An oil scavenge pump was found to have failed when a protective shear neck fractured during the start of a jet engine. Visual inspection revealed that the driven gear in one of the bearing compartments was frozen as was the corresponding drive gear. Spacer wear and thermal discoloration (particularly on the driven gear) were also observed. The gears were made from 32Cr-Mo-V13 steel, hardened and nitrided to 750 to 950 HV. Micrographic inspection of the gear teeth revealed microstructural changes that, in context, appear to be the result of friction heating. The spacers consist of Cu alloy (AMS4845) bushings force fit into AA2024-T3 Al alloy spacing elements. It was found that uncontrolled fit interference between the two components had led to Cu alloy overstress. Thermal cycling under operating conditions yielded the material. The dilation was directed inward to the shaft, however, because the bushing had only a few microns of clearance. The effect caused the oil to squeeze out, resulting in metal-to-metal contact, and ultimately failure.

Airfoil-shape impingement cooling tubes were fabricated of 0.25 mm (0.010 in.) thick Hastelloy X sheet stock, then pulse-laser-beam butt welded to cast Hastelloy X base plugs. Each weldment was then inserted through the base of a hollow cast turbine blade for a jet engine. The weldments were finally secured to the bases of the turbine blades by a brazing operation. One of the laser beam attachment welds broke after a 28-h engine test run. Exposure of the fracture surface for study under the electron microscope revealed the joint had broken in stress rupture. Failure was caused by tensile overload from stress concentration at the root of the laser beam weld, which was caused by the sharp notch created by the lack of full weld penetration. Radiographic inspection of all cooling-tube weldments was made mandatory, with rejection stipulated for joints containing subsurface weld-root notches. In addition, all turbine blades containing cooling-tube weldments were reprocessed by back-brazing. Back brazed turbine blades were reinstalled in the engine and withstood the full 150-h model test run without incident.

The engine on a jet aircraft was shut down immediately as it produced excessive vibration. Complete failure of the cage in one of the two main-shaft ball bearings (placed side by side in the engine) was revealed in the dismantled engine. The ball bearings (made of vacuum-melted 52100 steel) were both of the single-row deep-groove type with split inner rings and were designed to operate at a maximum temperature of 175 deg C. Overtempering of the rings was indicated by the reduced hardness in comparison to unfailed rings. Severe damage to approximately 20% of the load-bearing surface, with more damage on one of the shoulders of the groove, was revealed during examination of the outer raceway of the bearing which indicated misalignment of the bearing. No damage other than spalling cavities in the inner-ring raceway, caused by the elongated subsurface inclusion revealed by metallographic examination of circumferential section of the largest cavity, was exhibited by the second bearing. It was concluded that the fracture of the cage was caused by overheating and misalignment caused excessive stressing of the bearing on the main shaft.

The failure of an ATAR engine accessory angle drive gear assembly caused an engine flame-out in a Mirage III aircraft of the Royal Australian Air Force (RAAF) during a landing. Stripping of the engine revealed that the bevel gear locating splines (16 NCD 13) had failed. Visual and low-power microscope examination of the spline of the shaft showed evidence of fretting wear debris; similar wear was observed on the splines of the mating bevel gear. It was concluded that the splines had failed by severe fretting wear. Fretting damage was also observed on the shaft face adjacent to the splines and on the bevel gear abutment shoulder. Additional tests included a metrological inspection of the shaft, bevel gear and support ring; metallographic examination of a section from the shaft; chemical analysis of the shaft material (16 NCD 13); and hardness testing of a sample of the yoke material. The wear had been caused by incorrect machining of the shaft splines, which prevented the bevel gear nut from locating correctly against the gear.