This invention relates to metal-to-metal spherical bearings.
Metal-to-metal spherical bearings are used in the aerospace industry and have particular application to landing gear bearings in aircraft. Conventional materials used in the production of landing gear metal-to-metal bearings are stainless steel and copper alloy for the inner and outer races. These materials are used because they do not gall in areas where no lubricant is present. However, these are relatively heavy materials, so solutions have been sought in the past to use more lightweight materials. In this respect, it is possible to replace such relatively heavy materials on a strength basis with titanium alloy bearings which are approximately 40% lighter. Although there is a significant weight saving, these materials tend to gall very quickly under the loading conditions experienced in landing gears. U.S. Pat. No. 4,848,934 gives an example of a titanium alloy bearing. Since the titanium alloy is too soft for metal-to-metal contact, it is provided with a hard coating of chromium oxide.
Other coatings have been investigated but considerable problems are encountered with the adhesion of the hard coating to the relatively soft titanium alloy.
In a conventional metal-to-metal bearing, the bearing housing is manufactured from copper or a copper alloy and the ball held within the housing manufactured from stainless steel. Lightweight versions are available where the stainless steel component is replaced by a titanium alloy but this is a relatively soft material and is therefore provided with a relatively hard coating of chromium oxide so as to provide a good bearing surface at the interface between the bearing housing and the ball.
The use of chromium oxide and chromium plating techniques are environmentally unfriendly and entail considerable manufacturing costs so it is preferred not to use chromium oxide.
One aspect of the present invention provides a metal-to-metal bearing comprising a bearing housing of a first material and a ball of a second material held within the housing, the interface between the housing and the ball comprising two bearing surfaces, wherein one of the first and second materials is a titanium alloy having a diffusion zone near its surface to which is adhered a coating of titanium nitride to provide one of the bearing surfaces.
In order that the present invention may be more readily understood, embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:
Referring to
A nitride diffusion zone 6 (see
It has been found that the above processing is best carried out under vacuum at 700° C., at which temperature there is minimal detrimental effect to the titanium alloy properties but bonding of the coating 5 to the diffused zone 6 in the titanium alloy outer surface of the ball 5 is greatly enhanced as opposed to simply depositing the coating on an untreated (i.e. a surface with no nitride diffusion zone) titanium alloy surface.
The resultant PVD coating 5 has considerable advantages over other coatings that might be employed because components can be finished and machined prior to coating making the parts interchangeable without further machining after coating. The coating and the titanium alloy have very good corrosion resistance which removes the need for any cadmium plating.
Surprisingly, results (see
In the above example, the ball is manufactured from titanium alloy and the housing from a copper alloy. This material selection can be reversed with the titanium nitride coating being applied to the surface of the titanium alloy bearing housing adjacent the ball. Additionally, it is also possible for both the bearing housing and the ball to be manufactured from titanium alloy, in which case, either the bearing housing, the ball or both can be provided with the diffusion zone and PVD titanium nitride coating as in the example results shown in
Number | Name | Date | Kind |
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4434189 | Zaplatynsky | Feb 1984 | A |
4848934 | Blakely et al. | Jul 1989 | A |
6828041 | Ueda et al. | Dec 2004 | B2 |
Number | Date | Country |
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0209925 | Jan 1987 | EP |
1114940 | Jul 2001 | EP |
Number | Date | Country | |
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20050220381 A1 | Oct 2005 | US |