Nested damping device with relative motion

The invention relates to the field of damping in load-carrying members, particularly those load-carrying members that carry a load between a plurality of masses that are subject to induced cyclic distortion or vibration. According to an aspect of the invention, a damped structural member is provided that carries a load between a first mass and a second mass. The damped structural member comprises a load-carrying member that carries the load between the first and second masses, and at least one damping member nested with the load-carrying member. The damping member and the load-carrying member move relative to each other during cyclic distortion of the load-carrying member thereby dissipating distortion energy at a damping interface between the damping member and the load-carrying member. The damping member bears essentially only cyclic loads induced by cyclic bending mode movement of load-carrying member. A method for damping a structural member is also provided.

BACKGROUND
 The invention relates to the field of damping in load-carrying members,
 particularly those load-carrying members that carry a load between a
 plurality of masses that are subject to induced cyclic distortion or
 vibration.
 The invention applies to damping in any of a number of mechanical devices
 that carry a load between two masses. The invention is particularly useful
 for damping in vehicle braking systems. Vehicle braking systems are one of
 a class of mechanisms that can be induced to vibrate by the action of
 friction. In some situations, the vibration may become unstable and grow
 to levels severe enough to cause excessive noise, passenger discomfort,
 and/or structural failure of system components. Part of the art of
 designing such systems lies in increasing the stabilizing effects of
 damping within and between the components of the system to counteract the
 destabilizing effects of braking friction.
 The wheel and brake assemblies of an aircraft landing gear system are an
 example of a vehicle braking system that are subject to friction-induced
 vibration of various system components during braking. Aircraft brakes
 generally have a structure called a "torque tube" that transfers brake
 disk generated torque to the brake housing, and thence to the landing
 gears. In landing gears having more than two brakes, the wheel and brake
 assemblies are typically mounted in pairs on two or more tandem axles,
 which are in turn fixed to a "bogie" or "truck" beam that pivots about a
 point on the inner cylinder of the landing gear's shock strut. The brake
 torque generated by each of the fore and aft wheel and brake assemblies is
 reacted by forces at its axle and through a brake rod that links the brake
 housing to the inner cylinder of the landing gear at a point above or
 below the bogie pivot. The fore brake rods act in compression and the aft
 rods act in tension to transmit braking forces to the bogie. For each
 assembly, the rod forms one of the four links of a parallel four-bar
 linkage that operates in the pitch plane of the aircraft.
 The dominant modes of friction-induced vibration in aircraft brakes are
 "squeal" (a torsional oscillation of the non-rotatable brake parts) and
 "whirl" (a rotating bending oscillation). During both modes of vibration,
 an oscillating load is superimposed on the mean torsional load carried by
 the torque tube, and the mean compression or tension loads carried by the
 brake rods that may cause the rod to bend. Workers in the art have
 recently attempted to solve the brake vibration problem in various ways,
 including providing an axial coulomb damper in a brake rod. The brake rod
 essentially acts as a shock absorber. This solution was not entirely
 satisfactory. In addition, the torque tube may develop torsional and/or
 bending modes of vibration, and other components may also be induced to
 vibrate. Therefore, means of reducing or eliminating friction-induced
 vibrations are generally desired. Copending application Ser. No.
 08/592,816 now U.S. Pat. No. 5,806,794 entitled "Aircraft Braking System
 With Damped Brake Rod" and copending application Ser. No. 08/559,354 now
 U.S. Pat. No. 5,915,503 entitled "Brake Rod Having a Bending Mode Coulomb
 Damper" are directd to damped brake rods.
 The invention disclosed herein is a simple, lightweight, inexpensive, and
 effective solution to the brake vibration problem. However, it is not
 intended to limit the invention to application in aircraft brakes and
 landing gear, or friction-induced vibration, as improved damping devices
 are generally desired in the mechanical arts. The invention is useful for
 damping vibration in many types of load carrying members.
 SUMMARY OF THE INVENTION
 According to an aspect of the invention, a damped structural member is
 provided that carries a load between a first mass and a second mass,
 comprising:
 a load-carrying member that carries the load between the first and second
 masses; and,
 at least one damping member nested with the load-carrying member, the
 damping member and the load-carrying member moving relative to each other
 during cyclic distortion of the load-carrying member thereby dissipating
 distortion energy at a damping interface between the damping member and
 the load-carrying member, the damping member bearing essentially only
 cyclic loads induced by cyclic distortion of the load-carrying member.
 According to another aspect of the invention, a method is provided for
 carrying a load between a first mass and a second mass with damping,
 comprising the steps of:
 damping a load-carrying member that carries a load between the first mass
 and the second mass with a damping member nested with the load carrying
 member, wherein the damping member and the load-carrying member move
 relative to each other during cyclic distortion of the load-carrying
 member thereby dissipating distortion energy at a damping interface
 between the damping member and the load-carrying member, the damping
 member bearing essentially only cyclic loads induced by cyclic distortion
 of the load-carrying member.
 According to yet another aspect of the invention, a damped structural
 member is provided that carries a load between a first mass and a second
 mass, comprising:
 load-carrying member means for carrying the load between the first and
 second masses; and,
 damping member means nested with the load-carrying member means for damping
 cyclic distortion of the load-carrying member means, the damping member
 means and the load-carrying member means moving relative to each other
 during cyclic distortion of the load-carrying member means thereby
 dissipating distortion energy at a damping interface between the damping
 member means and the load-carrying member means, the damping member means
 bearing essentially only cyclic loads induced by cyclic distortion of the
 load-carrying member means.

DETAILED DESCRIPTION
 Several embodiments illustrating various aspects of the invention are
 presented in FIGS. 1-13, which are not drawn to scale, and wherein like
 numbered components and features are numbered alike. Referring now to FIG.
