TECHN ICAL FI ELD OF THE I NVENTION

The present invention refers to a vibration absorbing device for reducing vibrations and sounds in a structure according to the preamble of claim 1 , see WO 2006/083223. In various technical applications, such as in aircraft, motor vehicles, ships, machines and industrial plants, it is desirable to reduce vibrations and sounds. Such vibrations or sounds can have one or several main fundamental frequencies. In aircraft, at least one motor speed , which offers an economically advantageous balance between fuel cost and speed , is typically utilized . This motor speed results in vibrations and sounds with relatively well defined frequencies. In order to reduce these vibrations, it is known to use vi bration absorbing devices. The basic principal of these vi bration absorbing devices is to create a resonant system having a mass and spring connected to the object or the structure from which the vibration energy is to be absorbed . These vibration absorbing devices may be passive and tuned for an efficient absorption of vibrations and sounds having this defined fundamental frequency. US-A-2004/0134733 discloses such a passive vibration absorbing device.

Alternative vi bration absorbing devices with means for adapting to variations in the vibration frequency exist, see US-5,873,559. Such devices are realized with an actuator to change the dynamics of the vibration absorbing device, a sensor to detect the vibration frequency and control logics to control the adaptation . Such adaptive vibration absorbing devices require power for the actuator, the sensor, and the control logics. This makes the existing adaptive vibration absorbing devices more complex, less flexi ble for installation , and more expensive than conventional vibration absorbing devices. In various aircraft contexts, for instance propeller-driven aircraft, two or several motor speeds are typically used during flight to optimize performance, fuel consumption or comfort at various flight states. Each motor speed results in dynamic tonal excitation with certain frequencies and consequently a change of the motor speed results in a change of these frequencies. The known passive vi bration absorbing devices only give the desired reduction of vi brations and sounds at one of the motor speeds and related frequency. At the other motor speeds the influence of a passive vibration absorbing device may be an increase of the vibrations and sounds.

This problem has been addressed in WO 2006/083223 showing a vibration absorbing device, which thanks to the rotatable spring element will be adaptive. The spring element will adjust itself to such a rotary position that a maximum vibration amplitude is achieved for the oscillating mass of the spring element at vibration excitation at, or in the proximity of, one of the resonance frequencies of the device. The known device may thus, without any actuating members, absorb two different vibration frequencies. WO 2006/083222 shows another similar adaptive vibration absorbing device. SUM MARY OF THE I NVENTI ON

One problem which could occur with the known vibration absorbing devices is that the dynamic element may be in a disadvantageous position when a vibration is excited in the structure. Such a disadvantageous position is when the vibration direction of the vi bration in the structure is perpendicular to the desired oscillation direction of the dynamic element for a specific frequency. Thus, the dynamic element may be prevented from rotating to the one of the first rotary position and the second rotary position in which the oscillation direction is parallel with the vibration direction , or the adaptation will be so slow that the function of the vibration absorbing device is significantly reduced .

This problem is solved by the device initially defined , which is characterized in that the device comprises a positioning mechanism configured to position the dynamic element in a neutral rotary position , different from the first rotary position and the second rotary position , when the structure does not vibrate. Such a positioning mechanism will thus prevent the dynamic element from getting locked in an unfavorable rotary position in which its desired oscillation direction is perpendicular to the vibration direction of the excited vibration of the structure. It is to be noted that the vibration direction of the vibration of the structure is normally known . The positioning mechanism may thus be easily adjusted to a preferred neutral rotary position in a non-vibrating state of the structure when mounting the vibration absorbing device to the structure.

The dynamic element will adjust itself from the neutral rotary position to one of the first rotary position and the second rotary position so that a maximum vibration amplitude is achieved for the oscillating mass of the dynamic element at vibration excitation at, or in the proximity of, one of the resonance frequencies of the vibration absorbing device.

In some cases, for example when the vibration amplitude is very low, the adjusting may be slow. Energy absorption is however, acquired also for intermediate rotary positions between the first rotary position and the second rotary position .

The positioning mechanism may be designed to have a very specific relation between the rotation relative to the neutral rotary position and the moment (torque) given by the positioning mechanism. Such specific relations between rotary position of the dynamic element and the moment may thus include a progressive behavior with a low moment for low rotation and a significant increase of the moment at a certain rotation . According to an embodiment of the invention , the dynamic element comprises an elastic element extending along the axis of rotation .

