Downloaded from https://www.everyspec.com ADS-79D-HDBK FIGURE D-5. An example of a typical schematic of intermediate gearbox used to understand physical parameters Because the failure is related to material fatigue resulting in crack propagation, there are two major ways to detect the crack: 1) changes in the vibration sensed by an accelerometer; or, 2) monitoring oil debris for pieces of gear tooth that fall away and collect in the lubrication fluid.
Experience with the oil analysis program and maintenance history have shown there is relatively little operating time from the point where small bits of metal collect in the lubrication oil until the gears become so dysfunctional that loss of tail rotor thrust occurs. Clearly, detecting the crack prior to physical separation of portions of the gear tooth would be more beneficial. This requires data from the accelerometer, which, while installed, may not be sampling data and recording the right data stream for use by signal processing algorithms. D.5.2.2 Determine the best means of measurement.
From a review of the physical and functional models of the IGB, engineers know that the input assembly rotates at a specific frequency, and that a crack in a single tooth would be detected on a once-per-revolution basis by an accelerometer with sufficient sensitivity and dynamic range. D.5.2.3 Determine the existing system capabilities. The helicopter has an existing vibration data collection system with the capability of sampling accelerometer data at 40 kHz.
The processor and storage capacity of the Vibration Measurement Unit have the capability of storing an additional 4 mB of data, which should be sufficient for sampling data in at least 3 established flight regimes (flat pitch on the ground/flight idle, in hover and at 100 kts straight and level flight) per flight. The accelerometers are identical to those placed on the main gearbox casing, and these accelerometers have been proven capable of detecting cracks on the planetary gear assembly as well as the accessory drive shaft. Changing the software in the in-flight data collection equipment is executable as a limited software release 86