A $20,000.00 installation of
additional cabin sound proofing is
less effective than a $500.00 fan
trim balance for reducing noise
caused by an out of balance fan.
The balance job is also 100% more
effective in reducing vibration!
Creating better aviation maintenance solutions...
Turbine Fan
Trim Balance
.
Creating better aviation maintenance solutions...
Goals for this Seminar
Each attendee should
Understand Basic Vibration
Understand the Fundamentals of
Fan Trim Balancing
Understand Why Balancing is Beneficial
Creating better aviation maintenance solutions...
Overview
What is vibration?
What are the effects of vibration?
How is vibration measured?
What are the options in reducing vibration?
How is vibration analyzed when balancing?
Creating better aviation maintenance solutions...
What is Vibration?
For the purpose of fan trim balance in a
Turbofan Engine, vibration can be described as
the unwanted, unproductive,
cyclic oscillation of the
fan assembly about its
rotational axis.
Creating better aviation maintenance solutions...
REMEMBER...
All noise and/or vibration is not generated by an
imbalance in the fan.
To verify the vibration source, a vibration
survey should be conducted
and manufacturers’
limitations adhered to.
Creating better aviation maintenance solutions...
Examples of
Vibration-Related Complaints
Passenger complaints of noise in the cabin
Higher than normal EVM (Engine Vibration
Monitoring) system indications
Physical movement of airframe (buzz in the
seat, yoke, rudder pedals)
Malfunctioning or failed avionics
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What Are the Effects of Vibration?
Vibration excites natural frequencies causing
significant vibration at the components.
The expended energy from vibration causes
wear of components, reduced performance,
passenger discomfort and reduced fuel
economy.
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How Is Vibration Measured?
Sensor Types
Sensor Engineering Units (EUs)
Characteristics of Different Sensor Types
Sensor Specifications
Sensor Mounting
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How Is Vibration Measured?
Sensor Types
Displacement - Measures physical
change of position.
Velocity - Measures the rate
of change of displacement with time.
Acceleration - Measures the
rate of change of velocity with time.
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How Is vibration measured?
Engineering Units
Mils (0.001 inches) - displacement
IPS (Inches Per Second) - velocity
gs (equivalent gravities) - acceleration
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How Is Vibration Measured?
Modifiers
Peak
Peak to Peak
Average
RMS
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How Is vibration measured?
Sensor Characteristics
Displacement Sensor
Measure change in position
Typically reported in mils Peak to Peak
More sensitive to low frequencies
Directly related to movements due to imbalance
Seldom used in balance. (However, displacement units
(Mils) are sometimes used)
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How Is Vibration Measured?
Sensor Characteristics
Velocimeters/Velometers
Measure velocity
Typically reported in IPS Peak
More sensitive to medium frequencies
Directly related to energy from imbalance
Often used in balance
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How Is Vibration Measured?
Sensor Characteristics
Accelerometers
Measure acceleration
Typically reported in g’s
More sensitive to higher frequencies
Directly related to force caused by imbalance
Used in balancing (after conversion to velocity or
displacement)
Creating better aviation maintenance solutions...
How Is vibration measured?
Sensor Characteristics
Specifications
Sensitivity (millivolts per Engineering Unit)
Temperature range
Physical size
Physical weight
Mounted resonance frequencies
Use manufacturer recommendations for engine
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How Is Vibration Measured?
Creating better aviation maintenance solutions...
How Is Vibration Measured?
Sensor Characteristics
Sensor Mounting
Use engine manufacturer recommended mount
Mount sensor per manufacturer instructions
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Options for Reducing Vibration
Remove and replace the faulty component.
Repair the faulty component.
Use active cancellation systems.
Dynamically balance the fan.
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How Is Vibration Analyzed
A vibration sensor measures vibration in a given
direction.
The once per revolution tach signal provides a
timing reference from which the phase angle is
calculated.
The relationship between the two is used to identify
the magnitude of the vibration and the phase angle
or radial location of the heavy spot on the fan.
Creating better aviation maintenance solutions...
How Is Vibration Analyzed
The vibration sensor generates an
electrical signal as the heavy spot on
the fan passes its location and sends
the signal to the analyzer. The analyzer
measures its voltage, the time of its
occurrence and records these in
memory.
The reflective tape triggers a response
in the LASETACH® as it passes the
optical laser beam. The LASETACH
then sends an electrical signal to the
analyzer. The time of arrival is
recorded in the analyzer’s memory.
Creating better aviation maintenance solutions...
How Is Vibration Analyzed
The vibration sensor and reflective
tape are installed on the engine. The
LASETACH is mounted up to 30 feet in
front of the inlet and aimed at the
spinner where the tape is to trigger the
tach event. The mass (heavy spot on
the fan) is located by relative
occurrence of the tach event
(reflective tape passing in front of
laser beam) and mass passage at the
radial sensor location. In this slide,
the tach event is about to occur and
the vibration event is near zero
between the positive and negative
vibration events.
Creating better aviation maintenance solutions...
How Is Vibration Analyzed
As rotation of the fan continues, the
mass is now located opposite the
vibration sensor. This is the point of
the maximum negative going peak in
the sine wave. From this point, the
mass will move back toward the zero
crossing (between negative and
positive) then upward toward the
sensor. This compresses the
piezoelectric element inside which
generates the voltage output. That
voltage is sent along the connecting
cable to the analyzer where it is
measured and converted to
engineering units (Mils, gs, IPS).
Creating better aviation maintenance solutions...
How Is Vibration Analyzed
The mass is about to enter a point in
the rotation where the positive upward
movement begins in the vibration
event. The strength of the electrical
signal increases from this point and
peaks as the mass reaches the sensor
location.