The oscillation of the elastic element may be obtained through an energy-absorbing bending of the elastic element. Alternatively, or supplementary, the oscillation of the elastic element may be obtained through an energy-absorbing shearing of the elastic element. According to a further embodiment of the invention , the elastic element provides said different rigidity of the dynamic element.

According to a further embodiment of the invention , the dynamic element has a mass element provided on the elastic element and forming a dynamic mass.

According to a further embodiment of the invention , the mass element provides said different rigidity. The different rigidity of the dynamic element may be obtained through a geometric design of the dynamic element, i .e. the elastic element or the mass element, with a different geometric shape along the two main axes, e.g . the dimension along the first main axis may be different from the dimension along the second main axis. The different rigidity of the dynamic element may also be obtained through an orthotropic material of the dynamic element, i .e. the elastic element or the mass element.

According to a further embodiment of the invention , the elastic element comprises an intermediate segment, a first outer segment and a second outer segment, which extend along the axis of rotation and are rotatable in relation to each other and the attachment. Advantageously, the intermediate segment, the first outer segment and the second outer segment may provide said different rigidity. By such an elastic element, the vibration absorbing device may adapt itself to possibly eight different angular resonance frequencies.

According to a further embodiment of the invention , the first outer segment and the second outer segment are mechanically coupled to each other. With such a coupling , a symmetric elastic element is obtained , which may adapt itself to four different angular resonance frequencies.

According to a further embodiment of the invention , the vibration absorbing device comprises two rotary bearings, which are mounted to the attachment and thus are mechanically connected to the structure, wherein the dynamic element at a respective end thereof is journalled in the two rotary bearings to be rotatable around the axis of rotation .

With such a double support of the dynamic element, two elastic elements are formed , one on each side of the mass element or on each side of a central position of the dynamic element, which elastic elements will act as a combined spring and thus reduce the required accuracy of the mounting of the dynamic element. This will allow for proper functioning of the vibration absorbing device even if there is a small difference in properties for the two elastic elements. Advantageously, at least one of the rotary bearings comprises a pin bearing .

According to a further embodiment of the invention , the positioning mechanism comprises a weight provided on the dynamic element to rotate the dynamic element to the neutral rotary position by means of the gravity force when the structure does not vibrate.

According to a first variant of this embodiment, the weight is attached to the elastic element.

According to a second variant of this embodiment, the weight is attached to the mass element. According to a further embodiment of the invention , the positioning mechanism comprises a spring arrangement acting on the dynamic element to rotate the dynamic element to the neutral rotary position when the structure does not vibrate. Advantageously, the spring arrangement may comprise at least one spring connected to the dynamic element.

According to a first variant of this embodiment, the at least one spring is attached to the elastic element.

According to a second variant of this embodiment, the at least one spring is attached to the mass element.

According to a further embodiment of the invention , the at least one spring comprises a spring having a non-linear characteristic and a progressive behavior with a low moment for low rotation of the dynamic element and a significantly higher moment at a certain rotation of the dynamic element. With such a non-linear spring , the spring arrangement may thus be designed to have the very specific relation between the rotation relative to the neutral rotary position and the moment (torque) given by the spring arrangement. According to a further embodiment of the invention , the positioning mechanism comprises a weight and spring arrangement both acting on the dynamic element to rotate the dynamic element to the neutral rotary position when the structure does not vibrate. According to a further embodiment of the invention , the positioning mechanism comprises a magnet arrangement acting on the dynamic element to rotate the dynamic element to the neutral rotary position when the structure does not vibrate. Advantageously, the magnet arrangement comprises a first magnet connected to the dynamic element and a second magnet connected to the attachment.

According to a second variant of this embodiment, the first magnet is attached to the mass element.

According to a first variant of this embodiment, the first magnet is attached to the elastic element. According to a further embodiment of the invention , the positioning mechanism comprises a weight and magnet arrangement both acting on the dynamic element to rotate the dynamic element to the neutral rotary position when the structure does not vibrate.

According to a further embodiment of the invention , the positioning mechanism comprises a spring arrangement and a magnet arrangement both acting on the dynamic element to rotate the dynamic element to the neutral rotary position when the structure does not vibrate.

