(1)
Do the trim balance procedure to decrease the vibration levels of the low (N1) rotor to a satisfactory level.
B.
If the vibration occurred after the replacement of a fan blade, you must examine the replaced blades to make sure the moment weight pairs are not more than 1.0 ounce-inch in difference. The moment weight is written on the mid-span shroud. If it is necessary, you can replace the pair with a different set of fan blades that have more equal moment weights.
C.
Use the trim balance to correct the stable vibration of the N1 rotor.
D.
Description of the three methods which will decrease the low rotor
vibration levels in the engine are as follows:
(1)
The LPC Trim Balance - PBS-4100 Dual Pick-Up procedure uses the PBS-4100 Trim-balance system that collects engine vibration data from two engine installed vibration pick-ups; one on "A" flange and one on "P" flange. The PBS-4100 also calculates the balance solution. This is the preferred method because it will produce the lowest engine vibration levels. This method can stop vibration in the front balance plane of the LPC and the rear balance plane of the LPT. This method must be used when reductions in cabin noise are necessary.
(2)
The Trim-Balance - Airborne Vibration Monitoring (AVM) procedure uses the aircraft AVM system only. It is not necessary to install additional equipment. You can manually get the engine vibration data on the EICAS Performance maintenance page. This method is used to lower the AVM "A" flange pick-up vibration levels only. This method limits the quantity of weight that can be installed to the fan balance flange. This method cannot balance the turbine plane.
(3)
The LPC Trim Balance - PBS-4100/AVM procedure uses the PBS-4100 balancing system and the aircraft AVM system. The PBS-4100 is plugged into the AVM Signal Conditioner. The PBS-4100 collects the vibration data and calculates the balance solution. This method is used to lower the AVM "A" flange pick-up vibration levels only. This method limits the quantity of weight that can be installed to the fan balance flange. This method cannot balance the turbine plane.
E.
Engines that have a stable total vibration level can be trim balanced
more to decrease the N1 rotor vibration.
(1)
A stable vibration has a peak amplitude that does not change more than 0.5 units (1.0 mils DA) after 30 seconds of engine operation.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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TASK 72-31-00-725-081-N00
2. Balance the Front and Rear Balance Plane (Trim Balance - PBS-4100 Dual Pick-
____________________________________________________________________________ Up Procedure)
_____________
A. General
(1)
It is recommended that this trim balance procedure be done by personnel that are trained in the use and operation of the PBS-4100 portable trim balancing system and have experience with engine trim balance.
(2)
This procedure uses the Mechanical Technologies Portable Balancing System (PBS-4100) to find the quantity and the angular location of the correct weight(s) necessary to trim balance the engine. The weight is added to stop vibration in the front balance plane of the LPC and the rear balance plane of the LPT.
(3)
The weights for the engine trim balance are installed on the compressor (front balance plane) and the turbine (rear balance plane).
(4)
A fan plane balance will affect the low rotor vibration levels on both the vibration pick-ups ("A" and "P" Flanges). Also, fan plane trim balance counter-weights are easier to install. Therefore, it is recommended you try a fan plane balance first.
(5)
Trim balance can decrease engine vibration. The trim balance procedure adds a small amount of weight to correct the unbalance. The weight(s) are installed on the compressor front balance plane and the turbine rear balance plane. The trim balance procedure can be done on the engine while it is on the airplane.
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