GB2322684A

GB2322684A - Method and arrangement for damping vibration

Info

Publication number: GB2322684A
Application number: GB8925231A
Authority: GB
Inventors: David John Small
Original assignee: Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1989-11-08
Filing date: 1989-11-08
Publication date: 1998-09-02
Anticipated expiration: 2009-11-08
Also published as: GB2322684B; GB8925231D0

Abstract

In a method of damping vibration in a mechanical structure, such as blades or fins in a propulsion system, a slug (23) of steel or heavier metal is mounted in a cavity (13) formed in the structure. The slug is suspended in the cavity by means of two or more viscoelastic O-rings (27,29) which encircle the slug and are compressed between the slug and the cavity wall. The O-rings are positioned asymetrically relative to the length of the slug.

Description

Method and Arrangement for Damping Vibration This invention relates to a method and arrangement for damping vibration, particularly, but not exclusively, applicable to the damping of vibration in the blades and fins of propulsion systems.

It is a modification of the invention disclosed in our British Application No: 8814454.8.

The requirements of noise control often demand that vibration in a solid structure be reduced by damping the structure. For instance it is sometimes necessary to control a form of vibration known as singing" on propeller blades and other fin-like structures on ships and other marine vehicles. The vibrations are set in motion by the flow of water over the propeller blade or fin.

It has been proposed to damp vibrations of solid structures by means of 'dynamic vibration absorbers', i.e. devices that control vibration at one specified frequency. Such a device may consist of a compact mass, m, usually small compared to that of the structure, attached to the structure by a spring of stiffness, k, such that the natural resonance frequency of the combined mass and spring, f, given by: f = 1 (k/m)+ 2lt is at or very close to the frequency of the vibration to be controlled.

When vibration control is required only at one frequency it is usual to make 'm' a very small fraction of the mass of the structure and to invest the absorber with only a low level of its own internal damping so that it responds to excitation at its resonance with vibrations of high amplitude.

Some vibration control problems however may call for a high level of damping over a wider frequency range. For example, in the case of the singing of ships propellers, individual blades on the same rotor may resonate at somewhat different frequencies because of manufacturing tolerances. Furthermore, singing may occur in two or more completely different modes, especially if the propeller is to operate at more than one speed.

An object of the present invention is to provide a method of damping vibration which is applicable to situations in which vibration control is required over a significant range of frequencies. A further object of at least certain embodiments of the invention is to provide a damping arrangement in which the outer surface of the structure is unchanged, i.e. left continuous in the case of a propeller blade or like fluid flow structure.

According to the invention, there is provided a method of damping vibration in a component of a mechanical structure, wherein a cavity is provided in the component and an inertial slug is mounted in the cavity, the slug being suspended within the cavity by means of two or more viscoelastic O-rings each encompassing the slug and being compressed between the slug and the cavity wall; and wherein the O-rings are positioned asymmetrically relative to the length of the slug.

It will be appreciated that the term "viscoelastic O-rings" inplies O-rings of a material having both stiffness and damping characterisitics. Moreover, while standard O-ring sections are circular, the term is hereby defined to include toroidal viscoelastic members of square or other cross-section.

The slug and the cavity may each be cylindrical and may be of circular cross-section.

A plurality of machine components structurally coupled to each other may each be damped by a method as aforesaid so tending to suppress the mutual coupling and propagation of vibration.

In a method of damping vibration of the trailing edges of the stator blades of a propulsion unit, where the stator comprises a multiplicity of blades extending radially between a hub and an annular cowl, radial cavities are drilled in the trailing edges of a number of the blades, the radial cavities being drilled through the annular cowl. Vibration of the drilled blades is damped by a method as aforesaid, so tending to suppress the mutual coupling and propagation of vibration. Where the cavity extends to a surface of the component the cavity may be plugged to provide surface continuity.

The cavity may be provided by a capsule within which the inertial slug is mounted, the capsule being moulded into the machine component, and the machine component consisting of a material which is mouldable at a temperature which the O-ring can withstand.

In cases in which the invention is concerned, where vibration control is required over a wider range of frequencies, it has been found necessary to invest the dynamic vibration absorber with a higher degree of damping, usually in association with its spring element, so that its resonant response as a function of frequency appears as a broad hump spread across the frequency spectrum rather than a narrow peak. Such a damper will in fact continue to be quite effective at frequencies well above resonance. The mass, m, is a somewhat larger fraction of the mass of the structure, suitably, say, at least 5; or 10% of the structure mass.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawing, of which: Figure 1 is a sectional view of an inertial slug fitted with O-rings in accordance with our British Patent Application NO: 8814454.8 in position in a cavity of a machine component; Figure 2 is a sectional view of a slug in accordance with the present invention in position in a cavity of a machine component, and Figure 3 is a part-sectional view of a damping slug in position in a cavity in a stator blade.

Referring to the drawings, Figure 1 shows a cylindrical slug 1 of metal. Steel may provide sufficient inertia but if space is limited some heavier metal may be used. The cylinder is provided with grooves 3 in which O-rings 5 are seated in known manner so as to protrude from the groove proud of the cylindrical surface.

