Tham khảo tài liệu 'gear noise and vibration episode 2 part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 240 Chapter IS Root crack vibrations are submerged in similar ones generated by manufacturing errors and external disturbances. Very short shocks do not occur due to anything other than asperities or dirt and are of high frequency so they behave as shock fronts radiating out rather than as lumped mass vibrations. As mentioned previously high frequency vibrations will not transmit satisfactorily through bearings whether rolling or plain because the pressure wave fronts reflect at the bearings instead of passing through. This means that it is not possible to detect these asperity Smith shocks using accelerometers mounted conventionally on the bearing housings. Experiments were carried out to check the link between Smith shock measurements on a rotor and on the bearing housing 3 and showed neligible coherence with variations by a factor of 7 on the one giving only 20 variation on the other. This dictates that the detection system must be mounted on the pinion or wheel rotor to be effective. It is then remarkably sensitive. The system was used to monitor gearbox flank condition after lubrication failure in the form of removal of the oil system 4 . The instrumentation showed failure with local recorded acceleration levels exceeding 40 g about 125 minutes after oil removal. 40 g corresponds to saturation of the shock detection system so the levels would have exceeded this value. Fig. shows some of the test traces obtained. The information for each of the 20 teeth is displayed separately for each four second batch of vibration and the levels have been staggered down 40 g for each tooth for clarity. The interesting observations were that there were indications of failure some 45 minutes before the final failure indication but more surprisingly that though the instrumentation showed high Smith shock levels the surface damage was barely visible and nowhere near the level which would have been noticed with a normal routine visual inspection. To see where the damage was .
