SERVICE ENGINEERING • BOEING COMMERCIAL AIRPLANE • P.O. BOX 3707 • SEATTLE • WASHINGTON 98124-2207
SUBJECT: ATA: 3211-20 13 October 1999 IMPROVED MAIN LANDING GEAR (MLG) SHOCK STRUT BEARING DESIGN, FOR LADDER CRACK REDUCTION AND ENHANCED DYNAMIC SEAL PERFORMANCE. All 767-200 and 767-300 Airplanes All 767-200 and 767-300 AirplanesCum Line 1 through 737 (Upper and Lower bearings); and Cum Line 738 through 745 (Upper bearing only) a) 777-SL-32-012-A, Smear Marks on Landing gear Inner Cylinder Chrome Due to DU Bearing Liner Rubbing operation. b) Drawing 161T1100 Revision E, sheets 2 and 3. Component Assembly – Main Landing Gear.
SUMMARY: This service letter informs operators of a mainlanding gear (MLG) design improvement to minimize ladder cracking, on 767 inner and outer cylinders, and enhance shock strut seal performance. The information herein will assist operators’ retrofitting the improved bearings and overhaul planning. BACKGROUND: Ladder cracking of the shock strut inner cylinder (outside diameter) and the outer cylinder (inside diameter) are long standing problemswith large diameter cantilevered shock struts. Fleet overhaul reports indicate virtually all 767 MLG inner cylinders, configured with the solid aluminum-nickelbronze P/N 161T1152 lower bearing with P/N 161T1151 seal carrier, experience some amount of ladder cracking. The solid aluminum-nickel-bronze lower bearings were installed on 767 airplanes cum line 001- through 737. Page 2 of attachment showsthe shock strut with solid lower bearing. Outer cylinders configured with the P/N 161T1155 solid aluminum-nickel-bronze upper bearing also experience ladder cracking, although to a lesser degree than the inner cylinders. The P/N 161T1155 upper bearings were installed on 767 airplanes cum line 001 through 744.
13 October 1999 Page 2 of 5 Ladder cracking is currently addressed atoverhaul. Cylinders with ladder cracks are typically repairable. While very few cylinders were scrapped due to severe ladder cracking, the safe life limits on about six inner cylinders were reduced, due to the extent of the repairs. Several operators experienced operational delays and were burdened with periodic conditional inspections after finding inner cylinder ladder cracking, during sealchanges and other routine maintenance. In at least one case, the ladder crack findings resulted in early overhaul. Page 1 of attachment shows ladder crack patterns on a MLG inner cylinder. DISCUSSION: The first step toward developing and implementing a plan to reduce or eliminate the damage was understanding ladder cracking and the associated heat damage mechanism. Ladder cracks are caused by localizedhot spots on the cylinder. The hot spots are created by increased friction between the bearings and cylinder walls, as relative motion occurs. Increased friction is mostly attributable to ovalization (elastic deformation) of cylinders. Temperatures estimated to be between 600 and 1600 degrees Fahrenheit are typically localized at the surface of the cylinder, adjacent to the bearing. The heatrapidly dissipates away from the localized region. The factors contributing (together or separately) to ladder crack formation include: 1. Microcracks caused by the expansion and contraction of the substrate steel from the temperature gradient. 2. Microcracks caused by the expansion and contraction of the substrate steel from metallurgical phase changes (if untempered martensite is formed). 3.Residual tensile stresses in the steel induced by the thermal gradient and metallurgical changes. A reduction in friction between the bearings and the cylinders, to reduce ladder crack causing heat, is the key to reducing or eliminating ladder cracking. After several years of research, a new technology material (called DU) was found to reduce or eliminate friction. Garlock Bearings, Inc. manufactures...