A cavity 7 is drilled or otherwise formed in a machine component 9 the vibrations of which are to be damped. The cavity has a section conforming to that of the slug. Both sections may be circular or may both be elliptical or oval where space is limited in one direction. The slug is clear of the wall around the cavity by an amount sufficient to permit all likely vibration of the slug to take place without the slug coming into contact with the wall. On fitting the slug and its two O-rings into the cavity 7, the O-rings are compressed between the slug and the wall, so that the slug is held in the suspended condition shown. The cavity is then sealed off with a cap 11 which may be a force fit in the cavity. Alternatively the cap may be threaded and screwed into the cavity or fixed with adhesive. The outer surface of the machine part is thereby maintained continuous, which is clearly of importance in the case of a propeller or turbine blade or fin.

The diameter of the cavity is selected, along with the diameter of the grooved part of the slug, such that the desired percentage squeeze or compression is applied to the thickness of the O-ring. The percentage squeeze and the specific gravity of the material of the slug are selected such that the values of m and k in the formula above give a resonance at the desired frequency.

Whereas the equation above relates to a purely translational mode of vibration, it is also possible to make use of a torsional mode, whereby the cylindrical slug rotates about a transverse axis through its centre of gravity. To design the damper so that this mode resonates at the same frequency as the other, it can be shown that the O-rings should be located at a distance 2$ from the centre of the cylinder where L is the length of the cylinder. The O-rings are therefore spaced apart by a distance 23 . This confers increased damping at the design frequency, if the vibration of the structure is such as to excite both the translational and torsional modes.

The damper of the present invention is intended to resolve design conflicts entailed in fitting the slug of Figure 1 to hydrofoil blades in propulsion systems.

The conflicts arise in the course of attempting to maximise the amount of damping conferred on particular modes of vibration of a blade. To achieve this end it is necessary to apply the largest possible volume of high density metal to that part of the blade which vibrates at high amplitude in the mode needing to be damped. The O-rings in particular must be in contact with the metallic mass and also with the structure of the blade at points where the blade vibration amplitude is near its maximum level. This is illustrated in Figure 1, where the dotted line represents the variation of vibration amplitude along the span of the blade. The location of the two O-rings is seen to straddle the broad peak of the amplitude curve.

Several practical constraints limit the volume that can be allowed for accommodating the slug. The diameter of the cavity 7 is limited by the thickness of the blade and the need to retain no less than an acceptable minimum thickness of blade material between the cavity and the exterior surface. The depth of the cavity is also limited by drilling difficulties. In addition, the need to position the O-rings at points of high amplitude (as explained above), and also the symmetry of design, result in the mass being located well down into the depth of the cavity, leaving an unfilled space 13 above the slug, as can be seen in Figure 1.

If this space 13 is utilised, then the damper mass can be increased within the constraints imposed by drilling difficulties.

The design procedure is then to increase O-ring stiffness in the same proportions, so that the damper resonance, f, given by the above-mentioned formula f = 1 (k/m)1/2 27 remains the same. The damper mass can be made to extend into the space 13 if the requirement for symmetry can be relaxed. This requirement was observed in the devices disclosed in our British Patent Application No: 8814454.8 to ensure that a purely translational mode of damper vibration existed. Such a mode would readily be excited by blade vibration as shown in Figure 1, where both of the O-rings 5 are subjected to blade vibrations of the same amplitude and phase. The movement of the slug in Figure 1 is then purely horizontal, so that equal strains and stresses are imparted to the two O-rings.

In accordance with the present invention, an asymmetrical design with a translational mode is developed as follows. Referring to Figure 2, the length of the metal slug 23 is extended, at constant diameter, into the space 13. Consequently the centre of gravity 25 of the slug, which in the original design was equidistant between the O-rings, moves nearer to the upper O-ring 27. The distances of the centre of gravity from the upper and lower O-rings 27, 29 can be referred to as XU and xl, respectively. To maintain a translational mode, the reaction forces generated at the O-rings then need to be in inverse proportion to XU and xl. This can be ensured by increasing the stiffness of the upper O-ring 27 relative to that of the lower O-ring 29 in the ratio xitxu.

The increased stiffness of the O-ring 27 can be achieved by increasing the squeezing action on that O-ring, or by increasing the number of O-rings at this point. However, the former method confers an additional advantage, in that the damper becomes less sensitive to errors in the diameter of the cavity. The squeezing action can be increased by adjustment of the relative diameters of the cavity and the slot or by increasing the dimensions of the O-ring.

The increase in the length of the slug has to be limited, otherwise the position of the centre of gravity 25 may become very close to the upper 0being. The mode of damper vibration would then become sensitive to the exact position of the O-ring in its slot 31.

This position cannot be exactly controlled, because the width of the slot has to be larger than the thickness of the O-ring.

The resonance of the damper will tend to increase as a result of these modifications, as the overall stiffness will increase faster than the total mass. However, damper resonance does not need to be controlled to a high degree of accuracy, and small changes, consistent with the limits imposed by consideration of the position of the centre of gravity, will not affect the resonance unduly. The damper of the present invention provides levels of damping up to about 30-402 more than the original symmetrical design.

