WO1995002131A1

WO1995002131A1 - Split tubular elastomer damper

Info

Publication number: WO1995002131A1
Application number: PCT/US1994/007128
Authority: WO
Inventors: Francis E. Byrnes; David N. Schmaling
Original assignee: United Technologies Corporation
Priority date: 1993-07-08
Filing date: 1994-06-23
Publication date: 1995-01-19

Abstract

A split tubular elastomer damper (20) has a pair of co-axial aligned elastomer members (29, 30) located in a split housing (21, 22), each member located in a respective, separate co-axially aligned housing with each having a lug (24, 27). A common independent shaft (34) attaches the members to each other such that strain induced displacements are split between the members. This maximizes the displacement capacity of the tubular elastomer damper, particularly in high stroking applications. Essentially, the amount of elastomer required to tolerate a designated strain is half that of a conventional damper, substantially reducing size and weight requirements while maintaining full functional properties.

Description

SPLIT TUBULAR ELASTOMER DAMPER TECHNICAL FIELD

This invention relates to dampers suited for use in a rotor system of a helicopter aircraft and more particularly to a tubular elastomer damper having co-axial separated damping portions to optimize strain properties for improved damping efficiency and to minimize damper diameter.

BACKGROUND

The rotor head of a helicopter typically includes means for articulating one or more blades through flapping, lag and pitch motions. To dampen the angular lag oscillations of the blades, resilient braces with viscoelastic, hydraulic or other damping means are used, typically referred to as lead/lag dampers. These are usually interposed between a peripheral edge of a rotor hub to which each blade is connected and the blade root.

One such damper incorporates a piston cylinder arrangement having hydraulic fluid disposed between a pair of chambers separated by an orifice. As the blade twists fore and aft (leads and lags) about a vertical hinge or axis, the fluid is driven to pass through the estrictive orifice at a controlled rate thus damping the lead/lag effect. However, such dampers are expensive to manufacture and lose damping ability if fluid leakage occurs.

Another type of damper relies on a plurality of elastomer laminates which dissipate energy through shear deformation. Such a damper is shown in Fig. la. The damper has a central shaft having a lug for attachment to a first structure such as a blade. A portion of the shaft has an elastomer laminate firmly bonded to it. A cylindrical housing envelops the elastomer laminate and extends axially with means for attachment to a second structure such as a rotor hub. For example, the central shaft may mount to the rotor hub and the housing may mount to the leading or trailing edge of the rotor blade. However, the large edgewise excursions of the rotor blade impose a high axial stroking requirement on the damper. Since fatigue life is a function of shear strain, a large diameter damper is required to distribute the strain across the plurality of elastomer laminates. This results in a severe weight penalty for the aircraft. One alternative is to use a higher strain elastomer but typically damping efficiency or loss factor is reduced.

SUMMARY OF THE INVENTION

This invention has as its object the provision of a tubular elastomer damper which has increased stroke capacity with a minimized diameter. It is another object to provide a tubular elastomer damper of simple construction using high loss factor elastomer.

These and other objects of the present invention are achieved by a tubular elastomer damper having a pair of co¬ axial separated elastomer members having a gap therebetween. In one embodiment the members are disposed in separated co¬ axial housing members, each member bonded on an inner diameter thereof to a common shaft, and on their outer diameters to a respective housing member. By splitting and separating the elastomer members, elastomer displacement is reduced by half, reducing strain a corresponding amount. Consequently, the damper diameter can be substantially reduced while still accommodating a high stroking requirement at a comparable axial displacement.

In another embodiment of the invention, the pair of elastomer members are each associated with a respective, separate shaft but are each bonded to a unitary housing, in essence, a reversal of the separated and unitary structures of the previous embodiment, with the same functional result. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. la is a cross-sectional view of a prior art damper; Fig. lb shows the effective displacement of the prior art damper. Fig. 2a is a cross-sectional view of one embodiment of the damper of the invention; Fig. 2b shows the effective displacement of the inventive damper.

Fig. 3 is an alternative embodiment of the invention. Fig. 4 is a cross-sectional view of an alternative embodiment of the damper of Figs. 2a or 3, including cooling passages.