 1, a structural member 10 is presented that carries a load between a first
 mass 12 and a second mass 14. The load may place the structural member 10
 in one or more of tension, compression, and torsion. The structural member
 10 may be attached to the first and second masses 12 and 14 by means known
 in the mechanical arts, in a manner that allows the structural member 10
 to carry a load between the first and second masses 12 and 14. The mode of
 attachment may permit or prevent relative rotation of the structural
 member 10 relative to the first and second masses 12 and 14, depending on
 type of load to be carried.
 Referring now to FIG. 2, a cross-sectional view of the structural member 10
 along line A--A of FIG. 1 is presented. The structural member 10 comprises
 a load-carrying member 16 for carrying the load between the first and
 second masses 12 and 14, and at least one damping member 20 nested with
 the load-carrying member 16. The load-carrying member 16 and damping
 member 20 may be formed from materials typically used for structural load
 bearing applications, such as metals, including steel alloys and aluminum
 and its alloys, and fiber reinforced plastic composites, including glass
 fiber reinforced epoxies and carbon fiber reinforced epoxies. The
 materials listed are disclosed as examples of suitable materials, and it
 is not intended to limit the invention to a specific material. The damping
 member 20 may have an axial length less than, equal to, or greater than
 the length of the load-carrying member 16, depending on the particular
 application of the invention.
 According to aspect of the invention, the damping member 20 and the
 load-arraying member 16 move relative to each other during cyclic
 distortion or vibration of the load-carrying member 16 thereby dissipating
 distortion energy at a damping interface between the damping member 20 and
 the load-carrying member 16. According to one embodiment, the damping
 interface comprises frictional contact between two surfaces. This type of
 damping is generally known as "coulomb damping." The FIG. 2 embodiment
 utilizes coulomb damping. According to another embodiment, the damping
 interface may comprise a constrained viscoelastic damping layer disposed
 between the load-carrying member 16 and the damping member 20. This
 embodiment will be discussed with more detail in relation to FIG. 12.
 Still referring to FIG. 2, the actual load is transferred through the
 load-carrying member 16 rather than the damping member 20, according to an
 aspect of the invention. Loading of the damping member 20 consists
 essentially of cyclic distortion or vibration loads induced by cyclic
 distortion of the load-carrying member 16. The damping member is otherwise
 passive, meaning that the damping member 20 experiences "essentially" no
 loading in the absence of cyclic distortion or vibration of the
 load-carrying member 16. In operation, the load-carrying member 16 bears
 the actual load transferred between the first mass and the second mass,
 which may generate a cyclic distortion or vibration in the load-carrying
 member superposed upon the actual load. Conceptually, the actual load may
 be viewed as the average load transferred between the first mass 12 and
 second mass 14 during the load-carrying event. The damping member 20 bears
 essentially only cyclic loads induced by the cyclic bending mode movement
 or vibration of the load-carrying member 16. However, a small amount of
 the actual load carried between the first and second masses 12 and 14 by
 the load-carrying member 16 may be incidently transferred to the damping
 member 20 through the damping interface 20, and the term "essentially" is
 intended to encompass such effects.
 The coulomb damping embodiment of FIG. 2 may be implemented by providing
 the load-carrying member 16 with an axially distributed circumferential
 surface 18, and by providing the damping member with a second surface 22
 that engages the first surface 18. The first surface 18 and second surface
 may be generally coterminous. As used herein, the term "axial" refers to
 the lengthwise direction, as indicated at 11 in FIG. 1, and may or may not
 include the entire length of the referenced component. In the FIG. 2
 embodiment, the damping interface comprises frictional contact between the
 first surface 18 and the second surface 22. The second surface 22 slides
 relative to the first surface 18 during cyclic distortion of the
 load-carrying member 16 thereby dissipating distortion energy at the
 damping interface between the first surface 18 and the second surface 22.
 In coulomb damping, energy is dissipated as work exerted against friction
 forces during relative cyclic distortion between two bodies. The friction
 force is normally independent of displacement or rates of displacement and
 dependent only on the interface pressure, the coefficient of friction, and
 the direction of relative motion, between the bodies. For analytical
 purposes, it is proportional to friction and interface pressure and
 inversely proportional to vibration amplitude and frequency. Thus, it is a
 nonlinear form of damping which has the characteristic that it is
 relatively great for relatively small distortion amplitudes and relatively
 small for relatively great distortion amplitudes. In this invention, there
 may be an additional coulomb effect independent of amplitude and
 proportional to the bending stiffness of the damping member 20. The
 friction effect is small if the interference contact pressure between the
 load-carrying member 16 and damping member 20 is small. The additional
 bending effect is small if the bending stiffness of the damping member is
 small. The bending effect is non-existent if the distortion does not
 comprise a bending mode. According to a preferred embodiment, the
 load-carrying member 16 receives the damping member 20 with an
 interference fit resulting in the second surface 22 pressing against the
 first surface 18. The interference pressure is preferably great enough to
 provide significant damping at low amplitudes, but low enough to allow
 relative sliding motion to occur between the two bodies in contact.
 The cyclic distortion that is damped at the damping interface may comprise
 various forms of distortion, including torsional modes and/or bending
 modes. For bending in a plane parallel to the axial direction 11, the
 load-carrying member 16 has a load-carrying member neutral axis 28, and
 the damping member 20 has a damping member neutral axis 26. The
 load-carrying member 16 also has a load-carrying member shear center 38
 for the bending mode, and the damping member 20 has a damping member shear
 center 36 for the bending mode. Still referring to FIG. 2, at least a
 component of the bending mode may occur in a reference plane 24 parallel
 to the axial direction 11. An additional reference plane 30 is also
 presented, normal to the reference plane 24. Reference plane 24 and
 additional reference plane 3a are interchangeable, and are intended to
 provide a reference frame for purposes of defining the invention without
 limiting the invention to the specific orientations of the reference
 planes presented in the figures. According to an aspect of the invention,
 the damping member neutral axis 26 and the load-carrying neutral axis 28
 have a predetermined neutral axis misalignment 32, parallel to the
 reference plane 24. The neutral axis misalignment causes relative sliding
 movement between surfaces 18 and 22 during bending of the load-carrying
 member 10. This type of sliding action is referred to herein as "axial
 sliding" because it occurs in the axial direction.