It will be appreciated that in some circumstances, for example where an existing cavity is available, it might be possible to use, say, a square section cavity and a circular section slug, it being essential only that free movement is not possible in any direction.

Figure 3 shows a partial sectional view of a propulsion unit stator blade 15 extending between a hub portion 17 and an annular shroud 19. A hole 7 is drilled through the shroud and down into the trailing edge of the blade 15. The slug with its O-rings is inserted, and the remainder of the hole is filled by a plug 21. The hole is capped, as described above, to preserve the continuity of the surface. The hole may be drilled into substantially the whole length of the trailing edge of the blade 15 and two or more slugs inserted at spaced positions.

Two O-rings 27, 29 may be provided for each groove, thereby providing increased stiffness with the same inertial mass. Clearly, variations in the number of O-rings, and indeed in the number of grooves, are possible.

Where the vibrating structure is made of a material of relatively low moulding temperature, for example a plastics material, it may be more convenient to incorporate the slug and the cavity by moulding in on manufacture of the component. For this purpose, the cavity may be formed as the inside of a metal (or other relatively refractory material) capsule. The capsule is then sealed against ingress of the body material by a screw cap. It is of course essential that the temperature to which the O-rings are subjected during the moulding process is not sufficient to damage them, or at least is not maintained for sufficient time to damage them.

Claims

1. A method of damping vibration in a component of a mechanical structure wherein a cavity is provided in the component and an inertial slug is mounted in the cavity, the slug being suspended within the cavity by means of two or more viscoelastic O-rings each encompassing the slug and being compressed between the slug and the cavity wall; and wherein the O-rings are positioned asymmetrically relative to the length of the slug.

2. A method according to Claim 1, wherein the slug and the cavity are cylindrical.

3. A method according to Claim 2, wherein the slug and the cavity are of circular cross-section.

4. A method according to Claim 2 or Claim 3, wherein the O-rings are mounted in grooves in the slug.

5. A method according to Claim 4, wherein the'grooves are positioned on the length of the slug such that they are centred at positions spaced from the centre of the gravity of the slug by distances xu and xl, respectively, where XU is smaller than xl; and wherein the stiffness of the O-ring which is mounted in that slot which is closer to the centre of gravity is xl/xU times the stiffness of the O-ring which is mounted in the other slot.

6. A method of damping vibration in a rotary machine, wherein a plurality of machine components structurally coupled to each other are each damped by a method according to any one of Claims 1 to 5, thereby tending to suppress the mutual coupling and propagation of vibration.

7. A method of damping vibration of the trailing edges of the blades of a propulsion unit having a stator comprising a multiplicity of blades extending radially between a hub and an annular cowl, wherein radial cavities are drilled in the trailing edges of a number of the blades, the radial cavities being drilled through the annular cowl, and wherein vibration of the drilled blades is damped by a method according to any one of Claims 1 to 5, thereby tending to suppress the mutual coupling and propagation of vibration.

8. A method of damping vibration of the trailing edges of the blades of a propulsion unit having a stator comprising a multiplicity of blades extending radially from a hub, wherein radial cavities are drilled in the trailing edges of a number of the blades, and wherein vibration of the drilled blades is damped by a method according to any one of Claims 1 to 5, thereby tending to suppress the mutual coupling and propagation of vibration.

9. A method according to any preceding claim, wherein the cavity extends to a surface of the component and the cavity is plugged to provide surface continuity.

10. A method according to any one of Claims 1 to 5, wherein said cavity is provided by a capsule within which the inertial slug is mounted; wherein the capsule is moulded into the machine component; and wherein the machine component consists of a material which is mouldable at a temperature which the O-ring can withstand.

11. A method of damping vibration, substantially as hereinbefore described with reference to the accompanying drawings.

12. A vibration damping arrangement provided by a method according to any one of Claims 1 to 6 and 9.

13. A propulsion unit stator provided with vibration damping according to the method of Claim 7 or Claim 8.

14. A propellor provided with vibration damping according to the method of Claim 7 or Claim 8.

15. A damping arrangement substantially as hereinbefore described with reference to the accompanying drawings.

Amendments to the Claims have been filed as follows 1. A method of damping vibration in a component of a mechanical structure wherein a cavity is provided in the component and an inertial slug is mounted in the cavity, the slug being suspended within the cavity by means of two or more viscoelastic O-rings each encompassing the slug and being compressed between the slug and the cavity wall; and wherein the O-rings are positioned asymmetrically relative to the length of the slug.

5. A method according to Claim 4, wherein the grooves are positioned on the length of the slug such that they are centred at positions spaced from the centre of the gravity of the slug by distances xu and xl, respectively, where Xu is smaller than xl; and wherein the stiffness of the O-ring which is mounted in that slot which is closer to the centre of gravity is xl/xU times the stiffness of the O-ring which is mounted in the other slot.

11. A method of damping vibration, substantially as hereinbefore described with reference to Figures 2 and 3 of the accompanying drawings.