Fig. 5 is another alternative embodiment of the damper of Figs. 2a or 3, including cooling passages.

DETAILED DESCRIPTION OF THE INVENTION

Referring to Fig. la, a prior art elastomer damper 1 is shown. The damper has a tubular housing 2 with a first end 3 having a cap 4 attached thereto by bolts 5. A lug 6 is mounted to the cap, the lug serving to attach the damper to an adjacent structure such as a rotor blade (not shown) . An inside surface 7 of the cylinder has an elastomer member 8 bonded thereto. The elastomer member may be composed of a single unitary elastomer layer or be produced of a plurality of bonded laminate plys composed of a common elastomer or several elastomers, such as a natural or synthetic rubber. The elastomer member 8 has a central passage 9 therein within which a shaft 10 is disposed. The shaft has an internal chamber 11 and an outside surface 12 with the surface 12 bonded to the inner surface of the passage 9. The shaft has a threaded portion 13 which allows attachment of a lug 14 thereto, which is used to attach the damper to another adjacent structure, such as a rotor blade hub (not shown) . A pin 15 extends from the cap 4 and has a portion 16 which resides within the chamber 11. A head 17 of the pin is engagable with a shoulder 18 in the chamber to act as a stop in the event of an overstrain or elastomer failure. This prevents the damper from being pulled apart.

Referring to Fig. lb, the strained and unstrained conditions of the elastomer damper are illustrated. The elastomer has a thickness T_E which at its maximum strain allowable, is displaced by the quantity Δ in response to strain γ. This follows the relationship γ = A

^LE

To accommodate this displacement in a high stroking environment, a thick elastomer must be provided. Preferably low loss factor materials are used, such as natural rubber (about 0.35) or synthetic rubber (about 0.40). A lower loss factor material has a higher strain allowable, more suitable for high motion dampers. For example, a typical damper constructed in accordance with Fig. la may be five inches in diameter, consuming substantial space on the rotor head and adding a substantial weight penalty. In addition, such a thick elastomer has decreased radial stiffness, typically requiring interlaminate cylindrical shims, further increasing weight and complexity.

Referring to Fig. 2a, one embodiment of the invention is shown. A damper 20 has a first housing 21 and a second housing 22. The first housing 21 has a cap 23 with a lug 24, and an inner surface 25. The second housing 22 has a cap 26, a lug 27 and an inner surface 28. The first housing 21 has a first elastomer member 29 incorporated therein, bonded to the inner surface 25. The second housing 22 has a second elastomer member 30 incorporated therein and bonded to the inner surface 28 thereof. The housings and respective elastomer members are co-axially aligned and separated by a gap 31.

Each elastomer member has a central passage 32 and 33, wherein a shaft 34 resides. The shaft has an outer surface 35 with a portion 36 bonded to the first elastomer member and a portion 37 bonded to the second elastomer member. The shaft has stop nuts 38 and 39 which maintain the shaft within the elastomer members to prevent separation during repetitive displacement. However, the shaft is not connected to any external structures, and acts as a common independent element.

The shaft has a chamber 40 which also incorporates a shoulder 41 which operates with a pin 42 to act as a stop in the event of an overstrain or elastomer failure. The chamber has a similar shoulder at the opposite end thereof which operates with another pin in an identical manner, which is not shown simply to ease illustration. Referring to Fig. 2b, the strain and displacement for the inventive damper are shown. The strain induced displacement is split between the two elastomer members. When pulled apart, or when pushed together, the displacement operates oppositely in each elastomer member, doubling the displacement for a given strain, thus reducing by half the thickness of elastomer necessary. This allows either the same size elastomer to accommodate twice the displacement and strain or, to reduce the thickness of the elastomer member by about half. Thus, a two to three inch diameter split damper would replace a five to six inch prior art damper. This constitutes a substantial savings in weight and space requirements. Utilizing a smaller diameter elastomer member provides improved radial stiffness, which may also reduce the need for incorporating axial shims which are necessary with larger diameter elastomers.