 According to another aspect of the invention, shifting the shear center of
 the load-carrying member 16 out of the plane of bending also causes
 relative sliding movement between surfaces 18 and 22 during distortion of
 the structural member 10. Still referring to FIG. 2, at least a component
 of the bending mode may occur in the reference plane 30. The damping ember
 shear center 36 and the load-carrying member shear center 38 have a
 predetermined shear center misalignment 40. The shear center misalignment
 40 is normal to the reference plane 30. Shifting the load-carrying member
 shear center 38 out of the plane of bending causes the ad-carrying member
 to twist during bending. Likewise, shifting the damping member shear
 center 36 out of the plane of bending causes the damping member to twist
 during bending. Misaligning the two shear centers causes one member to
 twist more than the other member, resulting in relative sliding movement
 between the first surface 18 and second surface 22. Either or both shear
 centers 36 and 38 may be shifted out of the plane of bending as long as
 there is a misalignment 40. This type of sliding will be referred to
 herein as "rotational sliding" since it occurs in a plane generally normal
 to the axial direction in a rotational manner.
 Note that bending in the reference plane 24 will not cause rotational
 sliding because the misalignment 40 of the shear centers 36 and 38 is
 parallel to the reference plane 24. Likewise, note that bending in the
 reference plane 30 will not cause axial sliding because misalignment 32 of
 the neutral axes 26 and 28 is normal to the to reference plane 30.
 Therefore, according to the example presented, neutral axis misalignment
 causes relative sliding movement between the first surface 18 and second
 surface 22 for bending in the reference plane 24, and shear center
 misalignment causes relative sliding movement between the first surface 18
 and second surface 22 for bending in the reference plane 30. Bending in
 the reference plane 24 induces axial sliding, and bending in the reference
 plane 30 induces rotational sliding.
 The predetermined neutral axis misalignment 32 and predetermined shear
 center misalignment 40 are established by the cross-sections of the
 damping member 20 and load-carrying member 16. In the FIG. 2 example, an
 axial slot 44 is provided that shifts both the neutral axis 26 and shear
 center 36 of the damping member 20 from the neutral axis 28 and shear
 center 38 of the load-carrying member. The cross-section of the damping
 member 20 depends on the cross-section of the load-carrying member 16 and
 the damping characteristics desired in the damped member 10. It is not
 intended to limit the invention to the specific cross-sections presented
 in the figures, since many cross-sections are evident to an artisan
 skilled in the art, and may be designed according to the principles
 provided herein. Any such variations are considered to fall within the
 purview the invention.
 Referring now to FIG. 3, a cross-section of another embodiment of the
 invention along line A--A of FIG. 1 is presented, wherein the damping
 member 20 is rotated on the order of 90.degree. from the position
 presented in FIG. 2. Relative sliding motion between the first and second
 surfaces 18 and 22 still occurs during bending of the load-carrying member
 10 in both reference planes 24 and 30, but now bending in reference plane
 24 produces rotational sliding due to shear center misalignment 40 normal
 the reference plane 24, and bending in reference plane 30 produces axial
 sliding due to neutral axis misalignment 30 parallel to reference plane
 30. Rotating the damping member 20 to provide axial sliding versus
 rotational sliding in one reference plane versus another may optimize
 damping in one or both planes since each type of sliding may generate a
 different amount of damping, depending on the cross-section of the damping
 member 20, and the relative magnitudes of bending distortion.
 Referring now to FIG. 4, a cross-section of another embodiment of the
 invention along line A--A of FIG. 1 is presented. In this embodiment, the
 damping member 20 is rotated less than 90.degree. from the position
 presented in FIG. 2. This orientation causes a combination of sliding
 effects for bending in both the reference plane 24 and the reference plane
 30. In the embodiment presented the damping member neutral axis 26 and the
 load-carrying neutral axis 28 have a predetermined neutral axis
 misalignment 32. At least a component 33 of the predetermined neutral axis
 misalignment 32 is parallel to the reference plane 24, which causes
 relative axial sliding between first surface 18 and second surface 22
 during bending in said first plane. In addition, the damping member shear
 center 36 and the load-carrying member shear center 38 have a
 predetermined shear center misalignment 40. At least a component 41 of the
 predetermined shear enter misalignment 40 is normal to the reference plane
 24. The omponent 41 of the shear center misalignment 40 causes relative
 rotational sliding between first surface 18 and second surface 22 during
 bending in the reference plane 24. Therefore, bending in the reference
 plane 24 causes a combination of axial sliding and rotational sliding. The
 same is also true for the reference plane 30. At least a component 34 of
 the predetermined neutral axis misalignment 32 is parallel to the
 reference plane 30, which causes relative axial sliding between first
 surface 18 and second surface 22 during bending in reference plane 30.
 Likewise, at least a component 42 of the predetermined shear center
 misalignment 40 is normal to the additional reference plane 30, which
 causes relative rotational sliding between the first surface 18 and the
 second surface 22 during bending in reference plane 30. Therefore, bending
 in the reference plane 30 causes a combination of axial sliding and
 rotational sliding between first surface 18 and second surface 22.
