Titanium Bearings Take to the Skies

Every kilogram counts in aircraft design, so SKF subsidiary Sarma's announcement that its new titanium bearing with a bronze bushing will save more than 100 kg in weight per A380 landing gear over traditional bi-metal (bronze and steel) bearings is big news indeed. SKF has received a process patent on the bearing and the flanged bush construction. Six of these self-aligning, spherical plain bearings per aircraft provide a structural attachment between the landing gear and fuselage, and facilitate the rotation of the support pivot axis. In all, SKF engineers tested more than dozens of material combinations, many of which they admit were "improbable." In fact, they weren't even sure at the start that titanium would be a feasible material choice. Engineering Manager Yves Maheo says that the key to the bearing was the development of a coating to prevent galling, a common phenomenon in titanium parts because of the material's relatively high friction coefficient. "Due to its low theoretical tensile and shear strength, the dry sliding coefficient of TI-6AI-4V against steel, for example, is 0.6," Maheo says. "Making matters worse, the great affinity of titanium for oxygen results in the formation of an oxide surface layer that is transferred to the sliding surface during contact, forming wear debris." While Maheo declined to divulge the exact composition of the coating or its thickness, he says the design involves two different coatings that eliminate the galling issue and make possible titanium/titanium sliding contact for the self-aligning function. SKF will offer the bearings in a range of diameters from 20 to 155 mm, and is evaluating various applications including the pylon-to-wing attachment and aileron reaction strut rod end. For information on Sarma's line of aerospace bearings, go to http://rbi.ims.ca/3854-532.