GB2316732A

GB2316732A - Vibration reduction

Info

Publication number: GB2316732A
Application number: GB9617765A
Authority: GB
Inventors: Mark Francis Lucien Harper
Original assignee: Mecon Ltd
Current assignee: Mecon Ltd
Priority date: 1996-08-24
Filing date: 1996-08-24
Publication date: 1998-03-04
Also published as: GB9617765D0

Abstract

A detuner comprises a rigid reaction mass 2 attached by three pairs of springs to a rigid housing 20, such that it is able to oscillate at a single unique frequency in any of its six degrees of freedom. In a modification (figure 2, not shown) the springs are replaced by six rubber rings, one attached to each face of the mass.

Description

Device for Vibration Reduction This invention relates to the reduction of vibration of mechanical structures.

Many man-made structures incorporate rotating parts which exert appreciable forces on the rest of the structure as they rotate. For example cars and aeroplanes have engines with rotating shafts and machine tools such as lathes have rotating chucks. It is typical of such rotating parts that there is at least one rate of rotation at which they cause significant unwanted vibrations in the structure to which they are attached. This may be because a shaft itself vibrates at a critical speed, or because the structure has a lightly-damped mode of vibration at the frequency corresponding to the rate of rotation, or simply because the rotating part is governed so that it rotates at a constant fixed rate. It is often desirable to reduce these vibrations, and this is sometimes done by means of a device known as a detuner or dynamic absorber. This device essentially consists of a reaction mass supported on a spring which is attached to the structure.

Vibrations of the structure at the point of attachment which produce movement parallel to the axis of the spring cause the mass to oscillate. This produces a force of reaction at the point of attachment of the spring to the structure which opposes vibrations of the structure which have the same fundamental period of oscillation as the mass on the spring and will usually reduce the amplitude of such vibration. This means of reducing vibration is well known to those knowledgeable in the field of structural vibration. A limitation of such a detuner is that it will only reduce vibrations in one degree of freedom, that is, parallel to the axis of the spring. In general a point on a structure may vibrate in any of its six primitive degrees of freedom: linear vibration parallel to any of the three Cartesian axes, and rotational vibration about any of the three Cartesian axes. It may equally vibrate in a manner which combines vibrations in several or all of its primitive degrees of freedom with arbitrary combinations of amplitude and phase.

According to the present invention there is provided a detuner comprising a substantially rigid mass attached to a system of resilient members which allows the said mass to oscillate at a unique single frequency in any of its six degrees of freedom of motion independently of its motion in any other degree of freedom when at least one anchor point on the said system of resilient members is held stationary so that in effect the said mass and the said system of resilient members behave as six independent oscillators in the six degrees of freedom of motion of the said mass.

When rigidly attached to a structure the invention will impose forces on the structure that tend to reduce vibrations at the said unique single frequency in all six degrees of freedom of motion at its point of attachment to a structure. It thereby performs a function which would otherwise require at least six conventional detuners, affording considerable saving both in complexity and in the total mass which must be added to the structure.

Two specific embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows the cubic reaction mass with helical steel springs attached; Figure 2 shows the cubic reaction mass with elastomeric resilient members attached; and Figure 3 shows the housing in cutaway, with the reaction mass contained within it and the resilient members attached to its inner surface.

In the first embodiment of the invention, six linear helical steel springs 9,11,13,15,17,19 of identical stiffness are attached at their ends to a mass 2, as shown in figure 1. The springs 9,11,13,15,17,19 have negligible stiffness in directions normal to their axes. The springs 9,11,13,15,17,19 behave substantially as ideal springs at the resonant frequency co0ofthe mass 2 oscillating on the springs 9,11,13,15,17,19, which is to say that the frequencies of any internal resonances they may possess must be much higher than the said resonant frequencyco0. The mass 2 is in the form of a cube.

