To meet the stringent requirements of aircraft manufacturers and airlines for longer component service life, The Torrington Company/Fafnir Bearings Division, has dramatically increased the corrosion resistance of critical airframe control bearings and, in the process, has also improved their dynamic fatigue life.
Torrington, a subsidiary of Ingersoll-Rand, can offer an upgrade to its Fafnir airframe rod end bearings and stud-type track roller bearings with improved alloys used in their construction.
Instead of using an assembly consisting of a conventional carburizing bearing steel specified in MIL-B-6039 (such as SAE Type 8620) and a through-hardened SAE 52100 steel, the Torrington, CT, company decided to use Pyrowear® 675 Stainless alloy, a carburizing, corrosion resistant steel from Carpenter Technology Corp., Reading, PA, in combination with through-hardened Type 440C stainless steel.
Torrington has released a study documenting the improved results obtained when the outer rings of Fafnir rod end bearings were made of Pyrowear 675 Stainless alloy and the inner rings and balls were made of the Type 440C stainless.
New in the component mix is Carpenter's recently developed Pyrowear 675 Stainless alloy, a double-vacuum-melted alloy that, after carburizing, offers a hard case combined with a tough, ductile core and the ability to withstand both corrosion and the heat encountered under heavy duty service conditions. Its corrosion resistance is similar to that of Type 440C stainless, but with substantially greater core toughness.
The Torrington tests and alloy changes reflected the fact that lack of corrosion resistance, not rolling contact fatigue, was the primary reason that rod end bearings have to be replaced at scheduled maintenance intervals. Airlines would like these intervals to be extended and bearing life to be considerably increased.
Extensive testing by Torrington showed that the new Fafnir rod end bearings combining Pyrowear 675 Stainless alloy and Type 440C stainless exceeded all the performance requirements of MIL-B-6039. Mechanical tests showed a 17% improvement in dynamic life over rod ends of SAE 8620/52100 steels.
Alternate immersion tests and salt-spray tests verified that the corrosion resistance of the Pyrowear 675 Stainless alloy/Type 440C match was "considerably greater" than that of rod ends made of the two standard bearings steels. This was good news for airframe control bearings exposed to water, salt-water mist, washing solutions, deicing solutions and dirt (mud).
In the alternate immersion corrosion test, three samples each of the stainless steel bearings and the 8620/52100 bearings were tested for 1,000 hours according to ASTM G 44-88. The specimens were immersed in an aqueous solution of 3.5 wt. percent sodium chloride for 10 minutes, and then allowed to dry for 50 minutes. This 1-hr. cycle was continued 24 hours a day for 1,000 hours.
Tests were conducted during the 1,000-hr. exposure to determine the amount of starting torque needed to rotate the inner ring. After 176 hours, the balls and races of the 8620/52100 alloys seized from extreme corrosion build-up. The rod ends of Pyrowear 675 Stainless/Type 440C stainless exhibited no corrosion after 1,000 hours. Salt deposits caused only a minimal increase in the torque required to rotate throughout the test period.
In the more severe ASTM B117 salt-spray-corrosion test, sealed and greased rod end bearings of both alloy combinations were exposed for 1,000 hours in a chamber held at 35°C (95°F) with a 5 wt. percent sodium chloride reservoir. Compressed air agitated the solution, and atomizing nozzles maintained a foggy atmosphere.
The 8620/52100 rod ends showed considerable evidence of red corrosion. The Pyrowear 675 Stainless/Type 440C stainless rod ends exhibited no evidence of red corrosion, although some white corrosion appeared on the plating. Internal surfaces of all test samples were protected from corrosion by the seals and/or grease.
Carpenter reports that the core of Pyrowear 675 Stainless alloy has corrosion resistance similar to that of Type 410 stainless, which is somewhat better than that of Type 440C stainless. Polished core and carburized case samples of Pyrowear 675 Stainless alloy subjected to a humidity cabinet environment of 35°C (95°F) temperature and 95% humidity have shown no signs of rusting after a 200-hr test.
Fracture/Dynamic Load Tests
Rod end bearings, used to control moving elements in the aircraft, must have a hard case to provide good rolling contact fatigue resistance and a tough ductile core so the shaft can resist bending forces and shock loads. Carpenter designed its Pyrowear 675 Stainless alloy to have a core hardness of HRC 39/40, and a case hardness of HRC60 at 204°C (400°F), while maintaining HRC58 at 363°C (675°F). This level of hardness is achieved without sacrificing any of the high toughness required.
With initial assistance from Carpenter, Torrington developed and produced optimal case and core properties. Hardness profiles were measured on three finish-ground bearings in the raceway area. Fafnir rod ends easily met the industry's specification that the case depth to a hardness of 50HRC must fall between 25 to 50% of the wall thickness.
To confirm the mechanical properties of the stainless steel bearings, Torrington conducted a radial fracture load test, and two radial dynamic load tests.
In the radial fracture load test, ten stainless steel and ten 8620/52100 rod end bearings were exposed to a radial load of 2,580 pounds for one minute, as defined in MIL-B-6039. After removing the load, the bearings were rotated by hand to verify smoothness. None of the bearings showed any evidence of fracture.
Similarly, all bearings were subjected to an axial load of 520 pounds for one minute in accordance with the same specification. Again, after rotating to check smoothness, no evidence of fracture or raceway damage appeared on any of the bearings tested. In the first radial dynamic load test, the stainless steel bearings and the conventional steel bearings were subjected to a maximum radial limit load of 1,720 lbs., with an inner ring oscillation for 10,000 cycles, as indicated in MIl-B-6039. All 10,000 cycles were run at 28 cycles per minute (cpm) at room temperature. There was no increase in roughness or looseness in any of the bearings, and there was no evidence of pitting or surface fatigue. All bearings passed the test.
The second radial dynamic load test was similar to the first one, except that the bearings were tested to failure. Failure occurred when a ring or ball spalled. In terms of Weibull statistical results, the Pyrowear 675 Stainless alloy/Type 440C stainless bearings demonstrated 17% longer life than the bearings made of the 8620/52100 bearing steels.
The L10 (90% survival) life of 131,825 cycles for the stainless bearings compared with 112,330 cycles for the standard bearings. Moreover, the L50 (average) life of 225,139 cycles for the stainless bearings exceeded the 209,539 cycles for the 8620/52100 bearings.
Carpenter noted that the core fracture toughness (Kq >= 142 KSI of Pyrowear 675 Stainless alloy is superior to that of other stainless bearing steels, and at least as good as, if not better than that of non-corrosion-resistant carburizing bearing steels such as SAE Types 8620 and 9310. The company further explained that the chemical composition of its Pyrowear 675 Stainless alloy is carefully balanced to make the carburized case and uncarburized core properties compatible, a result clearly demonstrated in the Torrington products.