BRI EF DESCRI PTION OF THE DRAWI NGS

The present invention is now to be explained more closely through a description of various embodiments and with reference to the drawings attached hereto. discloses schematically a side view of a vibration absorbing device according a first embodiment of the present invention .

discloses schematically a cross-section through the vibration absorbing device in a first rotary position , discloses schematically a cross-section through the vibration absorbing device in a second rotary position .

discloses schematically a cross-section through the vibration absorbing device in a neutral rotary position .

discloses schematically cross-section through a variant of the vibration absorbing device in Fig 1 in a neutral rotary position .

discloses schematically a cross-section through the vibration absorbing device according to a second embodiment of the present invention .

discloses schematically a cross-section through the vibration absorbing device according to a third embodiment of the present invention .

discloses schematically a cross-section through the vibration absorbing device according to a fourth embodiment of the present invention .

discloses schematically a cross-section through the vibration absorbing device according to a fifth embodiment of the present invention .

discloses schematically a side view of a vi bration absorbing device according to a sixth embodiment of the invention .

discloses schematically a side view of a vi bration absorbing device according to a seventh embodiment of the invention .

discloses schematically a side view of a vi bration absorbing device according to an eight embodiment of the invention . Fig 13 discloses schematically a side view of a vibration absorbing device according to a ninth embodiment of the invention . DETAI LED DESCRI PTION OF VARI OUS EMBOD I MENTS

Fig 1 discloses a first embodiment of a vibration absorbing device for reducing vibrations and sounds in a structure 1 . The structure 1 may be a vehicle, for instance an aircraft, a car, or a ship, or any stationary structure, for instance a building , a machine tool or any other stationary installation , where it is desirable to reduce sounds or vibrations.

The vibration absorbing device comprises an attachment 2, which is configured to be attached to the structure 1 in any suitable manner, for instance by means of one or more screw joints 3, schematically indicated .

Furthermore, the vibration absorbing device comprises a dynamic element 4. The dynamic element 4 has two opposite ends. In the first embodiment, the two ends are supported by and journalled in a respective one of two rotary bearings 5 to be rotatable around an axis x of rotation . The dynamic element 4 is at least partly elastic and has a spring constant k.

The two rotary bearings 5 are mounted to the attachment 2. The rotary bearings 5 are thus mechanically connected to the structure 1 . One, or possibly both , of the two rotary bearings 5 comprises a pin bearing .

In the first embodiment, the dynamic element 4 comprises an elastic element 6 extending along the axis x of rotation . The elastic element 6 may be formed by a rod or any similar elongated element and may be made of any suitable elastic material , for instance spring steel , composite material , etc. , to provide elastic properties with said spring constant k permitting the elastic element 6 to flex and oscillate. In the embodiments disclosed , the elastic element 6 is in the form of an elastic rod .

The oscillation of the elastic element 6 may be obtained through an energy-absorbing bending of the elastic element 6. Alternatively, or supplementary, the oscillation of the elastic element 6 may be obtained through an energy-absorbing shearing of the elastic element 6. In this case, the elastic element 6, may have a smaller longitudinal extension along the axis x of rotation than in the embodiments disclosed .

As has been indicated in Fig 1 , the two ends held by the rotary bearings 5 are comprised by the elastic element 6. In the first embodiment, the dynamic element 4 also comprises a mass element 7 provided on the elastic element 6 at a central position of the elastic element 6. The mass element 7 forms a dynamic mass of the dynamic element 4. It should be noted that it is possi ble to dispense with the mass element 7. The dynamic mass of the dynamic element 4 may then be provided by the elastic element 6.

The dynamic element 4 has at least partly a cross-section , see Fig 2 , which defines a first main axis y, perpendicular to the axis x of rotation , and a second main axis z, perpendicular to the first main axis y and to the axis x of rotation . The dynamic element 4 has a different rigidity with respect to the first main axis y and the second main axis z.

In the first embodiment, the elastic element 6 provides the different rigidity of the dynamic element 4 along the first main axis y and the second main axis z. Fig 2 discloses a cross- section through the elastic element 6. The different rigidity of the dynamic element 4 may be obtained through a geometric design of the dynamic element 4, i .e. the elastic element 6 and/or the mass element 7. In Figs 2 and 3, the cross-sectional dimension of the elastic element 6 is longer along the first main axis y than along the second main axis z.

The different rigidity of the dynamic element 4 may also be obtained through an orthotropic material of the dynamic element 4, i .e. the elastic element 6 and/or the mass element 7.

In case of a vibration in the structure 1 , the dynamic element 4 is brought to take a first rotary position , in which the dynamic element 4 will oscillate along the first main axis y, or a second rotary position , in which the dynamic element 4 will oscillate along the second main axis z.