 The damping effects of a coulomb damper according to the invention may be
 quantified through experiment and/or analysis. The predetermined neutral
 axis misalignment may provide a predetermined quantity of bending damping
 in a desired reference plane. Similarly, the predetermined shear center
 misalignment may provide a predetermined quantity of bending damping in a
 desired reference plane. In addition, a predetermined neutral axis
 misalignment may be used with or without a predetermined shear center
 misalignment. Likewise, a predetermined shear center misalignment may be
 used with or without a predetermined neutral axis misalignment. A
 predetermined neutral axis misalignment and predetermined shear center
 misalignment may be used together to provide a predetermined quantity of
 bending damping in a desired reference plane. The magnitude and
 orientation of a neutral axis misalignment and/or shear center
 misalignment may be manipulated and combined to provide a predetermined
 quantity of damping, and relative degree of damping, in each reference
 plane. Referring to FIG. 4, for example, the load-carrying member 16 and
 damping member 20 may cyclically follow a deflection path 58 (shown in
 phantom) during the cyclic distortion. The deflection path 58 represents
 the path a point on the load-carrying member 16 or damping member 20 may
 follow during the cyclic distortion, and is shown greatly exaggerated in
 FIG. 4 for the sake of clarity. The magnitude and orientation of the
 predetermined neutral axis misalignment 32 and predetermined shear center
 misalignment 40, along with the angular orientation of the damping member
 20, may be manipulated and combined to provide a predetermined quantity of
 damping, and relative degree of damping, in each reference plane that
 optimally damps the cyclic distortion causing the deflection path 58.
 Directional damping may also be provided according to an aspect of the
 invention. For example, a damping member and load-carrying member may be
 designed having coincident neutral axes, and misaligned shear centers. The
 shear centers together define a reference plane, and bending in a plane
 normal to the reference plane is damped, while bending in a plane parallel
 to the reference plane is not damped. The bending damping is entirely due
 to rotational sliding induced by the shear center misalignment.
 Referring now to FIG. 5, another embodiment along line A--A of FIG. 1 is
 presented having multiple nested damping members. The damping member 20
 has a third surface 48 axially distributed along the damping member 20. At
 least a second damping member 50 is provided having a fourth surface 52
 engaging the third surface 48. The fourth surface 52 slides against the
 third surface 48 during bending of the load-carrying member 16 thereby
 providing coulomb damping. The damping member 20 may receive the second
 damping member 50 with an interference fit that forces the fourth surface
 52 against the third surface 48, thereby enhancing the coulomb damping
 effect. The second damping member 50 functions the same as the damping
 member 20, and the principles discussed in relation to FIGS. 1-4 also
 apply to the damping member 50. The neutral axis 60 of the damping member
 50 and the neutral axis 26 of the damping member 20 may have a
 predetermined neutral axis misalignment 62, which induces relative axial
 sliding movement between the third surface 48 and the fourth surface 52.
 The shear center 64 of the damping member 50 the shear center 36 of the
 damping member 20 may also have a predetermined shear center misalignment
 66, which induces relative rotational sliding movement between the third
 surface 48 and the fourth surface 52. Neutral axis misalignment and shear
 center misalignment may be used individually or in combination to generate
 a predetermined amount of damping in one or more reference frames. The
 neutral axis misalignment 62 and shear center misalignment 66 may be
 established with an appropriate cross-section of the damping member 50,
 for example by providing an axial slot 68. As discussed previously, other
 cross-sections are evident to artisans skilled in the art that provide a
 neutral axis misalignment and/or shear center misalignment, any of which
 are considered to fall within the purview of the invention.
 According to a preferred embodiment, the damping member 20 is oriented to
 provide optimized damping for bending in one plane, and the second damping
 member 50 is oriented to provide optimized damping for bending in a
 different plane. Nesting multiple damping members also greatly increases
 the quantity of damping without increasing overall size or weight. For
 example, three nested damping members having the same overall wall
 thickness as damping member 20 of FIG. 2 produce a far greater damping
 effect because coulomb damping is generated at three interfaces instead of
 only one interface. It is not intended to limit the invention to a
 specific number of nested damping members.
 Referring now to FIG. 6, a cross-section of an alternative embodiment along
 line A--A of FIG. 1 is presented. Damping member 21 has an axially
 extending reduced thickness portion 46. The reduced thickness portion 46
 shifts the neutral axis 26 and the shear center 36 of the damping member
 21 a desired distance, thereby generating the predetermined neutral axis
 misalignment 32 and predetermined shear center misalignment 40. Though
 shown as an axially extending flat, the reduced thickness portion 46 may
 have various shapes, that shift the neutral axis 26 and/or the shear
 center 36 to create a misalignment, any of which are considered to fall
 within the purview of the invention.
 Referring now to FIG. 7, a cross-section of another embodiment along line
 A--A of FIG. 1 is presented. In this embodiment, the damping member
 comprises three axially elongated shells 76 adjacent each other and nested
 inside the load-carrying member 16 that together form an annulus. The
 shells 76 move relative to the load-carrying member 16 and may move
 relative to each other during cyclic distortion of the load-carrying
 member 16. In the example presented, three identical shells are provided,
 each shell 76 occupying about 120.degree. of the circumference of the
 damping member 20. However, it is not intended to limit the invention to a
 particular number of shells 76, and it is not necessary that each shell be
 identical or occupy the same angular space. The damping member 20 may
 comprise only two shells.
 Referring now to FIG. 8, a cross-section of another embodiment along line
 A--A of FIG. 1 is presented. In this embodiment, the damping member
 comprises four tubes 78, 80, 82 and 84 adjacent each other and nested
 inside the load-carrying member 16. The tubes 78, 80, 82 and 84 move
 relative to the load-carrying member 16 causing frictional interaction
 between surfaces 18 and 22, and may move relative to each other causing
 additional frictional energy dissipation effect during cyclic distortion
 of the load carrying member 16. In the example presented, four tubes are
 provided, with tubes 78 and 82 being the same size, and tubes 80 and 84
 being the same size. It is not intended to limit the invention to a
 specific number of tubes, or to tubes having specific sizes. As few as two
 tubes may be utilized, and tubes having the same and/or different sizes
 may be utilized in the practice of the invention.