The attachment points 3,4 of the springs 9,11 to the mass 2 lie on the vertical line bisecting the face ABCD, being displaced vertically away from the centre of face in opposite directions by the same distance which we shall denote by d. The attachment points 5,6 of the springs 13,15 to the mass 2 lie on the horizontal line bisecting the face ABFE of the mass 2, being displaced horizontally away from the centre of face in opposite directions by the distance d. The attachment points 7,8 of the springs 17,19 to the mass 2 lie on the horizontal line parallel to side FG bisecting the face BCGF of the mass 2, being displaced away from the centre of face in opposite directions by the distance d parallel to side FG. The springs 9,11 are parallel to the side FB; the springs 13,15 are parallel to the side BC; the springs 17,19 are parallel to the side AB. The other ends of the springs 9,11,13,15,17,19 are attached to the inner surface of the housing 20 which is shown in figure 2; it is hollow, cubic and concentric with the mass 2. Both the mass 2 and the housing 20 are substantially rigid at the resonant frequency of the mass 2 oscillating on the springs 9,11,13,15,17,19.

The springs 9,11,13,15,17,19 are disposed so that the mass 2 will, if excited into oscillation in any one of its six degrees of freedom of motion while the housing 20 is held stationary, continue to oscillate in that degree of freedom without oscillations developing in any other degree of freedom. This is a necessary and sufficient condition for the invention to behave as desired, that is to say that it may act as a conventional detuner in counteracting vibrations in any one degree of freedom of motion of the point on a structure at which it is attached, but that it may also counteract any arbitrary combination of vibrations in any combination of the six degrees of freedom of motion of the point of attachment. The magnitude of the distance d is determined by the requirement that the resonant frequency coO of the mass 2 on the springs 9,11,13,15,17,19 is to be the same in all six degrees of freedom of motion of the mass 2 and may be calculated by a competent mechanical engineer with a knowledge of rigid body dynamics.

The principles of operation of the invention may thus be understood as a generalisation of those of the conventional detuner from one degree of freedom to six.

In the second embodiment of the invention, six identical resilient members 23,25,27,29,31,34 made of elastomeric material such as rubber are attached to the reaction mass 2. As in the first embodiment, the mass 2 is cubic and is substantially rigid at the resonant frequency of the mass 2 oscillating on the said resilient members.

Five of these resilient members 23,25,27,29,31 are illustrated attached to the mass 2 in figure 2. They are shaped as hollow, right circular-section cylinders and are attached one per face of the cubic mass 2, their axes being concentric with and normal to the face to which they are attached. The attachment is made over the whole of the area of the annular end of each resilient member. The sixth resilient member 34 is identically attached to the sixth face of the cubic mass 2 which is hidden in figure 2.

As in the first embodiment of the invention the cubic mass 2 and the resilient members 23,25,27,29,31,34 are contained within a hollow cubic housing which is substantially rigid at the resonant frequency of the mass 2 oscillating on the said resilient members, and the mass 2 is concentric with the said housing. The ends of the resilient members 23,25,27,29,31,34 not attached to the mass 2 are attached to the inner face of the said housing and the attachment is made over the whole of the area of the annular end of each resilient member.

Linear oscillations of the mass 2 parallel to any of the x, y and z axes indicated in figure 2 cause the two resilient members having their axes parallel to the direction of oscillation to deform in compression along their axes, while the remaining four resilient members deform in shear normal to their axes. The resonant frequency of linear oscillation may be adjusted by adjusting the ratio of cross-sectional area of the resilient members to their length, since this ratio controls the resilient members' stiffness in both compression parallel to their axes and shear normal to their axes. By symmetry, the resonant frequency of the mass 2 in linear oscillation on the resilient members 23,25,27,29,31,34 will be the same for oscillations parallel to any of the x, y and z axes indicated in figure 2.

Rotational oscillation of the mass 2 about any of the x, y and z axes indicated in figure 2 cause the two resilient members having their axes parallel to the axis of rotation to deform in torsion about their axes while the remaining resilient members deform in shear and torsion normal to their axes. The resonant frequency of rotational motion may be adjusted by adjusting the inner and outer radii of the resilient members, which between them control the torsional stiffnesses parallel and normal to the resilient member axis and the ratio of torsional stiffnesses to shear stiffness of the resilient members. By symmetry, the resonant frequency of the mass 2 in rotational oscillation on the resilient members 23,25,27,29,31,34 will be the same for oscillations about any of the x, y and z axes indicated in figure 2.

