic material har-dens, can escape through the bores 4, thus preventing such gases to cause overpressure and poorer adhesion between the parts.
Surprisingly enough the invention has proven to result in an - equaIly good damping - and in many cases a considerably better dam-ping - than prior art. The prior art with viscoelastic damping is based on the fact that, in consequence of vibration, shearing takes place between two spaced structure parts. If there is a viscoelastic layer between these parts, which layer adheres to the parts, the shea-ring movement is transmitted to this layer s border surfaces against the parts, whereby the shearing work in the viscoelastic material re-sults in losses in the form of heat and the vibrations are damped. The losses as a rule become greater the bigger the shearing angle, i.e.
the thinner - upto certain limits - the viscoelastic layer is. The - reasons for achieving a surprisingly good damping even if the visco-elastic material,according to the invention,is placed in bores is not yet quite clear. One reason could be, especielly in the embodiments according to fig. la-f with bores through part 2, that the bores them-selves are deformed at the bending of the part in consequence of vibra-tion, whereby also the viscoelastic material in the bores is deformed and, thus, absorbes energy. In order to obtain total damping, the distance between adjacent bores may not be greater than the bending : ~, ' , ' ' ' ' . , ' "

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wave length ~or the ~re~uency to be damped. The boxe~ should, pre~e-rablyl extend from the surface between the paxts and at least to half the thlckness of the respective part. For damping of metal plates a degree of perforation less than 30~ is preferred, while a degree of perforation of 1-2~ has proven sufficient when damping light concrete.
Generally speaking the damping methods according to figs la, b and e as well as figs 2a, b and e are best suited for damping of me-tals, while the methods according to figs lc and f in particular and also figs 2c and f are best suited for damping of such materials as concrete, light concrete and plaster.
As examples of experimental results the following can be men-tioned:
I. Two concrete slabs with a length of 2,4 m, a width of 0,6 m and a thickness of about 7 cm were placed against each other (according to fig. lc) and ten evenly distributed bores with a diameter of about 15 mm were drilled through the upper slab and down to quite half the thickness of the lower slab. The bores were fllled with a viscoelas-tic damping compound, which was allowed to harden during a certain pe-riod of time. After the hardening the slabs were excited and the dam-ping was measured within the frequency-range of 100-1000 ~z. The result showed a total loss-factor of 0,17 at the resonance frequency (125 Hz) of the slab, which is to be compared to a loss-factor of 0,035 achieved by prior art.
II. Two plaster-boards with a langth of 0,5 m, a width of 0,1 m and a thickness of 13 mm were placed against each other according to fig. lf, an intermediate layer 7 comprising felt was arranged between the boards and eight e~enly distributed bores were drilled through the up-per board and down to quite half the thickness of the lower board. The bores were filled with a viscoelastic dampincJ compound, ~hic~ was allowed to harden, whereafter the boards were excited and the damping was measured at the resonance frequency oE the board of about 500 Hz.
The result showed a total loss-factor of 0,25, which is to be compared to a loss-factor of 0,10 when damping according to prior art.
Conventional damping only results in damping of vibration oscil-lations perpendicularly to a sur~ace of a board. The above mentioned experiments, however, have surprisingly proven that damping also takes place at excltation and oscillation in directions parallel to the plane of the slabs or boards.

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, ' ' , 91~3 _.~ b One example of application of -the inven-tion is indicated in fig. 3, which showsa section throu~h two superinposed slabs 1 and 2 of concrete or light concrete. The damping material is applied accor-ding to fig. lc, i.e. in bores 4, which extend through the entire slab 2 and continues in bores 6, which bores reach down to quite a good half of the thickness of the slab 1. In the right hand portion of fig. 3 is indicated that the damping material not necessarily has to fill more than quite half of the height of the bores in slab 2. The remainder of the bores can, thus, be filled with concrete or cement mortar.
The invention could be used for damping by limited damping fields, which are applied to or in a structure or an object. Examples of damping by limited damping fields are shown in figs 4-6 (the same reference numerals as in figs 1-2 being used), fig. 4 showing a dam-ping field arranged according to fig. 2b and applied outside a struc~
ture or an object, fig. 5 showing a damping field arranged according to fig. lb and let in a structure or an object in such a manner, that the surface of the part 2 is in alignment with the upper surface of the structure or object 1, and fig. 6 showing a dampin~ field arran~ed according to fig. la and let to such a depth in the surface of a structure or an object, that a covering layer 8, e.g. a thin plate, applied on top of the damping field, is in alignment with the upper surface of the structure or object 1. The application of such a covering layer has shown to ~urther improve the damping. An explana-tlon hereof could be, tha~ the covering layer prevents the bulge of the damping material in the bores, which is the consequence of the bending of the structure when vlbrating, and, thus, that the dampin~
material undergoes a certain loss-brlnging compression instead of being allowed to expand freely and convexly out of the bores.
- Some examples of damping objects of metal will now be described.
Figs 7 and 8 show a saw blade 11, which can be a conventional one - e.g. for rock sawing - with teeth 12and a shaft bore 13. Preferably in both the plane blade surfaces 14 and 15, parallel to each other, are evenly distributed milled recesses 16, the bottom surfaces of which are parallel to the surfaces 14 and 15 of the blade. In each recess is applied a layer o~ an adhesive viscoelastic material 17, preferably a hardening one of a two component type. Outside the viscoelastic layer 17 a perforated plate 18 is applied in each recess 16, the .:

' ': .', ,'' ' ' ' ' ;''' 7 10~963 shapes of the pla-tes corresponding or being similar to the respective recess 16. Further, the riyidity of the plates isgreater than that of the layer 17. The combined thickness of the viscoelastic layers 17 and the plates 18 suitably should be such, that the plates 18 are aligned with or somewhat below the respective surface of the blade 11.
The damping fields shown in fig. 7 have more or less the shape of a longitudinal section of a pear and are distributed in four damping fields on each side of the blade, the damping fields on the two sides of the blade being displaced angu]arly relative each other. This is shown by the damping fields, indicated by dotted lines in fig. 7, on the backside of the blade.
Fig. 8 shows on an enlarged scale a section along llne VIII-VIII
in fig. 8, wherefrom appears that the viscoelastic material 17 - except for being present between the blade 11 and the plate - during the manu-facture has been pressed into the bores 19 to totally fill same as the plate was applied to the damping material not yet hardened.
Figs 9 and 10 show how an extended object can be damped. The object in this case is a drill 20 of a rock drilling machine. The cross section of the drill, as is shown in ~ig. 10, is a regular hexa-- gonal and in all of the six longitudinal side-surfaces of the drill damping fields 21 are let in according to fig. 5. Depending on the length of the object, the frequency or frequencies to be damped and -the location of antinode loops, several damping fields can be applied one after the other along the object.
Fi~. 11 shows a cross section through an extended cylindrical shaft 22/ in the periphery of which are shown six damping ~ields 23, similarly let in and applied evenly distributed. These are, as in the preceding case, extended in the longitudinal direction of the shaft.
In practical tests with a saw blade for rock sawing, damped according to the invention, a reduction of sound emitted from the b~ade by up to 18 dB(A) was aahieved, which means a decrease to 1/8 of the level of acoustic pressure in comparison with an equal, not damped blade operated under equal conditions. The sound disturbance is thus perceived by the ear as about 1/4 of the disturbance from a non-damped blade.
Figs 12 and 13 show sections through damped structures or objects embodying the invention, the viscoelastic material 24 being adhered to the outside o~ solid, rigid bodies 25, e.g. of steel, and to the ..

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i3 outside of tubes 26 respectively. The boAies 25 and the tubes 26 are applied in bores in the structures or objects 2, 1 to enable the - viscoelastic material 24 to adhere to the walls of the bores. These embodiments need less quantities of the viscoelastic material, than when said material totally fills the cross sections o~ the bores, as in the embodiments earlier described.
Normally, the thicknesses of the viscoelastic damping layers 5 are about 0,1 - 1 mm, while the cross sectional areas of the bores can vary depending on which material is to be damped, its rigidity etc.
The least complicated method is to make the bores circularly c~lindrical, e.g. throuyh drilling or punching, but a further improved effect could be achieved by giving the bores another geometrical form.
In spite of the fact that mainly symmetrical objects have been described here, the invention is equally applicable on non-symmetrical objects, as e.g. engine foundations. Likewise, objects with double bent surfaces can be damped in accordance with the invention.

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Claims (19)

Claims:

1. A structure or object, wherein vibration energy is absorbed in a viscoelastic material, characterized in that the viscoelastic material is applied in firstborns in a rigid part which is decoupled from at least one part of the struc-ture or object and may be a detail of the structure or object said part bearing on the structure or object in such a manner that it participates in vibrational oscillations of the structure or object, said viscoelastic material adhering to at least the limiting surfaces of the bores.

2. A structure or object according to claim 1, characterized in that the bores extend from the border surface between the part and the structure or object to at least half the thickness of the part.

3. A structure or object according to claim 1, characterized in that the bores extend through the entire rigid part.

4. A structure or object according to claim 1, characterized by second bores in the structure or object said second bores being co-axial to the bores in the rigid part, the viscoelastic material filling up also the second bores and adhering to the limiting surfaces thereof.

5. A structure or object according to claim 4, characterized in that said second bores extend down to a depth of at least half the thickness of the structure or object.

6. A structure or object according to any one of claims 1-3, characterized in that the viscoelastic material is applied adhesively to the outside of rigid elements introduced into the bores.

7. A structure or object according to any one of claims 1-3, characterized in that the same or another viscoelastic material is applied as a layer between the rigid part and the structure or object.

8. A structure or object according to any one of claims 1-3, characterized in that a non-viscoelastic material is placed between the rigid part and the structure or object.

9. A structure or object according to any one of claims 1-3, characterized in that the rigid part is applied outside the structure or object.

10. A structure or object according to any one of claims 1-3, characterized in that the rigid part is let into the structure or object.

11. Vibration damping apparatus comprising a structure subject to vibrations, a rigid part having plural bores, said rigid part being mechanically decoupled from the struc-ture but bearing thereon such that it participates in vibrational oscillations thereof, said first bores extending from a border surface between said rigid part and the struc-ture to at least half the thickness of said rigid part, viscoelastic damping material applied in said bores in said rigid part and extending therefrom at least to the border surface, and said viscoelastic material adhering to surfaces of said bores.

12. Vibration damping apparatus according to claim 11, wherein said bores extend entirely through said rigid part.

13. Vibration damping apparatus according to claim 11, comprising plural second bores in the structure, said second bores being coaxial to said plural bores in said rigid part, said viscoelastic material also filling up said second bores and adhering to surfaces thereof.

14. Vibration damping apparatus according to claim 13, wherein said second bores extend to a depth of at least half the thickness of the structure.

15. Vibration damping apparatus according to claim 14, further comprising plural rigid elements in the bores and wherein said viscoelastic material adhesively is applied to outsides of said rigid elements between the elements and surfaces of said bores.

16. Vibration damping apparatus according to claim 12, wherein a viscoelastic material is applied as a layer between the rigid part and the structure.

17. Vibration damping apparatus according to claim 12, further comprising a non-viscoelastic material placed between said rigid part and the structure.

18. Vibration damping apparatus according to claim 12, wherein said rigid part is outside the structure.

19. Vibration damping apparatus according to claim 12, wherein said rigid part extends into the structure.