 Referring now to FIG. 9, a damping member 20 according to an alternative
 embodiment is presented. Damping member 20 is provided with at least one
 hole 86 through the wall of the damping member 20. FIG. 10 presents a
 cross-section of the damped structural member 10 along line A--A of FIG. 1
 that utilizes the damping member 20 of FIG. 9. The holes may be arranged
 to shift the neutral axis and/or shear center of the damping member 20 as
 previously described in order to induce relative sliding motion between
 the damping member 20 and the load carrying member 16. Different numbers
 of holes may be provided, and the holes may have the same or different
 sizes. The holes may be arranged in one or more rows, and the rows may be
 staggered relative each other. The holes may be arranged in other ways,
 such as a helical pattern, and the holes need not be circular. The holes
 may also be arranged such that the position of the neutral axis and/or
 shear center changes with axial position along the axis of the damping
 member 20, and this effect may be employed via other geometric
 configurations, such as a helical damping member similar to a helical
 spring.
 Referring now to FIG. 11, a cross-section of another embodiment along line
 A--A of FIG. 1 is presented. In this embodiment, the damping member 20
 comprises a first tube-like member 88 and second tube-like member 92
 concentrically disposed inside the first tube-like member 88. A
 constrained viscoelastic layer 90 is disposed between and bonded to the
 first tube-like member and the second tube-like member 92. This embodiment
 is actually a hybrid between coulomb damping and viscoelastic damping.
 During cyclic bending of the load-carrying member 16, surface 18 slides
 relative to surface 22, which provides coulomb damping. In addition, the
 viscoelastic layer is flexed during cyclic bending of the load-carrying
 member 16, which adds viscoelastic damping to the system. This embodiment
 may or may not use neutral axis misalignment and/or shear center
 misalignment since viscoelastic damping relies on distortion of the
 viscoelastic layer 56 to dissipate vibration energy. This occurs even
 without neutral axis misalignment and/or shear center misalignment.
 However, neutral axis misalignment and/or shear center misalignment may be
 employed to increase distortion of the viscoelastic layer, and a slot 94
 may be provided for this purpose. As previously described, other
 cross-sectional shapes may be employed to utilize neutral axis
 misalignment and/or shear center misalignment, and any such variations are
 considered to fall within the purview of the invention.
 Referring now to FIG. 12, a cross-section of another alternative embodiment
 along line A--A of FIG. 1 is presented. In this embodiment the damping
 interface comprises a constrained viscoelastic damping layer 56 disposed
 between and bonded to the first surface 18 and the second surface 22.
 Viscoelastic damping is a different damping mechanism than coulomb
 damping, and relies on distortion of the viscoelastic layer 56 rather than
 frictional interaction between surfaces 18 and 22. The viscoelastic layer
 56 is distorted by relative movement between the load-carrying member 16
 and damping member 20. The relative movement may be generated according to
 any method disclosed herein. Increased relative movement between the
 load-carrying member 16 and damping member 20 through neutral axis
 misalignment and/or shear center misalignment generates a greater
 distortion of the viscoelastic layer 56 in comparison to known
 viscoelastic damping devices, resulting in greater damping with equivalent
 (or less) size and weight. Neutral axis misalignment and shear center
 misalignment are particularly preferred for damping bending in a plane
 parallel to the axis of the load-carrying member 16.
 According to a preferred embodiment, wherein the load-carrying member has
 an elongated axial cavity 54, as shown in FIG. 1, with the tube-like
 sleeve being received within the elongated axial cavity 54. However,
 outside mounting of the damping member is also envisioned. Referring FIG.
 13, for example, a cross-section along line A--A of FIG. 1 is provided
 wherein the damping member 20 is provided on the outside, rather than the
 inside, of the load-carrying member 16. Any of the configurations
 presented in FIGS. 2-12 may be attached to an outer surface of the
 load-carrying member in such manner, but an additional structure is
 required for the FIG. 7 and 8 embodiments in order to hold the damping
 member 20 in contact with the load-carrying member 16. According to a
 preferred embodiment, the load-carrying member receives the damping member
 with an interference fit, whether the damping member is attached inside or
 outside the load-carrying member.
 In FIGS. 2-13, the load-carrying member 16 is cylindrical, with a
 cylindrical axial cavity 54 as shown in FIG. 1. In such case, the damping
 member 20 may be configured as a tube-like sleeve concentric with the
 load-carrying member. This arrangement is particularly simple and cost
 effective. However, the invention is also useful with non-cylindrical
 load-carrying members having symmetric cross-sections or asymmetric
 cross-sections. In addition, the invention is useful with non-constant
 cross-sections in the axial direction, and axially tapered cross-sections
 and surfaces. Any such variations are considered to fall within the
 purview of the invention.
 The axial slot 44 of FIGS. 2-5 and 7-8 is shown having a width. Varying the
 width of the axial slot changes the position of the neutral axis 26, and
 may thereby vary the quantity of coulomb damping generated by the damping
 member. Replacing the slot 44 with an axial slit of non-existent or
 negligible width does not change the position of the neutral axis relative
 to a damping member without an axial slit. However, an axial slit of
 non-existent or negligible width does cause the shear center to shift to a
 position about two times the diameter diametrically opposite the axial
 slit. A damping member configured in this manner may be utilized to
 generate entirely rotational sliding during bending distortion. For
 example, with a cylindrical damping member 20 having an axial slit of
 negligible width and a cylindrical load-carrying member 16, the neutral
 axes are aligned at the axis of revolution of the cylindrical members, and
 the shear centers are separated by a distance of about two times the mean
 diameter of the damping member 20. In this example, rotational sliding is
 generated during bending unless the shear center lays in the plane of
 bending. This effect may be useful for damping some bending modes.