In Fig 2, the dynamic element 4 is adjusted to the first rotary position , in which the first main axis y is parallel to the vibration direction v. The structure 1 then vibrates with an angular resonance frequency co _y = (k _y /m) ^½ of the vi bration absorbing device, wherein k _y corresponds to the spring constant of the dynamic element 4 with respect to the first main axis y and m is the effective modal mass. The dynamic element 4 will oscillate in the direction of the first main axis y.

In Fig 3, the dynamic element 4 is rotated 90° and has thus adjusted itself to the second rotary direction in which the second main axis z is parallel to the vibration direction v. The structure 1 then vi brates with an angular resonance frequency ω _ζ = ( kz/m) ^½ of the vibration absorbing device, wherein k _z corresponds to the spring constant of the dynamic element 4 with respect to the second main axis z. The dynamic element 4 will oscillate in the direction of the second main axis z. If the structure 1 vibrates with the angular frequency ω _ζ and the dynamic element 4 is in the first rotary position , the dynamic element 4 may not rotate to the second rotary position in which the oscillation direction is parallel with the vi bration direction v.

In the same way, if the structure 1 vi brates with the angular frequency co _y and the dynamic element 4 is in the second rotary position , the dynamic element 4 may not rotate to the first rotary position in which the oscillation direction is parallel with the vibration direction v. In order to prevent the dynamic element 4 from being locked in such a disadvantageous position , the vi bration absorbing device comprises a positioning mechanism configured to position the dynamic element 4 in a neutral rotary position , different from the first rotary position and the second rotary position , when the structure 1 does not vibrate.

Consequently, in case of a vibration in the structure 1 , the dynamic element 4 is brought to take the first rotary position , in which the dynamic element 4 will oscillate along the first main axis y, or the second rotary position , in which the dynamic element 4 will oscillate along the second main axis z, if the frequency of the vibration is such that it matches one of the angular resonance frequencies ω _ζ , o _y of the vibration absorbing device.

The vibration absorbing device is thus self-adaptable to achieve maximum vibrations absorption of two different vibration frequencies. According to the first embodiment, the positioning mechanism comprises a weight 1 1 provided on the dynamic element 4. The weight 1 1 will rotate the dynamic element 4 to the neutral rotary position , see Fig 4, by means of the gravity force when the structure 1 does not vibrate. The weight 1 1 is attached to the mass element 7 as can be seen in Figs 2-4. It should be noted , that the weight 1 1 alternatively could be attached to the elastic element 6.

Fig 5 illustrates a variant of the positioning mechanism in Figs 2-4 , which differs therefrom in that the different rigidity is obtained through a geometric design of the mass element 7. In Fig 5, the cross-sectional dimension of the mass element 7 is longer along the first main axis y than along the second main axis z, in particular oval as illustrated in Fig 5. The elastic element 6 may have a square cross-section , or circular cross- section .

According to a second embodiment, the positioning mechanism comprises a spring arrangement 12 acting on the dynamic element 4 to rotate the dynamic element 4 to the neutral rotary position when the structure 1 does not vibrate, see Fig 6. In the second embodiment, the spring arrangement 12 comprise two springs 13, which are connected to the dynamic element 4. The two springs 12 are attached to the mass element 7 at a respective first end and are attached to the attachment 2 at a respective second end .

Fig 7 discloses a third embodiment, which differs from the fourth embodiment in that the spring arrangement 12 comprises only one spring 13 holding the dynamic element 4 in the neutral rotary position . The spring 13 is attached to the mass element 7 at a first end and to the attachment 2 at a second end .

It should be noted , that the springs 13, or said one spring 13, instead of being attached to the mass element 7, alternatively, could be attached to the elastic element 6.

The spring or springs 13 may have a non-linear characteristic. I n particular, the spring or springs 13 may have a progressive behavior with a low moment (torque) for low rotation of the dynamic element 4 and a significantly higher moment (torque) at a certain higher rotation of the dynamic element 4.

According to a fourth embodiment, the positioning mechanism comprises a magnet arrangement 14 acting on the dynamic element 4 to rotate the dynamic element 4 to the neutral rotary position , see Fig 8, when the structure 1 does not vibrate. The magnet arrangement 14 comprises a first magnet 15 attached to the mass element 7 and two second magnets 16 attached to the attachment 2. The first and second magnets 15 and 16 have the same polarity, positive or negative, i .e. they are repelling each other.

