Downloaded from https://www.everyspec.com ADS-79D-HDBK E.4.1 Shaft condition indicators. Shaft CIs are mathematically simpler compared to gear and bearing CIs because the shaft faults are detected through simple harmonics of the shaft operating speed (Table E-I). The key indicators of shaft faults can be calculated through either asynchronous or synchronous means, using a synchronous time average (STA). The following is a non-exhaustive list of CIs for shaft faults that are proven both on test stands and in the field environment: E.4.2 Shaft balance and rotor smoothing. Shaft balance and rotor smoothing diagnostics are basic CBM functions.
Detailed discussion regarding rotor smoothing is in Appendix F. E.4.2.1 Shaft balance Shaft balancing procedures are required on some aircraft platforms. The system may use permanently installed accelerometers to monitor the condition of shafts throughout the drive train, especially shafts operating at very high frequencies (greater than 200 Hz). An example would be the engine output shaft. Small mass imbalance on a high frequency shaft induces high vibration levels that can be destructive to the surrounding equipment, potentially causing the catastrophic loss of the aircraft. Shaft balance is achieved using a combination of the shaft condition indicators and balancing algorithms.
The system should be capable of using linear balance coefficients and applying basic shaft balance techniques to reduce vibrations below determined thresholds. E.4.3 Bearing condition indicators. Bearing faults are typically associated with the rolling elements, cages, and races which make up the bearing and their associated fundamental fault frequencies (Table E-II). Faults also appear as increases in energy bands. In current practice, there are two distinct methods for calculating CIs that use energy based algorithms. The methods differ in their use of an enveloping technique. 23,24 The following CIs are for bearings: 23 Bracewell, R.M.
―The Fourier Transform and its Applications‖, McGraw-Hill, 1965. 24 McFadden, P.D. ―Analysis of the Vibration of the Input Bevel Pinion in RAN Wessex Helicopter Main Rotor Gearbox WAK143 Prior to Failure‖ Aero Propulsion Report 169, Department of Defense, Defense Science and Technology Organization, Aeronautical Research Laboratories. 106 TABLE E-I. Shaft condition indicators Asynchronous Shaft Order ½ (SO½) Synchronous Shaft Order 2 (SO2) Asynchronous Shaft Order 1 (SO1) Synchronous Shaft Order 3 (SO3) Asynchronous Shaft Order 2 (SO2) STA RMS Asynchronous Shaft Order 3 (SO3) STA Peak to Peak Synchronous Shaft Order ½ (SO½) STA Kurtosis Synchronous Shaft Order 1 (SO1) TABLE E-II.
Bearing condition indicators Envelope Ball Energy Envelope Base Energy Envelope Cage Energy Envelope High Frequency Energy (15 – 20 kHz) Envelope Inner Race Energy Peak Pick Envelope Outer Race Energy Frequency Band Energy Envelope Tone Energy