The split tubular elastomer damper of the invention thus provides a marked improvement in damper efficiency, with reduced weight and space requirements. Referring to Fig. 3, an alternative embodiment of the invention is shown. A damper 50 has a unitary housing 51. A first elastomer member 52 is incorporated in the housing, having an outer surface 53 bonded to a portion of an inner surface 54 of the housing. A second elastomer member 55, co- axial with the first but separated by a gap 56, is similarly provided within the housing. Each elastomer member has an axial passage 57 and 58 therein, and each member has a respective shaft 59 and 60 disposed within each passage. Each shaft has an end 61 and 62 projecting beyond the respective ends of the housing, each end incorporating a threaded portion 63 and 64 to accept mounting lugs 65 and 66 therein. Each shaft also has at the opposite end thereof an abutment 67 and 68 which is engagable with a respective stop 69 and 70 in the housing, to prevent failure in an overstrain condition. These positions are shown in phantom. Optionally, a pin 71 may extend from one shaft for sliding engagement in the other shaft to assure coaxial alignment between the separate shafts. As with the previous embodiment, the split elastomer has the same strain displacement profile, but instead of a split housing working with split elastomer members and a unitary shaft as a common independent element, this design uses a unitary housing as the common independent element, with split elastomer members and split shafts. The result is the same, a smaller damper with increased damper efficiency using low loss factor elastomers.

The elastomer member may be composed of a single unitary elastomer layer or be produced as a plurality of bonded laminate plys composed of a common elastomer such as natural or synthetic rubber. The member optionally includes one or more cylindrical shims between plys to increase radial stiffness. The elastomer member may be composed of a blend of natural and synthetic rubbers, about 75-90% natural rubber and most preferably 85% natural rubber, 15% synthetic rubber, such as polybutadiene, or be composed of synthetic rubber alone. Among the synthetic rubbers usable are polybutadiene, polybutyl and butyl rubbers. The elastomer members could also incorporate silicone. Such a member may be produced by bonding the first layer to the shaft, bonding the second layer to the housing, assembling the shaft and housing in proximity to each other and filing the void between the elastomer layers with the intermediate material. Such manufacturing techniques are well known in the art. A plurality of cooling passages is optionally incorporated within each elastomer member to provide air cooling to minimize the potential for thermal effects which may be detrimental to the damping properties of the elastomer, particularly in high stroking environments. Where the elastomer damper utilizes a plurality of laminate plys, it is typical to include cylindrical metal shims between the elastomer plys to increase radial stiffness to resist bending moments.

Referring to Fig. 4, an alternative embodiment of the present invention is shown. The damper elastomer 52 has the shaft 59 therein and housing 51 surrounding it. The elastomer further includes four passages 72a-d which extend axially through the elastomer member. Each passage reduces the amount of elastomer but notably maintains a full continuous shaft to elastomer interface 73, though with a reduced housing to elastomer interface 74. The passages 72 are tapered to provide a constant cross sectional area between the inner and outer radius throughout the tubular elastomer material so that the incremental axial stiffness is constant, producing a uniformly strained tubular elastomer damper. Air is free to circulate through the damper, to remove heat generated by damper oscillations, to further improve damper efficiency.

The cooling passages may be provided by locating the central shaft in the housing, incorporating structures in the shape of the passages within the space between the shaft and housing, and utilizing a flowable elastomer material to fill the void spaces between the shaft and the housing. The shaped structures are then removed to provide the cooling passages. Of course, other means for providing the passages such as by drilling or machining may be used though these are somewhat more labor intensive.

Referring to Fig. 5, four cooling passages 75a-d are provided within the tubular elastomer damper member 55. However, a cylindrical metal shim 76 is incorporated within the elastomer member. While a single shim is shown, this is done for ease of illustration, and a plurality of such shims may be used. The shim extends circumferentially between the air passages and the elastomer. Since metal has a higher thermal conductivity than the damper elastomers, the shim acts as a heat sink in the elastomer and conducts heat into the air passage to provide an enhanced means for removing heat from the elastomer, in essence acting as a cooling fin. Consequently, a tubular damper provided according to the embodiment of Fig. 5 will have optimum thermal properties by minimizing temperature gradients, with operational damper characteristics close to the design optimum with an increase in useful life.