Fig 9 discloses a fifth embodiment, which differs from the fourth embodiment in that the second magnets 16 have been replaced by one magnet 1 7. In this embodiment, the first magnet 15 and the second magnet 1 7 have different polarity, i .e. they are attracting each other. It should be noted that the positioning mechanism may comprise a combination of a weight 1 1 and spring arrangement 12, a weight 1 1 and a magnet arrangement 14 , a spring arrangement 12 and magnet arrangement 14, or even a weight 1 1 , a spring arrangement 12 and magnet arrangement 14. Such combinations may act on the dynamic element 4 to rotate the dynamic element 4 to the neutral rotary position when the structure 1 does not vibrate.

The sounds and vibrations may emanate directly from an engine, transmission or the like of a vehicle or a stationary installation . However, the vibration absorbing device may also be used to alter and reduce the peak of a resonance in the structure 1 , for instance squeal from brakes. This as a result of the inherent energy absorption capability for the two angular resonance frequencies oa _y and ω _ζ . The vibration absorbing device will influence vi brations with any vibration frequency, not only when the device is in a specific rotary position . The magnitude of the influence will depend on the excitation frequency and the rotary position , with particularly high influence when the excitation frequency coincides with resonance of the device, as given by the specific rotary position .

The positioning mechanism assures that the vibration absorbing device is forced towards the neutral rotary position when no vibrations are present. The neutral rotary position can be selected for each specific structure in order to adapt for specific structural resonances and specific excitation frequencies. For example the neutral position me be selected to improve the adaptation to a frequency of vi bration that normally is first in an operating cycle like a flight envelope. Similarly the neutral position may be selected to improve the adaptation to reduce the statistically most likely resonance while the adaptation to less likely resonances thus becomes slower. Fig 10 discloses a sixth embodiment, which differs from the previous embodiments in that the dynamic element 4 is supported by one rotary bearing 5 only. In a first variant, the rotary bearing 5 engages one end of the dynamic element 4, i .e. the elastic element 6, whereas the other end of the dynamic element 4 is free. A mass element 7 may be provided at the other free end of the elastic element 6.

In a second variant, not disclosed , the rotary bearing 5 engages the dynamic element 4, i .e. the elastic element 6, at a central position thereof, wherein both ends of the dynamic element 4 are free. A mass elements 7 may be provided at a respective one of the free ends of the elastic element 6.

These variants are explained more closely in WO 2006/083223 discussed above, which is incorporated herein by reference. Fig 1 1 discloses a seventh embodiment, which differs from the previous embodiments in that the elastic element 6 comprises three elastic segments, namely an intermediate segment 6a, a first outer segment 6b and a second outer segment 6c, which extend along the axis x of rotation and are rotatable in relation to each other and the attachment 2. The intermediate segment 6a is connected to the first outer segment 6b by means of a rotary bearing 5' and to the second outer segment 6b by means of a rotary bearing 5' . All of the segments 6a-6b provides the different rigidity as the elastic element 6 of the previous embodiments. The independently rotatable segments 6a-6c may permit the vibration absorbing device to adapt itself to possibly eight different angular resonance frequencies. In the seventh embodiment, the intermediate segment 6a is provided with a mass element 7. A positioning mechanism comprising a weight 1 1 is provided on the intermediate segment 6a. Also the positioning mechanisms discussed above in the second to fifth embodiments may be employed . It is also to be noted that also the first and second outer segments 6b, 6c may, but do not have to, comprise a positioning mechanism as indicated in Fig 1 1 .

Fig 12 discloses an eight embodiment, which differs from the seventh embodiment in that the first outer segment 6b and the second outer segment 6c are coupled to each outer by means of a suitable mechanical element 18, schematically indicated as a dashed line. Thanks to the mechanical element 18, symmetry is achieved . The first outer segment 6b and the second outer segment 6c will act as one elastic element, which , together with the intermediate segment 6a , permits the vi bration absorbing device to adapt itself to four different angular resonance frequencies. It should be understood , the dynamic element 4 may comprise another number of elastic elements than disclosed above in order to create symmetry and/or to permit the vi bration absorbing device to adapt itself to several different angular frequencies. Fig 13 discloses a ninth embodiment of the vibration absorbing device, which differs from the first embodiment in that the positioning mechanism is provided on the rod or elastic element 6 outside the rotary bearings 5. In the ninth embodiment, the positioning mechanism comprises a weight 1 1 . It is to be noted that the positioning mechanism of the ninth embodiment may also comprise a spring arrangement 12 and/or a magnet arrangement 14 as discussed above.

The present invention is not limited to the embodiments disclosed but may be varied or modified within the scope of the following claims.

It should be noted that the progressive behavior mentioned above may also be achieved with a positioning mechanism comprising the magnet arrangement 14, or the weight 1 1 .