Creating better aviation maintenance solutions...
How Is Vibration Analyzed
In this slide the mass, or heavy spot
on the fan, is approaching the position
of the vibration sensor. Notice that the
reflective tape (now at the 3:00
position) has traveled 270 degrees
since the tach event. The maximum
amplitude of the vibration, as
measured by the vibration sensor
occurs here.
Creating better aviation maintenance solutions...
Creating better aviation maintenance solutions...
Fundamentals of Balancing
Overview
What information is required
How the data is collected
How the balance solution is computed
How the balance weight is installed
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Fundamentals of Balancing
What information is required?
Engine speed(s) for balancing
The vibration amplitude for each speed
The angular reference (phase) of the vibration for
each speed
Influence on the fan of weight addition for each
speed
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Fundamentals of Balancing
How to select balance speeds.
Use Manufacturers Recommended Speeds
Use Speed of the complaint
Use Vibration Survey to Select Speed(s)
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Fundamentals of Balance
What is an Influence?
A ratio of how much weight is required to counter a
measured out of balance condition
Typical units are (grams/IPS) with a phase lag
It is used to compute a balance solution
It can be estimated for the first run
It should be refined on subsequent runs from
actual vibration measurements
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Fundamentals of Balance
How is the required data collected?
Install vibration sensor
Install speed sensor
Run aircraft
Collect average magnitude and phase data for each
speed and sensor of interest
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Fundamentals of Balance
How is balance solution calculated?
The balancer calculates a solution based on the vibration
magnitude and phase data collected for each speed
It presents the solution to the user, such as “Place 2
Grams of weight at 90 degrees” or “Install a -2 weight in
hole number 2”
Install/record the weight added and its location
Run the engine again to verify predicted results
Refine solution if necessary
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Fundamentals of Balance
How is the solution refined?
Actual vibration changes and weight additions are used to
compute a more accurate estimate of the influence at each
speed
This recomputed “accurate” influence is used to estimate a
new balance weight
Balancer should record actual weights added
Influence should be further refined on each additional run
Creating better aviation maintenance solutions...
Fundamentals of Balance
Notes on calculated solutions
All turbine fans are different!
Sophisticated balance equipment adjusts for these
differences after the first weight placement
Vibration will not always go down with the first weight
placement
If the vibration is not reduced by the second adjustment,
check mechanical condition and/or balance process
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Fundamentals of Balance
How are the balance weights installed?
On first run, place closest weight in closest hole
On later runs, split weights between holes for more accurate
solution
If removing the spinner, index it so it can be reinstalled in
the same location and orientation
Beware of different length bolts used in weight placement or
spinner attachment
Use the engine manufacturers instructions
Creating better aviation maintenance solutions...
Fundamentals of Balance
Where to place the weights
NOTE: If your balancing equipment gives you
specific bolt or hole locations and class weight
information for addition of trim weights, these steps
are not necessary.
Creating better aviation maintenance solutions...
Fundamentals of Balance
Where to place the weights
1. Index the fan back to the
point where the tach event
occurs. (0 or 360 degrees)
2. Rotate the fan assembly, in
the direction of rotation, the
number of degrees specified in
the balance solution. The
location for the weight is now
adjacent to the vibration
sensor.
Creating better aviation maintenance solutions...
Fundamentals of Balance
How is weight split between holes?
Compute weights between holes so that the total
effective weight is the same as the desired weight
If class weights (a fixed set of available weights are
used on the engine), use an optimizing algorithm to
select the best weight combination to approximate
the desired weight.
Creating better aviation maintenance solutions...
Fundamentals of Balance
Summary of Steps:
Install vibration sensor
Install the once per revolution tachometer
Perform a vibration survey and verify balance necessary
Remove any existing trim balance weights
Collect magnitude and phase data for each speed of
interest
Compute weight necessary for balance
Creating better aviation maintenance solutions...
Fundamentals of Balance
Summary of Steps: (continued)
Convert the solution into a class weight for the
required weight and to a bolt or hole number for the
angle.
Present the solution to user as “Place -2 weight at
hole #3" for instance
Ask user for the actual weights installed
Run the engine again and collect vibration data.
Creating better aviation maintenance solutions...
Fundamentals of Balance
Summary of Steps: (continued)
Verify predicted results.
If acceptable, balance job is complete
If not, compute refined estimate of the influence
and continue the process at computing weight
Creating better aviation maintenance solutions...
Why Is Balancing Beneficial?
Reduces vibration and noise induced stress on crew
and passengers.
Airframe, avionics, and engine systems sustain less
damage. Useful life is extended.
Operational efficiency is increased because the energy
previously used to generate noise and vibration is now
used to generate thrust.
Creating better aviation maintenance solutions...
Review
Vibration
What is vibration
Examples of vibration and related complaints
Effects of vibration
How vibration is measured
How vibration is analyzed for balancing
Creating better aviation maintenance solutions...
Review
Why is Balancing Beneficial
Reduces vibration and noise induced stress on
crew and passengers.
Airframe, avionics, and engine systems sustain
less damage. Useful life is extended.
Operational efficiency is increased because the
energy previously used to generate noise and
vibration is now used to generate thrust.
Creating better aviation maintenance solutions...
Conclusion
If you’ve already bought additional insulation
for your company jet to reduce noise, you didn’t
get what you paid for. For much less money
and time a Fan Trim Balance is the best choice
for a quiet smooth engine.
Creating better aviation maintenance solutions...
Contact
www.acessystems.com
1-865-671-2003
sales@acessystems.com
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