 The invention is particularly useful for damping various components in a
 landing gear and/or wheel and brake assembly. A general description of a
 typical prior art landing gear and wheel and brake assembly follows as a
 general background to facilitate describing further embodiments of the
 invention. Referring now to FIG. 14, a typical prior art aircraft landing
 gear 2 includes a strut 3 and a multi-wheel truck 4 pivotally connected to
 the strut, and one or more brake rods 1. The beam 6 of the truck carries
 at opposite ends thereof respective axles for a plurality of wheel and
 brake assemblies 9. One end of each brake rod 1 is pivotally connected to
 a respective one of the wheel and brake assemblies at a torque arm lug 5
 while the other end is pivotally connected to an attachment lug 7 at the
 lower end of the strut 3. The pivot connection may be below the truck's
 pivot pin 8 as shown, or otherwise such as above the pivot pin 8 while
 still achieving the same functionality.
 Referring now to FIG. 15, a detailed sectional view of a typical prior art
 wheel and brake assembly 9 along line 10--10 of FIG. 14 is presented,
 comprising a friction brake mechanism 110 and a cylindrical wheel 111. The
 tire is not shown in FIG. 15 for the sake of clarity. The wheel 111 has
 matching inboard wheel section 112 and outboard wheel section 113. Each of
 the wheel sections 112, 113 has a corresponding respective rim member 114,
 115, web member 116, 117, and hub member 118, 119. The wheel sections 112
 and 113 are fastened together by suitable bolts (not illustrated) disposed
 in aligned bores (not illustrated) within web members 116 and 117 to form
 an integral unit. Friction brake mechanism 110 is generally symmetrical
 about its central axis of rotation 133.
 The hub members 118 and 119 are rotatably supported by bearings 122 mounted
 on a nonrotatable axle member 123. A stationary carrier or boss 124
 provided with a circumferentially-extending flange 125 is suitably mounted
 on stationary axle 123. Flange 125 has a plurality of circumferentially
 spaced bores 121 to receive bolts 126 for securing such flange to one end
 of a cylindrical torque tube 127. The other (outboard) end of torque tube
 127 has an annular and radially outwardly extending reaction member 128.
 The reaction member 128 may be made integrally with the torque tube 127 as
 illustrated in FIG. 1 or may be made as a separate annular piece and
 suitably connected to the torque tube 127.
 Torque tube 127 has on its exterior a plurality of circumferentially
 spaced, axially extending splines 130. Inboard wheel section 112 has a
 plurality of circumferentially spaced torque-transmitting bars 135 each
 connected to the rim flange portion 185 of wheel section 112 at their
 inboard ends by respective spacer means 162 and at their outboard ends to
 the radially outward portion of web member 116 by seating in respective
 annular recesses in such web member. The torque bars 135 may be varied in
 design from those shown and secured to the wheel section 112 by other
 suitable means such as is described in U.S. Pat. No. 5,024,297 to Russell
 to provide an integral connection therebetween.
 Splines 130 support an axially non-rotatable piston end disc or stator disc
 138 and inner discs 139, 140 and 141. All of such non-rotatable discs 138,
 139, 140 and 141 have slotted openings at circumferentially spaced
 locations on their respective inner peripheries for captive engagement by
 the splines 130, as is old and well-known in the art. A non-rotatable
 annular disc or annular braking element 142 is suitably connected to the
 torque plate or reaction member 128 and acts in concert with the stator
 discs 138, 139, 140 and 141 which discs (138, 139, 140, 141 and 142)
 constitute the stators for the friction brake 110. A suitable manner of
 connection of disc 142 to reaction member 128 is described in U.S. Pat.
 No. 4,878,563 to Baden et al.
 Each of a plurality of axially-spaced discs (rotor discs) 144, 145, 146 and
 147 interleaved between the stator discs 138 through 142, has a plurality
 of circumferentially spaced openings along its respective outer periphery
 for engagement by the corresponding wheel torque bar 135, as is old and
 well known in the art, thereby forming the rotor discs for the friction
 brake 110. All of the non-rotatable discs (138, 139, 140, 141 and 142) and
 rotatable discs (144, 145, 146 and 147) may be made from a suitable brake
 material such as steel or other metal or other wear-resistant material
 such as carbon for withstanding high temperatures and providing a heat
 sink. The number and size of discs may be varied as necessary for the
 application involved. Those stator discs and rotor discs that have
 circumferentially spaced openings on their respective inner and outer
 peripheries may accommodate reinforcing inserts to provide reinforcement
 to the walls of such slotted openings and to enhance the life of such
 slots, as is old and well-known in the art.
 The actuating mechanism or power means for the brake includes a plurality
 of circumferentially spaced cylinders 150 suitably mounted on or connected
 to the flange 125. Within each of the cylinders 150 is a hydraulic piston
 or electromechanical actuators, which is operative to move the stator
 discs 138 through 141 axially into and out of engagement with their
 respective associated rotatable discs 144 through 147, which in turn
 causes the facing radial surfaces of all of the brake discs to
 frictionally engage their radial surfaces as they are forced toward but
 are resisted by the end stationary annular disc 142 and the reaction
 member 128 on torque tube 127. During this period of brake disc
 engagement, the friction forces among all the rotatable and non-rotatable
 discs generate considerable heat energy within the discs. It is the
 frictional engagement of these stator and rotor discs which produces the
 braking action for the aircraft wheel.
 An interior wheel heat shield 160 is cylindrically shaped and is located
 between the inner surface 120 of wheel section 112 and the
 torque-transmitting bars 135. Interior wheel heat shield 160 may be formed
 as a single cylindrical piece or by joining together a plurality of
 arcuate pieces. The interior wheel heat shield may be formed by laminating
 a layer of ceramic fibrous material between two layers of stainless steel
 in a manner well known in the art. As described above, each torque bar 135
 at its outboard (wheel web) end is connected to the web member 116 by
 seating in an annular recess 143. The inboard (piston) end of each torque
 bar 135 and the adjacent portion of the heat shield 160 is secured to
 inboard rim member 114 of inboard wheel section 112 by a spacer 162.