The dimensions of the resilient members 23,25,27,29,31,34 which will be necessary to achieve the desired resonant frequency o, in all six degrees of motion may be calculated by a competent mechanical engineer with a knowledge of rigid body dynamics and of the properites of elastomeric materials.

In constructing a detuner according to the invention in either of the two embodiments already described or in any other embodiment, it should be noted that the force of gravity acting on the reaction mass 2 will cause a downward static deflection of the mass 2. In many cases this deflection will be too small to be of practical consequence (it is less than lmm if the resonant frequency coO is above 100 rad/sec) but in some cases it may be necessary to adjust the attachment points of the resilient members to the housing to compensate for the said deflection in order to avoid a deterioration in performance of the detuner.

Claims

DEVICE FOR VIBRATION REDUCTION

CLAIMS 1. A detuner comprising a substantially rigid mass attached to a system of resilient members which allows it to oscillate at a unique single frequency in any of its six degrees of freedom of motion independently of its motion in any other degree of freedom when at least one anchor point on the said system of resilient members is held stationary no matter in which of its six degrees of freedom of motion the said mass is oscillating so that in effect the said mass and the said system of resilient members may behave as six independent oscillators in the said six degrees of freedom of motion.
2. A detuner according to claim 1 in which the said system of resilient members is attached at its anchor point or points to a substantially rigid frame hereinafter to be referred to as the housing.
3. A detuner according to claim 2 in which means is provided for the said housing to be rigidly attached to a structure at a point where a reduction of vibration of the said structure is required.
4. A detuner according to claim 2 or claim 3 in which the said system of resilient members consists of one or more resilient members each of which is attached at one end to the said rigid mass and at the other to the said housing.
5. A detuner according to any of claims 1 to 4 in which the said rigid mass has a regular shape in order to simplify the design of the system of resilient members.
6. A detuner according to claim 5 in which the said rigid mass is a cube.
7. A detuner according to any of claims 4 to 6 in which all the elements of the said system of resilient members are identical.
8. A detuner according to claim 4 in which each of the said resilient members is a unidirectional linear spring which is to say that the resilient member is significantly stiff when extended or compressed along the axis which joins the two points at which attachment is made to it but the resilient member has negligible stiffness in response to small displacements normal to the said axis.
9. A detuner according to claims 6,7 and 8.
10.A detuner according to claim 9 in which the system of resilient members comprises six unidirectional linear springs and two springs are parallel to each other and are attached to one face of the said rigid mass while two further springs are parallel to each other and are attached to an adjoining face of the said rigid mass and the remaining pair of springs are parallel to each other and are attached to the face of the said rigid mass which adjoins the two faces already referred to and each spring has its stiffness axis normal to the face of the said rigid mass to which it is attached.
11 A detuner according to claim 10 in which the attachment points of any two springs which are attached to the same face of the said rigid mass lie on a line passing through the centre of the said face and lie on opposite sides of the said centre and are displaced away from the said centre by the same distance.
12.A detuner according to any of claims 1 to 4 in which the said system of resilient members consists of one or more resilient members at least one of which provides a significant stifness in response to relative displacement of its points of attachment to the said rigid mass and the said housing in more than one degree of freedom of motion.
13. A detuner according to claim 12 in which the said system of resilient members consists of one or more shaped pieces of elastomeric material.
14.A detuner according to claims 2, 6 and 13 in which the said shaped pieces of elastomeric material are identical hollow right circular cylinder sections and are six in number.
15. A detuner according to claim 14 in which one of the said elastomeric cylinders is attached to each face of the said cubic rigid mass with the axis of the cylinder normal to the said face to which it is attached and each cylinder being attached at its other end to the said housing.