Experimental determination of AH-64 Apache tailshaft hanger bearing vibration characteristics with seeded faults

Original publisher: Cleveland, OH: Army Research Laboratory: NASA Glenn, [2009] OCLC Number: (OCoLC)470814736 Subject: Apache (Attack helicopter) -- Maintenance and repair. Excerpt: ... For all three flaw sizes, the respective tests showed elevated overall vibration and an increase in the " Fwd HB Shock Pulse Energy, " condition indicator, as shown in figure 6. Other condition indicators did not respond as strongly when compared to the baseline ( healthy ) bearings. Because of the abrupt or sharp nature of the trench flaws, it is not surprising that the condition indicator tracking the highest frequency band responds most strongly to the flaw. However, among the three flaw sizes it should be noted that the condition indicator values did not necessarily trend with flaw size. The intermediate flaw size showed the largest increase in shock pulse energy and the largest " steady " RMS vibration values. It is unclear whether this result is due to poor repeatability or the nature of the flaw, but further investigation was not conducted during the course of testing. 8 ( g ) 7 Energy 8 6 rms ) 7 5 lse 6 ( g Pu vibration4 5 Shock 4 3 3 aring HB2 2 be FWD 1 1 test 0 0 14: 24 14: 31 14: 38 14: 45 14: 52 15: 00 0 2 4 6 8 10 Data Point time ( hh:mm ) ( a ) ( b ) Figure 6. Time traces of ( a ) RMS acceleration signal and ( b ) HB Shock Pulse Energy CI for medium trench spall size. It is notable that for all flaw sizes, irrespective of the specific RMS vibration or CI values, an fast Fourier transform ( FFT ) of the vibration signal showed two distinct bands of increased vibration compared to the baseline bearings. These bands were centered around approximately 2000 Hz and 5200 Hz, where figure 7 shows the spectral response of the bearing with the intermediate trench size. The baseline bearings also show content in these bands, as evidenced by the FFT of bearing 002 in figure 7, and appear to correspond with some of the natural frequencies of the hanger assembly. These natura...