 Spacer 162 is a rectangular shaped member that is recessed on its upper
 and lower surfaces to present an upper flat surface with a pair of spaced
 abutments or shoulders that receive the sides of torque bar 135 and
 present a lower surface with a lower pair of abutments or shoulders. With
 the interior wheel heat shield 160 firmly in place, the protective heat
 shield 190 effectively protects the wheel and its supporting structure
 from the transfer of heat energy from the heat sink.
 With this background, examples of further embodiments of the invention are
 presented in FIGS. 16-18. Referring now specifically to FIG. 16, a damped
 brake rod 200 according to an aspect of the invention is presented for
 attachment to a structure, the structure having at least one wheel and
 brake assembly. The structure may take various forms such as a high speed
 locomotive, or an aircraft landing gear, to the extent that relative
 rotation between the structure and the wheel and brake assembly is
 prevented by a brake rod or a similar structure regardless of the specific
 terminology employed. In the example presented, the structure is an
 aircraft landing gear 2, as presented in FIG. 1, but it is not intended to
 limit the invention to the an aircraft landing gear.
 During braking, some brakes develop an unacceptable level of vibration
 which may result in passenger discomfort, and/or damage to components of
 the landing gear 2 or wheel and brake assembly 9. Presently, the source of
 brake vibration is generally regarded to lie in the nature of friction
 itself, and its sensitivity to various conditions of its operating
 environment, such as load, speed, temperature, and surface irregularity.
 The vibration may originate due to a dynamically unstable state at the
 time braking which results in an unacceptable level of kinetic energy from
 the motion of the aircraft feeding into vibration modes in one or more
 components of the landing gear 2 and/or wheel and brake assembly 9 rather
 than being dissipated as heat energy in the heat sink (disks 138-142 and
 144-147 of FIG. 15). Regardless of the cause of the vibration, it was
 discovered that bending mode vibration of the brake rod 1 was often the
 greatest vibration level in some aircraft. In addition, it was discovered
 that coulomb damping is particularly effective in damping bending mode
 vibration in a brake rod induced during braking. Finally, it was also
 discovered that the wheel and brake assembly 9 and the landing gear 2
 behave as a dynamic system, and that damping the brake rod decreases the
 overall amount of kinetic energy feeding vibration modes in the landing
 gear 2 and/or wheel and brake assembly 9. Thus, the significance of at
 least one source of the problem, bending mode vibration of the prior art
 brake rod 1, eluded prior workers in the art, and identifying this
 significance is an aspect of the invention.
 Referring again to FIG. 16, a sectional view of a damped brake rod 200 is
 presented, that solves the brake and brake rod vibration problem, and
 which may be substituted for the prior art undamped brake rod 1 of FIG. 14
 as original or replacement equipment. Damped brake rod 200 comprises a
 rod-like member 202 for attachment to the brake assembly 9 and the
 structure 2 to resist rotation of the brake assembly 9 relative to the
 structure 2. The rod-like member may be provided with forked knuckles 70
 (only one fork shown) that are pinned to the torque arm lug 5 and
 attachment lug 7 (FIG. 14). A bending mode coulomb damper 204 is attached
 to the brake rod that damps cyclic bending mode movement of the rod-like
 member 202 during braking. The rod-like member 202 may be formed as a
 single piece, or as an assembly of at least two pieces. The rod-like
 member 202 preferably behaves as a continuous beam in dynamic bending.
 Therefore, the rod-like member 202 is preferably formed from a single
 piece of material, but two or more pieces may be assembled with joints
 that are suitably rigid in bending, and the rod-like member as whole will
 behave as a continuous beam in dynamic bending.
 According to a preferred embodiment, the rod-like member 202 has a first
 surface 206 axially distributed along the rod-like member 202, and the
 bending mode damper 204 comprises at least one elongated member 210 having
 a second surface 208 engaging the first surface 206. The elongated member
 210 generates coulomb damping by means of the second surface 208 sliding
 against the first surface 206 during bending of the rod-like member 202,
 as previously described in relation to FIGS. 1-4. The elongated member 210
 is functionally equivalent to the damping member 20, the rod-like member
 202 is functionally equivalent to the load-carrying member 16, the wheel
 and brake assembly 9 (FIG. 14) is functionally equivalent to one of the
 masses 12 or 14, and the attachment lug 7 and strut 3 (FIG. 14) are
 functionally equivalent to the other of the masses 12 or 14. Therefore,
 the rod-like member 202 and elongated member 210 may be configured
 according to any of the embodiments presented in FIGS. 2-11, thereby
 utilizing a predetermined neutral axis misalignment and/or predetermined
 shear center misalignment according to the teachings of those embodiments.
 Any of the embodiments of FIGS. 2-11 are representative of the view along
 line B--B of FIG. 16. One or more elongated members may be nested, as
 presented in FIG. 5, and the elongated members may disposed inside the
 rod-like member, as presented in FIGS. 2-11, or outside the rod-like
 member, as presented in FIG. 13. All of the teachings provided in relation
 to those figures are applicable to the damped brake rod 200 of FIG. 16. In
 some brake rods, the cyclic bending occurs predominantly in a vertical
 plane, and the bending mode damper is preferably oriented to optimize
 damping in that plane. The damped brake rod 200 may also have a
 constrained viscoelastic damping layer configured according to FIG. 12.
 However, coulomb damping is believed to more effective in damping the
 brake rod vibration induced in a landing gear and wheel and brake assembly
 system. In addition, it is not intended to limit the damped brake rod
 according to the invention to the specific bending mode coulomb dampers
 presented in FIGS. 2-11 and 13, as other forms of bending mode coulomb
 dampers may provide effective damping. The bending mode coulomb dampers
 disclosed herein are particularly effective, lightweight, simple, and
 inexpensive.
