Major takeaways from vibration and engine balancing seminar by Bentley Nevada
Held at Aeronautical society of India, February 28, 2020
- ICP accelerometers are not rugged. They are sensitive to temperature fluctuations.
- Charge type accelerometers are good for field use
- The gap in peaks between rotor and stator vibration signals is of the order of 10 microseconds
- Techniques to measure rotor angle position in aero engines:
- Rolls Royce: One tooth in gear wheel is shorter/taller than others
- Pratt and Whitney: Gap between successive gear teeth is smaller for only one pair.
- Pratt and Whitney's method requires a high sampling rate.
- PBS 4100 is the Industry standard for vibration analysis.
- 32% of fuel in aero engines goes to turning the fan in turbofans
- Vibration survey for Aero engines is a plot between the amplitude of vibrations and speed. 2 curves are typically presented:
- N1 signal amplitude (lower).
- Broadband (BB) (higher)
- BB is all the vibration measured by the sensor
- N1 is the vibration amplitude between 45 and 55 Hz for a 50 Hz shaft.
- N1 signal plotting requires a tracking filter.
- BB > N1, typically.
- If all vibration is caused by N1 alone, N1=BB
- PBS 4100:
- Number of channels used is 4 for portable instruments, 20 for test cells
- TCP / IP available
- Labview drivers available
- Online data retrieving possible
- C, C++ API available
- Digital output available
- Safety limits & triggers can be configured
- Balancing of the engine requires an influence coefficient matrix of the engine and one set of acceleration vibration surveys.
- 4 or 5 points on the vibration survey are taken for calculation of compensating masses.
- CFM 56 old TBO 5000 Hours. Now TBO is 7500 hrs. The current target is 12000 hrs. Fan RPM is 500 to 5750 RPM
- For balancing, the software can be forced to use standard weights.
- PBS4100 can be told to minimize vibration at a particular speed and not to care about other higher speeds.
- Imbalance in turbine plane can be corrected by masses in compressor plane
- Twin spool rotor (HP spool of CFM56 in 737 airplanes) core balancing is also done in the same fashion using optical pickups in the inspection window using a blade that is painted white in color.
- 1500CS calibrator can be used to apply 0 to 10,000 pico coulombs to the cable of the accelerometer. 1500CS has 2 function generators for sine, square voltage waveforms. The phase difference between 2 channels of PBS also needs to be calibrated upto 0.1 degrees using 1500CS.
- Simultaneous sampling by PBS has a delay in the order of nanoseconds.
- PBS is a rugged system and uses an accelerometer on static casing. Eddy current/proximity probes are not rugged and cannot be targeted on rotating shaft in aircraft engines.
- If phase of signal is steady, the mounting of accelerometer is good. If phase signal is jittery, mounting is not good.
- EGT and oil temperature have to be stabilized before collecting data for balancing. The phase of the accelerometer signal can drift due to temperature changes and due to non settled EGT levels.
- Shaft runout can be seen in the bode plot.
- Journal bearings are difficult to balance with this system. You need to see spectrum of responses to make a decision on feasibility.
- Comment by Sanjay Barad GTRE: Typically for journal bearings, we take response at low rpm (1000 RPM) and subtract it from the response at 25000 RPM. Subtraction has to be done vectorially. ADRE has such a provision. Squeeze film damper for bearings also to be done similarly.
- The lunch served was very simple.
Few photos:
