2. Engine Vibration Sensor
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A. No. 1 Bearing Vibration Sensor
(1) General The No. 1 bearing vibration sensor permanently monitors the vibrations from No. 1 bearing. It also senses vibrations from LPT and HPT shafts, though it is less sensitive to LPT and HPT shaft vibrations. It is also used for trim balance operations.
(2) Description (Ref. Fig. 001) The accelometer part of the vibration sensor is located at the 9:00 o'clock position on No. 1 and No. 2 bearing support (near No. 1 bearing). The sensor cable is routed through the fan frame. It comes out at the
3:00 o'clock position on fan frame mid-box structure aft face. The No.1 bearing vibration sensor consists of the following items :
(a) A housing including :
- a 100 pc/g sensing element (accelerometer)
- a bracket with 2 holes ensuring housing attachment to the engine with 2 bolts.
(b) A semi-rigid cable including :
- 2 conductors inserted in an inner sheath and an outer sheath
- 2 metal tubes providing for cable rigidity
- a metal plate for cable attachment to the forward flange of fan frame mid-box structure
- 5 shock absorbers to guide and dampen the cable in a tube through fan frame
- a 3-pin connector with a nut at one end. The nut ensures connector attachment to the fan frame aft face. The connector body has 2 flats which ensure its positioning with a lockplate secured by 2 bolts
- 4 clamps for attachment of the tube to fan frame.
(3) Operation The sensor is of the piezo-electric type. It includes a stack of piezo-electric discs placed between a mass and a base. When the accelerometer is subjected to a vibration, the mass exerts a variable force on the discs : this generates a potential difference directly proportional to the acceleration in a certain frequency range.
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No. 1 Bearing Vibration Sensor and Cable Assembly Figure 001
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This potential difference delivers a signals. The analysis of the signal enables identification of vibration characteristics of the monitored part.
B. Turbine Rear Frame Vibration Sensor
(Ref. Fig. 002, 003, 004)
(1) General The Turbine Rear Frame (TRF) vibration sensor is used in conjunction with the No. 1 bearing vibration sensor to monitor and, if necessary, reduce the engine vibration level using the trim balance procedure. The vibration signal is used by the aircraft Engine Vibration Monitoring Unit (EVMU).
(2) Description The TRF vibration sensor is installed at 12 o'clock on the front flange of the turbine rear frame. It consists in a hermetically sealed housing that encloses the sensing element. A flange with two holes is used to attach the housing to the engine. One electrical connector at the end of semi-rigid cable provides the interface with an aircraft harness. The accelerometer sensitivity is 50 pC/g.
(3) Operation The sensor is of the piezo-electric type. A piezo-electric material generates electrical charges when it is submitted to a mechanical stress. An inert mass is attached to a piece of piezo-electric material. When the assembly is submitted to vibrations, the inertia of the mass generates mechanical stresses in the piezo-electric material. It is therefore possible to collect an electrical signal the frequency and intensity of which are representative of the vibration level to be measured.
3. Engine Vibration Monitoring Unit (EVMU)
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(Ref. Fig. 005, 006)
A. General The Engine Vibration Monitoring Unit (EVMU) is located in the avionics compartment shelf 86VU. The EVMU receives analog signals from :
- the 4 engine accelerometers (2 per engine)
- and the N1 and N2 speed sensors of each engine.
It also receives digital input from CFDS through ARINC 429 data bus.
The EVMU sends signals through the digital ARINC 429 data bus to :
- SDAC1 and 2 for cockpit indication
- the CFDIU
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Turbine Rear Frame Vibration Sensor Figure 002
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