Bearings

A sliding contact bearing, especially an aircraft landing gear trunnion beating, comprises two bearing members (1, 3) composed of titanium or titanium alloy with cooperating sliding surfaces (2, 4), a first one of which is chromium plated and the second one of which is coated with nickel and fluorocarbon polymer. Preferably, the nickel coating is applied by an electroless plating process, and the fluorocarbon is infused into pores in the nickel plating. Preferably, the nickel and fluorocarbon coating is 0.025 to 0.041 mm thick, and has a hardness in the range 750-1000 VPN.

TECHNICAL FIELD 
This invention relates to bearings, especially sliding contact bearings 
used in aircraft landing gear and incorporating titanium components in 
surface-to-surface sliding contact. 
Titanium is used in aircraft construction because it is lightweight 
compared with steel, but when used in bearings such as landing gear 
trunnion bearings that experience large shock loads, there can be problems 
with galling and seizure between titanium surfaces sliding on one another. 
It has been proposed in U.S. Pat. No. 4848934 to overcome this problem by 
coating the surface of one titanium bearing component with chromium oxide 
and forming the other component of copper. Although chromium oxide is a 
brittle material, it is considered satisfactory provided the complementary 
bearing surface with which it cooperates includes copper or copper alloys. 
DISCLOSURE OF THE INVENTION 
According to the present invention a sliding contact bearing comprises two 
bearing members composed of titanium or a titanium alloy with cooperating 
sliding surfaces, characterised in that a first one of said surfaces is 
chromium plated and the second one of said surfaces is coated with nickel 
and fluorocarbon polymer. 
The nickel is preferably applied by an electroless plating process and 
provides a high level of wear and corrosion resistance. The fluorocarbon 
polymer is deposited on the metal surface so as to provide permanent 
lubrication between the two surfaces. A suitable nickel/fluorocarbon 
polymer coating may be provided by the General Magnaplate Corporation 
under their trade mark "NEDOX". 
Comparative tests indicate that a bearing according to the invention offers 
comparable performance and durability to that of conventional bearings 
comprising a chromium plated steel member in sliding cooperation with an 
aluminium bronze member and superior performance compared to bearings 
involving chromium plated, or chromium oxide coated or bronze coated 
bearing surfaces. Further, bearings according to the invention offer the 
advantage of limiting the friction between the bearing surfaces due to the 
effect of the fluorocarbon polymer used.

MODE OF CARRYING OUT THE INVENTION 
The aircraft landing gear trunnion bearing of FIG. 1 comprises an outer 
ring-shaped bearing race 1 with an inner bearing surface 2 formed on a 
sphere, and an inner ring-shaped bearing member 3 located within the outer 
bearing race 1 with an outer bearing surface 4 formed on a sphere so as to 
cooperate with the inner bearing surface 2 of the race 1 and allow 
relative rotary and sliding contact between the two bearing members 1, 3. 
The outer bearing race 1 is adapted for connection to one part of aircraft 
landing gear, and the inner bearing member 3 is formed with a bore 5 to 
receive the journal of a trunnion on the leg of the landing gear. 
The two bearing members 1 and 3 are composed of titanium or titanium alloy, 
and to avoid galling between the cooperating surfaces of these bearing 
members, the inner surface 2 of the outer bearing race 1 is chromium 
plated, and the outer surface 4 of the inner bearing member 3 is coated 
with nickel and fluorocarbon polisher. The nickel is applied by an 
electroless plating process and the fluorocarbon polymer is subsequently 
infused into pores in the plating, and the coating is heat treated to 
create a smooth surface. This nickel/polymer coating is typically that 
provided by General Magnaplate Corporation under their trade mark "NEDOX". 
Typically, the coating is 0.025 to 0.041 mm thick and has a hardness in 
the range 750-1000 VPN. 
For comparison purposes, the bearing as described above and a similar 
"standard" bearing with the outer bearing race 1 composed of chromium 
plated steel and the inner bearing 3 composed of aluminium bronze, were 
subjected to successive simulated flight tests with a maximum load of 214 
KN. In each case, the cooperating surfaces of the bearing were in good 
condition after 30,000 flight tests, and were still in good condition 
after 60,000 flight tests with the bearing according to the invention, but 
were showing slight wear with the "standard" bearing having aluminium 
bronze and steel/chromium plated members. Furthermore, the friction 
between the cooperating surfaces of both bearings were measured at regular 
intervals and the results shown in FIG. 2 obtained, the friction units 
shown being dimensionless scaled friction coefficient units used for ease 
of comparison. This clearly shows that the friction in the bearing 
according to the invention (curve A) increases to a maximum level after 
200 flight tests and remains substantially constant thereafter, whereas 
the friction in the "standard" bearing (curve B) continues to increase 
beyond this point and eventually exceeds the maximum level of the bearing 
according to the invention. 
Another series of comparative tests were conducted employing smaller 
diameter bearings than in the previous tests and in which the outer 
bearing race 1 was composed of chromium plated titanium and the inner 
bearing 3 was composed of titaniumwith different surface finishes as 
indicated below. In each case, a test load of 69 MPa was applied and the 
inner bearing was made to rotate back and forth through an angle of 
50.degree. at a frequency of 0.25 Hz. The relative distance of sliding 
between the two members 1 and 3 was 25 mm each cycle. The life of the 
bearings as determined from wear between their cooperating surfaces were 
assessed as follows: 
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CYCLES 
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Chromium plate 260 
Chromium oxide 249 
Tungsten carbide 340 
Aluminium bronze 1553 
Bronze 4000 
"NEDOX" 8593 
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These results clearly demonstrate the superior life achieved when using a 
"NEDOX" surface finish as compared with the other surface finishes.