Utilizing the present invention, tubular elastomer dampers are provided which minimize localized strain. The split elastomer members provide a means for accommodating strain induced displacement in a high stroking environment, while minimizing damper diameter. This improves overall damper efficiency. The damper also has a reduced propensity for developing high temperatures which are detrimental to the elastomer damping properties, as means are optionally provided for removing heat from the damper to prevent the subsequent heat up of the elastomer with a resultant loss of damping properties. Consequently, tubular elastomer dampers produced in accordance with the invention have optimal damping properties and increased useful life.

While preferred embodiments of the present invention have been shown and described, it will be understood by those skilled in the art that various changes and/or modifications could be made without varying from the scope of the invention.

Claims

Claim 1. A tubular elastomer damper comprising: a first elastomer member and a second elastomer member, the first and second members being co-axially aligned and separated by a gap, each elastomer member having a central passage longitudinally extending therethrough, a unitary support located within the central passage, each elastomer member attached at an inner surface thereof to the unitary support, and, first and second housings, each elastomer member bonded at an outer surface thereof to its respective housing, said first and second housings each having means for attachment to an adjacent structure, such that during axial damper displacement, each elastomer member moves in a direction opposite to the other.

Claim 2. The tubular elastomer damper of claim 1 wherein the unitary support is a shaft.

Claim 3. The tubular elastomer damper of claim 2 wherein the shaft has a chamber therein, a pin extending from the first or second housing into the chamber, the chamber containing a shoulder, the pin having a head such that the pin head engages the shoulder if an overstrain or an elastomer failure occurs.

Claim 4. The tubular elastomer damper of claim 1 wherein the elastomer is composed of a material selected from the group consisting of natural rubber, synthetic rubber, silicone and combinations thereof.

Claim 5. The tubular elastomer damper of claim 1 wherein each elastomer member is composed of a plurality of laminate plies.

Claim 6. The tubular elastomer damper of claim 5 further comprising intermediate rigid shims incorporated between the plurality of plies to provide resistance to bending moments.

Claim 7. The tubular elastomer damper of claim 1 wherein a plurality of axial passages are provided within each elastomer member to allow cooling air to flow through the elastomer members.

Claim 8. The tubular elastomer damper of claim 6 wherein a plurality of axial passages are provided within each elastomer member to allow cooling air to flow through the elastomer members, the rigid shims extend through the passages to conduct heat out of the elastomer members.

Claim 9. A tubular elastomer damper comprising a first elastomer member and a second elastomer member, the first and second members being co-axially aligned and separated by a gap, each member having a central passage and an outer surface, a unitary support located about the elastomer members, each member bonded at its outer surface to the unitary support, first and second interior support structures, each located within a respective elastomer member central passage, each interior support structure having an end which extends beyond the unitary support, and having means for attachment to an adjacent structure, such that during axial damper displacement each elastomer member moves in a direction opposite to the other.

Claim 10. The tubular elastomer damper of claim 9 wherein the unitary support structure is a housing bonded to the outer surfaces of the elastomer members.

Claim 11. The tubular elastomer damper of claim 9 wherein the first and second interior support structures comprise first and second shafts, each shaft disposed within a respective central passage in a respective elastomer member.

Claim 12. The tubular elastomer damper of claim 10 wherein the housing contains a pair of stops therein for engaging the first and second interior support structures in the event of an overstrain or an elastomer failure.

Claim 13. The tubular elastomer damper of claim 9 wherein the elastomer member is composed of a material selected from the group consisting of natural rubber, synthetic rubber, silicone and combinations thereof.

Claim 14. The tubular elastomer damper of claim 9 wherein each elastomer member is composed of a plurality of laminate plies.

Claim 15. The tubular elastomer damper of claim 14 further comprising intermediate rigid shims incorporated between the plurality of plies to provide resistance to bending moments.

Claim 16. The tubular elastomer damper of claim 9 wherein a plurality of axial passages are provided within each elastomer member to allow cooling air to flow through the elastomer members.

Claim 17. The tubular elastomer damper of claim 15 wherein a plurality of axial passages are provided within each elastomer member to allow cooling air to flow through the elastomer members, the rigid shims extend through the axial passages to conduct heat out of the elastomer members.