SHOCK STRUT

Abstract

A shock strut for a landing gear assembly that includes a hollow cylinder having a first bearing surface and a piston having a second bearing surface. The piston is configured to move within the hollow cylinder such that the second bearing surface slides relative to the first bearing surface. One of the first and second bearing surfaces includes a non-metallic material and the other of the first and second bearing surfaces includes cold worked titanium.

Claims

1. A shock strut for a landing gear assembly, the shock strut including: a hollow cylinder having a first bearing surface; and a piston having a second bearing surface, the piston being configured to move within the hollow cylinder such that the second bearing surface slides relative to the first bearing surface; wherein one of the first bearing surface and the second bearing surface includes a non-metallic material and the other of the first bearing surface and the second bearing surface includes cold worked titanium.

2. The shock strut of claim 1, wherein the cold worked titanium is bare titanium or titanium metal matrix composite.

3. The shock strut of claim 1, wherein the cold worked titanium is peened titanium, shot peened titanium, or laser peened titanium

4. The shock strut of claim 1, wherein the cold worked titanium comprises a peened and honed surface.

5. The shock strut of claim 1, wherein the cold worked titanium is frettaged titanium or autofrettaged titanium.

6. The shock strut of claim 1, wherein the cold worked titanium is burnished titanium, roller burnished titanium or low plasticity burnished titanium.

7. The shock strut of claim 1, wherein the first bearing surface includes the cold worked titanium.

8. A landing gear assembly including a shock strut according to claim 1.

9. A method of improving the service life of a shock strut including a hollow cylinder having a first bearing surface and a piston having a second bearing surface, the piston being configured to move within the hollow cylinder such that the second bearing surface slides relative to the first bearing surface, wherein one of the first bearing surface and the second bearing surface includes a non-metallic material and the other of the first bearing surface and the second bearing surface includes a metallic material, the method including mechanically modifying the metallic material.

10. The method of claim 9, wherein mechanically modifying the metallic material comprises cold working the metallic material.

11. The method of claim 10, wherein cold working the metallic material comprises peening, shot peening or laser peening the metallic material.

12. The method of claim 10, wherein cold working the metallic material comprises peening the metallic material and then honing a peened surface of the metallic material.

13. The method of claim 10, wherein cold working the metallic material comprises frettaging the metallic material or autofrettaging the metallic material.

14. The method of claim 10, wherein cold working the metallic material comprises burnishing the metallic material, roller burnishing the metallic material or low plasticity burnishing the metallic material.

15. The method of claim 9, wherein the first bearing surface comprises the metallic material.

16. The method of claim 15, wherein mechanically modifying the metallic material comprises mechanically modifying the metallic material following a step of forming the hollow cylinder

17. The method of claim 16, wherein mechanically modifying the metallic material comprises mechanically modifying the metallic material in situ.

18. The method of claim 9, wherein the metallic material includes titanium, bare titanium, or titanium metal matrix composite.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0031] FIG. 1 is a schematic cross sectional view of an aircraft landing gear assembly;

[0032] FIG. 2 is a schematic cross sectional view of the dynamic seal assembly of the landing gear assembly of FIG. 1; and

[0033] FIG. 3 is a burnishing tool for mechanically modifying the inner surface of the cylinder of the landing gear assembly of FIG. 1.

SPECIFIC DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0034] Referring first to FIG. 1, a known aircraft landing gear assembly comprising an oleo-pneumatic shock absorber is shown generally at 10. The shock absorber 10 forms the main strut of the aircraft landing gear.

[0035] The shock absorber comprises an inner housing portion 12, slidably coupled in an outer housing portion 14 via bearings 26. The inner housing portion is known in the art as a slider, sliding tube, inner cylinder, or piston, and the outer housing portion is known as a main fitting, or outer cylinder.

[0036] The sliding piston 12 and main fitting 14 together define an internal cavity or chamber 16 which contains shock absorber fluid. In the illustrated embodiment the chamber 16 contains oil 20 in a lower portion thereof and gas 22 in an upper portion thereof. The oil 20 and gas 22 together make up the shock absorber fluid.

[0037] The region where the sliding piston 12 and main fitting 14 overlap defines an annulus A between adjacent surfaces of the sliding piston 12 and main fitting 14. The annulus A varies in size in accordance with the extension state of the shock absorber 10. The term annulus can mean a ring-like space which has a cylindrical or non-cylindrical cross sectional profile.

[0038] Referring additionally to FIG. 2, an annular ring 18 is housed within the annulus A, adjacent to the open end of the main fitting 14. The annular ring 18 carries seals to confine the shock absorber fluid to the chamber 16.

[0039] A pair of dynamic seals 24 are mounted on the inner cylindrical face 18a of the annular ring 18 and arranged such that one or both of them press against the sliding piston 12 as the shock absorber extends and retracts, inhibiting the passage of shock absorber fluid from the chamber 16 to the outside environment.

[0040] A pair of static seals 28 are mounted on the outer cylindrical face 18b of the annular ring 18 to bear against the corresponding inner face 14b of the main fitting 14.

[0041] The annular ring 18 is locked in place within the annulus A between a shoulder portion 14c of the main fitting 14 and a gland nut 32 which is screwed into engagement with threaded end portion 14d of the main fitting 14.

[0042] In order to prevent dirt and other contaminants from entering the annulus A, an outer environmental seal 34 known in the art as a scraper seal or an extruder seal is provided. The scraper seal 34 is mounted in groove formed in the inner surface of the gland nut 32 between an outer flange 32a and an inner flange 32b so that its position is fixed relative to the cylinder 14.

[0043] The outer flange 32a also prevents larger objects from entering the annulus A through the gap between the outer surface of the sliding piston 12 and the inner surface of the main fitting 14. The outer flange 32a extends so that it is proximal to the outer surface of the piston 12, leaving a very small gap G between the inner edge of the flange 32a and the outer circumference of the sliding piston 12. This gap G may be engineered to accommodate for deflections in the sliding piston 12 during normal use such that during maximum lateral deflection of the sliding piston 12, it does not come into contact with the outer flange 32a.

[0044] The outer housing portion 14 is made from a metallic material, for example a metallic material including titanium such as bare titanium or a metal matrix composite including titanium.

[0045] The bearing surface 27 of the inner housing portion 12 is made from a non-metallic material, for example a polymer such as polytetrafluoroethylene (PTFE).

[0046] The inner surface 36 of the outer housing portion 14 is cold worked by roller burnishing prior to assembly of the aircraft landing gear assembly using a burnishing tool 60 as shown in FIG. 3.

[0047] The burnishing tool 60 has a handle 62 and a mandrel 64. The mandrel has an outer surface 65 and a plurality of rollers 66a, 66b, 66c, 66d that are arranged circumferentially around the outer surface 65 of the mandrel 64.

[0048] The outer housing portion 14 is held in a lathe (not shown) and the burnishing tool 60 is inserted into the outer housing portion 14. The mandrel 64 is moved axially and radially within the outer housing portion 14 such that the rollers 66a, 66b, 66c, 66d are pressed into and smeared across the inner surface 36 of the outer housing portion 14. In this way, the inner surface 36 of the outer housing portion 14 is cold worked, thereby imparting compressive residual stresses to the inner surface 36 of the outer housing portion 14. The compressive residual stresses improve the resistance of the outer housing portion 14 to fatigue and cracking. The inner surface 36 of the outer portion 14 is also hardened and polished. The combination of compressive residual stresses, increased hardness and improved (polished) surface finish improves the tribological (wear) properties of the titanium outer housing portion 14.

[0049] In use, for example during landing of the aircraft, the shock strut 10 is compressed such that the inner housing portion 12 is moved within the outer housing portion 14 and the volume of the elongate chamber 22 is reduced. Any gas within the elongate chamber 22 is compressed and provides an energy store.

[0050] The bearing surface 27 of the inner housing portion 12 slides relative to the inner surface 36 of the outer housing portion 14. As the bearing surface 27 of the inner housing portion 12 is formed from a non-metallic material and the inner surface 36 of the outer housing portion 14 includes cold worked titanium, excessive wearing of the outer surface of the outer housing portion 14 during use is prevented.

[0051] In the example described above the outer housing portion 14 is made from a metallic material, for example a metallic material including titanium such as bare titanium or a metal matrix composite including titanium. In alternative embodiments, the cylinder may be made from aluminium, stainless steel or any other metal.

[0052] In the example described above, the inner surface 36 of the outer housing portion 14 is mechanically modified or cold worked by roller burnishing. In alternative embodiments, other burnishing methods may be employed, for example low plasticity burnishing or ball burnishing. It will also be understood that other cold working methods may be employed, for example peening or autofrettaging.

[0053] In the example described above, the formed outer housing portion 14 is cold worked. In alternative embodiments, the material from which the cylinder is formed may be cold worked prior to forming into a cylinder.

[0054] While the foregoing description has focused on the aircraft landing gear assembly, it will be appreciated that the shock strut according to embodiments of the invention can usefully be applied to various vehicle support assemblies or other assemblies which require a shock absorber.

[0055] It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be capable of designing many alternative embodiments without departing from the scope of the invention as defined by the appended claims. In the claims, any reference signs placed in parenthesis shall not be construed as limiting the claims. The word comprising does not exclude the presence of elements or steps other than those listed in any claim or the specification as a whole. The singular reference of an element does not exclude the plural reference of such elements and vice-versa. Parts of the invention may be implemented by means of hardware comprising several distinct elements. In a device claim enumerating several parts, several of these parts may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.