 Many brake rods have an internal or external cylindrical portion of
 substantial length between the knuckles 70 that defines the first surface
 206. With such brake rods, the bending mode damper 204 preferably
 comprises at least one tube-like sleeve 210 having a second surface 208
 engaging the first surface 206, and wherein the second surface 208 slides
 against the first surface 206 during bending of the rod-like member 202
 thereby providing coulomb damping. Many brake rods have an elongated axial
 cavity 72 cylindrical in cross-section as a matter of brake rod design.
 The invention is particularly useful with this type of brake rod because
 the tube-like sleeve 210 fits conveniently and compactly inside the
 elongated axial cavity 72, without adding a significant amount of weight
 to the brake rod 200. The tube-like sleeve 210 preferably comprises a
 cylindrical tube having an axial slot (as shown in FIG. 2), and is
 preferably configured such that the rod-like member 202 receives the
 tube-like sleeve 210 with an interference fit that forces the second
 surface 208 against the first surface 206. Two or more cylindrical tubes
 may be nested to maximize damping in a compact space.
 According to one embodiment, a stock Boeing 747-400 brake rod having an
 internal sandblasted axial bore of about 2.25 inches in diameter was
 provided with a single tube-like sleeve about 14.5 inches long, having an
 axial slot about 0.070 to 0.130 inches wide, an inside diameter of about 2
 inches, and an outside diameter of about 2.255 inches resulting in about a
 0.005 inch interference between the sleeve and the bore. The sleeve was
 formed from 4340 steel, weighed about 3.4 pounds, and protruded about 11/4
 inches from the brake rod bore. The damped brake rod according to this
 embodiment greatly reduced brake induced vibration during braking, and
 brought the vibration well within acceptable levels.
 Referring now to FIG. 17, a sectional view of a brake torque tube 220 is
 presented, that may also reduce or eliminate some forms of brake
 vibration, and which may be substituted for the prior art undamped torque
 tube 127 of FIG. 16 as original or replacement equipment. In this
 embodiment, the damping interface preferably comprises frictional contact
 between a first surface 226 and a second surface 228 because of the heat
 generated in the brake disks 138-142 and 144-147 (FIG. 16) during braking.
 The brake torque tube 220 comprises a generally cylindrical cavity 222,
 and the damping member preferably comprises a cylindrical sleeve 224
 engaging the cylindrical cavity 222. The brake torque tube has a first
 surface 226, and the damping member or cylindrical sleeve 224 has a second
 surface 228. The damping interface comprises frictional contact between
 the first surface 226 and the second surface 228.
 Several modes of vibration may be induced in the torque tube 220 during
 braking. Such vibrations may include cyclic torsional distortions about
 the axis of rotation 74 of the torque tube 220 and/or cyclic bending
 distortions in an axial plane. With bending modes, neutral axis
 misalignment and/or shear center misalignment may be utilized, as
 previously described in relation to FIGS. 2-4, and FIGS. 2-6 are
 representative of the cross-sectional views of various embodiments along
 line C--C of FIG. 17, wherein the brake torque tube 220 is functionally
 equivalent to the load-carrying member 16, and the cylindrical sleeve 224
 is functionally equivalent to the damping member 20. With pure torsional
 modes, the cylindrical sleeve 224 may be circumferentially continuous with
 a circumferentially uniform wall thickness, as shown in FIG. 18. Relative
 sliding between surfaces 226 and 228 is due to the fact that most or all
 of the torsional deflection occurs in the torque tube 220 rather than the
 cylindrical sleeve 224, and generates the desired sliding action. Neutral
 axis misalignment and shear center misalignment provide no advantage with
 purely torsional modes of vibration. However, bending and torsional modes
 may occur together, and in such case, the cylindrical sleeve 224 may be
 configured according to the teachings provided in relation to FIGS. 2-10.
 Referring again to FIG. 17, the cylindrical sleeve 224 may be permitted to
 float within the torque tube 220, or it may be attached in some manner to
 the torque tube 220. For example, the cylindrical sleeve 224 and the brake
 torque tube 220 define a first end 230 axially spaced from a second end
 232. The cylindrical sleeve 224 and the brake torque tube 220 may be
 engaged together against rotation at only one of the first and second ends
 230 and 232. The cylindrical sleeve 224 and torque tube 220 may be
 attached by bolts, or an equivalent means, at the torque tube mounting
 flange 76 where the torque tube 220 attaches to the stationary carrier or
 boss 124 (FIG. 16). If the cylindrical sleeve 224 floats, sliding is
 induced by the inertia of the cylindrical sleeve 224. In either case, the
 brake torque tube 220, rather than the cylindrical sleeve 224, transfers
 the braking load to the landing gear.
 In the example presented in FIG. 17, the cylindrical sleeve 224 is provided
 with a bottom flange 94 that may be utilized to mount the cylindrical
 sleeve 224 to the torque tube 220 at first end 230, as previously
 described. Bottom flange 94 is optional since the cylindrical sleeve 224
 may be mounted to the torque tube 220 in other ways, and is preferably
 eliminated if the cylindrical sleeve 224 is permitted to float within the
 torque tube 220 (the "inertial" embodiment). In addition, the cylindrical
 sleeve 224 is shown provided with a support flange 96. The support flange
 96 engages the axle 123 (FIG. 15) and provides additional support for the
 wheel and brake assembly. The support flange 96 transmits axle bending
 distortions to the cylindrical sleeve 224 and torque tube 220, and may
 contribute to bending mode damping generated by the cylindrical sleeve
 224. However, not all wheel and brake assemblies utilize a support flange,
 in such case, flange 96 may be eliminated.
 Though described in relation to an aircraft landing gear and wheel and
 brake assembly, it is not intended to limit application of the invention
 to these particular components, or to aircraft in general. It is evident
 that many variations of the invention are apparent to persons skilled in
 the art, any of which are considered to fall within the purview of the
 invention according to the true scope and spirit of the invention as
 defined by the claims the follow.