GB2322426A

GB2322426A - Arrangement for damping vibration

Info

Publication number: GB2322426A
Application number: GB8814454A
Authority: GB
Inventors: David John Small
Original assignee: GEC Marconi Ltd; Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1988-06-17
Filing date: 1988-06-17
Publication date: 1998-08-26
Anticipated expiration: 2008-06-17
Also published as: GB8814454D0; GB2322426B

Abstract

For damping vibration in, for example, propeller blades or turbine fins, a slug of metal (1) is embraced by two viscoelastic O-rings (5) and forced into a cylindrical cavity (7) formed, e.g. by drilling in a blade (15), the slug being supported by the compression of the O-rings. The compression of the O-rings (5) is chosen, in conjunction with the mass of the slug (1), for resonance in a translational mode at the frequency at which vibrations need to be damped. Resonance at the same frequency in a torsional mode can be achieved by spacing the O-rings apart symmetrically a distance equal to 1/#3 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 vibration in the blades and fins of propulsion systems.

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) 2TZ 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 one aspect of the present invention, in a method of damping vibration in a component of a mechanical structure, a cavity is provided in the component and an inertial slug is mounted in the cavity, the slug being suspended between the walls of 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.

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

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

The O-rings may be mounted in grooves positioned symmetrically on the length of the cylindrical slug, the grooves being centred at positions spaced apart by the length of the cylindrical slug divided by 13.

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, the stator comprising 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.

A method of damping vibration and several damping arrangements in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, of which: Figure 1 is a perspective view of an inertial slug fitted with O-rings; Figure 2 is a sectional view of the slug of Figure 1 in position in a cavity of a machine component; Figure 3 is a similar view of a modification employing multiple O-rings; Figure 4 is a sectional view of a slug mounted in a capsule; and Figure 5 is a part-sectional view of a damping slug in position 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 the machine component 9 whose vibrations are to be damped. The cavity has a section conforming to that of the slug, both being circular in the drawings shown. They might, however, both be elliptical or oval where space is limited in one direction. The slug is clear of the cavity wall by an amount that will permit all likely vibration of the slug without it contacting the wall. On fitting the slug and its two O-rings into the cavity 7 as shown in Figure 2, the O-rings are compressed between slug and wall, so holding the slug in the suspended condition shown. The cavity is then sealed off with a cap or slug 11 which may be a force fit or the cap may be threaded or fixed with adhesive. The outer surface of the machine part is thus 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 spring. The percentage squeeze and 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 distance3 from the centre of the cylinder where L is the length of the cylinder, as illustrated in Figure 2. The O-rings are thus spaced apart J 3 . This confers increased damping at the design frquency, if the vibration of the structure is such as to excite both the translational and torsional modes.

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 circular slug, it being essential only that free movement in any direction is not possible.

Figure 3 illustrates an embodiment in which two O-rings 5 are provided for each (widened) groove 3 thus providing increased stiffness with the same inertial mass. Clearly variations on the number of wrings, and indeed the number of grooves, are possible.

Where the vibrating structure is made of a material of relatively low moulding temperature, a plastics material for example, it may be more convenient to incorporate the slug and 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 13 as shown in Figure 4. The capsule is sealed against ingress of the body material by a screw cap 15. 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.

Figure 5 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 edgewise into the trailing edge of the blade 15. The slug and its O-rings are inserted and the remainder of the hole filled by a plug 21 and the hole capped as before 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.

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 between the walls of 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.

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

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

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

5. A method according to Claim 4, wherein said grooves are positioned symmetrically on the length of the cylindrical slug the grooves being centred at positions spaced apart by the length of the cylindrical slug divided by 23.

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 of Claims 1 to 5 so tending to suppress the mutual coupling and propagation o-vibration.

7. A method of damping vibration of the trailing edges of the blades of a propulsion unit, the 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 in which vibration of the drilled blades is damped by a method according to any of Claims 1 to 5, so tending to suppress the mutual coupling and propagation of vibration.

8. A propulsion unit stator in which vibration is damped by a method according to Claim 2.

8. A method of damping vibration of the trailing edges of the blades of a propulsion unit, the 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 in which vibration of the drilled blades is damped by a method according to any of Claims 1 to 5, so 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 of Claims 1 to 5, wherein said cavity is 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.

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

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

9.

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

14. A propeller provided with vibration damping means according to the method of Claim 8 or Claim 9 as appended to Claim 8.

15. A damping arrangement substantially as hereinbefore described with reference to Figures 1 and 2, 3, 4 or 5 of the accompanying drawings.

Amendments to the Claims have been filed as follows 1. A method of damping vibration in a rotary machine, wherein a plurality of machine components structurally coupled to each other are damped by providing a cavity in each component and mounting an inertial slug in the cavity, the slug being suspended between the walls of 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.

2. A method of damping vibration of the trailing edges of stator blades of a propulsion unit, the 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 stator blades, the radial cavities being drilled through the annular cowl, and in which vibration of the drilled blades is damped by mounting an inertial slug in the cavity, the slug being suspended between the walls of 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.

3. A method of damping vibration of the trailing edges of the stator blades of a propulsion unit, the 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 stator blades, and wherein vibration of each drilled blade is damped by mounting an inertial slug in the cavity, the slug being suspended between the walls of 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.

4. A method according to any preceding claim, wherein the O-rings are mounted in grooves in the slug, the grooves being positioned symmetrically on the length of the slug and being centred at positions spaced apart by the length of the slug divided by 13.

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

6. A method according to any preceding claim, wherein said cavity is provided by a capsule within which the inertial slug is mounted, the capsule being moulded into the component or stator blade, and the component or stator blade being formed of a material which is mouldable at a temperature which the O-ring can withstand.

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