Measuring and testing – Vibration – By mechanical waves
Reexamination Certificate
1999-09-27
2001-10-09
Kwok, Helen (Department: 2856)
Measuring and testing
Vibration
By mechanical waves
Reexamination Certificate
active
06298725
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to processing of vibration data obtained from epicyclic gear trains, and more particularly to a method of performing selection time filtered signal averaging on vibration data from such gear trains.
The multiplicity of identical planet gears in an epicyclic gearbox, combined with the fact that the planet gear axes move in relation to the gearbox casing, make traditional vibration analysis techniques ineffective in the diagnosis of planet gear faults. The inability to effectively diagnose faults in epicyclic gearboxes is of particular concern in helicopters, most of which use epicyclic reduction gears as the final drive to the main rotor. Accordingly, there is need for more effective fault diagnosis techniques, and particularly as respects helicopter epicyclic gear trains, for reasons that include enhancement of safety.
SUMMARY OF THE INVENTION
A new method of separating the individual planet vibration signatures in an epicyclic gearbox has now been developed, and has significant advantages over previous methods. One advantage is a significant reduction in the time required to perform sufficient separation of the planet signatures for diagnostic purposes, which makes implementation of the new method feasible in operational aircraft.
It is a major object of the invention to provide a method of performing selective time filtered signal averaging on vibration data from epicyclic gear trains. The method provides separate signal averages for the individual planets based on an estimate of each planet's contribution to the total vibration signal. This method has far superior performance to an earlier method of extracting epicyclic planet gear vibration signatures.
Signal averaging has proved to be the most useful vibration analysis tool for detecting faults in gears. However, there has been a problem in the past in applying the technique to epicyclic gearboxes. An epicyclic gearbox has a number of planet gears which all mesh with a sun and ring gear. The problems encountered when attempting to perform a signal average for components within an epicyclic gearbox are twofold. Firstly, there are multiple tooth contacts, with each planet being simultaneously in mesh with both the sun and ring gear, and secondly, the axis of the planets move with respect to both the sun and ring gear.
An earlier method of performing selective signal averaging on epicyclic gearboxes was developed and tested at the Aeronautical and Maritime Research Laboratory Defense Science and Technology Organization, Melbourne, Victoria 3001, Australia. That method was useful in detecting faults on individual planet gears; however, it was tedious to implement, required an excessively long time to perform even a small number of averages and required a selective chopping up of the time signal which proved to introduce discontinuities in the signal average.
Another method attempted to extract a representative signal average for each planet by taking a narrow window or snap-shot of the vibration signal each time a planet came past the transducer. These small packets of vibration were then assembled into appropriate buckets in a stored signal average. Typically each packet of vibration would represent a single tooth mesh and only one of these could be collected for a particular planet for each revolution of the planet carrier. To create one complete ensemble, representing a single revolution of a planet, required N, revolutions of the planet carrier, where N
p
is the number of teeth on a planet gear, and p is the planet gear.
Although this windowing technique appeared to work, being capable of detecting seeded faults in a test rig, there are a number of problematic aspects to it, including the following:
a) The amount of time taken to generate a usable signal average is impractical for in-service use. For instance, the Sea King helicopter main rotor gearbox has an epicyclic gear train with 196 teeth on the ring gear, 71 teeth on each of five planets and the planet carrier rotates at approximately 3.8 Hertz. This would take 71/3.8=18.68 seconds of data for each ensemble; requiring over nine minutes to perform a signal average of thirty ensembles for a single planet gear. During this time, it would be necessary to hold the torque relatively constant to ensure correct signal averaging which is totally impractical for an operational aircraft.
b) The partitioning of the vibration data into small packets creates some discontinuities at the boundaries of each bucket on reassembly. The theoretical assumption was that as the number of averages approached infinity, any discontinuities would be eliminated. However, as it is not practical to take more than a few averages, these discontinuities remain. The most worrying aspect of this is that, if the packets are exactly one tooth wide, the discontinuities appear at the mesh frequency, making any attempt to eliminate them, or even detect them, by post processing impossible.
c) In creating the signal averages for each planet, a large portion of the vibration data is neglected.
The herein disclosed alternative method for performing signal averaging for epicyclic gearbox components overcomes the problems of the earlier method. A mathematical derivation of the method is provided which shows that the averaging can be performed with no loss of information, by proportionally dividing the vibration data amongst the individual gear meshes. Practical examples are presented which show that the method has far superior performance than the earlier method and requires no longer to perform than would be required for conventional signal averaging. The present invention is also an improvement upon Australian Patent 40638/95, incorporated herein by reference.
The basic method of the invention concerns detecting a fault in an epicyclic gear train having ring, planet and sun gears, and a planet carrier, and the steps of the method include:
a) locating a vibration monitoring detector in association with the ring gear,
b) operating the gear train to produce vibration detected by the detector to obtain an overall vibration signal,
c) and timewise dividing the overall vibration signal into contributions from each planet gear.
Additional steps include determining time filtered signal average values with respect to the planet gears; and determining such average values substantially in accordance with an equation or equations disclosed herein.
An additional object concerns determining a time window value b(t), substantially in accordance with a further equation or equations disclosed herein.
With respect to the basic method referred to above, it is a further object to employ a rotating member in the drive to or from the gear train, employing sampling of the detector vibration, and including synchronizing such sampling with an angular reference associated with the rotating member. Such synchronizing may be effected employing one of the following: frequency multiplication or signal resampling.
A further object is to provide means elements corresponding to the steps of the method or methods referred to.
These and other objects and advantages of the invention, as well as the details of an illustrative embodiment, will be more fully understood form the following specification and drawings, in which:
REFERENCES:
patent: 4931949 (1990-06-01), Hernandez et al.
patent: 5698788 (1997-12-01), Mol et al.
patent: 40638/95-B (1996-09-01), None
patent: 56168518-A (1981-12-01), None
patent: 56168519-A (1981-12-01), None
patent: 56168520-A (1981-12-01), None
patent: 56168521-A (1981-12-01), None
patent: 56168522-A (1981-12-01), None
Aeronautical and Maritime Research Laboratory, Defence Science a
Haefliger William W.
Kwok Helen
Miller Rose M.
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